Prevalence of anti-Fab antibodies in patients with autoimmune and infectious diseases.

Sensitive ELISA were devised to examine the specificity of circulating IgM and IgA autoantibodies for whole human IgG, Fc and Fab fragments of human IgG. Sera from patients with autoimmune and infectious conditions such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), tuberculosis (TB), infectious mononucleosis (IM) and cystic fibrosis (CF) were studied. Results of the ELISA assays using whole human IgG as antigen revealed that a proportion of patients in each of the groups studied had circulating IgM and IgA rheumatoid factors (RF). Fifteen normal individuals studied were negative. In the latex positive RA group, IgM RF and IgA RF had primarily anti-Fc reactivity (100% and 93% respectively), although 3/15 patients also showed IgM anti-Fab reactivity and one patient had high IgA anti-Fab activity. Patients with SLE and TB who had detectable RF levels also revealed predominantly anti-Fc specificity. In contrast, examination of 25 patients with IM showed positivity for IgM RF activity in 8% of patients using whole IgG as antigen, 24% positivity using purified Fc fragments as antigen and 45% positivity when plates were coated with Fab fragments. Similarly, a large number of CF patients (54%) also showed predominantly IgM anti-Fab activity. Of interest, 69% of the CF patients who were all studied at the time of bacterial infection had detectable IgA RF levels, with 46% of these patients showing both IgA anti-Fc and anti-Fab activity. These findings suggest that autoantibody specificities in autoimmune and infectious diseases are different.