High-affinity binding of melatonin by human circulating T lymphocytes (CD4+).

This paper shows the presence of high-affinity binding sites for melatonin in human circulating T lymphocytes, but not in B lymphocytes. The binding of melatonin to T cells was dependent on time, stable, reversible, saturable, specific, and inversely correlated to the production of melatonin, expressed as the nocturnal 12 h production of its urinary metabolite 6-sulfatoxymelatonin. The affinity of these binding sites (Kd = 0.27 nM) suggests that they may recognize the physiological concentrations of melatonin in serum. Moreover, among the lymphocyte subpopulations studied, binding of melatonin was mostly found in CD4+ cells rather than in CD8+ cells. Results suggest that CD4+ cells may be the target of melatonin among the human circulating lymphocytes.