Regulatory peptides, such as vasoactive intestinal peptide (VIP), somatostatin (SS), or substance P (SP), are considered to play a role in immune regulation. To localize the targets of these peptides in the human immune system, their receptors have been evaluated with in vitro receptor autoradiography in lymph nodes, tonsils, appendix, Peyer's patches, spleen, and thymus. The three peptide receptors were detected in all lymphoid tissues tested, but, unexpectedly, usually in distinct compartments. In lymph nodes, palatine tonsils, vermiform appendix, and Peyer's patches, VIP receptors were found in the CD3 positive zone around lymphoid follicles; SS receptors in the germinal centers of secondary follicles; and SP receptors mainly in interfollicular blood vessels. In the spleen, VIP receptors were detected in periarterial lymphatic sheaths, SS receptors in the red pulp, and SP receptors in the central arteries. In the thymus, VIP receptors were present in cortex and medulla, SS receptors in the medulla, and SP receptors in blood vessels. For comparison, cholecystokinin (CCK)-A and -B receptors were not demonstrated in any of these tissues. These results suggest a strong compartmentalization of the three peptide receptors in human lymphoid tissues and represent the molecular basis for the understanding of a very complex and interactive mode of action of these peptides.
Cycloheximide induces accumulation of vasoactive intestinal peptide (VIP) binding sites at the cell surface of a human colonic adenocarcinoma cell line (HT29-D4). Evidence for the presence of an intracellular pool of VIP receptors
