Lymphocyte activation in discordant multiple sclerosis twin pairs.

Phytohemagglutinin-induced lymphocyte activation sequence was studied in monozygotic and dizygotic discordant multiple sclerosis (MS) twin pairs in a quiescent disease phase. The study group included all available 11 pairs listed in a nation-wide twin register. Lymphocyte activation markers, DNA synthesis and gamma-interferon secretion were studied using avidin-biotin-peroxidase complex (ABC) stainings, [3H]thymidine incorporation, and a solid-phase double-antibody immunoradiometric assay (IRMA), respectively. The level and kinetics of interleukin-2 receptor expression, DNA synthesis, gamma-interferon secretion, and major histocompatibility complex (MHC) locus II antigen expression were similar (Wilcoxon's test for paired samples) in both the diseased and healthy monozygotic and dizygotic twins. Our results suggest that the cell-mediated immune system may not be primarily at fault, but rather that both MS itself and its exacerbations are caused by an unknown triggering stimulus facing a properly functioning immune system.