Temperature and Season Influence Phagocytosis by B1 Lymphocytes in the Mojave Desert Tortoise

Lymphocytes are usually interpreted as functioning in adaptive immunity despite evidence that large proportions of these cells (B1 lymphocytes) have innate immune functions, including phagocytosis, in the peripheral blood of ectothermic vertebrates. We used a recently optimized assay to assess environmental influences on phagocytic activity of lymphocytes isolated from the Mojave desert tortoise (Gopherus agassizii). Previous studies suggest that lymphocytes in this species are associated with reduced pathogen loads, especially in cooler climates, and that lymphocyte numbers fluctuate seasonally. Thus, we evaluated thermal dependence of phagocytic activity in vitro and across seasons. While B1 lymphocytes appeared to be cold-adapted and always increased phagocytosis at cool temperatures, we also found evidence of thermal acclimation. Tortoises upregulated these lymphocytes during cooler seasons in the fall as their preferred body temperatures dropped, and phagocytosis also increased in efficiency during this same time. Like many other ectothermic species, populations of desert tortoises are in decline, in part due to a cold-adapted pathogen that causes chronic respiratory disease. Future studies, similarly focused on the function of B1 lymphocytes, could serve to uncover new patterns in thermal acclimation of immune functions and disease ecology across taxa of ectothermic vertebrates.