Card9 promotes pathogenic neutrophil responses that induce Th17-mediated autoimmune arthritis and spondylitis

Abstract

Abstract Caspase recruitment domain-containing protein 9 (Card9) promotes anti-fungal immunity following C-type lectin receptor Dectin-1 activation by b-glucans. CARD9 has also been implicated in the pathogenesis of T cell-mediated inflammatory diseases including rheumatoid arthritis and ankylosing spondylitis, but how Card9 controls non-infectious inflammation is poorly understood. Using genetically susceptible SKG mice that develop b-glucan (zymosan)-induced, Th17-mediated autoimmune arthritis and spondylitis we found that Card9-deficiency (Card9−/− SKG mice) conferred complete protection from disease. Card9 promoted arthritis via a Dectin-1-mediated mechanism, as the Dectin-1-specific ligand (curdlan) induced disease and bacterial/fungal ligands specific for Dectin-2 or Mincle did not. At 8 weeks post-zymosan Th17 cells and neutrophils were dramatically reduced in Card9−/− SKG mouse joints, indicating an interplay between Card9, neutrophils and Th17 cells in promoting arthritis. Importantly, pre-arthritic Card9−/− SKG mice had reduced neutrophils in lavage fluid 4 hours post-zymosan compared to SKG mice, but no changes in dendritic cells or macrophages. Lavage fluid neutrophils from Card9−/− SKG mice had similar Dectin-1 expression to SKG mice, yet decreased activation (retention of CD62L) and degranulation of primary (CD63) and secondary (CD66b). Akin to Card9−/− SKG mice, neutrophil-deplete SKG mice had decreased Th17 cells and delayed arthritis onset, supporting a pathogenic function for neutrophils in induction of arthritis. These data suggest that Card9 functions downstream of Dectin-1 to promote pathogenic neutrophil responses necessary to induce Th17-mediated arthritis and spondylitis.