Localization of haptoglobin in normal human skin and some skin diseases

Haptoglobin (Hp) is an acute-phase reactant, known to be produced mainly in the liver. Haptoglobin can also be detected in the cytoplasm of normal epidermal Langerhans cells (LCs), and can prevent their functional maturation. The synthesis of Hp in skin cells has not been well studied.

We examined Hp expression at mRNA and protein levels by in situ hybridization and immunohistochemistry, respectively, in normal human skin and in the skin of patients with psoriasis, lichen planus, erythroderma, seborrheic keratosis, verruca vulgaris, basal cell carcinoma, systemic lupus erythematosus, pemphigus and bullous pemphigoid.

(1) Haptoglobin mRNA was expressed in the epidermal keratinocytes (KCs), the epithelial cells of hair follicles, sebaceous glands and eccrine glands in normal skin and all dermatoses investigated. (2) Whereas compared with normal skin, the Hp mRNA in KCs of patients with psoriasis, lichen planus, erythroderma, seborrhoea keratosis and verruca vulgaris was significantly intensified, it was weaker in patients with systemic lupus erythematosus, pemphigus and bullous pemphigoid. (3) Haptoglobin protein only stained positively in some KCs of patients with psoriasis, lichen planus and erythroderma. (4) Although some but not all epidermal LCs were positively stained with anti-Hp antibody in normal skin and in skin samples from all patients, the ratios of Hp-positive LCs/total LCs were significantly higher in those diseases with intensified Hp mRNA in KCs.

Skin is another extrahepatic organ where Hp can be synthesized by KCs. The expression of Hp mRNA in KCs and the Hp protein in both LCs and KCs appears to be correlated with the amount of inflammation, which might indicate that skin itself is involved in down-regulating the local inflammatory reaction by KC-synthesized Hp.