In situ evidence that mast cells release mutant NLRP1 in keratoacanthomas from multiple self-healing palmoplantar carcinoma

NLRP1 is an inflammasome sensor protein expressed in barrier tissues of humans. Its activation in response to microbes or cellular stress triggers a cascade of molecular events, leading up to IL1{beta}-driven inflammation and pyroptosis. Rare germline mutations of NLRP1 cause its persistent activation, resulting in autoinflammatory syndromes. Multiple self-healing palmoplantar carcinoma (MSPC) is one such syndrome, characterized by the appearance of recurrent keratoacanthomas (KAs) on the palms and soles. Here, we aimed to compare the subcellular localization of mutant NLRP1 in MSPC-associated lesions, to wild-type NLRP1 in non-MSPC KAs and in skin from healthy donors. Using mass spectrometry, immunohistochemistry and immunoelectron tomography, we found that NLRP1 localized to mast cell (MC) granules in all samples, a novel finding which implicates MCs in NLRP1-associated responses in human skin. Moreover, we found that MCs expressing the A66V pathogenic variant of NLRP1 overpopulated MSPC-KAs, infiltrated the epidermis and degranulated, a behavior not seen in other lesions from this study. The released granules had the highest NLRP1 protein content and also contained NLRP3 and IL1{beta}, indicating coexistence of inflammasome pathways within MCs. Taken together, our findings establish cutaneous MCs as a NLRP1 reservoir in health and disease, opening a new area of research in NLRP1-related syndromes.