EGFR/MEK inhibitor therapy induces a distinct inflammatory hair follicle response that includes a collapse of immune privilege.

Inhibitors of epidermal growth factor receptor (EGFRi) or mitogen-activated protein kinase (MEKi) induce a folliculitis in 75-90% of patients, whose pathobiology remains insufficiently understood. (1) Characterize changes in the skin immune status and global transcriptional profile of EGFRi-treated patients (2) Probe whether EGFRi affects the hair follicle's (HF) immune privilege (IP) (3) Identify early pro-inflammatory signals induced by EGFRi/MEKi in human scalp HFs ex vivo. Scalp biopsies were taken from long-term EGFRi-treated patients exhibiting folliculitis (Chronic-EGFRi, n=9) vs normal scalp skin (n=9) and patients prior to commencing EGFRi therapy and after two weeks of EGFRi therapy (Acute-EGFRi, n=5). Healthy organ-cultured scalp HFs were exposed to EGFRi (Erlotinib) or MEKi (Cobimetinib) (n=5 patients, each). Samples were assessed by quantatitive immunohistomorphometry, RNAseq and in situ hybridization. The Chronic-EGFRi cohort showed CD8+ T cell infiltration of the bulge alongside a partial collapse of the HF's IP, evidenced by upregulated MHC class I, ß2-microglobulin and MHC class II and decreased TGF-ß1 protein expression. Healthy HFs treated with EGFRi/MEKi ex vivo also showed partial HF IP collapse and increased transcription of HLA-A, HLA-DR, ß2-microglobulin transcripts. RNAseq anlysis showed increased transcription of chemokines (CXCL1, CXCL13, CCL18, CCL3, CCL7) and IL-26 in Chronic-EGFRi biopsies, as well as increased interlukin IL-33 and decreased IL-37 expesssion in both Acute-EGFRi biopsies and organ-cultured HFs. These data show that EGFRi/MEKi compromise the physiological IP of human scalp HFs and suggest that future clinical management of EGFRi/MEKi-induced folliculitis requires HF IP protection and inhibition of IL-33.

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