Different Photoperiods Affect Proliferation of Lymphocytes but Not Expression of Cellular, Humoral, or Innate Immunity in Hamsters

Abstract

In seasonal mammals, photoperiod change is associated with a suite of alterations in physiology. It has recently been proposed that the immune response is one of the systems regulated by changes in photoperiod, although this hypothesis has not been rigorously challenged by assays of functional immune responses. The aim of this study was to test the hypothesis that photoperiod modulates immune responsiveness in Syrian (Mesocricetus auratus) and Siberian (Phodopus sungorus) hamsters. Consistent with previously reported data, short-day-housed (SD) animals exhibited a significant increase in lymph node cell (LNC) numbers and increased cellular proliferation in response to the polyclonal mitogen concanavalin A compared to long-day-housed (LD) animals. In contrast, LNC numbers from intact or gonadectomized SD animals that had been sensitized with the antigen dinitrofluorobenzene (DNFB) exhibited a reduced ex vivo proliferative response and reduced production of interleukin-6 (IL-6) compared to LD animals. In vivo studies of the contact hypersensitivity response of animals that had previously been sensitized, and subsequently challenged, with DNFB were similar in SD and LD animals, as was the proliferative activity of LNC recovered from these animals. There were also no photoperiodic differences in the antidinitrophenyl antibody response of animals sensitized with DNFB, or the anti-sheep red blood cell (srbc) response of animals immunized with srbc. Furthermore, no differences could be detected in the activity of natural killer cells from spleens of LD and SD Siberian hamsters, or in lipopolysaccharide-induced IL-6 production by LD and SD Syrian hamsters in vivo. Thus, although photoperiod is able to influence factors regulating the gross number and non-antigen-specific proliferation of lymphocytes in seasonally breeding mammals, day length does not directly influence activation of an effective immune response. The authors conclude, therefore, that expression of the immune response is not directly modified or compromised by photoperiod in these seasonally breeding hamster species.

Form over Function?

“…Implicit in the authors' discussion of the data, and in the conclusion that photoperiod has no direct effect on immune responses per se, are ambiguous distinctions between measures of the immune system (measures of relatively static traits considered to be components of the entire immune system [e.g., tissue weights, cell numbers], including traits of the adaptive and the innate immune system), immune responses (measures of adaptive changes in these traits in response to challenges [e.g., phagocytosis, fever dynamics, clonal selection of lymphocytes, lymphocyte trafficking]), and immune function (measures of the manner in which changes in adaptive and innate immune responses preserve host integrity). Zhou et al (2002) provide evidence for effects of day length on immune measures (auricular lymphocyte counts) and responses (IL-6 production in sensitized lymphocytes, mitogen-stimulated T-cell blastogenesis). This is, of course, not the first report indicating effects of photoperiod on immune responses in a reproductively photoperiodic species (e.g., , nor is it the first report indicating the absence thereof (e.g., Prendergast et al, 2001).…”

“…In common with Zhou et al (2002), we routinely observe lower basal lymphocyte proliferation after exposure to short photoperiods. We are at a loss, however, to explain their failure to see photoperiodic effects on circulating IL-6, DTH, or NK cell activity, measures that we, and others, have observed to vary in response to changes in day length.…”

“…Randy J. Nelson 1 and Brian J. Prendergast *Departments of Psychology and Neuroscience,The Ohio State University,Columbus,OH 43210 USA In this issue of the Journal of Biological Rhythms, Zhou et al (2002) present data from several experiments aimed at identifying effects of photoperiod on the immune systems of two well-established photoperiodic model species: Siberian hamsters (Phodopus sungorus) and Syrian hamsters (Mesocricetus auratus). Their observations are unambiguous: after adaptation to short days, Syrian hamsters exhibited higher numbers of resident auricular lymphocytes, lower rates of spontaneous lymphocyte proliferation, and less interleukin-6 (IL-6) production.…”

“…While Zhou et al (2002) was apparently under review, several published reports demonstrated striking effects of photoperiod on immune responses with direct relevance to organismal immune function. For example, we recently reported that symptoms of infection, such as fever, anorexia, and lethargy, are affected by photoperiod (Bilbo, Drazen, et al, 2002).…”

“…The conjecture by Zhou et al (2002) with regard to the role of leptin as a neuroendocrine mediator of photoperiodic effects on the immune system appears untenable. Zhou et al measured immune parameters and immune cell responses in both Siberian and Syrian hamsters.…”