The role of interleukin-17 (IL-17) in the pathogenesis of discoid lupus erythematosus and lichen planopilaris: is immunohistochemistry for IL-17 a promising way to differentiate these entities?

BACKGROUND Interleukin-17 (IL-17) is a pro-inflammatory mediator which its excess secretion by immune cells is found in many chronic autoimmune diseases. This survey is aimed to compare the amount and pattern of distribution of IL-17 positive cells in lichen planopilaris (LPP) and discoid lupus erythematosus (DLE) which are of the most frequent causes of primary cicatricial alopecia (CA). METHODS Biopsies of lesional scalp skin from adult LPP (n = 30), DLE patients (n = 19), and control scalp skin (n = 18) were analyzed by immunohistochemistry (IHC) method using rabbit polyclonal antibody against IL-17. Cells stained positive for IL-17 and their pattern of distribution were assessed and compared between these groups. RESULTS The mean of IL-17 positive cell counts per high power field (HPF) in both LPP and DLE groups was significantly higher in comparison with the control group (both p < .0001). The LPP group had higher values of IL-17 positive cell counts per HPF (47.56 ± 13.37) compared to the DLE group (22.21 ± 11.06) (p < .0001). More than 30 IL-17 positive cell counts in IHC have a sensitivity of 90% and a specificity of 78% for differentiating LPP from DLE. CONCLUSIONS Our findings suggest that IL-17 is effective in the pathogenesis of DLE and LPP and its role is more prominent in LPP. IL-17 positive cells in IHC can distinguish DLE from LPP with satisfactory sensitivity and specificity.