Exogenous melatonin enhances cell-mediated, but not humoral, immune function in adult male deer mice (Peromyscus maniculatus)

Melatonin mediates seasonal changes in immune function

Field studies indicate that immune function is compromised and the prevalence of many diseases are elevated during winter when energetic stressors are extensive. Presumably, individuals would enjoy a survival advantage if seasonally recurring stressors could be anticipated and countered by shunting energy reserves to bolster immune function. The primary environmental cue that permits physiological anticipation of season is daily photoperiod, a cue that is mediated by melatonin. However, other environmental factors, including low food availability and ambient temperatures, may interact with photoperiod to affect immune function and disease processes. This paper will review laboratory studies that consistently report enhanced immune function in short day lengths. Prolonged melatonin treatment mimics short days, and both in vitro and in vivo melatonin treatment enhances various aspects of immune function, especially cell-mediated immune function, in nontropical rodents. Reproductive responsiveness to melatonin appears to affect immune function. In sum, melatonin may be part of an integrative system to coordinate reproductive, immunologic, and other physiological processes to cope successfully with energetic stressors during winter.

Seasonal variation in host susceptibility and cycles of certain infectious diseases

Seasonal cycles of infectious diseases have been variously attributed to changes in atmospheric conditions, the prevalence or virulence of the pathogen, or the behavior of the host. Some observations about seasonality are difficult to reconcile with these explanations. These include the simultaneous appearance of outbreaks across widespread geographic regions of the same latitude; the detection of pathogens in the off-season without epidemic spread; and the consistency of seasonal changes, despite wide variations in weather and human behavior. In contrast, an increase in susceptibility of the host population, perhaps linked to the annual light/dark cycle and mediated by the pattern of melatonin secretion, might account for many heretofore unexplained features of infectious disease seasonality. Ample evidence indicates that photoperiod-driven physiologic changes are typical in mammalian species, including some in humans. If such physiologic changes underlie human resistance to infectious diseases for large portions of the year and the changes can be identified and modified, the therapeutic and preventive implications may be considerable.

Exogenous melatonin: quantitative enhancement in vivo of cells mediating non-specific immunity

Melatonin (MLT), a biogenic indoleamine and neuromodulator produced by the pineal gland, is known to activate T helper cells by means of direct binding to melatonin receptors on both Th1 and Th2 cells. The present in vivo study aimed to investigate the effect of exogenously administered MLT on the hemopoietic and immune cell populations of the bone marrow and spleen in healthy, young adult male mice at two distinct MLT exposure intervals. The neurohormone, administered daily through the diet (7-14 days), was homogenized into finely ground chow. Control mice received ground chow without MLT. The results revealed cell lineage-specific, quantitative, MLT exposure-time-dependent changes in both the bone marrow and spleen. NK cells and monocytes (both components of the non-specific immune system functioning as the first line of defense against neoplasia and virus infected cells) were significantly increased in the bone marrow by both 7 and 14 days of dietary melatonin. The quantitative increment in these two cell populations, in the organ of their production, i.e. the bone marrow, indicates that new cell proliferation/production may have been stimulated by MLT. In the spleen, as in the bone marrow, NK cell levels remained significantly elevated at both 7 and 14 days after melatonin exposure. However, the number of monocytes in the spleen did not maintain, at day 14 of MLT exposure, the high levels observed after 7 days of MLT, in spite of their sustained, high numbers at 14 days in the bone marrow. This suggests that the progeny of the apparently increased monocyte production in the bone marrow (elevated absolute numbers therein), had localized in anatomical sites (other than the spleen) also common to these cells. Thus, the selective, positive influences of in vivo administered, exogenous melatonin on cells mediating non-specific immunity suggests a plausible mechanism for numerous claims that it is responsible for tumor amelioration in patients.

In vitro melatonin treatment enhances splenocyte proliferation in prairie voles

The seasonal effects of photoperiod on reproduction are mediated by melatonin, and it is hypothesized that increased immune function in short days is due to the increase in the duration of nightly melatonin secretion. Melatonin can act both directly and indirectly on target tissue within the immune system. The present study sought to tease apart the direct and indirect effects of melatonin on one aspect of immune function by examining the influence of in vitro melatonin on splenocyte proliferation in female prairie voles held in long (LD 16:8) or short (LD 8:16) days. Splenocyte proliferation in response to the T-cell mitogen concanavalin A was enhanced by the addition of melatonin in vitro, as compared to cultures receiving no melatonin. Body mass increased in short-day housed prairie voles, indicating that the animals were responsive to photoperiod. However, photoperiod did not affect splenocyte proliferation in the present study. These results support the hypothesis that melatonin exerts a direct effect on splenocyte proliferation, potentially via high-affinity melatonin receptors localized on splenocytes. The findings also indicate that, irrespective of photoperiod, melatonin exerts direct effects on splenocytes to enhance immune function.