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

    September 2019 in “ Journal of Investigative Dermatology ”
    David Rutkowski, J. Hardman, Richard B. Warren, C.E.M. Griffiths, Ralf Paus
    TLDR EGFR and MEK inhibitors reduce PD-L1 in hair follicles, possibly causing inflammation.
    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.
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