Reporters of TCR signaling identify arthritogenic T cells in murine and human autoimmune arthritis
How pathogenic CD4 T cells in Rheumatoid Arthritis (RA) develop remains poorly understood. We used Nur77--a marker of T cell antigen receptor (TCR) signaling--to identify antigen-activated CD4 T cells in the SKG mouse model of autoimmune arthritis and in patients with RA. Using a fluorescent reporter of Nur77 expression in SKG mice, we found that higher levels of Nur77-eGFP in SKG CD4 T cells marked their autoreactivity, arthritogenic potential, and ability to more readily differentiate into IL-17 producing cells. Moreover, the enhanced autoreactivity was associated with upregulation of IL-6 cytokine signaling machinery. As a result, the more autoreactive GFPhi CD4 T cells from SKGNur mice were hyper-responsive to IL-6. This suggests that, despite impaired TCR signaling, autoreactive T cells exposed to chronic antigen stimulation exhibit heightened sensitivity to IL-6 which contributes to the arthritogenicity in SKG mice, and perhaps in patients with RA. Additionally, endogenous Nur77 protein expression was enriched in a subset of synovial CD4 T cells from arthritic joints in SKG mice, consistent with antigen-activated T cells. This was was extended to patients with seropositive RA, suggesting that pathogenic CD4 T cells are enriched in RA joints and are likely recognizing an intra-articular auto-antigen(s).