Selkctive expression of a receptor tyrosine kinase, HTK on human erythroid progenitor cells

In a survey of receptor tyrosme kinases expressed on hematopoietic cells, we have cloned a novel receptor tyrosine kinase, HTK from a human megakaryoblastic cell line, UT-7 by PCR. HTK belongs to the Eph family and its molecular weight is approximately 120 kD. Unlike most of the Eph family members. HTK is significantly expressed on hematopoietic cells. We prepared monoclonal antibodies against human HTK and investigated its expression on human bone marrow cells by flow cytometry. About 5'/t of bone marrow mononuclear cells were HTK', which were also Kit. CD34"'" and glycophonn-A"""". Kit' fraction was divided into HTK' and HTK" fractions. F,ach fraction was sorted and assayed for hematopoietic progenitor cells. HTK" Kit' fraction formed GM colonies and erythroid bursts. On the other hand, HTK' Kit4 cells formed only erythroid colonies and bursts in the presence of SCF. IL-3 and erythropoietin, whereas few colonies were formed without SCF, indicating that they contained exclusively BFU-E. During further erythroid differentiation, HTK expression was down-regulated as the increase of glycophorin expression. These findings suggest HTK' cells are the subpopulation of erythroid progenitors, and anti-HTK monoclonal antibody enables to purify erythroid progenitors. We have also cloned HTK ligand (HTKJL), which is approximately 42 kD transmembrane protein, using BIAcore system. HTKL induced the phosphorylation of HTK only m the transmembrane form ur polymerized soluble form. It suggests that cell-cell contact is required in transducing signals for HTKL. The mitogenic activity of HTKL on erythroid progenitor celis was nol apparent. HTK may be involved in a novel cell function of these cells. HTK is the first surface antigen that is selectively expressed in erythroid committed progenitors.