Donor CD4⁺CD25⁺ T Cells Require Syngeneic MHC Class II for Initial Support of MHC-Mismatched Hematopoietic Engraftment.

Despite the potential to cure both acquired and inherited disorders involving the hematopoietic compartment, application of allogeneic bone marrow transplantation (BMT) is limited by the frequent and severe outcome of Graft vs. Host Disease (GVHD). Unfortunately, efforts to reduce GVHD by purging the donor graft of T cells have resulted in poor engraftment and elevated disease recurrence. Alternative cell populations capable of supporting allogeneic engraftment without inducing GVHD could increase the potential for donor-recipient matching and decrease treatment associated risks. We have observed that GVHD-suppressive donor CD4⁺CD25⁺ T cells are capable of supporting allogeneic hematopoietic engraftment, as demonstrated by initial donor progenitor activity and long-term chimerism and tolerance. Using a murine MHC mismatched model transplanting 0.5–2x10⁶ GFP⁺ C57BL/6 (B6) T cell-depleted bone marrow cells into 7.0 Gy sublethally irradiated BALB/c recipients, splenic CFU assessment demonstrated that co-transplantation of 1x10⁶ B6 CD4⁺CD25⁺ T cells lead to increased donor lineage-committed GM (p<.01) and multi-potential HPP (p<.05) progenitors seven days post-BMT compared to transplantation of BM alone. Furthermore, co-transplantation of CD4⁺CD25⁺ T cells lead to lymphoid and myeloid chimerism in peripheral blood (lineage specific mean donor chimerism ± SE: B220, 67.7±15.2 vs. 0.3±0.3; CD4, 38.3±10.5 vs.0.9±0.9; CD8, 48.3±11.0 vs. 1.0±1.0; Mac-1, 58.8±16.5 vs. 0.3±0.3) and the presence of donor GM and HPP progenitors in recipient marrow two months post-BMT (mean CFU chimerism ± SE: CFU-GM, 54.5±12.8 vs. 0.0; CFU-HPP, 63.0±17.8 vs.0.0). Donor chimerism persisted six months post-BMT and was associated with tolerance to donor and host antigens by acceptance of donor and host skin grafts >50 days post-homotopic grafting. Characterization of the initial invents of engraftment support demonstrated that augmentation of donor progenitors did not require CD4⁺CD25⁺ T cell IL-10, as co-transplantation of B6-wt and B6-IL-10^−/− CD4⁺CD25⁺ T cells both significantly increased total CFU-GM (mean CFU±SE: BM alone, 657.5±248.2; BM + wt, 1972±331.5; BM + IL-10^−/−, 1965±401.7; both p<.05 vs. BM alone). Assessment of the antigenic requirements for activation of progenitor support demonstrated that donor CD4⁺CD25⁺ T cells did not require alloreactivity to support progenitors, as BALB/c x B6 F1 CD4⁺CD25⁺ T cells significantly increased B6 CFU-GM in BALB/c recipients (p<.001 vs. BM alone). However, B6 CD4⁺CD25⁺ T cells failed to augment C3H/HeJ CFU-GM in BALB/c recipients (p>.05 vs. BM alone), suggesting that donor CD4⁺CD25⁺ T cells might require recognition of syngeneic MHC for progenitor support. Indeed, augmentation of donor CFU-GM was abrogated when B6 CD4⁺CD25⁺ T cells were co-transplanted with B6-MHC class II^−/− marrow into BALB/c recipients (p>.05 vs. BM alone). In conclusion, donor CD4⁺CD25⁺ T cells capable of promoting long-term engraftment and tolerance do not require IL-10 for support of initial donor progenitor activity, however progenitor support does require co-transplantation with syngeneic MHC class II expressing marrow. Donor CD4⁺CD25⁺ T cells may thus represent a useful alternative to unfractionated T cells for promotion of engraftment following allogeneic hematopoietic transplantation.