The extracellular domain of the Epstein-Barr virus BZLF2 protein binds the HLA-DR beta chain and inhibits antigen presentation.

The Epstein-Barr virus BZLF2 gene encodes a glycoprotein that associates with gH and gL and facilitates the infection of B lymphocytes. In order to determine whether the BZLF2 protein recognizes a B-cell-specific surface antigen, a soluble protein containing the extracellular portion of the BZLF2 protein linked to the Fc portion of human immunoglobulin G1 (BZLF2.Fc) was expressed from mammalian cells. BZLF2.Fc was used in an expression cloning system and found to bind to a beta-chain allele of the HLA-DR locus of the class II major histocompatibility complex (MHC). Analysis of amino- and carboxy-terminal deletion mutants of the BZLF2.Fc protein indicated that the first 90 amino acids of BZLF2.Fc are not required for HLA-DR beta-chain recognition. Site-directed mutagenesis of an HLA-DR beta-chain cDNA and subsequent immunoprecipitation of expressed mutant beta-chain proteins using BZLF2.Fc indicated that the beta1 domain, which participates in the formation of peptide binding pockets, is required for BZLF2.Fc recognition. The addition of BZLF2.Fc to sensitized peripheral blood mononuclear cells in vitro abolished their proliferative response to antigen and inhibited cytokine-dependent cytotoxic T-cell generation in mixed lymphocyte cultures. Flow-cytometric analysis of Akata cells induced to express late Epstein-Barr virus antigens indicated that expression of BZLF2 did not result in reduced surface expression levels of MHC class II. The ability of BZLF2.Fc to bind to the HLA-DR beta chain suggests that the BZLF2 protein may interact with MHC class II on the surfaces of B cells.