Inhibition by human serum of lymphocyte proliferation stimulated by purified protein derivative of tuberculin and bacillus Calmette-Guérin.

The addition of normal human serum to murine lymphocyte cultures consistently depressed mitogen-induced transformation, as measured by deoxyribonucleic acid synthesis. Stimulation by the B-cell mitogens purified protein derivative-tuberculin, bacillus Calmette-Guérin, and lipopolysaccharide was consistently affected, but there was no inhibition of T-cells when human serum was added to concanavalin A-stimulated cultures. The inhibitory effects were not due to cytotoxic factors for B-lymphocytes or to specific antibodies in serum directed against the mitogens. Analogous results were found with guinea pig serum. Contact of the lymphocytes with the serum within the first 24 h of culture was necessary for inhibition.