Seasonal variation in proliferative response and subpopulations of lymphocytes from mice housed in a constant environment.

A seasonal variation in the proliferative response to mitogens and in the proportion of splenic lymphocyte subpopulations was found in mice housed in a constant environment. The lymphoproliferative responses to T-cell and B-cell mitogens reached maximum values in autumn and summer. Identification of lymphocyte subpopulations by flow cytometry demonstrated that the proportion of T cytotoxic-suppressor (Tcs) lymphocytes was significantly higher in autumn and summer than in spring and winter. However, the proportion of B lymphocytes was significantly lower in spring than in the other three seasons, whereas the proportions of T and T helper (Th) cells did not show any seasonal variation. On the other hand, we observed a significant correlation between the level of mitogenic responsiveness and the proportion of Tcs cells, but not between the former and the proportions of B, T or Th cells. These data suggest that the seasonal variation in murine lymphoproliferative responses may depend on the cyclic changes in the proportion of Tcs lymphocytes; these changes, in turn, may be predetermined by the inherent internal biological rhythms of the animal.