Equine herpesvirus-1 (EHV-1), 2 an âŁ-herpesvirus of the family Herpesviridae, is the causative agent of respiratory disease, abortion, and a serious neurological disease known as encephalomyelitis in horses. Currently available vaccines are insufficient to protect against neurological disorders and abortions caused by EHV-1 (1). Outbreaks of EHV-1 encephalomyelitis can devastate farms, riding schools, and veterinary hospitals (2-4). Therefore, EHV-1 is one of the most important pathogens in the equine industry.EHV-1 attaches to heparan sulfate on the cell surface (5, 6). Viral entry into cells occurs through either direct fusion at the plasma membrane or endocytosis (7,8). Among the viral glycoproteins that play important roles in EHV-1 entry, glycoprotein D (gD) is essential for viral entry in all cases (9 -12). Recently, the equine MHC class I molecule was identified as a host factor involved in the entry process of EHV-1 (13, 14). NIH3T3 mouse fibroblast cells and B78H1 mouse melanoma cells are naturally resistant to EHV-1 infection; however, these cell lines become susceptible to EHV-1 infection by the exogenous expression of equine MHC class I (13, 14). The equine MHC class I molecule interacts with EHV-1 gD and mediates EHV-1 entry into equine cell types that are naturally susceptible to EHV-1 infection (14).MHC class I molecules are known primarily for their role in presenting peptides from intracellular antigens to cytotoxic T cells. MHC class I is polygenic and polymorphic (15). Fifty equine MHC class I genes were identified among 10 different serologically defined equine MHC class I haplotypes (16), and seven MHC class I loci were transcribed in an MHC homozygous individual (17). Although previous studies have shown that three equine MHC class I molecules can act as EHV-1 receptors (13,14), it is unclear whether all MHC class I molecules are involved in EHV-1 infection.Chinese hamster ovary (CHO)-K1 cells are known to be susceptible to EHV-1 infection (10). Unlike equine cells, inhibition of the cell surface expression of MHC class I molecules by â€2-microglobulin knockdown does not reduce the susceptibility of CHO-K1 cells to EHV-1 infection (14). This result suggests that hamster MHC class I has no role in EHV-1 entry into CHO-K1 cells.In this study, we expressed hamster MHC class I on NIH3T3 cells and confirmed the inability of hamster MHC class I to mediate the cellular entry of EHV-1. To clarify the region of equine MHC class I responsible for EHV-1 entry, we performed domain exchange and site-directed mutagenesis experiments on the hamster MHC class I backbone, in which parts of the extracellular region were replaced with corresponding sequences from equine MHC class I. Results from these experiments revealed an important amino acid residue of MHC class *