Reduced TCR‐dependent activation through citrullination of a T‐cell epitope enhances Th17 development by disruption of the STAT3/5 balance

Citrullination is a post‐translational modification of arginine that commonly occurs in inflammatory tissues. Because T‐cell receptor (TCR) signal quantity and quality can regulate T‐cell differentiation, citrullination within a T‐cell epitope has potential implications for T‐cell effector function. Here, we investigated how citrullination of an immunedominant T‐cell epitope affected Th17 development. Murine naïve CD4+ T cells with a transgenic TCR recognising p89‐103 of the G1 domain of aggrecan (agg) were co‐cultured with syngeneic bone marrow‐derived dendritic cells (BMDC) presenting the native or citrullinated peptides. In the presence of pro‐Th17 cytokines, the peptide citrullinated on residue 93 (R93Cit) significantly enhanced Th17 development whilst impairing the Th2 response, compared to the native peptide. T cells responding to R93Cit produced less IL‐2, expressed lower levels of the IL‐2 receptor subunit CD25, and showed reduced STAT5 phosphorylation, whilst STAT3 activation was unaltered. IL‐2 blockade in native p89‐103‐primed T cells enhanced the phosphorylated STAT3/STAT5 ratio, and concomitantly enhanced Th17 development. Our data illustrate how a post‐translational modification of a TCR contact point may promote Th17 development by altering the balance between STAT5 and STAT3 activation in responding T cells, and provide new insight into how protein citrullination may influence effector Th‐cell development in inflammatory disorders. Citrullination of an aggrecan TCR epitope leads to lower levels of IL‐2‐mediated STAT5 signaling during naïve Th cell priming, hence (i) abrogating Th2 responses and (ii) facilitating Th17 differentiation in response to STAT3‐activating cytokines by reducing the inhibitory effect of STAT5 on IL‐17 production.