Pathogenesis of Pseudocheckerboard Pattern in Mechanic's Hands.

Among the relatively common manifestations of dermatomyositis (DM), "mechanic's hands" refer to symmetric hyperkeratotic erythematous lesions on the ulnar aspect of the thumbs and the radial aspect of the index and middle fingers. The "pseudocheckerboard pattern" is a distinct histopathological finding associated with mechanic's hands. This pattern consists of vertical keratotic columns in the stratum corneum, some of which display a striped appearance characterized by alternating bands of orthokeratosis and parakeratosis in both vertical and horizontal orientations. Although this phenomenon is well documented, its underlying mechanism remains poorly understood. In this study, we collected tissue samples from 14 patients with DM accompanied by mechanic's hands and reviewed their clinical presentations and histopathological features. We identified several common histopathological findings of DM, including the characteristic pseudocheckerboard pattern associated with these lesions. Parakeratotic foci of the pseudocheckerboard pattern were observed within the stratum corneum, corresponding to the suprapapillary epidermis, where both CD4- and CD8-positive lymphocytes infiltrated the papillary dermis and suprapapillary epidermis. A portion of these lymphocytes expressed interleukin (IL)-17, and the number of IL-17-positive cells was comparable to that in psoriatic skin lesions. The inflammatory response was relatively confined to the papillary dermis and suprapapillary epidermis, suggesting that Th17-mediated epidermal hyperplasia may be involved in the pathogenesis of mechanic's hands.