Potential Role of Decoy B7-H4 in the Pathogenesis of Rheumatoid Arthritis: A Mouse Model Informed by Clinical Data

Background: A pathogenic hallmark of rheumatoid arthritis (RA) is persistent inflammatory responses in target tissues and organs. Immune responses mediated by T cells and autoantibodies are known to play pivotal roles. A possible interpretation for this observation is a loss of negative regulation of autoimmune responses. Here we sought to investigate whether B7-H4, a cell surface inhibitory molecule of the B7-CD28 signaling pathway, may play a role in the pathogenesis of RA. Methods and Findings: In a cross-sectional study of a clinical convenience sample using monoclonal antibodies against human B7-H4 molecules, we detected high levels of the soluble form of B7-H4 (sH4) in the sera of 65% of patients with RA (n = 68) versus only 13% of healthy donors (n = 24). Elevated sH4 was associated with an increased disease severity score (DAS28) in a cross-sectional analysis. In a mouse model of RA, transgenic expression of sH4 or genetic deletion of B7-H4 accelerated the progression of collagen-induced arthritis, accompanied by enhanced T and B cell-mediated autoimmune responses as well as increased activity of neutrophils. Expression in vivo of an agonist, a B7-H4-immunoglobulin Fc fusion protein, profoundly suppressed disease progression in the mouse model. Conclusions: Our findings in mice indicate that sH4 acts as a decoy molecule to block the inhibitory functions of cell-surface B7-H4, leading to exacerbation of collagen-induced arthritis. If the preliminary correlation between sH4 levels and disease activity in patients with RA can be confirmed to reflect a similar mechanism, these findings suggest a novel target for treatment approaches.