Normal keratinocytes express Toll-like receptors (TLRs) 1, 2 and 5: modulation of TLR expression in chronic plaque psoriasis

BACKGROUND: Toll-like receptors (TLRs) are part of the innate immune system involved in the response to microbial pathogens. TLR2 recognizes various ligands expressed by Gram-positive bacteria, while TLR3, TLR4 and TLR5 are specific for double-stranded RNA, Gram-negative lipopolysaccharides and bacterial flagellin, respectively. OBJECTIVES: To determine, firstly, whether epidermal keratinocytes of normal skin express TLRs and, secondly, whether modulation of TLR expression occurs in psoriasis, an inflammatory skin disease associated with certain microorganisms such as streptococci, staphylococci and yeasts. METHODS: Eight samples of normal, and 15 samples of lesional and nonlesional psoriatic skin were stained with polyclonal antibodies specific for TLR1-5 using an avidin-biotin-peroxidase technique. RESULTS: Epidermal keratinocytes in normal skin constitutively expressed TLR1, TLR2 and TLR5, while TLR3 and TLR4 were, in most cases, barely detectable. Cytoplasmic TLR1 and TLR2 were expressed throughout the epidermis, with higher staining of the latter on basal keratinocytes, while TLR5 expression was concentrated in the basal layer. In contrast, in lesional epidermis from patients with psoriasis, TLR2 was more highly expressed on the keratinocytes of the upper epidermis than on the basal layer, while TLR5 was downregulated in basal keratinocytes compared with corresponding nonlesional psoriatic epidermis. In addition, nuclear TLR1 staining was observed in the upper layers of both nonlesional and lesional psoriatic epidermis, but not in that of normal skin. CONCLUSIONS: These findings suggest that TLRs expressed by epidermal keratinocytes constitute part of the innate immune system of the skin. The relevance of altered keratinocyte TLR expression in psoriasis remains to be determined.