Expansion of tumor-reactive CD8+ T cell clonotypes occurs in the spleen in response to immune checkpoint blockade.

Immune checkpoint blockade (ICB) enhances T cell responses against cancer, leading to long-term survival in a fraction of patients. CD8+ T cell differentiation in response to chronic antigen stimulation is highly complex, and it remains unclear precisely which T cell differentiation states at which anatomic sites are critical for the response to ICB. We identified an intermediate-exhausted population in the white pulp of the spleen that underwent substantial expansion in response to ICB and gave rise to tumor-infiltrating clonotypes. Increased systemic antigen redirected differentiation of this population toward a more circulatory exhausted KLR state, whereas a lack of cross-presented tumor antigen reduced its differentiation in the spleen. An analogous population of exhausted KLR CD8+ T cells in human blood samples exhibited diminished tumor-trafficking ability. Collectively, our data demonstrate the critical role of antigen density within the spleen for the differentiation and expansion of T cell clonotypes in response to ICB.

Batf3+ DCs and the 4-1BB/4-1BBL axis are required at the effector phase in the tumor microenvironment for PD-1/PD-L1 blockade efficacy.

The cellular source of positive signals that reinvigorate T cells within the tumor microenvironment (TME) for the therapeutic efficacy of programmed death-1 (PD-1)/programmed death-ligand 1 (PD-L1) blockade has not been clearly defined. We now show that Batf3-lineage dendritic cells (DCs) are essential in this process. Flow cytometric analysis, gene-targeted mice, and blocking antibody studies revealed that 4-1BBL is a major positive co-stimulatory signal provided by these DCs within the TME that translates to CD8+ T cell functional reinvigoration and tumor regression. Immunofluorescence and spatial transcriptomics on human tumor samples revealed clustering of Batf3+ DCs and CD8+ T cells, which correlates with anti-PD-1 efficacy. In addition, proximity to Batf3+ DCs within the TME is associated with CD8+ T cell transcriptional states linked to anti-PD-1 response. Our results demonstrate that Batf3+ DCs within the TME are critical for PD-1/PD-L1 blockade efficacy and indicate a major role for the 4-1BB/4-1BB ligand (4-1BBL) axis during this process.

A prime editor mouse to model a broad spectrum of somatic mutations in vivo.

Genetically engineered mouse models only capture a small fraction of the genetic lesions that drive human cancer. Current CRISPR-Cas9 models can expand this fraction but are limited by their reliance on error-prone DNA repair. Here we develop a system for in vivo prime editing by encoding a Cre-inducible prime editor in the mouse germline. This model allows rapid, precise engineering of a wide range of mutations in cell lines and organoids derived from primary tissues, including a clinically relevant Kras mutation associated with drug resistance and Trp53 hotspot mutations commonly observed in pancreatic cancer. With this system, we demonstrate somatic prime editing in vivo using lipid nanoparticles, and we model lung and pancreatic cancer through viral delivery of prime editing guide RNAs or orthotopic transplantation of prime-edited organoids. We believe that this approach will accelerate functional studies of cancer-associated mutations and complex genetic combinations that are challenging to construct with traditional models.

Overcoming lung cancer immunotherapy resistance by combining nontoxic variants of IL-12 and IL-2.

Engineered cytokine-based approaches for immunotherapy of cancer are poised to enter the clinic, with IL-12 being at the forefront. However, little is known about potential mechanisms of resistance to cytokine therapies. We found that orthotopic murine lung tumors were resistant to systemically delivered IL-12 fused to murine serum albumin (MSA, IL12-MSA) because of low IL-12 receptor (IL-12R) expression on tumor-reactive CD8+ T cells. IL2-MSA increased binding of IL12-MSA by tumor-reactive CD8+ T cells, and combined administration of IL12-MSA and IL2-MSA led to enhanced tumor-reactive CD8+ T cell effector differentiation, decreased numbers of tumor-infiltrating CD4+ regulatory T cells, and increased survival of lung tumor-bearing mice. Predictably, the combination of IL-2 and IL-12 at therapeutic doses led to significant dose-limiting toxicity. Administering IL-12 and IL-2 analogs with preferential binding to cells expressing Il12rb1 and CD25, respectively, led to a significant extension of survival in mice with lung tumors while abrogating dose-limiting toxicity. These findings suggest that IL-12 and IL-2 represent a rational approach to combination cytokine therapy whose dose-limiting toxicity can be overcome with engineered cytokine variants.

Leveraging immune resistance archetypes in solid cancer to inform next-generation anticancer therapies.

Anticancer immunotherapies, such as immune checkpoint inhibitors, bispecific antibodies, and chimeric antigen receptor T cells, have improved outcomes for patients with a variety of malignancies. However, most patients either do not initially respond or do not exhibit durable responses due to primary or adaptive/acquired immune resistance mechanisms of the tumor microenvironment. These suppressive programs are myriad, different between patients with ostensibly the same cancer type, and can harness multiple cell types to reinforce their stability. Consequently, the overall benefit of monotherapies remains limited. Cutting-edge technologies now allow for extensive tumor profiling, which can be used to define tumor cell intrinsic and extrinsic pathways of primary and/or acquired immune resistance, herein referred to as features or feature sets of immune resistance to current therapies. We propose that cancers can be characterized by immune resistance archetypes, comprised of five feature sets encompassing known immune resistance mechanisms. Archetypes of resistance may inform new therapeutic strategies that concurrently address multiple cell axes and/or suppressive mechanisms, and clinicians may consequently be able to prioritize targeted therapy combinations for individual patients to improve overall efficacy and outcomes.

Tissue-specific abundance of interferon-gamma drives regulatory T cells to restrain DC1-mediated priming of cytotoxic T cells against lung cancer.

Local environmental factors influence CD8+ T cell priming in lymph nodes (LNs). Here, we sought to understand how factors unique to the tumor-draining mediastinal LN (mLN) impact CD8+ T cell responses toward lung cancer. Type 1 conventional dendritic cells (DC1s) showed a mLN-specific failure to induce robust cytotoxic T cells responses. Using regulatory T (Treg) cell depletion strategies, we found that Treg cells suppressed DC1s in a spatially coordinated manner within tissue-specific microniches within the mLN. Treg cell suppression required MHC II-dependent contact between DC1s and Treg cells. Elevated levels of IFN-γ drove differentiation Treg cells into Th1-like effector Treg cells in the mLN. In patients with cancer, Treg cell Th1 polarization, but not CD8+/Treg cell ratios, correlated with poor responses to checkpoint blockade immunotherapy. Thus, IFN-γ in the mLN skews Treg cells to be Th1-like effector Treg cells, driving their close interaction with DC1s and subsequent suppression of cytotoxic T cell responses.

Something old, something new: The marriage of PD-1 and IL-2.

IL-2 remains a promising candidate for immunotherapy of cancer, but its use is hampered by systemic toxicities. In this issue of Immunity, Tichet, Hanahan, and colleagues demonstrate that an IL-2 variant fused to an anti-PD-1 antibody overcomes these limitations to promote impressive tumor control. This approach may be a path to treat tumors that do not respond to anti-PD-1 monotherapy.

Lack of CD8+ T cell effector differentiation during priming mediates checkpoint blockade resistance in non-small cell lung cancer.

In non­small cell lung cancer (NSCLC), response to immune checkpoint blockade (ICB) is associated with programmed cell death ligand 1 expression that is induced by interferon-γ­producing, tumor-infiltrating CD8+ T cells. However, not all tumors with a CD8+ T cell infiltrate respond to ICB, and little is known about the mechanisms governing ICB resistance in T cell­infiltrated NSCLC. We used an orthotopic NSCLC mouse model to study ICB-refractory CD8+ T cell responses. Single-cell RNA sequencing of the NSCLC mouse tumors revealed that lung cancer­specific tumor-infiltrating CD8+ T cells exhibited clonal expansion but lacked expression of genes associated with effector and exhausted T cell responses, indicating that they underwent a differentiation program distinct from conventional T cell exhaustion. This lung cancer­specific T cell dysfunction program was established early during priming in the mediastinal lymph node and was characterized by robust proliferation but a failed up-regulation of effector and exhausted T cell characteristics. Intriguingly, CD8+ T cells from patients with NSCLC expressed an analogous gene expression program, which appeared distinct from conventional T cell exhaustion. Administration of recombinant interleukin-2 (IL-2) and IL-12 was sufficient to restore effector T cell differentiation and induce control of KP lung tumors. These findings imply that a CD8+ T cell differentiation trajectory, activated during T cell priming in the mediastinal lymph node, limits the response of CD8+ T cells to ICB and thereby may contribute to failure of ICB in a subset T cell­infiltrated NSCLC.

Formation of Human Neuroblastoma in Mouse-Human Neural Crest Chimeras.

Neuroblastoma (NB), derived from the neural crest (NC), is the most common pediatric extracranial solid tumor. Here, we establish a platform that allows the study of human NBs in mouse-human NC chimeras. Chimeric mice were produced by injecting human NC cells carrying NB relevant oncogenes in utero into gastrulating mouse embryos. The mice developed tumors composed of a heterogenous cell population that resembled that seen in primary NBs of patients but were significantly different from homogeneous tumors formed in xenotransplantation models. The human tumors emerged in immunocompetent hosts and were extensively infiltrated by mouse cytotoxic T cells, reflecting a vigorous host anti-tumor immune response. However, the tumors blunted the immune response by inducing infiltration of regulatory T cells and expression of immune-suppressive molecules similar to escape mechanisms seen in human cancer patients. Thus, this experimental platform allows the study of human tumor initiation, progression, manifestation, and tumor-immune-system interactions in an animal model system.

Tissue Site and the Cancer Immunity Cycle.

Checkpoint blockade immunotherapy (CBT) has revolutionized cancer treatment; however, the cellular and molecular factors that govern responsiveness to immunotherapy remain poorly understood. One emerging area of clinical importance is differential responsiveness to CBT across different tissue sites of tumor growth. Each tissue site in the body can contain unique tissue-resident immune cells from both the lymphoid and the myeloid compartment and differences in tissue-specific immune cell composition might predispose tumors in certain tissue sites to be more or less responsive to immunotherapy. Understanding the interplay between tissue-resident and systemic immune responses against tumors will help to determine how to better therapeutically target the immune system to fight cancer. This review summarizes clinical and preclinical investigations of tissue-specific antitumor immune responses and how they influence the tumor immune microenvironment and the efficacy of immunotherapy.

A Tumor Cell-Intrinsic Yin-Yang Determining Immune Evasion.

Pancreatic adenocarcinoma (PDAC) is mostly refractory to immunotherapies. In this issue of Immunity, Li et al. (2018) generate a library of clonal PDAC tumors to examine the tumor-intrinsic features shaping the anti-tumor immune response and find that tumor cell-derived CXCL1 directly blunts T cell infiltration and reduces responsiveness to immunotherapy.

Intratumoral CD8+ T-cell Apoptosis Is a Major Component of T-cell Dysfunction and Impedes Antitumor Immunity.

Subsets of human tumors are infiltrated with tumor antigen-specific CD8+ T cells [tumor-infiltrating lymphocytes (TILs)] despite tumor progression. These TILs are thought to be inactivated by the immunosuppressive tumor microenvironment, through the engagement of inhibitory receptors such as CTLA-4 and PD-1. However, antigen-specific CD8+ TILs are not functionally inert but are undergoing activation in situ Here, we show that antigen-specific CD8+ TILs are actively proliferating, yet also undergo high rates of apoptosis, leading to a vicious cycle of activation and death that limits immune efficacy. Preventing CD8+ TIL apoptosis by Bcl-xL overexpression enabled accumulation and improved tumor control. Effective combination immunotherapy with an agonist 4-1BB mAb plus either CTLA-4 or PD-L1 neutralization led to a marked accumulation of specific CD8+ TILs through decreased apoptosis rather than increased T-cell entry or proliferation. Our data suggest that antigen-driven apoptosis of CD8+ TILs is a barrier to effective spontaneous antitumor immunity and should be considered as a critical factor in the development of cancer immunotherapies. Cancer Immunol Res; 6(1); 14-24. ©2017 AACR.

Cancer Immunotherapy Targets Based on Understanding the T Cell-Inflamed Versus Non-T Cell-Inflamed Tumor Microenvironment.

Most cancers express tumor antigens that can be recognized by T cells of the host. The fact that cancers become clinically evident nonetheless implies that immune escape must occur. Two major subsets of human melanoma metastases have been identified based on gene expression profiling. One subgroup has a T cell-inflamed phenotype that includes expression of chemokines, T cell markers, and a type I IFN signature. In contrast, the other major subset lacks this phenotype and has been designated as non-T cell-inflamed. The mechanisms of immune escape are likely distinct in these two phenotypes, and therefore the optimal immunotherapeutic interventions necessary to promote clinical responses may be different. The T cell-inflamed tumor microenvironment subset shows the highest expression of negative regulatory factors, including PD-L1, IDO, FoxP3+ Tregs, and evidence for T cell-intrinsic anergy. Therapeutic strategies to overcome these inhibitory mechanisms are being pursued, and anti-PD-1 mAbs have been FDA approved. The presence of multiple inhibitory mechanisms in the same tumor microenvironment argues that combination therapies may be advantageous, several of which are in clinical testing. A new paradigm may be needed to promote de novo inflammation in cases of the non-T cell-infiltrated tumor microenvironment. Natural innate immune sensing of tumors appears to occur via the host STING pathway, type I IFN production, and cross-priming of T cells via CD8α+ DCs. New strategies are being developed to engage this pathway therapeutically, such as through STING agonists. The molecular mechanisms that mediate the presence or absence of the T cell-inflamed tumor microenvironment are being elucidated using parallel genomics platforms. The first oncogene pathway identified that mediates immune exclusion is the Wnt/ß-catenin pathway, suggesting that new pharmacologic strategies to target this pathway should be developed to restore immune access to the tumor microenvironment.

Tumor-Residing Batf3 Dendritic Cells Are Required for Effector T Cell Trafficking and Adoptive T Cell Therapy.

Effector T cells have the capability of recognizing and killing cancer cells. However, whether tumors can become immune resistant through exclusion of effector T cells from the tumor microenvironment is not known. By using a tumor model resembling non-T cell-inflamed human tumors, we assessed whether adoptive T cell transfer might overcome failed spontaneous priming. Flow cytometric assays combined with intra-vital imaging indicated failed trafficking of effector T cells into tumors. Mechanistically, this was due to the absence of CXCL9/10, which we found to be produced by CD103+ dendritic cells (DCs) in T cell-inflamed tumors. Our data indicate that lack of CD103+ DCs within the tumor microenvironment dominantly resists the effector phase of an anti-tumor T cell response, contributing to immune escape.

The EGR2 targets LAG-3 and 4-1BB describe and regulate dysfunctional antigen-specific CD8+ T cells in the tumor microenvironment.

Although the presence of tumor-infiltrating lymphocytes (TILs) indicates an endogenous antitumor response, immune regulatory pathways can subvert the effector phase and enable tumor escape. Negative regulatory pathways include extrinsic suppression mechanisms, but also a T cell-intrinsic dysfunctional state. A more detailed study has been hampered by a lack of cell surface markers defining tumor-specific dysfunctional TILs, and PD-1 alone is not sufficient. Recently, we identified the transcription factor Egr2 as a critical component in controlling the anergic state in vitro. In this study, we show that the Egr2-driven cell surface proteins LAG-3 and 4-1BB can identify dysfunctional tumor antigen-specific CD8+ TIL. Co-expression of 4-1BB and LAG-3 was seen on a majority of CD8+ TILs, but not in lymphoid organs. Functional analysis revealed defective IL-2 and TNF production yet retained expression of IFN-γ and regulatory T cell-recruiting chemokines. Transcriptional and phenotypic characterization revealed coexpression of multiple additional co-stimulatory and co-inhibitory receptors. Administration of anti-LAG-3 plus anti-4-1BB mAbs was therapeutic against tumors in vivo, which correlated with phenotypic normalization. Our results indicate that coexpression of LAG-3 and 4-1BB characterize dysfunctional T cells within tumors, and that targeting these receptors has therapeutic utility.

The non-T-cell-inflamed tumor microenvironment: contributing factors and therapeutic solutions.

The recent successes of cancer immunotherapy, first and foremost checkpoint blockade therapy, illustrate the power of the immune system to control cancer. As the number of patients receiving this therapy is increasing, the number of patients being resistant or establishing resistance toward immunotherapy is also increasing. We, therefore, need to further understand the mechanisms mediate resistance in order to prevent or overcome those mechanisms. Increasing evidence is being reported that alterations in tumor cell-intrinsic signaling pathways, including the activation of the WNT/ß-catenin pathway, are associated with blunted T-cell infiltration. Infiltration of tumor by CD8 T cells is one of the most predictive biomarkers for the response toward immunotherapy and therefore the notion that alterations of certain tumor cell-intrinsic signaling pathways might mediate resistance should be considered. Understanding the molecular and immunological mechanisms mediating resistance will ultimately facilitate the development of effective treatment strategies counteracting immune evasion.

Mechanism of tumor rejection with doublets of CTLA-4, PD-1/PD-L1, or IDO blockade involves restored IL-2 production and proliferation of CD8(+) T cells directly within the tumor microenvironment.

BACKGROUND: Blockade of immune inhibitory pathways is emerging as an important therapeutic modality for the treatment of cancer. Single agent treatments have partial anti-tumor activity in preclinical models and in human cancer patients. Inasmuch as the tumor microenvironment shows evidence of multiple immune inhibitory mechanisms present concurrently, it has been reasoned that combination therapies may be required for optimal therapeutic effect. METHODS: To test this notion, we utilized permutations of anti-CTLA-4 mAb, anti-PD-L1 mAb, and/or the IDO inhibitor INCB23843 in the murine B16.SIY melanoma model. RESULTS: All three combinations showed markedly improved tumor control over single treatments, with many mice achieving complete tumor rejection. This effect was seen in the absence of vaccination or adoptive T cell therapy. The mechanism of synergy was investigated to examine the priming versus effector phase of the anti-tumor immune response. Only a minimal increase in priming of anti-tumor T cells was observed at early time points in the tumor-draining lymph nodes (TdLN). In contrast, as early as three days after therapy initiation, a marked increase in the capacity of tumor-infiltrating CD8(+) T cells to produce IL-2 and to proliferate was found in all groups treated with the effective combinations. Treatment of mice with FTY720 to block new T cell trafficking from secondary lymphoid structures still enabled restoration of IL-2 production and proliferation by intratumoral T cells, and also retained most of the tumor growth control. CONCLUSIONS: Our data suggest that the therapeutic effect of these immunotherapies was mainly mediated through direct reactivation of T cells in situ. These three combinations are attractive to pursue clinically, and the ability of intratumoral CD8(+) T cells to produce IL-2 and to proliferate could be an important biomarker to integrate into clinical studies.