Donor Th2 Cells to Prevent Graft-Versus-Host Disease in Bone Marrow Transplants

Allogeneic peripheral blood stem cell transplantation (PBSCT) is primarily limited by graft-versus-host disease (GVHD). In murine models, we have demonstrated that donor CD4+ T cells of Th1 cytokine phenotype (defined by their secretion of IL-2 and IFN-gamma) mediate GVHD. In contrast, donor CD4+ T cells of Th2 phenotype (defined by their secretion of IL-4, IL-5, and IL-10) do not generate GVHD, and abrogate Th-1-mediated GVHD. Importantly, we have demonstrated that enrichment of murine allografts with Th2 cells reduces GVHD without impairing the ability of donor T cells to prevent graft rejection. These studies indicate that the administration of Th2 cells after allogeneic transplantation represents a strategy for achieving alloengraftment with reduced GVHD.

In addition to GVHD, allogeneic PBSCT has been limited by the toxicity associated with conventional myeloablative preparative regimens. Such regimens, which typically utilize total body irradiation (TBI) and high-dose chemotherapy, were once considered essential for the prevention of graft rejection. However, recent clinical studies have shown that non-myeloablative doses of fludarabine-based chemotherapy can result in alloengraftment. In murine models, we have demonstrated that severe host T cell depletion induced by combination fludarabine and cytoxan can prevent even fully-MHC mismatched marrow graft rejection. Although non-myeloablative regimens may reduce regimen-related toxicity, such transplants have been associated with a 30 to 40% incidence of severe acute GVHD that is similar to rates observed with myeloablative regimens. Because non-myeloablative regimens appear to be associated with reduced regimen-related toxicity, we have elected to conduct this phase I study of Th2 cells in the setting of an immunoablative (non-myeloablative) preparative regimen.

Patients with leukemia in clinical remission, and patients with refractory lymphoid malignancy will be candidates for this HLA-matched allogeneic PBSCT protocol. Patients will receive novel induction regimen (fludarabine and EPOCH) and transplant preparative regimen (fludarabine and cytoxan) designed to maximally deplete host immune T cells capable of mediating graft rejection. After induction and preparative regimen chemotherapy, patients will receive an unmanipulated, G-CSF mobilized PBSC graft. In the initial six patients receiving this transplant procedure at the NCI, graft rejection has been successfully prevented (100% donor chimerism by day 30 post-transplant). Importantly, GVHD has been observed in all six patients, with three of the six patients developing severe GVHD (grade III). Given that this regimen successfully achieves donor engraftment, and is associated with significant GVHD, this transplant regimen represents an excellent clinical setting for the evaluation of Th2 cells.

Using this non-myeloablative allogeneic PBSCT approach, we will perform a Phase I study to evaluate the safety and feasibility of administering donor Th2 cells on day 1 post-transplant. Prior to transplantation, donor CD4+ T cells will be stimulated in vitro using culture conditions that support the generation of donor CD4 cells of the Th2 cytokine profile. If this Phase I study demonstrates that Th2 cell administration is safe and feasible, a Phase III study will be performed to evaluate whether Th2 cell administration reduces the incidence and severity of GVHD. Successful implementation of this Th2 strategy will greatly reduce the morbidity and mortality associated with allogeneic PBSCT, and may also represent an approach to stem cell transplantation in patients lacking an HLA-matched donor.