A metabolic alarmin from keratinocytes potentiates systemic humoral immunity

How a local infection triggers systemic humoral immunity remains unclear. Here we identify farnesyl pyrophosphate (FPP), a mevalonate pathway metabolic intermediate¹, as an endogenous alarmin that enhances IgG antibody responses through keratinocyte-derived IL-6 and CCL20. This signalling axis potentiates the differentiation of T follicular helper cells and migratory dendritic cells^2,3. FPP accumulates within keratinocytes after infection or ultraviolet irradiation through the activation of the mevalonate pathway mediated by the unfolded protein response-SREBF pathway, amplifying germinal centre (GC) responses in draining lymph nodes. Mechanistically, accumulated FPP in the cytosol engages transient receptor potential vanilloid 3 (TRPV3) by binding to its intracellular domains, inducing Ca²⁺ influx that subsequently activates the calmodulin-calcineurin-NFAT and PYK2-RAS-ERK pathways to enhance IL-6 and CCL20 production. This FPP-TRPV3-IL-6/CCL20-GC axis potentiates pathogen-specific antibody production, conferring protection in wild-type but not TRPV3-deficient mice. Single-cell RNA-sequencing analyses of systemic lupus erythematosus (SLE) skin lesions and pathogen-infected mouse skin demonstrate hyperactivation of this signalling axis, particularly in the TRPV3^high keratinocyte subset. In mouse models of SLE, the activation of this axis correlates with exacerbated disease pathology. Thus, FPP potentiates systemic humoral immunity through the TRPV3-IL-6/CCL20-GC signalling axis, providing insights for the development of vaccine adjuvants and potential therapeutics for SLE.