Localization of rat CD1 transcripts and protein in rat tissues - An analysis of rat CD1 expression by in situ hybridization and immunohistochemistry

CD1 molecules are cell surface glycoproteins non-covalently associated with β₂-microglobulin. Recently, functional features of the CD1 family such as a target ligand and an antigen-presenting structure for T cells have been reported. In the current study, tissue distribution of rat CD 1 was analysed by in situ hybridization (ISH) in conjunction with immunohistochemistry to clarify the precise localization of both transcripts and proteins. CD1 transcripts were detected by ISH in a variety of organs: spleen, thymus, liver, lung, heart, kidney, small intestine and skin. In most organs, CD1 immunoreactivity paralleled the amount of CD1 mRNA expression and was localized in the same regions as its gene transcripts. However, there was a clear difference between the level of rat CD1 transcription and protein expression in the small intestine. CD1 mRNA was detected in the enterocytes of crypts of Lieberkuhn, but not in those of the intestinal villi, while immunoreactivity of CD1 protein was observed in the cells of the intestinal villi but not in those of crypts of Lieberkuhn. This suggests that CD1 gene transcription occurs in the enterocytes of intestinal crypts, and that as the cells of intestinal crypts migrate from the crypts to the intestinal villi, CD1 proteins are synthesized and accumulated in the intestinal villi. Such CD1 expression in the enterocytes appears to be consistent with migration-associated differentiation, and suggests that rat CD1 may take part in mucosal immunity as a first line of defence. In addition, clear cell membrane CD1 immunoreactivity on lymphoid cells raises the possibility that intercellular interaction via rat CD1 and T cell receptors may be involved in both lymphoid cell differentiation and immunoregulation.