Circadian phagocytic activity of neutrophils and its modulation by light

The experimental data reviewed herein is aimed at an evaluation of the circadian dynamics of neutrophil phagocytosis (basal phagocytosis, phagocytic response, and adherence) in mice and rats and attempts to establish whether the functional pinealectomy accomplished through constant light exposure affects the level and/or circadian oscillation of the parameters. In both species, basal phagocytic activity appears as a periodic function, peaking in the second half of the night (03:00– 04:00). Adherence to nylon fibres displays a circadian oscillation with two maxima (10:00 and 22:00). Under constant light, the mean phagocytic activity is lowered by 40%, but its circadian amplitude increases, while the acrophase occurs earlier. Both the mean adherence level and its circadian variation are depressed, the latter to a larger extent. The amplitude and shape of the phagocytic response in non-immunised mice appears to depend on the time of the day when antigenic stimulation occurs, thus on the phase of the basal rhythm, drawing attention to the necessity for careful circadian scheduling of immunomodulatory interventions. The results suggest that rhythmic phagocytosis is part of the immune system’s circadian oscillation, controlled by hypothalamic centres and susceptible to modulation under the impact of endogenous and exogenous factors, including melatonin and the light-dark cycle.