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 li ver of 13-day-old fetal rats was achieved by flow cytometry in an attempt t o further characterize the rat lymphoid progenitor cells. A small fraction of rat 13-day-old fetal liver (r13FL) cells, which weakly expressed the leu kocyte common antigen CD45, constituted a homogeneous Thy-1(hl), CD71(-), C D44(+), MHC class I+, CD43(+) cell subpopulation negative for CD45RC, CD3, TCR alpha beta, TCR gamma delta, CD2, CD5, CD4, CD8, CD25, CD28, NKR-P1a an d slg. On the contrary, the CD45(-) cells were a heterogeneous cell subset which expressed Thy-1, CD71 and CD44 at distinct levels. After MACS separat ion, the Cd45(+) r13FL cells, but not the CD45(-) cell subset, in vitro rep opulated 14-day-old SCID mouse fetal thymic lobes providing rat T cells, bo th TCR alpha beta and TdRyG, NK cells, and thymic dendritic cells but not B lymphocytes. Interestingly, NKR-P1a(lo) TCR alpha beta(+) or TCR gamma del ta(+) cells developed in the xenogeneic cultures, and a rare CD4(+)CD8(+) d ouble-positive subpopulation among the TCR gamma delta-expressing cells acc umulated in the oldest cultures. These results are discussed from the doubl e perspective of the nature of the precursor cells which colonize the fetal thymus and the relevance of the xenogeneic SCID mouse fetal thymic microen vironment for supporting rat lymphopoiesis.