Cellular competition modulates survival and selection of CD8+ T cells.

In this investigation we compare the repopulation of the CD8+ T cell compartments of bone marrow (BM) chimeras by either normal nontransgenic or T cell receptor (TcR) alpha beta-transgenic (TG) CD8+ T cells, the fate of TG and non-TG CD8+ T cells in different parabionts and the survival of TG and non-TG peripheral CD8+ T cells after transfer into athymic hosts. We found that cellular competition among CD8 T cells occurs at several steps of T cell differentiation including a) during the DN to DP transition, b) positive selection in the thymus, c) export from the thymus and d) in the periphery. Comparison of the results obtained in the BM chimeras and in the parabionts shows that an important step of T cell selection occurs during seeding of peripheral lymphoid tissues. Once established, peripheral T cells resist replacement by recent thymus migrants, i.e. in the periphery, selection of T cell repertoires follows the rule "first come, first served". Peripheral dominance correlates with T cell activation and division. Cell cycling and CD44 expression are more frequent among non-TG CD8 T cells than TG CD8 T cells and within the latter, more frequent among P14 TG CD8 T cells than anti-HYTG CD8 T cells. Thus, in the absence of intentional immunization, the frequencies of CD8+ T cells follow a hierarchy of selection in which non-TG > or = P14 TG > anti-HY TG. We also show that the equilibrium size and the fate of one CD8 T cell population differs according to the presence or absence of other CD8 T cell populations. Under these circumstances, selection of T cell repertoires and T cell survival and memory rely not only on the interactions of each T cell with their respective ligands, but also on the nature and number of other competing cells.