DPP4 and graft versus host disease: Inhibition of CD26 by a neutralising mAb or CD28 by the inhibitory ligand cytotoxic T-lymphocyte-associated protein 4 (CTLA-4; CD152) reduced GVHD, which corresponded to a reduction in a number of human pro-inflammatory cytokines including IFN-γ, TNF-α, IL-2, IL-4 and IL-17A in humanised mice [141].