Analysis of the functional WT1-specific T-cell repertoire in healthy donors reveals a discrepancy between CD4(+) and CD8(+) memory formation

Analysis of Wilms' tumor protein 1 specific TCR repertoire in AML patients uncovers higher diversity in patients in remission than in relapsed

The Wilms' tumor protein 1 (WT1) is a well-known and prioritized tumor-associated antigen expressed in numerous solid and blood tumors. Its abundance and immunogenicity have led to the development of different WT1-specific immune therapies. The driving player in these therapies, the WT1-specific T-cell receptor (TCR) repertoire, has received much less attention. Importantly, T cells with high affinity against the WT1 self-antigen are normally eliminated after negative selection in the thymus and are thus rare in peripheral blood. Here, we developed computational models for the robust and fast identification of WT1-specific TCRs from TCR repertoire data. To this end, WT137-45 (WT1-37) and WT1126-134 (WT1-126)-specific T cells were isolated from WT1 peptide-stimulated blood of healthy individuals. The TCR repertoire from these WT1-specific T cells was sequenced and used to train a pattern recognition model for the identification of WT1-specific TCR patterns for the WT1-37 or WT1-126 epitopes. The resulting computational models were applied on an independent published dataset from acute myeloid leukemia (AML) patients, treated with hematopoietic stem cell transplantation, to track WT1-specific TCRs in silico. Several WT1-specific TCRs were found in AML patients. Subsequent clustering analysis of all repertoires indicated the presence of more diverse TCR patterns within the WT1-specific TCR repertoires of AML patients in complete remission in contrast to relapsing patients. We demonstrate the possibility of tracking WT1-37 and WT1-126-specific TCRs directly from TCR repertoire data using computational methods, eliminating the need for additional blood samples and experiments for the two studied WT1 epitopes.

Human/mouse CD137 agonist, JNU-0921, effectively shrinks tumors through enhancing the cytotoxicity of CD8+ T cells in cis and in trans

Agonistic antibodies against CD137 have been demonstrated to completely regress established tumors through activating T cell immunity. Unfortunately, current CD137 antibodies failed to benefit patients with cancer. Moreover, their antitumor mechanisms in vivo remain to be determined. Here, we report the development of a small molecular CD137 agonist, JNU-0921. JNU-0921 effectively activates both human and mouse CD137 through direct binding their extracellular domains to induce oligomerization and signaling and effectively shrinks tumors in vivo. Mechanistically, JNU-0921 enhances effector and memory function of cytotoxic CD8+ T cells (CTLs) and alleviates their exhaustion. JNU-0921 also skews polarization of helper T cells toward T helper 1 type and enhances their activity to boost CTL function. Meanwhile, JNU-0921 attenuates the inhibitory function of regulatory T cells on CTLs. Our current work shows that JNU-0921 shrinks tumors by enhancing the cytotoxicity of CTLs in cis and in trans and sheds light on strategy for developing CD137 small molecular agonists.

Cytotoxicity of WT1-reactive T cells against Wilms tumor: An implication for antigen-specific adoptive immunotherapy

Introduction

T cells that recognize WT1 peptides have been shown to efficiently eliminate WT1-expressing tumor cells. This study was designed to investigate the feasibility of isolating WT1-reactive T cells from peripheral blood mononuclear cells (PBMCs) from healthy donors and patients with Wilms tumor, and to assess the cytotoxicity mediated by these cells against Wilms tumor cells (WiTu cells).

Methods

WT1-reactive T cells were enriched and isolated by stimulating PBMCs with a WT1 peptide pool and interferon-γ capture-based immunomagnetic separation (IMS). Using the lactate dehydrogenase release assay, the in vitro cytotoxicity of the isolated cells and standard chemotherapy was evaluated on WiTu cells.

Results

Higher proportions of WT1-reactive T cells were isolated from patients with Wilms tumor compared to those isolated from HDs. WT1-reactive T cells produced > 50% specific lysis when co-cultured with WT1+ WiTu cells at the highest effector-to-target (E:T) ratio in this study (i.e., 5:1), compared to <23% when co-cultured with WT1- WiTu cells at the same ratio. WT1-reactive T cells showed anti-tumoral activity in a dose-dependent manner and mediated significantly greater cytotoxicity than the non-WT1-reactive fraction of PBMCs on WT1+ WiTu cells. The cytotoxicity of standard chemotherapy was significantly lower than that of WT1-reactive T cells when co-cultured with WT1+ WiTu cells at E:T ratios of 2:1 and 5:1.

Conclusion

WT1-reactive T cells can be effectively enriched from the PBMCs of patients with Wilms tumor. Ex vivo generated WT1-reactive T cells might be considered an adoptive immunotherapeutic option for WT1+ Wilms tumors.

Antigen-specificity measurements are the key to understanding T cell responses

Antigen-specific T cells play a central role in the adaptive immune response and come in a wide range of phenotypes. T cell receptors (TCRs) mediate the antigen-specificities found in T cells. Importantly, high-throughput TCR sequencing provides a fingerprint which allows tracking of specific T cells and their clonal expansion in response to particular antigens. As a result, many studies have leveraged TCR sequencing in an attempt to elucidate the role of antigen-specific T cells in various contexts. Here, we discuss the published approaches to studying antigen-specific T cells and their specific TCR repertoire. Further, we discuss how these methods have been applied to study the TCR repertoire in various diseases in order to characterize the antigen-specific T cells involved in the immune control of disease.

Combined PD-L1 and TIM3 blockade improves expansion of fit human CD8+ antigen-specific T cells for adoptive immunotherapy

Antigen-specific T cell expansion ex vivo followed by adoptive transfer enables targeting of a multitude of microbial and cancer antigens. However, clinical-scale T cell expansion from rare precursors requires repeated stimulation, which may lead to T cell dysfunction and limited therapeutic potential. We used a clinically compliant protocol to expand Epstein-Barr virus (EBV) and Wilms tumor 1 (WT1) antigen-specific CD8⁺ T cells, and leveraged T cell exhaustion-associated inhibitory receptor blockade to improve T cell expansion. Several inhibitory receptors were expressed early by ex vivo-expanded antigen-specific CD8⁺ T cells, including PD-1 and TIM3, with co-expression matching evidence of T cell dysfunction as the cultures progressed. Introduction of anti-PD-L1 and anti-TIM3 blockade in combination (but not individually) to the culture led to markedly improved antigen-specific T cell expansion without inducing T cell dysfunction. Single-cell RNA sequencing (RNA-seq) and T cell receptor (TCR) repertoire profiling revealed that double blockade does not impart specific transcriptional programs in T cells or alterations in TCR repertoires. However, combined blockade may affect gene expression in a minority of clonotypes in a donor-specific fashion. We conclude that antigen-specific CD8⁺ T cell manufacturing can be improved by using TIM3 and PD-L1/PD-1 axis blockade in combination. This approach is readily applicable to several adoptive immunotherapy strategies.

Impact of T-cell immunity on chemotherapy response in childhood acute lymphoblastic leukemia

Although acute lymphoblastic leukemia (ALL) is highly responsive to chemotherapy, it is unknown how or which host immune factors influence the long-term remission of this cancer. To this end, we systematically evaluated the effects of T-cell immunity on Ph+ ALL therapy outcomes. Using a murine Arf-/-BCR-ABL1 B-cell ALL model, we showed that loss of T cells in the host drastically increased leukemia relapse after dasatinib or cytotoxic chemotherapy. Although ABL1 mutations emerged early during dasatinib treatment in both immunocompetent and immunocompromised hosts, T-cell immunity was essential for suppressing the outgrowth of drug-resistant leukemia. Bulk and single-cell transcriptome profiling of T cells during therapy pointed to the activation of type 1 immunity-related cytokine signaling being linked to long-term leukemia remission in mice. Consistent with these observations, interferon γ and interleukin 12 directly modulated dasatinib antileukemia efficacy in vivo. Finally, we evaluated peripheral blood immune cell composition in 102 children with ALL during chemotherapy and observed a significant association of T-cell abundance with treatment outcomes. Together, these results suggest that T-cell immunity plays pivotal roles in maintaining long-term remission of ALL, highlighting that the interplay between host immunity and drug resistance can be harnessed to improve ALL chemotherapy outcomes.

CRISPR-based gene disruption and integration of high-avidity, WT1-specific T cell receptors improve antitumor T cell function

T cell receptor (TCR)-based therapy has the potential to induce durable clinical responses in patients with cancer by targeting intracellular tumor antigens with high sensitivity and by promoting T cell survival. However, the need for TCRs specific for shared oncogenic antigens and the need for manufacturing protocols able to redirect T cell specificity while preserving T cell fitness remain limiting factors. By longitudinal monitoring of T cell functionality and dynamics in 15 healthy donors, we isolated 19 TCRs specific for Wilms' tumor antigen 1 (WT1), which is overexpressed by several tumor types. TCRs recognized several peptides restricted by common human leukocyte antigen (HLA) alleles and displayed a wide range of functional avidities. We selected five high-avidity HLA-A*02:01-restricted TCRs, three that were specific to the less explored immunodominant WT1_37-45 and two that were specific to the noncanonical WT1_-78-64 epitopes, both naturally processed by primary acute myeloid leukemia (AML) blasts. With CRISPR-Cas9 genome editing tools, we combined TCR-targeted integration into the TCR α constant (TRAC) locus with TCR β constant (TRBC) knockout, thus avoiding TCRαβ mispairing and maximizing TCR expression and function. The engineered lymphocytes were enriched in memory stem T cells. A unique WT1_37-45-specific TCR showed antigen-specific responses and efficiently killed AML blasts, acute lymphoblastic leukemia blasts, and glioblastoma cells in vitro and in vivo in the absence of off-tumor toxicity. T cells engineered to express this receptor are being advanced into clinical development for AML immunotherapy and represent a candidate therapy for other WT1-expressing tumors.

Induced dendritic cells co-expressing GM-CSF/IFN-α/tWT1 priming T and B cells and automated manufacturing to boost GvL

Acute myeloid leukemia (AML) patients with minimal residual disease and receiving allogeneic hematopoietic stem cell transplantation (HCT) have poor survival. Adoptive administration of dendritic cells (DCs) presenting the Wilms tumor protein 1 (WT1) leukemia-associated antigen can potentially stimulate de novo T and B cell development to harness the graft-versus-leukemia (GvL) effect after HCT. We established a simple and fast genetic modification of monocytes for simultaneous lentiviral expression of a truncated WT1 antigen (tWT1), granulocyte macrophage-colony-stimulating factor (GM-CSF), and interferon (IFN)-α, promoting their self-differentiation into potent “induced DCs” (iDCtWT1). A tricistronic integrase-defective lentiviral vector produced under good manufacturing practice (GMP)-like conditions was validated. Transduction of CD14⁺ monocytes isolated from peripheral blood, cord blood, and leukapheresis material effectively induced their self-differentiation. CD34⁺ cell-transplanted Nod.Rag.Gamma (NRG)- and Nod.Scid.Gamma (NSG) mice expressing human leukocyte antigen (HLA)-A∗0201 (NSG-A2)-immunodeficient mice were immunized with autologous iDCtWT1. Both humanized mouse models showed improved development and maturation of human T and B cells in the absence of adverse effects. Toward clinical use, manufacturing of iDCtWT1 was up scaled and streamlined using the automated CliniMACS Prodigy system. Proof-of-concept clinical-scale runs were feasible, and the 38-h process enabled standardized production and high recovery of a cryopreserved cell product with the expected identity characteristics. These results advocate for clinical trials testing iDCtWT1 to boost GvL and eradicate leukemia.

Ex vivo enrichment of PRAME antigen-specific T cells for adoptive immunotherapy using CD137 activation marker selection

Objective

Adoptive immunotherapy with ex vivo expanded tumor‐specific T cells has potential as anticancer therapy. Preferentially expressed antigen in melanoma (PRAME) is an attractive target overexpressed in several cancers including melanoma and acute myeloid leukaemia (AML), with low expression in normal tissue outside the gonads. We developed a GMP‐compliant manufacturing method for PRAME‐specific T cells from healthy donors for adoptive immunotherapy.

Methods

Mononuclear cells were pulsed with PRAME 15‐mer overlapping peptide mix. After 16 h, activated cells expressing CD137 were isolated with immunomagnetic beads and cocultured with irradiated CD137^neg fraction in medium supplemented with interleukin (IL)‐2, IL‐7 and IL‐15. Cultured T cells were restimulated with antigen‐pulsed autologous cells after 10 days. Cellular phenotype and cytokine response following antigen re‐exposure were assessed with flow cytometry, enzyme‐linked immunospot (ELISPOT) and supernatant cytokine detection. Detailed phenotypic and functional analysis with mass cytometry and T‐cell receptor (TCR) beta clonality studies were performed on selected cultures.

Results

PRAME‐stimulated cultures (n = 10) had mean expansion of 2500‐fold at day 18. Mean CD3⁺ percentage was 96% with CD4:CD8 ratio of 4:1. Re‐exposure to PRAME peptide mixture showed enrichment of CD4 cells expressing interferon (IFN)‐γ (mean: 12.2%) and TNF‐α (mean: 19.7%). Central and effector memory cells were 23% and 72%, respectively, with 24% T cells expressing PD1. Mass cytometry showed predominance of Th1 phenotype (CXCR3⁺/CCR4^neg/CCR6^neg/Tbet⁺, mean: 73%) and cytokine production including IL‐2, IL‐4, IL‐8, IL‐13 and GM‐CSF (2%, 6%, 8%, 4% and 11%, respectively).

Conclusion

PRAME‐specific T cells for adoptive immunotherapy were enriched from healthy donor mononuclear cells. The products were oligoclonal, exhibited Th1 phenotype and produced multiple cytokines.

Clinical Grade Production of Wilms' Tumor-1 Loaded Cord Blood-Derived Dendritic Cells to Prevent Relapse in Pediatric AML After Cord Blood Transplantation

Hematopoietic cell transplantation (HCT) is a last resort, potentially curative treatment option for pediatric patients with refractory acute myeloid leukemia (AML). Cord blood transplantation (CBT) results in less relapses and less graft-versus-host disease when compared to other sources. Nevertheless, still more than half of the children die from relapses. We therefore designed a strategy to prevent relapses by inducing anti-AML immunity after CBT, using a CB-derived dendritic cell (CBDC) vaccine generated from CD34+ CB cells from the same graft. We here describe the optimization and validation of good manufacturing practice (GMP)-grade production of the CBDC vaccine. We show the feasibility of expanding low amounts of CD34+ cells in a closed bag system to sufficient DCs per patient for at least three rounds of vaccinations. The CBDCs showed upregulated costimulatory molecules after maturation and showed enhanced CCR7-dependent migration toward CCL19 in a trans-well migrations assay. CBDCs expressed Wilms’ tumor 1 (WT1) protein after electroporation with WT1-mRNA, but were not as potent as CBDCs loaded with synthetic long peptides (peptivator). The WT1-peptivator loaded CBDCs were able to stimulate T-cells both in a mixed lymphocyte reaction as well as in an antigen-specific (autologous) setting. The autologous stimulated T-cells lysed not only the WT1+ cell line, but most importantly, also primary pediatric AML cells. Altogether, we provide a GMP-protocol of a highly mature CBDC vaccine, loaded with WT1 peptivator and able to stimulate autologous T-cells in an antigen-specific manner. Finally, these T-cells lysed primary pediatric AML demonstrating the competence of the CBDC vaccine strategy.

Impact of mature dendritic cells pulsed with a novel WT1 helper peptide on the induction of HLA‑A2‑restricted WT1‑reactive CD8+ T cells

The proliferation and activation of CD4+ T helper 1 (Th1) cells and CD8+ cytotoxic T lymphocytes (CTLs) that produce interferon‑γ (IFN‑γ) is an essential action of effective cancer vaccines. Recently, a novel Wilms' tumor 1 (WT1) helper peptide (WT1 HP34‑51; amino acid sequence, WAPVLDFAPPGASAYGSL) applicable for various human leukocyte antigen (HLA) subtypes (HLA‑DR, HLA‑DP and HLA‑DQ) was reported to increase peptide immunogenicity; however, the function of WT1 HP34‑51 remains unclear. In the present study, mature dendritic cells (mDCs) pulsed with WT1 HP34‑51 (mDC/WT1 HP34‑51) activated not only WT1‑specific CD4+ T cells but also CD8+ T cells that produced IFN‑γ following stimulation with immature dendritic cells (imDCs) pulsed with WT1 killer peptide (imDC/WT1 KP37‑45) in an HLA‑A*02:01‑ or HLA‑A*02:06‑restricted manner. Furthermore, the activated WT1‑reactive CD4+ Th1 cells were predominantly effector memory (EM) T cells. In 5 of 12 (41.7%) patients with cancer carrying the HLA‑A*02:01 or HLA‑A*02:06 allele, WT1‑reactive CD8+ T cells stimulated with mDC/WT1 HP34‑51 enhanced their levels of WT1 KP37‑45‑specific IFN‑γ production, with an increase >10%. Simultaneous activation of CD4+ and CD8+ T cells occurred more often when stimulation with mDC/WT1 HP34‑51 was combined with imDC/WT1 KP37‑45 restimulation. These results indicated that the novel mDC/WT1 HP34‑51 combination induced responses by WT1‑specific EM CD4+ Th1 cells and HLA‑A*02:01‑ or HLA‑A*02:06‑restricted CD8+ CTLs, suggesting its potential as a WT1‑targeting cancer vaccine.

A minority of T cells recognizing tumor-associated antigens presented in self-HLA can provoke antitumor reactivity

Tumor-associated antigens (TAAs) are monomorphic self-antigens that are proposed as targets for immunotherapeutic approaches to treat malignancies. We investigated whether T cells with sufficient avidity to recognize naturally overexpressed self-antigens in the context of self-HLA can be found in the T-cell repertoire of healthy donors. Minor histocompatibility antigen (MiHA)-specific T cells were used as a model, as the influence of thymic selection on the T-cell repertoire directed against MiHA can be studied in both self (MiHApos donors) and non-self (MiHAneg donors) backgrounds. T-cell clones directed against the HLA*02:01-restricted MiHA HA-1H were isolated from HA-1Hneg/HLA-A*02:01pos and HA-1Hpos/HLA-A*02:01pos donors. Of the 16 unique HA-1H–specific T-cell clones, five T-cell clones derived from HA-1Hneg/HLA-A*02:01pos donors and one T-cell clone derived from an HA-1Hpos/HLA-A*02:01pos donor showed reactivity against HA-1Hpos target cells. In addition, in total, 663 T-cell clones (containing at least 91 unique clones expressing different T-cell receptors) directed against HLA*02:01-restricted peptides of TAA WT1-RMF, RHAMM-ILS, proteinase-3-VLQ, PRAME-VLD, and NY-eso-1-SLL were isolated from HLA-A*02:01pos donors. Only 3 PRAME-VLD–specific and one NY-eso-1-SLL–specific T-cell clone provoked interferon-γ production and/or cytolysis upon stimulation with HLA-A*02:01pos malignant cell lines (but not primary malignant samples) naturally overexpressing the TAA. These results show that self-HLA–restricted T cells specific for self-antigens such as MiHA in MiHApos donors and TAAs are present in peripheral blood of healthy individuals. However, clinical efficacy would require highly effective in vivo priming by peptide vaccination in the presence of proper adjuvants or in vitro expansion of the low numbers of self-antigen–specific T cells of sufficient avidity to recognize endogenously processed antigen.

Inhibition of Heme Oxygenase-1 Activity Enhances Wilms Tumor-1-Specific T-Cell Responses in Cancer Immunotherapy

Wilms tumor protein-1 (WT1) is an attractive target for adoptive T-cell therapy due to its expression in solid tumors and hematologic malignancies. However, T cells recognizing WT1 occur in low frequencies in the peripheral blood of healthy donors, limiting potential therapeutic possibilities. Tin mesoporphyrin (SnMP) is known to inhibit heme oxygenase-1 (HO-1), which has been shown to boost the activation and proliferation of human virus-specific T cells. We analyzed the influence of this effect on the generation of WT1-specific T cells and developed strategies for generating quantities of these cells from healthy donors, sufficient for adoptive T-cell therapies. HO-1 inhibition with SnMP increased WT1-specific T-cell frequencies in 13 (26%) of 50 healthy donors. To assess clinical applicability, we measured the enrichment efficiency of SnMP-treated WT1-specific T cells in response to a WT1-specific peptide pool and a HLA-A*02:01-restricted WT1 peptide by cytokine secretion assay. SnMP treatment resulted in a 28-fold higher enrichment efficacy with equal functionality. In conclusion, pharmacological inhibition of HO-1 activity with SnMP results in more efficient generation of functionally active WT1-specific T cells. This study demonstrates the therapeutic potentials of inhibiting HO-1 with SnMP to enhance antigen-specific T-cell responses in the treatment of cancer patients with WT1-positive disease.

PRAME peptide-specific CD8+ T cells represent the predominant response against leukemia-associated antigens in healthy individuals

Antigen-specific T cells isolated from healthy individuals (HIs) have shown great therapeutic potential upon adoptive transfer for the treatment of viremia in immunosuppressed patients. The lack of comprehensive data on the prevalence and characteristics of leukemia-associated antigen (LAA)-specific T cells in HIs still limits such an approach for tumor therapy. Therefore, we have investigated T-cell responses against prominent candidates comprising Wilms' tumor protein 1 (WT1), preferentially expressed antigen in melanoma (PRAME), Survivin, NY-ESO, and p53 by screening PBMCs from HIs using intracellular IFN-γ staining following provocation with LAA peptide mixes. Here, we found predominantly poly-functional effector/effector memory CCR7^- /CD45RA^+/- /CD8⁺ LAA peptide-specific T cells with varying CD95 expression in 34 of 100 tested HIs, whereas CD4⁺ T cells responses were restricted to 5. Most frequent LAA peptide-specific T cell responses were directed against WT1 and PRAME peptides with a prevalence of 20 and 17%, respectively, showing the highest magnitude (0.16% ± 0.22% (mean ± SD)) for PRAME peptides. Cytotoxicity of PRAME peptide-specific T cells was demonstrated by specific killing of PRAME peptide-pulsed T2 cells. Furthermore, the proliferative capacity of PRAME peptide-specific T cells was confined to HIs responsive toward PRAME peptide challenge corroborating the accuracy of the screening results. In conclusion, we identified PRAME as a promising target antigen for adoptive leukemia therapy.

Enumeration of WT1-specific CD8+ T cells for clinical application using an MHC Streptamer based no-wash single-platform flow-cytometric assay

The advent of novel strategies to generate leukemia-associated-antigen (LAA)-specific T cells for adoptive immunotherapies creates a demand for standardized good laboratory practice (GLP)-compliant enumeration assays to provide a secure clinical environment-whether it is to identify potential donors, define therapeutic doses for transplantation, or monitor clinical success. Here, we introduce a no-wash assay based on single-platform cell enumeration and Streptamer staining to determine the Wilms' tumor antigen 1 (WT1)-specific T cell immunity in clinical samples. We analyzed the performance of the WT1-specific MHC Streptamers in direct comparison to CMV- and EBV-specific MHC Streptamer staining by spiking antigen-specific T cells in PBMCs. The accuracy of the assay was high for all performed experiments with a mean recovery of 94% and a linear regression of 0.988. Differences were apparent regarding the limit of detection/quantification (LOD/LOQ). While results obtained for WT1 yielded an LOD/LOQ of 0.08 ± 0.04% and 0.11 ± 0.06% (1.33 ± 0.32 cells/µl and 1.9 ± 0.14 cells/µl), the overall LOD/LOQ was notably lower and accounted to 0.02 ± 0.02% and 0.05 ± 0.03% (0.60 ± 0.03 cells/µl and 1.27 ± 0.58 cells/µl). Subsequent screening of 22 healthy individuals revealed significantly higher values for WT1 (0.04 ± 0.02% and 1.5 ± 0.9 cells/µl) than for the irrelevant HIV pol (0.016 ± 0.01% and 0.5 ± 0.4 cells/µl). In contrast, no increased frequencies were observed for WT1-specific T cells compared to HIV-specific T cells using a classical wash-protocol. These findings strongly suggest the use of no-wash single-platform assays in combination with MHC Streptamer staining for the detection of low affinity LAA-specific T cells due to its high accuracy and sensitivity. © 2017 International Society for Advancement of Cytometry.

Understanding CD8+ T-cell responses toward the native and alternate HLA-A*02:01-restricted WT1 epitope

The Wilms' tumor 1 (WT1) antigen is expressed in solid and hematological malignancies, but not healthy tissues, making it a promising target for cancer immunotherapies. Immunodominant WT1 epitopes, the native HLA-A2/WT1_126-134 (RMFPNAPYL) (HLA-A2/RMFPNAPYL epitope (WT1A)) and its modified variant YMFPNAPYL (HLA-A2/YMFPNAPYL epitope (WT1B)), can induce WT1-specific CD8⁺ T cells, although WT1B is more stably bound to HLA-A*02:01. Here, to further determine the benefits of those two targets, we assessed the naive precursor frequencies; immunogenicity and cross-reactivity of CD8⁺ T cells directed toward these two WT1 epitopes. Ex vivo naive WT1A- and WT1B-specific CD8⁺ T cells were detected in healthy HLA-A*02:01⁺ individuals with comparable precursor frequencies (1 in 10⁵–10⁶) to other naive CD8⁺ T-cell pools (for example, A2/HIV-Gag_77-85), but as expected, ~100 × lower than those found in memory populations (influenza, A2/M1_58-66; EBV, A2/BMLF1_280-288). Importantly, only WT1A-specific naive precursors were detected in HLA-A2.1 mice. To further assess the immunogenicity and recruitment of CD8⁺ T cells responding to WT1A and WT1B, we immunized HLA-A2.1 mice with either peptide. WT1A immunization elicited numerically higher CD8⁺ T-cell responses to the native tumor epitope following re-stimulation, although both regimens produced functionally similar responses toward WT1A via cytokine analysis and CD107a expression. Interestingly, however, WT1B immunization generated cross-reactive CD8⁺ T-cell responses to WT1A and could be further expanded by WT1A peptide revealing two distinct populations of single- and cross-reactive WT1A⁺CD8⁺ T cells with unique T-cell receptor-αβ gene signatures. Therefore, although both epitopes are immunogenic, the clinical benefits of WT1B vaccination remains debatable and perhaps both peptides may have separate clinical benefits as treatment targets.

Rationale for anti-CD137 cancer immunotherapy

The consideration of the complex interplay between the tumour microenvironment (TME) and the immune response is the key for designing effective immunotherapies. Therapeutic strategies that harness co-stimulatory receptors have recently gained momentum in the clinic. One such strategy with promising clinical applications is the targeting of CD137, a member of the tumour necrosis factor receptor superfamily. Its expression on both innate and adaptive immune cells, coupled with its unique ability to potentiate antitumour responses through modulating the TME and to ameliorate autoimmune responses, has established it as an appealing target. In this review, we will discuss the various CD137-targeted immunotherapeutics that have reached clinical development, with a focus on recent advances and novel modalities such as CD137 chimeric antigen receptors and CD137 bispecific antibodies. We will also highlight the effect of CD137 targeting on the TME and discuss the importance of probing TME changes for predicting and testing the efficacy of CD137-mediated immunotherapy.