The appearance of the thymus and the integrated evolution of adaptive immune and neuroendocrine systems

The immune system may be considered as a sensory organ able to respond to different kinds of danger signals that are not detected by nervous cells. The immune response is not autonomous but also regulated by the central and peripheral nervous system, as well as by neuropeptides, vitamin D and neuroendocrine axes such as the corticotrope, somatotrope, thyrotrope and gonadotrope axes. During evolution, the thymus emerged concomitantly with recombinase-dependent adaptive immunity as an'immune brain' or a'master class' highly specialized in the orchestration of central immunological self-tolerance. This was an absolute requirement for survival of species because of the high risk of autotoxicity inherent to the stochastic generation of extreme diversity characterizing this novel adaptive type of immune defenses against non-self. The thymus now appears to be an obligatory intersection for the integrated evolution of the major systems of cell-to-cell signalling, the nervous, endocrine and immune systems. The presentation of many self-peptides by thymic major histocompatibility complex (MHC) proteins is controlled by the autoimmune regulator (AIRE) gene/protein and is responsible for the clonal deletion of self-reactive T cells. In the same time, by still unexplained mechanisms, MHC presentation of the same self-peptides in the thymus promotes the generation of self-specific FOXP3+ CD4+CD25+ natural regulatory T cells (nTreg) that are able to inhibit in periphery self-reactive CD4+ and CD8+ T cells having escaped the thymus censorship. Moreover, a thymus dysfunction is more and more established as the primary event driving the development of organ-specific autoimmunity, which is the tribute paid, mainly by mankind, for the preservation of self against non-self. Our novel knowledge about thymus physiology and physiopathology already serves as the basis for the development of various innovative and efficient immunomodulating strategies in pharmacology.