Entry Mechanisms of Herpes Simplex Virus 1 into Murine Epidermis: Involvement of Nectin-1 and Herpesvirus Entry Mediator as Cellular

Skin keratinocytes represent a primary entry site for herpes simplex virus 1 (HSV-1) in vivo. The cellular proteins nectin-1 and herpes-virus entrymediator (HVEM) act as efficient receptors for both serotypes ofHSVand are sufficient for disease developmentmediated byHSV-2 inmice.HowHSV-1 enters skin andwhether both nectin-1 andHVEMare involved are not known.We addressed the im-pact of nectin-1 during entry ofHSV-1 intomurine epidermis and investigated the putative contribution ofHVEM.Using ex vivo in-fection ofmurine epidermis, we showed thatHSV-1 entered the basal keratinocytes of the epidermis very efficiently. In nectin-1-defi-cient epidermis, entrywas strongly reduced. Almost no entrywas observed, however, in nectin-1-deficient keratinocytes grown in culture. This observation correlatedwith the presence ofHVEMon the keratinocyte surface in epidermis andwith the lack ofHVEM expression in nectin-1-deficient primary keratinocytes.Our results suggest that nectin-1 is the primary receptor in epidermis, while HVEMhas amore limited role. For primarymurine keratinocytes, onwhich nectin-1 acts as a single receptor, electronmicroscopy suggested thatHSV-1 can enter both by direct fusionwith the plasmamembrane and via endocytic vesicles. Thus, we concluded that nectin-1 directs internalization into keratinocytes via alternative pathways. In summary,HSV-1 entry into epidermiswas shown t