Abstract

EXOGENOUS MELATONIN INHIBITS IMMUNE FUNCTION IN SHAW’S JIRD (MERIONES SHAWI)

Hanan A. El-Bakry*

ABSTRACT

The pineal hormone melatonin is reported to be a fundamental immunomodulator in a variety of species. Nevertheless, evidence supporting the immunoregulatory role of melatonin is conflicting, and information concerning its role in desert animals is scarce. The present study addressed the effects of exogenous melatonin administration on immune function in a wild-caught desert rodent species (Shaw’s jird; Meriones shawi). The serum levels of cytokines (TNFα, IL-1β, IL-6, IFNγ and IL-4) and immunoglobulins (IgA and IgE) were measured by ELISA assay; the frequency of CD8+ cells (cytotoxic T-lymphocytes) or IgM+ (immunoglobulin M) cells was assessed by immunohistochemistry; the percentage of splenic lymphocyte populations in the white pulp was assessed by quantitative image analysis. In addition, the levels of TNFα and IL-4 mRNAs in the thymus and spleen were assessed by Semi-quantitative RT-PCR. Daily administration of melatonin (50 μg/ml of drinking water for 8 weeks) decreased both humoral and cell-mediated immunity in Meriones shawi. It significantly decreased the secretion of Th1 cytokines (IFNγ and TNFα) and the production of Th2 cytokines (IL4). It also lowered the serum levels of IL1β and IL6 which are secreted by activated macrophages, and decreased the serum levels of IgA and Ig E. Similarly, the cellularity of splenic white pulp and red pulp, and the frequency of CD8+ and IgM+ cells were markedly decreased. Melatonin also down-regulated the level of gene expression of TNFα and IL-4 in thymus. Male jirds did not differ from female jirds in any of these parameters. Altogether, these results demonstrate for the first time that exogenous melatonin has immunosuppressive effects in Meriones shawi.

Keywords: melatonin, cytokines, immunoglobulins, cytotoxic T-lymphocytes, immunohistochemistry, RT-PCR.