Differentiationassociated expression of myeloid antigens

The changing expression of some of the myeloid antigens during granulocytic and monocytic differentiation is illustrated in Figure 1. Use of antibodies reactive with these antigens has assisted in the identification and purification of cell populations at specific stages of differentiation. Primitive myeloid cells express the CD34 antigen as do the primitive lymphoid progenitor cells. Successful hematopoietic stem cell transplants can be carried out, using purified CD34+ bone marrow or mobilized peripheral blood cells, although it has been suggested recently that a small population of the most primitive stem cells are CD34 negative. Considerable attention has been focused on the phenotypic and functional subdivision of the CD34+ hematopoietic cells. The more primitive stem cells, that give rise to long-term bone marrow cultures, are CD33", CD38", CD45RA/U'W, CD71"ow, c-kit (SCF receptor CD117)'"W. There is controversy as to the major histocompatibility complex (MHC) class II status of primitive stem cells. In fetal marrow those cells with the highest plating and replating efficiency are CD34+, CD38", HLA-DR+, whereas in adult marrow those cells capable of giving rise to long-term cultures are HLA-DR". The vast majority of colony-forming cells are HLA-DR+, but the expression of this antigen then ceases between the myeloblast and promyelocyte stage. The CD 13 antigen is expressed on a proportion of progenitor cells, and is most strongly expressed on those cells which are actively cycling. Antibodies to the CD33 and CD13 antigens have proved valuable in the diagnosis of myeloid leukemias.

A number of antigens appear in the identifiable granulocytic and monocytic precursor pool including CD14 and CD15. CD14 was initially thought to be monocyte specific but is expressed weakly on at least a proportion of granulocytes. CD 15 is expressed on all neutrophils, most monocyte precursor cells, but only a small proportion of circulating monocytes. Of particular interest is the CD10 antigen (CALLA), originally thought to be restricted within the hematopoietic system to early B lymphoid cells and now known to be expressed as a late event in neutrophil differentiation and also to be present on bone marrow stromal cells.

The phenotype of eosinophils and basophils has some similarities with neutrophils but there are also distinct differences. Both eosinophils and basophils express the leukocyte integrins (CDlla, CD lib, CDllc, CD18), CD13, CD32, (FcyRII), CD35, CD43, CD44, and CD45, as do neutrophils. In contrast to neutrophils, eosinophils express CD9 and lack CD 10, CD16 (FcyRIII) and CDwl7. Basophils, unlike neutrophils, express CD25 (IL-2R), CD38, CD40 and CD54 (ICAM-1), but lack CD10, CD16 (FcyRIII), CDw65, CD66a and CD66b. CD15 expression is weak.

The phenotype of early erythroid progenitor cells is similar to that of early granulocytic and monocytic progenitor cells, although expression of the transferrin receptor (CD71) may be higher on the erythroid cells. With erythroid differentiation into the recognizable precursor pool, there is acquisition of a large number of red cell-specific antigens, including many blood group antigens, 'band III' and glyco-phorin. Similarly, megakaryocytes and platelets acquire a number of differentiation-associated antigens, such as gpllb (CD41), gpIX (CD42a), gplbct (CD42b), gplbp (CD42c), gpV (CD42d), the vitronectin receptor (CD51), the integrin |3, subunit (CD61) which can associate with CD41 or CD51, and P-selectin or PADGEM (CD62P). Some platelet lysosomal proteins only become expressed on the cell surface after platelet activation (e.g. CD63, CD107a, CD107b) and antibodies to these antigens are being increasingly used in the assessment of platelet status and function.

Table 1 Selected antigens expressed on myeloid cells

CD Mol. wt Biochemical nature Reactivity with mature Reactivity with other tissue cells no. (kDa) hematopoietic cells

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