Cytokines, leptin, and the hypothalamo-pituitary-adrenal axis

The endocrine and immune systems are linked via an elaborated communication system constituted by an array of cytokines and neuropeptides which interact to modulate the integrated response of an organism to infection. Weight loss and anorexia, probably secondary to cytokine release, frequently accompany infection, but leptin could also play a role. Like cytokines, leptin serves as a peripheral messenger to convey signals to the brain. Expression of leptin is stimulated by glucocorticoids, endotoxins, and cytokines; on the other hand, leptin seems to inhibit the activation of the hypothalmo-pituitary-adrenal (HPA) axis. Indeed leptin exerts a direct, dose-dependent inhibition of stimulated cortisol secretion by normal human and rat adrenal cells in vitro. These effects are mediated by the long isoform of the leptin receptor, because its transcript is expressed in the adrenal tissue. In addition we investigated the role played by the glucocorticoids in the development of tolerance of the hypothalamo-corticotropic, immune and adipose system responses to repeated endotoxin administration. Unlike that of the corticotropic axis, tolerance of the immune and adipose systems is at least partially glucocorticoid-independent. This crosstalk between the endocrine, immune, and adipose systems may be of prime importance to homeostasis in pathophysiological events occurring during infection.