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CD5- CD8 alpha beta intestinal intraepithelial lymphocytes (IEL) are induced to express CD5 upon antigen-specific activation: CD5- and CD5+ CD8 alpha beta IEL do not represent separate T cell lineages.

We followed alpha beta T cell receptor (TCR) usage in subsets of gut intraepithelial lymphocytes (IEL) in major histocompatibility complex class 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 conditions of maintenance. Further fractionation of CD8 alpha beta IEL according to the expression level of surface CD5 revealed that in conventionally housed animals tg TCR+ CD5- CD8 alpha beta IEL are as frequent as in specific pathogen-free (SPF) mice, whereas tg TCR+ CD5int or, even more pronounced, tg TCR+ CD5hi CD8 alpha beta IEL are greatly diminished when compared with mice kept under SPF conditions. Upon antigen-specific stimulation of CD5- CD8 alpha beta IEL in vitro, CD5 surface expression is up-regulated on a large fraction of cells within 48 h. Up-regulation of CD5 surface expression is further enhanced by the presence of the anti-alpha IEL monoclonal antibody 2E7. This clearly demonstrates that CD5-, and CD5+ CD8 alpha beta IEL cannot be considered as separate T cell lineages.