IDENTIFICATION OF PRO-THYMOCYTES IN MURINE FETAL BLOOD - T-LINEAGE COMMITMENT CAN PRECEDE THYMUS COLONIZATION

Phenotype and commitment of thymus-colonizing precursors are unknown. Here we report the identification of T lineage-committed precursors (d esignated prothymocytes) in murine fetal blood at day 15.5 of developm ent, Fetal blood pro-thymocytes are Thy-1(+)c-kit(low)CD3(-) in contra st to fetal blood-derived pluripotent hematopoietic progenitors which are Thy-1(-)c-kit(+). Upon transfer into the thymus, fetal blood pro-t hymocytes generate a single wave of CD4(+) CD8(+) thymocytes and subse quently mature TCR alpha beta(+) peripheral T cells, However, fetal bl ood pro-thymocytes lack multipotent progenitor potential since,they fa il to reconstitute B lymphocytes and myeloid and erythroid lineages. i n contrast, T and B lymphocytes as well as myeloid and erythroid linea ges are reconstituted from fetal blood-derived pluripotent progenitors . Pro-thymocytes are equally present in peripheral blood of athymic fe tal mice, suggesting that this novel precursor population is T lineage -committed prior to thymus colonization and represents the earliest T lineage precursor identified.