Expression, regulation and function of AC133, a putative cell surface marker of primitive human haematopoietic cells

To explore the physiological significance of AC133 expression on human haematopoietic cells, we phenotyped normal and malignant human haematopoietic cells for AC133 expression, evaluated the utility of AC133 for isolating human stem/progenitor cells in comparison to other known early haematopoietic cell markers, investigated the role of AC133 in regulating hematopoiesis, and evaluated the possibility that MYB might regulate AC133. We found that while human CD34⁺ progenitor cells expressed AC133, expression was rapidly downregulated during differentiation. In apparent contrast, AC133 mRNA was detectable in cells isolated from CFU-Mix, BFU-E, CFU-GM and CFU-Meg colonies. Human cord blood CD34⁺ cells expressed AC133 at higher levels than their normal bone marrow counterparts. In apparent contrast to normal primitive haematopoietic cells, the AC133 protein was undetectable on cells from 24 different human haematopoietic cells lines, even though the majority of these cells expressed AC133 mRNA. Since CD34, AC133 and the c-kit (KIT) receptor are all co-expressed on human stem/progenitor cells, we compared the ability of monoclonal antibodies directed against each of these proteins to isolate early progenitor cells. Using these antibodies and magnetized particles in a standard immunoaffinity isolation protocol, we found that anti-CD34 and anti-KIT MoAbs could isolate >80-90% of the clonogeneic cell population present in a given marrow sample. Anti-AC133 MoAbs recovered ~75-80% of CFU-GM and CFU-Meg, but only about 30% of CFU-Mix and BFU-E. Perturbation of AC133 expression with antisense oligodeoxynucleotides (AS ODN) resulted in transient downregulation of AC133 protein on human CD34⁺ cells but no apparent effect on cell survival of cloning efficiency ex vivo. Finally, downregulation of MYB expression with AS ODN had no effect on the AC133 expression at either the mRNA or protein level. Based on these results, we conclude that AC133 offers no distinct advantage over CD34 or c-kit as a target for immunoaffinity based isolation of primitive hematopoietic cells, that AC133 expression is not required for normal hematopoietic progenitor cell development in vitro, and finally that AC133 expression may not be MYB-dependent.