Background: High risk human papillomaviruses (HPV) infect basal keratinocytes (KCs), and can lead, if persistent, to malignant transformation. Persistent infection and transformation are associated with local immune suppression. We previously observed that Langerhans cells (LCs), the primary antigen-presenting cell of epithelium, are impaired in antigen-presentation and induction of adaptive immunity in the K14E7 mouse model of HPV-induced dysplastic epithelium. LC impairment is associated with a deficiency of interleukin 34 (IL-34), a cytokine expressed by KCs and a critical molecule for LC homeostasis. Low IL-34 expression is also associated with poor prognosis in epithelial cancers of the cervix and head/neck. Aims of this study were to investigate the molecular underpinnings of IL-34 alteration, and to develop tools to restore Il-34 expression in hyperproliferative epithelium.
Methods and results: Single cell RNA sequencing identified significant reduction of IL-34 in basal and differentiated KCs in hyperplastic K14E7 skin. Reduction of IL-34 transcript and protein was confirmed by PCR and ELISA. Using the Assay for Transposase-Accessible Chromatin sequencing (ATAC-seq), we investigated changes in transcriptional regulation of KCs and LCs in K14E7 epithelium. The chromatin accessibility landscape was significantly altered in K14E7 KCs and LCs, revealing more than 30.000 differentially enriched sites (DES) in each cell type. We observed 8 DES in chromatin accessibility at the Il34 locus, including key sites within the skin specific Il34 promoter. Furthermore, we constructed Il34 encoding lentiviruses (LV-Il34), to test whether restoration of IL-34 in hyperproliferative epithelium can repair the function of LCs and subsequent adaptive immune responses.
Conclusion: Our data suggests that a difference in occupancy of site-specific transcription factors underpins the observed reduction in Il34 transcript and protein content. LV-Il34 is a promising tool for further investigations into the role of IL-34 in skin immunity and its potential use as gene-therapy for epithelial cancers.