Effect of met-enkephalin on the proliferative response of equine peripheral blood mononuclear cells and purified T lymphocytes

Enkephalins (ENK) are synthesized by neurons, glia, diverse carcinomas and various cells of the immune system. Work in other species indicates that exercise affects plasma concentrations of ENK. Various studies suggest that this may result in altered resis- tance to disease and progression of existing diseases. The present study was performed in order to determine whether Met-ENK may regulate the proliferative response of equine peripheral blood mononuclear cells (PBMCs) or isolated T cells. Quadruplicate aliquots of PBMCs from two geldings (G), T cells from one gelding (G) and PBMCs from one adult mare (M) were incubated with suboptimal concentrations of the T cell mitogen phytohemaggluti- nin (PHA), in the presence or absence of 10"s, 10"7, 10-9 and 10"11 M Met-ENK. Treatments were added at the beginning of culture and the incorporation of 3H-thymidine was measured on day 3. The l o-s M ENK treatment inhibited mitogen-induced proliferation of PBMCs from G whereas the PHA response of G PBMCs was stimulated by 10^-11 M ENK treatments. For cells collected from M, PHA-blastogenesis was significantly reduced by 10^-5, 10^-9 and 10^-11 M ENK treatments. There was no stimulatory effect of ENK on PHA-responsiveness of PBMCs from M. The PHA-stimulated response of T cells was enhanced by 10^-7 and 10^-9 M ENK treatments. Lower and higher ENK concentrations had no effect on the proliferation of T cells. Results from this experiment suggest that physiological to superphysiological concentrations of ENK may effect the non-specific proliferative response of equine im- mune cells. Whether endogenous enkephalins act to modify the resistance of horses to various infectious diseases requires further study. Veterinary Medicine, Baton Rouge, LA 70803.