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 lamella r bodies also found in the synovial joint. Both SP-A and C1q contain collag en-like regions, and SP-A and C1q have some structural similarities, both h aving 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 autoantib odies, is present in synovial fluid (SF) isolated from patients with rheuma toid arthritis (RA); (ii) in absorption experiments only a limited degree o f cross-reactivity between autoantibodies reactive with C1q and SP-A is obs erved; (iii) there is no cross-reactivity between autoantibodies reactive w ith type II collagen (CII) and those reactive with SP-A or C1q; (iv) autoan tibodies react with polymeric (dimers and larger) SP-A, but not with monome ric SP-A subunits, indicating that a degree of quaternary structure is requ ired for antibody binding. Unlike CII, which not accessible in the normal j oint, both SP-A and C1q are available within the SF in patients with RA and may therefore provide antigens driving an autoimmune response directed aga inst collagen-like structures.