Nutritive Stress as an Immunomodulator in the African Clawed Frog (Xenopus Laevis)

Samuel J. Black, Vassar College ’12 and Prof. Erica J. Crespi

Amphibian populations around the globe have been steadily declining since the 1950s. This decline has been partially attributed to human development and the degradation of necessary natural resources. These explanations, while pertinent, still leave the majority of the decline unexplained. This inability to fully explain the phenomenon stems from the reality that research regarding the amphibian immune system is limited. Previous research has shown that the rate of pre-metamorphic amphibian development is dependent on nutritional condition and stress. In this study, three sizes of X. laevis were studied: 1.5g (n=16), 4.75g (n=12), and 6g (n=24). Each size class was separated into two groups. One group was fed as usual, while the other group’s food intake was reduced to induce nutritional stress. A minimum of one month later, these groups were divided again; both groups received bromodeoxyuridine (BrdU), a cell-proliferative maker, and the experimental group also received a treatment of phytohaemagglutinin (PHA), a mitogen, known for activating and proliferating specific cell types. Forty-eight hours after treatment, blood and spleens were collected and analyzed. Using hematological smears, the number of erythrocytes (RBC) and leukocytes (WBC) were quantified. Confocal laser scanning microscopy was used to analyze the spleen’s incorporation of BrdU to determine if nutritive condition and mitogenic treatments caused splenocyte proliferation. The BrdU incorporation could not be reliably quantified across treatments. Animals from the smallest size group that were nutritionally stressed responded to the mitogenic treatment and trended towards an up-regulation of leukocytes (p = .070). While the animals in the medium sized group exhibited a significant down-regulation of erythrocytes in response to the mitogenic treatment (p < .01). Among all size groups, erythrocyte abundance is strongly correlated with body mass (p < .01). While leukocyte abundance approaches a significant correlation with body weight (p = .056). Our findings suggest that younger animals under nutritional stress are more responsive to factors meant to up-regulate immune function. While larger animals seem to remain unaffected. Larger animals seem to have more robust circulatory systems and tend to have greater numbers of circulating erythrocytes and leukocytes. These results and previous research suggest that animals subjected to nutritional and environmental stressors in early post-metamorphic development may have heightened immune responses and reduced susceptibility to disease than pre-metamorphic amphibians.