Human Brain Endothelial Cells (HUBEC) Can Expand Both Human Bone Marrow and Cord Blood SCID-Repopulating Cells (SRC) through Cell Contact Rather Than Soluble Factors.

Human brain endothelial cells (HUBEC), a U.S. Navy proprietary cell line, was reported previously by Chute et al as a promising co-culture ex vivo expansion system for both adult bone marrow (ABM) and cord blood (CB) hematopoietic stem cells (HSC).^a,b,c We report here our results of using HUBEC in ex vivo expansion and in vivo engraftment assay using NOD-SCID mice. CD34⁺ enriched fresh ABM was obtained using the method as described previously.^a,b However, we used frozen CB and the same cytokines for both ABM and CB expansion whereas Chute et al used fresh CB and different cytokines. Ex vivo expansion studies for both ABM and CB were performed for 7 days in the HUBEC coated plates with previously reported cell density and cytokine cocktail containing GM-CSF, IL-3, IL-6, SCF, and flt-3 (GM36SF) in IMDM 10% FBS media.^a HSC injections and BM harvesting of NOD-SCID mice as well as flow cytometric analysis were performed using the methods of Chute et al.^a NOD-SCID mice were transplanted with limiting doses of either fresh ABM CD34⁺ cells or freshly thawed CB CD34⁺. The progeny of the identical doses of ABM CD34+ or the progeny of the identical doses of CB CD34⁺ cells was then transplanted. Culture with GM36SF alone resulted in a 15.5-fold and 70-fold increase in total cells, a 3.4-fold and 32-fold increase in CD34+ cells, and a 4.8-fold and 4.1-fold increase in CD34+/CD38- cells for ABM and CB, respectively. In contrast, HUBEC co-culture with GM36SF yielded a 25-fold and 48-fold increase in total cells, a 8.9-fold and 13-fold increase in CD34+ cells, and 114-fold and 106-fold increase in CD34+/CD38- cells for ABM and CB, respectively. HUBEC co-culture without GM36SF supported a 1.0-fold and 1.0-fold increase in total cells, a 0.06-fold and 0.1-fold increase in CD34+ cells, and 0.25-fold and 0.2-fold increase in CD34+/CD38- cells for ABM and CB. HUBEC co-culture with GM36SF and transwell (non-contact culture) resulted in a 20-fold and 48-fold increase in total cells, a 6-fold and 8-fold increase in CD34+ cells, and a 32-fold and 38-fold increase in CD34+/CD38- cells for ABM and CB. Overall, the transwell expansion of CD34+/CD38- population in both ABM and CB was reduced to 30% of that achieved in the contact culture. ABM CD34+ cells (5 x 10⁵) engrafted 60% and the progeny of 5 x 10⁵ cultured in the HUBEC monolayer with GM36SF engrafted in 90% of transplanted mice. CB CD34+ cells (1 x 10⁴) engrafted 27% and the progeny 1 x 10⁴ CB CD34+ cells cultured in the HUBEC monolayer with GM36SF engrafted 64% of NOD-SCID mice. SRC frequencies calculated as a 3.12-fold and 2.7-fold increase in CD34+ enriched ABM and CB, respectively, which was less than reported previously.^a,b In summary, HUBEC supports and expands SRC mainly through cell-to-cell contact between HSC and endothelial cells, with HUBEC-secreted factors playing a minor role.