Transfer of T cells to freshly irradiated allogeneic recipients leads to their rapid recruitment to nonlymphoid tissues, where they induce graft-versus-host disease (GVHD). In contrast, when donor T cells are transferred to established mixed chimeras (MCs), GVHD is not induced despite a robust graft-versus-host (GVH) reaction that eliminates normal and malignant host hematopoietic cells. We demonstrate here that donor GVH-reactive T cells transferred to MCs or freshly irradiated mice undergo similar expansion and activation, with similar up-regulation of homing molecules required for entry to nonlymphoid tissues. Using dynamic two-photon in vivo microscopy, we show that these activated T cells do not enter GVHD target tissues in established MCs, contrary to the dogma that activated T cells inevitably traffic to nonlymphoid tissues. Instead, we show that the presence of inflammation within a nonlymphoid tissue is a prerequisite for the trafficking of activated T cells to that site. Our studies help to explain the paradox whereby GVH-reactive T cells can mediate graft-versus-leukemia responses without inducing GVHD in established MCs.

Following bone marrow transplantation, delayed donor leukocyte infusions (DLIs) can induce graft-versus-leukemia (GVL) effects without graft-versus-host disease (GVHD). These antitumor responses are maximized by the presence of host hematopoietic antigen-presenting cells (APCs) at the time of DLI. Using a tumor-protection model, we demonstrate here that GVL activity following administration of DLIs to established mixed chimeras is dependent primarily on reactivity to allogeneic MHC antigens rather than minor histocompatibility or tumorassociated antigens. CD8 ؉ T-celldependent GVL responses against an MHC class II-negative tumor following delayed DLI require CD4 ؉ T-cell help and are reduced significantly when host APCs lack MHC class II expression. CD4 ؉ T cells primed by host APCs were required for maximal expansion of graft-versus-host reactive CD8 ؉ T cells but not their synthesis of IFN-␥. In contrast, the GVL requirement for CD4 ؉ T-cell help was bypassed almost completely when DLI was administered to freshly irradiated recipients, indicating that the host environment is a major factor influencing the cellular mechanisms of GVL. IntroductionFollowing allogeneic bone marrow transplantation (BMT), donor T-cell alloreactivity can be co-opted to generate powerful antitumor activity, an effect termed the graft-versus-leukemia (GVL) response. 1,2 The GVL effect is associated with the presence of graft-versus-host disease (GVHD) and is linked to the degree of major MHC disparity and the presence of T cells within the graft, indicating that graft-versus-host (GVH) alloreactive donor T cells are important for this effect. 3 We have previously shown that administration of delayed donor leukocyte infusion (DLI) to established mixed chimeras (MCs; in which hematopoietic elements from both the donor and recipient are present) produces dramatically improved GVL effects compared with those seen following delayed DLI to full chimeras (FCs). 4 Host hematopoietic antigen-presenting cells (APCs) expressing MHC class I molecules are necessary for this optimization of GVL effects in MCs. 4 The importance of host APCs in inducing GVL has recently been confirmed in freshly irradiated mice, 5 and previous studies have shown their importance in inducing GVHD under such conditions. 6,7 The marked overlap of GVL and GVHD limit the wider application of allogeneic BMT, especially in those individuals who lack an HLA-identical donor. However, GVL can be achieved without GVHD by administration of DLI to established MCs that lack proinflammatory stimuli from recent conditioning. 4,8 A precise definition of the mechanisms that underlie the GVL effect of DLI in MCs will be important for the rational development of this strategy for achieving maximal GVL effects without GVHD in humans. Using a tumor protection model, we demonstrate here that GVL responses of DLI are due to alloresponses against recipient MHC antigens. We also demonstrate a requirement for CD4 ϩ T-cell help in generating maximal CD8 ϩ T-cell-mediated GVL activity against MH...

Surprisingly, antitumor responses can occur in patients who reject donor grafts following nonmyeloablative hemopoietic cell transplantation. In murine mixed chimeras prepared with nonmyeloablative conditioning, we previously showed that recipient leukocyte infusions (RLI) induced loss of donor chimerism, IFN-γ production, and antitumor responses against host-type tumors. However, the mechanisms behind this phenomenon remain to be determined. We now demonstrate that the effects of RLI are mediated by distinct and complex mechanisms. Donor marrow rejection is induced by RLI-derived alloactivated T cells, which activate non-RLI-derived, recipient IFN-γ-producing cells. RLI-derived CD8 T cells induce the production of IFN-γ by both RLI and non-RLI-derived recipient cells. The antitumor responses of RLI involve mainly RLI-derived IFN-γ-producing CD8 T cells and recipient-derived CD4 T cells and do not involve donor T cells. The pathways of donor marrow and tumor rejection lead to the development of tumor-specific cell-mediated cytotoxic responses that are not due to bystander killing by alloreactive T cells.