Tracing interactions of thymocytes with individual stromal cell partners.

Cellular interactions in T cell development can be analyzed using thymus chimeras prepared in vitro, in which stromal cells and T cell precursors are manipulated separately. In an earlier study, we showed that for optimal T cell maturation most--if not all--stromal cells must display appropriate (selecting) major histocompatibility complex (MHC) molecules: the substitution of selecting by nonselecting stromal cells leads to a proportional decrease in mature T cell production. These data imply that the availability of selecting stromal micro-environments is rate limiting for positive selection, and that in positive selection each thymocyte engages only one (rather than multiple) stromal cell partners. To test this hypothesis, we developed a tracing system for thymocyte/stromal cell interactions, based on the acquisition by thymocytes of stroma-derived MHC class II determinants. When MHC class II-deficient precursors are placed in H-2b x k F1 environments (where all stromal cells co-express H-2b and H-2k), individual thymocytes acquire class II determinants of both haplotypes. In striking contrast, when placed in mosaic stromal environments (where stromal cells express either H-2b or H-2k evenly interspersed), individual thymocytes preferentially acquire MHC class II determinants of one or the other haplotypes, but rarely both. This provides strong evidence that thymocytes have intimate interactions with individual stromal cells: having engaged one stromal cell niche, thymocytes do not (or only rarely) have promiscuous liaisons with others.