Investigating the role of interleukin-17 in radiation dermatitis.
Authors: Mieczkowska Karolina , Deutsch Alana , McLellan Beth , Kabarriti Rafi , Brodin N. Patrik , Koba Wade R. , Shinoda Kosaku 
Published: June 2021

e24094 Background: Up to 95% of patients receiving radiation therapy (RT) develop radiation dermatitis (RD), which can be therapy-limiting and detrimental to quality of life. Despite its ubiquity, no evidence-based gold standard for the management of RD exists, which highlights the inadequate understanding of its pathogenesis. Prior studies have suggested a role of the pro-inflammatory cytokine interleukin 17 (IL-17) in RD pathogenesis. Its pathway is known to be vital in other inflammatory dermatoses and anti-IL-17 antibodies are already in clinical use. Our goal is to mechanistically understand how RT leads to inflammation in the skin and demonstrate that we can inhibit these pathways and effectively manage RD. Methods: To test the effect of irradiation on the IL-17 pathway, mice were anesthetized and flank skin was stretched to form an exposure area. A single dose of 25 Gy was given. Controls were sham-irradiated and subjected to the same anesthesia and skin stretching. Three weeks after irradiation, we analyzed mRNA levels of IL-17 target genes by qRT-PCR. Furthermore, we utilized single cell RNA-sequencing (scRNA-seq) to profile cells from sham and irradiated skin. To elucidate the role of IL-17, we tested the effect of IL-17A blockade on RD severity in our mouse model. Of note, IL-17A utilizes IL-17 Receptor Type C (IL-17RC) as the membrane-bound receptor in its signaling cascade. Three treatment groups were established: sham receiving no radiation or drug, control receiving saline and radiation, and experimental receiving IL-17A neutralizing antibody and radiation. Mice receiving saline or neutralizing antibody had an intravenous infusion prior to irradiation and weekly thereafter. At the end of week 4, radiated skin was dissected and used for gene expression analysis and histology. To more meticulously study the contribution and requirement of keratinocyte IL-17RC for the development of RD, we generated keratinocyte-specific IL-17RC knockout mice and applied the abovementioned protocol to knockout and control cohorts. RD severity was assessed 8 weeks post irradiation. Results: mRNA levels of S100a8 and S100a9, markers of IL-17 pathway activation, are upregulated in irradiated skin and their expression strongly correlates with increased RD severity ( P < 0.001). From the scRNA-seq, we found that CD103+ dendritic cells and natural killer cells, known immunologic cell types of cutaneous radiation syndrome, were more abundant in irradiated skin. We also identified a novel keratinocyte subtype with abundant IL-17RC mRNA that was exclusive to the irradiated group. Mice receiving IL-17A neutralizing antibody as well as keratinocyte-specific IL-17RC knockout mice, showed a significant reduction in RD severity as compared to controls ( P = 0.0022). Conclusions: The IL-17 pathway plays a significant role in the pathogenesis of RD. Inhibition of this signaling within keratinocytes prevented the development of severe RD in a murine model.