LAUSR

Diurnal variations in immune cell number and function are regarded important for immune competence and are thought to be mediated by rest-activity rhythms and hormones. Moreover, the photoperiod is also known to modulate the immune system and considered to affect seasonal disease susceptibility. Whereas few studies investigated seasonal effects, the present study is the first investigating the specific effect of the photoperiod on diurnal rhythms in cell numbers of peripheral leukocyte types in any species. Domestic pigs were held either under long day (LD, 16L:8D, lights-on 07:00-23:00, n=9) or short day conditions (SD, 8L:16D, lights-on 07:00-15:00, n=11) and fed concentrate at 07:30 + 14:00 (ad libitum hay/water). Blood samples were taken every 2 h over periods of 50 h via indwelling vein catheters. Rest-activity behavior of the pigs was analyzed using continuous camera recordings. Cosinor analyses (p<.05) revealed photoperiodic differences in diurnal rhythms of cell numbers of various peripheral leukocyte subtypes, rest-activity behavior, and cortisol concentration. Cell numbers of total leukocytes, NK cells, T cells, and eosinophils in blood, rest-activity behavior, and cortisol concentration peaked earlier relative to lights-on under SD (p<.05). Relative amplitudes in rest-activity behavior and cell counts of total leukocytes, NK cells, T cells, and monocytes were higher under SD (p<.05). However, there was no photoperiodic effect on diurnal rhythms in neutrophil counts and mesor in any leukocyte type. Generalized linear mixed models revealed associations of leukocyte counts with rest-activity behavior and cortisol concentration in most cell types (p<.05). Moreover, the found photoperiodic effects on diurnal rhythms in rest-activity behavior and cortisol concentration are in agreement with research in humans and primates. The present study revealed photoperiodic effects on diurnal rhythms in the immune system, rest-activity behavior, and cortisol concentration in pigs and strengthens the importance of the domestic pig as suitable model for chronoimmunology. German Research Foundation DFG (grant provided to SS, grant number SCHM3162/1-1), Federal Ministry of Education and Research, Germany (grant number 01PL16003), Faculty of Agricultural Sciences, University of Hohenheim (scholarship provided to LE).