Evidence for a disease specific antigen in circulating immune complexes in ankylosing spondylitis

The presence of circulating immune complexes (CIC) has been documented in systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and ankylosing spondylitis (AS) by various investigators. It has been suggested that these may be used as probes to identify antigens playing a role in these pathological processes. Using a solid-phase cross-reaction assay to establish if these complexes reacted with each other in specific disease groups, it was found that polyethylene glycol (PEG) precipitates of six AS patients cross reacted in 29 of 36 tests, but reacted with SLE and RA PEG precipitates in only two of 24 tests in each case. SLE PEG precipitates cross-reacted in four of 14 tests and reacted with none of the six AS and four RA precipitates. Similarly, RA PEG precipitates did not cross-react (none of 16 tests), nor did they react with AS (none of 24 or SLE precipitates (none of 16). Similar results were observed when IgG, obtained after acid dissociation on sucrose density gradients of CIC isolated by PEG precipitation and staphylococcal protein A chromatography, was used as the solid phase component. F (ab')2 fragments with similar antibody specificity were obtained by pepsin digestion of isolated CIC from three of six AS patients. These bound radiolabelled AS PEG precipitates (2.02-2.40%) significantly more than SLE (0.22-0.28%) or RA (0.29-0.35%) precipitates. These studies demonstrate the feasibility of obtaining F (ab')2 fragments with antibody activity from isolated CIC and the presence of a disease specific antibody specificity in AS CIC. The nature of the antigen involved remains to be elucidated. Such a cross-reactive antibody specificity was not found in RA nor SLE CIC.