Investigating the role of macrophage-epithelial cell interaction in salivary gland regeneration

Abstract

Head and neck cancer patients treated by radiotherapy commonly suffer from a devastating side effect known as xerostomia (chronic dry-mouth), which results from irreversible salivary gland hypofunction. Although xerostomia has a detrimental impact on patient quality of life, there are currently no long-term solutions to restore salivary gland function, and patients have to rely on short-term solutions which currently only alleviate the symptoms.

A growing body of evidence suggests that tissue-resident macrophages in a variety of different organs interact with the epithelium to promote tissue regeneration and repair. We previously demonstrated that in the absence of macrophages epithelial repair after irradiation injury is aberrant, suggesting that macrophages are essential for successful tissue repair in the salivary gland. However, the precise function of macrophages in this process currently remains unknown. Here, I show that CX3CR1+ macrophages home specifically to ductal cells following radiation injury, and that DNA damage is more severe upon the genetic depletion of macrophages compared to radiation alone. In addition, via in vivo analysis and a novel method for ex vivo live imaging, I show that macrophages associate intimately with KRT14+ ductal progenitor cells after injury. Using our recently generated scRNAseq data and CellChat, a tool that is able to quantitatively infer and analyse intercellular communication networks, I propose that such interactions between progenitor cells and tissue-resident macrophages occur via the CX3CL1-CX3CR1 axis. Using CX3CL1 reporter mice I confirm that KRT14+ cells produce CX3CL1, the sole ligand for CX3CR1, providing robust evidence that macrophages directly communicate with KRT14+ cells during the repair process. Indeed, upon the depletion of macrophages KRT14-mediated ductal cell replacement is reduced, implicating the importance of macrophages in the replacement of damaged tissue via communication with epithelial progenitor cells.

Taken together, this study suggests that macrophage-epithelial cell interaction is essential for regeneration after salivary gland injury, and likely occurs through the direct communication between tissue-resident macrophages and lineage-restricted progenitor cells. Importantly, this provides strong rationale for further understanding this communication for the development of a future therapeutic strategy for xerostomia.