Antigen-presenting cells (APCs) such as dendritic cells (DCs) are crucial for initiation of adequate inflammatory responses, which critically depends on the cooperated engagement of different receptors. In addition to pattern recognition receptors (PRRs), Fc gamma receptors (FcγRs) have recently been identified to be important in induction of inflammation by DCs. FcγRs that recognize IgG immune complexes, which are formed upon opsonization of pathogens, induce pro-inflammatory cytokine production through cross-talk with PRRs such as Toll-like receptors (TLRs). While the physiological function of FcγR-TLR cross-talk is to provide protective immunity against invading pathogens, undesired activation of FcγR-TLR cross-talk, e.g., by autoantibodies, also plays a major role in the development of chronic inflammatory disorders such as rheumatoid arthritis (RA). Yet, the molecular mechanisms of FcγR-TLR cross-talk are still largely unknown. Here, we identified that FcγR-TLR cross-talk-induced cytokine production critically depends on activation of the transcription factor interferon regulatory factor 5 (IRF5), which results from induction of two different pathways that converge on IRF5 activation. First, TLR stimulation induced phosphorylation of TBK1/IKKε, which is required for IRF5 phosphorylation and subsequent activation. Second, FcγR stimulation induced nuclear translocation of IRF5, which is essential for gene transcription by IRF5. We identified that IRF5 activation by FcγR-TLR cross-talk amplifies pro-inflammatory cytokine production by increasing cytokine gene transcription, but also by synergistically inducing glycolytic reprogramming, which is another essential process for induction of inflammatory responses by DCs. Combined, here we identified IRF5 as a pivotal component of FcγR-TLR cross-talk in human APCs. These data may provide new potential targets to suppress chronic inflammation in autoantibody-associated diseases that are characterized by undesired or excessive FcγR-TLR cross-talk, such as RA, systemic sclerosis, and systemic lupus erythematous.

C-reactive protein (CRP) is an acute-phase protein produced in high quantities by the liver in response to infection and during chronic inflammatory disorders. Although CRP is known to facilitate the clearance of cell debris and bacteria by phagocytic cells, the role of CRP in additional immunological functions is less clear. This study shows that complexed CRP (phosphocholine [PC]:CRP) (formed by binding of CRP to PC moieties), but not soluble CRP, synergized with specific TLRs to posttranscriptionally amplify TNF, IL-1β, and IL-23 production by human inflammatory macrophages. We identified FcγRI and IIa as the main receptors responsible for initiating PC:CRP–induced inflammation. In addition, we identified the underlying mechanism, which depended on signaling through kinases Syk, PI3K, and AKT2, as well as glycolytic reprogramming. These data indicate that in humans, CRP is not only a marker but also a driver of inflammation by human macrophages. Therefore, although providing host defense against bacteria, PC:CRP–induced inflammation may also exacerbate pathology in the context of disorders such as atherosclerosis.