The hematopoietic microenvironment is critical for the self-renewal, proliferation, and differentiation of pluripotent hematopoietic stem cells. Within the hematopoietic microenvironment, whether it is embryonic yolk sac, fetal liver, or adult bone marrow, microvascular endothelium not only acts as a gatekeeper controlling the trafficking and homing of hematopoietic progenitors, but also provides cellular contact and secretes cytokines that allow for the preservation of the steady state hematopoiesis. We have developed a technique for the isolation and cultivation of adult bone marrow microvascular endothelium (BMEC) and fetal liver endothelial cells (FLEC). We have shown that BMEC and FLEC monolayers support the trafficking as well as long-term proliferation and terminal-differentiation of CD34+ hematopoietic progenitors. Direct cellular contact between endothelial monolayers and progenitor cells enhances the survival and regeneration of pluripotent hematopoietic progenitor cells. Although we have shown that binding of CD34+ progenitor cells to BMEC monolayers is partially mediated through interaction between CD34/L-selectin, b1, b2 integrins, and membrane-bound kit-ligand/c-kit receptor ligand pairs, as yet unrecognized membrane-bound adhesion molecule/chemoines are responsible for the self-renewal and homing of the pluripotent stem cells.

The major focus of our laboratory is to isolate and characterize known and novel adhesion and membrane-bound cytokines expressed by endothelium that regulate proliferation and adhesion of hematopoietic stem cells and their progenitors. To this end, we have utilized expression cloning strategy using BMEC and FLEC cDNA libraries to screen for known and novel adhesion/homing receptor and membrane-bound cytokines that regulate proliferation of hematopoietic progenitors. In collaboration with Dr. R. Crystal, adenoviral vectors overexpressing cytokines, and adhesion molecules are being used to examine their function in long-term CD34+ progenitor-endothelial coculture studies. Direct introduction of adenoviral vectors expressing stem cell active cytokines, into hematopoietic microenvironment provides novel approaches for the treatment of acquired or congenital hematological disorders.