Citations

Abstract Cytotoxic T lymphocytes (CTL) are indispensable in anti-tumor immunity. Although CTLs are prone to exhaustion in patients with advanced cancer, T cell resiliency explains the presence of tumor-reactive CTLs that are less exhausted, capable of cytolytic function, expansion, and rebound in response to immunotherapy to reject metastatic malignancies. However, the features of resilient T cells have not been clearly defined. In this report, we demonstrate that peripheral CX3CR1+ CD8+ T cells with low mitochondrial membrane potential rebounded CTL function quickly after radiation therapy in patients with large tumor burden portraying their functional resiliency. Furthermore, CX3CR1+ CD8+ T cell with low, but not high, mitochondrial membrane potential are highly cytotoxic, accumulate less reactive oxygen species (ROS), and express more Malic enzyme 1 (ME1). ME1 overexpression increases ATP production in a glycolysis-independent manner while concurrently curtailing excessive ROS in activated CD8+ T cells; and expands CX3CR1+NKG7+ effector CD8+ T cells with enhanced cytotoxicity. Importantly, transfection of ME1 mRNA promotes tumoricidal activity in CD8+ T cells from patients with advanced cancers. Our study reveals a mechanism used by CTLs to balance excessive ROS via ME1 to maintain a metabolic and functional resiliency. Modification of ME1 expression in CTLs may be a novel method to improve the efficacy of cancer immunotherapy by preventing T cell exhaustion. Supported by grants from Mayo Clinic Foundation and NIH (R01CA 256927-2)

Abstract Although immune checkpoint blockade that targets the PD-L1 has revolutionized the treatment of human cancers, only a small proportion of patients positively respond to this form of treatment. A limitation of these therapies is that they only target and inhibit PD-L1’s extracellular function, which is to bind its receptor on PD-1 on T cells and inhibit T cell cytotoxicity. Recent discoveries have shown that PD-L1 expressed on tumor cells can signal intrinsically and independently of PD-1 ligation to promote tumor cell survival. Besides tumor cells, host myeloid cells have been shown to have high expression of PD-L1, therefore contributing to the immunosuppressive nature of the tumor. However, PD-L1 intrinsic signaling in human myeloid cells has not been studied. Therefore, the purpose of our work is to determine the intrinsic functions of PD-L1 in myeloid cells and how this impacts anti-tumor immunity. To study this, we used a novel PD-L1 antibody (H1A) that does not block PD-L1’s binding with PD-1, but directs PD-L1 for degradation, thereby decreasing total PD-L1 expression. Additionally, we developed a chimeric mouse model in which the murine PD-1/PD-L1 has been replaced with the human version. This allows us to evaluate PD-L1’s intrinsic function in vivo. In our preliminary studies, the degradation of PD-L1 in human monocytes led to increased chemokine release as well as increased conversion of effector T cells. Additionally, H1A treatment of tumor-bearing mice led to lower tumor burden in a CD8 T cell dependent manner. Based on these results, PD-L1 intrinsic signaling has the potential to regulate the immune-stimulatory response of monocytes, thereby limiting effector T cell responses against cancer. Supported by grants from NIH (R0192389) and Mayo Clinic graduate school of biomedical sciences

Abstract Only a small cohort of patients show durable responses to immune checkpoint inhibitor (ICI) therapy and the mechanisms regulating CD8 T cell responses to ICIs are yet to be elucidated. To that end, we explored T cell intrinsic factors that may be attributed to low ICI therapy response rates. We have identified Dual Specificity Phosphatase 2 (DUSP2) as a potential negative regulator of ICI response within CD8 T cells. We demonstrated that DUSP2 expression in human CD8 T cells is transient, peaking at two-hours post-T cell stimulation with rapid waning thereafter. Downregulation of DUSP2, via siRNA transfection, increased cytotoxicity of human CD8 T cells activated for 48-hours prior to inclusion in a four-hour tumor killing co-culture assay, despite the return of DUSP2 expression to baseline levels at this time-point. Additionally, DUSP2 overexpression, via mRNA transfection, decreased release of effector molecules (Granzyme B and Interferon-gamma) from human CD8 T cells at 24-hours post-activation when compared to control mRNA. When we utilized RNA-sequencing to analyze and compare differentially expressed genes between responders and non-responders of ICI therapy, we found DUSP2 expression in CX3CR1 positive CD8 T cells to be consistently decreased in responders but not in non-responders. Since CX3CR1 positive CD8 T cells identify a highly resilient and cytotoxic subset in response to ICI therapy, we hypothesize that DUSP2 negatively regulates ICI-responsive effector CD8 T cells. Thus, uncovering the role of DUSP2 in CD8 T cells may provide a target to improve the efficacy of ICIs in patients with limited therapeutic alternatives. Mayo Clinic Department of Immunology Pilot Award Mayo Clinic Center for Individualized Medicine

Abstract Though anti-PD-1/PD-L1 immune checkpoint blockade therapy has demonstrated robust anti-tumor immunity in human patients with advanced cancers such as melanoma, only a fraction of patients achieved such durable anti-tumor responses. Among all the non-responders, some patients responded initially but developed resistance to the therapy in the later treatments. This reflects an urgent need to understand the role of PD-1 signals in CD8+ T cells at priming and at effector stages during tumor development for the benefit of immunotherapy. The controversy of rapid tumor progression in PD-1 conditional knockout in regulatory T cells and tumor regression in PD-1 targeted deletion in regulatory T cells indicated germline PD-1 knockout mice might not be sophisticated enough to study the role of PD-1 in CD8+ T cells during tumor development. Thus, we produced a CD8 T cell-specific PD-1 knockout mouse model (CD8 E8ICre-PD-1fl/fl, also called CD8-PD-1 cKO mice) in which PD-1 is deleted in CD8+ T cells in the peripheral tissues. When CD8-PD-1 cKO mice were challenged with B16-OVA mouse melanoma cells, we found a delayed tumor growth and prolonged survival of CD8-PD-1 cKO compared to control mice. However, after 20 days of tumor challenge, the delayed tumor growth rebound in CD8-PD-1 cKO mice. In an OVA antigen immunization model, we found increased effector cells and tissue resident cells in the spleen of CD8-PD-1 cKO mice compared with control mice on day 7 after immunization. Taken together, our results suggest that PD-1 may restrain the expansion of effector T cells that contribute to the early phase of tumor control, but the augmented effector T cell response may cause an attrition of memory T cell pool that will be needed for long term protection. R01CA256927

Abstract Seven different anti–PD-1 and PD-L1 mAbs are now widely used in the United States to treat a variety of cancer types, but no clinical trials have compared them directly. Furthermore, because many of these Abs do not cross-react between mouse and human proteins, no preclinical models exist in which to consider these types of questions. Thus, we produced humanized PD-1 and PD-L1 mice in which the extracellular domains of both mouse PD-1 and PD-L1 were replaced with the corresponding human sequences. Using this new model, we sought to compare the strength of the immune response generated by Food and Drug Administration–approved Abs. To do this, we performed an in vivo T cell priming assay in which anti–PD-1/L1 therapies were given at the time of T cell priming against surrogate tumor Ag (OVA), followed by subsequent B16-OVA tumor challenge. Surprisingly, both control and Ab-treated mice formed an equally robust OVA-specific T cell response at the time of priming. Despite this, anti–PD-1/L1-treated mice exhibited significantly better tumor rejection versus controls, with avelumab generating the best protection. To determine what could be mediating this, we identified the increased production of CX3CR1+PD-1+CD8+ cytotoxic T cells in the avelumab-treated mice, the same phenotype of effector T cells known to increase in clinical responders to PD-1/L1 therapy. Thus, our model permits the direct comparison of Food and Drug Administration–approved anti–PD-1/L1 mAbs and further correlates successful tumor rejection with the level of CX3CR1+PD-1+CD8 + T cells, making this model a critical tool for optimizing and better utilizing anti–PD-1/L1 therapeutics.

Abstract Though anti-PD-1/L1 antibody therapies have generated unprecedented responses in a limited number of patients, the biological and immunological parameters that define their effective use are still largely unknown. The absence of appropriate pre-clinical models has limited our ability to compare the mechanistic differences between the approved PD-1/L1 antibodies in vivo and to determine the context in which each might be most effective. To address this critical need, we have produced humanized PD-1 and PD-L1 mice in which the extracellular domains (antibody binding sites) of mouse PD-1 and PD-L1 are replaced with the sequences of human PD-1 and PD-L1. The PD-1 and PD-L1 in the resulting “HuPD” mice can be recognized by FDA-approved antibodies, while the intracellular domains are kept intact for normal downstream signals. Following characterization of the new mouse model, in our first in vivo treatment studies, HuPD mice were immunized with OVA/Poly I:C in the context of FDA-approved PD-1/L1 antibodies and antigen-specific T cell production was quantified. Nivolumab, Pembrolizumab, and Atezolizumab all increased effector CD8+T cell production against the OVA antigen as expected given their immune stimulating function. Surprisingly, OVA-tetramer staining was unable to detect antigen-specific CD8+T cells in the spleens of immunized mice treated with Avelumab. This may reflect the unique ability of Avelumab to induce ADCC, and indicates that our model is capable of distinguishing key differences among the antibodies in vivo. Ongoing studies will identify how these differences might be exploited to render the most effective results for patients in a variety of tumor types and immune environments.

Abstract The immune response to anti-PD-1 therapy in cancer patients is not completely defined due to lack of reliable biomarkers that can determine T cell function. Bim, a member of the Bcl-2 family, was recently identified as a downstream signaling molecule in the PD-1 pathway. Within CD8 T cells, Bim appears to have a dual function, and can be pro-apoptotic or promote effector activity. Our clinical data has shown that high levels of Bim in CD8 T cells are predictive of response to anti-PD-1 therapy in metastatic melanoma whereas in colorectal cancer, a low level of Bim is associated with treatment response. Thus, we sought to determine what molecular partners might influence the role of Bim. Using co-immunoprecipitation, high resolution microscopy and FRET we identified ANT2 (Adenine Nucleotide Translocase 2), an inner membrane protein of mitochondria, as a protein that interacts with Bim. Accordingly, we found CD8 T cells with low and high mitochondrial potential demonstrated different ANT2/Bim ratios, and a low ANT2/Bim ratio was present in effector T cells with low mitochondrial potential but high cytotoxic activity. Given the pro-survival function of ANT2, our results indicate that ANT2 may balance the pro-apoptotic and effector roles of Bim in mitochondria in order to preserve effector T cells before contraction. Understanding the molecular mechanism of Bim and ANT2 interactions in CD8 T cells may provide a novel marker to predict clinical response to anti-PD-1 therapy.

Abstract B7-H1 (also named PD-L1) is up-regulated on activated CD8 T cells at the late stage of antigen stimulation, and is required for T cell survival during contraction phase. Since more anti-B7-H1 (PD-L1) antibody has been used in clinical trials of treatment of human cancers, it is necessary to know whether antibody ligation of B7-H1 expressed by T cells would affect T cell function and survival. After screening several anti-B7-H1 antibodies, we found that some antibody triggered the activation of p38 MAPK in activated CD8 T cells. In addition, the levels of activated p38 MAPK increased in T cells isolated from B7-H1 knockout mice compared to wild type mice. Inhibition of DNA-PKcs activation increased the activation of p38 MAPK in wild type T cells but not in B7-H1 KO T cells, suggesting B7-H1 may negatively regulate p38 MAPK through DNA-PKcs pathway. To that end, we identified an association of B7-H1 with DNA-PKcs in activated human T cells but not in resting T cells. Since p38 MAPK inhibition rescued activated CD8 T cells from apoptosis, the pro-survival function of T cell intrinsic B7-H1 may be mediated by suppression of p38 MAPK. However, ligation of B7-H1 by agonist antibody may disrupt B7-H1’s pro-survival function by increasing p38 MAPK activation. Our results identified a link of B7-H1 with p38 MAPK pathway and may help to define agonist function of anti-B7-H1 antibody prior to clinical application.

Abstract A promising strategy in tumor immunotherapy is the use of activated dendritic cells as vehicles for tumor vaccines. Clinical trials of dendritic cell-based immunotherapies have proven successful, resulting in recent FDA approval for their use in the treatment of late-stage prostate cancer. Current formulations for dendritic cell-based immunotherapies are capable of prolonging patient survival by only four months, providing motivation for further investigation of ways to enhance dendritic cell priming of anti-tumor T cell responses. Since the discovery of B7-H1 by our laboratory, we have been investigating the regulatory function of B7-H1 expressed by immune cells. In preliminary studies investigating the use of B7-H1 knockout dendritic cells as tumor vaccines, we found that B7-H1 knockout dendritic cells prime enhanced anti-tumor CD8 T cell responses. Further investigations revealed that the enhanced CD8 T cell response raised by B7-H1 knockout dendritic cells is not due to a lack of signaling through the receptors for B7-H1 on CD8 T cells. We observed that B7-H1 knockout dendritic cells produced increased levels of IL-12, a key cytokine that is centrally involved in both innate and adaptive immunity. Our results suggest that B7-H1 expressed by dendritic cells has a role in negatively regulating IL-12 production, and as a result B7-H1 deficient dendritic cells are able to prime enhanced anti-tumor CD8 T cell responses.

Abstract We utilized CD11a, an integrin up-regulated on effector and memory CD8+ T cells, as a biomarker to identify, measure, and monitor endogenous tumor-reactive cytotoxic lymphocytes (CTLs) in two mouse tumor models and in melanoma patients. CD11ahigh CD8+ T cells were detected in primary and metastatic tumor sites after tumor cell injection, as well as in spontaneous murine breast cancer tissues. In the peripheral blood of melanoma patients, tumor antigen-specific T cells were detected within the population of CD11ahigh CD8+ T cells that express high levels of PD-1. Infusion of lethally irradiated tumor cells failed to stimulate the expansion of CD11ahigh CD8+ T cells, suggesting that the access of tumor cells to target tissues is required to induce CD11ahigh CD8+ T cell responses. Phenotypic analysis revealed that the CD11ahigh CD8+ T cells detected in our studies were proliferating (Ki67+) activated T cells (CD62L-, CD69+). Although CD11ahigh CD8+ T cells were differentiated effector cells with CTL effector functions, they were typically unable to control tumor growth in our mouse models, possibly due to high expression of PD-1. Our results demonstrate that CD11a can be used as a biomarker to identify and track endogenous tumor-reactive CTLs.