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 are associated with inflammatory
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. Several
clones were highly specific for MDA-modification with no cross-reactivity to
other antigen modifications. The mAbs recognized MDA-adducts in a variety of
proteins. Interestingly, the most reactive clone, originated from an
IgG1-bearing memory B cell, was encoded by germline variable genes, and showed
similarity to previously reported natural IgM. Other anti-MDA clones display
somatic hypermutations and lower reactivity. 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