Specific inhibition of graft rejection by soluble MHC superdimers

We have previously described a genetically engineered soluble divalent analog of an MHC molecule (soluble superdimer). This protein, H2Kb/IgG, was generated as a fusion protein between the extracellular domains of a murine class I polypeptide, H-2Kb, and an immunoglobulin heavy chain polypeptide. Soluble MHC superdimers inhibit lysis of target cells by alloreactive cytotoxic T cells at nanomolar concentrations. A direct binding assay showed high affinity binding, ICso of 1.2 nM, between the H-2Kb/IgG molecule and an H-2K"-specific alloreactive T cell clone. To analyze the influence of H-2Kb/IgG on in vivo responses, we have studied the effect of H-2Kb/IgG on skin graft rejection. Anti-H-2Kb specific responses were studied by engrafting skin from C57B1/6 mice (H2Kb) onto H-2Kbn" recipients. Median survival times increased by 7-10 days when animals were injected with H-2Kb/IgG, 500 ng/dose, every other day for a total of only six doses; several animals demonstrated long term graft survival, up to 60 days. There was no influence of H-2K"/IgG on graft rejection mediated by either a different class I MHC molecule, H2D , or by a full MHC mismatch. We are currently studying the mechanism of prolongation of graft survival using in vitro MLR derived from treated mice. These experiments demonstrate' selective in vivo immunosuppression of grafts rejection using soluble MHC superdimers. Thus soluble MHC superdimers effectively regulate in vivo allospecific immune responses.