DELINEATION OF T-PROGENITOR CELL-ACTIVITY WITHIN THE CD34(-MARROW() COMPARTMENT OF ADULT BONE)

T-cell production is largely dependent on the presence of a thymus gla nd where CD34(+) precursors mature into T lymphocytes. Prethymic stage s of T-cell development are less defined. Therefore, this study aims t o delineate T-progenitor cell potential within the CD34(+) Lineage- (L in(-)) cell compartment of adult bone marrow (ABM). Fractionation of C D34(+) Lin(-) ABM cells with CD45RA, Thy-1, CD38, and HLA-DR failed to absolutely segregate T-cell reconstituting ability, indicating broad distribution of T-progenitor cell potential. Titration experiments sho wed that low numbers of CD34(+) Lin(-) CD45RA(+) (RA(+)) cells had gre ater thymus repopulating ability than CD34(+) Lin(-) CD45RA(-) cells ( RA(-)). The great majority (>95%) of RA(+) cells expressed CD38, HLA-D R and 70% to 90% of RA(+) cells lacked Thy-1 surface expression. RA(+) cells contained colony-forming unit granulocyte-macrophage (CFU-GM) p rogenitor cells but were depleted of erythroid potential, did not prov ide hematopoietic reconstitution of human bone fragments implanted int o SCID mice, and did not efficiently maintain CD34(+) cells with secon dary clonogenic potential in bone marrow cultures. Thus, RA(+) cells a re oligopotent (nonprimitive) CD34(+) progenitors with T-cell reconsti tuting ability. In contrast, these same assays indicated that CD34(+) Lin(-) CD45RA(-) cells (RA(-) cells) comprised hematopoietic stem cell s (HSC) with primitive multilineage (T, B, myeloid, and erythroid) hem atopoietic potential. It was confirmed that HSC-containing populations , such as CD34(+) Lin(-) CD45RA(-) Thy-1(+) cells had thymus repopulat ing ability. Culture of RA- cells on murine bone marrow stromal cells in the presence of interleukin (IL)-3, IL-6, and leukemia inhibitory f actor (LIF) generated CD34(+) CD45RA(+) progeny engrafting in a second ary severe combined immunodeficiency (SCID)-hu thymus assay. Altogethe r, our results underscore the fact that T-cell reconstituting potentia l can be dissociated from HSC activity. Furthermore, we speculate that HSC might develop into the T lineage indirectly, via differentiation into an intermediate oligopotent CD34(+) CD45RA(+) stage. Finally, T-p rogenitor cells can be cultured in vitro. (C) 1995 by The American Soc iety of Hematology.