Palmoplantar pustulosis: Current understanding of disease definition and pathomechanism

•The term “PPP” sometimes is NOT used for the exact meaning of “palmoplantar pustulosis”.•There are two types of PPP (Type A and Type B), and the difference between them has been debated.•The lesion begins as a small vesicle in (or near) the acrosyringium.•The over-expression of sweat antimicrobial peptide is responsible for PPP pathomechanism.•The responsible proinflammatory cytokines are similar to those of psoriasis. Pustulosis palmaris et plantaris, or palmoplantar pustulosis (PPP), is a chronic pustular dermatitis involving the palms and soles and is characterized by vesicles, pustules, erythema, lichenification, and abnormal desquamation. It is one of the most common skin diseases in Japan but its pathomechanism is unclear and the disease remains poorly defined. Consequently, adequate treatment for PPP is lacking. As a localized type of pustular psoriasis, PPP has long been treated with the conventional therapies used for plaque-type psoriasis, especially in Western countries. However, PPP may be a distinct entity, with a much lower prevalence in Western countries than in Japan. Furthermore, while treatment has yielded insights into the underlying pathology in plaque-type psoriasis, the pathogenesis of PPP has yet to be elucidated. In 2018, Gulselkumab, a monoclonal antibody against interleukin (IL)-23, was certified for use in Japan and is the first biologic effective in PPP both in Japanese and other patients. In this review, we summarize the current understanding of PPP, including the revised definition and possible pathomechanism. The information presented herein provides a more complete picture of PPP and may facilitate the development of improved treatment options.