Allogeneic Effector Memory T Cells Enhance Hematopoietic Engraftment and Immune Reconstitution After Stem Cell Transplantation

Abstract 78

Allogeneic effector memory T Cells (T_EM) are able to kill host radioresistant T cells and host-type tumor cells without causing GVHD, suggesting that allogeneic T_EM may facilitate hematopoietic engraftment. We investigated the effects of allogeneic T_EM on hematopoietic engraftment and immune reconstitution using a rejection model. T_EM were prepared after negative selection of T cells followed by depletion of CD62L⁺ cells using magnetic beads. Lethally irradiated (8.5Gy) BALB/c mice were transplanted with minimum number (5×10⁵) of C57BL/6 T cell depleted bone marrow (TCD BM) cells. Only 40% of TCD BM recipients survived more than 100 days after transplantation. In contrast, addition of donor T_EM in the stem cell graft rescued 90% of the recipients (P<0.01, compared with TCD BM alone group). While all the recipients receiving TCD BM alone were mixed chimera (37.6±14.6% donor), all the recipients of additional donor T_EM became full chimera (99.7±0.07% donor). These data suggest that facilitation of engraftment is likely due to donor T_EM activation. Upon transfer into BALB/c recipients, donor T_EM were activated, secreted multiple inflammatory cytokines, and expressed all known cytotoxic molecules. Facilitation of engraftment may also lead to enhanced immune reconstitution. Faster stem-cell-derived T cell generation was observed in allogeneic T_EM recipients compared with the recipients of stem cells (c-Kit⁺ Thy1.1^low Lin⁻ Sca-1⁺) alone (P<0.01). Enhancement of stem-cell-derived T cell reconstitution was dramatically decreased in SCID recipients (lacking T and B cells) and could not be observed in NOG recipients (lacking T, B, and NK cells), further indicating that allogeneic T_EM enhance immune reconstitution through depletion of host radioresistant immune cells. To confirm that allogeneic T_EM enhance functional immune reconstitution, we injected BCL1 cells to the recipients at day +7 after transplantation. While 6 out of 8 C57BL/6 TCD BM alone recipients died of tumor, all allogeneic T_EM recipients were tumor-free and survived more than 100 days after transplantation (P<0.001). We hypothesize that donor T_EM can react to alloantigens initially but the alloresponses can not be sustained upon transfer to GVHD recipients, thus explaining the dichotomy of lack of GVHD with preservation of GVL. To test this hypothesis, we injected BCL1 cells into lethally irradiated BALB/c recipients of Rag-2^−/−gC^−/− BM and T_EM from C57BL/6 mice at different time points (day +7, +14, +21, +28) after transplantation. When injected at day +7, allogeneic T_EM recipients survived significantly longer than Rag-2^−/−gC^−/− BM only controls (medium survival time: 66 days vs. 26 days). When injected after day +14, the development of tumor and survival were comparable between Rag-2^−/−gC^−/− BM only and allogeneic T_EM groups. These data demonstrate that allogeneic T_EM enhance hematopoietic engraftment and immune reconstitution by reducing host resistance. The activation of allogeneic T_EM does not lead to GVHD because the alloresponse induced by T_EM can not be sustained in allogeneic stem cell recipients.