Pamela M. Odorizzi

Radiation and dual checkpoint blockade activate non-redundant immune mechanisms in cancer

Immune checkpoint inhibitors result in impressive clinical responses, but optimal results will require combination with each other and other therapies. This raises fundamental questions about mechanisms of non-redundancy and resistance. Here we report major tumour regressions in a subset of patients with metastatic melanoma treated with an anti-CTLA4 antibody (anti-CTLA4) and radiation, and reproduced this effect in mouse models. Although combined treatment improved responses in irradiated and unirradiated tumours, resistance was common. Unbiased analyses of mice revealed that resistance was due to upregulation of PD-L1 on melanoma cells and associated with T-cell exhaustion. Accordingly, optimal response in melanoma and other cancer types requires radiation, anti-CTLA4 and anti-PD-L1/PD-1. Anti-CTLA4 predominantly inhibits T-regulatory cells (Treg cells), thereby increasing the CD8 T-cell to Treg (CD8/Treg) ratio. Radiation enhances the diversity of the T-cell receptor (TCR) repertoire of intratumoral T cells. Together, anti-CTLA4 promotes expansion of T cells, while radiation shapes the TCR repertoire of the expanded peripheral clones. Addition of PD-L1 blockade reverses T-cell exhaustion to mitigate depression in the CD8/Treg ratio and further encourages oligoclonal T-cell expansion. Similarly to results from mice, patients on our clinical trial with melanoma showing high PD-L1 did not respond to radiation plus anti-CTLA4, demonstrated persistent T-cell exhaustion, and rapidly progressed. Thus, PD-L1 on melanoma cells allows tumours to escape anti-CTLA4-based therapy, and the combination of radiation, anti-CTLA4 and anti-PD-L1 promotes response and immunity through distinct mechanisms.

Progenitor and Terminal Subsets of CD8+ T Cells Cooperate to Contain Chronic Viral Infection

Chronic infections strain the regenerative capacity of antiviral T lymphocyte populations, leading to failure in long-term immunity. The cellular and molecular events controlling this regenerative capacity, however, are unknown. We found that two distinct states of virus-specific CD8+ T cells exist in chronically infected mice and humans. Differential expression of the T-box transcription factors T-bet and Eomesodermin (Eomes) facilitated the cooperative maintenance of the pool of antiviral CD8+ T cells during chronic viral infection. T-bethi cells displayed low intrinsic turnover but proliferated in response to persisting antigen, giving rise to Eomeshi terminal progeny. Genetic elimination of either subset resulted in failure to control chronic infection, which suggests that an imbalance in differentiation and renewal could underlie the collapse of immunity in humans with chronic infections. Chronic viral infections like HIV are kept in check by two functionally distinct types of T lymphocyte. Chronic infections like hepatitis C virus (HCV) or HIV are hard on the immune system. In the face of a constant threat, some immune cells like T lymphocytes become “exhausted”; although present, they can no longer mount responses that are effective enough to eliminate the virus. These responses, however, are still important because in many cases they do keep the virus relatively controlled. The mechanisms underlying the population dynamics of T cell responses during chronic viral infection, however, are not well understood. Paley et al. (p. 1220) now demonstrate that the T-box transcription factors T-bet and Eomesodermin differentially regulate two phenotypically and functionally distinct subsets of antiviral CD8+ T cells in mice. The cooperation of these subsets may be important for antiviral immunity during chronic viral infections in humans.

Molecular and Transcriptional Basis of CD4+ T Cell Dysfunction during Chronic Infection

T cell exhaustion is common during chronic infections. Although CD4(+) T cells are critical for controlling viral load during chronic viral infections, less is known about their differentiation and transcriptional program. We defined the phenotypic, functional, and molecular profiles of exhausted CD4(+) T cells. Global transcriptional analysis demonstrated a molecular profile distinct from effector and memory CD4(+) T cells and also from exhausted CD8(+) T cells, though some common features of CD4(+) and CD8(+) T cell exhaustion were revealed. We have demonstrated unappreciated roles for transcription factors (TFs) including Helios, type I interferon (IFN-I) signaling, and a diverse set of coinhibitory and costimulatory molecules during CD4(+) T cell exhaustion. Moreover, the signature of CD4(+) T cell exhaustion was found to be distinct from that of other CD4(+) T cell lineage subsets and was associated with TF heterogeneity. This study provides a framework for therapeutic interventions targeting exhausted CD4(+) T cells.

Epigenetic stability of exhausted T cells limits durability of reinvigoration by PD-1 blockade

Epigenetic profiling suggests that exhausted T cells are a distinct cell linage. The epigenetics of exhaustion During cancer or chronic infection, T cells become dysfunctional, eventually acquiring an “exhausted” phenotype. Immunotherapies aim to reverse this state. Using a mouse model of chronic infection, two studies now show that the epigenetic profile of exhausted T cells differs substantially from those of effector and memory T cells, suggesting that exhausted T cells are a distinct lineage (see the Perspective by Turner and Russ). Sen et al. defined specific functional modules of enhancers that are also conserved in exhausted human T cells. Pauken et al. examined the epigenetic profile of exhausted T cells after immunotherapy. Although there was transcriptional rewiring, the cells never acquired a memory T cell phenotype. Thus, epigenetic regulation may limit the success of immunotherapies. Science, this issue p. 1104, p. 1165; see also p. 1160 Blocking Programmed Death–1 (PD-1) can reinvigorate exhausted CD8 T cells (TEX) and improve control of chronic infections and cancer. However, whether blocking PD-1 can reprogram TEX into durable memory T cells (TMEM) is unclear. We found that reinvigoration of TEX in mice by PD-L1 blockade caused minimal memory development. After blockade, reinvigorated TEX became reexhausted if antigen concentration remained high and failed to become TMEM upon antigen clearance. TEX acquired an epigenetic profile distinct from that of effector T cells (TEFF) and TMEM cells that was minimally remodeled after PD-L1 blockade. This finding suggests that TEX are a distinct lineage of CD8 T cells. Nevertheless, PD-1 pathway blockade resulted in transcriptional rewiring and reengagement of effector circuitry in the TEX epigenetic landscape. These data indicate that epigenetic fate inflexibility may limit current immunotherapies.

Induction of T cell immunity overcomes complete resistance to PD-1 and CTLA-4 blockade and improves survival in pancreatic carcinoma

Winograd and colleagues used an engineered KPC mouse model of pancreatic ductal adenocarcinoma (PDA) reflecting that of human PDA to show that baseline refractoriness to checkpoint inhibitors can be rescued by priming a T-cell response with αCD40 plus chemotherapy with gemcitabine and nab-paclitaxel. Disabling the function of immune checkpoint molecules can unlock T-cell immunity against cancer, yet despite remarkable clinical success with monoclonal antibodies (mAb) that block PD-1 or CTLA-4, resistance remains common and essentially unexplained. To date, pancreatic carcinoma is fully refractory to these antibodies. Here, using a genetically engineered mouse model of pancreatic ductal adenocarcinoma in which spontaneous immunity is minimal, we found that PD-L1 is prominent in the tumor microenvironment, a phenotype confirmed in patients; however, tumor PD-L1 was found to be independent of IFNγ in this model. Tumor T cells expressed PD-1 as prominently as T cells from chronically infected mice, but treatment with αPD-1 mAbs, with or without αCTLA-4 mAbs, failed in well-established tumors, recapitulating clinical results. Agonist αCD40 mAbs with chemotherapy induced T-cell immunity and reversed the complete resistance of pancreatic tumors to αPD-1 and αCTLA-4. The combination of αCD40/chemotherapy plus αPD-1 and/or αCTLA-4 induced regression of subcutaneous tumors, improved overall survival, and conferred curative protection from multiple tumor rechallenges, consistent with immune memory not otherwise achievable. Combinatorial treatment nearly doubled survival of mice with spontaneous pancreatic cancers, although no cures were observed. Our findings suggest that in pancreatic carcinoma, a nonimmunogenic tumor, baseline refractoriness to checkpoint inhibitors can be rescued by the priming of a T-cell response with αCD40/chemotherapy. Cancer Immunol Res; 3(4); 399–411. ©2015 AACR.

Genetic absence of PD-1 promotes accumulation of terminally differentiated exhausted CD8+ T cells

Using PD-1–deficient mice and co-adoptive transfer approaches, Odorizzi et al. demonstrate that PD-1 is not required for the induction of CD8+ T cell exhaustion (TEX) in chronic LCMV infection. The absence of PD-1 leads to more cytotoxic, but terminally differentiated TEX, with compromised long-term durability. PD-1 may well serve to protect TEX from excessive overstimulation, proliferation, and terminal differentiation.

Disruption of Intestinal CD4+ T Cell Homeostasis Is a Key Marker of Systemic CD4+ T Cell Activation in HIV-Infected Individuals

HIV infection is associated with depletion of intestinal CD4+ T cells, resulting in mucosal immune dysfunction, microbial translocation, chronic immune activation, and progressive immunodeficiency. In this study, we examined HIV-infected individuals with active virus replication (n = 15), treated with antiretroviral therapy (n = 13), and healthy controls (n = 11) and conducted a comparative analysis of T cells derived from blood and four gastrointestinal (GI) sites (terminal ileum, right colon, left colon, and sigmoid colon). As expected, we found that HIV infection is associated with depletion of total CD4+ T cells as well as CD4+CCR5+ T cells in all GI sites, with higher levels of these cells found in ART-treated individuals than in those with active virus replication. While the levels of both CD4+ and CD8+ T cell proliferation were higher in the blood of untreated HIV-infected individuals, only CD4+ T cell proliferation was significantly increased in the gut of the same patients. We also noted that the levels of CD4+ T cells and the percentages of CD4+Ki67+ proliferating T cells are inversely correlated in both blood and intestinal tissues, thus suggesting that CD4+ T cell homeostasis is similarly affected by HIV infection in these distinct anatomic compartments. Importantly, the level of intestinal CD4+ T cells (both total and Th17 cells) was inversely correlated with the percentage of circulating CD4+Ki67+ T cells. Collectively, these data confirm that the GI tract is a key player in the immunopathogenesis of HIV infection, and they reveal a strong association between the destruction of intestinal CD4+ T cell homeostasis in the gut and the level of systemic CD4+ T cell activation.

The transcription factor BATF operates as an essential differentiation checkpoint in early effector CD8 + T cells

The transcription factor BATF is required for the differentiation of interleukin 17 (IL-17)-producing helper T cells (TH17 cells) and follicular helper T cells (TFH cells). Here we identified a fundamental role for BATF in regulating the differentiation of effector of CD8+ T cells. BATF-deficient CD8+ T cells showed profound defects in effector population expansion and underwent proliferative and metabolic catastrophe early after encountering antigen. BATF, together with the transcription factors IRF4 and Jun proteins, bound to and promoted early expression of genes encoding lineage-specific transcription-factors (T-bet and Blimp-1) and cytokine receptors while paradoxically repressing genes encoding effector molecules (IFN-γ and granzyme B). Thus, BATF amplifies T cell antigen receptor (TCR)-dependent expression of transcription factors and augments the propagation of inflammatory signals but restrains the expression of genes encoding effector molecules. This checkpoint prevents irreversible commitment to an effector fate until a critical threshold of downstream transcriptional activity has been achieved.

Inhibitory Receptors on Lymphocytes: Insights from Infections

Costimulatory and inhibitory receptors are critical regulators of adaptive immune cell function. These pathways regulate the initiation and termination of effective immune responses to infections while limiting autoimmunity and/or immunopathology. This review focuses on recent advances in our understanding of inhibitory receptor pathways and their roles in different diseases and/or infections, emphasizing potential clinical applications and important unanswered mechanistic questions. Although significant progress has been made in defining the influence of inhibitory receptors at the cellular level, relatively little is known about the underlying molecular pathways. We discuss our current understanding of the molecular mechanisms for key inhibitory receptor pathways, highlight major gaps in knowledge, and explore current and future clinical applications.

An Interferon Paradox

Interferons must balance antiviral actions against immunosuppressive effects during acute and chronic infections. [Also see Reports by Wilson et al. and Teijaro et al.] Type 1 interferons (IFN-α/β) are a major first line of host defense against viral infection. Because of this potent antiviral activity, IFN-based therapies have been developed for chronic infections with hepatitis B and C viruses, as well as for HIV. However, a poorly understood phenomenon has been the persistence of virus despite induction of antiviral immune responses by type 1 IFNs. On page 207 and 202 in this issue, Teijaro et al. (1) and Wilson et al. (2) address this long-standing question and find that IFN-α/β can also suppress the immune system in ways that promote viral persistence. This paradoxical finding should spur a reassessment of the fundamental roles of IFN-α/β during chronic infections.