Human epidermal Langerhans cells emigrate and induce a tolerogenic immune response in response to tick feeding and tick-borne pathogens

Langerhans cells (LC) induce immunity as well as tolerance at the epidermal barrier of the human skin in response to environmental stimuli and pathogens. Here, we investigate the migration and polarization patterns of human LC in response to clinical and experimental tick bite and skin infection with B. burgdorferi (acute Lyme borreliosis) using flow cytometry, migration assays, multi-color imaging and single-cell RNA sequencing. We observed a strong emigration of epidermal LC after tick feeding on human skin and in an experimental tick bite model using tick saliva injection. Consequently, LC were increased in the dermis, especially in proximity to dermal lymph vessels. LC over-expressed the migration marker CXCR4 as well as the lymph node homing molecule CCR7, indicating their capability for lymphatic migration. In line with this, LC stimulated with tick saliva showed increased potential to emigrate from epidermal sheets and invade towards collagen gels supplemented with the CCR7-ligand CCL19. Similarly, acute Lyme borreliosis skin harbored LC expressing CXCR4 and CCR7. Interestingly, monocyte-derived LC exposed to tick saliva or infected with B. burdorferi exhibited a tolerogenic phenotype characterized by the upregulation of the transcription factors IDO1 and IRF4, when compared to uninfected cells or S. aureus infection. This could be verified via single-cell RNA sequencing of patients with Lyme borreliosis, where LC exhibited increased expression of IRF4, IDO1 and IL4I1 compared to LC from healthy skin of the same individuals. Collectively, our results indicate that tick-feeding induces emigration of LC to the lymphatic system and a tolerogenic LC response, which may result in a decreased adaptive immune response to tick-borne pathogens. These findings explain the low immunogenic response and high transmission rates of Lyme disease and other tick-borne infections observed in human skin.