Exploration of Tumor-Intrinsic NLRP3 Signaling Regulators

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Abstract. Despite the positive impact that checkpoint inhibitor immunotherapy has had on the field of oncology, the majority of our cancer patients do not respond to this treatment strategy. It is now generally believed that a more complete understanding of the mechanisms driving resistance to checkpoint inhibitor immunotherapy will lead to the development of more effective immunotherapy regimens and to improved patient selection for specific therapies. However, our understanding of active tumor-mediated resistance mechanisms that are more responsive to pharmacologic targeting remains poor. Myeloid-derived suppressor cells (MDSCs) are an immunosuppressive cell population that have been correlated with inferior responses to checkpoint inhibitor therapy. Using pre-clinical models of different tumor types as well as clinical specimens harvested from melanoma patients, we have determined that resistance to anti-PD-1 antibody (ab) immunotherapy is associated with the recruitment of granulocytic MDSCs (PMN-MDSCs) into the tumor bed. Our subsequent studies have shown that this accumulation of PMN-MDSCs in tumors is dependent upon the upregulation of CXCR2- dependent chemokines via a Wnt5a-YAP1 signaling axis and that this pathway is, in turn, triggered by the release of heat shock protein-70 (HSP70) by tumors in response to anti-PD-(L)1 antibody-mediated CD8+ T cell activation. We have also determined that effector CD8+ T cells activate this signaling cascade by stimulating a tumor intrinsic PD-L1-NLRP3 inflammasome pathway which facilitates the release of soluble HSP70. Our studies further indicate that plasma levels of HSP70 are more significantly elevated in those patients who fail to respond to anti-PD-1 antibody immunotherapy. Our cumulative data indicate that either alterations in the expression or function of various regulators of the NLRP3 inflammasome may dictate the activation threshold of this resistance pathway and therefore determine whether certain tumors respond or fail to respond to anti-PD-(L)1 checkpoint inhibitor immunotherapies. While activation and regulation of the NLRP3 inflammasome has been well characterized in various innate immune cell populations such as macrophages and dendritic cells, much less is understood regarding the regulation of this signaling pathway in tumors. We have recently found that unique NLRP3 binding partners exist in tumor tissues relative to innate immune cell populations. In silico studies also show that mutations in the NLRP3 gene and various inflammasome regulators have been identified in various cancer types, including melanoma. We therefore hypothesize that the identification of tumor-specific NLRP3- binding proteins which regulate the activation state of this NLRP3 inflammasome-dependent signaling pathway would provide putative novel pharmacologic targets capable of reversing this immunotherapy resistance mechanism without impacting the innate immune system of the host. We further propose that genetic mutations involving regulators of the NLRP3 inflammasome will explain certain forms of tumor resistance to anti-PD-(L)1 checkpoint inhibitor immunotherapy.