Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by destructive changes in bone and cartilage of multiple joints, in which both the activation of autoimmune T cells and the increased production of inflammatory cytokines, especially TNFα, play central pathogenetic roles [1].

Dendritic cells (DCs) are a family of innate immune cells involved in cytokine production, antiviral response, antigen presentation and tolerance, with DCs of the myeloid subset specialized in bridging innate and adaptive immunity. Depending on their activation status, DCs discriminate between appropriate and excessive immune responses possibly leading to autoimmunity. Myeloid DCs deriving from infiltrating monocytes are emerging key players in RA as proinflammatory cells as well as crucial inducers of autoimmunity via the fueling of adaptive immune responses and breaking of self-tolerance [2]. Aside of TNFα production and priming of autoreactive T cells, myeloid DCs participate to RA pathogenesis also by differentiating into aggressive osteoclasts (OCs), thus contributing to the imbalance of bone homeostasis and exacerbating bone disruption of RA joints. Despite RA synovial fluid, rich in inflammatory and autoimmune mediators, is known to play a role in this process [3], the precise mechanisms activating both immune and non-immune functions of DCs in RA remain poorly characterized [2].

In several pathological conditions, including autoimmunity, fluctuations in the expression of microRNAs (miRNAs) play a role in disease pathogenesis and exacerbation, such as induction of joint inflammation and destruction in the case of RA [4,5]. MiRNAs are short, noncoding single stranded RNAs that are present in the cell cytoplasm but are also abundantly released in extracellular fluids. By reaching neighboring cells, extracellular miRNAs work as intercellular regulators, both via “classical” post-transcriptional mechanisms [6] and via the triggering of innate immune receptors (namely, Toll-like Receptors 7 and 8, TLR7 and TLR8) which, in turn, activate the secretion of proinflammatory mediators [5,7]. At difference with epigenetic regulation, requiring the accumulation of several hundreds of copies of one or few selected miRNAs in every recipient cell, TLR triggering is exerted by any miRNA, or miRNA pool, containing a sufficient amount of guanosines and uridines (GU-rich miRNAs), without any requirement of sequence specificity [7,8]. In addition, the induction of proinflammatory mediators potently amplifies the regulatory potential of even few miRNAs. These aspects are relevant since they overcome the hypothetical limitations of the regulatory potential of extracellular miRNAs related to their low concentration in extracellular fluids. Indeed, TLR activation by extracellular miRNA is emerging as a new trigger of undesired immune activation and tissue damage in different contexts where the pool of extracellular miRNAs is altered, including autoimmune diseases [7].

Based on these premises, we investigated whether GU-rich extracellular miRNAs upregulated in RA (Ex-miRNAs) [[9], [10], [11], [12]] may activate monocyte-derived DCs via TLR7/8 in terms of TNFα secretion, T cell activation and OC differentiation, the three main contributions of DCs to RA pathogenesis and progression.