Neuroimmunoendocrinology of the rheumatic diseases: past, present, and future

Adaptation to stressful stimuli, maintenance of homeostasis, and ultimately, survival require bidirectional feedback communication among components of the stress response and immune and endocrine systems. Substantial progress has been made in delineating molecular, cellular, and systemic physiologic mechanisms underlying this communication, particularly mechanisms that target the immune system. For example, our understanding of the immunomodulatory activities of numerous neuroendocrine mediators, such as cortisol, estrogen, testosterone, DHEA, catecholamines, corticotropin-releasing hormone, and adenosine, has advanced substantially. Substantial progress has also been made in defining how abnormalities involving these factors may contribute to the initiation, progression, and severity of autoimmune rheumatic diseases, particularly rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). For RA, the available data support the view that inflammatory and immune system inhibitory mechanisms, involving the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic nervous system are deficient. Age, gender, and reproductive status acting, in part, through gonadal hormonal effects on disease susceptibility genes also appear likely to modulate the inhibitory stress response systems and immune function. Animal model data also have provided direct evidence that many autoimmune disease regulatory genes are gender influenced. For SLE, a growing body of recent data indicates that estrogens and androgens exert contrasting effects on B-lymphocytes (i.e., estrogens enhance and testosterone suppresses autoantibody production). These observations provide potential new insights into SLE pathogenesis and gender differences in prevalence. Continued investigation will refine our understanding of these observations and will uncover even more extensive interactions of the nervous, immune, and endocrine systems. Moreover, it is highly likely that improved understanding of these interactions will translate into improved therapy for the rheumatic diseases.