Selective expression of cholera toxin-receptor in rat medullary thymocytes.

Cholera toxin binding sites in frozen sections of rat thymus were studied by indirect immunofluorescence and were found to be localized selectively in the medulla of thymic lobules. By flow cytometric analysis, two populations of thymocytes differing in size and reactivity towards cholera toxin were detected; the small thymocytes comprising 15% of the total thymocytes were found to express the receptor. The two kinds of cells isolated were not however, distinctly different in morphology. But ganglioside GM1, the cholera toxin binding site in the small thymocytes, when quantified by TLC-immunostaining, was contained at a higher concentration than in the large thymocytes. These results indicate that GM1 should be a useful marker for analysis of thymocyte differentiation, and that cholera toxin-mediated thymocyte proliferation might only occur in the small medullary thymocytes.