4-COLOR FLOW CYTOMETRIC INVESTIGATION OF TERMINAL DEOXYNUCLEOTIDYL TRANSFERASE-POSITIVE LYMPHOID PRECURSORS IN PEDIATRIC BONE-MARROW - CD79A EXPRESSION PRECEDES CD19 IN EARLY B-CELL ONTOGENY

Terminal deoxynucleotidyl transferase (TdT)-positive cells in human bo ne marrow (BM) are a phenotypically inhomogeneous population of precur sor cells. In their majority, these TdT(+) cells are unambiguously com mitted to the B lineage, as evidenced by CD19 expression. However, TdT (+) precursors that lack CD19 also exist and these may encompass a dif ferentiation potential for the B as well as for other lineages. Becaus e recent data suggested that CD19 expression is not the earliest diffe rentiation event in B-cell ontogeny, we sought to reevaluate TdT(+) ly mphoid precursors in pediatric BM to define the phenotypic denominator of B-lineage affiliation upstream of CD19. Using four-color flow cyto metry, we focused on the assessment of the CD79a antigen, which is hig hly B-cell specific and which may also be expressed very early in B-ce ll ontogeny. We found that a majority of TdT(+) cells coexpressed CD19 and CD79a in addition to CD10 and CD34, whereas, in all investigated samples, some TdT(+) precursors lacked CD19 but expressed CD79a, which suggestively indicates also their B-lineage affiliation. In contrast to the CD19(+) precursors, which were usually CD10(hi) and CD79b(+), t hese CD19(-)CD79a(+) putative B-cell precursors preferentially express ed CD10 at row levels and were CD79b(+) in only 41%. About 17% of thes e TdT(+)CD19(-)CD79a(+) precursors also coexpressed CD33 and CD7, but not myeloperoxidase, CD14, or cytoplasmic CD3, which is discussed in t he light of cellular activation rather than lineage promiscuity. Our d ata confirm that the earliest differentiation stages of B cells can be dissected upon expression of the lineage antigens CD79a and CD19 and imply that CD79a is earlier expressed than CD19. This raises the chanc e to follow the sequential events heralding B-cell commitment in the m ost immature precursors by correlating phenotypic and genetic differen tiation markers. (C) 1998 by The American Society of Hematology.