Characterizing the effect of bariatric surgery on circulating S100A9

Bariatric surgery (BS) is associated with improved cardiovascular (CV) outcomes in individuals with obesity. One proposed mechanism is reduced inflammation. S100A9, a pro-inflammatory cytokine, is elevated in obesity. S100A9, particularly expression in platelets, has been associated with CV risk. The impact of BS on circulating and platelet S100A9 in obesity is unknown.

We studied serum, plasma, and platelet supernatants from subjects with obesity pre- and post-BS (n = 23) and lean volunteers (n = 8). S100A9 levels were quantified using an S100A9 immunoassay. Wilcoxon, Mann-Whitney, and t-tests were performed to assess changes in S100A9 levels pre- and post-operatively and compare levels across sample and subject types. Spearman tests were used to assess correlations between S100A9 levels in different sample types and neutrophil/platelet counts.

Serum and plasma S100A9 concentrations were elevated in individuals with obesity relative to lean individuals. Levels decreased to lean subject levels at 1-year post-BS, despite subjects with obesity remaining overweight. Circulating neutrophil counts also decreased post-BS, and post-BS differences in serum S100A9 were eliminated when calculated per-neutrophil. Platelet supernatant S100A9 levels were lower than in serum and plasma and did not change post-BS. Platelet supernatant S100A9 correlated with plasma, but not serum, levels.

We found that S100A9 concentrations differ substantially between blood components, are elevated in obesity, and normalize post-BS. Reductions in circulating S100A9 may contribute to reduced inflammation and be largely driven by resolution of obesity-associated neutrophilia. Our data suggest minimal platelet contribution to circulating (or systemic) S100A9, but a local level inflammatory impact cannot be excluded.