Intrathymic differentiation: thymocyte heterogeneity and the characterization of early T-cell precursors.

Multiparameter FCA of unfractionated or isolated subpopulations of thymocytes reveals at least seven subpopulations in the mouse thymus. One of these subpopulations, designated dLy1 has been isolated and characterized extensively. The data reviewed here indicate that the dLy1 thymocyte subpopulation, whether derived from the adult or fetal thymus, represents an early stage in intrathymic differentiation. The immature status of dLy1 cells was suggested by the extensive similarity to a predominant cell type that occurs early in fetal thymic ontogeny. Its precursor role was demonstrated by its capacity to generate cortical and medullary-type thymocytes in vivo. Its expression of Ly1, Thy-1 and mRNA specific for the beta-chain of the T-cell antigen receptor support its commitment to a T-cell developmental pathway. In summary, dLy1 thymocytes appear to be the earliest committed T cells yet to be described, isolated and characterized. Further investigation should reveal whether this subpopulation of thymocytes contains subsets of cells in earlier states of maturation and/or precursors already committed to more than one T-cell lineage.