Novel mechanisms regulating PD-1 signaling and function

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ABSTRACT Cancer immunotherapies represent a powerful and newly emergent therapeutic paradigm, both because of their durable clinical responses and applicability to a wide variety of tumors. Immune checkpoint therapies block inhibitory receptors on T cells in order to augment anti-tumor immune responses. Programmed cell death protein 1 (PD-1) is a critical inhibitory checkpoint for T cells. The identification and clearance of malignant cells can be brought about by antibodies which block PD-1. Despite the success of these antibodies, most patients do not respond to PD-1 blockade, and many experience immune-related adverse events. New studies indicate that some 5-10% of patients demonstrate accelerated cancer progression after anti-PD-1 treatment, in contrast to predicted responses based on current mechanistic models. With both the potential successes and failures of PD-1 being so significant, the need to understand PD-1 signaling is evidently very urgent, both for explaining the mechanism of clinical responses and for developing therapeutics that go beyond simply interfering with ligand binding. We have developed a new method to analyze mass spectrometry data and discovered novel effectors of PD-1 signaling. Most excitingly, we have identified multiple candidates, including the kinase VRK2 which we intend to investigate further. The overarching goal of this proposal is to study PD-1-associated kinases in order to better define novel signaling pathways and to uncover T cell-intrinsic mechanisms which contribute to resistance to PD-1 blockade. In the first aim, we will discover the molecular mechanism by which VRK2 controls PD-1 signaling in T cells. We will perform structure-function analyses to test the hypothesis that VRK2’s enzymatic domain is required for mediating specific PD-1 functions and utilize biochemical and imaging approaches to uncover the contribution made by VRK2 interactions with MAPK8IP1 and MAP3K7 towards PD-1 signaling. In the second aim we will test the hypothesis that VRK2 is required for PD-1 inhibition of cellular functions in vivo. We will utilize VRK2 KO mice to uncover the mechanism by which VRK2 supports tumor growth in vivo in the context of PD-1 blockade and resistance. Given the large impact of PD- 1 on public health, the proposed work is incredibly significant.