The role of antigen cross-presentation from leukemia blasts on immunity to the leukemia-associated antigen PR1

Hu8F4-CAR T cells with mutated Fc spacer segment improve target-specificity and mediate anti-leukemia activity in vivo

Hu8F4 is a T cell receptor (TCR)-like antibody with high affinity for leukemia-associated antigen PR1/HLA-A2 epitope. Adapted into a chimeric antigen receptor (CAR) format, Hu8F4-CAR is comprised of the Hu8F4 scFv, the human IgG1 CH2CH3 extracellular spacer domain, a human CD28 costimulatory domain, and the human CD3ζ signaling domain. We have demonstrated high efficacy of Hu8F4-CAR-T cells against PR1/HLA-A2-expressing cell lines and leukemic blasts from AML patients in vitro. Previous studies have shown that modification of the Fc domains of IgG4 CH2CH3 spacer regions can eliminate activation-induced cell death and off-target killing mediated by mouse Fc gamma receptor (FcgR)-expressing cells. We generated Hu8F4-CAR(PQ) with mutated Fc receptor binding sites on the CH2 domain of Hu8F4-CAR to prevent unwanted interactions with FcgR-expressing cells in vivo. The primary human T cells transduced with Hu8F4-CAR(PQ) can specifically lyse HLA-A2+ PR1-expressing leukemia cell lines in vitro. Furthermore, both adult donor-derived and cord blood-derived Hu8F4-CAR(PQ)-T cells are active and can eliminate U937 leukemia cells in NSG mice. Herein, we demonstrate that modification of the IgG1-based spacer can eliminate Fc receptor-binding-induced adverse effects and Hu8F4-CAR(PQ)-T cells can kill leukemia in vivo.

Autoimmune HLA Alleles and Neoepitope Presentation Predict Post-Allogenic Transplant Relapse

Introduction

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) can cure patients with high-risk myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). However, many patients relapse or develop debilitating graft-versus-host disease. Transplant restores T-cell reactivity against tumor cells, implicating patient human leukocyte antigen (HLA)-dependent antigen presentation via the major histocompatibility complex as a determinant of response. We sought to identify characteristics of the HLA genotype that influence response in allo-HSCT patients.

Methods

We collected HLA genotype and panel-based somatic mutation profiles for 55 patients with AML and MDS and available data treated at the University of California San Diego Moores Cancer Center between May 2012 and January 2019. We evaluated characteristics of the HLA genotype relative to relapse-free time and overall survival (OS) post-allo-HSCT using univariable and multivariable regression.

Results

In multivariable regression, the presence of an autoimmune allele was significantly associated with relapse-free time (hazard ratio [HR], 0.25; p = 0.01) and OS (HR, 0.16; p < 0.005). The better potential of the donor HLA type to present peptides harboring driver mutations trended toward better relapse-free survival (HR, 0.45; p = 0.07) and significantly correlated with longer OS (HR, 0.33; p = 0.01) though only a minority of cases had an HLA mismatch.

Conclusion

In this single institution retrospective study of patients receiving allo-HSCT for relapsed AML/MDS, characteristics of an individual's HLA genotype (presence of an autoimmune allele and potential of the donor HLA to better present peptides representing driver mutations) were significantly associated with better outcomes. These findings suggest that HLA type may guide the optimal application of allo-HSCT and merit evaluation in larger cohorts. ClinicalTrials.gov Identifier: NCT02478931.

Bringing Together the Power of T Cell Receptor Mimic and Bispecific Antibodies for Cancer Immunotherapy: Still a Long Way to Go

Antibody-based cancer immunotherapy has revolutionized oncology. The first successful therapeutic antibodies relied on eliciting immune-mediated cytotoxicity (rituximab) or modulation of intracellular signaling (trastuzumab). Further attempts to enhance the antitumor effects led to the development of immunoconjugates with radioactive or cytotoxic compounds (tositumomab, brentuximab vedotin). Another line of research led to the bispecific antibodies that can enhance the formation of immunological synapse between cancer and cytotoxic T cells (blinatumomab). Despite the constant advances in design and production, the application of monoclonal antibodies in cancer treatment remains limited by the presence of specific cell surface markers. A rational approach to target intracellular cancer antigens was proposed almost two decades ago by the development of anti-peptide human leukocyte antigen (HLA) complex (T cell receptor-like/mimic) antibodies. They could recognize specifically cancer neoantigens expressed in the context of specific HLA molecules theoretically providing unprecedented specificity. Furthermore, they can be developed in a semigeneric format, that is, to target common neoantigens expressed in the context of common HLA molecules. It is rationale to expect that the development of such antibodies in the format of bispecific antibody constructs can bring together the power of specific antibody-based recognition and that of T cell-mediated lysis. There are already some preliminary reports that suggest such constructs would be an achievable goal. In this brief review, we discuss some of the successful preclinical developments in the field and the major challenges that are yet to be addressed.

Cytomegalovirus infection is associated with AML relapse after allo-HSCT: a meta-analysis of observational studies

Cytomegalovirus (CMV) infection and primary disease relapse remain challenging problems after allogeneic hematopoietic stem cell transplantation (allo-HSCT). We sought to assess the association between CMV infection and disease relapse after transplantation. PubMed, EMBASE, the Cochrane Library, SCI, and Chinese Biomedicine Databases were searched up to July 1, 2018, for all studies that investigate pre-transplant CMV serostatus, CMV replication, and primary disease relapse in allo-HSCT patients with hematologic malignancies. Meta-analysis of 24 eligible cohort studies showed a significantly lower relapse risk after allo-HSCT in patients with CMV replication in acute myeloid leukemia (AML) (HR = 0.64, 95% CI, 0.50-0.83; P < 0.001) subgroup. However, CMV replication was associated with increased non-relapse mortality (NRM) in AML patients (HR = 1.64, 95% CI, 1.46-1.85; P < 0.001), but not associated with overall survival (OS) or graft-versus-host disease for AML patients (P > 0.05). There was no association between pre-transplant CMV serostatus and disease relapse, although D-/R- was associated with better OS in acute leukemia patients (HR = 0.89, 95% CI, 0.83-0.96; P = 0.003). In AML patients, CMV replication may be a protective predictor against disease relapse, although the potential benefit of CMV replication is offset by increased NRM.

A Novel T-Cell Engaging Bi-specific Antibody Targeting the Leukemia Antigen PR1/HLA-A2

Despite substantial advances in the treatment of acute myeloid leukemia (AML), only 30% of patients survive more than 5 years. Therefore, new therapeutics are much needed. Here, we present a novel therapeutic strategy targeting PR1, an HLA-A2 restricted myeloid leukemia antigen. Previously, we have developed and characterized a novel T-cell receptor-like monoclonal antibody (8F4) that targets PR1/HLA-A2 and eliminates AML xenografts by antibody-dependent cellular cytotoxicity (ADCC). To improve the potency of 8F4, we adopted a strategy to link T-cell cytotoxicity with a bi-specific T-cell-engaging antibody that binds PR1/HLA-A2 on leukemia and CD3 on neighboring T-cells. The 8F4 bi-specific antibody maintained high affinity and specific binding to PR1/HLA-A2 comparable to parent 8F4 antibody, shown by flow cytometry and Bio-Layer Interferometry. In addition, 8F4 bi-specific antibody activated donor T-cells in the presence of HLA-A2⁺ primary AML blasts and cell lines in a dose dependent manner. Importantly, activated T-cells lysed HLA-A2⁺ primary AML blasts and cell lines after addition of 8F4 bi-specific antibody. In conclusion, our studies demonstrate the therapeutic potential of a novel bi-specific antibody targeting the PR1/HLA-A2 leukemia-associated antigen, justifying further clinical development of this strategy.

Membrane-Associated Proteinase 3 on Granulocytes and Acute Myeloid Leukemia Inhibits T Cell Proliferation

Proteinase 3 (P3), a serine protease expressed by myeloid cells, localized within azurophil granules, and also expressed on the cellular membrane of polymorphonuclear neutrophils (PMN), is the target of autoimmunity in granulomatosis with polyangiitis. PR1, an HLA-A2 restricted nonameric peptide derived from P3, has been targeted effectively in myeloid leukemia. We previously showed (Molldrem et al. 2003. JClinInvest 111: 639-647) that overexpression of P3 in chronic myeloid leukemia induces apoptosis of high-affinity PR1-specific T cells, leading to deletional tolerance and leukemia outgrowth. In this study, we investigated the effect of membrane P3 (mP3)-expressing PMN and acute myeloid leukemia (AML) blasts on the proliferation of CD4 and CD8 T cells in vitro. We demonstrate that mP3-expressing PMN significantly inhibits autologous healthy donor T cell proliferation but does not affect cytokine production in activated T cells and that this effect requires cell proximity and was abrogated by P3 blockade. This inhibition required P3 enzyme activity. However, suppression was not reversed by either the addition of catalase or the inhibition of arginase I. In addition to P3 blockade, anti-low density lipoprotein receptor-related protein 1 (LRP1) Ab also restored T cells' capacity to proliferate. Last, we show dose-dependent inhibition of T cell proliferation by mP3-expressing AML blasts. Together, our findings demonstrate a novel mechanism whereby PMN- and AML-associated mP3 inhibits T cell proliferation via direct LRP1 and mP3 interaction, and we identify P3 as a novel target to modulate immunity in myeloid leukemia and autoimmune disease.

Targeting the Leukemia Antigen PR1 with Immunotherapy for the Treatment of Multiple Myeloma

Purpose: PR1 is a human leukocyte antigen (HLA)-A2 nonameric peptide derived from neutrophil elastase (NE) and proteinase 3 (P3). We have previously shown that PR1 is cross-presented by solid tumors, leukemia, and antigen-presenting cells, including B cells. We have also shown that cross-presentation of PR1 by solid tumors renders them susceptible to killing by PR1-targeting immunotherapies. As multiple myeloma is derived from B cells, we investigated whether multiple myeloma is also capable of PR1 cross-presentation and subsequently capable of being targeted by using PR1 immunotherapies.Experimental Design: We tested whether multiple myeloma is capable of cross-presenting PR1 and subsequently becomes susceptible to PR1-targeting immunotherapies, using multiple myeloma cell lines, a xenograft mouse model, and primary multiple myeloma patient samples.Results: Here we show that multiple myeloma cells lack endogenous NE and P3, are able to take up exogenous NE and P3, and cross-present PR1 on HLA-A2. Cross-presentation by multiple myeloma utilizes the conventional antigen processing machinery, including the proteasome and Golgi, and is not affected by immunomodulating drugs (IMiD). Following PR1 cross-presentation, we are able to target multiple myeloma with PR1-CTL and anti-PR1/HLA-A2 antibody both in vitro and in vivoConclusions: Collectively, our data demonstrate that PR1 is a novel tumor-associated antigen target in multiple myeloma and that multiple myeloma is susceptible to immunotherapies that target cross-presented antigens. Clin Cancer Res; 24(14); 3386-96. ©2018 AACR.

Cathepsin G Is Expressed by Acute Lymphoblastic Leukemia and Is a Potential Immunotherapeutic Target

Cathepsin G (CG) is a myeloid azurophil granule protease that is highly expressed by acute myeloid leukemia (AML) blasts and leukemia stem cells. We previously identified CG1 (FLLPTGAEA), a human leukocyte antigen-A2-restricted nonameric peptide derived from CG, as an immunogenic target in AML. In this report, we aimed to assess the level of CG expression in acute lymphoid leukemia (ALL) and its potential as an immunotherapeutic target in ALL. Using RT-PCR and western blots, we identified CG mRNA and protein, respectively, in B-ALL patient samples and cell lines. We also examined CG expression in a large cohort of 130 patients with ALL via reverse-phase protein array (RPPA). Our data show that CG is widely expressed by ALL and is a poor prognosticator. In addition to endogenous expression, we also provide evidence that CG can be taken up by ALL cells. Finally, we demonstrate that patient ALL can be lysed by CG1-specific cytotoxic T lymphocytes in vitro. Together, these data show high expression of CG by ALL and implicate CG as a target for immunotherapy in ALL.

Cross-platform comparison of independent datasets identifies an immune signature associated with improved survival in metastatic melanoma

Platform and study differences in prognostic signatures from metastatic melanoma (MM) gene expression reports often hinder consensus arrival. We performed survival/outcome-based pairwise comparisons of three independent MM gene expression profiles using the threshold-free algorithm rank-rank hypergeometric overlap analysis (RRHO). We found statistically significant overlap for genes overexpressed in favorable outcome (FO) groups, but no overlap for poor outcome (PO) groups. This “favorable outcome signature” (FOS) of 228 genes coinciding on all three overlapping gene lists showed immune function predominated in FO MM. Surprisingly, specific cell signature-enrichment analysis showed B cell-associated genes enriched in FO MM, along with T cell-associated genes. Higher levels of B and T cells (p<0.05) and their relative proximity (p<0.05) were detected in FO-to-PO tumor comparisons from an independent MM patients cohort. Finally, expression of FOS in two independent Stage III MM tumor datasets correctly predicted clinical outcome in 12/14 and 44/70 patients using a weighted gene voting classifier (area under the curve values 0.96 and 0.75, respectively). This RRHO-based, cross-study analysis emphasizes the RRHO approach power, confirms T cells relevance for prolonged MM survival, supports a favorable role for B cells in anti-melanoma immunity, and suggests B cells potential as means of intervention in melanoma treatment.

Trastuzumab Increases HER2 Uptake and Cross-Presentation by Dendritic Cells

Early-phase clinical trials evaluating CD8⁺ T cell-eliciting, HER2-derived peptide vaccines administered to HER2⁺ breast cancer patients in the adjuvant setting suggest synergy between the vaccines and trastuzumab, the mAb targeting the HER2 protein. Among 60 patients enrolled in clinical trials evaluating the E75 + GM-CSF and GP2 + GM-CSF vaccines, there have been no recurrences in patients vaccinated after receiving trastuzumab as part of standard therapy in the per treatment analyses conducted after a median follow-up of greater than 34 months. Here, we describe a mechanism by which this synergy may occur. Flow cytometry showed that trastuzumab facilitated uptake of HER2 by dendritic cells (DC), which was mediated by the Fc receptor and was specific to trastuzumab. In vitro, increased HER2 uptake by DC increased cross-presentation of E75, the immunodominant epitope derived from the HER2 protein, an observation confirmed in two in vivo mouse models. This increased E75 cross-presentation, mediated by trastuzumab treatment, enabled more efficient expansion of E75-specific cytotoxic T cells (E75-CTL). These results demonstrate a mechanism by which trastuzumab links innate and adaptive immunity by facilitating activation of antigen-specific T cells. On the basis of these data, we conclude that HER2-positive breast cancer patients that have been treated with trastuzumab may experience a more robust antitumor immune response by restimulation of T cells with the E75 peptide vaccine, thereby accounting for the improved disease-free survival observed with combination therapy. Cancer Res; 77(19); 5374-83. ©2017 AACR.

Neuropilin-1 mediates neutrophil elastase uptake and cross-presentation in breast cancer cells

Neutrophil elastase (NE) can be rapidly taken up by tumor cells that lack endogenous NE expression, including breast cancer, which results in cross-presentation of PR1, an NE-derived HLA-A2-restricted peptide that is an immunotherapy target in hematological and solid tumor malignancies. The mechanism of NE uptake, however, remains unknown. Using the mass spectrometry-based approach, we identify neuropilin-1 (NRP1) as a NE receptor that mediates uptake and PR1 cross-presentation in breast cancer cells. We demonstrated that soluble NE is a specific, high-affinity ligand for NRP1 with a calculated K_d of 38.7 nm Furthermore, we showed that NRP1 binds to the RRXR motif in NE. Notably, NRP1 knockdown with interfering RNA or CRISPR-cas9 system and blocking using anti-NRP1 antibody decreased NE uptake and, subsequently, susceptibility to lysis by PR1-specific cytotoxic T cells. Expression of NRP1 in NRP1-deficient cells was sufficient to induce NE uptake. Altogether, because NRP1 is broadly expressed in tumors, our findings suggest a role for this receptor in immunotherapy strategies that target cross-presented antigens.

Serine Proteases Enhance Immunogenic Antigen Presentation on Lung Cancer Cells

Immunotherapies targeting immune checkpoints have proven efficacious in reducing the burden of lung cancer in patients; however, the antigenic targets of these reinvigorated T cells remain poorly defined. Lung cancer tumors contain tumor-associated macrophages (TAM) and neutrophils, which release the serine proteases neutrophil elastase (NE) and proteinase 3 (P3) into the tumor microenvironment. NE and P3 shape the antitumor adaptive immune response in breast cancer and melanoma. In this report, we demonstrate that lung cancer cells cross-presented the tumor-associated antigen PR1, derived from NE and P3. Additionally, NE and P3 enhanced the expression of human leukocyte antigen (HLA) class I molecules on lung cancer cells and induced unique, endogenous peptides in the immunopeptidome, as detected with mass spectrometry sequencing. Lung cancer patient tissues with high intratumoral TAMs were enriched for MHC class I genes and T-cell markers, and patients with high TAM and cytotoxic T lymphocyte (CTL) infiltration had improved overall survival. We confirmed the immunogenicity of unique, endogenous peptides with cytotoxicity assays against lung cancer cell lines, using CTLs from healthy donors that had been expanded against select peptides. Finally, CTLs specific for serine proteases-induced endogenous peptides were detected in lung cancer patients using peptide/HLA-A2 tetramers and were elevated in tumor-infiltrating lymphocytes. Thus, serine proteases in the tumor microenvironment of lung cancers promote the presentation of HLA class I immunogenic peptides that are expressed by lung cancer cells, thereby increasing the antigen repertoire that can be targeted in lung cancer. Cancer Immunol Res; 5(4); 319-29. ©2017 AACR.

HLA class II antigen-processing pathway in tumors: Molecular defects and clinical relevance

The human leukocyte antigen (HLA) class II antigen-processing machinery (APM) presents to cognate CD4⁺ T-cells antigenic peptides mainly generated from exogeneous proteins in the endocytic compartment. These CD4⁺ T cells exert helper function, but may also act as effector cells, thereby recognizing HLA class II antigen-expressing tumor cells. Thus, HLA class II antigen expression by tumor cells influences the tumor antigen (TA)-specific immune responses and, depending on the cancer type, the clinical course of the disease. Many types of human cancers express HLA class II antigens, although with marked differences in their frequency. Some types of cancer lack HLA class II antigen expression, which could be due to structural defects or deregulation affecting different components of the complex HLA class II APM and/or from lack of cytokine(s) in the tumor microenvironment. In this review, we have summarized the information about HLA class II antigen distribution in normal tissues, the structural organization of the HLA class II APM, their expression and regulation in malignant cells, the defects, which have been identified in malignant cells, and their functional and clinical relevance.

Functional exhaustion of CD4+ T cells induced by co-stimulatory signals from myeloid leukaemia cells

To cope with immune responses, tumour cells implement elaborate strategies such as adaptive resistance and induction of T-cell exhaustion. T-cell exhaustion has been identified as a state of hyporesponsiveness that arises under continuous antigenic stimulus. Nevertheless, contribution of co-stimulatory molecules to T-cell exhaustion in cancer remains to be better defined. This study explores the role of myeloid leukaemia-derived co-stimulatory signals on CD4⁺ T helper (Th) cell exhaustion, which may limit anti-tumour immunity. Here, CD86 and inducible T-cell co-stimulator ligand (ICOS-LG) co-stimulatory molecules that are found on myeloid leukaemia cells supported Th cell activation and proliferation. However, under continuous stimulation, T cells co-cultured with leukaemia cells, but not with peripheral blood monocytes, became functionally exhausted. These in vitro-generated exhausted Th cells were defined by up-regulation of programmed cell death 1 (PD-1), cytotoxic T-lymphocyte antigen 4 (CTLA-4), lymphocyte activation gene 3 (LAG3) and T-cell immunoglobulin and mucin domain-containing protein 3 (TIM-3) inhibitory receptors. They were reluctant to proliferate upon re-stimulation and produced reduced amounts of interleukin-2 (IL-2), tumour necrosis factor-α (TNF-α) and interferon-γ (IFN-γ). Nonetheless, IL-2 supplementation restored the proliferation capacity of the exhausted Th cells. When the co-stimulation supplied by the myeloid leukaemia cells were blocked, the amount of exhausted Th cells was significantly decreased. Moreover, in the bone marrow aspirates from patients with acute myeloid leukaemia (AML) or myelodysplastic syndrome (MDS), a subpopulation of Th cells expressing PD-1, TIM-3 and/or LAG3 was identified together with CD86⁺ and/or ICOS-LG⁺ myeloid blasts. Collectively, co-stimulatory signals derived from myeloid leukaemia cells possess the capacity to facilitate functional exhaustion in Th cells.

Autophagy and proteasome interconnect to coordinate cross-presentation through MHC class I pathway in B cells

Cross-presentation of exogenous protein antigens by B cells through the major histocompatibility complex (MHC) class I pathway in lymphoid malignancies, and transplant setting has been recognised as an important mediator of immune pathogenesis and T cell-mediated immune regulation. However, the precise mechanism of cross-presentation of exogenous antigens in B cells has remained unresolved. Here we have delineated a novel pathway for cross-presentation in B cells, which involves synergistic cooperation of the proteasome and autophagy. After endocytosis, protein antigen is processed through an autophagy- and proteasome-dependent pathway and CD8⁺ T-cell epitopes are loaded onto MHC class I molecules within the autophagolysomal compartment rather than the conventional secretory pathway, which requires transporters associated with antigen processing-dependent transport. Interestingly, this cross-presentation was critically dependent on valosin-containing protein (VCP)/p97 ATPase through its participation in autophagy. Loss of VCP/p97 ATPase was coincident with accumulation of LC3-II and marked reduction in antigen presentation. These observations provide unique insight on how the autophagy and proteasomal degradation systems interconnect to coordinate MHC class I-restricted cross-presentation in B cells.

Neutrophil elastase enhances antigen presentation by upregulating human leukocyte antigen class I expression on tumor cells

Neutrophil elastase (NE) is an innate immune cell-derived inflammatory mediator that we have shown increases the presentation of tumor-associated peptide antigens in breast cancer. In this study, we extend these observations to show that NE uptake has a broad effect on enhancing antigen presentation by breast cancer cells. We show that NE increases human leukocyte antigen (HLA) class I expression on the surface of breast cancer cells in a concentration and time-dependent manner. HLA class I upregulation requires internalization of enzymatically active NE. Western blots of NE-treated breast cancer cells confirm that the expression of total HLA class I as well as the antigen-processing machinery proteins TAP1, LMP2, and calnexin does not change following NE treatment. This suggests that NE does not increase the efficiency of antigen processing; rather, it mediates the upregulation of HLA class I by stabilizing and reducing membrane recycling of HLA class I molecules. Furthermore, the effects of NE extend beyond breast cancer since the uptake of NE by EBV-LCL increases the presentation of HLA class I-restricted viral peptides, as shown by their increased sensitivity to lysis by EBV-specific CD8+ T cells. Together, our results show that NE uptake increases the responsiveness of breast cancer cells to adaptive immunity by broad upregulation of membrane HLA class I and support the conclusion that the innate inflammatory mediator NE enhances tumor cell recognition and increases tumor sensitivity to the host adaptive immune response.

Targeting cathepsin G in myeloid leukemia

Cathepsin G (CG) is a serine protease normally found within the azurophil granules of neutrophils. CG is expressed during the early stages of normal myeloid differentiation and—aberrantly—by myeloid leukemia cells. We have recently identified CG-derived HLA-A*0201-binding peptides that constitute promising targets for the immunotherapy of myeloid leukemia.

Dendritic cell-based immunotherapy for myeloid leukemias

Acute and chronic myeloid leukemia (AML, CML) are hematologic malignancies arising from oncogene-transformed hematopoietic stem/progenitor cells known as leukemia stem cells (LSCs). LSCs are selectively resistant to various forms of therapy including irradiation or cytotoxic drugs. The introduction of tyrosine kinase inhibitors has dramatically improved disease outcome in patients with CML. For AML, however, prognosis is still quite dismal. Standard treatments have been established more than 20 years ago with only limited advances ever since. Durable remission is achieved in less than 30% of patients. Minimal residual disease (MRD), reflected by the persistence of LSCs below the detection limit by conventional methods, causes a high rate of disease relapses. Therefore, the ultimate goal in the treatment of myeloid leukemia must be the eradication of LSCs. Active immunotherapy, aiming at the generation of leukemia-specific cytotoxic T cells (CTLs), may represent a powerful approach to target LSCs in the MRD situation. To fully activate CTLs, leukemia antigens have to be successfully captured, processed, and presented by mature dendritic cells (DCs). Myeloid progenitors are a prominent source of DCs under homeostatic conditions, and it is now well established that LSCs and leukemic blasts can give rise to "malignant" DCs. These leukemia-derived DCs can express leukemia antigens and may either induce anti-leukemic T cell responses or favor tolerance to the leukemia, depending on co-stimulatory or -inhibitory molecules and cytokines. This review will concentrate on the role of DCs in myeloid leukemia immunotherapy with a special focus on their generation, application, and function and how they could be improved in order to generate highly effective and specific anti-leukemic CTL responses. In addition, we discuss how DC-based immunotherapy may be successfully integrated into current treatment strategies to promote remission and potentially cure myeloid leukemias.

A novel HLA-A*0201 restricted peptide derived from cathepsin G is an effective immunotherapeutic target in acute myeloid leukemia

PURPOSE

Immunotherapy targeting aberrantly expressed leukemia-associated antigens has shown promise in the management of acute myeloid leukemia (AML). However, because of the heterogeneity and clonal evolution that is a feature of myeloid leukemia, targeting single peptide epitopes has had limited success, highlighting the need for novel antigen discovery. In this study, we characterize the role of the myeloid azurophil granule protease cathepsin G (CG) as a novel target for AML immunotherapy.

EXPERIMENTAL DESIGN

We used Immune Epitope Database and in vitro binding assays to identify immunogenic epitopes derived from CG. Flow cytometry, immunoblotting, and confocal microscopy were used to characterize the expression and processing of CG in AML patient samples, leukemia stem cells, and normal neutrophils. Cytotoxicity assays determined the susceptibility of AML to CG-specific cytotoxic T lymphocytes (CTL). Dextramer staining and cytokine flow cytometry were conducted to characterize the immune response to CG in patients.

RESULTS

CG was highly expressed and ubiquitinated in AML blasts, and was localized outside granules in compartments that facilitate antigen presentation. We identified five HLA-A*0201 binding nonameric peptides (CG1-CG5) derived from CG, and showed immunogenicity of the highest HLA-A*0201 binding peptide, CG1. We showed killing of primary AML by CG1-CTL, but not normal bone marrow. Blocking HLA-A*0201 abrogated CG1-CTL-mediated cytotoxicity, further confirming HLA-A*0201-dependent killing. Finally, we showed functional CG1-CTLs in peripheral blood from AML patients following allogeneic stem cell transplantation.

CONCLUSION

CG is aberrantly expressed and processed in AML and is a novel immunotherapeutic target that warrants further development.

Broad cross-presentation of the hematopoietically derived PR1 antigen on solid tumors leads to susceptibility to PR1-targeted immunotherapy

PR1 is a HLA-A2-restricted peptide that has been targeted successfully in myeloid leukemia with immunotherapy. PR1 is derived from the neutrophil granule proteases proteinase 3 (P3) and neutrophil elastase (NE), which are both found in the tumor microenvironment. We recently showed that P3 and NE are taken up and cross-presented by normal and leukemia-derived APCs, and that NE is taken up by breast cancer cells. We now extend our findings to show that P3 and NE are taken up and cross-presented by human solid tumors. We further show that PR1 cross-presentation renders human breast cancer and melanoma cells susceptible to killing by PR1-specific CTLs (PR1-CTL) and the anti-PR1/HLA-A2 Ab 8F4. We also show PR1-CTL in peripheral blood from patients with breast cancer and melanoma. Together, our data identify cross-presentation as a novel mechanism through which cells that lack endogenous expression of an Ag become susceptible to therapies that target cross-presented Ags and suggest PR1 as a broadly expressed tumor Ag.

TCL1: a shared tumor-associated antigen for immunotherapy against B-cell lymphomas

Immunotherapy with therapeutic idiotype vaccines offers promise for treatment of B-cell malignancies. However, identification of novel immunogenic lymphoma-associated antigens that are universally expressed is necessary to overcome the barriers of patient-specific idiotype vaccines. Here, we determined whether T-cell leukemia/lymphoma 1 (TCL1) oncoprotein encoded by the TCL1 gene could be a target for immunotherapy of B-cell malignancies. We show that TCL1 mRNA and protein are selectively expressed in normal B cells but markedly hyperexpressed in multiple human B-cell lymphomas, including follicular lymphoma, chronic lymphocytic leukemia, mantle cell lymphoma, diffuse large B-cell lymphoma, and splenic marginal zone B-cell lymphoma. We demonstrated that TCL1-specific CD8(+) T cells can be generated from HLA-A*0201 (HLA-A2)(+) normal donors and identified TCL1(71-78) (LLPIMWQL) as the minimal epitope recognized by these T cells. More importantly, TCL1(71-78) peptide-specific T cells were present in the peripheral blood and tumor-infiltrating lymphocytes of lymphoma patients, could be expanded in vitro, and lysed autologous tumor cells but not normal B cells in an HLA-A2-restricted manner. Our results suggest that TCL1 is naturally processed and presented on the surface of lymphoma cells for recognition by cytotoxic T cells and can serve as a novel target for development of immunotherapeutic strategies against common B-cell lymphomas.