Two abortifacient
proteins, alpha- and
beta-momorcharin, have been purified from the seeds of the bitter melon (Momordica charantia). It was found that non-cytotoxic concentrations of these
plant proteins can significantly inhibit the mitogenic responses of mouse splenocytes to
concanavalin A, phytohaemagglutinin and
lipopolysaccharide in a dose-dependent manner. In addition, the
alloantigen-induced lymphoproliferation and the in vitro generation of a primary cytotoxic lymphocyte response were severely suppressed in the presence of these
proteins. In contrast, the cytolytic activity of cytotoxic lymphocytes and natural killer cells was unimpaired by in vitro exposure to
momorcharin. On the other hand, a clear decrease in the functional capacity of macrophages, such as the
cytostatic and phagocytic activities, was observed under similar conditions. In vivo studies have shown that single
injections of nontoxic microgram amounts of
momorcharin into mice resulted in a significant depression of the delayed-type
hypersensitivity response as well as the humoral antibody formation to sheep red blood cells. Similarly, the thioglycollate-induced in vivo migration of macrophages was also suppressed. Interestingly, the in vivo activation of natural killer cells was not appreciably affected. Our data suggests that the observed potent immunosuppressive effect of alpha- and
beta-momorcharin is unlikely to be due to direct lymphocytotoxicity or due to a shift in the kinetic parameter of the immune response.