CD8 T cell derived Granzyme K activates complement that drives synovial fibroblast inflammation

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The role of T cells in RA is established, but nearly all research has focused on CD4 T cells. We have shown that CD8 T cells are similarly expanded and activated in inflamed RA synovium yet do not belong to the typical granzyme (Gzm) B+ cytotoxic T lymphocyte (CTL) subset. Instead, they express high levels of GzmK whose function is not well defined. Remarkably, we find that GzmK drives a new pathway of complement activation. We show that GzmK can cleave C4 and C2 into C4b and C2a to generate an active C3 convertase (C4b2a) that cleaves C3 to C3a and C3b. We show that fibroblasts express the highest levels of complement components C2, C3 and C4 in the synovium and can secrete these proteins in response to CD8 T cell-derived TNF and IFNg. We show that GzmK can elicit formation of an active C3 covertase by cleaving complement components secreted by fibroblasts. Further, we demonstrate that GzmK generated C3a and C3b are bioactive and can drive mast cell degranulation and opsonize cells, respectively. We hypothesize that GzmK-mediated complement activation by GzmK in vivo occurs on cell surfaces as we show GzmK binds strongly to plasma membranes. Recently published research by others shows that complement activation drives inflammatory priming of murine synovial fibroblasts, mediated by mTOR activation with downstream metabolic changes and production of pro-inflammatory molecules. We propose to determine if human CD8 T cell-derived GzmK can induce inflammatory priming of human synovial fibroblasts by mediating local cellular complement activation and whether GzmK can drive complement activation and inflammatory arthritis in murine models. In Aim 1, we study GzmK binding to cell surfaces and determine its ability to assemble a C5 convertase, resulting in the generation of the anaphylatoxin C5a and a C5b-9 membrane attack complex. In Aim 2, we define the ability of GzmK to induce inflammatory priming of synovial fibroblasts via complement activation, by determining the effects of 1) recombinant and live CD8 T cell-derived GzmK on the inflammatory and metabolic activation of synovial fibroblasts. Then, using CRISPR GzmK-deleted T cells or CRISPR complement factor or complement receptor deleted fibroblasts, we confirm if these effects are mediated directly by GzmK and complement. In Aim 3, we use GzmK globally deficient mice and targeted CD8 T cell GzmK-deficient mice to demonstrate the role of GzmK in vivo in driving inflammation, complement activation, joint damage and fibroblast activation in the CIA mouse model of inflammatory arthritis. Together, these studies define a new pathway of complement activation mediated by lymphocyte- derived GzmK, and its role in inflammatory fibroblast activation and joint pathology. These studies will unravel key elements of how the pathway works and determine its role in arthritis in vivo. These findings provide new insights into the immunopathology of RA and point to GzmK and CD8 T cells as potential new therapeutic targets.