MHC Disparity, Dose of Irradiation and T Cells in the Graft Determine the Persistence of Residual Host DCs, Mostly Skin-Derived Langerhans Cells (LCs) That Are Targets of DLI-Mediated Alloresponse.

Host antigen presenting cells (APCs), particularly DCs have a critical role in mediating allogeneic T-cell responses, however, the mechanisms that influence the donor DC reconstitution and persistence of residual host APCs after alloBMT are not characterized. We hypothesize that MHC disparity, pre-transplant conditioning and donor T cells directly influence donor DC reconstitution and amount of residual host DCs after alloBMT. To determine the role of major and minor histocompatibility antigens, syngeneic, B6Ly5.2 (H2^b, CD45.1⁺) → C57BL/6 (H2^b, CD45.2⁺), MHC-matched B6Ly5.2 (H2^b, CD45.1⁺) → C3H.SW (H2^b, CD45.2⁺) and MHC-mismatched C57BL/6 (H2^b) → BALB/c (H2^d) chimeras were constructed after lethal TBI (900 cGy) and transplantation of 1 x 10⁷ bone marrow (BM) cells from corresponding donors. Six weeks later DC chimerism was determined in the spleen and subcutaneous axilary lymph nodes (LNs) based on the differential expression of CD45 marker. In all three sets of chimeras full reconstitution with donor-derived CD11c⁺ DCs was found in the spleen but significant amount of residual host DCs was found in the subcutaneous LNs of syngeneic (^≈15%) and MHC-matched (^≈10%) but not MHC-mismatched chimeras (<1%). Since the percentage of host DCs in the LNs of all three sets of chimeras varied despite receiving same amount of TBI and BM we concluded that the level of the residual host DCs elimination is directly influenced by the MHC disparity. To determine the role of conditioning we titrated the dose of TBI (200–900cGy) in the syngeneic model and measured donor DC chimerism 6 weeks after transplantation. Donor DC chimerism in the spleen increased proportionally to the TBI dose and reached 100% at 700 cGy dose. Increase in donor DC chimerism in subcutaneous LNs was slower and never exceeded 90%. Since there is no immunologic barrier in this donor → recipient pair we concluded that the level of donor DC chimerism is directly affected by the dose of TBI and that complete donor DC chimerism is never achieved in subcutaneous LNs. Next, we addressed the role of donor T cells. We focused on the MHC-matched alloBMT setting since the full donor chimerism in the spleen and subcutaneous LNs was readily achieved in the MHC-mismatched setting. B6Ly5.2 mice were conditioned with titrating dose of TBI (500–900) and received MHC-matched C3H.SW marrow with or without added 1 x 10⁷ donor T cells. Presence of donor T cells in the graft clearly decreased the minimal dose of TBI required for engraftment of MHC-matched marrow from 700 to 500 cGy (p<0.001). However, once the MHC-matched marrow engrafted with or without added donor T cells there was a rapid conversion to full donor DC chimerism in the spleen but not in LNs. Increasing the dose of TBI from 900 to 1100 cGy did not significantly decrease the amount of residual host DCs in subcutaneous LNs. However, the amount of residual host DC chimerism in the LNs was clearly influenced by the presence of the donor T cells in the graft (900 cGy, 12.7% vs 4.4%). Next, by phenotypic analysis we characterized that dominant residual host DC populations in LNs are dermally-derived DCs and/or epidermally-derived LCs based on the high expression of DEC-205 marker. Finally, we tested whether residual host LCs are the target of DLI-mediated GVH-response. Administration of DLI resulted in decreased number of residual host LCs in the LNs (p< 0.01) providing the first evidence that residual host LCs are the targets of DLI-mediated GVH-response.