The earliest steps of intrathymic differentiation recently have been elucidated. It has been reported that both CD410 (CD44(+) CD25(-) c-kit(+) CD3(-) CD4(lo) CD8(-)) and pro-T cells (CD44(+) CD25(+) c-kit(+) CD3(-) GD4- CD8(-), representing the next step in maturation) exhibit germline T-cell receptor beta and gamma loci, suggesting that neither population is exclusively committed to the T-cell lineage. Several groups have shown that CD4(lo) cells retain the capacity to generate multiple lymphoid lineages in vivo; however, the lineage commitment status of pro-T cells is unknown. To determine when T-cell lineage commitment occurs, we examined the ability of sorted CD4(lo) and pro-T cells to generate lymphoid lineage cells in vivo or in fetal thymic organ cultures (FTOCs). When intravenously injected into scid mice, CD4(lo) cells generated both T and B cells, whereas the progeny of pro-T cells contained T cells exclusively. Fetal thymic organ cultures repopulated with CD4(lo) cells contained both T and natural killer (NK) cells, whereas cultures repopulated with pro-T cells contained T cells almost exclusively. These observations strongly suggest that T-cell lineage commitment occurs during the transition of CD4(lo) to pro-T cells. Because it is likely that the thymic microenvironment plays a critical role in T-cell commitment, we compared the responses of CD4(lo) and pro-T cells to various cytokine combinations in vitro, as well as the ability of the cultured cells to repopulate organ cultures. Cytokine combinations that maintained T-cell repopulation potential for both CD4(lo) and pro-T cells were found. CD4(lo) cells proliferated best in response to the combination containing interleukin-1 (IL-1), IL-3, IL-6, IL-7, and stem cell factor (SCF). Unlike CD4(lo) cells, pro-T cells were much more dependent upon IL-7 for proliferation and FTOC repopulation. However, combinations of cytokines lacking IL-7 were found that maintained the T-cell repopulating potential of pro-T cells, suggesting that, whereas this cytokine is clearly very important for normal pro-T cell function, it is not an absolute necessity during early T-cell expansion and differentiation. (C) 1995 by The American Society of Hematology.
