Objectives Autoantibody production and aberrant immune complex formation belong to the pathological hallmarks of systemic lupus erythematosus (SLE). This study aimed to determine the occurrence of autoantibodies against human serum albumin (HSA) and their potential association with antibodies against bovine serum albumin (anti-BSA) in patients with SLE.

Methods Sera of 180 well-defined SLE patients included into the Swiss SLE Cohort Study and 188 age- and sex-matched healthy controls were evaluated. Levels of anti-HSA IgG and anti-BSA IgG were quantified by ELISA. Selected samples were further characterised with regard to the occurrence of monomeric versus albumin-complexed IgG using serum fractions obtained by fast liquid chromatography (FPLC).

Results SLE patients had increased levels of antibodies against HSA (p=0.002) but similar levels of anti-BSA IgG as compared to matched healthy controls. Anti-HSA IgG levels correlated with the SLE Disease Activity Index (SLEDAI), which was more pronounced in patients with an additional physician’s global assessment (PGA) of greater or equal 1 (r=0.309, p=0.0066). The analysis of selected samples indicates that anti-HSA IgG are partially complexed with serum albumin but also occur as monomeric autoantibodies in highly positive SLE patients. However, SLE patients were not found to have strikingly increased levels of albumin-IgG complexes compared to matched controls. Interestingly, a positive correlation between anti-HSA IgG and anti-BSA IgG was found that was stronger in SLE patients than in healthy controls (r=0.3172, p<0.001 vs r=0.2122, p<0.0035). This might be due to a partial cross-reaction as binding of anti-BSA IgG could partially be inhibited by the presence of HSA in samples with double positivity for anti-HSA and anti-BSA (median inhibition 47.9%, range 0.9%–100%).

Conclusions SLE patients were found to have an increased prevalence of anti-HSA antibodies that are associated with SLE disease activity. As anti-HSA were also found to be associated with the occurrence of anti-BSA antibodies, anti-HSA might be triggered by food-derived bovine albumin.