Photoperiod, reproduction, and immunity in select strains of inbred mice

The purpose of this study was to determine whether decreased day lengths affect reproduction or the immune system in inbred mice. Irrespective of a nocturnal pineal melatonin rise, the signal for day length information, body and testis weights were the same in various strains 8 weeks after transfer from long to short days (16 to 8 h of light/day) compared to mice that remained in long days. Serum testosterone was unaffected by the photoperiod shift. The second goal was to determine whether the shift from long to short days influenced lymphocyte populations in spleen or blood, as well as innate and cell-mediated immune cell functions in C3H/HeN mice, an inbred strain with a robust melatonin rhythm. By flow cytometry, a stable percentage and number of B cells, T cells, and natural killer cells were identified in spleen from mice in both long and short days during the day and night. This complement of immunophenotypes in spleen suggests that equivalent functional capabilities persist in secondary lymphoid tissue of mice irrespective of day length. This was supported by findings that cytolytic activity by splenic natural killer cells (innate immunity) and antigen-induced T cell-dependent B cell antibody production (adaptive immunity) were similar in mice in long and short days. In blood, cell numbers but not helper T cell subset percentages (i.e., naive, memory, cytotoxic, or activated) were augmented in mice in short compared to long days, a consequence of increased circulating B cells. Day length differences in certain immunophenotypes in circulation may forecast photoperiod-mediated alterations in responsiveness to pathogens that are associated with a change in season. At night, the reduced proportion of cytotoxic T cells (long and short days), as well as increases in the percentage of activated T cells (long days), B cells (short days), and NK cell activity (long and short days) relative to daytime, suggests that surveillance and function by select immunophenotypes may adapt to circadian transitions even in highly inbred species. Thus, inbred mice retain capabilities for photoperiod to influence trait-specific aspects of immune cell but not reproductive function.