The precise contribution(s) of skin dendritic cells (DCs) (epidermal Langerhans cells (LCs) and dermal dendritic cells (dDCs)) to graft-versus-host-disease (GvHD) in the skin has not been well delineated. We therefore developed an intradermal (id) injection model whereby CD8+ T (OT-I) cells that express ovalbumin (OVA)-peptide-specific T-cell receptors (Vα2/Vβ5) are delivered directly to the dermis of transgenic (Tg) mice expressing OVA targeted to the epidermis under a keratin 14 (K14) promoter. To determine the relative contribution of LCs to the GvHD reaction, we have generated K14-OVA x Langerin-diphtheria-toxin-receptor (Langerin-DTR) Tg mice to conditionally ablate LCs in the epidermis. Additionally, to identify the role of dDCs in the reaction we generated K14-OVA Tg chimeras using β2-microglobulin-deficient (b2m) congenic donor bone marrow cells. These mice were chimerized with donor b2m-derived MHC II high cells in the spleen (> 98%), LN (> 98%), and dermis (> 90%), whereas the majority of epidermal LCs remained recipient derived (> 95%). The dDCs in these mice do not have the capacity to present OVA to OT-I cells, whereas the LCs maintain their antigen-presenting functions, as previously reported. Following id injection of OT-I cells, the K14-OVA Tg mice developed localized skin GvHD on day 7 regardless of the level of OVA expression in the epidermis. Local injection of OT-I cells into the b2m→K14-OVA chimeric mice and LC-depleted K14-OVA x Langerin-DTR double Tg mice resulted in skin GvHD by day 7, comparable to normal K14-OVA Tg mice. Unexpectedly, OT-I cell injection into diptheria toxin (DT)-treated b2m→K14-OVA x Langerin-DTR Tg mice also resulted in skin GvHD. Thus, in vivo, even in the absence of both LCs in the epidermis and dDCs that can present OVA peptide in association with MHC I molecules, GvHD ensued. Furthermore, in vitro, OVA-expressing epidermal cells depleted of LCs (> 98% depletion) induced OT-I cell priming but did not induce a detectable allogeneic epidermal lymphocyte reaction. These results indicate that keratinocytes can function as accessory cells to induce the priming of naive autoreactive T cells in the skin.
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