SAA1/FPR2 signaling between keratinocytes and neutrophils sustains chronic inflammation in Sweet syndrome.

Sweet syndrome (also known as acute febrile neutrophilic dermatosis) is a rare inflammatory skin disorder characterized by erythematous plaques with a dense dermal neutrophilic infiltrate. First-line therapy remains oral corticosteroids, which suppresses inflammation non-specifically. Although neutrophils are typically short-lived, how they persist in Sweet syndrome skin and contribute to disease pathogenesis remains unclear. Here, we identify a previously unrecognized population of antigen-presenting cell (APC)-like neutrophils expressing MHC class II genes that are uniquely present in Sweet syndrome skin but absent from healthy tissue and circulation. Keratinocytes extended neutrophil lifespan 10-fold in co-culture experiments and drove the emergence of an APC-like phenotype in approximately 30% of neutrophils, mirroring observations in patient lesions. Mechanistically, keratinocyte-derived serum amyloid A1 (SAA1) signals through the formyl peptide receptor 2 (FPR2) on neutrophils to promote their survival. These long-lived neutrophils actively orchestrate local immune responses by recruiting T cells and inducing cytokine production. Strikingly, dual blockade of SAA1-FPR2 signaling restores neutrophil turnover to baseline levels, with efficacy comparable to high-dose corticosteroids. These findings uncover a keratinocyte-neutrophil-T cell axis that sustains chronic inflammation in Sweet syndrome and highlight the SAA1/FPR2 pathway as a promising target for precision therapy.