PREDOMINANT EXPRESSION OF A RECEPTOR TYROSINE KINASE, TIE, IN HEMATOPOIETIC STEM-CELLS AND B-CELLS

A receptor tyrosine kinase (RTK), TIE (tyrosine kinase that contains i mmunoglobulin-like loops and epidermal growth factor [EGF] homology do mains), is expressed in vascular endothelial and hematopoietic cells. We generated monoclonal antibodies (MoAbs) against the extracellular d omain of TIE and a polyclonal antibody against the TIE carboxy-terminu s and used them to analyze expression of TIE in hematopoietic cells. W estern blotting detected two forms of TIE protein with a molecular mas s of 135 and 130 kD in hematopoietic and endothelial cells. Northern b lotting analysis revealed that TIE was expressed preferentially in und ifferentiated cell lines, especially when megakaryocytic, but not eryt hroid differentiation was induced. Reverse transcriptase-polymerase ch ain reaction (RT-PCR) showed that TIE was predominantly expressed in t he human hematopoietic progenitor fraction, CD34(+) cells. Fluorescenc e-activated cell sorting (FACS) showed that 42% of CD34(+) and 17% of KIT-positive (KIT+) cells were TIE-positive (TIE(+)). The majority (81 %) of the primitive hematopoietic stem cells, CD34(+)CD38(-) cells, we re TIE(+). Assays of progenitor cells and long-term culture-initiating cells (LTC-IC) showed that the TIE(+) fraction contained more primiti ve cells than the TIE(-) fraction. Some TIE(+) cells were in the CD34( -) fraction, which were CD19(+) and CD20(+) (B cells). These findings indicate that TIE has a unique spectrum of expression in primitive hem atopoietic stem cells and B cells. Although its ligand has not been id entified, TIE and its ligand may establish a novel regulatory pathway not only in early hematopoiesis, but also in the differentiation and/o r proliferation of B cells. (C) 1996 by The American Society of Hemato logy.