NORMAL THYMIC SELECTION OF TCR TRANSGENIC CD4 T-CELLS, BUT IMPAIRED SURVIVAL IN THE PERIPHERY DESPITE THE PRESENCE OF SELECTING MHC MOLECULES

In this paper, we investigate selection in the thymus and survival in the periphery of CD4 T cells, which carry a major histocompatibility c lass Ii-restricted transgenic TCR (A18 TCRtg) specific for a natural s elf Ag, the fifth component of complement (C5), A18 TCRtg thymocytes d evelop normal numbers of CD4 single-positive (SP) thymocytes, but do n ot show pronounced overselection as do some other TCR transgenic strai ns. CD4 SE cells are mature as judged by termination of CD8 synthesis, resistance to cortisone, and functional competence. The kinetics of p ositive selection, determined by BrdU labeling, are very fast, CD4 SP thymocytes are demonstrable within 2 days of labeling, and within 8 da ys after labeling a large proportion (20 %) of lymph node T cells are recent thymic emigrants, The high number of recent thymic emigrants su ggests rapid turnover of CD4 T cells in the periphery, which was confi rmed by thymectomy and determination of CD4 T cell life spans. Alg TCR tg T cells have a t(1/2) of similar to 6 wk, despite the presence of s electings MHC molecules, This explains the failure to accumulate high numbers of peripheral T cells and suggests that the MHC-bound ligand(s ) responsible for initiating survival signals is limiting for the sele ction and maintenance of A18 transgenic CD4 T cells.