Development of rat CD45+ 13-day-old fetal liver cells in SCID mouse fetal thymic organ cultures.

A phenotypic analysis of the lympho-hemopoietic cells which occur in the liver of 13-day-old fetal rats was achieved by flow cytometry in an attempt to further characterize the rat lymphoid progenitor cells. A small fraction of rat 13-day-old fetal liver (r13FL) cells, which weakly expressed the leukocyte common antigen CD45, constituted a homogeneous Thy-1(hi), CD71(-), CD44(+), MHC class I+, CD43(+) cell subpopulation negative for CD45RC, CD3, TCRalphabeta, TCRgammadelta, CD2, CD5, CD4, CD8, CD25, CD28, NKR-P1a and sIg. On the contrary, the CD45(-) cells were a heterogeneous cell subset which expressed Thy-1, CD71 and CD44 at distinct levels. After MACS separation, the CD45(+) r13FL cells, but not the CD45(-) cell subset, in vitro repopulated 14-day-old SCID mouse fetal thymic lobes providing rat T cells, both TCRalphabeta and TCRgammadelta, NK cells, and thymic dendritic cells but not B lymphocytes. Interestingly, NKR-P1a(lo) TCRalphabeta+ or TCRgammadelta+ cells developed in the xenogeneic cultures, and a rare CD4(+)CD8(+) double-positive subpopulation among the TCRgammadelta-expressing cells accumulated in the oldest cultures. These results are discussed from the double perspective of the nature of the precursor cells which colonize the fetal thymus and the relevance of the xenogeneic SCID mouse fetal thymic microenvironment for supporting rat lymphopoiesis.