Jeanne Cheung

T cell costimulatory receptor CD28 is a primary target for PD-1-mediated inhibition

Immunotherapeutic PD-1–targeted therapies require CD28 to promote cancer cell killing. CD28 is a critical target for PD-1 blockade PD-1–targeted therapies have been a breakthrough for treating certain tumors and can rejuvenate T cells to unleash the anticancer immune response (see the Perspective by Clouthier and Ohashi). It is widely believed that PD-1 suppresses signaling through the T cell receptor (TCR). However, Hui et al. find instead that the TCR costimulatory receptor, CD28, is the primary target of PD-1 signaling. Independently, Kamphorst et al. show that CD28 is required for PD-1 therapies to kill cancer cells efficiently and eliminate chronic viral infections in mice. Lung cancer patients that responded to PD-1 therapy had more CD28+ T cells, which suggests that CD28 may predict treatment response. Science, this issue p. 1428, p. 1423; see also p. 1373 Programmed cell death–1 (PD-1) is a coinhibitory receptor that suppresses T cell activation and is an important cancer immunotherapy target. Upon activation by its ligand PD-L1, PD-1 is thought to suppress signaling through the T cell receptor (TCR). By titrating PD-1 signaling in a biochemical reconstitution system, we demonstrate that the co-receptor CD28 is strongly preferred over the TCR as a target for dephosphorylation by PD-1–recruited Shp2 phosphatase. We also show that CD28, but not the TCR, is preferentially dephosphorylated in response to PD-1 activation by PD-L1 in an intact cell system. These results reveal that PD-1 suppresses T cell function primarily by inactivating CD28 signaling, suggesting that costimulatory pathways play key roles in regulating effector T cell function and responses to anti–PD-L1/PD-1 therapy.

MAP Kinase Inhibition Promotes T Cell and Anti-tumor Activity in Combination with PD-L1 Checkpoint Blockade

Targeted inhibition of mitogen-activated protein kinase (MAPK) kinase (MEK) can induce regression of tumors bearing activating mutations in the Ras pathway but rarely leads to tumor eradication. Although combining MEK inhibition with T-cell-directed immunotherapy might lead to more durable efficacy, T cell responses are themselves at least partially dependent on MEK activity. We show here that MEK inhibition did profoundly block naive CD8(+) T cell priming in tumor-bearing mice, but actually increased the number of effector-phenotype antigen-specific CD8(+) T cells within the tumor. MEK inhibition protected tumor-infiltrating CD8(+) T cells from death driven by chronic TCR stimulation while sparing cytotoxic activity. Combining MEK inhibition with anti-programmed death-ligand 1 (PD-L1) resulted in synergistic and durable tumor regression even where either agent alone was only modestly effective. Thus, despite the central importance of the MAP kinase pathway in some aspects of T cell function, MEK-targeted agents can be compatible with T-cell-dependent immunotherapy.

Tumour and host cell PD-L1 is required to mediate suppression of anti-tumour immunity in mice

Expression of PD-L1, the ligand for T-cell inhibitory receptor PD-1, is one key immunosuppressive mechanism by which cancer avoids eradication by the immune system. Therapeutic use of blocking antibodies to PD-L1 or its receptor PD-1 has produced unparalleled, durable clinical responses, with highest likelihood of response seen in patients whose tumour or immune cells express PD-L1 before therapy. The significance of PD-L1 expression in each cell type has emerged as a central and controversial unknown in the clinical development of immunotherapeutics. Using genetic deletion in preclinical mouse models, here we show that PD-L1 from disparate cellular sources, including tumour cells, myeloid or other immune cells can similarly modulate the degree of cytotoxic T-cell function and activity in the tumour microenvironment. PD-L1 expression in both the host and tumour compartment contribute to immune suppression in a non-redundant fashion, suggesting that both sources could be predictive of sensitivity to therapeutic agents targeting the PD-L1/PD-1 axis.

Role of PD-1 during effector CD8 T cell differentiation

Significance PD-1–mediated inhibitory signals play a major role in T cell exhaustion during chronic infections and cancers, which makes PD-1 a valuable target of checkpoint blockade in cancer immunotherapy. However, the role of PD-1 in effector CD8 T cell differentiation during an acute viral infection is not well defined. Here, we show that PD-1 also functions as an inhibitory receptor during this early stage of T cell activation and that PD-1 blockade enhances effector function and results in faster clearance of infection. PD-1 (programmed cell death-1) is the central inhibitory receptor regulating CD8 T cell exhaustion during chronic viral infection and cancer. Interestingly, PD-1 is also expressed transiently by activated CD8 T cells during acute viral infection, but the role of PD-1 in modulating T cell effector differentiation and function is not well defined. To address this question, we examined the expression kinetics and role of PD-1 during acute lymphocytic choriomeningitis virus (LCMV) infection of mice. PD-1 was rapidly up-regulated in vivo upon activation of naive virus-specific CD8 T cells within 24 h after LCMV infection and in less than 4 h after peptide injection, well before any cell division had occurred. This rapid PD-1 expression by CD8 T cells was driven predominantly by antigen receptor signaling since infection with a LCMV strain with a mutation in the CD8 T cell epitope did not result in the increase of PD-1 on antigen-specific CD8 T cells. Blockade of the PD-1 pathway using anti–PD-L1 or anti–PD-1 antibodies during the early phase of acute LCMV infection increased mTOR signaling and granzyme B expression in virus-specific CD8 T cells and resulted in faster clearance of the infection. These results show that PD-1 plays an inhibitory role during the naive-to-effector CD8 T cell transition and that the PD-1 pathway can also be modulated at this stage of T cell differentiation. These findings have implications for developing therapeutic vaccination strategies in combination with PD-1 blockade.

Chemotherapy Combines Effectively with Anti–PD-L1 Treatment and Can Augment Antitumor Responses

Immunotherapy with checkpoint inhibitors has proved to be highly effective, with durable responses in a subset of patients. Given their encouraging clinical activity, checkpoint inhibitors are increasingly being tested in clinical trials in combination with chemotherapy. In many instances, there is little understanding of how chemotherapy might influence the quality of the immune response generated by checkpoint inhibitors. In this study, we evaluated the impact of chemotherapy alone or in combination with anti–PD-L1 in a responsive syngeneic tumor model. Although multiple classes of chemotherapy treatment reduced immune cell numbers and activity in peripheral tissues, chemotherapy did not antagonize but in many cases augmented the antitumor activity mediated by anti–PD-L1. This dichotomy between the detrimental effects in peripheral tissues and enhanced antitumor activity was largely explained by the reduced dependence on incoming cells for antitumor efficacy in already established tumors. The effects of the various chemotherapies were also agent specific, and synergy with anti–PD-L1 was achieved by different mechanisms that ultimately helped establish a new threshold for response. These results rationalize the combination of chemotherapy with immunotherapy and suggest that, despite the negative systemic effects of chemotherapy, effective combinations can be obtained through distinct mechanisms acting within the tumor.

Abstract A114: Effect of different standard of care chemotherapeutics on anti-PD-L1 responses in syngeneic mouse tumor models

Immunotherapy with checkpoint inhibitors has proven to be highly effective, with durable responses in a subset of patients. In an effort to broaden the number of responding individuals, overcome resistance to single-agent therapy and extend the duration of responses, combination of immunotherapy with other treatments such as chemotherapy is currently being tested in multiple clinical trials. Importantly, combination of immunotherapy with standard of care chemotherapy could bring the benefits of immunotherapy into earlier lines of treatment. Here, we have evaluated the effects of different classes of chemotherapeutic agents alone or in combination with anti-PD-L1 treatment in syngeneic murine tumor models. We have found that combination of anti-PD-L1 with various alkylating agents, taxanes and platins caused differential changes on the frequency and number of intratumoral immune cell subsets, but did not antagonize the functional changes mediated by anti-PD-L1 treatment. Importantly, by comparing the effects of these chemotherapies in different tumor models, we have observed that their effects are model-specific. These differences may be due to tumor-specific immune composition and tumor intrinsic responses to different chemotherapies. Our results demonstrate that in the models we have tested, chemotherapeutic agents do not appear to directly antagonize the activity of anti-PD-L1 as measured by pharmacodynamic changes in immune cell activation although some differences can be observed in the frequency and number of certain intratumoral immune cell subsets. Additionally, our findings also reveal that tumor models might respond differently to the same chemotherapeutic treatment adding complexity to how we evaluate the effect of combination treatments with chemotherapy. Nonetheless, our results suggest a rationale for combining immunotherapy with chemotherapy in the clinic, although consideration should be taken when generalizing results from mouse tumor models. Citation Format: Rafael Cubas, Marina Moskalenko, Jeanne Cheung, Shiuh-Ming Luoh, Erin McNamara, Marcia Belvin, Jeong Kim, Stephen Gould. Effect of different standard of care chemotherapeutics on anti-PD-L1 responses in syngeneic mouse tumor models [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A114.

Abstract 4905: Effects of MAPK pathway inhibitors in the tumor immune microenvironment

Immunotherapeutic agents have shown great promise in the clinic in recent years and this has led to their approval as single agents or as immune doublet combinations in melanoma and lung cancer. In order to increase the extent of benefit from these agents and to extend immunotherapies to additional patients, combinations are being evaluated of immunotherapeutic agents with chemotherapy and targeted agents. Inhibitors of the mitogen-activated kinase protein kinase (MAPK) pathway, including BRAF and MEK inhibitors, have been approved in melanoma, and are being evaluated in additional indications. We evaluated the effects of MEK and ERK inhibitors on human T cells cultured in vitro as well as on the tumor microenvironment in preclinical syngeneic mouse models. In studies with human T cells in vitro, we find that MEK or ERK inhibition reduced proliferation of naive T cells, but had little effect on the proliferation of central memory cells, suggesting a differential requirement on MAPK signaling for proliferation of these T cell subsets. In vivo, MEK and ERK inhibition resulted in an increase in CD8+ infiltration in the tumor, as well as a decrease of markers of T cell exhaustion such as PD-1 and EOMES. The combination of MEK or ERK inhibitors plus anti-PD-L1 resulted in increased CD8+ effector function as measured by increased interferon gamma (IFNg) levels. In some cases, the MEK and ERK inhibitors showed differences in their pharmacodynamic effects as single agents; however, in combination with anti-PD-L1, the effects were similar. For example, single agent MEK inhibition but not single agent ERK inhibition increased the number of CD4+ helper and regulatory T cells (Tregs) in the tumor. However, both of these CD4+ subsets, as well as CD4+ IFNg levels, were increased with MEK or ERK inhibitors combined with anti-PD-L1. Similarly, single agent MEK inhibition decreased the number of infiltrating CD11b+Ly6G+ myeloid cells whereas single agent ERK inhibition increased the number of infiltrating CD11b+Ly6C+ myeloid cells. However, in combination with anti-PD-L1, these changes in myeloid populations were less apparent. Taken together, these data show that MAPK pathway suppression can modulate multiple immune cell subtypes within the tumor, with many of these changes expected to activate the immune infiltrate. Despite the increase in Tregs in response to ERK and MEK inhibition in combination with anti-PD-L1, these combinations were efficacious. This suggests that the other immune stimulatory effects of these treatments are sufficient to drive efficacy despite the presence of increased Tregs. Our data suggest that the combination of MAPK inhibition plus anti-PD-L1 inhibition, such as with atezolizumab, is a promising hypothesis to test in the clinic. Citation Format: Marcia P. Belvin, Erin Williams, Shiuh-Ming Luoh, Jeanne Cheung, Christine Orr, Emily Chan, Peter Ebert, Ira Mellman, Jeong Kim, Mark Merchant. Effects of MAPK pathway inhibitors in the tumor immune microenvironment. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4905.

Abstract 4898: Effects of chemotherapeutic agents on the tumor immune microenvironment

Immunotherapeutic agents have shown dramatic success in oncology in recent years and several have been approved in melanoma and lung cancer. One strategy for extending the benefit of immunotherapy is to evaluate these agents in combination with standard of care chemotherapy, with the goal of bringing the benefit of immunotherapy into earlier lines of treatment. We have carried out a systematic evaluation of the effects of different classes of chemotherapeutic agents (including alkylating agents, platinum based agents, and taxanes) on the tumor immune microenvironment of syngeneic murine tumor models (CT26 and MC38) in two different strains of mice (BALB/c and C57BL/6). We found that different chemotherapeutic agents caused markedly different changes in the composition and cell phenotype of the tumor immune environment. The majority of chemotherapeutic agents resulted in a mild to moderate increase in CD8+ T cells in the tumor. Some agents also altered the activation state of these CD8+ cells as measured by the level of PD-1 and IFN-gamma. A subset of the chemotherapeutic agents resulted in profound changes in the number of CD4+ cells, including regulatory T cells, as well as changes in myeloid subpopulations including monocytes, neutrophils, and macrophages. In addition to single agents studies, we carried out combination studies of selected chemo agents with simultaneous addition of anti-PD-L1. Several of these combinations caused pronounced increases in efficacy above single agent treatment. We will present the combination pharmacodynamic marker changes and efficacy of these treatments. Our findings demonstrate that chemotherapeutic agents can stimulate the tumor immune environment, at least transiently, through multiple mechanisms. Different subsets of chemotherapeutic agents display unique changes in the tumor immune infiltrate, and may require different immunotherapeutic approaches. These findings provide a rationale for testing immunotherapy-chemotherapy combinations in the clinic. Citation Format: Marcia P. Belvin, Shiuh-Ming Luoh, Jeanne Cheung, Erin McNamara, Rafael Cubas, Jeong Kim. Effects of chemotherapeutic agents on the tumor immune microenvironment. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4898.

MAP kinase inhibitors stimulate T cell and anti-tumor activity in combination with blockade of the PD-L1/PD-1 interaction

Pharmacological inhibition of the MAPK pathway with MEK or BRAF antagonists has proved successful in inducing regression of melanoma tumors bearing the targeted activating mutations. Moreover, antibodies targeting T-cell immune checkpoint inhibitors CTLA-4 or PD-L1/PD-1 have demonstrated the capacity to generate durable responses in patients with multiple cancer types. Thus, combining MAPK pathway-targeted agents with antibodies that enhance anti-tumor immunity represents an increasingly attractive treatment paradigm for cancer. However, little is known about the impact of tumor-targeted agents on immune function as similar signaling pathways drive both T-cell activation and cancer cell proliferation. Accordingly, agents targeting MAPK-dependent tumor growth would be predicted to also inhibit T-cell immunity. Here we show that, unexpectedly, potent suppression of T-cell receptor (TCR) function by MEK inhibition can be largely overcome in the presence of co-stimulation by anti-CD28 in vitro or blockade of the inhibitory PD-L1/PD-1 pathway in T cells in vivo. The ability of anti-CD28 to override suppression of T-cell activation by MEK inhibitors was dependent on the PI3K/mTOR pathway. Enhanced anti-tumor activity was also observed combining MEK inhibition with PD-L1 blockade, which was likely potentiated by upregulation of tumor MHC Class I expression through inhibition of MEK. Interestingly, inhibitors targeting BRAF V600E mutations actually augmented TCR-driven proliferation in vitro and T-cell function in vivo when combined with a vaccine or blockade of PD-L1 exclusively in the context of a wildtype BRAF background. These data demonstrate that targeting the MAPK pathway can be compatible with or even enhance T-cell function and provide rationale for combining these inhibitors with immunotherapy in clinical trials.