Effects of photoperiod and melatonin on lymphocyte activities in male broiler chickens.

Understanding the role of the pineal gland in regulating the immune response and the role of photoperiod in influencing pineal gland secretions are becoming increasingly important. The purposes of the present experiments were to investigate the effects of different photoperiod regimens on T- and B-lymphocyte activities in broiler chickens. Next, the influence of different photoperiod regimens on the responsiveness of lymphocytes to melatonin in vitro was examined. The effect of melatonin in vitro on lymphocyte activities was also studied, regardless of the photoperiod received. Finally, the effects of photoperiod on the profiles of different splenocyte cell types were investigated. To study the effect of photoperiod on lymphocyte activities, different photoperiod regimens were used. These were: constant lighting, 23 h light:1 h darkness; intermediate lighting, 12 h light:12 h darkness; and intermittent lighting, 1 h light:3 h darkness. Peripheral blood and splenic lymphocyte activities were tested at 3 and 6 wk of age by performing a mitogen cell-proliferation assay with a polyclonal T-cell mitogen, concanavalin A (Con A), and T-dependent B-cell mitogen, pokeweed mitogen (PWM). To study the effect of photoperiod on the responsiveness of lymphocytes to melatonin in vitro or the effect of melatonin in vitro on lymphocyte activities regardless of photoperiod received, lymphocytes from the chickens that were exposed to the different photoperiod regimens were incubated with mitogen and different concentrations of melatonin. To study the effect of photoperiod on profiles of different cell types, the percentages of splenocyte subpopulations from birds exposed to different photo-periods were determined using flow cytometry with CD4+, CD8+, CD3+, and B-cell markers. The results of these studies indicate that splenic T and B lymphocytes from 6-wk-old chickens grown in intermittent lighting had higher activities than those from chickens grown in constant lighting. Peripheral blood and splenic lymphocytes from chickens raised under constant lighting were more responsive to melatonin in vitro than those from chickens raised under intermittent lighting. This difference in response may be due to lower levels of melatonin in birds receiving constant lighting, making them more sensitive to melatonin in vitro. Melatonin in vitro enhanced the mitogenic response of peripheral blood T lymphocytes from 6-wk-old chickens, splenic T lymphocytes from 3-wk-old chickens, and splenic T and possibly B lymphocytes from 6-wk-old chickens. Finally, intermittent lighting increased the percentages of splenic CD4+, CD8+, and CD3+ cells but not B-cell subpopulations at 6 wk of age, presumably because of increased levels of melatonin in birds receiving intermittent lighting. Our results re-emphasize the importance of melatonin in regulating host immune response; this regulation could be accomplished through exposing broiler chicks to intermittent lighting.