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Human blood cells, except for erythrocytes and platelets, express CD81, a member of the transmembrane 4 superfamily (TM4SF). CD81 is also expressed on most of human immature hematopoietic cells, CD34+ cells, which are divided into three populations according to the expression of CD34 and CD81; CD34+CD81+, CD34+CD81(High) and CD34(Low)CD81+. Myeloid and lymphoid progenitors exist in the CD34+CD81+ population, and megakaryocytic progenitors are only in CD34(Low)CD81+ population. Erythroid and multipotential progenitors are shared by CD34+CD81+ and CD34(Low)CD81+ populations, but multipotential progenitors in the CD34+CD81+ population have already lost most of their myeloid potential. NK cells and mast cells can be generated from all three populations. Long-term repopulating (LTR) lymphohematopoietic stem cells are present in the CD34+CD81+ population. Based on these findings, we propose a model for the development of CD34+CD81+ lymphohematopoietic stem cells. Along the differentiation cascade from CD34+CD81+ lymphohematopoietic stem cells, there appear to be pathways to CD34(Low)CD81 + or CD34+CD81(High) cells, even if they are indirect. CD34(Low)CD81+ pathways define the loss of LTR ability, and lymphoid and myeloid potentials, whereas CD34+CD81(High) pathways represent the exclusive commitment to NK cells and mast cells.

Interferon-gamma (IFN-gamma) exhibits various properties including antigrowth activity in neoplastic and normal cells and regulatory roles in immune responses and hematopoiesis, but studies of IFN-gamma effects on human megakaryopoiesis have been inconclusive. Recently we have used serum-free culture of purified CD34+ cells to demonstrate that IFN-gamma stimulates the proliferation of relatively mature megakaryocytic progenitors independently of thrombopoietin. It has been also shown that IFN-gamma stimulates the maturation of megakaryocytes, and has a significant synergism with stem cell factor in human megakaryopoiesis. Further studies are needed to clarify the in vivo effect of IFN-gamma on human megakaryopoiesis and the clinical relevance of IFN-gamma.

The 8p11 myeloproliferative syndrome (EMS) is an aggressive chronic myeloproliferative disorder (MPD) which is frequently accompanied by T or B lymphoblastic lymphoma (LBL), and rapidly transforms ...