Hormones, lymphohemopoietic cytokines and the neuroimmune axis

The classical distinction between hormones and cytokines has become increasingly obscure with the realization that homeostatic responses to infection involve coordinated changes in both the neuroendocrine and immune systems. The hypothesis that these systems communicate with one another is supported by the ever-accruing demonstrations of a shared molecular network of ligands and receptors. For instance, leukocytes express receptors for hormones and these receptors modulate diverse biological activities such as the growth, differentiation and effector functions. Leukocyte lineages also synthesize and secrete hormones, such as insulin-like growth factor-I (IGF-I), in response to both growth hormone (GH) and also to cytokines such as tumor necrosis factor-α (TNF-α). Since hormones share intracellular signaling substrates and biological activities with classical lymphohemopoietic cytokines, neuroendocrine and immune tissues share a common molecular language. The physiological significance of this shared molecular framework is that these homeostatic systems can intercommunicate. One important example of this interaction is the mechanism by which bacterial lipopolysaccharide, by eliciting a pro-inflammatory cytokine cascade from activated leukocytes, modulate pituitary GH secretion as well as other CNS-controlled behavioral and metabolic events. This article reviews the cellular and molecular basis for this communication system and proposes novel mechanisms by which neuroendocrine-immune interactions converge to modulate disease resistance, metabolism and growth.