Importance of CD44v7 isoforms for homing and seeding of hematopoietic progenitor cells

The adhesion molecule CD44 consists of many isoforms of which particularly CD44v7 is of major importance in hematopoietic progenitor cell homing. An increase of progenitor cells in the periphery was observed after treating mice with a CD44v7-specific antibody, concomitant with a substantially augmented marrow-repopulating ability (MRA). Because CD44v7 is expressed on a fraction of bone marrow cells (BMC) as well as on long-term bone marrow culture-derived stromal cells, we aimed to differentiate between the functional relevance of CD44v7 on either cell type by transferring CD44v7+/+ BMC into CD44v7-/- mice and vice versa. CD44v7+/+ BMC homed poorly in the bone marrow of CD44v7-/- mice and their MRA was severely impaired. CD44v7-/- BMC, instead, exhibited an improved MRA when transferred into the CD44v7+/+ host, although their MRA remained below that of CD44v7+/+ BMC. Thus, it is predominantly, but not exclusively, expression of CD44v7 on stromal cells which supports progenitor cell homing.