Investigation into the attenuation of graft versus host disease using in vivo models of preparative conditioning and dendritic cell depletion

Graft versus host disease (GVHD) is a major limitation to the widespread application of allogeneic haematopoietic stem cell transplantation (HSCT). Pre-transplant myeloablative conditioning (MAC) regimens control malignancy, ablate host immune responses and facilitate haematopoietic reconstitution, but they are also associated with significant transplant related mortality (TRM). In addition, conditioning drives host dendritic cell (DC) and donor T-cell interactions that result in GVHD. Reducing the intensity of the conditioning (RIC) regimen maintains anti-leukaemic activity and reduces TRM but the overall incidence of GVHD is unchanged. To date clinically relevant in vivo murine models of RIC allogeneic HSCT do not exist, preventing studies into the mechanism of RIC-associated delayed onset GVHD. We have developed two RIC HSCT models that result in delayed onset GVHD, recapitulating that seen in the clinic. The development of these models has allowed the effect of RIC on chimerism, DC chimerism and subsequent GVHD to be investigated. RIC activates and increases overall DC numbers post-HSCT without triggering early GVHD. Delayed onset GVHD in these RIC HSCT recipients is characterised by the persistence of host cells, a reduced inflammatory response early post- HSCT and the late emergence of activated donor DC. Better understanding of GVHD in the RIC HSCT setting will allow the development of strategies to eradicate the problem of GVHD whilst retaining the graft versus leukaemia (GVL) effect. Host DC play a vital role in the initiation of GVHD, we therefore proposed the strategic administration of DC depleting antibodies as an effective means to control GVHD. N418 the monoclonal antibody against mouse leukocyte integrin CD11c was chosen for the in vivo depletion of DC due to its expression on all murine DC including activated, immature and pDC. The in vivo injection of N418 eliminated 50% of splenic DC in mice. However, subsequent experiments demonstrated that administration of N418 was not sufficient to delay GVHD in either the MHC mismatched or MHC matched, miHA mismatched models of HSCT. Administration of N418 also decreased engraftment and the beneficial effects of GVL. These results suggest that antibody administration may need to be tailored towards a specific subset of DC to prevent GVHD in the absence of immunosuppression and disease relapse. The successful application of DC depletion to control GVHD would improve the safety of HSCT for patients with leukaemia and extend the immune benefits of this curative therapy to the wider patient population