The sialomicin CD164, a negative regulator of haemopoiesis, is expressed by CD34+ and erythroid subsets and is located on chromosome 6q21

CD 164 is an 80 kDa mucin-like adhesion molecule that may play a key role in the negative regulation of haemopoiesis. Four CD164 monoclonal antibodies, 103B2/9E10, 105A5, 67D2 and N6B6, have been generated. At least two of these antibodies, 103B2/9E10 and 105A5, exert functional effects on human CD34+ bone marrow cells in vitro. In particular 103B2/9E10 partially inhibits CD34+ haemopoielic cell adhesion to bone marrow stroma, while 105A5 and 103B2/9E10 block clonogenic growth of these cells. Cross-blocking experiments indicate that the epitopes recognised by 105AS and 103B2/9E10 are distinct from one another and from those recognised by N6B6 and 6702. The 105A5 epitope is sialic acid dependent. This implies that at least one of the epitopes required for the regulation of haemopoietic cell growth contains sialic acid residues in its recognition site. Furthermore, pre-labelling of cells with the 103B2/9E10 antibody partially inhibits 105A5 binding but not vice versa. This suggests that 103B2/9E10 may induce a conformational change in the CD164 molecular that might affect the functional activity the 105AS recognition site. Flow cytometric analyses revealed that the four monoclonal antibodies show distinct patterns of reactivity on CD34+ cells, with fewer cells being stained with 105A5. 30-90% of bone marrow and cord blood CD34+ cells are CD164 positive. Detailed analyses of CD34+ subsets expressing the 103B2/9E10 epitope revealed that the majority of CD34+ cells that co-express CD190 (Trtyf), CD117 (c-kit) and CD135 (FLT3R) are CD164 (103B2/9E10)+. This epitope is therefore expressed by the most primitive CD34+ progenitor cell subset. Similar analyses of the CD34lo/-CD164(103B2/9E10)+ subsets indicate that the most prominent populations consist of B cells and erythroid cells, with CD164 expression preceding the expression of the erythroid markers, glycophorin c, glycophorin a and band III. Although the majority of the B cells in bone marrow express the 103B2/9E10 epitope, the CD34+CD164 (103B2.9E10)lo/- cells are almost exclusively CD19+CD20lo/- B cell precursors. Thus, the most immature B cells do not express significant surface levels of the 103B2/9E10 epitope. Using the CD164 cDNA as a probe, we have isolated PAC clones and have shown that the CD164 gene is composed of 4 separate exons interrupted by intronic sequences. Furthermore, somatic cell hybrid and FISH analyses indicate that the CD164 gene is located on human chromosome 6q21.