Autoantibody cross-reactivity with a microbial protein from a prevalent human gut commensal in antiphospholipid syndrome

Given the vast antigenic source of the gut microbiota, we hypothesized that human gut commensal bacteria induce and sustain autoreactivity via cross-reactivity. As a paradigm, we studied the antiphospholipid syndrome (APS), which is a serious autoimmune clotting disorder of unknown etiology but with a well-defined major autoantigen, b 2 -glycoprotein I (b 2 GPI). Infectious triggers are implicated in transient autoantibody production, but the persistent stimuli for anti-b 2 GPI antibodies remain unknown. To this end, we characterized b 2 GPI-specific monoclonal autoantibody reactivity to an in silico identified candidate commensal, Roseburia intestinalis ( R.int ), that is abundant in human stool from APS patients as detected by a species-specific PCR. Using an APS-derived monoclonal antibody, P1-117, that binds to the major B cell epitope in domain I of b 2 GPI (RGGMR), we tested for cross-reactivity to anaerobically cultured R.int and its candidate cross-reactive R.int protein (WP_006857340.1). P1-117 bound significantly to R.int lysates whereas a control antibody specific for another epitope within b 2 GPI did not. Further, P1-117 displayed significant binding to the recombinantly expressed R.int cross-reactive candidate protein. BALB/cJ mice immunized with whole heat-killed R.int displayed significant cross-reactive splenocytes and low titers of cross-reactive IgG autoantibodies. In conclusion, we present here in vitro and in vivo studies supporting cross-reactivity of pathogenic b 2 GPI-specific autoantibody with a common gut commensal. This study provides a concept for persistently colonizing gut commensals as chronic cross-reactive triggers in genetically susceptible autoimmune hosts such as APS patients.