Autoreactivity to malondialdehyde-modifications in rheumatoid arthritis is linked to disease activity and synovial pathogenesis

Abstract

Oxidation-associated malondialdehyde (MDA) modification of proteins can generate immunogenic neo-epitopes that are recognized by autoantibodies. In health, IgM antibodies to MDA-adducts are part of the natural antibody pool, while elevated levels of IgG anti-MDA antibodies are associated with inflammatory and autoimmune conditions. Yet, in human autoimmune disease IgG anti-MDA responses have not been well characterized and their potential contribution to disease pathogenesis is not known. Here, we investigate MDA-modifications and anti-MDA-modified protein autoreactivity in rheumatoid arthritis (RA). While RA is primarily associated with autoreactivity to citrullinated antigens, we also observed increases in serum IgG anti-MDA in RA patients compared to controls. IgG anti-MDA levels significantly correlated with disease activity by DAS28-ESR and serum TNF-alpha, IL-6, and CRP. Mass spectrometry analysis of RA synovial tissue identified MDA-modified proteins and revealed shared peptides between MDA-modified and citrullinated actin and vimentin. Furthermore, anti-MDA autoreactivity among synovial B cells was discovered when investigating recombinant monoclonal antibodies (mAbs) cloned from single B cells, and 3.5% of memory B cells and 2.3% of plasma cells were found to be anti-MDA positive. Several clones were highly specific for MDA-modification with no cross-reactivity to other antigen modifications such as citrullination, carbamylation or 4-HNE-carbonylation. The mAbs recognized MDA-adducts in a variety of proteins including albumin, histone 2B, fibrinogen and vimentin. Interestingly, the most reactive clone, originated from an IgG1-bearing memory B cell, was encoded by near germline variable genes, and showed similarity to previously reported natural IgM. Other anti-MDA clones display somatic hypermutations and lower reactivity. Importantly, these anti-MDA antibodies had significant in vitro functional properties and induced enhanced osteoclastogenesis, while the natural antibody related high-reactivity clone did not. We postulate that these may represent distinctly different facets of anti-MDA autoreactive responses.

Depressed serum IgM levels in SLE are restricted to defined subgroups

“…We have previously reported that anti-PC and anti-MDA recognize distinct epitopes and these antibodies display nonoverlapping binding specificities [25,26,41]. Especially IgG anti-PC and anti-MDA have very different expression pattern in autoimmune patients and in rheumatoid arthritis IgG anti-MDA levels are associated with higher disease activity and potentially pathogenic properties [42]. IgM anti-MDA is a natural antibody specificity that is prevalent at birth but may also arise in part in response to inflammatory stimuli that are increased during autoimmune pathogenesis [25,26,41].…”

Force generated by myosin cross-bridges is reduced in myofibrils exposed to ROS/RNS

“…ROS can cause oxidative post-translational modifications (oxPTM) of amino acid residues of proteins. Oxidative stress has been shown to be part of the pathophysiology of RA (2,3,(38)(39)(40).…”

“…On the other hand, carbonylation, nitration and addition of a malondialdehyde (MDA) adduct are more stable changes to amino acids (41)(42)(43)(44)(45). Hallmarks of both nitration and MDA adducts have been found in RA patients (38,39) and rodents with arthritis (2,3). Due to the stability of nitration and MDA adducts, progressive protein damage may occur (46).…”

“…LOO • can alternatively cyclize to and endoperoxide which, by cleavage, will form MDA. Intriguingly, increased levels of auto-antibodies against MDA-modified proteins have been found in serum and the synovium from RA patients (38,39,53).…”

Reply

“…We continue to investigate targets and functions of this and other antibodies, being aware that the weak citrulline reactivity reported in the study by Amara et al is presumably not responsible for the functional effects of these antibodies. Notably, RA‐relevant pathogenic functionalities have been demonstrated also with RA patient–derived mAb that target proteins with modifications other than citrullination, including malondialdehyde modifications .…”