Imaging of bone and joints in vivo: pathological osteoclastogenesis in arthritis

Macrophages and nociceptor neurons form a sentinel unit around fenestrated capillaries to defend the synovium from circulating immune challenge

A wide variety of systemic pathologies, including infectious and autoimmune diseases, are accompanied by joint pain or inflammation, often mediated by circulating immune complexes (ICs). How such stimuli access joints and trigger inflammation is unclear. Whole-mount synovial imaging revealed PV1+ fenestrated capillaries at the periphery of the synovium in the lining-sublining interface. Circulating ICs extravasated from these PV1+ capillaries, and nociceptor neurons and three distinct macrophage subsets formed a sentinel unit around them. Macrophages showed subset-specific responses to systemic IC challenge; LYVE1+CX3CR1+ macrophages orchestrated neutrophil recruitment and activated calcitonin gene-related peptide+ (CGRP+) nociceptor neurons via interleukin-1β. In contrast, major histocompatibility complex class II+CD11c+ (MHCII+CD11c+) and MHCII+CD11c- interstitial macrophages formed tight clusters around PV1+ capillaries in response to systemic immune stimuli, a feature enhanced by nociceptor-derived CGRP. Altogether, we identify the anatomical location of synovial PV1+ capillaries and subset-specific macrophage-nociceptor cross-talk that forms a blood-joint barrier protecting the synovium from circulating immune challenges.

New insights into inflammatory osteoclast precursors as therapeutic targets for rheumatoid arthritis and periodontitis

Rheumatoid arthritis (RA) and periodontitis are chronic inflammatory diseases leading to increased bone resorption. Preventing this inflammatory bone resorption is a major health challenge. Both diseases share immunopathogenic similarities and a common inflammatory environment. The autoimmune response or periodontal infection stimulates certain immune actors, leading in both cases to chronic inflammation that perpetuates bone resorption. Moreover, RA and periodontitis have a strong epidemiological association that could be explained by periodontal microbial dysbiosis. This dysbiosis is believed to be involved in the initiation of RA via three mechanisms. (i) The dissemination of periodontal pathogens triggers systemic inflammation. (ii) Periodontal pathogens can induce the generation of citrullinated neoepitopes, leading to the generation of anti-citrullinated peptide autoantibodies. (iii) Intracellular danger-associated molecular patterns accelerate local and systemic inflammation. Therefore, periodontal dysbiosis could promote or sustain bone resorption in distant inflamed joints. Interestingly, in inflammatory conditions, the existence of osteoclasts distinct from "classical osteoclasts" has recently been reported. They have proinflammatory origins and functions. Several populations of osteoclast precursors have been described in RA, such as classical monocytes, a dendritic cell subtype, and arthritis-associated osteoclastogenic macrophages. The aim of this review is to synthesize knowledge on osteoclasts and their precursors in inflammatory conditions, especially in RA and periodontitis. Special attention will be given to recent data related to RA that could be of potential value in periodontitis due to the immunopathogenic similarities between the two diseases. Improving our understanding of these pathogenic mechanisms should lead to the identification of new therapeutic targets involved in the pathological inflammatory bone resorption associated with these diseases.

Pathological Osteoclasts and Precursor Macrophages in Inflammatory Arthritis

Macrophages comprise a variety of subsets with diverse biological functions, including inflammation, tissue repair, regeneration, and fibrosis. In the bone marrow, macrophages differentiate into multinucleated osteoclasts, which have a unique bone-destroying capacity and play key roles in physiological bone remodelling. In contrast, osteoclasts are also involved in inflammatory bone erosion in arthritis and it has been unclear whether the osteoclasts in different tissue settings arise from similar monocytoid precursors and share similar phenotypes. Rapid progresses in the sequencing technologies have provided many important insights regarding the heterogeneity of different types of osteoclasts. The application of single-cell RNA sequencing (scRNA-seq) to the osteoclast precursor-containing macrophages enabled to identify the specific subpopulation differentiating into pathological mature osteoclasts in joints. Furthermore, an intravital imaging technology using two-photon microscopy has succeeded in visualizing the real-time dynamics of immune cells in the synovial microenvironment. These technologies together contributed to characterize the unique macrophages in the inflamed synovium, termed “arthritis-associated osteoclastogenic macrophages (AtoMs)”, causing the pathological bone destruction in inflammatory arthritis. Here, we review and discuss how novel technologies help to better understand the role of macrophages in inflammatory arthritis, especially focusing of osteoclastogenesis at the pannus-bone interface.