To what extent different autoantibodies accumulate in systemic lupus erythematosus (SLE)
immune complexes (ICs), and whether such accumulation is associated with disease activity
has been investigated. ICs were isolated from SLE sera by both polyethylene glycol (PEG)
precipitation and C1q-binding. Autoantibody specificities were determined using a lineblot
assay quantified by densitometry. To compare the relative levels of autoantibodies, levels were
normalized to the total levels of IgG measured by ELISA in sera and parallel ICs. Samples
were investigated both in a cross-sectional design as well as in a paired design with samples
obtained during both active and inactive SLE. All investigated autoantibody specificities
except anti-dsDNA were enriched in circulating ICs as compared with parallel sera. The
group of antibodies against RNA-associated antigens (anti-RNP/Sm, anti-Sm, anti-SSA/
Ro60, anti-SSA/Ro52, anti-SSB/La) all exhibited higher median enrichment than the DNAassociated
(anti-dsDNA, anti-histones, anti-nucleosomes) or cytoplasmic (anti-ribosomal P)
antigens. In particular autoantibodies against RNP/Sm and SSA/Ro52 had the highest degree
of enrichment in SLE PEG precipitates. These findings were corroborated by analysis of autoantibody
content in C1q-bound ICs. There was no difference in degree of IC accumulation
of the investigated autoantibodies during active and inactive SLE. Our findings demonstrate
a difference in enrichment between autoantibodies against RNA- and DNA-associated
autoantigens in isolated SLE IC, suggesting that the RNA-associated autoantibodies
are more prone to form circulating ICs in SLE, in contrast to antibodies against
DNA-associated autoantigens such as dsDNA. These finding have implications in understanding
mechanisms of differential autoantibody accumulation in target organs
in SLE.