Activated macrophages produce the most biologically active form of vitamin D, 1,25-dihydroxyvitamin D3 (calcitriol), at inflammatory sites in considerable quantity. In certain disease states, macrophage-derived calcitriol is so great as to induce systemic hypercalcemia. Calcitriol is distinct from other macrophage-derived cytokines as it is a steroid hormone. Like many of the steroid hormones, it is thought to mediate its effects through binding to a high-affinity nuclear receptor for vitamin D (VDR). The presence of VDR in activated, but not resting lymphocytes led to the discovery by our laboratory that calcitriol was a potent inhibitor of T lymphocyte activation and lymphokine production. Recently, we have started to characterize the molecular mechanism by which calcitriol regulates T lymphocyte activation. We have determined that calcitriol treatment does not alter the transcription of lymphokine mRNA. Rather, calcitriol treatment decreases lymphokine mRNA accumulation by specifically increasing the rate of lymphokine mRNA degradation. Calcitriol is therefore the first agent demonstrated to selectively regulate lymphokine mRNA stability. Thus, calcitriol possesses two unique qualities relative to the immune system. First, it is the only steroid hormone produced by activated macrophages or any other immune cell. Second, it is the first cytokine that post-transcriptionally inhibits T cell reactivity. This immunoregulatory activity of calcitriol is demonstrable in vivo; calcitriol administration has been shown to ameliorate several different animal models of autoimmunity (autoimmune thyroiditis, systemic lupus, adjuvant-induced arthritis, and experimental allergic encephalomyelitis). Calcitriol has been shown to be equivalent to cyclosporine A (CSA) or glucocorticoid treatment in these autoimmune disease models. Thus, calcitriol may have substantial clinical application as an immunosuppressive agent. In this proposal, we plan to carefully examine the mechanisms by which calcitriol increases the degradation of lymphokine mRNA. As a result, considerable insight into the regulation of lymphokine mRNA stability will be obtained, the major process by which T lymphocytes activation is switched off. Thus, important information regarding the role of the activated macrophage (through calcitriol production) in modulating T cell reactivity will be derived. Moreover, the use of vitamin D analogs will enable analysis of the role of the nuclear vitamin D receptor in transducing this effect of calcitriol. These studies may also permit the definition of novel approaches to immunosuppressive therapy. While not a specific aim of this proposal, understanding of the post-transcriptional regulation of lymphokine production may allow the development of new pharmacologic strategies in the treatment of autoimmune disease and allograft rejection.
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