Neuroendocrine manifestations in Sjögren's syndrome

Molecular biology has had a major impact on our concepts of the immune system and its relation to neuroendocrine axes, in particular, the adrenal, gonadal, and thyroid axes. It is now well established that not only are the biosynthetic and catabolic pathways of glucocorticoids and sex hormones (estrogen, progesterone, and testosterone) closely related but that the receptors for these hormones are part of a supergene family of receptors which include (in addition to these hormone receptors) the mineralocorticoid receptor, thyroid hormone receptor, retinoic acid receptors, and vitamin D receptors. This suggests a complex network of steroid hormones and receptors for the control and integration of a multitude of physiologic functions at a systemic level. The immune system seems to be tightly integrated into this homeostatic neuroendocrine regulatory network. The neurophysiologic and biochemical events that promote successful adaptation during stressful situations are now identified for illnesses that seem to occur as a result of or are associated with dysregulation of the stress response. One difficulty in interpreting the mechanisms of HPA axis dysfunction in autoimmune-inflammatory syndromes arises from the plasticity of the hormonal systems involved. Levels of hormones produced and receptors reset rapidly with changes in the hormonal milieu (deficiency or excess) and have likely changed during the course of the chronic immune disorder. This, in turn, is further confounded by the pleomorphic natural history of most autoimmune-inflammatory diseases such as SS. The levels of sex hormones and their receptors are tightly linked to HPA axis function. It may be that significant changes in the estrogen-to-androgen ratio or the ratio of their receptors alter the activity of steroid-sensitive cells such as the individual immune cells or epithelial cells, thus providing a means for endocrine regulation of the immune response in SS. Studies in the closely related disorder RA support this hypothesis. Taken together, adrenal and gonadal steroid hormone deficiency plus elevated PRL levels probably greatly facilitate cellular immunity in SS patients. This hypothesis in SS is supported by a growing body of data indicating that RA develops as a consequence of a deficiency in adrenal and gonadal steroid hormone production. It is noteworthy that the findings in female SS patients indicated a central deficiency in all three neuroendocrine axes: adrenal, gonadal, and thyroid. At present, it is not clear if any one system plays a primary role in the expression of the disease. Rather, it is likely that the net effect involves the synergistic and antagonistic effects of multiple hormones, making the specific effects of individual hormones difficult to discern.