Normal human serum inhibits the lymphocytotoxicity of sera from patients with infectious mononucleosis.

The inhibitory effects of normal human serum (NHS) on the lymphocytotoxic activity (LCA) of sera from patients with infectious mononucleosis (IM) were investigated. Dilution of IM serum with complement fixation diluent (CFD) caused a significant rise in LCA at 1/10 dilution (P less than 0.001) followed by a steady decline at higher dilutions. In contrast, 1/10 dilution with pooled NHS caused a gross reduction in lymphocyte killing (P less than 0.01). This reduction occurred irrespective of the order of incubation of NHS and IM serum with target lymphocytes. Pre-dilution of the NHS showed this inhibitory effect to be dose-responsive. Further characterization of the inhibitor(s) showed it to reside in the exclusion peak of Sephadex G-200, to be abolished by treatment with the sulphydryl inhibitors, 2-mercaptoethanol and iodoacetamide and to be depleted selectively by incubation with monospecific anti-IgM (but not anti-C1q or anti-alpha 2-macroglobulin). The site of the inhibitory reaction was examined by indirect immunoperoxidase staining of the lymphocyte surface with monospecific anti-IgM and peroxidase conjugated swine anti-rabbit immunoglobulin. This showed that pre-incubation of IM serum with NHS caused a significant reduction in IgM positive cells compared to that observed with IM serum diluted in CFD alone. It is concluded that certain IgM molecules, present in NHS, inhibit the complement-mediated LCA of IM sera. This inhibition occurs by fluid phase interference with surface deposition of cytotoxic IgM rather than by competitive surface binding. The presence of such serum-serum interactions emphasises the complexity of the lymphocytotoxin reaction and the need for caution in attributing abnormalities in vivo to cytotoxic phenomena measured in vitro.