Sequential Blockade and Engagement of Co-Stimulatory Pathways: A Potential Strategy for Amplifying Graft-Versus-Leukemia Responses without GVHD.

One potential approach for the effective disengagement of graft-versus-leukemia (GVL) effects from graft-versus-host disease (GVHD) following BMT is the use of nonmyeloablative conditioning as a platform for the adoptive transfer of donor T cells. In pre-clinical models, donor CD8+ T cells can induce powerful responses against tumors of host origin, but the effect lacks durability such that a re-challenge with tumor inevitability leads to tumor progression and death. This deficit is associated with the failure of functional CD8+ effector/memory T cells (T_E/M) to survive long-term post-DLI. To examine the fate of GVH-reactive CD8+ T cells following DLI, we established mixed hematopoietic chimeras (MC) in a parent →F1 model using a nonmyeloablative protocol that incorporates co-stimulatory molecule blockade. B6D2F1 mice received 3Gy TBI and intra-peritoneal injections of anti-CD154 and anti-CD8 mAb on day 0 followed by infusion of 2 x 10⁷ C57BL/6 bone marrow cells. 10 weeks later, when mAb had cleared from the circulation, MC received DLI that included CD8+ T cells from 2C transgenic mice that bear TCR specific for recipient class I MHC L^d. Using a clonotypic marker to monitor the response, we observed expansion of 2C CD8+ cells, peaking in the spleen on day 7 and then rapidly declining such that 2C CD8+ T cells were <0.1% of splenocytes by day 60. The decline in GVH-reactive T cells was associated with marked apoptosis and a sustained reduction in the expression of IL-7Rα. By day 60, no CTL activity against host cells was detectable. We reasoned that strategies that augment the survival of GVH T_E/M might enhance the durability of the GVL response and, in the absence of tissue inflammation induced by conditioning, might not lead to GVHD. Co-stimulation through the tumor necrosis family receptor, OX40, which is expressed on activated T cells, is anti-apoptotic and enhances recruitment of T_E/M to the memory pool. Following DLI, OX40 expression on 2C CD8+ T cells peaked on day 7 with somewhat earlier and sustained expression on DLI-derived CD4+ T cells. Since OX40 expression was specific for GVH-reactive T cells, we examined the effect of giving agonistic anti-OX40 antibody on day +5 following DLI. This was associated with rapid and complete conversion to full donor chimerism by day +14, whereas DLI + control antibody recipients had only partially converted by day +28. By day 60 post-DLI, anti-host CTL activity was clearly detectable in anti-OX40 recipients but not in controls. No clinical evidence of GVHD was observed, although histological examination revealed transient mild lymphocytic infiltration of the lamina propria on day +13, which resolved completely by day +18. In further experiments, anti-OX40 administration was associated with marked increases in the numbers of 2C CD8+ T cells in spleen, lymph node and bone marrow following DLI. Furthermore, effector differentiation, as assayed by intracellular expression of interferon-γ by 2C CD8+ T cells, was increased in recipients of anti-OX40 antibody. Of note, we observed a complete inhibition IL-7Rα down-regulation that is normally observed on activated CD8+ T cells following DLI. We conclude that OX40 co-stimulation following delayed DLI to established MC represents a potential means to enhance the magnitude and duration of a GVH reaction without the induction of significant GVHD.