ALBERT

Description: Key Points Efforts to understand mechanisms of disease initiation in human adult pre-B ALL are hampered by lack of appropriate animal models. Optimized xenotransplant assays show that niche-based SDF-1/CXCR4 interaction is crucial for adult non-t(4;11) pre-B ALL leukemia initiation.

Description: Abstract 2450 Poster Board II-427 Following allogeneic bone marrow transplantation (BMT), delayed donor leukocyte infusion (DLI) at a point when conditioning-induced inflammation has resolved, is employed to induce graft-versus-leukemia (GVL) responses while limiting the risk of host injury in terms of graft-versus-host disease (GVHD). We have previously shown that host bone marrow (BM)-derived antigen presenting cells (APC) are required to prime GVL responses following delayed donor T cell transfer into MHC-mismatched allogeneic chimeras. The APC population(s) required for induction of graft-versus-host reactivity are not known, although it has been demonstrated by others that certain host-derived dendritic cell (DC) populations (in situ or transferred) are capable of inducing GVHD in models of immediate T cell transfer to freshly conditioned recipients. We hypothesised that host CD11c+ conventional DC would be required for the induction of graft-versus-host reactivity in a model of delayed T cell transfer to established mixed chimeras (MC). To test this hypothesis, we reconstituted lethally irradiated B6 (H2b) mice with a mixture of BALB/c (H2d) and B6.CD11c-DTR T cell depleted bone marrow. In the resulting MC, host CD11chigh cells (predominantly conventional DC) are uniquely sensitive to killing by diphtheria toxin (DT) due to their selective expression of a high affinity DT receptor. 8-10 week old established MC received DLI in form of congenic Thy1.1+ BALB/c splenocytes. MC were injected with DT (or PBS) every 72 hours from day -1 to day 10 following DLI. Depletion of host conventional DC (〉90%) abrogated accumulation of donor Thy1.1+ CD4 and CD8 T cells in recipient spleens and reduced in vivo cytotoxicity against host target B cells. These effects associated with a lack of conversion from mixed to full donor chimerism, demonstrating an absolute requirement for host CD11chigh DC in priming the lympho-haematopoietic graft-versus-host response in the steady state. We have shown previously that induction of systemic inflammation by injection of a synthetic TRL7 agonist (R-848) is sufficient to induce systemic GVHD in MC after delayed DLI (Chakraverty et al, JEM, 2006). We therefore considered the requirement for CD11chigh DC in priming graft-versus-host reactivity in the presence of inflammation. We generated BALB/c + B6.CD11c-DTR→ B6 MC as before and then used DT treatment to deplete host conventional DC in the presence of R-848 (given every 72h from day -1 to day 10 following DLI). In MC without depletion of CD11chigh DC, R-848 treatment enhanced accumulation of Thy1.1+ BALB/c CD4 and CD8 donor cells and increased in vivo cytotoxicity against host B cell targets as compared to non-R-848 treated MC. This was associated with histological evidence of GVHD in the skin, gut and liver. In MC with depletion of CD11chigh DC (〉90% deletion), R-848 treatment was associated with equivalent donor T cell accumulation and in vivo killing of host targets as observed in non-DC depleted controls. Furthermore, DC-depleted MC treated with R-848 also developed histological GVHD. In vitro experiments using CD11c+ stimulator cells purified from the spleens and lymph nodes of R-848 or PBS-treated mice suggested that priming of the allo-response in the presence of inflammation was not due to a residual population of CD11chigh DC remaining after DT treatment. Taken together, these data demonstrate for the first time that conventional host CD11chigh DC are required to prime lympho-haematopoietic graft-versus-host responses in the steady state following delayed DLI to MC. In contrast, their role is redundant in the context of TLR-agonist induced GVHD. This suggests that other APC populations can prime GVHD in the presence of systemic inflammation. Disclosures: No relevant conflicts of interest to declare.

Description: T cell immunotherapies for cancer should ideally generate high levels of anti-tumor activity, with minimal host injury and permit the prolonged survival of functional memory/effector cells to prevent tumor recurrence. Following allogeneic stem cell transplantation, delayed donor leukocyte infusion (DLI) is one strategy employed to induce graft-versus-leukemia (GVL) responses while limiting the risk of host injury in terms of graft-versus-host disease. However, patients remain at significant risk of relapse following DLI and murine models of delayed DLI indicate that this results from the eventual loss of functional, alloreactive cytotoxic T lymphocytes (CTL) [Mapara et al. Transplantation 2003]. We hypothesised that the loss of functional CTL is driven by persistent stimulation of donor CD8 cells by alloantigen expressed by peripheral tissues. In order to follow and characterise an alloreactive CD8 response under conditions in which alloantigen was present or absent in peripheral tissues, we employed a model in which either parental B6 (H2b) or B6 x DBA-2 F1 (BDF1, H2dxb) mice were lethally irradiated and reconstituted with a mixture of B6 and BDF1 T cell depleted bone marrow. 8-10 weeks later congenic CD45.1 B6 splenocytes were transferred into the established mixed chimeras. This allowed us to test the importance of peripheral antigen in the loss of alloreactive CTL responses, since alloantigen was either restricted to the hematopoietic system (B6 +BDF1 → B6) or was ubiquitously expressed (B6 +BDF1 → BDF1). Following transfer of CD45.1 B6 splenocytes, the ensuing alloantigenspecific T cell response in both groups led to the elimination of alloantigen-positive (BDF1-derived) hematopoietic elements. Thereafter, alloreactive CD8 cells resided in an environment in which peripheral alloantigen was present (PA+) or absent (PA-). We observed similar kinetics of initial CD45.1+ CD8 cell proliferation and expansion and similar acquisition of a CD44highCD62Llow phenotype. However, by day 60, there were striking differences in the phenotype and function of transferred CD8 cells. In PA- hosts, CD45.1+ CD8 cells killed allogeneic target cells effectively both in vitro and in vivo, underwent rapid proliferation in a mixed leukocyte reaction and produced the effector cytokine, IFN-γ. In contrast CD45.1+ CD8 cells from PA+ hosts had little or no cytotoxic activity, did not proliferate to alloantigen and were IFN-γlow. Moreover, CD45.1+ CD8 cells from PA+ hosts displayed high levels of the co-inhibitory receptor PD-1, low levels of the IL-7Rα chain and responded poorly to IL-7 and IL-15 in vitro, a phenotype typical of the ‘exhaustion’ signature observed in CTL following chronic antigen exposure. In comparison, CD45.1+ CD8 cells from PA- hosts expressed significantly lower levels of PD-1, higher levels of IL-7Rα and demonstrated better responsiveness to IL-7 and IL-15 in vitro. In vitro PD-1 or PD-L1 blockade restored IFN-γ generation to CD45.1+ CD8 cells from PA+ hosts, suggesting that the PD-1 pathway may play a functional role in driving exhaustion of these cells. Importantly we observed no loss of long-term alloreactive CD4 responses in either PA+ or PA- hosts. This finding is consistent with a model in which peripheral alloantigen drives exhaustion since the majority of cells expressing Class II alloantigens in PA+ and PA- hosts would be restricted to the hematopoietic system and thus, would have been cleared in the initial alloresponse. The full exhausted phenotype of alloreactive CD8 cells described above was not seen until at least 30 days after transfer to PA+ hosts. However, as early as day 14, CTL primed in PA+ hosts produced less IFN-γ in comparison to those primed in PA-hosts, even though they were still equivalent in terms of their cytotoxicity. Furthermore, when CD8 cells primed in PA+ hosts were transferred to secondary antigen-free hosts, they still displayed reduced ‘fitness’ compared to CTL originally primed in PA- hosts. These data show that peripheral alloantigen qualitatively affects donor CTL function during priming and drives their eventual exhaustion. Additionally they suggest that blockade of co-inhibitory signals may have potential in restoring function to such cells as has been demonstrated in models of chronic infection.

Description: Reactivation of cytomegalovirus (CMV) remains a serious complication after allogeneic stem cell transplantation, but the role of γδ T cells is undefined. We have studied the immune reconstitution of Vδ2negative (Vδ2neg) γδ T cells, including Vδ1 and Vδ3 subsets and Vδ2positive (Vδ2pos) γδ T cells in 40 patients during the first 24 months after stem cell transplantation. Significant long-term expansions of Vδ2neg but not Vδ2pos γδ T cells were observed during CMV reactivation early after transplantation, suggesting direct involvement of γδ T cells in anti-CMV immune responses. Similarly, significantly higher numbers of Vδ2neg γδ T cells were detected in CMV-seropositive healthy persons compared with seronegative donors; the absolute numbers of Vδ2pos cells were not significantly different. The expansion of Vδ2neg γδ T cells appeared to be CMV-related because it was absent in CMV-negative/Epstein-Barr virus-positive patients. T-cell receptor-δ chain determining region 3 spectratyping of Vδ2neg γδ T cells in healthy subjects and patients showed restricted clonality. Polyclonal Vδ2neg cell lines generated from CMV-seropositive healthy donors and from a recipient of a graft from a CMV-positive donor lysed CMV-infected targets in all cases. Our study shows new evidence for role of γδ T cells in the immune response to CMV reactivation in transplantation recipients.