Seasonal variations in cytokine expression and cell-mediated immunity in male rhesus monkeys

Our objectives in this study were to examine seasonal changes in immune responses including cytokine profiles of male rhesus monkeys housed under natural lighting conditions. We also monitored circannual changes in the secretion of several immunomodulatory hormones as potential mediators of the seasonal shifts in immune status. Retrospectively, the medical records of a large group of rhesus monkeys were examined to determine whether a common disease (campylobacteriosis) in this species shows a seasonal pattern of prevalence. Results of the study showed that there was a seasonal shift in the frequency of cells expressing TH1 cytokines (interleukin-2 and interferon-gamma) versus the TH2 prototype cytokine (interleukin-4) by peripheral blood mononuclear cells (PBMC) collected during the winter and summer. The frequency of TH1-type cytokine synthesis in the summer was markedly greater than in the winter whereas TH2-type cytokine expression did not vary between the two seasons. The proliferative response of PBMC to mitogens and natural killer cell activity of PBMC also varied with the season. Several hormones (testosterone, leptin, and prolactin) that modulate immune function exhibited circannual patterns of secretion. The prevalence of Campylobacter infections was higher in the spring than during the summer, fall, or winter. The data suggest that seasonal fluctuations in immune system status may alter the ability of primates to successfully respond to pathogens, and this may be related to circannual patterns of secretion of immunomodulatory hormones.