Type I Interferon promotes MxA-dependent IL-1b release in SLE monocytes

Systemic lupus erythematosus (SLE) is characterized by upregulation of Type Ι Interferon (IFN) and widespread inflammation. However, blocking the IFN pathway benefits a fraction of patients, pointing to additional pathogenic players. Here we describe monocytes (Mo) undergoing erythrophagocytosis and co-expressing IFN-inducible genes (ISGs) and interleukin-1b (IL-1b) in patients with active disease. This phenotype is recapitulated in vitro upon internalization of red blood cells carrying mitochondria (Mito+ RBCs), a feature of SLE. While ISG expression requires the interaction between Mito+ RBC-derived mitochondrial DNA (mtDNA) and cGAS, the production of IL-1b entails Mito+ RBC-derived mitochondrial RNA (mtRNA) triggering RIG-I-like receptor (RLR) activation. This leads to the cytosolic release of Mo-derived mtDNA and activation of the NLRP3 inflammasome. Importantly, the Type I IFN-inducible protein myxovirus resistant protein 1 (MxA) enables IL-1b release by routing this cytokine into a trans-Golgi network (TGN)-mediated unconventional secretory pathway. As Type I IFN and IL-1b are thought to counter-regulate each other, our study highlights an unprecedented synergy between these two cytokine pathways in SLE.