The long-term effect of metabolic bariatric surgery on innate immune cell phenotype and function

Obesity is an important risk factor for atherosclerotic cardiovascular disease. This cardiovascular risk remains increased even after substantial weight loss by bariatric surgery. Innate immune cells are important regulators of atherogenesis and can adopt a long-term hyperinflammatory phenotype via epigenetic reprogramming, called "trained immunity". In this translational observational case-cohort study, we investigated the persistence of innate immune cell hyperresponsiveness following bariatric surgery, and explored the potential contribution of adipose tissue in ex vivo models.

In a cohort of 27 patients with obesity blood was drawn before and six months after bariatric surgery, and compared to 20 healthy subjects with normal body weight. We assessed monocytes using flow cytometry, functional assays, and RNA sequencing and chromatin immunoprecipitation. In an accompanying series of in vitro studies, healthy donor monocytes were exposed for 24 h to adipose tissue obtained from patients with obesity during bariatric surgery. Cytokine production capacity was assessed after one week.

In our case-cohort study, although leukocyte numbers and systemic inflammatory markers such as hs-CRP decreased after bariatric surgery to the level of normal weight control subjects, the hyperresponsive pro-inflammatory monocyte phenotype was only partially reverted six months after bariatric surgery, which aligned with the RNA expression profiles. Exposure of monocytes to adipose tissue obtained from patients with obesity induces a persistent augmented cytokine production capacity.

Although in patients with obesity, six months after bariatric surgery systemic inflammatory markers and leukocyte numbers are decreased to the levels observed in healthy lean subjects, there is still a residual functional and transcriptional hyper-inflammatory monocyte phenotype. This might be caused, at least in part, by the capacity of adipose tissue to induce long-term pro-inflammatory effects in monocytes. These finding help to understand the long-term effects of obesity and bariatric surgery on cardiovascular health.