The Role of Peptidyl Arginine Deiminases in Regulating Anti-tumor Responses in Immune Cells

Abstract

Arginine deimination – also known as citrullination – is an understudied post-translational modification which continues to garner interest across biomedical research fields since the mechanism has been implicated in the progression of multiple diseases. While citrullination has been shown to control processes in pluripotent stem cells and cancer cells, studies on the regulatory roles of citrullination in immune cell subsets are limited. A large body of work has accumulated over the decades describing the process by which aberrant histone citrullination in neutrophils drives a number of autoimmune diseases – most notably rheumatoid arthritis ¬– however the disease-associated processes regulated by citrullination in mononuclear immune cells have yet to be comprehensively studied. A family of peptidyl arginine deiminases (PADs) citrullinate protein targets catalyzing the conversion of arginine residues into peptidylcitrulline changing protein charge from positive to neutral and as a consequence altering protein folding and function. The five PAD isozyme family members including PAD1-4 and 6, modify a wide range of protein targets, however only PAD2 and PAD4 have been shown to translocate into the nucleus and therefore may have more access to controlling fundamental functions of the cell via the citrullination of both cytoplasmic and nuclear proteins alike. In this body of work, we begin by surveying the current knowledge on PAD2 and PAD4-mediated citrullination in immune cell subsets including neutrophils, T-cells and macrophages. To substantiate our understanding on the range of consequences from protein citrullination, we will briefly discuss the outcomes of citrullination in cancer cells and stem cells. Importantly, we will present our original findings demonstrating a key role of PAD4 in macrophages and a preliminary understanding of an important role for PAD2 in lymphocytes. In macrophages, we discovered that PAD4 citrullinates transcription factor signal transducer and activator of transcription 1 (STAT1) facilitating the inhibitory interaction between STAT1 and the protein inhibitor of activated STAT1 (PIAS1), negatively regulating STAT1 binding to DNA and, consequently, restraining key downstream pathways. We observed that MHC-II–mediated antigen presentation was selectively restrained and that this had a major impact on tumor growth in vivo and the response to immunotherapy in mice and in humans. In this dissertation, more focus will be placed on the study of PAD4 activity in macrophages. These studies provide novel as well as useful mechanistic insights into how citrullination may be clinically targeted.

Through this work, we aim to elucidate the significance of citrullination in the regulation of immune cell functions for the purpose of suggesting means of targeting immune cell citrullination for the enhancement of anti-tumor immunity.