A SOLUBLE DIVALENT CLASS-I MAJOR HISTOCOMPATIBILITY COMPLEX MOLECULE INHIBITS ALLOREACTIVE T-CELLS AT NANOMOLAR CONCENTRATIONS

Genetically engineered or chemically purified soluble monovalent major histocompatibility complex (MHC) molecules, which have previously bee n used to study T cells, have not blocked cytotoxic T-cell responses. Here we describe a genetically engineered divalent class I MHC molecul e which inhibits lysis of target cells by alloreactive cytotoxic T cel ls. This protein, H-2K(b)/IgG, was generated as a fusion protein betwe en the extracellular domains of a murine class I polypeptide, H-2K(b), and an immunoglobulin heavy chain polypeptide. The chimeric protein h as serological and biochemical characteristics of both the MHC and IgG polypeptides. Nanomolar concentrations of H-2K(b)/IgG inhibited lysis of H-2K(b)-expressing target cells not only by alloreactive H-2K(b)-s pecific T-cell clones but also by alloreactive H-2K(b)-specific primar y T-cell cultures. A direct binding assay showed high-affinity binding between the H-2K(b)/IgG molecule and an H-2K(b)-specific alloreactive T-cell clone. Unlabeled H-2K(b)/IgG displaced I-125-labeled H-2K(b)/I gG from T cells with an IC50 of 1.2 nM.