Thymic selection events mediated by the pre-TCR do not depend upon a limiting ligand

Thymocyte differentiation requires the production of a functional TCR, the culmination of a carefully orchestrated series of events in which TCR beta chain gene rearrangement precedes that of TCR alpha genes. The product of a successful rearrangement of the TCR beta locus associates with an invariant protein in immature thymocytes to form the 'pre-TCR' complex, which is required for allelic exclusion at the TCR beta locus, the expression of CD4 and CD8 co-receptors, and the clonal expansion of immature thymocytes. The pivotal role for the beta chain protein during early thymocyte development led us to investigate the relative differentiation efficiency within the same thymus of cells which do and cells which do not possess productive TCR gene rearrangements. Using mixed radiation bone marrow chimeras to establish an in vivo competition between TCR beta transgenic (Tg) and non-Tg bone marrow cells, we show that the prior productive rearrangement of a TCR beta chain gene only subtly enhances the efficiency of intrathymic differentiation. Further, we have compared the relative differentiation efficiency of TCR alpha beta and TCR beta Tg cells within the mixed chimera system by altering the proportion of TCR Tg bone marrow cells in the reconstituting inoculum. As expected, Tg cells carrying both alpha and beta chains of a selectable TCR are developmentally hindered compared with their non-Tg counterparts by the lack of ample numbers of intrathymic positively selecting ligands or niches. In contrast, parallel experiments using TCR beta Tg bone marrow cells demonstrate that the early selection events mediated by the pre-TCR do not similarly depend upon a ligand present in limiting quantities.