Immunotherapeutic aspects of allogeneic peripheral progenitor cells

In a newly developed murine model of allogeneic peripheral progenitor transplantation (PBPCT) we investigated the immunotherapeutic potential of allogeneic peripheral stem cells. The following topics were addressed by our experiments: (1) comparison of the graft-versus-leukemia effect exerted by allogeneic PBPCT compared to allogeneic BMT; (2) the influence of T-lymphocytes on GVL activity; (3) the possibility to enhance the GVL activity of allogeneic PBPCT grafts by ex vivo cytokine incubation. Balb/c mice received cells of the syngeneic B-lymphatic leukemia A20 2 days prior to TBI (7.5 Gy) and the respective graft. The recipients received allogeneic bone marrow grafts or allogeneic peripheral progenitor cells obtained after mobilization of the donors (DBA/2) with either G-CSF in a dose of 250 microg/kg/day for 5 days. In some experiments T lymphocytes were removed by immunomagnetic depletion with CD3-coated beads. An additional group received T cell-depleted and IL-2/IL12-activated PBPCT grafts. The antileukemic activity of an allogeneic PBPCT graft was significantly greater than the antileukemic activity of an allogeneic BMT graft of the same size. Relapse rates were 80% in syngeneic PBPCT, 60% after allogeneic BMT and 34% after allogeneic PBPCT. This rise in antileukemic activity is not accompanied by a rise in GVHD mortality. Depletion of T lymphocytes by CD3-coated beads resulted in a nearly complete loss of the GVL activity with a relapse rate of 75%. Incubation of the T-depleted graft with IL-2 and IL-12 to enhance NK-based GVL activity has only limited success after MHC-matched transplantation with a relapse rate of 55%. Allogeneic PBPC exert a pronounced antileukemic effect. After MHC-matched PBPCT, this GVL effect resides mostly on the T cells of the graft. Ex vivo activation of T cell-depleted grafts by IL-2 and IL-12 is accompanied by an only limited reduction of relapse rate. PBPC are a valuable modality for primary transplantation in situations with high risk of relapse and for the treatment of relapse after BMT.