INTRATHYMIC LYMPHOID PRECURSOR CELLS DURING FETAL THYMUS DEVELOPMENT

Our previous studies have demonstrated the presence, in the adult mous e thymus, of a population of early precursor cells able to give rise t o T and B lymphocytes but not myeloid cells. This population of cells expresses low levels of CD4 and has been termed the ''low CD4 precurso rs.'' All these precursors were found to be c-kit positive, and they p recede the better known CD4-CD8- precursor stage. In this study, embry onic and neonatal thymuses were examined to see whether a similar low CD4 precursor was part of the pathway of T cell development during ont ogeny. A population with the phenotypic characteristics of the adult l ow CD4 precursor was found from day 15 of embryonic development, altho ugh the expression of low levels of CD4 was apparent only from embryon ic day 17. Functional tests of these putative precursors showed they h ad no thymus-reconstituting ability when isolated from thymuses at any time during embryonic life, and very low reconstituting ability even 24 days after birth. These results raise questions about the adult low CD4 precursor as an obligatory stage in the development of T cells in the thymus.