Small plaque psoriasis re‐visited: A type of psoriasis mediated by a type‐I interferon pathway

TNFα-inhibitor-induced psoriasis is mediated by the type-I interferon pathway, of which IFNα, LL37 and IL-36γ are major players. A subset of patients treated with TNFα inhibitors develop small plaque psoriatic lesions. Small plaque psoriasis is similarly observed in patients on immune checkpoint inhibitors (ICI), and with concurrent systemic lupus erythematosus (SLE) or positive antinuclear antibody (ANA). Small plaque psoriasis is also the predominant phenotype in Asian populations. The association between small plaque psoriasis morphology in various clinical scenarios and the type-I interferon pathway has not been previously studied. A cross-sectional study was conducted of patients who developed small plaque psoriasis and had a biopsy for diagnostic clarification between 2009 and 2017. We obtained skin specimens from 14 adults with small plaque psoriasis: four patients taking anti-TNFα treatment, four patients with antecedent SLE, three patients with concurrent ANA positivity and three patients taking ICI. Controls included three patients with chronic plaque psoriasis. Histology confirmed psoriasiform epidermal hyperplasia with focal lichenoid and spongiotic features. Immunohistochemical analysis revealed higher expression of IFNα-induced MXA, LL37 and IL-36γ in all clinical scenarios of small plaque psoriasis compared to chronic plaque psoriasis. There was decreased CD8 T-cell migration to the epidermis and variability in the number of LAMP3+ cytoplasmic dendritic cells in the dermis of small plaque psoriasis. The findings suggest that small plaque psoriasis is a unique type of psoriasis with a distinct morphology and immune-phenotype, primarily mediated by the type-I interferon pathway. Associating morphology and disease pathogenesis may help identify therapeutic targets for better disease control.