Bariatric surgery and subsequent weight loss lead to a reversal of peripheral blood monocytosis in obesity

Abstract Objective To examine changes in monocyte subpopulations and surface markers in people with obesity before and after bariatric surgery, and their relation to weight loss, inflammation markers, and comorbidities. Methods Peripheral blood mononuclear cells (PBMCs) were isolated from patients with obesity designated for bariatric surgery at three different time points: before surgery (=baseline), six months and twelve months after the intervention. PBMCs were analyzed using flow cytometry to distinguish the different monocytic subpopulations. At each visit, anthropometric measures and routine laboratory parameters (e.g., C-reactive protein) have been determined. Results 111 individuals with obesity (59.5% female, mean age 45.2±11.3 years) with a median body mass index (BMI) of 48.4 kg/m 2 were included into this study. Median weight loss was 44.5 kg. The absolute monocyte count decreased significantly after surgery within twelve months ( p = 0.0035). Classical monocytes, non-classical monocytes, intermediate monocytes, and monocytic myeloid-derived suppressor cells (M-MDSC) decreased significantly after the surgical intervention within six to twelve months. CD14 bright /CD56 + monocytes did not change significantly during twelve months of observation. Surface expression of CD14 increased in both classical and intermediate monocytes ( p = 0.0272 and 0.0087, respectively) within 6 months whereas CD16 declined across all monocyte subpopulations at every time point. The total monocyte counts as well as numbers of non-classical monocytes were significantly higher in patients with obesity and type 2 diabetes mellitus. COVID-19 containment measures resulted in a longitudinal reduction in the number of patient evaluations. Conclusions Following bariatric surgery and the resulting weight loss, the obesity-associated perturbation of the monocyte compartment was largely reversed. Normalization of both the total monocyte pool and of monocyte subpopulations, particularly those with pro-inflammatory properties such as intermediate monocytes, could contribute to a risk reduction of known co-morbidities of obesity such as chronic inflammation, impaired glucose regulation, and an increased risk of cancer.