Interleukin-15 is a hair follicle immune privilege guardian.

The autoimmunity-promoting cytokine, Interleukin-15 (IL-15), is often claimed to be a key pathogenic cytokine in alopecia areata (AA). Yet, rhIL-15 promotes human hair follicle (HF) growth ex vivo. We have asked whether the expression of IL-15 and its receptor (IL-15R) isoforms is altered in human AA and how IL-15 impacts on human HF immune privilege (HF-IP) in the presence/absence of interferon-γ (IFNγ), the well-documented key AA-pathogenic cytokine, as well as on hair regrowth after experimental AA induction in vivo. Quantitative immunohistomorphometry showed the number of perifollicular IL-15+ T cells in AA skin biopsies to be significantly increased compared to healthy control skin, while IL-15, IL-15Rα, and IL-15Rγ protein expression within the hair bulb were significantly down-regulated in AA HFs. In organ-cultured human scalp HFs, rhIL-15 significantly reduced hair bulb expression of MICA, the key "danger" signal in AA pathogenesis, and increased production of the HF-IP guardian, α-MSH. Crucially, ex vivo, rhIL-15 prevented IFNγ-induced HF-IP collapse, restored a collapsed HF-IP by IL-15Rα-dependent signaling (as documented by IL-15Rα-silencing), and protected AA-preventive immunoinhibitory iNKT10 cells from IFNγ-induced apoptosis. rhIL-15 even promoted hair regrowth after experimental AA induction in human scalp skin xenotransplants on SCID/beige mice in vivo. Our data introduce IL-15 as a novel, functionally important HF-IP guardian whose signaling is constitutively defective in scalp HFs of AA patients. Our data suggest that selective stimulation of intrafollicular IL-15Rα signaling could become a novel therapeutic approach in AA management, while blocking it pharmacologically may hinder both HF-IP restoration and hair re-growth and may thus make HFs more vulnerable to AA relapse.

Dandruff lesional scalp skin exhibits epidermal T cell infiltration and a weakened hair follicle immune privilege.

OBJECTIVE: Dandruff is characterised by the presence of perivascular leukocytes and mild inflammation; however, the immune microenvironment of dandruff-affected scalp skin and the potential changes to the hair follicle's (HF) physiological immune privilege (HF IP) remain unknown. Here, we characterised the HF immune microenvironment and immune privilege status in dandruff-affected scalp skin. METHODS: We assessed relevant key parameters in healthy versus dandruff-affected human scalp biopsies using quantitative immunohistomorphometry, laser capture microdissection, and RNA sequencing. RESULTS: The number of epidermal CD4+ and CD8+ T cells was increased in lesional dandruff scalp skin, while the number of MHC class II+ /CD1a+ Langerhans cells was decreased in the infundibulum. The number of intrafollicular and perifollicular CD4+ T cells and CD8+ T cells, perifollicular CD68+ macrophages, and tryptase+ mast cells remained unchanged. Interestingly, MHC class Ia and ß2-microglobulin protein expression were significantly increased specifically in the suprabulbar outer root sheath (ORS) compartment of dandruff-associated HFs. RNAseq analysis of laser capture micro-dissected suprabulbar ORS compartment revealed antigen presentation pathway as the top regulated canonical pathway, along with the upregulation of HF-IP genes such as HLA-C, HLA-DP, and TAP1, which are normally down-regulated in healthy HFs. Intrafollicular protein expression of known HF IP guardians (CD200 and α-MSH) and 'danger signals' (MICA and CXCL10) remained unaltered at the IP sites of dandruff lesional HFs compared to non-lesional and healthy HFs. Instead, the expression of macrophage migration inhibiting factor (MIF), another HF IP guardian, was reduced. CONCLUSION: Together, this work shows that dandruff is associated with epidermal T-cell infiltration and a weakened HF IP in the suprabulbar ORS of HFs in dandruff lesional scalp.

OBJECTIF: Les pellicules sont caractérisées par la présence de leucocytes périvasculaires et une légère inflammation. Cependant, le microenvironnement immunitaire de la peau du cuir chevelu affectée par les pellicules et les modifications potentielles du privilège immunitaire physiologique du follicule pileux (PI FP) restent inconnus. Ici, nous avons caractérisé le microenvironnement immunitaire du follicule pileux (FP) et l'état du privilège immunitaire de la peau du cuir chevelu affectée par les pellicules. MÉTHODES: Nous avons évalué les principaux paramètres pertinents dans des biopsies de cuir chevelu humain sain par rapport à ceux touchés par les pellicules, à l'aide d'une immuno-histomorphométrie quantitative, d'une microdissection au laser et d'un séquençage de l'ARN. RÉSULTATS: Le nombre de lymphocytes T CD4+ et CD8+ épidermiques a augmenté dans la peau du cuir chevelu atteinte de pellicules lésionnelles, tandis que le nombre de cellules de Langerhans du CMH de classe II+ /CD1a+ a diminué dans l'infundibulum. Le nombre de lymphocytes T CD4+ et de lymphocytes T CD8+ intrafolliculaires et périfolliculaires, de macrophages CD68+ périfolliculaires et de mastocytes tryptase+ est resté inchangé. Il est intéressant de noter que l'expression des protéines du CMH de classe Ia et de la ß2-microglobuline a augmenté de manière significative dans le compartiment suprabulbaire de la gaine radiculaire externe (GRE) en particulier des FP associés aux pellicules. L'analyse par séquençage ARN du compartiment suprabulbaire de la GRE micro-disséquée au laser a révélé que la voie de présentation de l'antigène était la voie canonique la plus régulée, ainsi que la régulation à la hausse des gènes PI-FP tels que HLA-C, HLA-DP et TAP1, qui sont normalement régulés à la baisse dans les FP sains. L'expression protéique intrafolliculaire des gardiens connus du PI FP (CD200 et α-MSH) et des « signaux de danger ¼ (MICA et CXCL10) est restée inchangée au niveau des sites du PI des FP à pellicules lésionnelles par rapport aux FP sans pellicules lésionnelles et sains. En revanche, l'expression du facteur d'inhibition de la migration des macrophages (MIF), un autre gardien du PI FP, a été réduite. CONCLUSION: L'ensemble de ces travaux montrent que les pellicules sont associées à une infiltration épidermique des lymphocytes T et à un affaiblissement du PI FP dans la GRE suprabulbaire des FP du cuir chevelu atteint de pellicules lésionnelles.

Activation of the integrated stress response in human hair follicles.

Unravelling how energy metabolism and stress responses are regulated in human scalp hair follicles could reveal novel insights into the controls of hair growth and provide new targets to manage hair loss disorders. The Mitochondrial Pyruvate Carrier (MPC) imports pyruvate, produced via glycolysis, into the mitochondria, fuelling the TCA cycle. Previous work has shown that MPC inhibition promotes lactate generation, which activates murine epithelial hair follicle stem cells (eHFSCs). However, by pharmacologically targeting the MPC in short-term human hair follicle ex vivo organ culture experiments using UK-5099, we induced metabolic stress-responsive proliferative arrest throughout the human hair follicle epithelium, including within Keratin 15+ eHFSCs. Through transcriptomics, MPC inhibition was shown to promote a gene expression signature indicative of disrupted FGF, IGF, TGFß and WNT signalling, mitochondrial dysfunction, and activation of the integrated stress response (ISR), which can arrest cell cycle progression. The ISR, mediated by the transcription factor ATF4, is activated by stressors including amino acid deprivation and ER stress, consistent with MPC inhibition within our model. Using RNAScope, we confirmed the upregulation of both ATF4 and the highly upregulated ATF4-target gene ADM2 on human hair follicle tissue sections in situ. Moreover, treatment with the ISR inhibitor ISRIB attenuated both the upregulation of ADM2 and the proliferative block imposed via MPC inhibition. Together, this work reveals how the human hair follicle, as a complex and metabolically active human tissue system, can dynamically adapt to metabolic stress.