Herpes Simplex Virus Type 1 Infects Langerhans Cells And The Novel Epidermal Dendritic Cell, Epi-Cdc2s, Via Different Entry Pathways

Skin mononuclear phagocytes (MNPs) provide the first interactions of invading viruses with the immune system. In addition to Langerhans cells (LCs), we recently described a second epidermal MNP population, Epi-cDC2s, in human anogenital epidermis that is closely related to dermal conventional dendritic cells type 2 (cDC2) and can be preferentially infected by HIV. Here we show that in epidermal explants topically infected with herpes simplex virus (HSV-1), both LCs and Epi-cDC2s interact with HSV-1 particles and infected keratinocytes. Isolated Epi-cDC2s support higher levels of infection than LCs in vitro, inhibited by acyclovir, but both MNP subtypes express similar levels of the HSV entry receptors nectin-1 and HVEM, and show similar levels of initial uptake. Using inhibitors of endosomal acidification, actin and cholesterol, we found that HSV-1 utilises different entry pathways in each cell type. HSV-1 predominantly infects LCs, and monocyte-derived DCs, via a pH-dependent pathway. In contrast, Epi-cDC2s are mainly infected via a pH-independent pathway which may contribute to the enhanced infection of Epi-cDC2s. Both cells underwent apoptosis suggesting that Epi-cDC2s may follow the same dermal migration and uptake by dermal DCs that we have previously shown for LCs. Thus, we hypothesize that the uptake of HSV and infection of Epi-cDC2s will stimulate immune responses via a different pathway to LCs, which in future may help guide HSV vaccine development and adjuvant targeting.Author summary Here we describe the first interactions of herpes simplex virus type 1 (HSV-1) with the human immune system as it enters the human genital skin. It was previously thought that this initial interaction of HSV-1 was only with 'Langerhans cells' (LCs) but we describe another important interaction with a new immune cell, Epi-cDC2s, which can also be infected by HIV and much more efficiently than LCs. Thus, there are now two of these types of immune cells resident in human epidermis. Therefore the way in which sexually transmitted and skin infecting viruses establish infection needs to be re-examined, including for HSV. We compared how these two cell types interacted with HSV-1 and showed Epi-cDC2s are more susceptible to HSV-1 infection than LCs and take up the virus via a different entry pathway. We speculate these cells may then carry HSV fragments to the dermis and then to lymph nodes to stimulate a protective immune response. These findings suggest similar routes may apply for chickenpox and cowpox viruses which also enter via the skin. Studying this pathway to establishment of natural immunity to HSV, may help guide the development of a successful vaccine.