Identification of autoantibodies targeting citrullinated CLEC12A in rheumatoid arthritis patients

Rheumatoid arthritis is an autoimmune disease characterized by anti-citrullinated protein antibodies (ACPA). The pathogenic and protective roles of ACPA of distinct specificities are emerging and remains poorly understood. Thus, it is crucial to define the range of ACPA specificities and determine their contribution to disease and their potential clinical relevance. Since extracellular citrullination occurs in RA, we investigated whether autoantibodies in RA patients bind a citrullinated form of the cell-surface receptor CLEC12A that is expressed on neutrophils, the most abundant leukocyte in inflamed joints.

We generated a FLAG-tagged, recombinant form of the extracellular portion of human CLEC12A. After purification, the tag was removed prior to citrullination by PAD2 that was confirmed by mass spectrometry. We developed an ELISA for citrullinated CLEC12A to screen for seropositivity in sera of 68 RA patients and 36 healthy controls. Potential associations between these autoantibodies and clinical variables were determined.

In our cohort, 40 % of RA patients were positive for anti-citrullinated CLEC12A autoantibodies. Those seropositive patients were younger than RA patients that tested negative for these autoantibodies (p = 0.0058). Most patients had antibodies to multiple citrullinated and homocitrullinated antigens; 17 % of patients negative for other ACPA were positive for anti-citrullinated CLEC12A autoantibodies.

This is the first report of seropositivity towards citrullinated CLEC12A in RA patients. A validation cohort will confirm our findings and identify additional correlations between these autoantibodies and clinical parameters. Citrullination may be a mechanism through which CLEC12A's inhibitory function is altered to exacerbate inflammation in RA. Identifying citrullinated neoantigens advances our understanding of the diverse molecular mechanisms that contribute to RA pathogenesis.