REQUIREMENT FOR THE THYMUS IN ALPHA-BETA T-LYMPHOCYTE LINEAGE COMMITMENT

We recently identified a fetal thymic developmental stage (NK1.1(+)/CD 117(lo)) that characterizes committed T/NK progenitors. We now report the existence of phenotypically and functionally identical T/NK progen itors in mouse fetal blood and spleen but not in fetal liver. These pr ecursors are indistinguishable from previously characterized fetal blo od ''prothymocytes'' (CD90(+)/CD117(lo)), with the exception that they express NK1.1, lack markers associated with T lineage commitment, mai ntain a germline TCR beta locus, and can give rise to both T and NK ce lls. Moreover, NK1.1(+)/CD90(+)/CD117(lo) fetal blood precursors are p resent in athymic nude mice. These results suggest that the T/NK linea ge commitment pathway is thymus-independent. In contrast, full commitm ent to the alpha beta T lineage does not precede thymus colonization.