Human CD8⁺/TCR⁻/CD56^dim/- Facilitating Cells Facilitate Homing and Engraftment of Human HSC in Vivo

Abstract 4093

Hematopoietic stem cell (HSC) transplantation has become a common procedure for treatment of hematopoietic malignancies and autoimmune disease. Despite significant advances in HSC transplantation, the morbidity and mortality of ablative conditioning and graft-versus-host disease (GVHD) remain limitations to application in the clinic. However, these risks can be overcome through less toxic nonmyeloablative conditioning and cell depletion strategies to remove GVHD causing-cells while retaining engraftment enhancing-tolerogeneic cells. We were the first to discover CD8⁺/TCR⁻ graft facilitating cells (FC) in mouse bone marrow. The addition of as few as 30,000 FC to 10,000 HSC significantly enhances engraftment of HSC in allogeneic recipients without causing GVHD. FC also potently enhance engraftment of limiting numbers of syngeneic HSC. Human CD8⁺/TCR^- FC comprised 1.1% ± 0.27% of total G-CSF-mobilized peripheral blood mononuclear cells (mPBMC). In the CD8⁺/TCR^- FC, 48% of cells expressed CD3ε⁺, 43% were FoxP3⁺, 43% were CD11c⁺, 19% were CD19⁺, and 30% were HLA-DR⁺. Approximately 55% of FC are also CD56^dim/-, and the remaining population is CD56^bright. The morphology of human CD8⁺/TCR⁻ FC with Wright-Giemsa staining under light microscopy suggested that the human FC population is heterogeneous. Here we evaluated if human FC enhance human HSC or progenitor homing to bone marrow of NOD/SCID/IL-2rγ^null (NSG) mouse recipients. CD45⁺CD34⁺ HSC and CD8⁺/TCR⁻/CD56^dim/-FC were sorted from mPBMC. NSG recipients were conditioned with 1100 cGy of total body irradiation (TBI). 24 hours after TBI, 100,000 HSC with or without 300,000 CD8⁺/TCR⁻/CD56^dim/- FC were transplanted into conditioned NSG recipients. Recipients were euthanized 16 hours after transplantation. Bone marrow was harvested from femurs and tibias of recipients and plated in Colony Forming Culture (CFC) Assays. Recipients of HSC plus FC generated significantly more colony formation (colonies = 110) compared with HSC alone (colonies = 65) (P = 0.011), suggesting that CD8⁺/TCR⁻/CD56^dim/- FC enhanced homing of HSC or progenitors to bone marrow. To test if human CD8⁺/TCR⁻/CD56^dim/- FC facilitate engraftment of human HSC in NSG mice, 300,000 CD8⁺/TCR⁻/CD56^dim/- FC were mixed with 100,000 HSC and transplanted into NSG recipient mice conditioned with 325 cGy TBI. Mice that received HSC alone served as controls. At 30 days after transplantation, PBL typing showed that 34% (10 of 29) recipients of HSC alone engrafted. In contrast, 78% of recipients (n = 23) of HSC plus CD8⁺/TCR⁻/CD56^dim/- FC engrafted, and donor chimerism in PB was 1.1% ± 0.8% and 4.1% ± 1.3% (P <0.05), respectively. At 6 months after transplantation, NSG recipients of HSC + CD8⁺/TCR⁻CD56^dim/- FC exhibited persistent donor chimerism in PB (9.1% ± 6% vs. 3.8% ± 3.5%) (P <0.05) and significantly higher levels of donor chimerism in spleen (26.3% ± 11.8% vs. 12.3% ± 9.8%) (P <0.05) and BM (11.6% ± 4.8% vs. 2.9% ± 1.3%) (P <0.05) compared to recipients of HSC alone. Our data indicate that CD8⁺/TCR⁻/CD56^dim/- FC facilitate homing of human HSC or progenitors and enhance engraftment of human HSC in NSG recipient mice.