Human Hair Follicles Express PD-L1, Whose Expression Is Down-Regulated by EGFR and MEK Inhibitors Ex Vivo: A Potential Mechanism for EGFR Inhibitor Induced Sterile Folliculitis

The study investigated the effects of epidermal growth factor receptor inhibitors (EGFRi) and mitogen activated protein kinase inhibitors (MEKi) on PD-L1 expression in human hair follicles, which could explain the sterile folliculitis observed in patients treated with these inhibitors. In normal human scalp skin, PD-L1 was found in immune cells within the dermis and in the hair follicle. Organ culture experiments with human facial skin showed that EGFRi and MEKi significantly decreased PD-L1 gene expression after 24 hours and reduced keratinocyte proliferation after 3 days. This suggested that the down-regulation of PD-L1 might contribute to the inflammatory reactions seen in patients, potentially due to a collapse in hair follicle immune privilege.