Host CD11c+ Dendritic Cells Are Required for Priming the Lympho-Haematopoietic Graft-Versus-Host Response but Not Graft-Versus-Host Disease.
Authors: Fallah-Arani Farnaz , Sivakumaran Janani , Flutter Barry R , Bennett Clare , Chakraverty Ronjon 
Published: October 2019

Abstract
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.