Non-viral precision T cell receptor replacement for personalized cell therapy

T cell receptors (TCRs) enable T cells to specifically recognize mutations in cancer cells1-3. Here we developed a clinical-grade approach based on CRISPR-Cas9 non-viral precision genome-editing to simultaneously knockout the two endogenous TCR genes TRAC (which encodes TCRα) and TRBC (which encodes TCRβ). We also inserted into the TRAC locus two chains of a neoantigen-specific TCR (neoTCR) isolated from circulating T cells of patients. The neoTCRs were isolated using a personalized library of soluble predicted neoantigen-HLA capture reagents. Sixteen patients with different refractory solid cancers received up to three distinct neoTCR transgenic cell products. Each product expressed a patient-specific neoTCR and was administered in a cell-dose-escalation, first-in-human phase I clinical trial ( NCT03970382 ). One patient had grade 1 cytokine release syndrome and one patient had grade 3 encephalitis. All participants had the expected side effects from the lymphodepleting chemotherapy. Five patients had stable disease and the other eleven had disease progression as the best response on the therapy. neoTCR transgenic T cells were detected in tumour biopsy samples after infusion at frequencies higher than the native TCRs before infusion. This study demonstrates the feasibility of isolating and cloning multiple TCRs that recognize mutational neoantigens. Moreover, simultaneous knockout of the endogenous TCR and knock-in of neoTCRs using single-step, non-viral precision genome-editing are achieved. The manufacture of neoTCR engineered T cells at clinical grade, the safety of infusing up to three gene-edited neoTCR T cell products and the ability of the transgenic T cells to traffic to the tumours of patients are also demonstrated.

Neoantigen-targeted CD8+ T cell responses with PD-1 blockade therapy

Neoantigens are peptides derived from non-synonymous mutations presented by human leukocyte antigens (HLAs), which are recognized by antitumour T cells1-14. The large HLA allele diversity and limiting clinical samples have restricted the study of the landscape of neoantigen-targeted T cell responses in patients over their treatment course. Here we applied recently developed technologies15-17 to capture neoantigen-specific T cells from blood and tumours from patients with metastatic melanoma with or without response to anti-programmed death receptor 1 (PD-1) immunotherapy. We generated personalized libraries of neoantigen-HLA capture reagents to single-cell isolate the T cells and clone their T cell receptors (neoTCRs). Multiple T cells with different neoTCR sequences (T cell clonotypes) recognized a limited number of mutations in samples from seven patients with long-lasting clinical responses. These neoTCR clonotypes were recurrently detected over time in the blood and tumour. Samples from four patients with no response to anti-PD-1 also demonstrated neoantigen-specific T cell responses in the blood and tumour to a restricted number of mutations with lower TCR polyclonality and were not recurrently detected in sequential samples. Reconstitution of the neoTCRs in donor T cells using non-viral CRISPR-Cas9 gene editing demonstrated specific recognition and cytotoxicity to patient-matched melanoma cell lines. Thus, effective anti-PD-1 immunotherapy is associated with the presence of polyclonal CD8+ T cells in the tumour and blood specific for a limited number of immunodominant mutations, which are recurrently recognized over time.

Abstract 4783: Highly efficient, non-viral precision genome engineering for the generation of personalized neoepitope-specific adoptive T cell therapies

Methods used to engineer cells for adoptive cell therapies (ACT) utilizing receptors that are constant across many patients (CAR or shared Ag TCRs) typically rely on Lenti-, retro-, or adeno-associated virus to deliver specificity-altering sequences to T cells. However, for personalized therapies such as the generation of neoepitope-specific TCR T cell therapies, use of viral vectors is not feasible due to long manufacturing timelines and prohibitive per-patient costs. PACT Pharma has developed a highly efficient, DNA-mediated (non-viral) proprietary precision genome engineering approach to engineer neoepitope-specific primary human T cells. This method can be widely utilized to generate T cells at research scale, as well as for ex vivo manufacturing.

Briefly, genomes of individual primary human CD8 and CD4 T cells are engineered with site-specific nucleases in a single-step transfection process to yield efficient, targeted replacement of the endogenous TCR with the therapeutic neoTCR sequences. In this way, the expression of the endogenous TCR is abolished ensuring natural expression and regulation of the inserted neoTCR. The precision of neoTCR-T cell genome engineering was evaluated by Targeted Locus Amplification (TLA) for off-target integration hot spots or translocations, and by next generation sequencing based off-target cleavage assays and found to lack evidence of unintended outcomes.

Engineered neoepitope-specific T cells are highly functional as demonstrated by antigen-specific proliferation, killing and cytokine production. Phenotype and detailed functional characterization of PACTs neoTCR-P1 T cells were performed and are described in a separate abstract.

PACT’s precision genome engineering approach enables highly efficient generation of bespoke NeoTCR T cells for personalized adoptive cell therapy for patients with solid tumors. Furthermore, PACT precision genome engineering method is not restricted to the use in T cells and has also been applied successfully to other primary cell types, including natural killer and hematopoietic stem cells.

Citation Format: Kyle Jacoby, Robert Moot, William Lu, Diana Nguyen, Barbara Sennino, Andrew Conroy, Bhamini Purandare, Adam J. Litterman, Fabrizia Urbinati, Susan P. Foy, Theresa Hunter, Albert Tai, Michael T. Bethune, Songming Peng, Olivier Dalmas, Alex Franzusoff, Stefanie J. Mandl. Highly efficient, non-viral precision genome engineering for the generation of personalized neoepitope-specific adoptive T cell therapies [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 4783.

Abstract LB-234: Poxvirus-based active immunotherapy synergizes with PD-1 plus LAG-3 immune checkpoint inhibition to enhance antitumor efficacy in preclinical models

Treatment with poxvirus-based active immunotherapies shows evidence of robust immune responses against a variety of tumor-associated antigens in preclinical and clinical studies. Poxvirus-based immunotherapies in development include PSA-targeted PROSTVAC, now in Phase 3 clinical development; CV-301 (targeting CEA and MUC-1); as well as MVA-BN-HER2 and MVA-BN-Brachyury (targeting HER-2 and the transcription factor Brachyury, respectively). Evidence of robust and productive anti-tumor efficacy in preclinical models was accompanied by treatment-emergent infiltration of tumors by activated cytotoxic CD8 T cells producing high amounts of IFNγ.

Treatment with immune checkpoint inhibitors such as anti-PD-1 antibodies is showing significant clinical benefit by re-activating dormant tumor-specific T cells. Furthermore, preclinical studies have shown further synergistic efficacy by combining PD-1 blockade with inhibition of LAG-3, which acts independently of PD-1 to modulate T cell function. We hypothesized that poxvirus-based active immunotherapy may provide even greater improvements to patient outcome when used in combination with immune checkpoint blockade, by inducing new productive tumor-specific responses. This may be especially important in patients lacking an endogenous T cell response against their tumors.

In therapeutic CT26-HER2 solid and metastatic tumor models, mice were administered MVA-BN-HER2 immunotherapy alone or in combination with anti-PD-1 and/or anti-LAG-3 antibodies. Synergistic benefit for anti-tumor efficacy was observed when combining MVA-BN-HER2 immunotherapy with anti-PD-1 alone, while combination with anti-LAG-3 alone had little effect. Notably, a further enhancement occurred when MVA-BN-HER2 immunotherapy was combined with PD-1 and LAG-3 blockade as shown by complete tumor regression in 20/20 mice. Subsequent rejection of HER-2 negative tumors 6 months after the original challenge revealed that immune responses were durable and included antigen spread to additional tumor antigens.

Flow cytometric analysis demonstrated that tumor infiltrating lymphocytes (TILs) in untreated tumors were PD-1hi and LAG-3+, a more exhausted phenotype. Poxvirus-based immunotherapy led to the induction of activated TILs characterized by low to mid-levels of PD-1 expression. While PD-1 blockade prevented binding to PD-L1 it also caused an increase in LAG-3 expression on T cells. Together these data provide further rationale for why combination therapy of poxvirus-based immunotherapy with inhibition of PD-1 plus LAG-3 resulted in synergistic efficacy in preclinical tumor models.

Overall these data demonstrate that combining complementary immune-based therapies such as poxvirus-based active immunotherapy and PD-1 plus LAG-3 immune checkpoint blockade result in synergistic anti-tumor efficacy.

Citation Format: Barbara Sennino, Susan P. Foy, Ryan B. Rountree, Tracy dela Cruz, Evan J. Gordon, Veronica Xavier, Felicia Kemp, Alex Franzusoff, James Breitmeyer, Stefanie J. Mandl. Poxvirus-based active immunotherapy synergizes with PD-1 plus LAG-3 immune checkpoint inhibition to enhance antitumor efficacy in preclinical models. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-234. doi:10.1158/1538-7445.AM2015-LB-234

Elucidating immunologic mechanisms of PROSTVAC cancer immunotherapy

BACKGROUND

PROSTVAC®, an active immunotherapy currently studied for the treatment of metastatic castration-resistant prostate cancer (mCRPC), consists of a heterologous prime-boost regimen with two different poxvirus-based vectors to provoke productive immune responses against prostate specific antigen (PSA) as the target tumor antigen. A Phase 2 study of PROSTVAC immunotherapy showed significantly improved median overall survival by 8.5 months and is currently being validated in a global Phase 3 study (PROSPECT; NCT01322490). Here, preclinical models were explored to investigate the mechanism of action and immune signatures of anti-tumor efficacy with PROSTVAC immunotherapy with the goal to identify potential immune correlates of clinical benefit.

METHODS

PROSTVAC-induced immune responses and anti-tumor efficacy were studied in male BALB/c mice. Functionality of the induced T cell response was characterized by interferon-gamma (IFNγ) ELISPOT, cytotoxic degranulation, multi-cytokine intracellular staining, and in vivo T cell depletion. Tumor infiltrating lymphocytes (TILs) were evaluated phenotypically by flow cytometry.

RESULTS

The heterologous prime-boost regimen of the two PROSTVAC vectors significantly enhanced the magnitude and quality of activated PSA-specific CD4 and CD8 T cell responses compared to homologous, single vector regimens. PROSTVAC-activated CD4 and CD8 T cells were highly functional as evidenced by expression of activation markers, production of multiple cytokines, and amplified cytotoxic T cell activity. Importantly, PROSTVAC immunotherapy resulted in significant anti-tumor efficacy in a transplantable prostate cancer mouse model. Antigen-spreading occurred in PROSTVAC-treated animals that rejected PSA-expressing tumors, as shown by subsequent rejection of PSA-negative tumors. In vivo CD4 and CD8 depletion revealed that both T cell subsets contributed to anti-tumor efficacy. Characterization of TILs demonstrated that PROSTVAC immunotherapy greatly increased the intra-tumoral ratio of activated effector to regulatory T cells.

CONCLUSIONS

PROSTVAC immunotherapy activates broad, highly functional T cell immunity to PSA and to endogenous tumor antigens via immune-mediated antigen spreading. These preclinical results further elucidate the mode of action of PROSTVAC immunotherapy and its potential causal relationship to extended overall survival as observed in the PROSTVAC Phase 2 study. The clinical validation is ongoing in the PROSPECT Phase 3 clinical study.

Abstract A21: Active immunotherapy with PROSTVAC® demonstrates potent antitumor efficacy in a mouse model of prostate cancer

BN ImmunoTherapeutics (BNIT) specializes in developing novel active immunotherapies for cancer. These therapies use recombinant poxviruses engineered to express tumor-associated antigens (TAAs), with the intent of generating effective immune responses against the patients' cancer. PROSTVAC® is a candidate product for the treatment of prostate cancer for which a global Phase III clinical trial (PROSPECT) was recently initiated. This product is composed of two different viral vectors derived from a recombinant vaccinia virus (PROSTVAC™-V) and a recombinant fowlpox virus (PROSTVAC™-F). Both vectors contain transgenes encoding prostate-specific antigen (PSA) and a triad of costimulatory molecules (B7-1, ICAM-1, and LFA-3), designated as TRICOM™. Patients are immunized using a prime-boost strategy consisting of an initial treatment with PROSTVAC™-V followed by repeated boosting with PROSTVAC™-F to maximize the immune responses against the PSA tumor-antigen.

Here we show preclinical data characterizing PROSTVAC® activity in mice. Treatment with either PROSTVAC™-V or PROSTVAC™-F induced PSA-specific antibody and T cell responses; however, PSA-specific responses were further increased by the prime/boost strategy, particularly with respect to the frequency of responding CD8 T cells. These CD8 T cells produced IFN-gamma and degranulated in an antigen-specific manner. Furthermore, PROSTVAC® treatment resulted in strong efficacy in a mouse model of prostate cancer. In this model, treatment with PROSTVAC resulted in anti-tumor efficacy accompanied by a Th1-biased response against PSA. In contrast, growth of tumors in control mice induced only non-protective PSA-specific responses with strong Th2 bias.

Overall, these animal studies help define the activity and mechanism of action of PROSTVAC® which is currently being evaluated in the clinic.

R.B. Rountree and S.J. Mandl contributed equally to this work.

Citation Format: Ryan B. Rountree, Stefanie J. Mandl, Joseph Cote, Tracy dela Cruz, Thierry Giffon, Evan Gordon, Susan P. Foy, John R. Lombardo, Erica Trent, Reiner Laus, Alain Delcayre. Active immunotherapy with PROSTVAC® demonstrates potent antitumor efficacy in a mouse model of prostate cancer. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr A21.

Oh the irony: Iron as a cancer cause or cure?

Iron-oxide nanoparticles facilitate cancer diagnosis through enhanced contrast, selectively enhance tumor cell death with magnetic hyperthermia, and improve drug delivery with magnetic drug targeting. One application that remains largely unexplored is using the iron-oxide nanoparticles themselves to selectively inhibit tumor growth. In this leading opinion paper, we propose that high doses of iron-oxide nanoparticles can be used as a treatment for cancer by generating an oxidative assault against cancer. This proposal may be met with resistance considering the controversy surrounding iron in the field of cancer. Iron generates reactive oxygen species through the Fenton reaction, which may both cause - or cure cancer. Additionally, high demand for iron by cancer cells leads to contradictory therapeutic approaches: iron deprivation or overdose are both potential cancer therapies.

Tumor ablation and nanotechnology

Next to surgical resection, tumor ablation is a commonly used intervention in the treatment of solid tumors. Tumor ablation methods include thermal therapies, photodynamic therapy, and reactive oxygen species (ROS) producing agents. Thermal therapies induce tumor cell death via thermal energy and include radiofrequency, microwave, high intensity focused ultrasound, and cryoablation. Photodynamic therapy and ROS producing agents cause increased oxidative stress in tumor cells leading to apoptosis. While these therapies are safe and viable alternatives when resection of malignancies is not feasible, they do have associated limitations that prevent their widespread use in clinical applications. To improve the efficacy of these treatments, nanoparticles are being studied in combination with nonsurgical ablation regimens. In addition to better thermal effect on tumor ablation, nanoparticles can deliver anticancer therapeutics that show a synergistic antitumor effect in the presence of heat and can also be imaged to achieve precision in therapy. Understanding the molecular mechanism of nanoparticle-mediated tumor ablation could further help engineer nanoparticles of appropriate composition and properties to synergize the ablation effect. This review aims to explore the various types of nonsurgical tumor ablation methods currently used in cancer treatment and potential improvements by nanotechnology applications.

Optical imaging and magnetic field targeting of magnetic nanoparticles in tumors

To address efficacy issues of cancer diagnosis and chemotherapy, we have developed a magnetic nanoparticle (MNP) formulation with combined drug delivery and imaging properties that can potentially be used in image-guided drug therapy. Our MNP consists of an iron-oxide magnetic core coated with oleic acid (OA) and stabilized with an amphiphilic block copolymer. Previously, we reported that our MNP formulation can provide prolonged contrast for tumor magnetic resonance imaging and can be loaded with hydrophobic anticancer agents for sustained drug delivery. In this study, we developed MNPs with optical imaging properties using new near-infrared dyes to quantitatively determine their long-term biodistribution and tumor localization with and without an external magnetic field in mice with xenograft breast tumors. MNPs localized slowly in the tumor, reaching a peak 48 h post-injection before slowly declining over the next 11 days. One hour exposure of the tumor to a magnetic field further enhanced MNP localization to tumors. Our MNPs can be developed with combined drug delivery and multimodal imaging properties to improve cancer diagnosis, provide sustained treatment, and monitor therapeutic effects in tumors over time.

PEG-functionalized magnetic nanoparticles for drug delivery and magnetic resonance imaging applications

PURPOSE

Polyethylene glycol (PEG) functionalized magnetic nanoparticles (MNPs) were tested as a drug carrier system, as a magnetic resonance imaging (MRI) agent, and for their ability to conjugate to an antibody.

METHODS

An iron oxide core coated with oleic acid (OA) and then with OA-PEG forms a water-dispersible MNP formulation. Hydrophobic doxorubicin partitions into the OA layer for sustained drug delivery. The T(1) and T(2) MRI contrast properties were determined in vitro and the circulation of the MNPs was measured in mouse carotid arteries. An N-hydroxysuccinimide group (NHS) on the OA-PEG-80 was used to conjugate the amine functional group on antibodies for active targeting in the human MCF-7 breast cancer cell line.

RESULTS

The optimized formulation had a mean hydrodynamic diameter of 184 nm with an ~8 nm iron-oxide core. The MNPs enhance the T(2) MRI contrast and have a long circulation time in vivo with 30% relative concentration 50 min post-injection. Doxorubicin-loaded MNPs showed sustained drug release and dose-dependent antiproliferative effects in vitro; the drug effect was enhanced with transferrin antibody-conjugated MNPs.

CONCLUSION

PEG-functionalized MNPs could be developed as a targeted drug delivery system and MRI contrast agent.

Magnetic resonance imaging of multifunctional pluronic stabilized iron-oxide nanoparticles in tumor-bearing mice

We are investigating the magnetic resonance imaging characteristics of magnetic nanoparticles (MNPs) that consist of an iron-oxide magnetic core coated with oleic acid (OA), then stabilized with a pluronic or tetronic block copolymer. Since pluronics and tetronics vary structurally, and also in the ratio of hydrophobic (poly[propylene oxide]) and hydrophilic (poly[ethylene oxide]) segments in the polymer chain and in molecular weight, it was hypothesized that their anchoring to the OA coating around the magnetic core could significantly influence the physical properties of MNPs, their interactions with biological environment following intravenous administration, and ability to localize to tumors. The amount of block copolymer associated with MNPs was seen to depend upon their molecular structures and influence the characteristics of MNPs. Pluronic F127-modified MNPs demonstrated sustained and enhanced contrast in the whole tumor, whereas that of Feridex IV was transient and confined to the tumor periphery. In conclusion, our pluronic F127-coated MNPs, which can also be loaded with anticancer agents for drug delivery, can be developed as an effective cancer theranostic agent, i.e. an agent with combined drug delivery and imaging properties.

The impact of intravenous corticosteroids with third molar surgery in patients at high risk for delayed health-related quality of life and clinical recovery

PURPOSE

To compare recovery for clinical and health-related quality of life (HRQOL) outcomes after third molar surgery in patients predicted to be at risk for delayed recovery, treated with or without intravenous (IV) corticosteroids at surgery.

PATIENTS AND METHODS

Patients at least 18 years of age and with all 4 third molars below the occlusal plane were given IV corticosteroids just before third molar surgery. Clinical and HRQOL outcomes of these patients were compared with those of a nonconcurrent control group who did not receive corticosteroids. No antibiotics were administered. The control group was selected using the same criteria and treated under the same surgical protocol as the corticosteroid group. Differences between the groups were assessed with Cochran-Mantel-Haenszel row mean score statistics.

RESULTS

Sixty patients were in each cohort. The incidence of delayed clinical recovery, a postsurgery visit with treatment, was higher in the control group compared with the corticosteroid group. In the corticosteroid group, 6 patients (10%) had 1 postsurgery visit with treatment. In the control group without corticosteroids, 17 patients (28%) had at least 1 postsurgery visit with treatment (P = .01). Compared with the control group, nausea tended to bother patients less on postsurgery day 1 (P = .07); sleep was improved on postsurgery days 1 through 4 (P < .05). Though not statistically significant, corticosteroids reduced the patients reported recovery by at least 1 day for pain, lifestyle, and oral function.

CONCLUSION

Administration of IV corticosteroids before third molar surgery without antibiotics does not hamper clinical recovery even when healthy adult patients are predicted to have delayed recovery. Overall, IV corticosteroid administration had a limited, but beneficial effect on HRQOL outcomes.

The impact of third molar symptoms, pain, and swelling on oral health-related quality of life

PURPOSE

This study was designed to assess the impact of "pain and swelling" associated with third molars on patients' quality of life before surgery.

PATIENTS AND METHODS

The data for these analyses were obtained from a larger ongoing study designed to examine the surgical and medical management of problems associated with third molars. Data from 480 patients with 4 third molars scheduled for removal were used in the analysis. Questionnaires administered presurgery assessed patients' medical and dental history, their reasons for seeking third molar removal, and sociodemographic characteristics. Adverse impacts on oral health-related quality of life were measured using the 14-item Oral Health Impact Profile (OHIP) questionnaire. The primary outcome variable was the percentage of people reporting 1 or more of the 12 non-pain-specific OHIP items "fairly often" or "very often" during the 3 months before enrollment.

RESULTS

One third (178 of 480) of patients said they were seeking third molar surgery because of current or previous symptoms of pain/swelling, and 17% reported 1 or more of the 12 non-pain-specific OHIP items. In the multivariate logistic regression model, the odds of one or more impacts was greater for people who presented because of symptoms (odds ratio [OR], 2.9; 95% confidence interval [CI], 1.7 to 4.8), who were aged 25 years or more (OR, 1.9; 95% CI, 1.1-3.3), and who had a self-reported history of tooth loss due to pathology or trauma (OR, 2.9; 95% CI, 1.9 to 5.5).

CONCLUSIONS

Adverse impacts on quality of life occurred for 1 in 8 patients seeking third molar surgery, and the odds increased 3-fold for patients who had experienced pain/swelling compared with those who were asymptomatic.

The impact of intravenous antibiotics on health-related quality of life outcomes and clinical recovery after third molar surgery

PURPOSE

We sought to compare recovery for clinical and health-related quality of life (HRQOL) outcomes after third molar surgery in patients treated with or without intravenous antibiotics at surgery.

PATIENTS AND METHODS

Fifty-six patients at least 18 years of age and with all 4 third molars below the occlusal plane, treated at 3 clinical centers, were given intravenous antibiotics just before third molar surgery. Clinical and HRQOL outcomes of these patients were compared with those of a nonconcurrent control group (n = 60 patients) who did not receive antibiotics. The control group was selected using the same criteria and treated under the same surgical protocol as the antibiotic group. Differences between the groups were assessed with Cochran-Mantel-Haenszel row mean score statistics.

RESULTS

The incidence of delayed clinical recovery defined as a postsurgery visit with treatment was higher in the control group compared with the antibiotic group. In the antibiotic group, 4% had 1 postsurgery visit with treatment; no patient had 2 visits. In the control group without antibiotics, 28% had at least 1 postsurgery visit with treatment (P <.0001) and 13% had at least 2 postsurgery visits with treatment. No statistically significant differences in HRQOL outcomes were found between the 2 groups.

CONCLUSIONS

Administration of intravenous antibiotics before third molar surgery may improve clinical recovery in healthy adult patients with all 4 third molars below the occlusal plane, a presenting characteristic that has been suggested as a risk factor for delayed recovery. The findings from this exploratory trial indicate that evaluation of the effectiveness of systemic antibiotic administration with third molar surgery in a randomized, multi-intervention, explanatory clinical trial is warranted.

Association of B lymphocyte antigen receptor polypeptides with multiple chaperone proteins

The B cell antigen receptor (BCR) is comprised of four different polypeptides, immunoglobulin (Ig) heavy chain, Ig light chain, and the two signaling subunits of this receptor, Ig-alpha and Ig-beta. These four chains must assemble correctly in the endoplasmic reticulum (ER) before the BCR can be transported to the cell surface. The roles of the different chaperone proteins in mediating the assembly of mIg with the Ig-alpha/beta are not fully understood. To gain insights into the roles of chaperone proteins in BCR assembly, we have generated transfected non-lymphoid cell lines that express various intermediate assembled forms of the BCR and used them to examine the interactions of chaperone proteins with subunits of the BCR. We examined the interactions of BiP (GRP78), GRP94 and calnexin with the mu heavy chain, lambda light chain, Ig-alpha and Ig-beta. We report for the first time that Ig-alpha associates with GRP94 and that this interaction increases dramatically when other BCR chains are co-expressed. In contrast, the mu heavy chain interacts strongly with BiP (GRP78) when expressed by itself but this interaction is reduced when the lambda light chain is expressed, with the resulting mu(lambda) complexes interacting with GRP94 and calnexin. Thus, our data are consistent with the idea that there is an ordered association of BCR components with different protein chaperones during BCR assembly.

Signal transduction by the B-cell antigen receptor

The antigen receptor of B lymphocytes (BCR) plays important roles in recognition of foreign antigens and self-components to allow the immune system to make appropriate antibody responses. The BCR is a complex between membrane immunoglobulin and the Ig-alpha and Ig-beta heterodimer. Site-directed mutagenesis experiments have shown that the mu heavy chain transmembrane domain plays a key role in the association of mIgM with Ig-alpha/Ig-beta. In the absence of complex formation, mIgM is retained in the endoplasmic reticulum, and this function is also specified by the mu chain transmembrane domain. The ability of various mutant mIgM molecules to associate with Ig-alpha/Ig-beta correlates well with their ability to induce signal transduction reactions such as protein tyrosine phosphorylation and phosphoinositide breakdown. Thus, the signaling ability of the BCR appears to reside in the Ig-alpha/Ig-beta heterodimer. The cytoplasmic domains of Ig-alpha and Ig-beta each contain an ITAM sequence, which is defined by its limited homology with subunits of the T-cell antigen receptor and of Fc receptors. Moreover, chimeric proteins containing these ITAMs and surrounding sequences from the cytoplasmic domains of Ig-alpha or Ig-beta exhibit signaling function characteristics of the intact BCR. The Ig-alpha and Ig-beta chimeras are each capable of inducing all of the BCR signaling events tested and thus represent redundant functions. Cross-linking these chimeras leads to their phosphorylation and to binding of the intracellular tyrosine kinases Lyn and Syk. The BCR expressed in the nonlymphoid AtT20 cells, which express the Src-family tyrosine kinase Fyn but not Syk, was not able to trigger vigorous signaling reactions. Introduction of the active form of Syk into these cells restored some signaling events. These results are consistent with a model in which the ITAMs act to initiate the BCR signaling reactions by binding and activating tyrosine kinases.

A mutation of the mu transmembrane that disrupts endoplasmic reticulum retention. Effects on association with accessory proteins and signal transduction

The mu heavy chain has an unusually high content of hydroxyl-containing amino acids in its membrane-spanning region. We have examined the involvement of two of these hydrophilic residues in endoplasmic reticulum (ER) retention, interactions with Ig-alpha/Ig-beta, and transmembrane signaling. Neighboring tyrosine and serine residues were mutated to either phenylalanine and alanine (mutant YS/FA) or valine and valine (mutant YS/VV). Membrane Ig (mIgM) molecules containing these mutant mu chains were expressed on the surface of transfected B lymphoma cells. Anti-Ig-induced signaling by the YS/FA mutant mIgM was equivalent to wild-type (wt) mIgM, whereas signaling by the YS/VV mutant mIgM was notably diminished. Association between mutant YS/VV mIgM and Ig-alpha/Ig-beta was detectable but reduced in comparison to YS/FA or wt mIgM. Signaling by YS/VV mutant mIgM appeared to involve Ig-alpha/Ig-beta, because these proteins were tyrosine phosphorylated on receptor cross-linking. When YS/VV and wt mu chains were cotransfected with light chains into nonlymphoid cells, mutant mIgM was expressed at the cell surface in the absence of Ig-alpha/Ig-beta, whereas wt mIgM was not. These data suggest that the mutated residues contribute to ER retention and directly or indirectly to association with Ig-alpha/Ig-beta. Moreover, ER retention can be disrupted without preventing functional association with Ig-alpha/Ig-beta. In addition, these data indicate that the hydroxyl groups of the mutated residues are not required for functional association between mu and Ig-alpha/Ig-beta because their removal did not reduce the ability of the YS/FA mutant mIgM to associate with accessory proteins or to participate in signal transduction.

A mutation of the mu transmembrane that disrupts endoplasmic reticulum retention. Effects on association with accessory proteins and signal transduction

The mu heavy chain has an unusually high content of hydroxyl-containing amino acids in its membrane-spanning region. We have examined the involvement of two of these hydrophilic residues in endoplasmic reticulum (ER) retention, interactions with Ig-alpha/Ig-beta, and transmembrane signaling. Neighboring tyrosine and serine residues were mutated to either phenylalanine and alanine (mutant YS/FA) or valine and valine (mutant YS/VV). Membrane Ig (mIgM) molecules containing these mutant mu chains were expressed on the surface of transfected B lymphoma cells. Anti-Ig-induced signaling by the YS/FA mutant mIgM was equivalent to wild-type (wt) mIgM, whereas signaling by the YS/VV mutant mIgM was notably diminished. Association between mutant YS/VV mIgM and Ig-alpha/Ig-beta was detectable but reduced in comparison to YS/FA or wt mIgM. Signaling by YS/VV mutant mIgM appeared to involve Ig-alpha/Ig-beta, because these proteins were tyrosine phosphorylated on receptor cross-linking. When YS/VV and wt mu chains were cotransfected with light chains into nonlymphoid cells, mutant mIgM was expressed at the cell surface in the absence of Ig-alpha/Ig-beta, whereas wt mIgM was not. These data suggest that the mutated residues contribute to ER retention and directly or indirectly to association with Ig-alpha/Ig-beta. Moreover, ER retention can be disrupted without preventing functional association with Ig-alpha/Ig-beta. In addition, these data indicate that the hydroxyl groups of the mutated residues are not required for functional association between mu and Ig-alpha/Ig-beta because their removal did not reduce the ability of the YS/FA mutant mIgM to associate with accessory proteins or to participate in signal transduction.

Mechanism of B cell antigen receptor function: transmembrane signaling and triggering of apoptosis

The antigen receptor of B lymphocytes (BCR) plays important roles in virtually every stage in the development, inactivation, or activation of B cells. The BCR is a complex of membrane immunoglobulin (mIg) and a heterodimer of two transmembrane polypeptides called Ig-alpha and Ig-beta. Site directed mutation of the mu immunoglobulin heavy chain has demonstrated that the mu transmembrane domain plays a key role in the assembly of mIgM with Ig-alpha/Ig-beta. In addition, there is a strong correlation between the ability of various mutant mIgM molecules to associate with Ig-alpha/Ig-beta and their ability to induce signal transduction reactions such as protein tyrosine phosphorylation and phosphoinositide breakdown. The cytoplasmic domains of Ig-alpha and Ig-beta share a region of limited homology with each other and with components of the T cell antigen receptor and of the Fc receptor. The presence of regions of the cytoplasmic domains of Ig-alpha or Ig-beta including this conserved amino acid sequence motif is sufficient to confer signaling function on chimeric transmembrane proteins. Both Ig-alpha and Ig-beta chimeras are capable of inducing all of the BCR signaling events tested. Based on these and related observations, we propose that the motifs act to initiate the BCR signaling reactions by binding and activating tyrosine kinases. Among the important events mediated by BCR signaling is induced expression of a series of genes referred to as early response genes. In B cells these include transcription factors and at least one component that regulates signaling events. One of these genes, c-myc, appears to play an important role in mediating apoptosis in B cells stimulated via the BCR complex.