Suppression of in vitro antibody response of human peripheral blood lymphocytes by a heat-labile factor in normal human serum.

When fresh autologous serum was added to normal human peripheral blood lymphocytes (PBL), it suppressed greater than 90% of the in vitro anti-SRBC response of these cells. Heating the serum for 30 min at 56 degrees C reversed this suppression, Serum from a patient with rheumatoid arthritis and circulating immune complexes had no suppressive effect on the anti-SRBC response of normal human PBL, but serum from patients having the same disease, without circulating immune complexes, did suppress over 90% of the plaque-forming cell response. Serum from an agammaglobulinaemic patient was also suppressive. Addition of serum from patients with congenital deficiencies of C2, C3, C5 and C8 also has a suppressive effect. Absorption of normal serum with immune complexes markedly decreased levels of C1 and C4, and also reversed the suppressive effect of this serum. These data suggest that a heat-labile factor in normal human serum which can be absorbed by immune complexes suppresses the antibody response to a T-dependent antigen. Other immune suppressors found in normal human serum are heat-stable or do not suppress in the presence of normal serum proteins. Thus the suppressive protein described in these studies may be unique. It is possible that either C1 or C4 or both may play a role in the suppression noted here.