TLR7 ligand R848 prevents mouse graft-versus-host disease and cooperates with anti-IL-27 antibody for maximal protection and Treg upregulation.

In spite of considerable therapeutic progress, acute graft-versus-host disease still limits allogeneic hematopoietic cell transplantation. We recently reported that mouse infection with nidovirus lactate dehydrogenase elevating virus impairs disease in non-conditioned B6D2F1 recipients of parental B6 spleen cells. As this virus activates TLR7, we tested pharmacologic TLR7 ligand R848 in this model and observed complete survival if donor and recipients were treated before transplantation. Mixed lymphocyte culture performed 48 h after R848-treatment of normal mice demonstrated that both T cell allo-responsiveness and antigen presentation by CD11b+ and CD8α+ dendritic cells were inhibited. These inhibitions were dependent on IFNAR-1 signaling. In the B6 to B6D2F1 transplantation model, R848 decelerated, but did not abrogate, donor T cell implantation and activation. However, it decreased IFNγ, TNFα and IL-27 while upregulating active TGF-β1 plasma levels. In addition, donor and recipient Foxp3+ regulatory T cell numbers were increased in recipient mice and their elimination compromised disease prevention. R848 also strongly improved survival of lethally irradiated BALB/c recipients of B6 hematopoietic cells and this also correlated with an upregulation of CD4 and CD8 Foxp3+ regulatory T cells that could be further increased by IL-27 inhibition. The anti-IL-27p28 monoclonal antibody and R848 combination also showed strong synergy in preventing disease in the B6 to B6D2F1 transplantation model when recipients were sublethally irradiated and this also correlated with Treg upregulation. We conclude that R848 modulates multiple aspects of graft-versus-host disease and offers potential for safe allogeneic bone marrow transplantation that can be further optimized by IL-27 inhibition.