Skin-derived myeloid precursors and joint-resident fibroblasts spread psoriatic disease from skin to joints

Psoriatic disease initially affects the skin and later extends to the joints. Here, we show a two-step process that orchestrates the spread of inflammation from the skin to the joints. Induction of psoriatic skin disease in photoconvertible mice, followed by sequencing and computational characterization of skin-derived cells in the joints, was used to identify a population of CD2+MHC-II+CCR2+ myeloid precursors that builds a skin-derived myeloid cell compartment in the joints. Single-cell cross-species reference mapping and mitochondrial variant tracing showed an orthologous human cell population. Interactome analysis of the joints showed that in a second step, resident regulatory CD200+ fibroblasts regulate the priming of CD2+MHC-II+CCR2+ myeloid precursors, which subsequently control IL-17 expression in T cells. Hence, the spread of inflammation requires a distinct migratory myeloid precursor population and a permissive local tissue environment, similar to tumor metastasis.