Induction of Graft-versus-Leukemia-Activity after Peripheral Blood Progenitor Cell Transplantation (PBPCT)

Abstract

Using a murine transplantation model we have investigated the induction of GvL activity with allogeneic peripheral blood progenitor cells (PBPCs). We compared the influence of allogeneic PBPC and BM grafts on GvHR and leukemic relapse in mice bearing a B-lymphoblastic leukemia (A20). Furthermore, we evaluated the impact of T cell depletion on the risk of relapse and determined the effectiveness of ex vivo treatment with NK-cell activating cytokines as a compensation for the loss of T-cell derived factors stimulating natural cytotoxicity. Methods: After pretreatment of Balb/c (H-2^d) recipients with 7.5 Gy of total body irradiation, 2 × 10⁷ rhG-CSF-mobilized PBPCs of syngeneic or MHC-identical DBA (H-2^d) mice were transfered. Selective T-cell depletion (TCD) was performed by immunomagnetic purging with a monoclonal antibody directed against CD3. In some ex-perimental groups, T-cell-depleted PBPCs were incubated with 200 U/ml IL-2 and 100 U/ml IL-12 for 24 hrs. To investigate anti-leukemic activity in vivo, recipient mice were inoculated with 1 × 10⁵A-20 cells (a Blymphoblastic leukemia of Balb/c origin) 2 days prior to PBPCT. Results: The mortality rate due to GVHD was identical after allogeneic BMT and allogeneic PBPCT, although PBPC grafts contained the fourfold amount of CD3+ T cells than BMC grafts (61% vs. 15%). The relapse rates were 80% after syngeneic PBPCT and 60% after allogeneic BMT. After allogeneic PBPCT, a relapse rate of 34% was observed, indicating significantly (p < 0.05)superior GVL activity of PBPCT. After TCD of allogeneic grafts with antiCD3, the incidence of GvH-related mortality was below 5% but leukemia free survival was decreased to 25% and thus was similar to syngeneic PBPCT (17%, p < 0.05). When CD3-depleted grafts were incubated with IL-2 and IL-12, 45% of the animals remained free from leukemia. However, the difference was statistcally not significant. Our results suggest that stronger GVL effects can be induced by transplantation of PBPC as compared to BM grafts and that the ex-vivo activation of residual MHC-matched NK-cells with IL-2 and IL-12 does not fully compen-sate for the abrogation of GvL-activity after depletion of CD3-positive T-cells.