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The hair follicle ostium represents a potential port of microbial entry into the skin and harbours a rich residential microflora, but only rarely shows clinical signs of infection. This suggests the presence of effective local antimicrobial defence systems. So far, research was mostly focussed on the cellular components of the hair follicle’s adaptive immune system, although some antimicrobial peptides (AMPs) had already been detected in this region.
Therefore the current study aimed at systematically analysing the immunoreactivity of four selected AMPs in normal human scalp hair follicles in situ: RNase 7, psoriasin, hornerin and LEKTI-2. Furthermore the two well characterized AMPs RNase 7 and psoriasin were additionally examined in isolated outer root sheath keratinocytes, via semi-quantitative RT-PCR and immunohistochemistry. In addition, functional studies in full thickness skin organ culture were conducted investigating whether hair follicle AMPs can be stimulated by prototypic bacterial pathogenicity factors [lipopolysaccharides (LPS), lipoteichoic acid (LTA) and protein A] and/ or the pro-inflammatory cytokine interferon-γ (IFNγ).
In this study, all tested AMPs could indeed be detected in the hair follicle epithelium as well as in the sebaceous gland, providing the first immunohistological expression pattern for RNase 7, hornerin and LEKTI-2. RNase 7 immunoreactivity was found throughout the entire outer root sheath (ORS), whereas psoriasin was only observed in the ORS distal of the orifice of the sebaceous duct. Hornerin immunoreactivity was revealed in the entire ORS with a very strong suprabasal staining pattern in the distal part and LEKTI-2 displayed in addition to the strong immunoreactivity in the ORS a very intense staining of the companion layer. Moreover, psoriasin and RNase7 mRNA could be detected in isolated ORS keratinocytes from the distal and the middle part of the hair follicle, but not from the proximal part. Furthermore isolated ORS keratinocytes from all different parts also showed a strong immunoreactivity for RNase 7, but not for psoriasin.
Stimulation of normal human scalp skin biopsies in serum-free organ culture with LPS from different gram-negative microbes significantly up-regulated RNase 7 and psoriasin immunoreactivity in the distal part of the ORS, whereas hornerin remained at about the same level. LTA and protein A both significantly increased psoriasin IR, while RNase 7 was only up-regulated by LTA, and hornerin was even down-regulated by LTA. IFNγ did not influence neither psoriasin nor RNase 7 immunoreactivity, but significantly down-regulated hornerin immunoreactivity.
To summarize, this study confirms that the epithelium of human hair follicles exhibits an elaborate and partially inducible antimicrobial defence system which includes the peptides RNase 7, psoriasin, hornerin and LEKTI-2. RNase 7 and psoriasin are inducible by bacterial cell wall components, whereas hornerin is constitutively expressed and does not appear to respond prominently to exposure to exemplary bacterial pathogenicity factors.