INFLUENCE OF DIETARY-PROTEIN ON SELECTED MEASURES OF HUMORAL AND CELLULAR-IMMUNITY IN THE COTTON RAT SIGMODON-HISPIDUS

Small-mammal populations are subject to unpredictable and often dramatic fluctuations in numbers, but the mechanisms underlying this phenomenon are largely unclear. It has been suggested that protein is an important nutritional factor limiting survival, so we explored the effect of protein intake on the immunocompetence of cotton rats (Sigmodon hipsidus). In vivo and in vitro measures of both humoral and cell-mediated immunity were examined in a series of four protein-feeding trials (with weanlings, juveniles, subadults, and adults), using animals randomly assigned to an isocaloric diet containing either 4 or 16% crude protein. The response of lymphoid organ mass and cellularity to low dietary protein was variable, the spleen being more sensitive to dietary protein than the thymus. The ability of splenic lymphocytes to respond to in vitro stimulation with mitogens was unaffected by dietary protein, while the capacity of weanling and subadult cotton rats fed 4% protein to mount a delayed-type hypersensitivity response was enhanced. Nonspecific immunity (complement activity) was reduced in juveniles fed 4% protein. Dietary protein did not influence the ability of subadults or adults to generate a specific antibody response. These data suggest that cotton rats are less sensitive to dietary protein level than many inbred strains of laboratory rodents, younger animals may be at greater risk, and nutritional history may influence the response of a population to a reduction in available dietary protein.