Selective regulation of T-cell development and function by calcitonin gene-related peptide in thymus and spleen - An example of differential regional regulation of immunity by the neuroendocrine system

In the course of our studies, we have shown the presence of calcitonin gene related peptide (CGRP) by immunocytochemistry in cell bodies and nerve fibers of the murine thymus and in a sparce innervation of the spleen. Receptors for CGRP have been characterized within these glands, and their activation by physiological levels of CGRP was found to suppress Con A-stimulated proliferation of thymocytes and splenic T cells as well as antigen-specific T-cell proliferation. This suppression is blocked by the antagonist for CGRP (CGRP 8-37). Within the thymus cultures, the antagonist CGRP (8-37) alone enhanced proliferation of thymocytes during Con A stimulation, most likely by inhibiting the endogenous release of CGRP into the culture medium by resident thymocytes. Some of the CGRP-induced suppression of mitogenic stimulation of thymocytes, but not of splenocytes, was due to apoptosis. The antagonist, CGRP(8-37), did not block apoptosis caused by Con A or CGRP but rather enhanced it. Flow cytometric analysis of CGRP-treated cultures using antibodies to cluster determinates (CD) showed that the majority of thymocytes undergoing apoptosis induced by CGRP were of the CD4/CD8 double-positive type. These data indicate that apoptosis in the thymocytes is mediated by a CGRP receptor not sensitive to the antagonist CGRP(8-37). Because proliferation of thymocytes and splenocytes induced by Con A is blocked by this antagonist and splenocytes are refractory to CGRP induced apoptosis, CGRP appears to mediate at least two separate functions on subpopulations of thymocytes and T cells via two different CGRP receptors within the gland. These effects of a neuropeptide exemplify the phenomenon of differential regional regulation of immunity by the autonomic and neuroendocrine systems.