Serum and synovial fluid inhibitors of antibody-mediated lymphocytotoxicity in rheumatoid arthritis and systemic lupus erythematosus.

Many sera from patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) inhibit lymphocyte-dependent antibody cytotoxicity (LDAC). RA synovial fluids also inhibit LDAC. RA serum inhibition was present in high molecular weight and 5S serum fractions, whereas in SLE it was confined to 7S fractions. A correlation between rheumatoid factor activity and LDAC inhibition was noted, and there was some evidence for inhibition of SLE serum acting on effector cells. Inhibition in RA synovial fluid was found in both high molecular weight and very low molecular weight fractions (less than 4S.)