Characterization of Human CD8+TCR- Facilitating Cells In Vitro and In Vivo in a NOD/SCID/IL2r[gamma]null Mouse Model

CD8+/TCR- facilitating cells (FCs) in mouse bone marrow (BM) significantly enhance engraftment of hematopoietic stem/progenitor cells (HSPCs). Human FC phenotype and mechanism of action remain to be defined. We report, for the first time, the phenotypic characterization of human FCs and correlation of phenotype with function. Approximately half of human FCs are CD8+/TCR-/CD56 negative (CD56neg); the remainder are CD8+/TCR-/CD56 bright (CD56bright). The CD56neg FC subpopulation significantly promotes homing of HSPCs to BM in nonobese diabetic/severe combined immunodeficiency/IL-2 receptor [gamma]-chain knockout mouse recipients and enhances hematopoietic colony formation in vitro. The CD56neg FC subpopulation promotes rapid reconstitution of donor HSPCs without graft-versus-host disease (GVHD); recipients of CD56bright FCs plus HSPCs exhibit low donor chimerism early after transplantation, but the level of chimerism significantly increases with time. Recipients of HSPCs plus CD56neg or CD56bright FCs showed durable donor chimerism at significantly higher levels in BM. The majority of both FC subpopulations express CXCR4. Coculture of CD56bright FCs with HSPCs upregulates cathelicidin and [beta]-defensin 2, factors that prime responsiveness of HSPCs to stromal cell-derived factor 1. Both FC subpopulations significantly upregulated mRNA expression of the HSPC growth factors and Flt3 ligand. These results indicate that human FCs exert a direct effect on HSPCs to enhance engraftment. Human FCs offer a potential regulatory cell-based therapy for enhancement of engraftment and prevention of GVHD. Human CD8+/TCR- facilitating cells enhance homing of hematopoietic stem/progenitor cells to bone marrow, promote clonogenicity, prevent apoptosis of hematopoietic stem/progenitor cells, and enhance durable chimerism in NOD/SCID/IL2[gamma]null mouse recipients.