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

The identification of rare stromal cells that support high levels of stem c ells has opened avenues to identify molecules that contribute to the mainte nance of these cells. We show that the maintenance of long-term culture ini tiating cells (LTC-IC) in stromal cell-supported cultures can be modulated via mAbs specific for CD44. mAb IM7.8.1 suppressed while mAb RAMBM44 enhanc ed LTC-IC levels in culture. Genetic polymorphisms in CD44 were used to sho w that the stromal cell compartment is targeted by mAb RAMBM44 and the hema topoietic compartment by mAb IM7.8. Neither of the CD44-specific mAbs inhib ited adhesion of LTC-IC to the stroma, suggesting alternative mechanisms of action. In support of this interpretation, we show that mAb RAMBM44 direct ly induces signal transduction in the stromal cell line S17 but not in hema topoietic cells. Conversely, mAb IM7.8 elicited the appearance of phosphory lated bands in hematopoietic cells, but not in stromal cells. Collectively, the data indicate that the opposing effects of CD44-mediated regulation ca n be explained by different cellular programs that are elicited in distinct cell compartments. The binding of the enhancing mAb RAMBM44 to CD44 is spe cifically inhibited by collagen TV, while binding of the suppressive mAb IM 7.8.1 is inhibited by a substance contained in the supernatant of the strom al cell line AC3.U. Thus, the CD44 epitopes defined by the mAbs bind distin ct ligands and the ligands provide a potential physiological counterpart fo r the regulatory actions Of the mAbs. (C) 2000 Academic Press.