Humoral autoreactivity directed against surfactant protein‐A (SP‐A) in rheumatoid arthritis synovial fluids
SP‐A is found principally in the lung, and has been associated with lamellar bodies also found in the synovial joint. Both SP‐A and C1q contain collagen‐like regions, and SP‐A and C1q have some structural similarities, both having a globular head region and a collagen‐like tail. Here we are able to show that (i) autoreactivity to SP‐A, as expressed by IgG and IgM autoantibodies, is present in synovial fluid (SF) isolated from patients with rheumatoid arthritis (RA); (ii) in absorption experiments only a limited degree of cross‐reactivity between autoantibodies reactive with C1q and SP‐A is observed; (iii) there is no cross‐reactivity between autoantibodies reactive with type II collagen (CII) and those reactive with SP‐A or C1q; (iv) autoantibodies react with polymeric (dimers and larger) SP‐A, but not with monomeric SP‐A subunits, indicating that a degree of quaternary structure is required for antibody binding. Unlike CII, which not accessible in the normal joint, both SP‐A and C1q are available within the SF in patients with RA and may therefore provide antigens driving an autoimmune response directed against collagen‐like structures.