Background/Purpose: Proteins subjected to post-translational modifications, such as citrullination, carbamylation, acetylation, malondialdehyde (MDA) or malondialdehyde/acetaldehyde (MDA/MAA) modification are targeted by autoantibodies in seropositive rheumatoid arthritis (RA). The pathogenicity of anti-modified protein antibodies has been suggested by both epidemiological and experimental studies. However, molecular mechanisms triggered by these antibodies have remained to be identified. Here we characterized in detail the cellular pathways and the associated in vivo pathogenicity of anti-MDA modified protein antibodies that were obtained from synovial B cells of RA patients.

Methods: We monitored gene expression, metabolism and cytokine release in anti-MDA/MAA antibody-treated macrophages and osteoclasts. The effect of anti-MDA/MAA antibodies was studied on bone erosion in mice, using X-ray microscopy, whereas their role in inflammation was evaluated using the collagen antibody induced arthritis model.

Results: Anti-MDA/MAA antibodies triggered a pro-inflammatory profile in macrophages, with robust upregulation of genes encoding cytokines, adhesion receptors and signal transduction components, increased production of pro-inflammatory cytokines and decreased levels of IL-10. Moreover, anti-MDA/MAA antibodies synergized with collagen-targeting antibodies in arthritis induction. Anti-MDA/MAA antibodies induced a robust osteoclast differentiation in vitro and bone loss in vivo via boosting glycolysis in the cells through FcγRI activation, and via the subsequent increase in oxidative phosphorylation. Interestingly, osteoclast development was underlied by robust phosphoglyceride and triacylglyceride biosynthesis, which was also enhanced by anti-MDA/MAA, due to an increased citrate production in the tricarboxylic acid cycle and the upregulation of glycerol-3-phosphate dehydrogenase 1 (GPD1) and glycerol-3-phosphate acyltransferase 2 (GPAT2) genes. Notably, only two of three tested anti-MDA/MAA clones shared the osteoclast and macrophage-acting features, both carrying somatic hypermutations in the variable regions, whereas a third, germline-encoded anti-MDA/MAA clone lacked activity in these models.

Conclusion: In healthy individuals natural anti-MDA/MAA antibodies, which are typically IgM and germline-encoded, might contribute to immune homeostasis. In seropositive RA class-switched and mutated anti-MDA/MAA antibodies can appear, typically at the height of inflammation. Here we showed that certain anti-MDA/MAA IgG clones accelerated bone erosion by stimulating osteoclast differentiation through reprogramming metabolism in the cells. Moreover, the pro-inflammatory cytokine release in macrophages and the increased joint inflammation in anti-MDA/MAA-treated mice suggested a key role for these antibodies in exacerbating inflammation. In summary, anti-MDA/MAA IgG may regulate various immune cells in RA acting as a key pathogenic mediator in the inflamed joints.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/autoantibodies-targeting-malondialdehyde-modifications-are-potential-mediators-of-inflammation-and-bone-loss-in-ra-acting-via-macrophage-and-osteoclast-regulatory-pathways/