The prognostic value of 19S ATPase proteasome subunits in acute myeloid leukemia and other forms of cancer

Introduction

The ubiquitin-proteasome system (UPS) is an intracellular organelle responsible for targeted protein degradation, which represents a standard therapeutic target for many different human malignancies. Bortezomib, a reversible inhibitor of chymotrypsin-like proteasome activity, was first approved by the FDA in 2003 to treat multiple myeloma and is now used to treat a number of different cancers, including relapsed mantle cell lymphoma, diffuse large B-cell lymphoma, colorectal cancer, and thyroid carcinoma. Despite the success, bortezomib and other proteasome inhibitors are subject to severe side effects, and ultimately, drug resistance. We recently reported an oncogenic role for non-ATPase members of the 19S proteasome in chronic myeloid leukemia (CML), acute myeloid leukemia (AML), and several different solid tumors. In the present study, we hypothesized that ATPase members of the 19S proteasome would also serve as biomarkers and putative therapeutic targets in AML and multiple other cancers.

Methods

We used data from The Cancer Genome Atlas (TCGA) and the Clinical Proteomic Tumor Analysis Consortium (CPTAC) available at UALCAN and/or GEPIA2 to assess the expression and prognostic value of proteasome 26S subunit, ATPases 1-6 (PSMC1-6) of the 19S proteasome in cancer. UALCAN was also used to associate PSMC1-6 mRNA expression with distinct clinicopathological features. Finally, cBioPortal was employed to assess genomic alterations of PSMC genes across different cancer types.

Results

The mRNA and protein expression of PSMC1-6 of the 19S proteasome were elevated in several cancers compared with normal controls, which often correlated with worse overall survival. In contrast, AML patients demonstrated reduced expression of these proteasome subunits compared with normal mononuclear cells. However, AML patients with high expression of PSMC2-5 had worse outcomes.

Discussion

Altogether, our data suggest that components of the 19S proteasome could serve as prognostic biomarkers and novel therapeutic targets in AML and several other human malignancies.

Loss of G0/G1 switch gene 2 (G0S2) promotes disease progression and drug resistance in chronic myeloid leukaemia (CML) by disrupting glycerophospholipid metabolism

Tyrosine kinase inhibitors (TKIs) targeting BCR::ABL1 have turned chronic myeloid leukaemia (CML) from a fatal disease into a manageable condition for most patients. Despite improved survival, targeting drug-resistant leukaemia stem cells (LSCs) remains a challenge for curative CML therapy. Aberrant lipid metabolism can have a large impact on membrane dynamics, cell survival and therapeutic responses in cancer. While ceramide and sphingolipid levels were previously correlated with TKI response in CML, the role of lipid metabolism in TKI resistance is not well understood. We have identified downregulation of a critical regulator of lipid metabolism, G0/G1 switch gene 2 (G0S2), in multiple scenarios of TKI resistance, including (1) BCR::ABL1 kinase-independent TKI resistance, (2) progression of CML from the chronic to the blast phase of the disease, and (3) in CML versus normal myeloid progenitors. Accordingly, CML patients with low G0S2 expression levels had a worse overall survival. G0S2 downregulation in CML was not a result of promoter hypermethylation or BCR::ABL1 kinase activity, but was rather due to transcriptional repression by MYC. Using CML cell lines, patient samples and G0s2 knockout (G0s2-/- ) mice, we demonstrate a tumour suppressor role for G0S2 in CML and TKI resistance. Our data suggest that reduced G0S2 protein expression in CML disrupts glycerophospholipid metabolism, correlating with a block of differentiation that renders CML cells resistant to therapy. Altogether, our data unravel a new role for G0S2 in regulating myeloid differentiation and TKI response in CML, and suggest that restoring G0S2 may have clinical utility.

19S Proteasome Subunits as Oncogenes and Prognostic Biomarkers in FLT3-Mutated Acute Myeloid Leukemia (AML)

26S proteasome non-ATPase subunits 1 (PSMD1) and 3 (PSMD3) were recently identified as prognostic biomarkers and potential therapeutic targets in chronic myeloid leukemia (CML) and multiple solid tumors. In the present study, we analyzed the expression of 19S proteasome subunits in acute myeloid leukemia (AML) patients with mutations in the FMS-like tyrosine kinase 3 (FLT3) gene and assessed their impact on overall survival (OS). High levels of PSMD3 but not PSMD1 expression correlated with a worse OS in FLT3-mutated AML. Consistent with an oncogenic role for PSMD3 in AML, shRNA-mediated PSMD3 knockdown impaired colony formation of FLT3+ AML cell lines, which correlated with increased OS in xenograft models. While PSMD3 regulated nuclear factor-kappa B (NF-κB) transcriptional activity in CML, we did not observe similar effects in FLT3+ AML cells. Rather, proteomics analyses suggested a role for PSMD3 in neutrophil degranulation and energy metabolism. Finally, we identified additional PSMD subunits that are upregulated in AML patients with mutated versus wild-type FLT3, which correlated with worse outcomes. These findings suggest that different components of the 19S regulatory complex of the 26S proteasome can have indications for OS and may serve as prognostic biomarkers in AML and other types of cancers.

Harnessing the Immune System with Cancer Vaccines: From Prevention to Therapeutics

Prophylactic vaccination against infectious diseases is one of the most successful public health measures of our lifetime. More recently, therapeutic vaccination against established diseases such as cancer has proven to be more challenging. In the host, cancer cells evade immunologic regulation by multiple means, including altering the antigens expressed on their cell surface or recruiting inflammatory cells that repress immune surveillance. Nevertheless, recent clinical data suggest that two classes of antigens show efficacy for the development of anticancer vaccines: tumor-associated antigens and neoantigens. In addition, many different vaccines derived from antigens based on cellular, peptide/protein, and genomic components are in development to establish their efficacy in cancer therapy. Some vaccines have shown promising results, which may lead to favorable outcomes when combined with standard therapeutic approaches. This review provides an overview of the innate and adaptive immune systems, their interactions with cancer cells, and the development of various different vaccines for use in anticancer therapeutics.

MS4A3 promotes differentiation in chronic myeloid leukemia by enhancing common β-chain cytokine receptor endocytosis

The chronic phase of chronic myeloid leukemia (CP-CML) is characterized by the excessive production of maturating myeloid cells. As CML stem/progenitor cells (LSPCs) are poised to cycle and differentiate, LSPCs must balance conservation and differentiation to avoid exhaustion, similar to normal hematopoiesis under stress. Since BCR-ABL1 tyrosine kinase inhibitors (TKIs) eliminate differentiating cells but spare BCR-ABL1-independent LSPCs, understanding the mechanisms that regulate LSPC differentiation may inform strategies to eliminate LSPCs. Upon performing a meta-analysis of published CML transcriptomes, we discovered that low expression of the MS4A3 transmembrane protein is a universal characteristic of LSPC quiescence, BCR-ABL1 independence, and transformation to blast phase (BP). Several mechanisms are involved in suppressing MS4A3, including aberrant methylation and a MECOM-C/EBPε axis. Contrary to previous reports, we find that MS4A3 does not function as a G1/S phase inhibitor but promotes endocytosis of common β-chain (βc) cytokine receptors upon GM-CSF/IL-3 stimulation, enhancing downstream signaling and cellular differentiation. This suggests that LSPCs downregulate MS4A3 to evade βc cytokine-induced differentiation and maintain a more primitive, TKI-insensitive state. Accordingly, knockdown (KD) or deletion of MS4A3/Ms4a3 promotes TKI resistance and survival of CML cells ex vivo and enhances leukemogenesis in vivo, while targeted delivery of exogenous MS4A3 protein promotes differentiation. These data support a model in which MS4A3 governs response to differentiating myeloid cytokines, providing a unifying mechanism for the differentiation block characteristic of CML quiescence and BP-CML. Promoting MS4A3 reexpression or delivery of ectopic MS4A3 may help eliminate LSPCs in vivo.

Abstract PO-200: Border differences on breast cancer incidence and survival between non-Hispanic white and Hispanic patients: A Texas population-based study

Background: Breast cancer (BC) is the leading cause of cancer related deaths in Hispanic women. Hispanics make up an estimated 82% of the US/Mexico border, a region characterized by socioeconomic inequity and barriers to healthcare access. Identifying disparities associated with BC incidence and overall survival (OS) is a priority for allocating resources and optimizing care in this medically underserved population. We hypothesized that differences in Hispanics and Non-Hispanic Whites (NHW), proximity to the border, BC subtype, and treatment are associated with poor outcomes. Methods: BC data was obtained from the Texas Cancer Registry (1996-2016). Kaplan-Meier curves of OS by ethnicity, location (border, non-border), subtype (ER+, PR+, HER2+, Triple Negative (TN)), age group (18-39, 40-69, >70 years), and treatments were constructed. Other covariates included rurality, insurance status, poverty indicators, and comorbidities. Adjustment of these variables for effect on relative risk and OS were assessed with Mantel-Haenszel and Cox regression methods. Results: Univariate Cox analysis noted significantly higher Hazard Ratios (HR) for age >65 years, HER2+ and TN subtype (HR 1.76 and 2.21, p<0.0001), and use of biologic response modifiers (HR 1.09, p=0.004). Lower HR were observed for patients in El paso compared to the rest of Texas (HR 0.73, p<0.0001), and use of hormone therapy, chemotherapy and radiation (HR 0.92, 0.96, 0.82, p<0.0001). Surgery and transplant/endocrine procedures were not associated with mortality. Kaplan Meier curves showed increased median survival (MS) for Hispanic patients compared to NHW (16 and 14 years, p<0.0001). Similar trends were seen for patients in El Paso compared to the rest of Texas (MS 8 and 6 years, p<0.0001). With ER+ and PR+ subtypes, Hispanics and NHWs had similar OS, while patients in El Paso had improved OS compared to the rest of Texas (ER+: MS of 7 and 5 years, p<0.00001; PR+: MS of 6 and 4 years, p<0.00001). Non-significant differences in OS were noted for HER2+ subtype. No difference in OS was found for Hispanics and NHWs with TN subtype while OS was higher in El Paso compared to the rest of Texas (MS 4 versus 2 years, p=0.024). Older adults had lower OS (MS 22, 19, and 8 years for patients 18-39, 40-69, and >70 years, p<0.00001). Hispanics age 18-39 and 40-69 had worse OS (p<0.00001). ER+ and PR+ Hispanics across age groups had similar OS compared to NHW. HER2+ and TN Hispanics 18-39 years had worse OS (p=0.0193 and p<0.00001), while those ages 40-69 had similar OS for HER2+ compared to NHWs but improved OS with TN (p<0.00001). Similar OS was seen across age groups for patients in El Paso compared to the rest of Texas. Smaller sample sizes of patients in El Paso and loss to follow-up, especially for ages 18-39 may affect interpretation of results. Conclusion: Overall, Hispanics had comparable OS to NHW patients. Those in the border region seem to have improved OS compared to the rest of Texas. Efforts should focus on screening, detection, and follow-up for patients with HER2+ and TN subtypes.

Citation Format: Vutha Nhim, Alfonso E. Bencomo-Alvarez, Alok K. Dwivedi, Shrikanth S. Gadad, Anna M. Eiring. Border differences on breast cancer incidence and survival between non-Hispanic white and Hispanic patients: A Texas population-based study [abstract]. In: Proceedings of the AACR Virtual Conference: 14th AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2021 Oct 6-8. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2022;31(1 Suppl):Abstract nr PO-200.

Abstract P150: PSMD1 and PSMD3 as putative targets for cancer therapy

Ever since the ubiquitin proteasome system was characterized, efforts have been made to manipulate its function to abrogate the progression of cancer. As a result, the anticancer drugs bortezomib, carfilzomib, and ixazomib targeting the 26S proteasome were developed to treat multiple myeloma, mantle cell lymphoma, diffuse large B-cell lymphoma, colorectal cancer, and thyroid carcinoma. Despite the success demonstrated by these treatments, adverse side effects and drug resistance are prominent, raising the need for alternative therapeutic options. The 26S proteasome is composed of two 19S regulatory subunits and a 20S catalytic core, leaving ample opportunity for new targets. We recently demonstrated that knockdown of the 19S regulatory subunits, proteasome 26S subunit, non-ATPases 1 (PSMD1) and 3 (PSMD3), resulted in increased apoptosis of tyrosine kinase inhibitor (TKI)-resistant chronic myeloid leukemia (CML) cells, but had no effect on normal cord blood controls, suggesting they may be good targets for therapy. Therefore, we hypothesized that PSMD1 and PSMD3 are potential targets for anticancer therapeutics and that their relevance stretches beyond CML to other types of cancers. In the present study, we analyzed PSMD1 and PSMD3 expression in other types of cancers versus normal controls using data from The Cancer Genome Atlas (TCGA) and the Clinical Proteomic Tumor Analysis Consortium (CPTAC), comparing expression with overall survival (OS). Data were analyzed using UALCAN (http://ualcan.path.uab.edu) and GEPIA2 (http://gepia2.cancer-pku.cn/). At the mRNA level, PSMD1 was found to be overexpressed in 14/24 (58%) TCGA cancers compared with normal controls. This was confirmed at the protein level for breast (p=4.2e-4), colon (p=7.1e-20), clear cell renal carcinoma (p=1.8e-4), and endometrial cancer (p=2.0e-22). PSMD3 was found upregulated at the mRNA level in 18/24 (75%) TCGA cancers, which was confirmed at the protein level in breast (p=8.3e-4), colon (p=1.0e-27), ovarian (p=4.2e-6), clear cell renal carcinoma (p=4.0e-19), and endometrial cancer (p=1.5e-19). Upon correlation of PSMD1 expression with OS, we saw that when overexpressed, individuals did significantly worse with adrenocortical carcinoma, lower grade glioma, lung adenocarcinoma, mesothelioma, and uveal melanoma. In contrast, diffuse large B cell lymphoma (DLBCL) and stomach adenocarcinoma patients with lower PSMD1 expression had a worse OS. Similarly, patients with high levels of PSMD3 mRNA expression had a significantly worse OS in kidney chromophobe, skin cutaneous melanoma, uveal melanoma, and mesothelioma. Our analysis revealed that differential expression of PSMD1 and PSMD3 is correlated with survival in several different cancer types. Future directions will identify PSMD1 and PSMD3 post-translational modifications that may be novel targets for anticancer therapeutics. In conclusion, we highlight PSMD1 and PSMD3 as potential therapeutic targets for the development of novel proteasome inhibitors to treat cancer patients with less toxicity.

Citation Format: Andres J. Rubio, Alfonso E. Bencomo-Alvarez, James E. Young, Vanessa V. Velazquez, Anna M. Eiring. PSMD1 and PSMD3 as putative targets for cancer therapy [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P150.

A Role for the Bone Marrow Microenvironment in Drug Resistance of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease with a poor prognosis and remarkable resistance to chemotherapeutic agents. Understanding resistance mechanisms against currently available drugs helps to recognize the therapeutic obstacles. Various mechanisms of resistance to chemotherapy or targeted inhibitors have been described for AML cells, including a role for the bone marrow niche in both the initiation and persistence of the disease, and in drug resistance of the leukemic stem cell (LSC) population. The BM niche supports LSC survival through direct and indirect interactions among the stromal cells, hematopoietic stem/progenitor cells, and leukemic cells. Additionally, the BM niche mediates changes in metabolic and signal pathway activation due to the acquisition of new mutations or selection and expansion of a minor clone. This review briefly discusses the role of the BM microenvironment and metabolic pathways in resistance to therapy, as discovered through AML clinical studies or cell line and animal models.

26S Proteasome Non-ATPase Regulatory Subunits 1 (PSMD1) and 3 (PSMD3) as Putative Targets for Cancer Prognosis and Therapy

Ever since the ubiquitin proteasome system was characterized, efforts have been made to manipulate its function to abrogate the progression of cancer. As a result, the anti-cancer drugs bortezomib, carfilzomib, and ixazomib targeting the 26S proteasome were developed to treat multiple myeloma, mantle cell lymphoma, and diffuse large B-cell lymphoma, among others. Despite success, adverse side effects and drug resistance are prominent, raising the need for alternative therapeutic options. We recently demonstrated that knockdown of the 19S regulatory components, 26S proteasome non-ATPase subunits 1 (PSMD1) and 3 (PSMD3), resulted in increased apoptosis of chronic myeloid leukemia (CML) cells, but had no effect on normal controls, suggesting they may be good targets for therapy. Therefore, we hypothesized that PSMD1 and PSMD3 are potential targets for anti-cancer therapeutics and that their relevance stretches beyond CML to other types of cancers. In the present study, we analyzed PSMD1 and PSMD3 mRNA and protein expression in cancerous tissue versus normal controls using data from The Cancer Genome Atlas (TCGA) and the Clinical Proteomic Tumor Analysis Consortium (CPTAC), comparing expression with overall survival. Altogether, our data suggest that PSMD1 and PSMD3 may be novel putative targets for cancer prognosis and therapy that are worthy of future investigation.

Discordant PET Findings and a High Relapse Rate Characterize Hispanics With Hodgkin's Lymphoma Treated With ABVD

BACKGROUND

Population-based studies on Hodgkin's lymphoma (HL) have shown reduced survival in Hispanics and non-Hispanic Blacks compared with non-Hispanic Whites. To better understand the factors contributing to this outcome discrepancy, we retrospectively reviewed the charts of patients with HL diagnosed and treated at a single institution located along the Texas-Mexico border.

PATIENTS AND METHODS

We performed a retrospective chart review of all patients with HL treated at our institution over an 8-year period (2011-2018). The International Prognostic Score was calculated for all patients and results of positron-emission tomography (PET) scans (interim and end of treatment) were also recorded. Variables analyzed included tumor-related findings (stage, subtype of HL), treatment history (chemotherapy regimen including number of cycles, dose intensity and radiation treatments) and neutrophil to lymphocyte ratio. Quantitative variables were described using median, interquartile range, minimum and maximum observations. Categorical variables were described using frequency and proportions. Kaplan-Meier curves were used to show relapse-free survival.

RESULTS

A total of 24 patients were treated in the time frame, of whom 23 were Hispanic. All were treated with doxorubicin, bleomycin, vinblastine and dacarbazine (ABVD) or an ABVD-like regimen. Dose intensity for chemotherapy exceeded 90%. After a median follow-up of 43 months, the relapse rate was 45.8%. Positive and negative predictive values for interim PET (0% and 50%) and end of therapy PET (80% and 58%) were suboptimal to allow for a PET-adapted therapeutic approach.

CONCLUSION

Hispanics have a high relapse rate following ABVD which is not fully explained by universally accepted prognostic factors. Performance of PET scan in predicting outcomes of HL needs to be further studied and optimized before adopting a PET-adapted treatment paradigm for underserved Hispanic populations.

Proteasome 26S subunit, non-ATPases 1 (PSMD1) and 3 (PSMD3), play an oncogenic role in chronic myeloid leukemia by stabilizing nuclear factor-kappa B

Tyrosine kinase inhibitors (TKIs) targeting BCR-ABL1 have revolutionized therapy for chronic myeloid leukemia (CML), paving the way for clinical development in other diseases. Despite success, targeting leukemic stem cells and overcoming drug resistance remain challenges for curative cancer therapy. To identify drivers of kinase-independent TKI resistance in CML, we performed genome-wide expression analyses on TKI-resistant versus sensitive CML cell lines, revealing a nuclear factor-kappa B (NF-κB) expression signature. Nucleocytoplasmic fractionation and luciferase reporter assays confirmed increased NF-κB activity in the nucleus of TKI-resistant versus sensitive CML cell lines and CD34+ patient samples. Two genes that were upregulated in TKI-resistant CML cells were proteasome 26S subunit, non-ATPases 1 (PSMD1) and 3 (PSMD3), both members of the 19S regulatory complex in the 26S proteasome. PSMD1 and PSMD3 were also identified as survival-critical genes in a published small hairpin RNA library screen of TKI resistance. We observed markedly higher levels of PSMD1 and PSMD3 mRNA in CML patients who had progressed to the blast phase compared with the chronic phase of the disease. Knockdown of PSMD1 or PSMD3 protein correlated with reduced survival and increased apoptosis in CML cells, but not in normal cord blood CD34+ progenitors. Luciferase reporter assays and immunoblot analyses demonstrated that PSMD1 and PSMD3 promote NF-κB protein expression in CML, and that signal transducer and activator of transcription 3 (STAT3) further activates NF-κB in scenarios of TKI resistance. Our data identify NF-κB as a transcriptional driver in TKI resistance, and implicate PSMD1 and PSMD3 as plausible therapeutic targets worthy of future investigation in CML and possibly other malignancies.

Blood cancer health disparities in the United States Hispanic population

Cancer is a challenging, multifaceted disease that involves a combination of biological and nonbiological factors. Aside from COVID-19, cancer is the second leading cause of death in the United States and the first among Hispanic Americans. The Hispanic population is the largest minority group in the United States, which is rapidly growing in size. Unfortunately, U.S. Hispanics and other minority groups experience many different health disparities, resulting in poor survival outcomes and a reduced quality of life. Factors such as genomic mutations, lower socioeconomic status, lack of education, reduced access to health care, comorbidities, and environmental factors all contribute to these health-care inequalities. In the context of blood cancer health disparities, Hispanic patients are often diagnosed at a younger age and have worse outcomes compared with non-Hispanic individuals. In this commentary, we highlight the existing knowledge about cancer health disparities in the Hispanic population, with a focus on chronic and acute leukemia. In our experience at the U.S./Mexican border, analysis of several different blood cancers demonstrated that younger Hispanic patients with acute lymphoid or myeloid leukemia have higher incidence rates and worse prognoses. A combined approach, involving improved health-care access and better knowledge of the underlying factors, will allow for more timely diagnoses and the development of intervention strategies aimed at reducing or eliminating the disparities.

Femoral Heads from Total Hip Arthroplasty as a Source of Adult Hematopoietic Cells

Normal human bone marrow cells are critical for studies of hematopoiesis and as controls to assess toxicity. As cells from commercial vendors are expensive, many laboratories resort to cancer-free bone marrow specimens obtained during staging or to umbilical cord blood cells, which may be abnormal or reflect a much younger age group compared to the disease samples under study. We piloted the use of femoral heads as an alternative and inexpensive source of normal bone marrow. Femoral heads were obtained from 21 successive patients undergoing elective hip arthroplasty. Mononuclear cells (MNCs) were purified with Ficoll, and CD3+, CD14+, and CD34+ cells were purified with antibody-coated microbeads. The median yield of MNCs was 8.95 × 107 (range, 1.62 × 105-2.52 × 108), and the median yield of CD34+ cells was 1.40 × 106 (range, 3.60 × 105-9.90 × 106). Results of downstream applications including qRT-PCR, colony-forming assays, and ex vivo proliferation analysis were of high quality and comparable to those obtained with standard bone marrow aspirates. We conclude that femoral heads currently discarded as medical waste are a cost-efficient source of bone marrow cells for research use.

Ethnic and border differences on blood cancer presentation and outcomes: A Texas population-based study

BACKGROUND

The Texas/Chihuahua (US/Mexico) border is a medically underserved region with many reported barriers for health care access. Although Hispanic ethnicity is associated with health disparities for many different diseases, the population-based estimates of incidence and survival for patients with blood cancer along the border are unknown. The authors hypothesized that Hispanic ethnicity and border proximity is associated with poor blood cancer outcomes.

METHODS

Data from the Texas Cancer Registry (1995-2016) were used to investigate the primary exposures of patient ethnicity (Hispanic vs non-Hispanic) and geographic location (border vs non-border). Other confounders and covariates included sex, age, year of diagnosis, rurality, insurance status, poverty indicators, and comorbidities. The Mantel-Haenszel method and Cox regression analyses were used to determine adjusted effects of ethnicity and border proximity on the relative risk (RR) and survival of patients with different blood cancer types.

RESULTS

Hispanic patients were diagnosed at a younger age than non-Hispanic patients and presented with increased comorbidities. Whereas non-Hispanics had a higher incidence of developing blood cancer compared with Hispanics overall, Hispanics demonstrated a higher incidence of acute lymphoblastic leukemia (RR, 1.92; 95% CI, 1.79-2.08; P < .001) with worse outcomes. Hispanics from the Texas/Chihuahua border demonstrated a higher incidence of chronic myeloid leukemia (RR, 1.28; 95% CI, 1.07-1.51; P = .02) and acute myeloid leukemia (RR, 1.17; 95% CI, 1.04-1.33; P = .0009) compared with Hispanics living elsewhere in Texas.

CONCLUSIONS

Hispanic ethnicity and border proximity were associated with a poor presentation and an adverse prognosis despite the younger age of diagnosis. Future studies should explore differences in disease biology and treatment strategies that could drive these regional disparities.

Abstract 4343: Retrospective study of incidence and survival for patients with hematological malignancies residing at the U.S./Mexico border

Introduction: Hispanics represent the largest minority group in the United States (U.S.), with 57.5 million individuals. The majority of Hispanics in the U.S. reside in the Southwest region, and &gt;11 million live in the state of Texas. Health disparities for Hispanic cancer patients have previously been linked to disproportionate poverty and other barriers to optimal healthcare. In the case of acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL), Hispanics were found to be diagnosed at a younger age and have worse overall survival (OS) compared with non-Hispanic whites (NHWs) (ACS Cancer Facts & Figures for Hispanics/Latinos 2018-2020). However, little is known about incidence and survival for Hispanic blood cancer patients residing at the U.S./Mexico border.

Methods: We retrospectively reviewed data from the Texas Cancer Registry for hematologic malignancies diagnosed in the state of Texas between 1995 and 2016, focusing our analysis on chronic and acute leukemia's (both myeloid and lymphoid), myelodysplastic syndrome (MDS), and myeloproliferative neoplasms (MPNs). Survival for Hispanic and NHW groups was compared using the log-rank test, and Cox regression analyses adjusting for age and diagnosis. Differences in age at diagnosis were evaluated using t-tests and generalized linear models. Research was conducted according to a local IRB-approved protocol in accordance with the Declaration of Helsinki.

Results: Of the 69,941 cases of hematologic malignancies with available information throughout the state of Texas, 18.29% identified as Hispanic. We found that Hispanic patients were diagnosed at a significantly younger age in all diseases analyzed (p&lt;0.0001). Surprisingly, in unadjusted analyses, Hispanics had significantly better OS than NHWs diagnosed with myeloid malignancies (p&lt;0.0001), but no significant differences for patients with other types of leukemia. After adjusting for age, there was a clear disparity in OS for Hispanic versus NHW patients with ALL (p&lt;0.0001) and acute promyelocytic leukemia (APL, p=0.03), with no significant differences in other diseases. We also compared Hispanic patients diagnosed in El Paso versus Hispanics from the rest of Texas. Hispanics in El Paso had a significant reduction in OS compared to Hispanics in other areas of Texas for ALL (p=0.0164), AML (p&lt;0.0001), and chronic myeloid leukemia (CML, p=0.016).

Conclusions: Hispanics are diagnosed at a significantly younger age than NHWs in all blood malignancies analyzed. Hispanic patients with AML, MDS, and CML had significantly better OS compared to NHWs in unadjusted analyses, which could be explained by the reduced age of diagnosis. Hispanics with ALL, AML, or CML diagnosed near the U.S./Mexico border demonstrate worse OS compared with Hispanics diagnosed in other areas of Texas. In age-adjusted analyses, Hispanic patients with ALL or APL have a worse OS compared with NHWs. There appears to be evidence that disparities in outcome by ethnicity may be different in El Paso compared with the rest of Texas.

Future Directions: Our data demonstrates blood cancer disparities present in our region. Further study is required to identify factors responsible for the disparity in OS, and to identify ways to address it.

Citation Format: Alfonso E. Bencomo, Andres J. Rubio, Mayra A. Gonzalez, Idaly M. Olivas, Joshua J. Lara, Kiran Ghimire, Osvaldo Padilla, Angelica Padilla, Nawar Hakim, Attilio Orazi, Javier Corral, Alexander Philipovskiy, Sumit Gaur, Anna M. Eiring. Retrospective study of incidence and survival for patients with hematological malignancies residing at the U.S./Mexico border [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 4343.

Abstract 648: Role of G0S2 in chronic myeloid leukemia and TKI resistance

Tyrosine kinase inhibitors (TKIs) targeting BCR-ABL1 have turned chronic myeloid leukemia (CML) from a fatal to a chronic disease. However, resistance is a clinical problem, and TKIs do not target CML leukemic stem cells (LSC), which are independent from BCR-ABL1 kinase activity. To understand mechanisms driving BCR-ABL1-independent resistance, we analyzed the transcriptional signature of TKI-naïve CD34+ cells from patients with or without a response to imatinib after 12 months of therapy (McWeeney et al. Blood 2010). Among the genes most profoundly downregulated in patients destined to fail imatinib was G0/G1 switch gene 2 (G0S2) (&gt;3-fold, n=59, p&lt;0.02). Retrospective analysis of survival data for patients whose samples were evaluated in a microarray revealed that high G0S2 expression (&gt;50%) correlated with a longer overall survival (n=30 responders, n=16 non-responders, p=0.036). We next analyzed G0S2 mRNA expression in CD34+ cells from normal cord blood and from primary CD34+ cells lacking BCR-ABL1 kinase domain mutations. G0S2 was 4-fold downregulated in newly diagnosed CML (n=6) compared to normal cord blood (n=5, p&lt;0.01), and further downregulated by &gt;3-fold in TKI-resistant (n=2) and BP-CML samples (n=5, p&lt;0.01).G0S2 mRNA was also lower in CD34+38- stem cells compared to committed CD34+38+ progenitor cells in CML (n=6, p&lt;0.01) but not normal cord blood (n=4, p=ns). CFSE staining revealed that G0S2 mRNA levels in CP-CML CD34+ cells are lowest in cell populations with the least number of cell divisions. To assess the role of G0S2 in CML, we cloned G0S2 amplified from normal human mononuclear cells into a lentiviral expression system, and confirmed ectopic expression by qRT-PCR and immunoblot. Ectopic G0S2 reduced colony formation in both TKI-sensitive and TKI-resistant CML cell lines and CD34+ CML patient samples. While G0S2 has been reported to induce apoptosis by direct inhibition of BCL-2, reduced colony formation was not associated with an increase of apoptosis, but did restore imatinib-induced apoptosis. These data are consistent with a tumor suppressor role for G0S2 in CML. G0S2 has also been shown to inhibit adipocyte triglyceride lipase (ATGL), the rate-limiting enzyme in the conversion of triglycerides to free fatty acids (FFAs). While ATGL is indeed expressed at the protein level in CML cells, shRNAs targeting ATGL do not phenocopy ectopic G0S2 expression. Furthermore, the effects of ectopic G0S2 are not impaired in cells with simultaneous ATGL knockdown, and ectopic G0S2 has no effect on intracellular FFA or ATP levels. Finally, G0S2 gene expression has been shown to be regulated by promoter DNA hypermethylation. However, DNA bisulfite and patch PCR sequencing in primary CD34+ cells from CML patients did not reveal G0S2 promoter hypermethylation in CML. Rather, ChIP-PCR revealed the presence of MYC/MAX at the G0S2 promoter in CML, suggesting MYC-mediated transcriptional repression. Altogether, these data suggest that G0S2 downregulation plays a functional role in CML disease progression and TKI response, but is independent from its role as an inhibitor of BCL-2 or ATGL. These data unravel a new role for G0S2 as a regulator of TKI response in CML, and suggest that restoring G0S2 expression may have clinical utility.

Citation Format: Mayra A. Gonzalez, Alfonso E. Bencomo, Christian Barreto-Vargas, Andres J. Rubio, Idaly M. Olivas, Joshua J. Lara, Anna Senina, Jonathan Ahmann, Katherine T. Varley, Luis F. Jave-Suarez, O'Hare Thomas, Michael W. Deininger, Anna M. Eiring. Role of G0S2 in chronic myeloid leukemia and TKI resistance [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 648.

Abstract 647: NF-kB associates with tyrosine kinase inhibitor resistance in chronic myeloid leukemia

Tyrosine kinase inhibitors (TKIs) targeting BCR-ABL1 are effective at eliminating most BCR-ABL1+ cells in chronic myeloid leukemia (CML), but do not target CML leukemic stem cells (LSCs), which are independent of BCR-ABL1 kinase activity. Our previous work showed that BCR-ABL1-independent resistance is largely driven by STAT3 (Eiring et al. Leukemia 2015). To understand how STAT3 contributes to TKI resistance, we performed RNA sequencing on TKI-sensitive K562-S cells versus TKI-resistant K562-R cells, which demonstrate kinase-independent resistance. Surprisingly, gene set enrichment analysis did not reveal STAT3-mediated transcription (p=1.0), but was reminiscent of TNFα signaling via NF-κB (p=0.024). Nucleocytoplasmic fractionation revealed higher levels of phospho-NF-κB in the nucleus of K562-R vs. K562-S controls, and in CD34+ progenitors from TKI-resistant CML patients (n=3) compared to TKI responders (n=2) and normal individuals (n=2). These data suggest NF-κB may be driving the transcriptional signature of TKI resistance, and implicate non-canonical functions for STAT3. To confirm increased NF-κB transcriptional activity in TKI resistance, K562-S and K562-R cells were transduced with an NF-κB luciferase reporter construct or a scrambled control. K562-R cells demonstrated increased NF-κB reporter activity compared to K562-S cells. To assess whether STAT3 activates NF-κB, reporter cells were co-infected with an inducible lentiviral shRNA targeting STAT3 (shSTAT3). STAT3 knockdown decreased NF-κB reporter activity in TKI-resistant cells, but increased reporter activity in TKI-sensitive controls. These data suggest that STAT3 may cooperate with NF-κB in the setting of TKI resistance. NF-κB and STAT3 have been shown to cooperatively bind to gene promoters of cytokines, specifically IL-6. ELISA assays demonstrated that K562-R cells produce autonomous IL-6, but not TNFα. Since IL-6 is a potent activator of STAT3, we treated TKI-resistant cells with the IL-6 receptor inhibitor, tocilizumab (100 and 1000 ng/ml). As expected, tocilizumab treatment reduced STAT3 Y705 phosphorylation as assessed by immunoblot analysis, but surprisingly had no effect on survival or NF-κB reporter activity in K562-R cells. These data suggest that STAT3 phosphorylation is not required for survival or NF-κB activation in TKI resistance. The role of unphosphorylated STAT3 in TKI resistance is currently being explored. We correlated our RNA sequencing data in TKI-resistant cell lines with microarray data in TKI-resistant patient samples (McWeeney et al. Blood 2010), identifying upregulation of the proteasome components, PSMD1 and PSMD3. qRT-PCR confirmed upregulation of PSMD3 but not PSMD1 in TKI resistance, and only the PSMD3 promoter was bound by NF-κB as assessed by ChIP. Lentiviral shRNAs targeting PSMD3 (shPSMD3) reduced colony formation of K562-S and K562-R cells, which correlated with induction of apoptosis, an increase of global protein ubiquitination, and decreased NF-κB activation, with little effects observed in normal cord blood. Altogether, our data implicate a positive feedback loop involving NF-κB and PSMD3 in TKI resistance and suggest that PSMD3 may be a novel target for treatment of TKI-resistant CML.

Citation Format: Andres J. Rubio, Idaly M. Olivas, Alfonso E. Bencomo, Mayra A. Gonzalez, Joshua J. Lara, Rebecca Ellwood, Carme Ripoll-Fiol, Georgios Nteliopoulos, Alistair Reid, Dragana Milojkovic, Jane Apperley, Jamshid Sorouri-Khorashad, Anna M. Eiring. NF-kB associates with tyrosine kinase inhibitor resistance in chronic myeloid leukemia [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 647.

Dasatinib overcomes stroma-based resistance to the FLT3 inhibitor quizartinib using multiple mechanisms

FLT3-ITD mutations occur in 20–30% of AML patients and are associated with aggressive disease. Patients with relapsed FLT3-mutated disease respond well to 2^nd generation FLT3 TKIs but inevitably relapse within a short timeframe. In this setting, until overt relapse occurs, the bone marrow microenvironment facilitates leukemia cell survival despite continued on-target inhibition. We demonstrate that human bone marrow derived conditioned medium (CM) protects FLT3-ITD⁺ AML cells from the 2^nd generation FLT3 TKI quizartinib and activates STAT3 and STAT5 in leukemia cells. Extrinsic activation of STAT5 by CM is the primary mediator of leukemia cell resistance to FLT3 inhibition. Combination treatment with quizartinib and dasatinib abolishes STAT5 activation and significantly reduces the IC₅₀ of quizartinib in FLT3-ITD⁺ AML cells cultured in CM. We demonstrate that CM protects FLT3-ITD⁺ AML cells from the inhibitory effects of quizartinib on glycolysis and that this is partially reversed by treating cells with the combination of quizartinib and dasatinib. Using a doxycycline-inducible STAT5 knockdown in the FLT3-ITD⁺ MOLM-13 cell line, we show that dasatinib-mediated suppression of leukemia cell glycolytic activity is STAT5-independent and provide a preclinical rationale for combination treatment with quizartinib and dasatinib in FLT3-ITD⁺ AML.

Abstract 3018: Mechanisms of tyrosine kinase inhibitor resistance in chronic myeloid leukemia

Tyrosine kinase inhibitors (TKIs) targeting BCR-ABL1 have turned chronic myeloid leukemia (CML) from a fatal to a chronic disease. Despite improved survival, resistance is a clinical problem, and TKIs do not target the quiescent CML leukemic stem cell (LSC), meaning that patients must be treated for life at a high economic burden and sometimes with significant side effects. TKI resistance is frequently characterized by mutations in the BCR-ABL1 kinase domain, but they explain only ~50% of clinical TKI failure. The remaining patients have BCR-ABL1-independent resistance, defined as survival despite BCR-ABL1 inhibition. We have previously reported a gene expression signature predictive of TKI failure in CD34+ cells from chronic phase CML patients (McWeeney et al. 2010). This gene expression signature demonstrated significant overlap with signatures of blast phase CML (Zheng et al. 2006), suggesting that similar biological processes may be driving TKI resistance and disease progression. We expanded our analysis, and also found significant overlap between the expression profiles of TKI resistance and quiescent CML LSCs reported by Graham et al. (p=4x10-14) and Cramer Morales et al. (p=7x10-6). These data suggest that there is a core of genes whose expression is consistently associated with multiple scenarios of BCR-ABL1-independent resistance. Our previous work has shown that BCR-ABL1-independent resistance is largely driven by STAT3, and that targeting STAT3 in combination with TKIs restores sensitivity in TKI-resistant CML stem and progenitor cells (Eiring et al. Leukemia 2015). Unexpectedly, gene set enrichment analysis revealed that our gene expression signature predictive of TKI failure (McWeeney et al. 2010) does not reveal a STAT3 transcriptional signature. Similarly, RNA sequencing data on TKI-resistant K562-R cells, which are resistant to TKIs but lack BCR-ABL1 kinase domain mutations, revealed that, while TKI resistance was not associated with a STAT3 transcriptional signature (p=1.0), it was correlated with a signature reminiscent of TNFα signaling via NFκB (p=0.024). Nucleocytoplasmic fractionation revealed higher levels of total- and phospho-NFκB in the nucleus of K562-R versus K562-S cells, and in CD34+progenitors from TKI-resistant CML patients (n=3) compared to TKI responders (n=2) and normal individuals (n=2). These data indicate that NFκB may be driving the gene expression signature associated with BCR-ABL1-independent resistance, and suggest non-canonical functions for STAT3 that go beyond its traditional role as a transcription factor.

Citation Format: Anna M. Eiring, Rebecca Ellwood, Carme Ripoll Fiol, Robert K. Hills, Georgios Nteliopoulos, Alistair Reid, Dragana Milojkovic, Jane Apperley, Jamshid Sorouri-Khorashad. Mechanisms of tyrosine kinase inhibitor resistance in chronic myeloid leukemia [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 3018.

Energy metabolism and drug response in myeloid leukaemic stem cells

Despite significant advances in the treatment of myeloid malignancies, many patients become resistant to therapy and ultimately succumb to their disease. Accumulating evidence over the past several years has suggested that the inadequacy of many leukaemia therapies results from their failure to target the leukaemic stem cell (LSC). For this reason, the LSC population currently represents the most critical target in the treatment of myeloid malignancies. However, while LSCs are ideal targets in the treatment of these diseases, they are also the most difficult population to target. This is due to both their heterogeneity within the LSC population, and also their phenotypic similarities with normal haematopoietic stem cells. This review will highlight the current landscape surrounding LSC biology in myeloid malignancies, with a focus on altered energy metabolism, and how that knowledge is being translated into clinical advances for the treatment of chronic and acute myeloid leukaemia and myelodysplastic syndromes.

Nuclear-Cytoplasmic Transport Is a Therapeutic Target in Myelofibrosis

PURPOSE

Myelofibrosis is a hematopoietic stem cell neoplasm characterized by bone marrow reticulin fibrosis, extramedullary hematopoiesis, and frequent transformation to acute myeloid leukemia. Constitutive activation of JAK/STAT signaling through mutations in JAK2, CALR, or MPL is central to myelofibrosis pathogenesis. JAK inhibitors such as ruxolitinib reduce symptoms and improve quality of life, but are not curative and do not prevent leukemic transformation, defining a need to identify better therapeutic targets in myelofibrosis.

EXPERIMENTAL DESIGN

A short hairpin RNA library screening was performed on JAK2^V617F-mutant HEL cells. Nuclear-cytoplasmic transport (NCT) genes including RAN and RANBP2 were among top candidates. JAK2^V617F-mutant cell lines, human primary myelofibrosis CD34⁺ cells, and a retroviral JAK2^V617F-driven myeloproliferative neoplasms mouse model were used to determine the effects of inhibiting NCT with selective inhibitors of nuclear export compounds KPT-330 (selinexor) or KPT-8602 (eltanexor).

RESULTS

JAK2^V617F-mutant HEL, SET-2, and HEL cells resistant to JAK inhibition are exquisitely sensitive to RAN knockdown or pharmacologic inhibition by KPT-330 or KPT-8602. Inhibition of NCT selectively decreased viable cells and colony formation by myelofibrosis compared with cord blood CD34⁺ cells and enhanced ruxolitinib-mediated growth inhibition and apoptosis, both in newly diagnosed and ruxolitinib-exposed myelofibrosis cells. Inhibition of NCT in myelofibrosis CD34⁺ cells led to nuclear accumulation of p53. KPT-330 in combination with ruxolitinib-normalized white blood cells, hematocrit, spleen size, and architecture, and selectively reduced JAK2^V617F-mutant cells in vivo.

CONCLUSIONS

Our data implicate NCT as a potential therapeutic target in myelofibrosis and provide a rationale for clinical evaluation in ruxolitinib-exposed patients with myelofibrosis.

Disarming an Electrophilic Warhead: Retaining Potency in Tyrosine Kinase Inhibitor (TKI)-Resistant CML Lines While Circumventing Pharmacokinetic Liabilities

Pharmacologic blockade of the activation of signal transducer and activator of transcription 3 (STAT3) in tyrosine kinase inhibitor (TKI)-resistant chronic myeloid leukemia (CML) cell lines characterized by kinase-independent resistance was shown to re-sensitize CML cells to TKI therapy, suggesting that STAT3 inhibitors in combination with TKIs are an effective combinatorial therapeutic for the treatment of CML. Benzoic acid- and hydroxamic acid-based STAT3 inhibitors SH-4-054 and SH-5-007, developed previously in our laboratory, demonstrated promising activity against these resistant CML cell lines. However, pharmacokinetic studies in murine models (CD-1 mice) revealed that both SH-4-054 and SH-5-007 are susceptible to glutathione conjugation at the para position of the pentafluorophenyl group via nucleophilic aromatic substitution (SN Ar). To determine whether the electrophilicity of the pentafluorophenyl sulfonamide could be tempered, an in-depth structure-activity relationship (SAR) study of the SH-4-054 scaffold was conducted. These studies revealed that AM-1-124, possessing a 2,3,5,6-tetrafluorophenylsulfonamide group, retained STAT3 protein affinity (Ki =15 μm), as well as selectivity over STAT1 (Ki >250 μm). Moreover, in both hepatocytes and in in vivo pharmacokinetic studies (CD-1 mice), AM-1-124 was found to be dramatically more stable than SH-4-054 (t1/2 =1.42 h cf. 10 min, respectively). AM-1-124 is a promising STAT3-targeting inhibitor with demonstrated bioavailability, suitable for evaluation in preclinical cancer models.

Age-related mutations and chronic myelomonocytic leukemia

Chronic myelomonocytic leukemia (CMML) is a hematologic malignancy nearly confined to the elderly. Previous studies to determine incidence and prognostic significance of somatic mutations in CMML have relied on candidate gene sequencing, although an unbiased mutational search has not been conducted. As many of the genes commonly mutated in CMML were recently associated with age-related clonal hematopoiesis (ARCH) and aged hematopoiesis is characterized by a myelomonocytic differentiation bias, we hypothesized that CMML and aged hematopoiesis may be closely related. We initially established the somatic mutation landscape of CMML by whole exome sequencing followed by gene-targeted validation. Genes mutated in ⩾10% of patients were SRSF2, TET2, ASXL1, RUNX1, SETBP1, KRAS, EZH2, CBL and NRAS, as well as the novel CMML genes FAT4, ARIH1, DNAH2 and CSMD1. Most CMML patients (71%) had mutations in ⩾2 ARCH genes and 52% had ⩾7 mutations overall. Higher mutation burden was associated with shorter survival. Age-adjusted population incidence and reported ARCH mutation rates are consistent with a model in which clinical CMML ensues when a sufficient number of stochastically acquired age-related mutations has accumulated, suggesting that CMML represents the leukemic conversion of the myelomonocytic-lineage-biased aged hematopoietic system.

β-Catenin is required for intrinsic but not extrinsic BCR-ABL1 kinase-independent resistance to tyrosine kinase inhibitors in chronic myeloid leukemia

Activation of nuclear β-catenin and expression of its transcriptional targets promotes chronic myeloid leukemia (CML) progression, tyrosine kinase inhibitor (TKI) resistance, and leukemic stem cell self-renewal. We report that nuclear β-catenin has a role in leukemia cell-intrinsic but not -extrinsic BCR-ABL1 kinase-independent TKI resistance. Upon imatinib inhibition of BCR-ABL1 kinase activity, β-catenin expression was maintained in intrinsically resistant cells grown in suspension culture and sensitive cells cultured in direct contact (DC) with bone marrow (BM) stromal cells. Thus, TKI resistance uncouples β-catenin expression from BCR-ABL1 kinase activity. In β-catenin reporter assays, intrinsically resistant cells showed increased transcriptional activity versus parental TKI-sensitive controls, and this was associated with restored expression of β-catenin target genes. In contrast, DC with BM stromal cells promoted TKI resistance, but had little effects on Lef/Tcf reporter activity and no consistent effects on cytoplasmic β-catenin levels, arguing against a role for β-catenin in extrinsic TKI resistance. N-cadherin or H-cadherin blocking antibodies abrogated DC-based resistance despite increasing Lef/Tcf reporter activity, suggesting that factors other than β-catenin contribute to extrinsic, BM-derived TKI resistance. Our data indicate that, while nuclear β-catenin enhances survival of intrinsically TKI-resistant CML progenitors, it is not required for extrinsic resistance mediated by the BM microenvironment.

shRNA library screening identifies nucleocytoplasmic transport as a mediator of BCR-ABL1 kinase-independent resistance

The mechanisms underlying tyrosine kinase inhibitor (TKI) resistance in chronic myeloid leukemia (CML) patients lacking explanatory BCR-ABL1 kinase domain mutations are incompletely understood. To identify mechanisms of TKI resistance that are independent of BCR-ABL1 kinase activity, we introduced a lentiviral short hairpin RNA (shRNA) library targeting ∼5000 cell signaling genes into K562(R), a CML cell line with BCR-ABL1 kinase-independent TKI resistance expressing exclusively native BCR-ABL1. A customized algorithm identified genes whose shRNA-mediated knockdown markedly impaired growth of K562(R) cells compared with TKI-sensitive controls. Among the top candidates were 2 components of the nucleocytoplasmic transport complex, RAN and XPO1 (CRM1). shRNA-mediated RAN inhibition or treatment of cells with the XPO1 inhibitor, KPT-330 (Selinexor), increased the imatinib sensitivity of CML cell lines with kinase-independent TKI resistance. Inhibition of either RAN or XPO1 impaired colony formation of CD34(+) cells from newly diagnosed and TKI-resistant CML patients in the presence of imatinib, without effects on CD34(+) cells from normal cord blood or from a patient harboring the BCR-ABL1(T315I) mutant. These data implicate RAN in BCR-ABL1 kinase-independent imatinib resistance and show that shRNA library screens are useful to identify alternative pathways critical to drug resistance in CML.

A coiled-coil mimetic intercepts BCR-ABL1 dimerization in native and kinase-mutant chronic myeloid leukemia

Targeted therapy of chronic myeloid leukemia is currently based on small-molecule inhibitors that directly bind the tyrosine kinase domain of BCR-ABL1. This strategy has generally been successful, but is subject to drug resistance due to point mutations in the kinase domain. Kinase activity requires transactivation of BCR-ABL1 following an oligomerization event, which is mediated by the coiled-coil (CC) domain at the N-terminus of the protein. Here, we describe a rationally engineered mutant version of the CC domain, called CC^mut3, which interferes with BCR-ABL1 oligomerization and promotes apoptosis in BCR-ABL1-expressing cells, regardless of kinase domain mutation status. CC^mut3 exhibits strong pro-apoptotic and anti-proliferative activity in cell lines expressing native BCR-ABL1, single kinase domain mutant BCR-ABL1 (E255V and T315I) or compound mutant BCR-ABL1 (E255V/T315I). Moreover, CC^mut3 inhibits colony formation by primary CML CD34⁺ cells ex vivo, including a sample expressing the T315I mutant. These data suggest that targeting BCR-ABL1 with CC mutants may provide a novel alternative strategy for treating patients with resistance to current targeted therapies.

Individualizing kinase-targeted cancer therapy: the paradigm of chronic myeloid leukemia

The success of tyrosine kinase inhibitors in treating chronic myeloid leukemia highlights the potential of targeting oncogenic kinases with small molecules. By using drug activity profiles and individual patient genotypes, one can guide personalized therapy selection for patients with resistance.

BCR-ABL1 compound mutations combining key kinase domain positions confer clinical resistance to ponatinib in Ph chromosome-positive leukemia

Ponatinib is the only currently approved tyrosine kinase inhibitor (TKI) that suppresses all BCR-ABL1 single mutants in Philadelphia chromosome-positive (Ph(+)) leukemia, including the recalcitrant BCR-ABL1(T315I) mutant. However, emergence of compound mutations in a BCR-ABL1 allele may confer ponatinib resistance. We found that clinically reported BCR-ABL1 compound mutants center on 12 key positions and confer varying resistance to imatinib, nilotinib, dasatinib, ponatinib, rebastinib, and bosutinib. T315I-inclusive compound mutants confer high-level resistance to TKIs, including ponatinib. In vitro resistance profiling was predictive of treatment outcomes in Ph(+) leukemia patients. Structural explanations for compound mutation-based resistance were obtained through molecular dynamics simulations. Our findings demonstrate that BCR-ABL1 compound mutants confer different levels of TKI resistance, necessitating rational treatment selection to optimize clinical outcome.

Combined STAT3 and BCR-ABL1 inhibition induces synthetic lethality in therapy-resistant chronic myeloid leukemia

Mutations in the BCR-ABL1 kinase domain are an established mechanism of tyrosine kinase inhibitor (TKI) resistance in Philadelphia chromosome-positive leukemia, but fail to explain many cases of clinical TKI failure. In contrast, it is largely unknown why some patients fail TKI therapy despite continued suppression of BCR-ABL1 kinase activity, a situation termed BCRABL1 kinase-independent TKI resistance. Here, we identified activation of signal transducer and activator of transcription 3 (STAT3) by extrinsic or intrinsic mechanisms as an essential feature of BCR-ABL1 kinase-independent TKI resistance. By combining synthetic chemistry, in vitro reporter assays, and molecular dynamics-guided rational inhibitor design and high-throughput screening, we discovered BP-5-087, a potent and selective STAT3 SH2 domain inhibitor that reduces STAT3 phosphorylation and nuclear transactivation. Computational simulations, fluorescence polarization assays, and hydrogen-deuterium exchange assays establish direct engagement of STAT3 by BP-5-087 and provide a high-resolution view of the STAT3 SH2 domain/BP-5-087 interface. In primary cells from CML patients with BCR-ABL1 kinase-independent TKI resistance, BP-5-087 (1.0 μM) restored TKI sensitivity to therapy-resistant CML progenitor cells, including leukemic stem cells (LSCs). Our findings implicate STAT3 as a critical signaling node in BCR-ABL1 kinase-independent TKI resistance, and suggest that BP-5-087 has clinical utility for treating malignancies characterized by STAT3 activation.

PP2A-activating drugs selectively eradicate TKI-resistant chronic myeloid leukemic stem cells

The success of tyrosine kinase inhibitors (TKIs) in treating chronic myeloid leukemia (CML) depends on the requirement for BCR-ABL1 kinase activity in CML progenitors. However, CML quiescent HSCs are TKI resistant and represent a BCR-ABL1 kinase-independent disease reservoir. Here we have shown that persistence of leukemic HSCs in BM requires inhibition of the tumor suppressor protein phosphatase 2A (PP2A) and expression--but not activity--of the BCR-ABL1 oncogene. Examination of HSCs from CML patients and healthy individuals revealed that PP2A activity was suppressed in CML compared with normal HSCs. TKI-resistant CML quiescent HSCs showed increased levels of BCR-ABL1, but very low kinase activity. BCR-ABL1 expression, but not kinase function, was required for recruitment of JAK2, activation of a JAK2/β-catenin survival/self-renewal pathway, and inhibition of PP2A. PP2A-activating drugs (PADs) markedly reduced survival and self-renewal of CML quiescent HSCs, but not normal quiescent HSCs, through BCR-ABL1 kinase-independent and PP2A-mediated inhibition of JAK2 and β-catenin. This led to suppression of human leukemic, but not normal, HSC/progenitor survival in BM xenografts and interference with long-term maintenance of BCR-ABL1-positive HSCs in serial transplantation assays. Targeting the JAK2/PP2A/β-catenin network in quiescent HSCs with PADs (e.g., FTY720) has the potential to treat TKI-refractory CML and relieve lifelong patient dependence on TKIs.

KIT signaling governs differential sensitivity of mature and primitive CML progenitors to tyrosine kinase inhibitors

Imatinib and other BCR-ABL1 inhibitors are effective therapies for chronic myelogenous leukemia (CML), but these inhibitors target additional kinases including KIT, raising the question of whether off-target effects contribute to clinical efficacy. On the basis of its involvement in CML pathogenesis, we hypothesized that KIT may govern responses of CML cells to imatinib. To test this, we assessed the growth of primary CML progenitor cells under conditions of sole BCR-ABL1, sole KIT, and dual BCR-ABL1/KIT inhibition. Sole BCR-ABL1 inhibition suppressed mature CML progenitor cells, but these effects were largely abolished by stem cell factor (SCF) and maximal suppression required dual BCR-ABL1/KIT inhibition. In contrast, KIT inhibition did not add to the effects of BCR-ABL1 inhibition in primitive progenitors, represented by CD34(+)38(-) cells. Long-term culture-initiating cell assays on murine stroma revealed profound depletion of primitive CML cells by sole BCR-ABL1 inhibition despite the presence of SCF, suggesting that primitive CML cells are unable to use SCF as a survival factor upon BCR-ABL1 inhibition. In CD34(+)38(+) cells, SCF strongly induced pAKT(S473) in a phosphoinositide 3-kinase (PI3K)-dependent manner, which was further enhanced by inhibition of BCR-ABL1 and associated with increased colony survival. In contrast, pAKT(S473) levels remained low in CD34(+)38(-) cells cultured under the same conditions. Consistent with reduced response to SCF, KIT surface expression was significantly lower on CD34(+)38(-) compared with CD34(+)38(+) CML cells, suggesting a possible mechanism for the differential effects of SCF on mature and primitive CML progenitor cells.

Pushing the limits of targeted therapy in chronic myeloid leukaemia

Tyrosine kinase inhibitor (TKI) therapy targeting the BCR-ABL1 kinase is effective against chronic myeloid leukaemia (CML), but is not curative for most patients. Minimal residual disease (MRD) is thought to reside in TKI-insensitive leukaemia stem cells (LSCs) that are not fully addicted to BCR-ABL1. Recent conceptual advances in both CML biology and therapeutic intervention have increased the potential for the elimination of CML cells, including LSCs, through simultaneous inhibition of BCR-ABL1 and other newly identified, crucial targets.

Advances in the treatment of chronic myeloid leukemia

Although imatinib is firmly established as an effective therapy for newly diagnosed patients with chronic myeloid leukemia (CML), the field continues to advance on several fronts. In this minireview we cover recent results of second generation tyrosine kinase inhibitors in newly diagnosed patients, investigate the state of strategies to discontinue therapy and report on new small molecule inhibitors to tackle resistant disease, focusing on agents that target the T315I mutant of BCR-ABL. As a result of these advances, standard of care in frontline therapy has started to gravitate toward dasatinib and nilotinib, although more observation is needed to fully support this. Stopping therapy altogether remains a matter of clinical trials, and more must be learned about the mechanisms underlying the persistence of leukemic cells with treatment. However, there is good news for patients with the T315I mutation, as effective drugs such as ponatinib are on their way to regulatory approval. Despite these promising data, accelerated or blastic phase disease remains a challenge, possibly due to BCR-ABL-independent resistance.

Abstract 1950: Suppression of RISC-independent decoy and RISC-mediated RNA-pairing activities of microRNA-328 is required for maturation-arrest and enhanced survival of blast crisis CML progenitors

MicroRNAs (miRs) and heterogeneous ribonucleoproteins (hnRNPs) are post-transcriptional gene regulators that bind to mRNA in a sequence-specific manner. We showed that hnRNP-E2 inhibits myeloid maturation of bone marrow (BM) progenitors from chronic myelogenous leukemia patients in myeloid blast crisis (CML-BC) by suppressing CEBPA mRNA translation. We report here that loss of miR-328 is induced by BCR/ABL and specifically occurs in CML-BC, and its restored expression rescues differentiation and impairs clonogenic potential of BCR/ABL+ BM progenitors. Accordingly, miR-328 increases during granulocytic differentiation of human CD34+ and mouse LSK BM stem/progenitor cells. Mechanistically, BCR/ABL uses the MAPK-hnRNP-E2 pathway to suppress C/EBPα and miR-328 expression as pharmacologic inhibition of and/or shRNAs against these molecules efficiently restore miR-328 expression. Interestingly, two functional C/EBPα binding sites are present in the miR-328 promoter and positively regulate its transcription.

We also show that maturation of differentiation-arrested BCR/ABL+ blasts requires direct interaction of hnRNP-E2 with the miR-328 C-rich regions. Moreover, imatinib treatment restores miR-328 expression, thus allowing its direct binding to hnRNP E2 independent from the RISC complex. Importantly, physiological miR-328 expression decreased hnRNP E2 binding to the uORF/spacer region of endogenous CEBPA mRNA (decoy activity). This, in turn, releases CEBPA mRNA from hnRNP E2 translation inhibition and allows in vitro and in vivo BCR/ABL+ cell differentiation.

Although hnRNP E2 was not found in complex with the basic RISC components in BCR/ABL+ cells, miR-328 was found associated to Dicer and Ago2, suggesting that miR-328 also acts through base-pairing with the 3′UTR of mRNA targets in a RISC-dependent manner. In fact, miR-328 suppresses PIM1 protein but not mRNA expression and this effect requires the integrity of the PIM1 3′UTR. Indeed, forced expression of a wild type, but not a kinase-deficient, PIM1 lacking the 3′UTR into miR-328-expressing cells fully rescues BCR/ABL clonogenicity, suggesting that miR-328-induced inhibition of PIM1 accounts for reduced survival of miR-328-infected CML-BCCD34+ blasts. To demonstrate that miR-328 acts on PIM1 in a RISC-dependent manner, we mutated the miR-328 in the seed sequence (miR-328-Mut) while retaining its C-rich character. As expected, miR-328-Mut interacted with hnRNP-E2 and rescued C/EBPα-mediated differentiation, but did not silence PIM1 expression. Thus, the discovery of dual activities for miR-328 which affect myeloid differentiation and survival not only adds a layer to the complexity of mechanisms regulating CML-BC but also highlights the ability of miRNAs to alter mRNA metabolism by acting as molecular decoys for RNA binding proteins.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1950.

Sp1/NFkappaB/HDAC/miR-29b regulatory network in KIT-driven myeloid leukemia

The biologic and clinical significance of KIT overexpression that associates with KIT gain-of-function mutations occurring in subsets of acute myeloid leukemia (AML) (i.e., core binding factor AML) is unknown. Here, we show that KIT mutations lead to MYC-dependent miR-29b repression and increased levels of the miR-29b target Sp1 in KIT-driven leukemia. Sp1 enhances its own expression by participating in a NFkappaB/HDAC complex that further represses miR-29b transcription. Upregulated Sp1 then binds NFkappaB and transactivates KIT. Therefore, activated KIT ultimately induces its own transcription. Our results provide evidence that the mechanisms of Sp1/NFkappaB/HDAC/miR-29b-dependent KIT overexpression contribute to leukemia growth and can be successfully targeted by pharmacological disruption of the Sp1/NFkappaB/HDAC complex or synthetic miR-29b treatment in KIT-driven AML.

miR-328 functions as an RNA decoy to modulate hnRNP E2 regulation of mRNA translation in leukemic blasts

MicroRNAs and heterogeneous ribonucleoproteins (hnRNPs) are posttranscriptional gene regulators that bind mRNA in a sequence-specific manner. Here, we report that loss of miR-328 occurs in blast crisis chronic myelogenous leukemia (CML-BC) in a BCR/ABL dose- and kinase-dependent manner through the MAPK-hnRNP E2 pathway. Restoration of miR-328 expression rescues differentiation and impairs survival of leukemic blasts by simultaneously interacting with the translational regulator poly(rC)-binding protein hnRNP E2 and with the mRNA encoding the survival factor PIM1, respectively. The interaction with hnRNP E2 is independent of the microRNA's seed sequence and it leads to release of CEBPA mRNA from hnRNP E2-mediated translational inhibition. Altogether, these data reveal the dual ability of a microRNA to control cell fate both through base pairing with mRNA targets and through a decoy activity that interferes with the function of regulatory proteins.

Alloreactive (CD4-Independent) CD8+ T cells jeopardize long-term survival of intrahepatic islet allografts

Despite success of early islet allograft engraftment and survival in humans, late islet allograft loss has emerged as an important clinical problem. CD8+ T cells that are independent of CD4+ T cell help can damage allograft tissues and are resistant to conventional immunosuppressive therapies. Previous work demonstrates that islet allografts do not primarily initiate rejection by the (CD4-independent) CD8-dependent pathway. This study was performed to determine if activation of alloreactive CD4-independent, CD8+ T cells, by exogenous stimuli, can precipitate late loss of islet allografts. Recipients were induced to accept intrahepatic islet allografts (islet 'acceptors') by short-term immunotherapy with donor-specific transfusion (DST) and anti-CD154 mAb. Following the establishment of stable long-term islet allograft function for 60-90 days, recipients were challenged with donor-matched hepatocellular allografts, which are known to activate (CD4-independent) CD8+ T cells. Allogeneic islets engrafted long-term were vulnerable to damage when challenged locally with donor-matched hepatocytes. Islet allograft loss was due to allospecific immune damage, which was CD8- but not CD4-dependent. Selection of specific immunotherapy to suppress both CD4- and CD8-dependent immune pathways at the time of transplant protects islet allografts from both early and late immune damage.

FTY720, a new alternative for treating blast crisis chronic myelogenous leukemia and Philadelphia chromosome-positive acute lymphocytic leukemia

Blast crisis chronic myelogenous leukemia (CML-BC) and Philadelphia chromosome-positive (Ph1-positive) acute lymphocytic leukemia (ALL) are 2 fatal BCR/ABL-driven leukemias against which Abl kinase inhibitors fail to induce a long-term response. We recently reported that functional loss of protein phosphatase 2A (PP2A) activity is important for CML blastic transformation. We assessed the therapeutic potential of the PP2A activator FTY720 (2-amino-2-[2-(4-octylphenyl)ethyl]-1,3-propanediol hydrochloride), an immunomodulator in Phase III trials for patients with multiple sclerosis or undergoing organ transplantation, in CML-BC and Ph1 ALL patient cells and in in vitro and in vivo models of these BCR/ABL+ leukemias. Our data indicate that FTY720 induces apoptosis and impairs clonogenicity of imatinib/dasatinib-sensitive and -resistant p210/p190(BCR/ABL) myeloid and lymphoid cell lines and CML-BC(CD34+) and Ph1 ALL(CD34+/CD19+) progenitors but not of normal CD34+ and CD34+/CD19+ bone marrow cells. Furthermore, pharmacologic doses of FTY720 remarkably suppress in vivo p210/p190(BCR/ABL)-driven [including p210/p190(BCR/ABL)(T315I)] leukemogenesis without exerting any toxicity. Altogether, these results highlight the therapeutic relevance of rescuing PP2A tumor suppressor activity in Ph1 leukemias and strongly support the introduction of the PP2A activator FTY720 in the treatment of CML-BC and Ph1 ALL patients.

High levels of the BCR/ABL oncoprotein are required for the MAPK-hnRNP-E2 dependent suppression of C/EBPalpha-driven myeloid differentiation

The inability of myeloid chronic myelogenous leukemia blast crisis (CML-BC) progenitors to undergo neutrophil differentiation depends on suppression of C/EBPalpha expression through the translation inhibitory activity of the RNA-binding protein hnRNP-E2. Here we show that "oncogene dosage" is a determinant factor for suppression of differentiation in CML-BC. In fact, high levels of p210-BCR/ABL are required for enhanced hnRNP-E2 expression, which depends on phosphorylation of hnRNP-E2 serines 173, 189, and 272 and threonine 213 by the BCR/ABL-activated MAPK(ERK1/2). Serine/threonine to alanine substitution abolishes hnRNP-E2 phosphorylation and markedly decreases its stability in BCR/ABL-expressing myeloid precursors. Similarly, pharmacologic inhibition of MAPK(ERK1/2) activity decreases hnRNP-E2 binding to the 5'UTR of C/EBPalpha mRNA by impairing hnRNP-E2 phosphorylation and stability. This, in turn, restores in vitro and/or in vivo C/EBPalpha expression and G-CSF-driven neutrophilic maturation of differentiation-arrested BCR/ABL(+) cell lines, primary CML-BC(CD34+) patient cells and lineage-negative mouse bone marrow cells expressing high levels of p210-BCR/ABL. Thus, increased BCR/ABL oncogenic tyrosine kinase activity is essential for suppression of myeloid differentiation of CML-BC progenitors as it is required for sustained activation of the MAPK(ERK1/2)-hnRNP-E2-C/EBPalpha differentiation-inhibitory pathway. Furthermore, these findings suggest the inclusion of clinically relevant MAPK inhibitors in the therapy of CML-BC.

Activation and maturation of alloreactive CD4-independent, CD8 cytolytic T cells

The goal of this study was to determine the in vivo conditions that promote activation of the (CD4-independent) CD8+ T cell-mediated rejection pathway. We have previously noted that hepatocellular but not islet allografts readily activate this rejection pathway. In the current study, we utilized these two cell transplant models to investigate whether differences in host cell recruitment to the graft site, expression of T-cell activation markers by CD8+ graft infiltrating cells (GICs), and/or development of delayed-type hypersensitivity (DTH) and cytotoxic T lymphocyte cell-mediated effector functions could account for the differential transplant outcomes. The collective results demonstrate that recruitment of CD8+ T cells to the site of transplant, CD103 or CD69 expression on CD8+ GICs, and activation of alloreactive DTH responses are insufficient to initiate CD4-independent, CD8-dependent transplant rejection. Instead, rejection by alloreactive (CD4-independent) CD8+ T cells correlated with expression of CD25, CD154 and CD43 by CD8+ GICs, in vitro alloproliferation by recipient CD8+ T cells, and the development of in vivo allospecific cytolytic effector function. These results suggest that tissue-derived factors influence the activation and maturation of (CD4-independent) CD8+ T cells into cytolytic effectors, which correlates with transplant rejection.

Targeting LFA-1 and cd154 suppresses the in vivo activation and development of cytolytic (cd4-Independent) CD8+ T cells

Short-term immunotherapy targeting both LFA-1 and CD40/CD154 costimulation produces synergistic effects such that long-term allograft survival is achieved in the majority of recipients. This immunotherapeutic strategy has been reported to induce the development of CD4+ regulatory T cells. In the current study, the mechanisms by which this immunotherapeutic strategy prevents CD8+ T cell-dependent hepatocyte rejection in CD4 knockout mice were examined. Combined blockade of LFA-1 and CD40/CD154 costimulation did not influence the overall number or composition of inflammatory cells infiltrating the liver where transplanted hepatocytes engraft. Expression of T cell activation markers CD43, CD69, and adhesion molecule CD103 by liver-infiltrating cells was suppressed in treated mice with long-term hepatocellular allograft survival compared to liver-infiltrating cells of untreated rejector mice. Short-term immunotherapy with anti-LFA-1 and anti-CD154 mAb also abrogated the in vivo development of alloreactive CD8+ cytotoxic T cell effectors. Treated mice with long-term hepatocyte allograft survival did not reject hepatocellular allografts despite adoptive transfer of naive CD8+ T cells. Unexpectedly, treated mice with long-term hepatocellular allograft survival demonstrated prominent donor-reactive delayed-type hypersensitivity responses, which were increased in comparison to untreated hepatocyte rejectors. Collectively, these findings support the conclusion that short-term immunotherapy with anti-LFA-1 and anti-CD154 mAbs induces long-term survival of hepatocellular allografts by interfering with CD8+ T cell activation and development of CTL effector function. In addition, these recipients with long-term hepatocellular allograft acceptance show evidence of immunoregulation which is not due to immune deletion or ignorance and is associated with early development of a novel CD8+CD25high cell population in the liver.

CD4+ T-cell-dependent immune damage of liver parenchymal cells is mediated by alloantibody

BACKGROUND

Allogeneic hepatocytes initiate both CD4- and CD8-dependent rejection responses. The current studies address the hypothesis that acute damage of allogeneic liver parenchymal cells by the CD4-dependent pathway is alloantibody-mediated and examines immune conditions which promote activation of this pathway.

METHODS

The role of alloantibody in CD4-dependent hepatocyte rejection was evaluated by assessing hepatocyte (FVB/N, H-2q) survival in CD8-depleted B-cell knockout (KO) (H-2b) recipients and by monitoring hepatocyte survival in C57BL/6.SCID (H-2b) recipients transfused with donor-reactive alloantibody. The development of donor-reactive alloantibody in C57BL/6 (H-2b), CD8-depleted C57BL/6, CD8 KO (H-2b), IFN-gamma KO (H-2b), perforin KO (H-2b), and FasL mutant gld/gld (H-2b) hepatocyte recipients was assessed.

RESULTS

Hepatocyte rejection in B-cell KO mice was significantly delayed by CD8+ T-cell depletion (median survival time [MST], 35 days) when compared to untreated (MST, 8 days) and CD4-depleted (MST, 10 days) recipient mice. Transfusion of donor-reactive alloantibody into SCID recipients with functional hepatocellular allografts was sufficient to precipitate rejection in a dose-dependent fashion. Donor-reactive alloantibody was minimal in the serum of C57BL/6 hepatocyte recipients, but was produced in significant quantities in hepatocyte recipients genetically deficient in or depleted of CD8+ T cells and in recipients with impaired cytotoxic effector mechanisms. In addition, recipients with defects in Th1 immunity, such as IFN-gamma KO recipients, also produced readily detectable alloantibody.

CONCLUSIONS

Collectively, these data support the hypothesis that acute immune damage of allogeneic hepatocytes by the CD4-dependent pathway is mediated by alloantibody and that this pathway is favored when Th1- or cell-mediated cytotoxic effector immune mechanisms are impaired.

Evidence for Tissue-Directed Immune Responses: Analysis of CD4- and CD8-Dependent Alloimmunity

BACKGROUND

Transplant rejection has generally been considered a CD4 T-cell-dependent immune process. CD4-independent, CD8 T-cell rejection pathways have recently gained attention because of their relative resistance to immunosuppression. In the current study, the role of the allograft tissue in activation of these distinct pathways was examined by comparing host-immune responses with allogeneic pancreatic islets or hepatocytes transplanted across the same genetic disparity.

METHODS

To compare activation of CD4-dependent versus CD8-dependent alloimmunity, islets or hepatocytes retrieved from FVB/N (H-2) mice were transplanted into CD8 or CD4 T-cell-reconstituted severe combined immunodeficiency mice, CD4 or CD8 knockout (KO) mice, and anti-CD4 monoclonal antibody (mAb) or anti-CD8 mAb treated C57BL/6 mice (all H-2). The ability to immunomodulate CD4-dependent allograft rejection (in CD8 KO mice) was examined in the context of several mechanistically distinct immunotherapeutic strategies, including anti-CD4 mAb, donor-specific transfusion and anti-CD154 mAb, and anti-lymphocyte function-associated antigen-1 mAb.

RESULTS

The studies demonstrate that, whereas hepatocytes evoke alloreactive CD4-dependent and (CD4-independent) CD8 T-cell immune responses, allogeneic islets only activate CD4-dependent immune pathways. CD4-dependent host-immune responses initiated by pancreatic islet allografts were readily suppressed by a variety of short-term immunotherapies, whereas hepatocyte-initiated CD4-dependent alloimmune responses were not.

CONCLUSIONS

These results demonstrate that immune characteristics of the specific allograft tissue uniquely influence the pattern of host immune responses such that the propensity to activate CD4- or CD8-dependent alloimmune responses can be distinguished. Furthermore, CD4-dependent immune responses activated by different tissues from the same donor strain are distinguished by their susceptibility to specific immunotherapy.

Critical role for CD8 T cells in allograft acceptance induced by DST and CD40/CD154 costimulatory blockade

Donor-specific transfusion (DST) and CD40/CD154 costimulation blockade is a powerful immunosuppressive strategy which prolongs survival of many allografts. The efficacy of DST and anti-CD154 mAb for prolongation of hepatocellular allograft survival was only realized in C57BL/6 mice that have both CD4- and CD8-dependent pathways available (median survival time, MST, 82 days). Hepatocyte rejection in CD8 KO mice which is CD4-dependent was not suppressed by DST and anti-CD154 mAb treatment (MST, 7 days); unexpectedly DST abrogated the beneficial effects of anti-CD154 mAb for suppression of hepatocyte rejection (MST, 42 days) and on donor-reactive alloantibody production. Hepatocyte rejection in CD4 KO mice which is CD8-dependent was suppressed by treatment with DST and anti-CD154 mAb therapy (MST, 35 days) but did not differ significantly from immunotherapy with anti-CD154 mAb alone (MST, 32 days). Induction of hepatocellular allograft acceptance by DST and anti-CD154 mAb immunotherapy was dependent on host CD8(+) T cells, as demonstrated by CD8 depletion studies in C57BL/6 mice (MST, 14 days) and CD8 reconstitution of CD8 KO mice (MST, 56 days). These studies demonstrate that both CD4(+) and CD8(+) T-cell subsets contribute to induction of hepatocellular allograft acceptance by this immunotherapeutic strategy.