Targeting TGFβ-activated kinase-1 activation in microglia reduces CAR T immune effector cell-associated neurotoxicity syndrome

Cancer immunotherapy with chimeric antigen receptor (CAR) T cells can cause immune effector cell-associated neurotoxicity syndrome (ICANS). However, the molecular mechanisms leading to ICANS are not well understood. Here we examined the role of microglia using mouse models and cohorts of individuals with ICANS. CD19-directed CAR (CAR19) T cell transfer in B cell lymphoma-bearing mice caused microglia activation and neurocognitive deficits. The TGFβ-activated kinase-1 (TAK1)-NF-κB-p38 MAPK pathway was activated in microglia after CAR19 T cell transfer. Pharmacological TAK1 inhibition or genetic Tak1 deletion in microglia using Cx3cr1CreER:Tak1fl/fl mice resulted in reduced microglia activation and improved neurocognitive activity. TAK1 inhibition allowed for potent CAR19-induced antilymphoma effects. Individuals with ICANS exhibited microglia activation in vivo when studied by translocator protein positron emission tomography, and imaging mass cytometry revealed a shift from resting to activated microglia. In summary, we prove a role for microglia in ICANS pathophysiology, identify the TAK1-NF-κB-p38 MAPK axis as a pathogenic signaling pathway and provide a rationale to test TAK1 inhibition in a clinical trial for ICANS prevention after CAR19 T cell-based cancer immunotherapy.

Antigen experience history directs distinct functional states of CD8+ CAR T cells during the anti-leukemia response

Chimeric antigen receptor T cells are an effective therapy for B-lineage malignancies. However, many patients relapse and this therapeutic has yet to show strong efficacy in other hematologic or solid tumors. One opportunity for improvement lies in the ability to generate T cells with desirable functional characteristics. Here, we dissect the biology of CD8+ CAR T cells (CAR8) by controlling whether the T cell has encountered cognate TCR antigen prior to CAR generation. We find that prior antigen experience influences multiple aspects of in vitro and in vivo CAR8 functionality, resulting in superior effector function and leukemia clearance in the setting of limiting target antigen density compared to antigen-inexperienced T cells. However, this comes at the expense of inferior proliferative capacity, susceptibility to phenotypic exhaustion and dysfunction, and inability to clear wildtype leukemia in the setting of limiting CAR+ cell dose. Epigenomic and transcriptomic comparisons of these cell populations identified overexpression of the Runx2 transcription factor as a novel strategy to enhance CAR8 function, with a differential impact depending on prior cell state. Collectively, our data demonstrate that prior antigen experience determines functional attributes of a CAR T cell, as well as amenability to functional enhancement by transcription factor modulation.

Children's Oncology Group's 2023 blueprint for research: Cellular therapy and stem cell transplantation

Since the publication of the last Cellular Therapy and Stem Cell Transplant blueprint in 2013, Children's Oncology Group cellular therapy-based trials advanced the field and created new standards of care across a wide spectrum of pediatric cancer diagnoses. Key findings include that tandem autologous transplant improved survival for patients with neuroblastoma and atypical teratoid/rhabdoid brain tumors, one umbilical cord blood (UCB) donor was safer than two UCB donors, killer immunoglobulin receptor (KIR) mismatched donors did not improve survival for pediatric acute myeloid leukemia when in vivo T-cell depletion is used, and the depth of remission as measured by next-generation sequencing-based minimal residual disease assessment pretransplant was the best predictor of relapse for acute lymphoblastic leukemia. Plans for the next decade include optimizing donor selection for transplants for acute leukemia/myelodysplastic syndrome, using novel engineered cellular therapies to target a wide array of malignancies, and developing better treatments for cellular therapy toxicities such as viral infections and graft-vs-host disease.

Chimeric Antigen Receptor T Cells as Salvage Therapy for Post-Chimeric Antigen Receptor T Cell Failure

Outcomes for post-chimeric antigen receptor (CAR) T cell therapy (CART) relapse are poor. The utilization of a unique CAR T cell construct for post-CART failure is increasing, but this approach is not well described. In this study, with CART-A the first unique CAR T cell construct received and CART-B the second, the primary objective was to characterize outcomes following CART-B. Secondary objectives included evaluating safety and toxicity with sequential CART infusions; investigating the impact of potential factors, such as antigen modulation and interval therapy, on CART-B response; and characterizing long-term outcomes in patients receiving multiple CARTs. This was a retrospective review (NCT03827343) of children and young adults with B cell acute lymphoblastic leukemia (B-ALL) undergoing CART therapy who received at least 2 unique CART constructs, excluding interim CART reinfusions of the same product. Of 135 patients, 61 (45.1%) received 2 unique CART constructs, including 13 who received >2 CARTs over time. Patients included in this analysis received 14 distinct CARTs targeting CD19 and/or CD22. The median age at CART-A was 12.6 years (range, 3.3 to 30.4 years). The median time from CART-A to CART-B was 302 days (range, 53 to 1183 days). CART-B targeted a different antigen than CART-A in 48 patients (78.7%), owing primarily to loss of CART-A antigen target. The rate of complete remission (CR) was lower with CART-B (65.5%; 40 of 61) than with CART-A (88.5%; 54 of 61; P = .0043); 35 of 40 (87.5%) CART-B responders had CART-B targeting a different antigen than CART-A. Among the 21 patients with a partial response or nonresponse to CART-B, 8 (38.1%) received CART-B with the same antigen target as CART-A. Of 40 patients with CART-B complete response (CR), 29 (72.5%) relapsed. For the 21 patients with evaluable data, the relapse immunophenotype was antigennegative in 3 (14.3%), antigendim in 7 (33.3%), antigenpositive in 10 (47.6%), and lineage switch in 1 (4.8%). The median relapse-free survival following CART-B CR was 9.4 months (95% confidence interval [CI], 6.1 to 13.2 months), and overall survival was 15.0 months (95% CI, 13.0 to 22.7 months). Given the limited salvage options for post-CART relapse, identifying optimizing strategies for CART-B is critical. We raise awareness about the emerging use of CART for post-CART failure and highlight clinical implications accompanying this paradigm shift.

CD22 CAR T-cell associated hematologic toxicities, endothelial activation and relationship to neurotoxicity

BACKGROUND

Hematologic toxicities, including coagulopathy, endothelial activation, and cytopenias, with CD19-targeted chimeric antigen receptor (CAR) T-cell therapies correlate with cytokine release syndrome (CRS) and neurotoxicity severity, but little is known about the extended toxicity profiles of CAR T-cells targeting alternative antigens. This report characterizes hematologic toxicities seen following CD22 CAR T-cells and their relationship to CRS and neurotoxicity.

METHODS

We retrospectively characterized hematologic toxicities associated with CRS seen on a phase 1 study of anti-CD22 CAR T-cells for children and young adults with relapsed/refractory CD22+ hematologic malignancies. Additional analyses included correlation of hematologic toxicities with neurotoxicity and exploring effects of hemophagocytic lymphohistiocytosis-like toxicities (HLH) on bone marrow recovery and cytopenias. Coagulopathy was defined as evidence of bleeding or abnormal coagulation parameters. Hematologic toxicities were graded by Common Terminology Criteria for Adverse Events V.4.0.

RESULTS

Across 53 patients receiving CD22 CAR T-cells who experienced CRS, 43 (81.1%) patients achieved complete remission. Eighteen (34.0%) patients experienced coagulopathy, of whom 16 had clinical manifestations of mild bleeding (typically mucosal bleeding) which generally subsided following CRS resolution. Three had manifestations of thrombotic microangiopathy. Patients with coagulopathy had higher peak ferritin, D-dimer, prothrombin time, international normalized ratio (INR), lactate dehydrogenase (LDH), tissue factor, prothrombin fragment F1+2 and soluble vascular cell adhesion molecule-1 (s-VCAM-1). Despite a relatively higher incidence of HLH-like toxicities and endothelial activation, overall neurotoxicity was generally less severe than reported with CD19 CAR T-cells, prompting additional analysis to explore CD22 expression in the central nervous system (CNS). Single-cell analysis revealed that in contrast to CD19 expression, CD22 is not on oligodendrocyte precursor cells or on neurovascular cells but is seen on mature oligodendrocytes. Lastly, among those attaining CR, grade 3-4 neutropenia and thrombocytopenia were seen in 65% of patients at D28.

CONCLUSION

With rising incidence of CD19 negative relapse, CD22 CAR T-cells are increasingly important for the treatment of B-cell malignancies. In characterizing hematologic toxicities on CD22 CAR T-cells, we demonstrate that despite endothelial activation, coagulopathy, and cytopenias, neurotoxicity was relatively mild and that CD22 and CD19 expression in the CNS differed, providing one potential hypothesis for divergent neurotoxicity profiles. Systematic characterization of on-target off-tumor toxicities of novel CAR T-cell constructs will be vital as new antigens are targeted.

TRIAL REGISTRATION NUMBER

NCT02315612.

Hypoimmune anti-CD19 chimeric antigen receptor T cells provide lasting tumor control in fully immunocompetent allogeneic humanized mice

Manufacturing autologous chimeric antigen receptor (CAR) T cell therapeutics is complex, and many patients experience treatment delays or cannot be treated at all. Although current allogeneic CAR products have the potential to overcome manufacturing bottlenecks, they are subject to immune rejection and failure to persist in the host, and thus do not provide the same level of efficacy as their autologous counterparts. Here, we aimed to develop universal allogeneic CAR T cells that evade the immune system and produce a durable response. We generated human hypoimmune (HIP) T cells with disrupted B2M, CIITA, and TRAC genes using CRISPR-Cas9 editing. In addition, CD47 and anti-CD19 CAR were expressed using lentiviral transduction. These allogeneic HIP CD19 CAR T cells were compared to allogeneic CD19 CAR T cells that only expressed the anti-CD19 CAR (allo CAR T). In vitro assays for cancer killing and exhaustion revealed no differences between allo CAR T and HIP CAR T cells, confirming that the HIP edits did not negatively affect T cell performance. Clearance of CD19⁺ tumors by HIP CAR T cells in immunodeficient NSG mice was comparable to that of allo CAR T cells. In fully immunocompetent humanized mice, HIP CAR T cells significantly outperformed allo CAR T cells, showed improved persistence and expansion, and provided lasting cancer clearance. Furthermore, CD47-targeting safety strategies reliably and specifically eliminated HIP CAR T cells. These findings suggest that universal allogeneic HIP CAR T cell-based therapeutics might overcome the limitations associated with poor persistence of allogeneic CAR T cells and exert durable anti-tumor responses.

Abstract 4091: Engineered hypoimmune CAR T cells provide lasting tumor control in immunocompetent allogeneic humanized mice even with re-challenge

Off-the-shelf CAR T cells potentially offer advantages over autologous strategies such as ease of manufacturing, quality control, off-the-shelf availability, and lack of T cell dysfunction, as well as the ability to generate a more consistent CAR T product from healthy T cells. However, the vigorous host-versus-graft immune response against histoincompatible T cells prevents expansion and persistence of allogeneic CAR T cells and mitigates the efficacy of this approach. A major challenge is that, while HLA deletion can result in adaptive immune evasion, innate reactivity is enhanced with this approach. CD47 overexpression can block both NK cell and macrophage killing (J Exp Med 2021;218(3):e20200839), and we hypothesized that T cells would lose their immunogenicity when human leukocyte antigen (HLA) class I and II genes are disrupted and CD47 is over-expressed. We describe here the engineering of human immune evasive CAR T cells building on our previously described hypoimmune technology (Nat Biotechnol 2019;37(3):252-258 and Proc Natl Acad Sci U S A 2021;118(28):e2022091118).Human T cells from healthy donors were obtained by leukapheresis. CRISPR/Cas12b technology was used to disrupt the B2M, CIITA, and TCR genes, and lentiviral transduction was used to overexpress CD47 and to express a CD19 CAR to generate hypoimmune (HIP) CD19 CAR T cells. Control T cells were unmanipulated except for overexpression of the CD19 CAR (unmodified).For 3 months persistence studies, allogeneic SGM3 humanized mice were injected with 1 × 106 Luc+ Nalm6 cells and received 7 × 106 control CD19 CAR T cells or HIP CD19 CAR T cells.In the mice treated with either unmodified CD19 CAR T cells and HIP CD19 CAR T cells, tumor control was initially rapidly achieved. However, unmodified CD19 CAR T cells were eventually rejected by the host and the loss of these cells resulted in re-growth of tumor. By contrast, in HIP CD19 CAR T injected mice, tumor control was maintained throughout the study, including following a rechallenge at day 83 with NALM6 cells without further administration of HIP CD19 CAR T cell. Flow cytometry at endpoint from bone marrow and spleen confirmed persistence of HIP CD19 CAR T cells.These findings show that HIP CD19 CAR T cells are immune evasive in allogeneic recipients and data suggest that HIP CD19 CAR T cells are able to persist and maintain efficacy without immunosuppression.

Citation Format: Xiaomeng Hu, Pascal Beauchesne, Karl Manner, Corie Gattis, Priscilla Ngo, Rowena De Jesus, Ramya Ankala, Chi Young, Frank Wells, Lindong Weng, Kathy White, William E. Dowdle, Aaron Foster, Terry J. Fry, Sonja Schrepfer. Engineered hypoimmune CAR T cells provide lasting tumor control in immunocompetent allogeneic humanized mice even with re-challenge. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4091.

Murine allogeneic CAR T cells integrated before or early after posttransplant cyclophosphamide exert antitumor effects

Relapse limits the therapeutic efficacy both of chimeric antigen receptor (CAR) T cells and allogeneic hematopoietic cell transplantation (allo-HCT). Patients may undergo these therapies sequentially to prevent or treat relapsed malignancy. However, direct integration of the 2 therapies has been avoided over concerns for potential induction of graft-versus-host disease (GVHD) by allogeneic CAR T cells. We have shown in murine T-cell-replete MHC-haploidentical allo-HCT that suppressive mechanisms induced immediately after posttransplant cyclophosphamide (PTCy), given on days +3/+4, prevent GVHD induction by alloreactive T cells infused as early as day +5. Therefore, we hypothesized that allogeneic CAR T cells given in a similarly integrated manner in our murine MHC-haploidentical allo-HCT model may safely exert antitumor effects. Indeed, allogeneic anti-CD19 CAR T cells given early after (day +5) PTCy or even prior to (day 0) PTCy cleared leukemia without exacerbating the cytokine release syndrome occurring from the MHC-haploidentical allo-HCT or interfering with PTCy-mediated GVHD prevention. Meanwhile, CAR T-cell treatment on day +9 or day +14 was safe but less effective, suggesting a limited therapeutic window. CAR T cells infused before PTCy were not eliminated, but surviving CAR T cells continued to proliferate highly and expand despite PTCy. In comparison with infusion on day +5, CAR T-cell infusion on day 0 demonstrated superior clinical efficacy associated with earlier CAR T-cell expansion, higher phenotypic CAR T-cell activation, less CD4+CD25+Foxp3+ CAR T-cell recovery, and transcriptional changes suggesting increased activation of CD4+ CAR T cells and more cytotoxic CD8+ CAR T cells. This study provides mechanistic insight into PTCy's impact on graft-versus-tumor immunity and describes novel approaches to integrate CAR T cells and allo-HCT that may compensate for deficiencies of each individual approach.

Potent preclinical activity of FLT3-directed chimeric antigen receptor T-cell immunotherapy against FLT3- mutant acute myeloid leukemia and KMT2A-rearranged acute lymphoblastic leukemia

Chimeric antigen receptor (CAR) T-cell immunotherapies targeting CD19 or CD22 induce remissions in the majority of patients with relapsed/refractory B-cell acute lymphoblastic leukemia (ALL), although relapse due to target antigen loss or downregulation has emerged as a major clinical dilemma. Accordingly, great interest exists in developing CAR T cells directed against alternative leukemia cell surface antigens that may help to overcome immunotherapeutic resistance. The fms-like tyrosine kinase 3 receptor (FLT3) is constitutively activated via FLT3 mutation in acute myeloid leukemia (AML) or wild-type FLT3 overexpression in KMT2A (lysine-specific methyltransferase 2A)-rearranged ALL, which are associated with poor clinical outcomes in children and adults. We developed monovalent FLT3-targeted CAR T cells (FLT3CART) and bispecific CD19xFLT3CART and assessed their anti-leukemia activity in preclinical models of FLT3-mutant AML and KMT2A-rearranged infant ALL. We report robust in vitro FLT3CART-induced cytokine production and cytotoxicity against AML and ALL cell lines with minimal cross-reactivity against normal hematopoietic and non-hematopoietic tissues. We also observed potent in vivo inhibition of leukemia proliferation in xenograft models of both FLT3-mutant AML and KMT2A-rearranged ALL, including a post-tisagenlecleucel ALL-to-AML lineage switch patient-derived xenograft model pairing. We further demonstrate significant in vitro and in vivo activity of bispecific CD19xFLT3CART against KMT2Arearranged ALL and posit that this additional approach might also diminish potential antigen escape in these high-risk leukemias. Our preclinical data credential FLT3CART as a highly effective immunotherapeutic strategy for both FLT3- mutant AML and KMT2A-rearranged ALL which is poised for further investigation and clinical translation.

Digital polymerase chain reaction strategies for accurate and precise detection of vector copy number in chimeric antigen receptor T-cell products

BACKGROUND AIMS

Vector copy number (VCN), an average quantification of transgene copies unique to a chimeric antigen receptor (CAR) T-cell product, is a characteristic that must be reported prior to patient administration, as high VCN increases the risk of insertional mutagenesis. Historically, VCN assessment in CAR T-cell products has been performed via quantitative polymerase chain reaction (qPCR). qPCR is reliable along a broad range of concentrations, but quantification requires use of a standard curve and precision is limited. Digital PCR (dPCR) methods were developed for absolute quantification of target sequences by counting nucleic acid molecules encapsulated in discrete, volumetrically defined partitions. Advantages of dPCR compared with qPCR include simplicity, reproducibility, sensitivity and lack of dependency on a standard curve for definitive quantification. In the present study, the authors describe a dPCR assay developed for analysis of the novel bicistronic CD19 × CD22 CAR T-cell construct.

METHODS

The authors compared the performance of the dPCR assay with qPCR on both the QX200 droplet dPCR (ddPCR) system (Bio-Rad Laboratories, Inc, Hercules, CA, USA) and the QIAcuity nanoplate-based dPCR (ndPCR) system (QIAGEN Sciences, Inc, Germantown, MD, USA). The primer-probe assay was validated with qPCR, ndPCR and ddPCR using patient samples from pre-clinical CAR T-cell manufacturing production runs as well as Jurkat cell subclones, which stably express this bicistronic CAR construct.

RESULTS

ddPCR confirmed the specificity of this assay to detect only the bicistronic CAR product. Additionally, the authors' assay gave accurate, precise and reproducible CAR T-cell VCN measurements across qPCR, ndPCR and ddPCR modalities.

CONCLUSIONS

The authors demonstrate that dPCR strategies can be utilized for absolute quantification of CAR transgenes and VCN measurements, with improved test-retest reliability, and that specific assays can be developed for detection of unique constructs.

CD19/22 CAR T cells in children and young adults with B-ALL: phase 1 results and development of a novel bicistronic CAR

Remission durability following single-antigen targeted chimeric antigen receptor (CAR) T-cells is limited by antigen modulation, which may be overcome with combinatorial targeting. Building upon our experiences targeting CD19 and CD22 in B-cell acute lymphoblastic leukemia (B-ALL), we report on our phase 1 dose-escalation study of a novel murine stem cell virus (MSCV)-CD19/CD22-4-1BB bivalent CAR T-cell (CD19.22.BBζ) for children and young adults (CAYA) with B-cell malignancies. Primary objectives included toxicity and dose finding. Secondary objectives included response rates and relapse-free survival (RFS). Biologic correlatives included laboratory investigations, CAR T-cell expansion and cytokine profiling. Twenty patients, ages 5.4 to 34.6 years, with B-ALL received CD19.22.BBζ. The complete response (CR) rate was 60% (12 of 20) in the full cohort and 71.4% (10 of 14) in CAR-naïve patients. Ten (50%) developed cytokine release syndrome (CRS), with 3 (15%) having ≥ grade 3 CRS and only 1 experiencing neurotoxicity (grade 3). The 6- and 12-month RFS in those achieving CR was 80.8% (95% confidence interval [CI]: 42.4%-94.9%) and 57.7% (95% CI: 22.1%-81.9%), respectively. Limited CAR T-cell expansion and persistence of MSCV-CD19.22.BBζ compared with EF1α-CD22.BBζ prompted laboratory investigations comparing EF1α vs MSCV promoters, which did not reveal major differences. Limited CD22 targeting with CD19.22.BBζ, as evaluated by ex vivo cytokine secretion and leukemia eradication in humanized mice, led to development of a novel bicistronic CD19.28ζ/CD22.BBζ construct with enhanced cytokine production against CD22. With demonstrated safety and efficacy of CD19.22.BBζ in a heavily pretreated CAYA B-ALL cohort, further optimization of combinatorial antigen targeting serves to overcome identified limitations (www.clinicaltrials.gov #NCT03448393).

Abstract 5598: Engineered hypoimmune allogeneic CAR T cells as potential off-the-shelf CAR T cell immunotherapies

Off-the-shelf CAR T cells may offer advantages over autologous strategies, including ease of manufacturing, improved quality control with avoidance of malignant contamination and T cell dysfunction, and the ability to generate a final product from healthy T cells. However, host-versus-graft immune response against histoincompatible T cells prevents the expansion and persistence of allogeneic CAR T cells and mitigates the efficacy of this approach. A major challenge is that, while HLA deletion can result in adaptive immune evasion, innate reactivity is enhanced. While T cells express CD47, we demonstrate here that CD47 expression above endogenous levels is important for immune evasion. We describe here the engineering of human immune evasive CAR T cells building on our previously described hypoimmune technology (Nat Biotechnol 2019;37(3):252-258 and Proc Natl Acad Sci U S A 2021;118(28):e2022091118). The goal is to achieve improved rates of durable complete remissions by improving allogeneic CD19CAR persistence, since it has been shown that autologous CAR T cells have greater durability over years than allogeneic CAR T cells. Human T cells from healthy donors were obtained by leukapheresis. To generate hypoimmune CD19CAR T cells, gene editing was used to eliminate HLA-I/II and TCR expression and lentiviral transduction was used to express CD47 and CD19CAR containing a 4-1BB costimulatory domain to generate hypoimmune CD19CAR T cells. Control CD19CAR T cells were unmanipulated, i.e., unedited, except for lentiviral transduction used to express CD19CAR. Hypoimmune CD19CAR T cells persist in allogeneic humanized mice and lack T cell activation measured using bioluminescence imaging and ELISPOT analysis, respectively. In contrast, transplantation of control CD19CAR T cells generated from the same human donor resulted in rejection (ELISPOT mean 59 and 558 spot frequencies for hypoimmune CD19CAR T cells and control CD19CAR T cells, respectively; p<0.0001 unpaired t-test). Innate immune cell assays show that CD47 overexpression protects hypoimmune CD19CAR T cells from NK cell and macrophage killing. A blocking antibody against CD47 made the hypoimmune CD19CAR T cells susceptible to macrophage and NK cell killing, confirming the importance of CD47 overexpression to evade innate immune clearance. Importantly, CD47 seemed to provide protection from all NK cell populations while other tested NK cell inhibitory molecules (such as HLA-E/G, PD-L1) seemed to prevent NK cell killing of only certain subpopulations rather than primary NK cells in total. Hypoimmune CD19CAR T cells retain their antitumor activity in the Nalm-6 B cell leukemia model in vitro and in vivo comparable to control CD19CAR T cells derived from various donors. Thus, hypoimmune edits seem to not impact CD19CAR T cell activity and have the potential to provide universal CAR T cells that are able to persist without immunosuppression.

Citation Format: Xiaomeng Hu, Kathy White, Corie Gattis, Ryan Clarke, Sam Landry, Ron Basco, Eleonore Tham, Emily Luo, Andrew Tucker, Christopher Bandoro, Elaine Chu, Chi Young, Karl Manner, Priscilla Nho, Ben Lam, Pascal Beauchesne, Aaron Foster, William E. Dowdle, Edward J. Rebar, Terry J. Fry, Sonja Schrepfer. Engineered hypoimmune allogeneic CAR T cells as potential off-the-shelf CAR T cell immunotherapies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5598.

EPEN-26. Chemokine receptor blockade reverses CCL2 mediated immunosuppression and restores CAR-T cell function in posterior fossa ependymoma

Trastuzumab-based HER2 CAR-T constructs have demonstrated preclinical efficacy in medulloblastoma and are being evaluated for use in children and young adults with recurrent or refractory CNS tumors. Preliminary results demonstrate immune activation but no objective tumor response in three patients, including two patients with posterior fossa (PF)-EPN. A key finding in the serum and CSF of all three patients was very high levels of the inflammatory chemokine CCL2 following treatment with CAR-T cells. Preclinical studies suggest that high levels of CCL2 may impede T cell mediated anti-tumor activity in CNS tumors. The role of CCL2 to enhance or diminish CAR-T cell efficacy for CNS tumors is unknown. We evaluated a second generation trastuzumab-based HER2 CAR construct with a 4-1BB co-stimulatory domain in two ultra-high-risk patient-derived xenograft (PDX) models that faithfully recapitulate PFA-EPN. In contrast to preclinical studies in other cancers, treatment with trastuzumab-based HER2 CAR-T cell alone causes only partial regression of tumors and robust infiltration of immunosuppressive monocytes in PFA-EPN PDX mouse models. We studied constitutive NF-kB activation because it is a hallmark of PFA-EPN that drives dysregulation of inflammatory genes and forms an immunosuppressive tumor microenvironment. Upon tumor recognition, CAR-T cells produce high amounts of the cytokine tumor necrosis factor-alpha, which is an extracellular stimulus that propagates NF-kB activation in PFA-EPN. We show that HER2 CAR-T cell treatment causes increased nuclear translocation of the RELA NF-kB subunit, which induces CCL2 gene transcription and chemokine release. This results in CCL2-CCR2 ligand/receptor mediated influx of inflammatory monocytes and regulatory T cells, impairing CAR-T cell effector function. Inhibition of CCR2 restores anti-tumor CAR-T cytotoxicity against bulky orthotopic tumors by decreasing the infiltration of inflammatory monocytes and regulatory T cells. Combinatorial strategies addressing tumor mediated immunosuppression should be evaluated in upcoming CAR-T cell trials for patients with high-risk CNS tumors.

CAR T-cells effective for post-CART relapse: A new treatment paradigm

e19508

Background: Chimeric antigen receptor T-cells (CART) induce remarkable responses in B-cell acute lymphoblastic leukemia (B-ALL), but relapse remains a challenge. Effective therapies for post-CART relapse are limited but may include infusion of a unique CART construct, a strategy distinct from reinfusion of the same CART product. Given limited data evaluating outcomes of a unique CART as salvage therapy for post-CART relapse, we report on the impact of prior CART constructs on subsequent CART responses. Methods: This was a retrospective review (NCT03827343) of children and young adults receiving CD19 and/or CD22 CART therapy for B-ALL between 7/1/12-12/30/21 at our center. Patients included received at least 2 unique CART constructs at some point in therapy, excluding interim CART reinfusions. CART-A was the first CART construct ever received and CART-B the second unique CART. The primary objective was to evaluate complete remission (CR) rates following CART-A versus CART-B. Results: Of 135 heavily pretreated patients, 54 (40%) received at least one prior CART. The majority (n=37, 68.5%) were male. Median age at CART-A and CART-B was 12.5 (range, 3.3-30.4) and 13.7 years (range, 4.5-30.7), respectively. In 42 (77.8%) patients, CART-B targeted a different antigen than CART-A, primarily due to loss of antigen target after CART-A. CR rate was substantially lower with CART-B (n=35, 64.8%) than with CART-A (n=48, 88.9%, p=0.006) (Table). Still, two (3.7%) patients with CART-A nonresponse attained CR with CART-B. Most CART-B responders (n=31, 88.6%) had CART-B targeting a different antigen than CART-A, suggesting limitations of same antigen targeting even with a unique CART construct. CART-B responses amongst 10 patients with interim hematopoietic stem cell transplantation (HSCT) after CART-A were similar to CART-B responses in those without interim HSCT (5 of 10, 50% vs. 30 of 44, 68.2%, p=0.3). In those where CRS grade was known for both CART infusions (n=40), CRS severity was milder with CART-B (≥grade 3, n=1, 2.5%) than with CART-A (≥grade 3, n=15, 37.5%, p=0.0001). Conclusions: Using an alternative CART for post-CART relapse is effective in a substantial proportion of patients, particularly when targeting a unique antigen. As post-CART relapse occurs more frequently and patients receive multiple CARTs, identifying optimization strategies for CART-B will be critical. Further investigation of indication for CART-B, role of interim HSCT, and optimal timing for sequential CART infusions is underway.[Table: see text]

Efficacy of second CAR-T (CART2) infusion limited by poor CART expansion and antigen modulation

Chimeric antigen receptor T-cells (CART) are active in relapsed/refractory (r/r) B-cell acute lymphoblastic leukemia (B-ALL), but relapse remains a substantial challenge. Reinfusion with the same CART product (CART2) in patients with suboptimal response or antigen positive relapse following first infusion (CART1) represents a potential treatment strategy, though early experiences suggest limited efficacy of CART2 with CD19 targeting. We report on our experience with CART2 across a host of novel CAR T-cell trials. This was a retrospective review of children and young adults with B-ALL who received reinfusion with an anti-CD19, anti-CD22, or anti-CD19/22 CART construct on one of 3 CAR T-cells trials at the National Cancer Institute (NCT01593696, NCT02315612, NCT0344839) between July 2012 and January 2021. All patients received lymphodepletion (LD) pre-CART (standard LD: 75 mg/m² fludarabine, 900 mg/m² cyclophosphamide; or intensified LD: 120 mg/m² fludarabine, 1200 mg/m² cyclophosphamide). Primary objectives were to describe response to and toxicity of CART2. Indication for CART2, impact of LD intensity, and CAR T-cell expansion and leukemia antigen expression between CART infusions was additionally evaluated. Eighteen patients proceeded to CART2 due to persistent (n=7) or relapsed antigen positive disease (n=11) following CART1. Seven of 18 (38.9%) demonstrated objective response (responders) to CART2: 5 achieved a minimal residual disease (MRD) negative CR, 1 had persistent MRD level disease, and 1 showed a partial remission, the latter with eradication of antigen positive disease and emergence of antigen negative B-ALL. Responders included four patients who had not achieved a CR with CART1. Limited cytokine release syndrome was seen following CART2. Peripheral blood CART1 expansion was higher than CART2 expansion (p=0.03). Emergence of antigen negative/dim B-ALL in 6 (33.3%) patients following CART2 contributed to lack of CR. Five of seven (71.4%) responders received intensified LD pre-CART2, which corresponded with higher CART2 expansion than in those receiving standard LD (p=0.029). Diminished CAR T-cell expansion and antigen downregulation/loss impeded robust responses to CART2. A subset of patients, however, may derive benefit from CART2 despite suboptimal response to CART1. Intensified LD may be one strategy to augment CART2 responses, though further study of factors associated with CART2 response, including serial monitoring of antigen expression, is warranted.

Optimizing CARs for ocular delivery

Effective methods for treating retinoblastoma while preserving vision are an unmet clinical need. Subretinal delivery of a hydrogel containing T cells that secrete the cytokine IL-15 and express a chimeric antigen receptor directed at the ganglioside protein GD2 completely controls retinoblastoma in immunocompromised mice, with no obvious damage to the surrounding retina.

Systematic preclinical evaluation of CD33-directed chimeric antigen receptor T cell immunotherapy for acute myeloid leukemia defines optimized construct design

Background

Successful development of chimeric antigen receptor (CAR) T cell immunotherapy for children and adults with relapsed/refractory acute myeloid leukemia (AML) is highly desired given their poor clinical prognosis and frequent inability to achieve cure with conventional chemotherapy. Initial experiences with CD19 CAR T cell immunotherapy for patients with B-cell malignancies highlighted the critical impact of intracellular costimulatory domain selection (CD28 vs 4-1BB (CD137)) on CAR T cell expansion and in vivo persistence that may impact clinical outcomes. However, the impact of costimulatory domains on the efficacy of myeloid antigen-directed CAR T cell immunotherapy remains unknown.

Methods

In this preclinical study, we developed six CAR constructs targeting CD33, a highly expressed and validated AML target, comprised of one of three single-chain variable fragments with CD3ζ and either CD28 or 4-1BB costimulatory domains. We systematically compared the preclinical in vitro and in vivo efficacy of T cells lentivirally transduced with CD33 CAR constructs (CD33CARTs) against human AML.

Results

We observed potent in vitro cytokine production and cytotoxicity of CD33CARTs incubated with human CD33+ AML cell lines, as well as robust in vivo antileukemia activity in cell line and childhood AML patient-derived xenograft (PDX) models. Gemtuzumab-based CD33CARTs were unexpectedly toxic in vivo in animal models despite observed in vitro anti-leukemia activity. CD28-based CD33CARTs consistently induced more robust inhibition of leukemia proliferation in AML cell line and PDX models than did 4-1BB-based CD33CARTs. A ‘best-in-class’ lintuzumab-CD28/CD3ζ CAR construct was thus selected for clinical translation.

Conclusions

CD33 is a critical antigen for potential immunotherapeutic targeting in patients with AML. Based on this rigorous preclinical evaluation, our validated clinical grade lintuzumab-CD28/CD3ζ CD33CART immunotherapy is now under evaluation in a first-in-child/first-in-human phase 1 clinical trial for children and adolescents/young adults with relapsed/refractory AML.

Trial registration number

clinicaltrials.gov; NCT03971799.

Combining Immunocytokine and Ex Vivo Activated NK Cells as a Platform for Enhancing Graft-Versus-Tumor Effects Against GD2+ Murine Neuroblastoma

Management for high-risk neuroblastoma (NBL) has included autologous hematopoietic stem cell transplant (HSCT) and anti-GD2 immunotherapy, but survival remains around 50%. The aim of this study was to determine if allogeneic HSCT could serve as a platform for inducing a graft-versus-tumor (GVT) effect against NBL with combination immunocytokine and NK cells in a murine model. Lethally irradiated C57BL/6 (B6) x A/J recipients were transplanted with B6 bone marrow on Day +0. On day +10, allogeneic HSCT recipients were challenged with NXS2, a GD2+ NBL. On days +14-16, mice were treated with the anti-GD2 immunocytokine hu14.18-IL2. In select groups, hu14.18-IL2 was combined with infusions of B6 NK cells activated with IL-15/IL-15Rα and CD137L ex vivo. Allogeneic HSCT alone was insufficient to control NXS2 tumor growth, but the addition of hu14.18-IL2 controlled tumor growth and improved survival. Adoptive transfer of ex vivo CD137L/IL-15/IL-15Rα activated NK cells with or without hu14.18-IL2 exacerbated lethality. CD137L/IL-15/IL-15Rα activated NK cells showed enhanced cytotoxicity and produced high levels of TNF-α in vitro, but induced cytokine release syndrome (CRS) in vivo. Infusing Perforin-/- CD137L/IL-15/IL-15Rα activated NK cells had no impact on GVT, whereas TNF-α-/- CD137L/IL-15/IL-15Rα activated NK cells improved GVT by decreasing peripheral effector cell subsets while preserving tumor-infiltrating lymphocytes. Depletion of Ly49H+ NK cells also improved GVT. Using allogeneic HSCT for NBL is a viable platform for immunocytokines and ex vivo activated NK cell infusions, but must be balanced with induction of CRS. Regulation of TNFα or activating NK subsets may be needed to improve GVT effects.

Abstract LB144: Overexpression of CD47 protects hypoimmune CAR T cells from innate immune cell killing

Off-the-shelf CAR T cells could offer advantages over autologous strategies, including ease of manufacturing, quality control and avoidance of malignant contamination and T cell dysfunction. TCR editing can effectively prevent graft-versus-host reactions. However, the vigorous host-versus-graft immune response against histoincompatible T cells prevents expansion and persistence of allogeneic CAR T cells and mitigates the efficacy of this approach. A major challenge is that, while HLA deletion can result in adaptive immune evasion, innate reactivity is enhanced. CD47 overexpression can block both NK cell and macrophage killing (J Exp Med (2021) 218 (3): e20200839), and we hypothesized that T cells would lose their immunogenicity when human leukocyte antigen (HLA) class I and II genes are inactivated and CD47 is over-expressed. We describe here the engineering of human immune evasive CAR T cells based on our previously described hypoimmune technology. Human T cells from healthy donors were obtained by leukapheresis. CRISPR/Cas9 technology was used to delete b2m, CIITA, and TCR and lentiviral transduction to overexpress CD47 and CD19CAR. Control T cells were unmanipulated except for overexpression of CD19CAR containing a 41BB costimulatory domain. When transplanted into allogeneic humanized mice, hypoimmunogenic HLA-I/II- TCR- CD47+ CD19CAR+ T cells evade immune recognition by T and B cells compared to CD19CAR+ T cells generated from the same human donor using ELISPOT and flow cytometry analysis. Innate immune cell assays show that CD47 overexpression protects HLA-I/II deficient CAR T cells from NK cell and macrophage killing in vitro and in vivo. Relative CD47 expression levels were analyzed to understand the relevance of CD47 for protection from macrophage and NK cell killing. A blocking antibody against CD47 made the hypoimmunogenic CAR T cells susceptible to macrophage and NK cell killing, confirming the importance of CD47 overexpression to evade innate immune clearance. The use of CD47 blocking could additionally be envisioned as a safety strategy for our hypoimmunogenic CAR T cells. Neither isolated CD47 overexpression nor all three hypoimmune modifications or knockout of the TCR showed any effect on the cytotoxic potential of CAR+ T cells. Hypoimmune CAR+ T cells retain their antitumor activity in the Nalm-6 B cell leukemia model in vitro and clear leukemic cells in NSG mice across a range of tumor cell: CAR T cell ratios comparable to unmodified CAR T cells. These findings show that hypoimmunogenic CAR T cells are functionally immune evasive in allogeneic recipients with cytotoxic anti-tumor capacity and suggest they could provide universal CAR T cells that is able to persist without immunosuppression. Blocking CD47 could additionally serve as safety strategy for our hypoimmunogenic CAR T cells.

Citation Format: Xiaomeng Hu, Mo Dao, Kathy White, Ryan Clarke, Sam Landry, Ron Basco, Corie Gattis, Eleonore Tham, Emily Luo, Andrew Tucker, Christopher Bandoro, Elaine Chu, Junmo Kim, Chi Young, William E. Dowdle, Edward J. Rebar, Terry J. Fry, Sonja Schrepfer. Overexpression of CD47 protects hypoimmune CAR T cells from innate immune cell killing [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB144.

Long-Term Follow-Up of CD19-CAR T-Cell Therapy in Children and Young Adults With B-ALL

PURPOSE

CD19 chimeric antigen receptor (CD19-CAR) T cells induce high response rates in children and young adults (CAYAs) with B-cell acute lymphoblastic leukemia (B-ALL), but relapse rates are high. The role for allogeneic hematopoietic stem-cell transplant (alloHSCT) following CD19-CAR T-cell therapy to improve long-term outcomes in CAYAs has not been examined.

METHODS

We conducted a phase I trial of autologous CD19.28ζ-CAR T cells in CAYAs with relapsed or refractory B-ALL. Response and long-term clinical outcomes were assessed in relation to disease and treatment variables.

RESULTS

Fifty CAYAs with B-ALL were treated (median age, 13.5 years; range, 4.3-30.4). Thirty-one (62.0%) patients achieved a complete remission (CR), 28 (90.3%) of whom were minimal residual disease-negative by flow cytometry. Utilization of fludarabine/cyclophosphamide-based lymphodepletion was associated with improved CR rates (29/42, 69%) compared with non-fludarabine/cyclophosphamide-based lymphodepletion (2/8, 25%; P = .041). With median follow-up of 4.8 years, median overall survival was 10.5 months (95% CI, 6.3 to 29.2 months). Twenty-one of 28 (75.0%) patients achieving a minimal residual disease-negative CR proceeded to alloHSCT. For those proceeding to alloHSCT, median overall survival was 70.2 months (95% CI, 10.4 months to not estimable). The cumulative incidence of relapse after alloHSCT was 9.5% (95% CI, 1.5 to 26.8) at 24 months; 5-year EFS following alloHSCT was 61.9% (95% CI, 38.1 to 78.8).

CONCLUSION

We provide the longest follow-up in CAYAs with B-ALL after CD19-CAR T-cell therapy reported to date and demonstrate that sequential therapy with CD19.28ζ-CAR T cells followed by alloHSCT can mediate durable disease control in a sizable fraction of CAYAs with relapsed or refractory B-ALL (ClinicalTrials.gov identifier: NCT01593696).

Robust Antitumor Activity and Low Cytokine Production by Novel Humanized Anti-CD19 CAR T Cells

Recent studies have described the remarkable clinical outcome of anti-CD19 chimeric antigen receptor (CAR) T cells in treating B-cell malignancies. However, over 50% of patients develop life-threatening toxicities associated with cytokine release syndrome which may limit its utilization in low-resource settings. To mitigate the toxicity, we designed a novel humanized anti-CD19 CAR T cells by humanizing the framework region of single-chain variable fragment (scFv) derived from a murine FMC63 mAb and combining it with CD8α transmembrane domain, 4-1BB costimulatory domain, and CD3ζ signaling domain (h1CAR19-8BBζ). Docking studies followed by molecular dynamics simulation revealed that the humanized anti-CD19 scFv (h1CAR19) establishes higher binding affinity and has a flexible molecular structure with CD19 antigen compared with murine scFv (mCAR19). Ex vivo studies with CAR T cells generated from healthy donors and patients with relapsed/refractory B-cell acute lymphoblastic leukemia (B-ALL) expressing either h1CAR19 or mCAR19 showed comparable antitumor activity and proliferation. More importantly, h1CAR19-8BBζ T cells produced lower levels of cytokines (IFNγ, TNFα) upon antigen encounter and reduced the induction of IL6 cytokine from monocytes than mCAR19-8BBζ T cells. There was a comparable proliferation of h1CAR19-8BBζ T cells and mCAR19-8BBζ T cells upon repeated antigen encounter. Finally, h1CAR19-8BBζ T cells efficiently eliminated NALM6 tumor cells in a preclinical model. In conclusion, the distinct structural modification in CAR design confers the novel humanized anti-CD19 CAR with a favorable balance of efficacy to toxicity providing a rationale to test this construct in a phase I trial.

Society for Immunotherapy of Cancer (SITC) clinical practice guideline on immune effector cell-related adverse events

Immune effector cell (IEC) therapies offer durable and sustained remissions in significant numbers of patients with hematological cancers. While these unique immunotherapies have improved outcomes for pediatric and adult patients in a number of disease states, as 'living drugs,' their toxicity profiles, including cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS), differ markedly from conventional cancer therapeutics. At the time of article preparation, the US Food and Drug Administration (FDA) has approved tisagenlecleucel, axicabtagene ciloleucel, and brexucabtagene autoleucel, all of which are IEC therapies based on genetically modified T cells engineered to express chimeric antigen receptors (CARs), and additional products are expected to reach marketing authorization soon and to enter clinical development in due course. As IEC therapies, especially CAR T cell therapies, enter more widespread clinical use, there is a need for clear, cohesive recommendations on toxicity management, motivating the Society for Immunotherapy of Cancer (SITC) to convene an expert panel to develop a clinical practice guideline. The panel discussed the recognition and management of common toxicities in the context of IEC treatment, including baseline laboratory parameters for monitoring, timing to onset, and pharmacological interventions, ultimately forming evidence- and consensus-based recommendations to assist medical professionals in decision-making and to improve outcomes for patients.

Diagnostic approach to the evaluation of myeloid malignancies following CAR T-cell therapy in B-cell acute lymphoblastic leukemia

Immunotherapeutic strategies targeting B-cell acute lymphoblastic leukemia (B-ALL) effectively induce remission; however, disease recurrence remains a challenge. Due to the potential for antigen loss, antigen diminution, lineage switch or development of a secondary or treatment-related malignancy, the phenotype and manifestation of subsequent leukemia may be elusive. We report on two patients with multiply relapsed/refractory B-ALL who, following chimeric antigen receptor T-cell therapy, developed myeloid malignancies. In the first case, a myeloid sarcoma developed in a patient with a history of myelodysplastic syndrome. In the second case, two distinct events occurred. The first event represented a donor-derived myelodysplastic syndrome with monosomy 7 in a patient with a prior hematopoietic stem cell transplantation. This patient went on to present with lineage switch of her original B-ALL to ambiguous lineage T/myeloid acute leukemia. With the rapidly evolving field of novel immunotherapeutic strategies, evaluation of relapse and/or subsequent neoplasms is becoming increasingly more complex. By virtue of these uniquely complex cases, we provide a framework for the evaluation of relapse or evolution of a subsequent malignancy following antigen-targeted immunotherapy.

Perforin-deficient CAR T cells recapitulate late-onset inflammatory toxicities observed in patients

Late-onset inflammatory toxicities resembling hemophagocytic lymphohistiocytosis (HLH) or macrophage activation syndrome (MAS) occur after chimeric antigen receptor T cell (CAR T cell) infusion and represent a therapeutic challenge. Given the established link between perforin deficiency and primary HLH, we investigated the role of perforin in anti-CD19 CAR T cell efficacy and HLH-like toxicities in a syngeneic murine model. Perforin contributed to both CD8+ and CD4+ CAR T cell cytotoxicity but was not required for in vitro or in vivo leukemia clearance. Upon CAR-mediated in vitro activation, perforin-deficient CAR T cells produced higher amounts of proinflammatory cytokines compared with WT CAR T cells. Following in vivo clearance of leukemia, perforin-deficient CAR T cells reexpanded, resulting in splenomegaly with disruption of normal splenic architecture and the presence of hemophagocytes, which are findings reminiscent of HLH. Notably, a substantial fraction of patients who received anti-CD22 CAR T cells also experienced biphasic inflammation, with the second phase occurring after the resolution of cytokine release syndrome, resembling clinical manifestations of HLH. Elevated inflammatory cytokines such as IL-1β and IL-18 and concurrent late CAR T cell expansion characterized the HLH-like syndromes occurring in the murine model and in humans. Thus, a murine model of perforin-deficient CAR T cells recapitulated late-onset inflammatory toxicities occurring in human CAR T cell recipients, providing therapeutically relevant mechanistic insights.

Abstract 601: Chimeric antigen receptors incorporating novel binding domains targeting CD123 direct potent antitumor activity of T cells: Correlation between affinity and activity

Unlike the tumor targets associated with initial CAR T cell successes, the vast majority of tumor associated antigens are not unique, but rather expressed at elevated levels compared to normal tissue. For example, while CD123 is overexpressed on many AML tumor cells, concerns regarding on-target, off-tumor toxicities - often associated low-level CD123 expression on normal endothelial cells - has hindered clinical success. Therefore, future generations of CAR T therapies will require novel strategies that have the ability to discriminate between normal and elevated antigen levels - potentially through mechanisms that involve 1) the simultaneous or sequential targeting of multiple tumor associated antigens and 2) agents that contain affinity-tuned targeting domains. Due to the large inventory of validated antibodies against a variety of therapeutic targets, scFv naturally emerged as the obvious and justifiable targeting domain for chimeric antigen receptors. However, many of the characteristics that have made antibodies versatile and effective recombinant therapeutics (e.g., high affinity, bivalency, antibody-dependent cytotoxicity, complement-dependent cytotoxicity, FcRn recycling, and low renal filtration rates) are not advantageous for membrane associated chimeric receptors. Furthermore, scFv are not native protein structures and their development, particularly as it pertains to solubility and aggregation, can be challenging. Therefore, we sought to develop a simple, highly selective targeting domain that could be engineered into complex, potentially multispecific therapeutics. We describe the design and development of non-scFv-derived binding domains. Using phage display and targeted mutagenesis, we identify a series of binding domains that target CD123 with high specificity - as characterized by functional and tissue binding studies. These domains exhibit affinities that range over 2 logs and demonstrate in vitro potencies that correlate with their affinities. CARs comprised of higher affinity binding domains mediate potent T-cell activation and cytolysis of CD123-expressing target cells and induce complete durable remission in two AML xenograft models. We also describe a strategy of engineering less immunogenic binding domains through the identification and removal of putative T cell epitopes and demonstrate that the resultant variants retain biological activity. Finally, we demonstrate further potential of our binding domains by generating functional, bi-specific CARs comprised of a CD123-specific binding domain and a CD19-specific scFv. The ability to incorporate our target-specific binding domains with a range of affinities into complex chimeric receptors, affords a viable alternative to scFv as targeting domains in CAR T cell therapeutics.

Citation Format: David W. LaFleur, Haiying Qin, Justin P. Edwards, Liubov Zaritskaya, Ankit Gupta, C. Jenny Mu, Laura K. Richman, Terry J. Fry, David M. Hilbert. Chimeric antigen receptors incorporating novel binding domains targeting CD123 direct potent antitumor activity of T cells: Correlation between affinity and activity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 601.

Abstract 4231: Factors predictive of CAR T cell associated hemophagocytic lymphohistiocytosis (HLH)

Introduction: HLH is a T cell mediated inflammatory syndrome associated with immune activation and NK cell dysfunction. In the context of CAR T cells, HLH occurs in a subset of those with cytokine release syndrome (CRS), yet predisposing factors are not yet well established. Utilizing our ongoing phase I study of CD22/4-1BB CAR T cells in children and young adults with CD22+ B cell malignancies (NCT02315612), we evaluated patient and product characteristics to elucidate factors associated with HLH.

Methods: We retrospectively reviewed multiple variables to identify correlates of HLH in those infused. Definitions for CRS and HLH are in Table 1. Multiple univariate analyses were performed to identify risk factors for HLH; variables with significance were incorporated into a multiple logistic regression model to predict those at risk of HLH-like CRS.

Table 1.Patient and Product Characteristics Amongst those with CRS, Distinguished by HLH versus non HLH-like toxicitiesNo HLH^HLH ^pBaseline DemographicsHLH manifestations amongst those with CRS, n, (%)31 (59.6%)21 (40.4%)Age (years), median (range)17 (4 - 30)13 (4 - 30)nsTime to CRS onset, day post infusion, median (range)8 (3-13)7.5 (3-13)nsTime to HLH onset, day post infusion, median (range)N/A14 (7-26)N/A# patients with prior HSCT, n (%)23/31 (74.2%)13/21 (61.9%)ns# patients with prior CAR, n (%)20/31 (64.5%)11/21 (52.4)nsPatient Factors; median (IQR)Baseline# PB12 (7.1-23.6)7.4 (4.4-14.6)0.04NK%Baseline# PB75 (58.5 - 88.5)83.9 (75.3-92.1)0.05CD3%Baseline# PB2.3 (1.2 - 4.3)6.1 (2.6 - 10.8)0.008CD8/NK ratioBaseline# BM3.6 (1.6 - 6.1)8.4 (4.9 - 15.7)0.0009CD3/NK ratioBaseline# Disease burden, % marrow blasts50 (18.9 - 90)71.4 (40.8 - 88.5)nsBaseline# Soluble IL2Ra (pg/mL)728 (460 - 1001)1176 (556.3 - 1550)0.07Product Factors; median (IQR)TCS of apheresis product, n, (%)13/31 (41.9%)18/21 (85.7%)0.002Product culture day 7, CD3%99.6 (99.3-99.8)99.8 (99.7 - 99.8)0.02Product culture day 7, NK%.1 (.01 - .23)0 (0 -.10)0.002Final product, CD3%99.7 (99.6 - 99.9)99.9 (99.8 - 99.9)0.01Final product, NK%.03 (0 - .12)0 (0 - .04)0.1Max Grade CRSGrade 121/31 (67.7%)5/21 (23.8%)0.004Grade 27/31 (22.3%)14/21 (66.7%)Grade 3&43/31 (9.6%)2/21 (9.5%)CAR T-cell Expansion (PB); median (IQR)Day 14, %CD871.6 (60.3 - 79.4)85.9 (61.1-92.9)0.02Day 14, %CD419.8 (11.9 - 27.2)7.8 (4.5 - 32.7)0.06Day 14, %NK4.0 (1.9 - 5.4)1.7 (.7 - 5.2)nsDay 14, CD8:NK ratio16.7 (10.5 - 41.1)42.1 (12.3 - 151.1).05Day 14, CD4:NK5.3 (1.8 - 8.1)5.5 (1.7 - 15)nsDay 28, %CD843.6 (37.8 - 55.7)69.8 (43.6 - 81.7)0.02Day 28, %CD435.8 (26 - 47.2)22.3 (10.7 - 44.9)0.07Day 28, %NK9 (4.5 - 15.5)1.9 (.7 - 11.7)0.02Day 28, CD8:NK ratio4.5 (2.6 - 12.8)21.5 (2.9 - 109.2)0.03Day 28, CD4:NK3.9 (1.8 - 7.6)7.7 (3.9 - 19.6)0.05Inflammatory markers and select cytokine profiling**; median (IQR)Ferritin (ng/mL)Baseline#1979 (992.5 - 3802)1942 (1397 - 4192)nsDay 51433 (888.5 - 2491)3621 (1587 - 5024)0.02Day 95920 (1097 - 18321)14063 (5367 - 61503)0.02IL18 (pg/mL)Day 0347.1 (251.8 - 544.4)645 (331.7 -863.3)0.02Day 5450 (232.5 - 589)578.6 (426.5 - 887.4)0.05Day 9835 (586.8 - 1035)894.4 (843.5 - 2971)nsDay 131494 (843.3 - 1970)2058 (1386 - 5001)0.04IL-1B (pg/mL)Day 0.29 (.28 - .33).28 (.28 - .29)nsDay 5.29 (.28 - .33).28 (.28 - .29)nsDay 9.33 (.28. - .82).53 (.28 - 3.36)nsDay 13.29 (.28 - .645).89 (.28 - 1.528)0.03IL-6** (pg/mL)Day 02.9 (1.4 - 8.9)4.1 (2.1 - 8.5)nsDay 53.3 (1.7 - 14.9)6.2 (2.7 - 9.3)nsDay 920.5 (5.0- 59.1)43.0 (15.5 - 983)0.04Day 139.025 (4.118 - 22.54)127.2 (20.92 - 597.4)0.0008IFNy (pg/mL)Day 011.1 (3.8 - 17.4)8.5 (3.2 - 15.4)nsDay 518.01 (5.0 - 46.9)13.6 (4.0 - 25.5)nsDay 9169.5 (93.3 - 665.4)485.2 (188.2 - 1887).06Day 1381.07 (21.1 - 132.8)287.3 (123 - 829.7)0.001Predictive model1.9882 x (TCS, 1=Yes/0=No) + 0.8013 x (max grade CRS) + 0.2368 x (bone marrow T/NK ratio); values > 4.47 were predictive for HLH.(Model based on 59/60 infused patients, with full data available for 55 subjects. 1 subject excluded due to rapidly progressive disease requiring steroids prior to CAR expansion)*CRS (cytokine release syndrome) was defined by Lee et al. (Blood 2014).^HLH was defined by the following criteria, modified from Neelapu et al.: peak ferritin >100,000 µg/L with at least two of the following criteria: a) hepatic transaminases or bilirubin > grade 3, b) creatinine > grade 3, c) pulmonary edema > grade 3 and/or d) evidence of hemophagocytosis on bone marrow aspirate/biopsy.#Baseline: Timepoint prior to apheresis.**IL-6R antagonist (Tocilizumab) administration confounds results.Abbreviations: PB: peripheral blood; BM: bone marrow; TCS: T-cell selection of the apheresis product using CD4/CD8 beads; IQR: interquartile range; N/A: not applicable; ns: not significant.P values determined using Fisher's exact and Mann Whitney U tests for categorical and continuous variables, respectively, except Max Grade CRS which utilized a Chi-Squared test.

Results: 52 of 60 (86.7%) treated subjects developed CRS; 21 (40.4%) of whom subsequently developed HLH. The following factors were associated with HLH: lower baseline blood and marrow NK% and higher CD3% cells; CD4/CD8 T cell selection (TCS) of the apheresis product (which increased CD3 and reduced NK cell populations); and higher IL18. Following infusion, distinct cytokine profiles and prolonged predominance of CD8 T cells distinguished those with HLH from those without it. Multiple logistic regression identified a) use of TCS, b) higher grade CRS and c) high baseline bone marrow T/NK ratio as most predictive of HLH; leading to a model with 75% sensitivity and 80% specificity.

Conclusion: Our results provide novel insights into factors associated with CAR T cell related HLH and implicate preexisting T and NK cell populations, baseline cytokines, apheresis selection methodologies and CAR T cell expansion parameters in the pathophysiology. With a goal of earlier intervention to prevent HLH, the predictive model will be prospectively evaluated. Biologic correlative studies and assessment of generalizability in alternate CAR T cell constructs are planned.

Citation Format: Daniel A. Lichtenstein, Seth M. Steinberg, Steven L. Highfill, Bonnie Yates, Ping Jin, Jianjian Jin, Sandhya Panch, Haneen Shalabi, David F. Stroncek, Terry J. Fry, Nirali N. Shah. Factors predictive of CAR T cell associated hemophagocytic lymphohistiocytosis (HLH) [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4231.

CD4/CD8 T-Cell Selection Affects Chimeric Antigen Receptor (CAR) T-Cell Potency and Toxicity: Updated Results From a Phase I Anti-CD22 CAR T-Cell Trial

PURPOSE

Patients with B-cell acute lymphoblastic leukemia who experience relapse after or are resistant to CD19-targeted immunotherapies have limited treatment options. Targeting CD22, an alternative B-cell antigen, represents an alternate strategy. We report outcomes on the largest patient cohort treated with CD22 chimeric antigen receptor (CAR) T cells.

PATIENTS AND METHODS

We conducted a single-center, phase I, 3 + 3 dose-escalation trial with a large expansion cohort that tested CD22-targeted CAR T cells for children and young adults with relapsed/refractory CD22⁺ malignancies. Primary objectives were to assess the safety, toxicity, and feasibility. Secondary objectives included efficacy, CD22 CAR T-cell persistence, and cytokine profiling.

RESULTS

Fifty-eight participants were infused; 51 (87.9%) after prior CD19-targeted therapy. Cytokine release syndrome occurred in 50 participants (86.2%) and was grade 1-2 in 45 (90%). Symptoms of neurotoxicity were minimal and transient. Hemophagocytic lymphohistiocytosis-like manifestations were seen in 19/58 (32.8%) of subjects, prompting utilization of anakinra. CD4/CD8 T-cell selection of the apheresis product improved CAR T-cell manufacturing feasibility as well as heightened inflammatory toxicities, leading to dose de-escalation. The complete remission rate was 70%. The median overall survival was 13.4 months (95% CI, 7.7 to 20.3 months). Among those who achieved a complete response, the median relapse-free survival was 6.0 months (95% CI, 4.1 to 6.5 months). Thirteen participants proceeded to stem-cell transplantation.

CONCLUSION

In the largest experience of CD22 CAR T-cells to our knowledge, we provide novel information on the impact of manufacturing changes on clinical outcomes and report on unique CD22 CAR T-cell toxicities and toxicity mitigation strategies. The remission induction rate supports further development of CD22 CAR T cells as a therapeutic option in patients resistant to CD19-targeted immunotherapy.

Disease detection methodologies in relapsed B-cell acute lymphoblastic leukemia: Opportunities for improvement

BACKGROUND

Accurate disease detection is integral to risk stratification in B-cell acute lymphoblastic leukemia (ALL). The gold standard used to evaluate response in the United States includes morphologic evaluation and minimal residual disease (MRD) testing of aspirated bone marrow (BM) by flow cytometry (FC). This MRD assessment is usually made on a single aspirate sample that is subject to variability in collection techniques and sampling error. Additionally, central nervous system (CNS) assessments for ALL include evaluations of cytopathology and cell counts, which can miss subclinical involvement.

PROCEDURE

We retrospectively compared BM biopsy, aspirate, and FC samples obtained from children and young adults with relapsed/refractory ALL to identify the frequency and degree of disease discrepancies in this population. We also compared CNS FC and cytopathology techniques.

RESULTS

Sixty of 410 (14.6%) BM samples had discrepant results, 41 (10%) of which were clinically relevant as they resulted in a change in the assignment of marrow status. Discrepant BM results were found in 28 of 89 (31.5%) patients evaluated. Additionally, cerebrospinal fluid (CSF) FC identified disease in 9.7% of cases where cytopathology was negative.

CONCLUSIONS

These results support further investigation of the role of concurrent BM biopsy, with aspirate and FC evaluations, and the addition of FC to CSF evaluations, to fully assess disease status and response, particularly in patients with relapsed/refractory ALL. Prospective studies incorporating more comprehensive analysis to evaluate the impact on clinical outcomes are warranted.

Chimeric Antigen Receptor T-Cell Therapy

Patients with relapsed or refractory (R/R) cancers have a poor prognosis and limited treatment options. The recent approval of 2 chimeric antigen receptor (CAR) autologous T-cell products for R/R B-cell acute lymphoblastic leukemia and non-Hodgkin's lymphoma treatment is setting the stage for what is possible in other diseases. However, there are important factors that must be considered, including patient selection, toxicity management, and costs associated with CAR T-cell therapy. To begin to address these issues, NCCN organized a task force consisting of a multidisciplinary panel of experts in oncology, cancer center administration, and health policy, which met for the first time in March 2018. This report describes the current state of CAR T-cell therapy and future strategies that should be considered as the application of this novel immunotherapy expands and evolves.

Abstract LB-146: Phase I CD22 CAR T-cell trial updates

Introduction: Patients who relapse after or are resistant to CD19 targeting are in need of novel therapies. We previously reported on the initial experience with our highly-active, first-in-human, first-in-child, CD22 CAR trial in the first 21 subjects with ALL. We now report on the cumulative experience with 52 treated subjects.

Methods: This phase I study of anti-CD22 CAR-T cells (Clinicaltrials.gov NCT02315612) enrolled subjects between the ages of 3-30 years with relapsed/refractory CD22+ disease. All subjects received fludarabine 25 mg/m2 x 3 days and cyclophosphamide 900 mg/m2 x 1 day for lymphodepletion. Disease assessments were performed prior to initiation of lymphodepletion and at day 28 (+/- 4 days) post CAR infusion. Three dose levels were explored; with an interim manufacturing modification incorporating CD4/CD8 bead T-cell selection (TCS); current dose is DL1-TCS. (Table 1)

Results: The median age was 18.1 years (range, 4.4-30.6). 36 (69.2%) subjects had undergone HSCT; 30 (57.7%) had prior CD19 CAR; 22 (42.3%) had prior blinatumomab; 28 (53.8%) subjects had a CD19 negative population, including two who were inherently partial CD19 negative without prior targeted therapy. 46 (88.4%) experienced CRS, 5 (10.9%) had grade 3-4 CRS. Unique toxicities apparent with expanded experience included capillary leak syndrome (n=3), including one grade 5 event; atypical HUS (n=2), symptomatic coagulopathy (n=8) and HLH-like manifestations (n=18). The complete remission rate was 72.5% overall; 84% at the current dose level. This included complete remissions seen in subjects who were non-responders to CD19 CAR and/or blinatumomab. The longest remission is > 3 years (n=1) post-CAR. Relapse occurred at a median of 6 months post CAR in 23 (64%) subjects primarily due to CD22 modulation. 12 proceeded to HSCT following CD22 CAR.

Conclusion: In the largest study of CD22 CAR T-cell therapy to date, this extended experience confirms the initial efficacy, while highlighting novel aspects of the toxicity profile that warrant special attention. Results from our study support further testing of this CD22 CAR in a phase 2 clinical trial.

Treated SubjectsDL1 (3 x 10e5)DL2 (1 x 10e6)DL3 (3 x 10e6)DL2 TCS (1 x 10e6)DL1 TCS> (3 x 10e5)n5261827#19n, (% of all subjects)526 (11.5%)18 (34.6%)2 (3.8%)7 (13.4%)19 (36.5%)DemographicsMedian age, (range, years)18.1 (4.4-30.6)21.3 (7.3-22.7)16.7 (8.0-30.7)17.1 (7.9-26.4)12.8 (4.4-28.9)18.5 (4.9-30.4)Prior HSCT, n (%)36 (69.2%)6 (100%)13 (72.2%)2 (100%)6 (85.7%)9 (47.3%)Prior CD19 CAR, n (%)30 (57.7%)6 (100%)11 (61.1%)1 (50%)5 (71.4%)7 (36.8%)Prior Blinatumomab, n (%)22 (42.3%)1 (16.7%)4 (22.2%)2 (100%)2 (28.6%)13 (68.4%)Prior Inotuzumab, n (%)14 (26.9%)1 (16.7%)4 (22.2%)1 (50%)3 (42.9%)5 (26.3%)Prior CD22 CAR, n (%)3 (5.8%)0002 (28.6%)1 (5.3%)Any CD19 negative population (<90%+), n (%)28 (53.8%)4 (66.7%)9 (50%)05 (71.4%)10 (52.6%)>M2 marrow, n (%)38 (73.0%)4 (66.7%)11 (61.1%)2 (100%)6 (85.7%)15 (78.9%)Toxicity ProfileTotal with CRS, n (%)46 (88.4%)3 (50%)16 (88.9%)2 (100%)6 (85.7%)19 (100%)Amongst all CRSCRS Grades 1-2, n (% of all with CRS)41 (89.1%)3 (100%)15 (93.8%)2 (100%)6 (100%)15 (78.9%)CRS Grades > 3, n (% of all with CRS)5 (10.9%)01 (5.6%)004 (21.1%)Received Tocilizumab, n (%)19 (36.5%)03 (16.7%)04 (57.1%)12 (63.2%)Received Steroids, n (%)17 (32.7%)02 (11.1%)1 (50%)4 (57.1%)10 (52.6%)Developed symptomatic coagulopathy, n (%)8 (15.4%)03 (16.7%)04 (57.1%)1 (5.3%)Developed HLH, n (%)18 (34.6%)03 (16.7%)05 (71.4%)10 (52.6%)Developed CLS, n (%)3 (5.8%)01 (5.6%)^002Developed aHUS, n (%)2 (3.8%)0001 (14.2%)2Grade 5 events, n (%)2 (3.8%)02 (11.1%)000Response RateComplete Remissions, n (%)@37 (72.5%)*1 (16.7%)13 (76.5)**1 (50%)6 (85.7%)16 (84.2%)MRD negative CR, n(%)@32 (62.7)*1 (16.7%)10 (58.8%)**0 (0%)6 (85.7%)15 (78.9%)CRS: cytokine release syndrome, as graded per Lee et al. HLH: hemophagocytic lymphohistiocytosis, retrospectively identified and defined as present if the following criteria were met: peak ferritin >100,000 with at least one of the following criteria: a) liver function tests > grade 3, b) creatinine > grade 3, c) pulmonary edema >grade 3 or d) evidence of hemophagocytosis on the bone marrow. *51 subjects evaluable for response. One subject had a grade 5 toxicity prior to disease restaging; **17 subjects evaluable for response. One subject had a grade 5 toxicity prior to disease restaging; ^CLS developed into fatal adult respiratory distress syndrome; #Subject 27 had stable disease with the first infusion with grade 1 CRS not requiring steroids or tocilizumab and limited CAR expansion. Notably he had received a CD22 CAR construct at an outside hospital prior to treatment on this protocol. Data presented in this table reflect the response and toxicity profile following the second infusion as it informed the toxicity and response profile at this dose. @Reflects the best response at any time point without any interval therapy; >Implementation of pre-emptive tocilizumab dosing initiated in this cohort.

Citation Format: Nirali N. Shah, Haneen Shalabi, Bonnie Yates, Constance Yuan, Haiying Qin, Amanda Ombrello, Hao-Wei Wang, Leah Hoffman, Minh Tran, Sandhya Panch, Maryalice Stetler-Stevenson, Jianjian Jin, Crystal Mackall, Steve Highfill, David Stroncek, Terry J. Fry. Phase I CD22 CAR T-cell trial updates [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-146.

Mechanisms of resistance to CAR T cell therapy

The successes with chimeric antigen receptor (CAR) T cell therapy in early clinical trials involving patients with pre-B cell acute lymphoblastic leukaemia (ALL) or B cell lymphomas have revolutionized anticancer therapy, providing a potentially curative option for patients who are refractory to standard treatments. These trials resulted in rapid FDA approvals of anti-CD19 CAR T cell products for both ALL and certain types of B cell lymphoma - the first approved gene therapies in the USA. However, growing experience with these agents has revealed that remissions will be brief in a substantial number of patients owing to poor CAR T cell persistence and/or cancer cell resistance resulting from antigen loss or modulation. Furthermore, the initial experience with CAR T cells has highlighted challenges associated with manufacturing a patient-specific therapy. Understanding the limitations of CAR T cell therapy will be critical to realizing the full potential of this novel treatment approach. Herein, we discuss the factors that can preclude durable remissions following CAR T cell therapy, with a primary focus on the resistance mechanisms that underlie disease relapse. We also provide an overview of potential strategies to overcome these obstacles in an effort to more effectively incorporate this unique therapeutic strategy into standard treatment paradigms.

Effect of Cryopreservation on Autologous Chimeric Antigen Receptor T Cell Characteristics

As clinical applications for chimeric antigen receptor T cell (CART) therapy extend beyond early phase trials, commercial manufacture incorporating cryopreservation steps becomes a logistical necessity. The effect of cryopreservation on CART characteristics is unclear. We retrospectively evaluated the effect of cryopreservation on product release criteria and in vivo characteristics in 158 autologous CART products from 6 single-center clinical trials. Further, from 3 healthy donor manufacturing runs, we prospectively identified differentially expressed cell surface markers and gene signatures among fresh versus cryopreserved CARTs. Within 2 days of culture initiation, cell viability of the starting fraction (peripheral blood mononuclear cells [PBMNCs]) decreased significantly in the cryo-thawed arm compared to the fresh arm. Despite this, PBMNC cryopreservation did not affect final CART fold expansion, transduction efficiency, CD3%, or CD4:CD8 ratios. In vivo CART persistence and clinical responses did not differ among fresh and cryopreserved final products. In healthy donors, compared to fresh CARTs, early apoptotic cell-surface markers were significantly elevated in cryo-thawed CARTs. Cryo-thawed CARTs also demonstrated significantly elevated expression of mitochondrial dysfunction, apoptosis signaling, and cell cycle damage pathways. Cryopreservation during CART manufacture is a viable strategy, based on standard product release parameters. The clinical impact of cryopreservation-related subtle micro-cellular damage needs further study.

Modulation of Target Antigen Density Improves CAR T-cell Functionality and Persistence

PURPOSE

Chimeric antigen receptor T-cell (CART) therapy targeting CD22 induces remission in 70% of patients with relapsed/refractory acute lymphoblastic leukemia (ALL). However, the majority of post-CD22 CART remissions are short and associated with reduction in CD22 expression. We evaluate the implications of low antigen density on the activity of CD22 CART and propose mechanisms to overcome antigen escape.

EXPERIMENTAL DESIGN

Using ALL cell lines with variable CD22 expression, we evaluate the cytokine profile, cytotoxicity, and in vivo CART functionality in the setting of low CD22 expression. We develop a high-affinity CD22 chimeric antigen receptor (CAR) as an approach to improve CAR sensitivity. We also assess Bryostatin1, a therapeutically relevant agent, to upregulate CD22 and improve CAR functionality.

RESULTS

We demonstrate that low CD22 expression negatively impacts in vitro and in vivo CD22 CART functionality and impairs in vivo CART persistence. Moreover, low antigen expression on leukemic cells increases naïve phenotype of persisting CART. Increasing CAR affinity does not improve response to low-antigen leukemia. Bryostatin1 upregulates CD22 on leukemia and lymphoma cell lines for 1 week following single-dose exposure, and improves CART functionality and in vivo persistence. While Bryostatin1 attenuates IFNγ production by CART, overall in vitro and in vivo CART cytotoxicity is not adversely affected. Finally, administration of Bryostain1 with CD22 CAR results in longer duration of in vivo response.

CONCLUSIONS

We demonstrate that target antigen modulation is a promising strategy to improve CD22 CAR efficacy and remission durability in patients with leukemia and lymphoma.See related commentary by Guedan and Delgado, p. 5188.

Chimeric Antigen Receptors Incorporating D Domains Targeting CD123 Direct Potent Mono- and Bi-specific Antitumor Activity of T Cells

Chimeric antigen receptor (CAR) T cell therapies have demonstrated impressive initial response rates in hematologic malignancies. However, relapse rates are significant, and robust efficacies in other indications, such as solid tumors, will likely require novel therapeutic strategies and CAR designs. To that end, we sought to develop simple, highly selective targeting domains (D domains) that could be incorporated into complex, multifunctional therapeutics. Herein, we describe the identification and characterization of D domains specific for CD123, a therapeutic target for hematologic malignancies, including acute myelogenous leukemia (AML). CARs comprised of these D domains mediate potent T cell activation and cytolysis of CD123-expressing target cells and induce complete durable remission in two AML xenograft models. We describe a strategy of engineering less immunogenic D domains through the identification and removal of putative T cell epitopes and investigate the binding kinetics and affinity requirements of the resultant D domain CARs. Finally, we extended the utility of D domains by generating functional, bi-specific CARs comprised of a CD123-specific D domain and a CD19-specific scFv. The properties of D domains suggest that this class of targeting domain may facilitate the development of multi-functional CARs where conventional, scFv-based designs may be suboptimal.

T cells genetically engineered to overcome death signaling enhance adoptive cancer immunotherapy

Across clinical trials, T cell expansion and persistence following adoptive cell transfer (ACT) have correlated with superior patient outcomes. Herein, we undertook a pan-cancer analysis to identify actionable ligand-receptor pairs capable of compromising T cell durability following ACT. We discovered that FASLG, the gene encoding the apoptosis-inducing ligand FasL, is overexpressed within the majority of human tumor microenvironments (TMEs). Further, we uncovered that Fas, the receptor for FasL, is highly expressed on patient-derived T cells used for clinical ACT. We hypothesized that a cognate Fas-FasL interaction within the TME might limit both T cell persistence and antitumor efficacy. We discovered that genetic engineering of Fas variants impaired in the ability to bind FADD functioned as dominant negative receptors (DNRs), preventing FasL-induced apoptosis in Fas-competent T cells. T cells coengineered with a Fas DNR and either a T cell receptor or chimeric antigen receptor exhibited enhanced persistence following ACT, resulting in superior antitumor efficacy against established solid and hematologic cancers. Despite increased longevity, Fas DNR-engineered T cells did not undergo aberrant expansion or mediate autoimmunity. Thus, T cell-intrinsic disruption of Fas signaling through genetic engineering represents a potentially universal strategy to enhance ACT efficacy across a broad range of human malignancies.

Preclinical Development of Bivalent Chimeric Antigen Receptors Targeting Both CD19 and CD22

Despite high remission rates following CAR-T cell therapy in B-ALL, relapse due to loss of the targeted antigen is increasingly recognized as a mechanism of immune escape. We hypothesized that simultaneous targeting of CD19 and CD22 may reduce the likelihood of antigen loss, thus improving sustained remission rates. A systematic approach to the generation of CAR constructs incorporating two target-binding domains led to several novel CD19/CD22 bivalent CAR constructs. Importantly, we demonstrate the challenges associated with the construction of a bivalent CAR format that preserves bifunctionality against both CD19 and CD22. Using the most active bivalent CAR constructs, we found similar transduction efficiency compared to that of either CD19 or CD22 single CARs alone. When expressed on human T cells, the optimized CD19/CD22 CAR construct induced comparable interferon γ and interleukin-2 in vitro compared to single CARs against dual-antigen-expressing as well as single-antigen-expressing cell lines. Finally, the T cells expressing CD19/CD22 CAR eradicated ALL cell line xenografts and patient-derived xenografts (PDX), including a PDX generated from a patient with CD19^- relapse following CD19-directed CAR therapy. The CD19/CD22 bivalent CAR provides an opportunity to test whether simultaneous targeting may reduce risk of antigen loss.

Preclinical Development of Bivalent Chimeric Antigen Receptors Targeting Both CD19 and CD22

Despite high remission rates following CAR-T cell therapy in B-ALL, relapse due to loss of the targeted antigen is increasingly recognized as a mechanism of immune escape. We hypothesized that simultaneous targeting of CD19 and CD22 may reduce the likelihood of antigen loss, thus improving sustained remission rates. A systematic approach to the generation of CAR constructs incorporating two target-binding domains led to several novel CD19/CD22 bivalent CAR constructs. Importantly, we demonstrate the challenges associated with the construction of a bivalent CAR format that preserves bifunctionality against both CD19 and CD22. Using the most active bivalent CAR constructs, we found similar transduction efficiency compared to that of either CD19 or CD22 single CARs alone. When expressed on human T cells, the optimized CD19/CD22 CAR construct induced comparable interferon γ and interleukin-2 in vitro compared to single CARs against dual-antigen-expressing as well as single-antigen-expressing cell lines. Finally, the T cells expressing CD19/CD22 CAR eradicated ALL cell line xenografts and patient-derived xenografts (PDX), including a PDX generated from a patient with CD19^- relapse following CD19-directed CAR therapy. The CD19/CD22 bivalent CAR provides an opportunity to test whether simultaneous targeting may reduce risk of antigen loss.

Induction of resistance to chimeric antigen receptor T cell therapy by transduction of a single leukemic B cell

We report a patient relapsing 9 months after CD19-targeted CAR T cell (CTL019) infusion with CD19^- leukemia that aberrantly expressed the anti-CD19 CAR. The CAR gene was unintentionally introduced into a single leukemic B cell during T cell manufacturing, and its product bound in cis to the CD19 epitope on the surface of leukemic cells, masking it from recognition by and conferring resistance to CTL019.

TCR engagement negatively affects CD8 but not CD4 CAR T cell expansion and leukemic clearance

Chimeric antigen receptor (CAR)-expressing T cells induce durable remissions in patients with relapsed/refractory B cell malignancies. CARs are synthetic constructs that, when introduced into mature T cells, confer a second, non-major histocompatibility complex-restricted specificity in addition to the endogenous T cell receptor (TCR). The implications of TCR activation on CAR T cell efficacy has not been well defined. Using an immunocompetent, syngeneic murine model of CD19-targeted CAR T cell therapy for pre-B cell acute lymphoblastic leukemia in which the CAR is introduced into T cells with known TCR specificity, we demonstrate loss of CD8 CAR T cell efficacy associated with T cell exhaustion and apoptosis when TCR antigen is present. CD4 CAR T cells demonstrate equivalent cytotoxicity to CD8 CAR T cells and, in contrast, retain in vivo efficacy despite TCR stimulation. Gene expression profiles confirm increased exhaustion and apoptosis of CD8 CAR T cells upon dual receptor stimulation compared to CD4 CAR T cells and indicate inherent differences between CD4 and CD8 CAR T cells in the use of T cell-associated signaling pathways. These results provide insights into important aspects of CAR T cell immune biology and indicate opportunities to rationally design CAR constructs to optimize clinical efficacy.

Abstract 1534: Analysis of CAR 41-BB versus CD28 co-stimulatory domains exposes emergence of extramedullary disease in acute myeloid leukemia

Phase I CD19 and CD22 targeting Chimeric Antigen Receptor (CAR) T cell trials have shown tremendous results against acute lymphoid leukemia (ALL). Across these trials there were variations in the types of costimulatory molecules included in the CAR constructs and this lead to the discovery that CAR T cells with CD28 result in earlier potency and activation with decreased persistence, while CARs with 4-1BB show increased expansion and persistence. There is great interest in translating these results in ALL trials into other diseases such as acute myeloid leukemia (AML). It is unclear which costimulatory domain in a CAR will be most effective in treating AML. To look at how co-stimulatory domains impact CAR functionality in AML, we developed CD33 targeting CD28 and 4-1BB CARs.

In vitro testing of the constructs revealed that CD33 CD28 CAR consistently produced more IL2 and Interferon-gamma than CD33 4-1BB across multiple AML cell lines. To translate these findings in vivo, xenograft models were injected with Molm 14 AML cells and treated with either CD33 CD28 or CD33 4-1BB CAR T cells. By bioluminescence imaging, CD33 CD28 treated mice had no detectable disease while CD33 4-1BB treated mice were ridden with leukemia. Combined, the in vitro and in vivo results suggest that the co-stimulatory domain does play a critical role in CAR T cell functionality and may improve CAR potency. To confirm the presence of AML in mice detected by bioluminescence, flow cytometry was performed on tissues from mock and CD33 4-1BB treated mice. No leukemia was found in the bone marrow of mock T cell treated mice. In contrast, CD33 4-1BB treated animals were clear of any leukemia in the bone marrow, suggesting the presence of extra medullary disease (EMD). The development of EMD in the less potent CD33 4-1BB CAR treated mice suggests that CAR immune pressure may be potent enough to clear primary sites of leukemia such as the bone marrow, but unable to eliminate disease in secondary tissues that AML can seed. This is not surprising since treatment of AML with chemotherapy often leads to the development of extramedullary disease in the form of chloromas.

To further investigate the effects of these two factors, we moved onto another AML model, THP1, that regularly presents with EMD even in the absence of CAR pressure. With CD33 CD28 against THP1, there was clearance in compartments bone marrow, however CD33 CD28 CAR was not able to prevent the development of EMD. These experiments suggest that although the CD28 costimulatory domain is more potent than 4-1BB in Molm14, the potency of CD28 is still not able to overcome EMD in all models. Using further studies of different AML models, we will continue to tease apart the contribution the impact of immune pressure from CARs, the natural progression of AML models, and effect of CAR potency on leukemia distribution.

Citation Format: Lila Yang, Samiksha Tarun, Christopher D. Chien, Mark E. Kohler, Haiying Qin, Terry J. Fry. Analysis of CAR 41-BB versus CD28 co-stimulatory domains exposes emergence of extramedullary disease in acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1534.

Abstract 1630: FLT3 chimeric antigen receptor T cell therapy induces B to T cell lineage switch in infant acute lymphoblastic leukemia

Though childhood acute lymphoblastic leukemia (ALL) is highly treatable, there remain subsets of pediatric ALL with very poor prognoses. Infant ALL, found in children under the age of 1, is difficult to treat due to the scarcity of cases impeding the ability of even the largest pediatric oncology centers from gaining experience in treating the disease, the more aggressive initial clinical presentation, as well as the inability for these young patients to tolerate toxicities associated with chemotherapeutic regimens and procedures. Despite being only 5% of total ALL cases, 80% of infant ALL cases are marked by mixed lineage leukemia (MLL/KMT2A) rearrangements. In KMT2A rearranged (KMT2Ar) ALL, FLT3 is the most differentially expressed gene that distinguishes KMT2Ar ALL from non-KMT2Ar ALL making FLT3 an attractive target for infant ALL. CD19 and CD22 targeting chimeric antigen receptor (CAR) T cell therapy has demonstrated outstanding responses in phase 1 clinical trials for relapsed/refractory B ALL, leading to tremendous interest in testing other CAR targets. Here we explore FLT3 CAR as a potential treatment for B ALL and the unexpected finding that FLT3 CAR T cells induce lineage switch of an infant ALL from a B to T cell phenotype. We generated a FLT3-targeting CAR consisting of a FLT3 binding domain derived from a well-characterized anti-human FLT3 antibody coupled to 4-1BB costimulatory and CD3-zeta activation domains. In vitro studies confirmed that human T cells expressing the FLT3 CAR produced interferon-gamma and interleukin-2 after co-culture with KMT2Ar B ALL SEM and infant B ALL KOPN8. FLT3 CAR T cells eliminated ALL in vivo in NOD-SCID-IL2Rγc-/- (NSG) mice engrafted with high FLT3 expressing SEM. KOPN8, which expresses lower levels of FLT3 when treated with FLT3 CAR T cells showed an initial clearance of leukemia in NSG mice, however relapsed with ALL that no longer expressed FLT3 or B cell marker CD19. Interestingly, this loss of FLT3 and CD19 happened concurrently with gain of T cell markers (CD3+ and either CD4+ or CD8+). The durability of this T cell phenotype was transient because when T lineage switched KOPN8 was cultured ex vivo without immune pressure, the KOPN8 cells reverted to the parental B ALL phenotype (FLT3+, CD19+, CD3 neg, CD4 neg, CD8 neg) suggesting that the ability to lineage switch is not a selection of a clone that genetically does not express B cell markers while expressing T cell markers, but rather a potential epigenetic mechanism driving the cell lineage change. Contrary to reports from CD19 CAR treated KMT2Ar B ALL that switched to a myeloid phenotype, these cells did not upregulate myeloid markers (CD33, CD11b). Taken together these data imply that lineage switch driven by CAR T cell immune pressure may cause different types of lineage switch based on the target of the CAR. Furthermore, using CAR T cell immunotherapy we may be able to interrogate the biology of leukemia.

Citation Format: Christopher D. Chien, Lila Yang, Sang M. Nguyen, Christopher T. Sauter, Kazusa Ishii, Feng Shen, Sarah K. Tasian, Terry J. Fry. FLT3 chimeric antigen receptor T cell therapy induces B to T cell lineage switch in infant acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1630.

Outcomes of Measurable Residual Disease in Pediatric Acute Myeloid Leukemia before and after Hematopoietic Stem Cell Transplant: Validation of Difference from Normal Flow Cytometry with Chimerism Studies and Wilms Tumor 1 Gene Expression

We enrolled 150 patients in a prospective multicenter study of children with acute myeloid leukemia undergoing hematopoietic stem cell transplantation (HSCT) to compare the detection of measurable residual disease (MRD) by a "difference from normal" flow cytometry (ΔN) approach with assessment of Wilms tumor 1 (WT1) gene expression without access to the diagnostic specimen. Prospective analysis of the specimens using this approach showed that 23% of patients screened for HSCT had detectable residual disease by ΔN (.04% to 53%). Of those patients who proceeded to transplant as being in morphologic remission, 10 had detectable disease (.04% to 14%) by ΔN. The disease-free survival of this group was 10% (0 to 35%) compared with 55% (46% to 64%, P < .001) for those without disease. The ΔN assay was validated using the post-HSCT specimen by sorting abnormal or suspicious cells to confirm recipient or donor origin by chimerism studies. All 15 patients who had confirmation of tumor detection relapsed, whereas the 2 patients with suspicious phenotype cells lacking this confirmation did not. The phenotype of the relapse specimen was then used retrospectively to assess the pre-HSCT specimen, allowing identification of additional samples with low levels of MRD involvement that were previously undetected. Quantitative assessment of WT1 gene expression was not predictive of relapse or other outcomes in either pre- or post-transplant specimens. MRD detected by ΔN was highly specific, but did not identify most relapsing patients. The application of the assay was limited by poor quality among one-third of the specimens and lack of a diagnostic phenotype for comparison.