Exploring the role of macrophage-niche crosstalk in radiation-induced salivary gland degeneration

Radiotherapy is a life-saving treatment for those with head and neck cancer. Although radiotherapy is very often successful in treating the cancer, a serious side-effect is damage to healthy tissue near the tumour(s), including the salivary glands. This tissue damage results in a reduced ability to produce saliva (a condition known as chronic dry mouth or xerostomia), which leads to difficulties with eating, speaking and sleeping and a risk of choking. Furthermore, it causes tooth decay and oral health problems. Thus, the side-effects of radiotherapy adversely affect a patient's quality of life.
Existing treatments for chronic dry mouth (e.g. saliva substitutes and mouthwashes) only give short-term relief and therefore there is a need to develop better, more effective treatments. The salivary glands degenerate over time following radiotherapy. However, why this happens is currently unknown.
In this proposal we will first extensively profile what happens in the salivary gland after radiotherapy, using glands isolated from both mice and humans. We already know that macrophages, a cell that has long been considered a key player in the repair and regeneration of many different organs, are essential for salivary gland regeneration after radiotherapy, and in their absence efficient regeneration does not occur. Conversely, over time macrophages are lost after radiotherapy, suggesting that something that supports the macrophages is missing. We know that salivary gland macrophages rely on a factor called Colony-Stimulating-Factor-1 (CSF1) and that CSF1 declines after radiotherapy, but we do not know which cells in the salivary gland produce CSF1. Here we will use genetically altered mice to deplete CSF1 in specific cells and explore the effect on macrophages. Finally, we will assess if we can rescue salivary gland function after radiotherapy by treating with CSF1 or by transplanting new macrophages into the salivary glands. Collectively, this will tell us which cells we should be focussing on to get better regeneration and whether we can ultimately rescue radiotherapy injury.

Radiotherapy remains a life-saving treatment for those with head and neck cancer (550,000 people annually worldwide). Although it is often effective at treating the cancer, it also destroys healthy organs, such as salivary glands (SGs), within the field of radiation, leading to difficulties in speaking, eating, and sleeping, and to significant oral health problems, severely affecting patient quality of life. Patients rely solely on short-term solutions which alleviate the symptoms but to date there is no long-term cure. The SGs undergo an initial period of regeneration but succumb to eventual degeneration and loss of function. However, why this occurs and the cells involved is unknown.

We will extensively profile the mouse and human SG using single cell RNA sequencing, to better understand the kinetics of tissue degeneration. In other tissues there is evidence that macrophages play a pivotal role in repair and regeneration, and we have recently shown that macrophages are necessary for initial epithelial regeneration of the mouse SG, and in their absence, function is not restored after injury. Furthermore, we have shown that SG macrophages are reliant on CSF1R signalling and that expression of the ligand Csf1 declines over time post-irradiation. However, we do not know which cells of the SG produce CSF1. In this proposal we seek to determine the cellular source(s) of CSF1, using sophisticated genetic knockout mouse models, and ascertain if cell-specific loss of CSF1 accelerates SG degeneration. Finally, we will explore if we can rescue degeneration by administering CSF1 or undertaking adoptive transfer of macrophages to irradiation-injured mouse SG.

Given the unrivalled ability of macrophages to orchestrate tissue repair, understanding the molecular basis of crosstalk between macrophages and other cells of the SG could ultimately provide new therapeutic avenues to promote regeneration in patients with radiation-induced injury of the SG.