Microglial activation during neuroinflammation is crucial for coordinating the immune response against neuronal tissue, and the initial response of microglia determines the severity of neuro-inflammatory diseases. The CD83 molecule has been recently shown to modulate the activation status of dendritic cells and macrophages. Although the expression of CD83 is associated with early microglia activation in various disease settings, its functional relevance for microglial biology has been elusive. Here, we describe a thorough assessment of CD83 regulation in microglia and show that CD83 expression in murine microglia is not only associated with cellular activation but also with pro-resolving functions. Using single-cell RNA-sequencing, we reveal that conditional deletion of CD83 results in an over-activated state during neuroinflammation in the experimental autoimmune encephalomyelitis model. Subsequently, CD83-deficient microglia recruit more pathogenic immune cells to the central nervous system, deteriorating resolving mechanisms and exacerbating the disease. Thus, CD83 in murine microglia orchestrates cellular activation and, consequently, also the resolution of neuroinflammation.

Fibroblasts are key orchestrators of inflammation. Little is known whether these cells change phenotype during resolution of inflammation. We adopted a method to visualise fibroblast activation during inflammation in humans in vivo, which is based on a fibroblast activation protein (FAP) tracer detected by positron emission tomography (PET). While tracer accumulation was high in active arthritis, it decreased significantly after TNF- and IL-17A inhibition. Biopsy-based scRNA-seq analyses in experimental arthritis demonstrated that FAP signal reduction reflected a phenotypic switch from pro-inflammatory MMP3+/IL6+ fibroblasts (high FAP internalisation) to pro-resolving CD200+DKK3+ fibroblasts (low FAP internalisation). Spatial transcriptomics of human joints revealed that pro-resolving niches of CD200+DKK3+ fibroblasts clustered with innate lymphoid cells (ILC)2, whereas MMP3+/IL6+ fibroblasts were co-localised with inflammatory immune cells. CD200+DKK3+ fibroblasts stabilised the ILC2 phenotype and induced resolution of arthritis via CD200/CD200R1 pathway. Taken together, these data suggest a dynamic molecular regulation of the mesenchymal compartment during resolution of inflammation.