Haemopoietic progenitor cell differentiation: flow cytometric assessment in bone marrow and thymus

We have recently shown that expression of any of the lineage-associated molecules CD2, CD7, CD10, CD19 or CD33 does not ensure lineage-commitment of CD34+ progenitor cells. Further, normal progenitor cells and leukaemic blast cells have been shown to coexpress molecules associated with more than one haemopoietic lineage. Five-dimensional flow cytometric analysis of normal bone marrow cells was exploited to investigate the hypothesis of a developmental stage in haemopoiesis comprising CD34+ cells coexpressing CD2, CD5, CD7, CD10, CD19 and CD33 or any combination of these molecules. We report on a subpopulation of CD34+ bone marrow cells constituting < 5% of the CD34+ cells and characterized by extensive coexpression of several molecules associated with the B lymphoid, T lymphoid and myeloid lineages. There is every probability that some cells display the CD34+ CD2+ CD5+ CD7+ CD10+ CD19+ CD33+ phenotype. Studies on postnatal thymocytes suggest that this may be the phenotype or one of a few phenotypes of a candidate thymus-seeding progenitor cell population. Finally, our findings that CD34+ as well as CD34+ CD5+ thymocytes can be driven into non-T-lymphoid differentiation by cytokines, support the notion that the thymus is seeded by uncommitted progenitors.