Expression and function of cell adhesion molecules on fetal liver, cord blood and bone marrow hematopoietic progenitors: Implications for anatomical localization and developmental stage specific regulation of hematopoiesis

The mechanism of localization, migration, and regulation of hematopoiesis at different stages of ontogeny is not well understood, but may relate to the specific adhesive interactions between hematopoietic stem cells and their microenvironment at different ontogenic stages. We studied the expression of cell adhesion molecules (CAM) on fetal liver (FL), umbilical cord blood (UCB) and adult bone marrow (ABM) CD34⁺ cells, and the adhesion of committed progenitors (CFC) from all three sources to ABM stromal layers and purified extracellular matrix proteins. Compared to ABM CFC, significantly more UCB CFC and fewer FL CFC adhered to ABM stroma. Adhesion of FL CFC to fibronectin (FN), the 75 kD RGD containing FN fragment and the 33-66 kD COOH-terminal heparin binding FN fragment was also significantly less than that of ABM CFC. Like ABM CFC, the adhesion of FL CFC was mediated through α4β1 and α5β1 integrins. Of note, more FL CD34⁺ cells expressed α5 integrins and the number of α4, α5 and β1 integins per cell (mean channel frequency) was similar or higher for FL CD34⁺ cells than ABM CD34⁺ cells. Further, treatment of FL CFC with a β1 integrin activating antibody (8A2), increased adhesion of FL CFC to FN to the same level as that of 8A2 treated ABM CFC. This suggests that the 2a4β1 and α5β1 integrins on FL CD34⁺ cells may be present in a low avidity/affinity state. We also show that unlike ABM, FL CD34⁺ cells expressed α2 and that approximately 20% FL CFC adhered to collagen IV. Further, α2β1 integrin on FL CFC was functional since their engagement, either by adhesion to collagen IV or through blocking α2 antibodies, transmitted proliferation inhibitory signals. In contrast to α4b and α5β1 integrin dependent adhesion, α2β1 dependent adhesion of FL CFC to collagen IV was not enhanced after treatment with 8A2. The reason for this is not clear but suggests that α2 integrins on FL CFC are maximally activated. This novel adhesive interaction with collagen IV, reminiscent of that described for CML progenitors, may have a role in the extramedullary localization of FL hematopoiesis or its developmental stage-specific regulation by its microenvironment. Studies to evaluate these possibilities are underway.