The origin and specificity of alphabeta TCR(+) T cells that express CD8alphaalpha have been controversial issues. Here we provide direct evidence that precursors of functional CD8alphaalpha T cells are positively selected in the thymus in the presence of agonist self-peptides. Like conventional positive selection, this agonist selection process requires functional TCR alpha-CPM, whereas it is independent of CD8beta expression. Furthermore, CD8alphaalpha expression on mature, agonist-selected T cells does not imply selection by MHC class I, and CD8alphaalpha(+) T cells can be either class I or class II restricted. Our data define a distinct agonist-dependent, positive selection process in the thymus, and they suggest a function for CD8alphaalpha distinct from the conventional TCR coreceptor function of CD8alphabeta or CD4.

We performed a comprehensive analysis of T cell receptor (TCR) ␥ rearrangements in T cell precursors of the mouse adult thymus. Using a sensitive quantitative PCR method, we show that TCR␥ rearrangements are present in CD44 ؉ CD25؉ Pro-T thymocytes much earlier than expected. TCR␥ rearrangements increase significantly from the Pro-T to the CD44 ؊ CD25؉ Pre-T cell transition, and follow different patterns depending on each V␥ gene segment, suggesting that ordered waves of TCR␥ rearrangement exist in the adult mouse thymus as has been described in the fetal mouse thymus. Recombinations of TCR␥ genes occur concurrently with TCR␦ and D-J␤ rearrangements, but before V␤ gene assembly. Productive TCR␥ rearrangements do not increase significantly before the Pre-T cell stage and are depleted in CD4 ؉ CD8 ؉ double-positive cells from normal mice. In contrast, double-positive thymocytes from TCR␦؊/؊ mice display random proportions of TCR␥ rearranged alleles, supporting a role for functional TCR␥/␦ rearrangements in the ␥␦ divergence process.Most ␣␤ and ␥␦ T cells develop in the thymus through a complex developmental program leading to the generation of a specific T cell receptor (TCR) repertoire. The diversity of this repertoire is insured by the expression of distinct TCR ␣/␤ or ␥/␦ chains and by their variable region encoded by several variable (V), joining (J), and diversity (D) gene segments (1). The diversity at the TCR␥ locus (limited by no D, and only a few V/J segments) is increased by the presence of three functional C regions. Relative to the expressed V␥-J␥ chain, ␥␦ T cells have been subdivided into distinct subsets. Overall, cells of the ␥␦ lineage (which predominate during the fetal life) represent only Ϸ1% of the thymic population in the adult mouse. Adult ␥␦ thymocytes mostly express V␥2/4-J␥1, V␥1.1/ 1-J␥4, and V␥1.2/2-J␥2 chains, also found in peripheral lymphoid organs (for nomenclature see Fig. 1). The V␥5/7 subset (whose origin is still obscure) is preferentially found in intestinal intraepithelial lymphocytes (for reviews, see refs. 2 and 3).The different subtypes of ␥␦ cells as well as the low frequency of adult ␥␦ thymocytes represent obstacles in our understanding of ␥␦ T cell development. Although it is known that ␥␦ lineage divergence is an early event, it remains to be defined at which cell stage ␥␦ cells leave the triple negative (TN) step of the adult ␣␤ differentiation pathway. TN thymocytes have been subdivided into four subpopulations according to the expression of the CD25 and CD44 antigens, which progress from the CD44 ϩ CD25 ϩ TN thymocytes in vitro (which include both the Pro-T and Pre-T subsets) suggested that most ␥␦ T cells split before or at the pre-T cell stage (6). In contrast, Petrie et al. described ␣␤/␥␦ divergence as a relative late event (i.e., at the CD25 Ϫ TN cell stage) (7). An alternate way to study this may be to follow progenitor cells through analysis of their TCR variable gene segments, which conserve their configuration once assembled.TCR gene rearrangements ...

We describe transgenic mice carrying germline variable gene segments associated with either the T cell receptor (TCR) beta or alpha gene enhancers (E beta or E alpha). Transgenic constructs underwent high rates of site‐specific rearrangements predominantly in T cells from independent mice. Rearrangements of the E beta‐containing transgenes began at different stages of T cell differentiation in embryonic and adult thymus than did the E alpha‐containing ones, with a pattern superimposable upon the patterns of TCR beta or TCR alpha gene expression, respectively. We demonstrate that sequences within the TCR beta and TCR alpha gene enhancers confer tissue‐ and stage‐specificity upon the V(D)J recombination events affecting adjacent gene segments. The patterns of transgene expression also gave information on developmental events and lineage relationships (gamma delta versus alpha beta) during T cell development.