B220 EXPRESSION BY T-LYMPHOID PROGENITOR CELLS IN MOUSE FETAL LIVER

The present study has characterized T lymphoid progenitor cells that r eside in mouse fetal liver. Day 14 fetal liver contains progenitor cel ls that can differentiate into mature T cells upon being transferred i nto the thymus by hanging drop cultures. Fractionation of fetal liver cells indicated that T progenitor cells were confined in TER119(-)CD45 (+)FcR(low) cells, To our surprise, B220(+) rather than B220(-) fracti on in TER119-CD45(+)FcR(low) fetal liver cells exhibited efficient pro genitor activity generating T cells. progenitor activity by the B220() fetal liver cells was restricted to T cells, B cells, and macrophage s at frequency similar to 1/10, similar to 1/10, and similar to 1/20, respectively, of isolated B220(+) cells. B220(+) fetal liver cells did not contain detectable D-J rearrangement of TCR-beta gene and were L- 7R(+)Thy-1(-)CD3(-)CD4(low)CD8(-)CD25(-)CD44(+). B220(+) fetal liver c ells expressed mRNAs encoding TCR-beta, pT alpha, Ig alpha, and VpreB. Interestingly, TCR beta-chains were expressed by B220(+) fetal liver cells in the VDJ-rearranged TCR-beta-transgenic mice, indicating that TCR-beta transcription and B220 expression are activated simultaneousl y by the transgenic B220(+) fetal liver cells. These results indicate that B220 is expressed by fetal liver lymphoid progenitor cells that c an become T cells, and suggest that lymphoid progenitor cells in fetal liver concurrently undergo T- and B-specific molecular events within a single cell.