Abstract
The photoperiod is known to modulate immune cell number and function and is regarded essential for seasonal disease susceptibility. In addition, diurnal variations in the immune system are regarded important for immune competence. Whereas few studies investigated the influence of season, none investigated the specific effect of the photoperiod on these diurnal immune rhythms until now. Therefore, the present study compared diurnal rhythms in cell numbers of peripheral leukocyte types in domestic pigs held either under long day conditions (LD) or short day conditions (SD). Cosinor analyses of cell numbers of various peripheral leukocyte subtypes investigated over periods of 50 h revealed distinct photoperiodic differences in diurnal immune rhythms. Relative amplitudes of cell numbers of total leukocytes, NK cells, T cells, and monocytes in blood were higher under SD than LD. In addition, cell counts of total leukocytes, NK cells, T cells including various T cell subtypes, and eosinophils peaked earlier relative to the time of lights-on under SD than LD. In contrast, diurnal rhythms of neutrophil counts did not show photoperiodic differences. Mesor values did not differ in any leukocyte type. Generalized linear mixed model analyses revealed associations of leukocyte counts with plasma cortisol concentration and activity behavior in most investigated cell types. Moreover, the present study demonstrated photoperiodic effects on diurnal rhythms in plasma cortisol concentrations and activity behavior, which is in agreement with human and primate studies. The results of the present study imply stronger rhythmicity in leukocyte counts in general under SD. Common intrinsic mechanisms seem to regulate photoperiodic effects on diurnal rhythms in leukocyte counts, except for neutrophils, in domestic pigs. Our results reveal considerable insights into the regulation of immune rhythms in diurnally active species.
Highlights
Diurnal and seasonal rhythms are important characteristics of physiology and behavior in humans and animals [1,2,3,4]
The cell numbers of total leukocytes, monocytes, and total T helper (Th) cells in blood exhibited diurnal rhythmicity (P < 0.05) in all animals held under short day conditions (SD) but only in a proportion of animals under long day conditions (LD), whereas neutrophil counts exhibited diurnal rhythmicity (P < 0.05) in all animals held under LD and in a proportion of 91% of animals under SD (Table S1)
The present study demonstrated that the photoperiod modulates diurnal rhythms in cell numbers of different immune cell types in blood of domestic pigs
Summary
Diurnal and seasonal rhythms are important characteristics of physiology and behavior in humans and animals [1,2,3,4]. Beside differences in physiology and behavior, seasonal differences were found in the incidence of disease and mortality in many species [11,12,13,14,15,16,17,18,19] In this respect, the photoperiod is regarded essential for seasonal disease susceptibility as it is known to modulate immune function [15, 20,21,22,23]. Whereas important mediators of diurnal rhythmicity in the immune system, such as glucocorticoids and the sympathetic nervous system, were already identified [37, 38], mechanisms driving seasonal changes in the immune system are not clearly defined yet, especially in diurnally active species
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