How can immunohistochemistry improve the diagnosis of pemphigus foliaceus?

Purpose: Pemphigus foliaceus (PF) is a rare, autoimmune blistering disorder characterized by the production of autoantibodies against desmoglein 1. The mainstay for diagnosis is the demonstration of immune complex deposition by direct immunofluorescence (DIF) in fresh tissue samples. Immunohistochemistry (IHC) recognizes autoantibodies in formalin-fixed paraffin-embedded specimens, but studies regarding its use in PF are scarce. This study aims to evaluate immunoglobulin and C3 deposition using IHC in patients with confirmed PF by DIF and indirect immunofluorescence (IIF). Material and methods: Six biopsies obtained from five patients with PF and six healthy individuals were included in this study. Anti-C3c, -IgG, -IgM, and -IgA antibodies were used for DIF and automated IHC. After digitalizing the slides, staining was classified as negative (0) or positive (1 = mild/2 = intense). Results: DIF revealed intraepidermal intercellular deposition of IgG and C3c (n = 6), without deposits in dermal structures. IHC was positive in the intercellular spaces between keratinocytes with anti-IgG (n = 6) and anti-C3c antibodies (n = 6); no intercellular immune complexes deposition was observed in healthy individuals. In patients with PF, inflammatory cells were tagged by anti-IgG and anti-C3c (n = 6), anti-IgM (n = 1), and anti-IgA (n = 1); and immune complexes at vessel walls were detected with anti-C3c, anti-IgG, anti-IgA (n = 6), and anti-IgM (n = 5) antibodies. Adnexal positivity occurred with anti-C3c and anti-IgG (n = 6), anti-IgM (n = 1), and anti-IgA (n = 3). Healthy individuals also presented positivity in inflammatory cells with anti-IgG and anti-C3c (n = 4), anti-IgM (n = 1), and anti-IgA (n = 3); vessels were stained with anti-IgG and anti-C3c (n = 5), anti-IgM and anti-IgA (n = 4); adnexa were not represented in all samples obtained from healthy individuals. Conclusion: IHC may serve as a reliable method to assess PF diagnosis. Immune deposits in dermal structures suggest their participation in autoimmune/inflammatory processes in PF. IHC may contribute to evaluate disease mechanisms, prognostic factors, and target-oriented treatment in PF