Granzyme K drives a new pathway of complement activation that contributes to pathology in inflammatory skin disease

Summary: Abstract Body: Granzyme K (GZMK) expressing CD8+ T-cells have recently been defined as the predominant CD8+ T-cell population in inflamed tissues in multiple autoimmune diseases. In human psoriasis and cutaneous lupus we see numerous GZMK+ T-cells by immunofluorescence. However, the overall function of GZMK has been relatively undefined. Here, we show that GZMK is a novel activator of the complement cascade. GZMK cleaves C2 to C2b and C4 to C4b to form an active C3 convertase that cleaves C3 into bioactive C3a and C3b. GZMK further forms active C5 convertases that generate C5a and the membrane attack complex, thus generating all key components of the complement cascade. GZMK is continuously released from CD8+ T-cells in an antigen-independent manner. Upon release, GZMK binds to negatively charged surface molecules through electrostatic interactions. Interference of GZMK cell surface binding limits C3b opsonization suggesting that GzmK must be membrane bound to elicit formation of an active C3 convertase. Within tissues, fibroblasts are the major producer of C2, C3 and C4. IFNg and TNFa, cytokines secreted by GZMK+ T-cells, drive fibroblasts to increase synthesis and release of C2-C4. Importantly, in an imiquimod induced dermatitis model, Gzmk-/- mice have less severe erythema, scaling and thickening of skin compared to Gzmk+/+ or Gzmk+/- mice. Strikingly, Gzmk-/- mice have markedly less C3d and C4d, products of complement pathway activation at the dermal epidermal junction following IMQ treatment. These data suggest GZMK is activating the complement cascade in vivo to drive inflammation. Taken together, we define a new pathway of complement activation independent of the previously well-described classical, alternative, and lectin pathways that is dependent on GZMK and has the potential to drive inflammation in a variety of tissues and disease states. Erin Theisen¹, Carlos Donado¹, Madison L. Fairfield¹, A. Helena Jonsson², Michael Brenner¹ 1. Brigham and Women's Hospital, Boston, MA, United States. 2. University of Colorado Anschutz Medical Campus School of Medicine, Aurora, CO, United States. Innate Immunity, Microbiology, and Microbiome