Expression of CD6 and the UCHL1-defined CD45 (p180) antigen by human colonic T lymphocytes

Cryostat sections of histologically normal human colon were studied by double-label immunofluorescence techniques using a panel of monoclonal antibodies to T-lineage antigens and activation markers, with particular reference to the CD6 antigen. In the lamina propria, the majority population of T cells was of the CD3+, CD6+, CD4+ subset, of which virtually all were of the UCHL1+, CD45R-, Leu 8- phenotype of antigen-committed helper T cells. This majority lamina propria population did not express markers associated with blastogenesis (CD7), activation (MHC class II, CD25, CD38) or proliferation (OKT9, Ki67). In both intra-epithelial and lamina propria compartments, a subpopulation of CD3+ T cells was identified which did not express either the CD6 or CD5 peripheral 'pan T' markers. Most of the CD3+, CD6- cells were of the CD8+ (cytotoxic-suppressor) subset which co-expressed the CD7 antigen with a much higher frequency than did the CD6+ subpopulation. Expression of CD25, CD38 and HLA-D antigens, although infrequent, was confined to this CD8+, CD5-, CD6- population. Our data thus imply that the colonic mucosa is largely populated by mature, antigen-committed resting T cells of the helper-inducer phenotype.