PHENOTYPIC VARIATION IN LYMPHOPROLIFERATIVE RESPONSIVENESS TO MITOGENIC STIMULATION IN COTTON RATS

Information on the diversity of function of the immune system and how it responds to environmental stimuli could provide insights into the mechanisms regulating survival within populations of small mammals. We evaluated the degree of diversity in cell-mediated immune function in cotton rats (Sigmodon hispidus) by measuring in vitro-proliferative responses of splenic-lymphocyte preparations to polyclonal activation with the mitogens concanavalin A (Con A), pokeweed mitogen (PWM), phytohemagglutinin (PHA), lipopolysaccharide (LPS), and Salmonella typhimurium (STM). Only Con A, PWM, and PHA showed a consistent pattern of stimulation in 3-day cultures of splenic lymphocytes; responses were minimal for STM and LPS. Spleen weight increased while lymphocyte cell-yields remained stable with age. Lymphoproliferative responses to PWM were significantly lower among juveniles than subadults and adults. Cotton rats collected from wild populations had heavier spleens with greater lymphocyte cellularity than captive-raised animals. In contrast, lymphoproliferative responses to PWM at all concentrations were lower; responses to Con A were similar (or greater at the highest concentrations) among wild-caught animals.