Natural CD4+ T-cell responses against indoleamine 2,3-dioxygenase

Long-term follow-up of anti-PD-1 naïve patients with metastatic melanoma treated with IDO/PD-L1 targeting peptide vaccine and nivolumab

BACKGROUND

We have previously published initial efficacy of the indoleamine 2,3-dioxygenase (IDO)/anti-programmed death ligand 1 (PD-L1) vaccine in combination with nivolumab in 30 anti-PD-1 therapy naïve patients with metastatic melanoma (cohort A). We now report long-term follow-up of patients in cohort A. Further, we report results from cohort B, where the peptide vaccine was added to anti-PD-1 therapy for patients with progressive disease during anti-PD-1 treatment.

METHODS

All patients were treated with a therapeutic peptide vaccine in Montanide targeting IDO and PD-L1 combined with nivolumab (NCT03047928). A long-term follow-up of safety, response rates, and survival rates were performed in cohort A including patient subgroup analyses. Safety and clinical responses were analyzed for cohort B.

RESULTS

Cohort A: At data cut-off, January 5, 2023, the overall response rate (ORR) was 80%, and 50% of the 30 patients obtained a complete response (CR). The median progression-free survival (mPFS) was 25.5 months (95% CI 8.8 to 39), and median overall survival (mOS) was not reached (NR) (95% CI 36.4 to NR). The minimum follow-up time was 29.8 months, and the median follow-up was 45.3 months (IQR 34.8-59.2). A subgroup evaluation further revealed that cohort A patients with unfavorable baseline characteristics, including either PD-L1 negative tumors (n=13), elevated lactate dehydrogenase (LDH) levels (n=11), or M1c (n=17) obtained both favorable response rates and durable responses. The ORR was 61.5%, 79%, and 88% for patients with PD-L1^- tumors, elevated LDH, and M1c, respectively. The mPFS was 7.1 months for patients with PD-L1^- tumors, 30.9 months for patients with elevated LDH, and 27.9 months for M1c patients. Cohort B: At data cut-off, the best overall response was stable disease for 2 of the 10 evaluable patients. The mPFS was 2.4 months (95% CI 1.38 to 2.52), and the mOS was 16.7 months (95% CI 4.13 to NR).

CONCLUSION

This long-term follow-up confirms the promising and durable responses in cohort A. Subgroup analyses of patients with unfavorable baseline characteristics revealed that high response rates and survival rates were also found in patients with either PD-L1 negative tumors, elevated LDH levels, or M1c. No meaningful clinical effect was demonstrated in cohort B patients.

TRIAL REGISTRATION NUMBER

NCT03047928.

IDO1 Protein Is Expressed in Diagnostic Biopsies from Both Follicular and Transformed Follicular Patients

Follicular lymphoma (FL) is a lymphoid neoplasia characterized by an indolent clinical nature. Despite generally favorable prognoses, early progression and histological transformation (HT) to a more aggressive lymphoma histology remain the leading causes of death among FL patients. To provide a basis for possible novel treatment options, we set out to evaluate the expression levels of indoleamine 2,3-dioxygenase 1 (IDO1), an immunoinhibitory checkpoint molecule, in follicular and transformed follicular biopsies. The expression levels of IDO1 were assessed using immunohistochemical staining and digital image analysis in lymphoma biopsies from 33 FL patients without subsequent HT (non-transforming FL, nt-FL) and 20 patients with subsequent HT (subsequently transforming FL, st-FL) as well as in paired high-grade biopsies from the time of HT (transformed FL, tFL). Despite no statistical difference in IDO1 expression levels seen between the groups, all diagnostic and transformed lymphomas exhibited positive expression, indicating its possible role in novel treatment regimens. In addition, IDO1 expression revealed a positive correlation with another immune checkpoint inhibitor, namely programmed death 1 (PD-1). In summary, we report IDO1 expression in all cases of FL and tFL, which provides the grounds for future investigations of anti-IDO1 therapy as a possible treatment for FL patients.

Pre-existing TGF-β-specific T-cell immunity in patients with pancreatic cancer predicts survival after checkpoint inhibitors combined with radiotherapy

BACKGROUND

Circulating transforming growth factor-β (TGF-β)-specific T cells that recognize TGF-β-expressing immune regulatory cells have been described in patients with cancer. TGF-β-derived peptide vaccination modulates the tumor microenvironment and has shown clinical effects in animal models of pancreatic cancer (PC). TGF-β-expressing regulatory cells are especially elevated in PC and may prevent the clinical response to immune checkpoint inhibitors (ICIs). Thus, in the present study we investigated the significance of TGF-β-specific T-cell immunity in patients with PC treated with ICI combined with radiotherapy in a randomized phase 2 study (CheckPAC).

METHODS

Immune responses to a TGF-β-derived epitope entitled TGF-β-15 as well as epitopes from Clostridium tetani (tetanus) and influenza were measured in peripheral blood mononuclear cells (PBMCs) with interferon-ɣ enzyme-linked immunospot assays. PBMCs were isolated before and after treatment. Correlations between immune response data and clinical data were evaluated with parametric and non-parametric statistical methods. Survival was analyzed with univariate and multivariate Cox-regression. TGF-β-15 specific T cells were isolated and expanded and examined for recognition of autologous regulatory immune cells by flow cytometry.

RESULTS

PBMCs from 32 patients were analyzed for immune responses to the TGF-β-derived epitope entitled TGF-β-15. Patients with a strong TGF-β-specific immune response at treatment initiation had longer progression-free and overall survival, compared with patients with a weak or no TGF-β-specific immune response. This remained significant in multivariate analysis. Patients with weak and strong TGF-β-specific responses displayed similar responses towards viral antigens. Furthermore, we show that TGF-β-specific T cells from a clinical responder specifically reacted to and lysed autologous, regulatory immune cells. Finally, mimicking a TGF-β-15 vaccination, we showed that repeated stimulations with the TGF-β-15 epitope in vitro enhanced the immune response to TGF-β-15.

CONCLUSION

A strong TGF-β-15 specific immune response was associated with clinical benefit and improved survival after ICI/radiotherapy for patients with PC. Importantly, the lack of TGF-β-specific T cells in some patients was not caused by a general immune dysfunction. TGF-β-specific T cells recognized regulatory immune cells and could be introduced in vitro in patients without spontaneous responses. Taken together, our data suggest that combining TGF-β-based vaccination with ICI/radiotherapy will be beneficial for patients with PC.

N6-methyladenosine in hematological malignancies: a concise review

PURPOSE OF REVIEW

Hematological malignancies are a kind of systemic cancers mostly related to abnormal differentiation of blood stem cells. Because of the poor prognosis, chemotherapy resistance and common recurrence, new mechanisms and treatment therapies are looking forward to be discovered.

RECENT FINDINGS

Over the years, epigenetic abnormalities have been known to act a key part in occurrence and development of hematological tumors. In the internal modifications on long noncoding eukaryotic mRNA, there is a common type called N6-methyladenosine that can change the expression of target genes and participate in the translation, degradation and splicing of mRNA. M6A is related to a wealth of cancers, such as HNRNPA2B1's elevation in multiple myeloma, METTLE3's elevation in acute myeloid leukemia and lung cancer. Immune cells, playing a significant role in hematological cancers, can also be regulated by m6A.

SUMMARY

In the review, we summarized the recent progress on hematological malignancies associating with m6A and immune cells, which may offer a new road for the treatment of them.

Tumor microenvironment antigens

The identification and characterization of tumor antigens are central objectives in developing anti-cancer immunotherapy. Traditionally, tumor-associated antigens (TAAs) are considered relatively restricted to tumor cells (i.e., overexpressed proteins in tumor cells), whereas tumor-specific antigens (TSAs) are considered unique to tumor cells. Recent studies have focused on identifying patient-specific neoantigens, which might be highly immunogenic because they are not expressed in normal tissues. The opposite strategy has emerged with the discovery of anti-regulatory T cells (anti-Tregs) that recognize and attack many cell types in the tumor microenvironment, such as regulatory immune cells, in addition to tumor cells. The term proposed in this review is "tumor microenvironment antigens" (TMAs) to describe the antigens that draw this attack. As therapeutic targets, TMAs offer several advantages that differentiate them from more traditional tumor antigens. Targeting TMAs leads not only to a direct attack on tumor cells but also to modulation of the tumor microenvironment, rendering it immunocompetent and tumor-hostile. Of note, in contrast to TAAs and TSAs, TMAs also are expressed in non-transformed cells with consistent human leukocyte antigen (HLA) expression. Inflammation often induces HLA expression in malignant cells, so that targeting TMAs could additionally affect tumors with no or very low levels of surface HLA expression. This review defines the characteristics, differences, and advantages of TMAs compared with traditional tumor antigens and discusses the use of these antigens in immune modulatory vaccines as an attractive approach to immunotherapy. Different TMAs are expressed by different cells and could be combined in anti-cancer immunotherapies to attack tumor cells directly and modulate local immune cells to create a tumor-hostile microenvironment and inhibit tumor angiogenesis. Immune modulatory vaccines offer an approach for combinatorial therapy with additional immunotherapy including checkpoint blockade, cellular therapy, or traditional cancer vaccines. These combinations would increase the number of patients who can benefit from such therapeutic measures, which all have optimal efficiency in inflamed tumors.

CCL22-based peptide vaccines induce anti-cancer immunity by modulating tumor microenvironment

CCL22 is a macrophage-derived immunosuppressive chemokine that recruits regulatory T cells through the CCL22:CCR4 axis. CCL22 was shown to play a key role in suppressing anti-cancer immune responses in different cancer types. Recently, we showed that CCL22-specific T cells generated from cancer patients could kill CCL22-expressing tumor cells and directly influence the levels of CCL22 in vitro. The present study aimed to provide a rationale for developing a CCL22-targeting immunotherapy. Vaccination with CCL22-derived peptides induced CCL22-specific T-cell responses in both BALB/c and C57BL/6 mice, assessed by interferon-γ secretion ex vivo. Anti-tumor efficacy of the peptides was evaluated in mouse models engrafted with syngeneic tumor models showing a reduced tumor growth and prolonged survival of the treated mice. Vaccination induced changes in the cellular composition of immune cells that infiltrated the tumor microenvironment assessed with multicolor flow cytometry. In particular, the infiltration of CD8⁺ cells and M1 macrophages increased, which increased the CD8/Treg and the M1/M2 macrophage ratio. This study provided preclinical evidence that targeting CCL22 with CCL22 peptide vaccines modulated the immune milieu in the tumor microenvironment. This modulation led to an augmentation of anti-tumor responses. This study provided a rationale for developing a novel immunotherapeutic modality in cancer.

The immunosuppressive role of indoleamine 2, 3-dioxygenase in glioblastoma: mechanism of action and immunotherapeutic strategies

Glioblastoma multiforme (GBM) is a fatal brain tumor in adults with a bleak diagnosis. Expansion of immunosuppressive and malignant CD4 + FoxP3 + GITR + regulatory T cells is one of the hallmarks of GBM. Importantly, most of the patients with GBM expresses the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO). While IDO1 is generally not expressed at appreciable levels in the adult central nervous system, it is rapidly stimulated and highly expressed in response to ongoing immune surveillance in cancer. Increased levels of immune surveillance in cancer are thus related to higher intratumoral IDO expression levels and, as a result, a worse OS in GBM patients. Conversion of the important amino acid tryptophan into downstream catabolite known as kynurenines is the major function of IDO. Decreasing tryptophan and increasing the concentration of immunomodulatory tryptophan metabolites has been shown to induce T-cell apoptosis, increase immunosuppressive programming, and death of tumor antigen-presenting dendritic cells. This observation supported the immunotherapeutic strategy, and the targeted molecular therapy that suppresses IDO1 activity. We review the current understanding of the role of IDO1 in tumor immunological escape in brain tumors, the immunomodulatory effects of its primary catabolites, preclinical research targeting this enzymatic pathway, and various issues that need to be overcome to increase the prospective immunotherapeutic relevance in the treatment of GBM malignancy.

IDO Vaccine Ablates Immune-Suppressive Myeloid Populations and Enhances Antitumor Effects Independent of Tumor Cell IDO Status

The immunosuppressive tumor microenvironment (TME) does not allow generation and expansion of antitumor effector cells. One of the potent immunosuppressive factors present in the TME is the indoleamine-pyrrole 2,3-dioxygenase (IDO) enzyme, produced mainly by cancer cells and suppressive immune cells of myeloid origin. In fact, IDO+ myeloid-derived suppressor cells (MDSC) and dendritic cells (DC) tend to be more suppressive than their IDO- counterparts. Hence, therapeutic approaches that would target the IDO+ cells in the TME, while sparing the antigen-presenting functions of IDO- myeloid populations, are needed. Using an IDO-specific peptide vaccine (IDO vaccine), we explored the possibility of generating effector cells against IDO and non-IDO tumor-derived antigens. For this, IDO-secreting (B16F10 melanoma) and non-IDO-secreting (TC-1) mouse tumor models were employed. We showed that the IDO vaccine significantly reduced tumor growth and enhanced survival of mice in both the tumor models, which associated with a robust induction of IDO-specific effector cells in the TME. The IDO vaccine significantly enhanced the antitumor efficacy of non-IDO tumor antigen-specific vaccines, leading to an increase in the number of total and antigen-specific activated CD8+ T cells (IFNγ+ and granzyme B+). Treatment with the IDO vaccine significantly reduced the numbers of IDO+ MDSCs and DCs, and immunosuppressive regulatory T cells in both tumor models, resulting in enhanced therapeutic ratios. Together, we showed that vaccination against IDO is a promising therapeutic option for both IDO-producing and non-IDO-producing tumors. The IDO vaccine selectively ablates the IDO+ compartment in the TME, leading to a significant enhancement of the immune responses against other tumor antigen-specific vaccines.

Peptide vaccination activating Galectin-3-specific T cells offers a novel means to target Galectin-3-expressing cells in the tumor microenvironment

Galectin-3 (Gal3) can be expressed by many cells in the tumor microenvironment (TME), including cancer cells, cancer-associated fibroblasts, tumor-associated macrophages, and regulatory T cells (Tregs). In addition to immunosuppression, Gal3 expression has been connected to malignant cell transformation, tumor progression, and metastasis. In the present study, we found spontaneous T-cell responses against Gal3-derived peptides in PBMCs from both healthy donors and cancer patients. We isolated and expanded these Gal3-specific T cells in vitro and showed that they could directly recognize target cells that expressed Gal3. Finally, therapeutic vaccination with a long Gal3-derived peptide epitope, which induced the expansion of Gal3-specific CD8⁺ T cells in vivo, showed a significant tumor-growth delay in mice inoculated with EO771.LMB metastatic mammary tumor cells. This was associated with a significantly lower percentage of both Tregs and tumor-infiltrating Gal3⁺ cells in the non-myeloid CD45⁺CD11b⁻ compartment and with an alteration of the T-cell memory populations in the spleens of Gal3-vaccinated mice. These results suggest that by activating Gal3-specific T cells by an immune-modulatory vaccination, we can target Gal3-producing cells in the TME, and thereby induce a more immune permissive TME. This indicates that Gal3 could be a novel target for therapeutic cancer vaccines.

Arginase 1-Based Immune Modulatory Vaccines Induce Anticancer Immunity and Synergize with Anti-PD-1 Checkpoint Blockade

Expression of the L-arginine catabolizing enzyme arginase 1 (ARG1) is a central immunosuppressive mechanism mediated by tumor-educated myeloid cells. Increased activity of ARG1 promotes the formation of an immunosuppressive microenvironment and leads to a more aggressive phenotype in many cancers. Intrinsic T-cell immunity against ARG1-derived epitopes in the peripheral blood of cancer patients and healthy subjects has previously been demonstrated. To evaluate the antitumor efficacy of ARG1-derived peptide vaccines as a monotherapy and as a combinational therapy with checkpoint blockade, different in vivo syngeneic mouse tumor models were utilized. To evaluate the antitumor effects, flow cytometry analysis and IHC were performed on tumors, and ELISPOT assays were performed to characterize immune responses. We show that ARG1-targeting therapeutic vaccines were able to activate endogenous antitumor immunity in several in vivo syngeneic mouse tumor models and to modulate the cell composition of the tumor microenvironment without causing any associated side effects or systemic toxicity. ARG1-targeting vaccines in combination with anti-PD-1 also resulted in increased T-cell infiltration, decreased ARG1 expression, reduced suppressive function of tumor-educated myeloid cells, and a shift in the M1/M2 ratio of tumor-infiltrating macrophages. These results indicated that the induced shift toward a more proinflammatory microenvironment by ARG1-targeting immunotherapy favors effective tumor control when combined with anti-PD-1 checkpoint blockade. Our data illustrate the ability of ARG1-based immune modulatory vaccination to elicit antigen-specific immunosurveillance and imply the feasibility of this novel immunotherapeutic approach for clinical translation.

The Kynurenine Pathway-New Linkage between Innate and Adaptive Immunity in Autoimmune Endocrinopathies

The kynurenine pathway (KP) is highly regulated in the immune system, where it promotes immunosuppression in response to infection or inflammation. Indoleamine 2,3-dioxygenase 1 (IDO1), the main enzyme of KP, has a broad spectrum of activity on immune cells regulation, controlling the balance between stimulation and suppression of the immune system at sites of local inflammation, relevant to a wide range of autoimmune and inflammatory diseases. Various autoimmune diseases, among them endocrinopathies, have been identified to date, but despite significant progress in their diagnosis and treatment, they are still associated with significant complications, morbidity, and mortality. The precise cellular and molecular mechanisms leading to the onset and development of autoimmune disease remain poorly clarified so far. In breaking of tolerance, the cells of the innate immunity provide a decisive microenvironment that regulates immune cells' differentiation, leading to activation of adaptive immunity. The current review provided a comprehensive presentation of the known role of IDO1 and KP activation in the regulation of the innate and adaptive arms of the immune system. Significant attention has been paid to the immunoregulatory role of IDO1 in the most prevalent, organ-specific autoimmune endocrinopathies-type 1 diabetes mellitus (T1DM) and autoimmune thyroiditis.

Therapeutic Cancer Vaccination With a Peptide Derived From the Calreticulin Exon 9 Mutations Induces Strong Cellular Immune Responses in Patients With CALR-Mutant Chronic Myeloproliferative Neoplasms

Background

The calreticulin (CALR) exon 9 mutations that are identified in 20% of patients with Philadelphia chromosome negative chronic myeloproliferative neoplasms (MPN) generate immunogenic antigens. Thus, therapeutic cancer vaccination against mutant CALR could be a new treatment modality in CALR-mutant MPN.

Methods

The safety and efficacy of vaccination with the peptide CALRLong36 derived from the CALR exon 9 mutations was tested in a phase I clinical vaccination trial with montanide as adjuvant. Ten patients with CALRmut MPN were included in the trial and received 15 vaccines over the course of one year. The primary end point was evaluation of safety and toxicity of the vaccine. Secondary endpoint was assessment of the immune response to the vaccination epitope (www.clinicaltrials.gov identifier NCT03566446).

Results

Patients had a median age of 59.5 years and a median disease duration of 6.5 years. All patients received the intended 15 vaccines, and the vaccines were deemed safe and tolerable as only two grade three AE were detected, and none of these were considered to be related to the vaccine. A decline in platelet counts relative to the platelets counts at baseline was detected during the first 100 days, however this did not translate into neither a clinical nor a molecular response in any of the patients. Immunomonitoring revealed that four of 10 patients had an in vitro interferon (IFN)-γ ELISPOT response to the CALRLong36 peptide at baseline, and four additional patients displayed a response in ELISPOT upon receiving three or more vaccines. The amplitude of the immune response increased during the entire vaccination schedule for patients with essential thrombocythemia. In contrast, the immune response in patients with primary myelofibrosis did not increase after three vaccines.

Conclusion

Therapeutic cancer vaccination with peptide vaccines derived from mutant CALR with montanide as an adjuvant, is safe and tolerable. The vaccines did not induce any clinical responses. However, the majority of patients displayed a marked T-cell response to the vaccine upon completion of the trial. This suggests that vaccines directed against mutant CALR may be used with other cancer therapeutic modalities to enhance the anti-tumor immune response.

Characterization of T-cell responses against IκBα in cancer patients

The nuclear factor κ light chain enhancer of activated B cells (NFκB) is constitutively active in most cancers, controlling multiple cellular processes including proliferation, invasion and resistance to therapy. NFκB is primarily regulated through the association with inhibitory proteins that are known as inhibitors of NFκB (IκBs). Increased NFκB activity in tumor cells has been correlated with decrease stability of IκB proteins, in particular IκBα. In responso to a large number of stimuli, IκB proteins are degraded by the proteasome. Cytotoxic T lymphocytes (CTLs) recognize HLA-restricted antigenic peptides that are generated by proteasomal degradation in target cells. In the present study, we demonstrate the presence of naturally occurring IκBα -specific T cells in the peripheral blood of patients suffering from several unrelated tumor types, i.e., breast cancer, malignant melanoma and renal cell carcinoma, but not of healthy controls. Furthermore, we show that such IBα-specific T cells are granzyme B-releasing, cytotoxic cells. Hence, the increased proteasomal degradation of IκBα in cancer induces IκBα-specific CTLs.

A phase 1/2 trial of an immune-modulatory vaccine against IDO/PD-L1 in combination with nivolumab in metastatic melanoma

Anti-programmed death (PD)-1 (aPD1) therapy is an effective treatment for metastatic melanoma (MM); however, over 50% of patients progress due to resistance. We tested a first-in-class immune-modulatory vaccine (IO102/IO103) against indoleamine 2,3-dioxygenase (IDO) and PD ligand 1 (PD-L1), targeting immunosuppressive cells and tumor cells expressing IDO and/or PD-L1 (IDO/PD-L1), combined with nivolumab. Thirty aPD1 therapy-naive patients with MM were treated in a phase 1/2 study ( https://clinicaltrials.gov/ , NCT03047928). The primary endpoint was feasibility and safety; the systemic toxicity profile was comparable to that of nivolumab monotherapy. Secondary endpoints were efficacy and immunogenicity; an objective response rate (ORR) of 80% (confidence interval (CI), 62.7-90.5%) was reached, with 43% (CI, 27.4-60.8%) complete responses. After a median follow-up of 22.9 months, the median progression-free survival (PFS) was 26 months (CI, 15.4-69 months). Median overall survival (OS) was not reached. Vaccine-specific responses assessed in vitro were detected in the blood of >93% of patients during vaccination. Vaccine-reactive T cells comprised CD4⁺ and CD8⁺ T cells with activity against IDO- and PD-L1-expressing cancer and immune cells. T cell influx of peripherally expanded T cells into tumor sites was observed in responding patients, and general enrichment of IDO- and PD-L1-specific clones after treatment was documented. These clinical efficacy and favorable safety data support further validation in a larger randomized trial to confirm the clinical potential of this immunomodulating approach.

The Changing Landscape of Therapeutic Cancer Vaccines-Novel Platforms and Neoantigen Identification

Therapeutic cancer vaccines, an exciting development in cancer immunotherapy, share the goal of creating and amplifying tumor-specific T-cell responses, but significant obstacles still remain to their success. Here, we briefly outline the principles underlying cancer vaccine therapy with a focus on novel vaccine platforms and antigens, underscoring the renewed optimism. Numerous strategies have been investigated to overcome immunosuppressive mechanisms of the tumor microenvironment (TME) and counteract tumor escape, including improving antigen selection, refining delivery platforms, and use of combination therapies. Several new cancer vaccine platforms and antigen targets are under development. In an effort to amplify tumor-specific T-cell responses, a heterologous prime-boost antigen delivery strategy is increasingly used for virus-based vaccines. Viruses have also been engineered to express targeted antigens and immunomodulatory molecules simultaneously, to favorably modify the TME. Nanoparticle systems have shown promise as delivery vectors for cancer vaccines in preclinical research. T-win is another platform targeting both tumor cells and the TME, using peptide-based vaccines that engage and activate T cells to target immunoregulatory molecules expressed on immunosuppressive and malignant cells. With the availability of next-generation sequencing, algorithms for neoantigen selection are emerging, and several bioinformatic platforms are available to select therapeutically relevant neoantigen targets for developing personalized therapies. However, more research is needed before the use of neoepitope prediction and personalized immunotherapy becomes commonplace. Taken together, the field of therapeutic cancer vaccines is fast evolving, with the promise of potential synergy with existing immunotherapies for long-term cancer treatment.

Cancer Immune Therapy for Philadelphia Chromosome-Negative Chronic Myeloproliferative Neoplasms

Philadelphia chromosome-negative chronic myeloproliferative neoplasms (MPN) are neoplastic diseases of the hematopoietic stem cells in the bone marrow. MPN are characterized by chronic inflammation and immune dysregulation. Of interest, the potent immunostimulatory cytokine interferon-α has been used to treat MPN for decades. A deeper understanding of the anti-cancer immune response and of the different immune regulatory mechanisms in patients with MPN has paved the way for an increased perception of the potential of cancer immunotherapy in MPN. Therapeutic vaccination targeting the driver mutations in MPN is one recently described potential new treatment modality. Furthermore, T cells can directly react against regulatory immune cells because they recognize proteins like arginase and programmed death ligand 1 (PD-L1). Therapeutic vaccination with arginase or PD-L1 therefore offers a novel way to directly affect immune inhibitory pathways, potentially altering tolerance to tumor antigens like mutant CALR and mutant JAK2. Other therapeutic options that could be used in concert with therapeutic cancer vaccines are immune checkpoint-blocking antibodies and interferon-α. For more advanced MPN, adoptive cellular therapy is a potential option that needs more preclinical investigation. In this review, we summarize current knowledge about the immune system in MPN and discuss the many opportunities for anti-cancer immunotherapy in patients with MPN.

Telomerase and CD4 T Cell Immunity in Cancer

Telomerase reverse transcriptase (TERT) is a conserved self-tumor antigen which is overexpressed in most tumors and plays a critical role in tumor formation and progression. As such, TERT is an antigen of great relevance to develop widely applicable immunotherapies. CD4 T cells play a major role in the anti-cancer response alone or with other effector cells such as CD8 T cells and NK cells. To date, efforts have been made to identify TERT peptides capable of stimulating CD4 T cells that are also able to bind diverse MHC-II alleles to ease immune status monitoring and immunotherapies. Here, we review the current status of TERT biology, TERT/MHC-II immunobiology, and past and current vaccine clinical trials. We propose that monitoring CD4 T cell immunity against TERT is a simple and direct way to assess immune surveillance in cancer patients and a new way to predict the response to immune checkpoint inhibitors (ICPi). Finally, we present the initial results of a systematic discovery of TERT peptides able to bind the most common HLA Class II alleles worldwide and show that the repertoire of MHC-II TERT peptides is wider than currently appreciated.

The metabolic enzyme arginase-2 is a potential target for novel immune modulatory vaccines

One way that tumors evade immune destruction is through tumor and stromal cell expression of arginine-degrading enzyme arginase-2 (ARG2). Here we describe the existence of pro-inflammatory effector T-cells that recognize ARG2 and can directly target tumor and tumor-infiltrating cells. Using a library of 34 peptides covering the entire ARG2 sequence, we examined reactivity toward these peptides in peripheral blood mononuclear cells from cancer patients and healthy individuals. Interferon-γ ELISPOT revealed frequent immune responses against several of the peptides, indicating that ARG2–specific self-reactive T-cells are natural components of the human T-cell repertoire. Based on this, the most immunogenic ARG2 protein region was further characterized. By identifying conditions in the microenvironment that induce ARG2 expression in myeloid cells, we showed that ARG2-specific CD4T-cells isolated and expanded from a peripheral pool from a prostate cancer patient could recognize target cells in an ARG2-dependent manner. In the ‘cold’ in vivo tumor model Lewis lung carcinoma, we found that activation of ARG2-specific T-cells by vaccination significantly inhibited tumor growth. Immune-modulatory vaccines targeting ARG2 thus are a candidate strategy for cancer immunotherapy.

Cancer immune therapy for myeloid malignancies: present and future

The myelodysplastic syndromes, the chronic myeloproliferative neoplasms, and the acute myeloid leukemia are malignancies of the myeloid hematopoietic stem cells of the bone marrow. The diseases are characterized by a dysregulation of the immune system as both the cytokine milieu, immune phenotype, immune regulation, and expression of genes related to immune cell functions are deregulated. Several treatment strategies try to circumvent this deregulation, and several clinical and preclinical trials have shown promising results, albeit not in the same scale as chimeric antigen receptor T cells have had in the treatment of refractory lymphoid malignancies. The use of immune checkpoint blocking antibodies especially in combination with hypomethylating agents has had some success-a success that will likely be enhanced by therapeutic cancer vaccination with tumor-specific antigens. In the chronic myeloproliferative neoplasms, the recent identification of immune responses against the Januskinase-2 and calreticulin exon 9 driver mutations could also be used in the vaccination setting to enhance the anti-tumor immune response. This immune response could probably be enhanced by the concurrent use of immune checkpoint inhibitors or by vaccination with epitopes from immune regulatory proteins such as arginase-1 and programmed death ligand-1. Herein, we provide an overview of current cancer immune therapeutic treatment strategies as well as potential future cancer immune therapeutic treatment options for the myeloid malignancies.

The T-win® technology: immune-modulating vaccines

The T-win® technology is an innovative investigational approach designed to activate the body's endogenous anti-regulatory T cells (anti-Tregs) to target regulatory as well as malignant cells. Anti-Tregs are naturally occurring T cells that can directly react against regulatory immune cells because they recognize proteins that these targets express, including indoleamine 2,3-dioxygenase (IDO), tryptophan 2,6-dioxygenase, arginase, and programmed death ligand 1 (PD-L1). The T-win® technology is characterized by therapeutic vaccination with long peptide epitopes derived from these antigens and therefore offers a novel way to target genetically stable cells with regular human leukocyte antigen expression in the tumor microenvironment. The T-win® technology thus also represents a novel way to attract pro-inflammatory cells to the tumor microenvironment where they can directly affect immune inhibitory pathways, potentially altering tolerance to tumor antigens. The modification of an immune regulatory environment into a pro-inflammatory milieu potentiates effective anti-tumor T cell responses. Many regulatory immune cells may be reverted into effector cells given the right stimulus. Because T-win® technology is based on the immune-modulatory function of the vaccines, the vaccines activate both CD4 and CD8 anti-Tregs. Of importance, in clinical trials, vaccinations against IDO or PD-L1 to potentiate anti-Tregs have so far proved to be safe, with minimal toxicity.

Cytomegalovirus induces HLA-class-II-restricted alloreactivity in an acute myeloid leukemia cell line

Cytomegalovirus (HCMV) reactivation is found frequently after allogeneic hematopoietic stem cell transplantation (alloSCT) and is associated with an increased treatment-related mortality. Recent reports suggest a link between HCMV and a reduced risk of cancer progression in patients with acute leukemia or lymphoma after alloSCT. Here we show that HCMV can inhibit the proliferation of the acute myeloid leukemia cell line Kasumi-1 and the promyeloid leukemia cell line NB4. HCMV induced a significant up-regulation of HLA-class-II-molecules, especially HLA-DR expression and an increase of apoptosis, granzyme B, perforin and IFN-γ secretion in Kasumi-1 cells cocultured with peripheral blood mononuclear cells (PBMCs). Indolamin-2,3-dioxygenase on the other hand led only to a significant dose-dependent effect on IFN-γ secretion without effects on proliferation. The addition of CpG-rich oligonucleotides and ganciclovir reversed those antiproliferative effects. We conclude that HCMV can enhance alloreactivity of PBMCs against Kasumi-1 and NB4 cells in vitro. To determine if this phenomenon may be clinically relevant further investigations will be required.

Frequent adaptive immune responses against arginase-1

The enzyme arginase-1 reduces the availability of arginine to tumor-infiltrating immune cells, thus reducing T-cell functionality in the tumor milieu. Arginase-1 is expressed by some cancer cells and by immune inhibitory cells, such as myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs), and its expression is associated with poor prognosis. In the present study, we divided the arginase-1 protein sequence into overlapping 20-amino-acid-long peptides, generating a library of 31 peptides covering the whole arginase-1 sequence. Reactivity towards this peptide library was examined in PBMCs from cancer patients and healthy individuals. IFNγ ELISPOT revealed frequent immune responses against multiple arginase-1-derived peptides. We further identified a hot-spot region within the arginase-1 protein sequence containing multiple epitopes recognized by T cells. Next, we examined in vitro-expanded tumor-infiltrating lymphocytes (TILs) isolated from melanoma patients, and detected arginase-1-specific T cells that reacted against epitopes from the hot-spot region. Arginase-1-specific CD4+T cells could be isolated and expanded from peripheral T cell pool of a patient with melanoma, and further demonstrated the specificity and reactivity of these T cells. Overall, we showed that arginase-1-specific T cells were capable of recognizing arginase-1-expressing cells. The activation of arginase-1-specific T cells by vaccination is an attractive approach to target arginase-1-expressing malignant cells and inhibitory immune cells. In the clinical setting, the induction of arginase-1-specific immune responses could induce or increase Th1 inflammation at the sites of tumors that are otherwise excluded due to infiltration with MDSCs and TAMs.

Indoleamine 2,3-dioxygenase and survivin peptide vaccine combined with temozolomide in metastatic melanoma

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO) and survivin have been identified as potential targets for cancer vaccination. In this phase II study a vaccine using the peptides Sur1M2 and IDO5 was combined with the chemotherapy temozolomide (TMZ) for treatment of metastatic melanoma patients. The aim was to simultaneously target several immune inhibiting mechanisms and the highly malignant cells expressing survivin.

METHODS

HLA-A2 positive patients with advanced malignant melanoma were treated biweekly with 150 mg/m² TMZ daily for 7 days followed by subcutaneous vaccination with 250 µg of each peptide in 500 µL Montanide solution at day 8. Granulocyte-macrophage colony-stimulating factor was used as an adjuvant and topical imiquimod was applied prior to vaccination. Treatment was continued until disease progression. Clinical response was evaluated by PET-CT and immunological outcome was assessed by ELISPOT and flow cytometry.

RESULTS

In total, 17 patients were treated with a clinical benefit rate of 18% including one patient with partial tumor regression. Immune analyses revealed a vaccine specific response in 8 (67%) of 12 patients tested, a significant decrease in the frequency of CD4+ T-cells during treatment, a tendency towards decreasing frequencies of naïve CD4+ and CD8+ T-cells, and increasing frequencies of memory CD4+ and CD8+ T-cells.

CONCLUSIONS

These results demonstrate that vaccine-induced immunity towards survivin and IDO-derived peptides can be achieved in combination with TMZ in patients mainly suffering from grade M1c melanoma including patients with brain metastases. A significant clinical activity could not be proven in this small study and a larger setup is needed to properly assess clinical efficacy.

Safety, immune and clinical responses in metastatic melanoma patients vaccinated with a long peptide derived from indoleamine 2,3-dioxygenase in combination with ipilimumab

BACKGROUND AIM

Indoleamine 2,3-dioxygenase (IDO) is an emerging new target in cancer therapy that can be targeted with active immunotherapy (e.g. through peptide vaccination). Furthermore, IDO has been identified as a key mechanism underlying resistance to treatment with the checkpoint blocking antibody ipilimumab (ipi).

METHODS

Ten patients with metastatic melanoma participated in a phase I first-in-human clinical study assessing safety of combining ipi with a 21-mer synthetic peptide vaccine from IDO denoted IDOlong. Secondary and tertiary end points included vaccine and clinical response.

RESULTS

Treatment was generally safe and well tolerated. Vaccine related adverse reactions included grade I and II erythema, oedema and pruritus at the vaccination site, which were manageable with mild topical corticosteroids. One patient developed presumed ipi-induced colitis. It initially responded to high-dose parenteral corticosteroids but later relapsed while the patient was admitted to a local hospital, where he died after receiving suboptimal therapy. Vaccine-specific T-cell responses were detectable ex vivo in three patients. At first evaluation, five of the 10 treated patients were in stable disease, one of whom had an unconfirmed partial response.

CONCLUSIONS

Treatment with IDOlong synthetic peptide vaccine in combination with ipi was generally safe and without augmented toxicity. The vaccine induced readily detectable T-cell responses in a subset of patients. Treatment showed signs of clinical activity, although not exceeding efficacy of ipi alone. Results should be confirmed in a larger study.

Anti-regulatory T cells

Our initial understanding of immune-regulatory cells was based on the discovery of suppressor cells that assure peripheral T-cell tolerance and promote immune homeostasis. Research has particularly focused on the importance of regulatory T cells (Tregs) for immune modulation, e.g. directing host responses to tumours or inhibiting autoimmunity development. However, recent studies report the discovery of self-reactive pro-inflammatory T cells-termed anti-regulatory T cells (anti-Tregs)-that target immune-suppressive cells. Thus, regulatory cells can now be defined as both cells that suppress immune reactions as well as effector cells that counteract the effects of suppressor cells and support immune reactions. Self-reactive anti-Tregs have been described that specifically recognize human leukocyte antigen-restricted epitopes derived from proteins that are normally expressed by regulatory immune cells, including indoleamine 2,3-dioxygenase (IDO), tryptophan 2,6-dioxygenase (TDO), programmed death-ligand 1 (PD-L1), and forkhead box P3 (Foxp3). These proteins are highly expressed in professional antigen-presenting cells under various physiological conditions, such as inflammation and stress. Therefore, self-reactive T cells that recognize such targets may be activated due to the strong activation signal given by their cognate targets. The current review describes the existing knowledge regarding these self-reactive anti-Tregs, providing examples of antigen-specific anti-Tregs and discussing their possible roles in immune homeostasis and their potential future clinical applications.

Novel understanding of self-reactive T cells

In a recent issue of Immunity, Mark Davis and colleagues describe that thymic selection does not eliminate but prunes self-reactive T cell clones. Self-reactive T cells are a natural part of the T-cell repertoire and may be important in the fight against pathogens in addition to being important immune regulators.

Potential roles of self-reactive T cells in autoimmunity: lessons from cancer immunology

The immune system is a complex arrangement of cells and molecules that preserve the integrity of the organism by eliminating all elements judged to be dangerous. Several regulatory mechanisms function to terminate immune responses to antigens, return the immune system to a basal state after the antigen has been cleared, and maintain unresponsiveness, or tolerance, to self-antigens. In recent years, reports have described T cell responses to several proteins involved in regulating the immune system, particularly under malignant conditions. The present review highlights specific T cells that recognize proteins involved in three, well-defined immunosuppressive mechanisms: (1) inhibitory T cell pathways (i.e., PD-L1), (2) regulatory T cells (i.e., Foxp3(+)), and (3) metabolic enzymes, like indoleamine-2,3-dioxygenase. Cytotoxic T cells can eliminate regulatory cells, thereby suppressing and/or delaying local immune suppression; conversely, regulatory CD4(+) and non-cytotoxic CD8(+) T cells enhance target-mediated immune suppression. The apparent lack of tolerance against endogenous proteins expressed by regulatory cells is intriguing, because it suggests that self-reactive T cells play a general role of fine-tuning the immune system. Thus, T cell responses may be generally used to maintain the homeostasis of the immune system. Further exploration is warranted to investigate the potential role of auto-reactive T cells under different physiological and/or pathological conditions.

Role of indoleamine 2,3-dioxygenase in health and disease

IDO1 (indoleamine 2,3-dioxygenase 1) is a member of a unique class of mammalian haem dioxygenases that catalyse the oxidative catabolism of the least-abundant essential amino acid, L-Trp (L-tryptophan), along the kynurenine pathway. Significant increases in knowledge have been recently gained with respect to understanding the fundamental biochemistry of IDO1 including its catalytic reaction mechanism, the scope of enzyme reactions it catalyses, the biochemical mechanisms controlling IDO1 expression and enzyme activity, and the discovery of enzyme inhibitors. Major advances in understanding the roles of IDO1 in physiology and disease have also been realised. IDO1 is recognised as a prominent immune regulatory enzyme capable of modulating immune cell activation status and phenotype via several molecular mechanisms including enzyme-dependent deprivation of L-Trp and its conversion into the aryl hydrocarbon receptor ligand kynurenine and other bioactive kynurenine pathway metabolites, or non-enzymatic cell signalling actions involving tyrosine phosphorylation of IDO1. Through these different modes of biochemical signalling, IDO1 regulates certain physiological functions (e.g. pregnancy) and modulates the pathogenesis and severity of diverse conditions including chronic inflammation, infectious disease, allergic and autoimmune disorders, transplantation, neuropathology and cancer. In the present review, we detail the current understanding of IDO1’s catalytic actions and the biochemical mechanisms regulating IDO1 expression and activity. We also discuss the biological functions of IDO1 with a focus on the enzyme's immune-modulatory function, its medical implications in diverse pathological settings and its utility as a therapeutic target.

Immune Regulation by Self-Recognition: Novel Possibilities for Anticancer Immunotherapy

Circulating T cells that specifically target normal self-proteins expressed by regulatory immune cells were first described in patients with cancer, but can also be detected in healthy individuals. The adaptive immune system is distinguished for its ability to differentiate between self-antigens and foreign antigens. Thus, it was remarkable to discover T cells that apparently lacked tolerance to important self-proteins, eg, IDO, PD-L1, and FoxP3, expressed in regulatory immune cells. The ability of self-reactive T cells to react to and eliminate regulatory immune cells can influence general immune reactions. This suggests that they may be involved in immune homeostasis. It is here proposed that these T cells should be termed antiregulatory T cells (anti-Tregs). The role of anti-Tregs in immune-regulatory networks may be diverse. For example, pro-inflammatory self-reactive T cells that react to regulatory immune cells may enhance local inflammation and inhibit local immune suppression. Further exploration is warranted to investigate their potential role under different malignant conditions and the therapeutic possibilities they possess. Utilizing anti-Tregs for anticancer immunotherapy implies the direct targeting of cancer cells in addition to regulatory immune cells. Anti-Tregs provide the immune system with yet another level of immune regulation and contradict the notion that immune cells involved in the adjustment of immune responses only act as suppressor cells.

Kynurenine and uric acid levels in chronic myeloid leukemia patients

Indoleamine 2,3-dioxygenase 1 (IDO1), IDO2 and tryptophan 2,3-dioxygenase (TDO) represent some of the key immune regulators. Their increased activity has been demonstrated in a number of human malignancies but not yet in chronic myeloid leukemia (CML). In the present study, the activity of these enzymes was tested in 29 CML patients and 28 healthy subjects by monitoring the kynurenine (KYN)/tryptophan ratio. Serum samples taken prior to the therapy displayed a highly significant difference in KYN levels between the patient and control groups. However, increased KYN levels were detected in only 13 (44.8%) of these CML patients. The KYN levels in pretreatment sera of the patients correlated with the tumor burden. There was also a strong correlation between KYN levels and uric acid levels (UA). This suggests but does not prove the possible involvement of UA in activating IDO family of enzymes. Whenever tested, the increased KYN levels normalized in the course of the therapy. Patients with normal KYN levels in their pretreatment sera and subsequently treated with interferon-α, showed a transitory increase in their KYN levels. The present data indicate that CML should be added to the malignancies with an increased activity of the IDO family of enzymes and suggest that IDO inhibitors may be used in the treatment of CML patients.

Tryptophan 2,3-dioxygenase (TDO)-reactive T cells differ in their functional characteristics in health and cancer

Tryptophan-2,3-dioxygenase (TDO) physiologically regulates systemic tryptophan levels in the liver. However, numerous studies have linked cancer with activation of local and systemic tryptophan metabolism. Indeed, similar to other heme dioxygenases TDO is constitutively expressed in many cancers. In the present study, we detected the presence of both CD8+ and CD4+ T-cell reactivity toward TDO in peripheral blood of patients with malignant melanoma (MM) or breast cancer (BC) as well as healthy subjects. However, TDO-reactive CD4+ T cells constituted distinct functional phenotypes in health and disease. In healthy subjects these cells predominately comprised interferon (IFN)γ and tumor necrosis factor (TNF)-α producing Th1 cells, while in cancer patients TDO-reactive CD4+ T-cells were more differentiated with release of not only IFNγ and TNFα, but also interleukin (IL)-17 and IL-10 in response to TDO-derived MHC-class II restricted peptides. Hence, in healthy donors (HD) a Th1 helper response was predominant, whereas in cancer patients CD4+ T-cell responses were skewed toward a regulatory T cell (Treg) response. Furthermore, MM patients hosting a TDO-specific IL-17 response showed a trend toward an improved overall survival (OS) compared to MM patients with IL-10 producing, TDO-reactive CD4+ T cells. For further characterization, we isolated and expanded both CD8+ and CD4+ TDO-reactive T cells in vitro. TDO-reactive CD8+ T cells were able to kill HLA-matched tumor cells of different origin. Interestingly, the processed and presented TDO-derived epitopes varied between different cancer cells. With respect to CD4+ TDO-reactive T cells, in vitro expanded T-cell cultures comprised a Th1 and/or a Treg phenotype. In summary, our data demonstrate that the immune modulating enzyme TDO is a target for CD8+ and CD4+ T cell responses both in healthy subjects as well as patients with cancer; notably, however, the functional phenotype of these T-cell responses differ depending on the respective conditions of the host.

The immune checkpoint regulator PD-L1 is a specific target for naturally occurring CD4+ T cells

Programmed cell death 1 ligand 1 (PD-L1) is an important regulator of T-cell responses and may consequently limit anticancer immunity. We have recently identified PD-L1-specific, cytotoxic CD8⁺ T cells. In the present study, we develop these findings and report that CD4⁺ helper T cells spontaneously recognize PD-L1. We examined the locality of a previously identified HLA-A*0201-restricted PD-L1-epitope for the presence of possible CD4⁺ T-cell epitopes. Thus, we identified naturally occurring PD-L1-specific CD4⁺ T cells among the peripheral blood lymphocytes of cancer patients and - to lesser extents - healthy donors, by means of ELISPOT assays. PD-L1-specific CD4⁺ T cells appeared to be T_H17 cells exhibiting an effector T-cell cytokine profile. Hence, PD-L1-specific CD4⁺ T cells released interferon γ (IFNγ), tumor necrosis factor α (TNFα) and interleukin-17 (IL-17) in response to a long PD-L1-derived peptide. Furthermore, we demonstrate that the specific recognition of PD-L1 by CD4⁺ T cells is MHC class II-restricted. Natural T-cell responses against PD-L1 are noteworthy as they may play a prominent role in the regulation of the immune system. Thus, cytokine release from PD-L1-specific CD4⁺ T cells may surmount the overall immunosuppressive actions of this immune checkpoint regulator. Moreover, PD-L1-specific T cells might be useful for anticancer immunotherapy, as they may counteract common mechanisms of immune escape mediated by the PD-L1/PD-1 pathway.

CD4 responses against IDO

Natural indoleamine-2,3-dioxygenase (IDO)-reactive CD4(+) T cells have been shown to release interferonγ (IFNγ), tumor necrosis factor α (TNFα), as well as interleukin 17 (IL-17). In some individuals, these cells also demonstrated the ability to suppress IL-10 production. IDO-specific CD4(+) helper T cells among peripheral blood lymphocytes may participate in immunoregulatory networks by delaying the immune suppressive actions of IDO. However, IDO-specific CD4(+) T cells may also have a regulatory phenotype, de facto exerting immunosuppressive functions.

Evolving role of tumor antigens for future melanoma therapies

ABSTRACT: 

Human tumor rejection antigens recognized by T lymphocytes were first defined in the early 1990s and the identification of shared tumor-restricted antigens sparked hopes for the development of a therapeutic vaccination to treat cancer, including melanoma. Despite decades of intense preclinical and clinical research, the success of anticancer vaccines based on these antigens has been limited. While melanoma is a highly immunogenic tumor, the ability to prime immunity with vaccines has not generally translated into objective disease regression. However, with the development of small molecules targeting oncogenic proteins, such as V600-mutated BRAF, and immune checkpoint inhibitors with demonstrable long-lasting clinical benefit, new opportunities for antigen-targeted directed therapies are emerging.

Self-reactive T cells: suppressing the suppressors

The immune system is a tightly regulated and complex system. An important part of this immune regulation is the assurance of tolerance toward self-antigens to maintain immune homeostasis. However, in recent years, antigen-specific cellular immune responses toward several normal self-proteins expressed in regulatory immune cells have been reported, especially in patients with cancer. The seemingly lack of tolerance toward such proteins is interesting, as it suggests a regulatory function of self-reactive T (srT) cells, which may be important for the fine tuning of the immune system. In particular, surprising has been the description of cytotoxic srT cells that are able to eliminate normal regulatory immune cells. Such srT cells may be important as effector cells that suppress regulatory suppressor cells. The current knowledge of the nature and function of srT cells is still limited. Still, the therapeutic targeting of srT cells offers a novel approach to harness immune-regulatory networks in cancer.

The targeting of immunosuppressive mechanisms in hematological malignancies

The adaptive immune system has the capacity to recognize and kill leukemic cells. However, immune tolerance mechanisms that normally protect healthy tissues from autoimmune effects prevent the development of effective antitumor immunity. Tumors use several different immunosuppressive mechanisms to evade otherwise effective T-cell responses. A growing number of immune evasion mechanisms have been characterized mainly in solid tumors. In hematological malignancies, less is known about how different immune escape mechanisms influence tumor immune evasion and the extent of their impact on ongoing immune responses. The present review highlights the potential role of three well-defined immunosuppressive mechanisms in hematological malignancies: (i) inhibitory T-cell pathways (especially programmed death ligand 1/programmed death 1 (PD-L1/PD-1)), (ii) regulatory immune cells, and (iii) metabolic enzymes such as indoeamine-2,3-dioxygenase (IDO). The possible therapeutic targeting of these pathways is also discussed. Exciting new strategies that might affect future antileukemia immunotherapy include monoclonal antibodies that block inhibitory T-cell pathways (PD-1/PD-L1) and the prevention of tryptophan depletion by IDO inhibitors. Furthermore, the clinical effect of several chemotherapeutic drugs may arise from the targeting of immunosuppressive cells. Evidence for a new feedback mechanism to suppress the function of regulatory immune cells was recently provided by the identification and characterization of spontaneous cytotoxic T lymphocyte (CTL) responses against regulatory immune cells. Such specific CTLs may be immensely useful in anticancer immunotherapy (for example, by anticancer vaccination). The targeting of one or more immunosuppressive pathways may be especially interesting in combination with antileukemic immunotherapy in cases in which immunosuppressive mechanisms antagonize the desired effects of the therapy.

HLA-restricted CTL that are specific for the immune checkpoint ligand PD-L1 occur with high frequency in cancer patients

PD-L1 (CD274) contributes to functional exhaustion of T cells and limits immune responses in patients with cancer. In this study, we report the identification of an human leukocyte antigen (HLA)-A2-restricted epitope from PD-L1, and we describe natural, cytolytic T-cell reactivity against PD-L1 in the peripheral blood of patients with cancer and healthy individuals. Notably, PD-L1-specific T cells were able not only to recognize and kill tumor cells but also PD-L1-expressing dendritic cells in a PD-L1-dependent manner, insofar as PD-L1 ablation rescued dendritic cells from killing. Furthermore, by incubating nonprofessional antigen-presenting cells with long peptides from PD-L1, we found that PD-L1 was rapidly internalized, processed, and cross-presented by HLA-A2 on the cell surface. Apparently, this cross-presentation was TAP-independent, as it was conducted not only by B cells but in addition by TAP-deficient T2-cells. This is intriguing, as soluble PD-L1 has been detected in the sera from patients with cancer. PD-L1-specific CTL may boost immunity by the killing of immunosuppressive tumor cells as well as regulatory cells. However, PD-L1-specific CTLs may as well suppress immunity by the elimination of normal immune cells especially PD-L1 expressing mature dendritic cells.

The specific targeting of immune regulation: T-cell responses against Indoleamine 2,3-dioxygenase

Indoleamine 2,3-dioxygenase (IDO) is an immunoregulatory enzyme that is implicated in suppressing T-cell immunity in many settings including cancer. In recent years, we have described spontaneous CD8⁺ as well as CD4⁺ T-cell reactivity against IDO in the tumor microenvironment of different cancer patients as well as in the peripheral blood of both cancer patients and to a lesser extent in healthy donors. We have demonstrated that IDO-reactive CD8⁺ T cells were peptide-specific, cytotoxic effector cells, which are able to recognize and kill IDO-expressing cells including tumor cells as well as dendritic cells. Consequently, IDO may serve as a widely applicable target for immunotherapeutic strategies with a completely different function as well as expression pattern compared to previously described antigens. IDO constitutes a significant counter-regulatory mechanism induced by pro-inflammatory signals, and IDO-based immunotherapy may consequently be synergistic with additional immunotherapy. In this regard, we have shown that the presence of IDO-specific T cells boosted immunity against CMV and tumor antigens by eliminating IDO⁺ suppressive cells and changing the regulatory microenvironment. The current review summarizes current knowledge of IDO as a T-cell antigen, reports the initial results that are suggesting a general function of IDO-specific T cells in immunoregulation, and discusses future opportunities.