CD5(-) CD8-ALPHA-BETA INTESTINAL INTRAEPITHELIAL LYMPHOCYTES (IEL) ARE INDUCED TO EXPRESS CD5 UPON ANTIGEN-SPECIFIC ACTIVATION - CD5(-) ANDCD5(-ALPHA-BETA IEL DO NOT REPRESENT SEPARATE T-CELL LINEAGES() CD8)

We followed alpha beta T cell receptor (TCR) usage in subsets of gut i ntraepithelial lymphocytes (IEL) in major histocompatibility complex c lass I-restricted alpha beta TCR-transgenic (tg) mice. The proportion of tg alpha beta TCR(+)CD8 alpha beta IEL is reduced compared with CD8 (+) splenocytes of the same animal, particularly under conventional co nditions of maintenance. Further fractionation of CD8 alpha beta IEL a ccording to the expression level of surface CD5 revealed that in conve ntionally housed animals tg TCR(+)CD5(-)CD8 alpha beta IEL are as freq uent as in specific pathogen-free (SPF) mice, whereas tg TCR(+)CD5(int ) or, even more pronounced, tg TCR(+)CD5(hi)CD8 alpha beta IEL are gre atly diminished when compared with mice kept under SPF conditions. Upo n antigen-specific stimulation of CD5(-)CD8 alpha beta IEL in vitro, C D5 surface expression is up-regulated on a large fraction of cells wit hin 48 h. Up-regulation of CD5 surface expression is further enhanced by the presence of the anti-alpha IEL monoclonal antibody 2E7. This cl early demonstrates that CD5(-), and CD5(+)CD8 alpha beta IEL cannot be considered as separate T cell lineages.