AbstractMHC class II haplotypes control the specificity of Th immune responses and susceptibility to many autoimmune diseases. Understanding the role of HLA class II haplotypes in immunity is hampered by the lack of animal models expressing these genes as authentic cis-haplotypes. In this study we describe transgenic expression of the autoimmune prone HLA DR3-DQ2 haplotype from a yeast artificial chromosome (YAC) containing an intact ∼320-kb region from HLA DRA to DQB2. In YAC-transgenic mice HLA DR and DQ gene products were expressed on B cells, macrophages, and dendritic cells, but not on T cells indicating cell-specific regulation. Positive selection of the CD4 compartment by human class II molecules was 67% efficient in YAC-homozygous mice lacking endogenous class II molecules (Aβnull/null) and expressing only murine CD4. A broad range of TCR Vβ families was used in the peripheral T cell repertoire, which was also purged of Vβ5-, Vβ11-, and Vβ12-bearing T cells by endogenous mouse mammary tumor virus-encoded superantigens. Expression of the HLA DR3-DQ2 haplotype on the Aβnull/null background was associated with normal CD8-dependent clearance of virus from influenza-infected mice and development of CD4-dependent protection from otherwise lethal infection with Salmonella typhimurium. HLA DR- and DQ-restricted T cell responses were also elicited following immunization with known T cell determinants presented by these molecules. These findings demonstrate the potential for human MHC class II haplotypes to function efficiently in transgenic mice and should provide valuable tools for developing humanized models of MHC-associated autoimmune diseases.