Phenotypic and functional characteristics of CD34+ cells are related to their anatomical environment: is their versatility a prerequisite for their bio-availability?

Human CD34+ hematopoietic progenitors (HP) are mainly resident in adult bone marrow (BM). However, their recent revelation in nonhematopoietic tissues implies their circulation through peripheral blood (PB). The intimate mechanisms of this physiological process are not yet understood. Our results showed that steady-state CD34+ HP exhibit a differential phenotypic profile according to their BM versus PB localization. We demonstrated that this phenotype could be modulated by incubation in the presence of their counterpart mononuclear cells (MNC) through cell interactions and cytokine production. Such a modulation mainly concerns migration-mediated cytokine and chemokine receptors as well as some adhesion molecules and partly results from MNC specificity. These phenotypic profiles are associated with distinct cell-cycle position, cloning efficiency, and migration capacity of CD34+ cells from the different anatomical sources. We therefore propose a definition for a circulating versus resident CD34+ cell profile, which mostly depends on their cellular environment. We suggest that blood would represent a supply of cells for which phenotypic and functional characteristics would be a prerequisite for their bio-availability.