Tissue- and epitope-specific mechanisms account for the diverse effects of anti-CD44 antibodies on the maintenance of primitive hematopoietic progenitors in vitro.

Abstract ABSTRACT The identification of rare stromal cells that support high levels of stem cells has opened avenues to identify molecules that contribute to the maintenance of these cells. We show that the maintenance of long-term culture initiating cells (LTC-IC) in stromal cell-supported cultures can be modulated via mAbs specific for CD44. mAb IM7.8.1 suppressed while mAb RAMBM44 enhanced LTC-IC levels in culture. Genetic polymorphisms in CD44 were used to show that the stromal cell compartment is targeted by mAb RAMBM44 and the hematopoietic compartment by mAb IM7.8. Neither of the CD44-specific mAbs inhibited adhesion of LTC-IC to the stroma, suggesting alternative mechanisms of action. In support of this interpretation, we show that mAb RAMBM44 directly induces signal transduction in the stromal cell line S17 but not in hematopoietic cells. Conversely, mAb IM7.8 elicited the appearance of phosphorylated bands in hematopoietic cells, but not in stromal cells. Collectively, the data indicate that the opposing effects of CD44-mediated regulation can be explained by different cellular programs that are elicited in distinct cell compartments. The binding of the enhancing mAb RAMBM44 to CD44 is specifically inhibited by collagen IV, while binding of the suppressive mAb IM7.8.1 is inhibited by a substance contained in the supernatant of the stromal cell line AC3.U. Thus, the CD44 epitopes defined by the mAbs bind distinct ligands and the ligands provide a potential physiological counterpart for the regulatory actions of the mAbs.