Obesity in Humans Is Characterized by Gut Inflammation as Shown by Pro-Inflammatory Intestinal Macrophage Accumulation

Abstract

Chronic low-grade inflammation is a hallmark of obesity and associated with cardiovascular complications. However, it remains unclear where this inflammation starts. As the gut is constantly exposed to food, gut microbiota, and metabolites, we hypothesized that mucosal immunity triggers an innate inflammatory response in obesity. We characterized five distinct macrophage subpopulations (P1-P5) along the gastrointestinal tract and blood monocyte subpopulations (classical, non-classical, intermediate), which replenish intestinal macrophages, in non-obese (BMI<27kg/m²) and obese individuals (BMI>32kg/m²). To elucidate factors that potentially trigger gut inflammation, we correlated these subpopulations with cardiovascular risk factors and lifestyle behaviors. In obese individuals, we found higher pro-inflammatory macrophages in the stomach, duodenum, and colon. Intermediate blood monocytes were also increased in obesity, suggesting enhanced recruitment to the gut. We identified unhealthy lifestyle habits as potential triggers of gut and systemic inflammation (i.e., low vegetable intake, high processed meat consumption, sedentary lifestyle). Cardiovascular risk factors other than body weight did not affect the innate immune response. Thus, obesity in humans is characterized by gut inflammation as shown by accumulation of pro-inflammatory intestinal macrophages, potentially via recruited blood monocytes. Understanding gut innate immunity in human obesity might open up new targets for immune-modulatory treatments in metabolic disease.

Keywords: Obesity; chronic inflammation; diabetes; intestinal inflammation; macrophages; metabolic disease; monocytes; mucosal immunity.

Figure 1

Enrollment flow by intervention group. A total of 54 study participants were recruited and the samples of 48 subjects were analyzed. Colon biopsy specimens were obtained from 13 non-obese and 10 obese individuals. Stomach and duodenum biopsies were derived from 12 non-obese and obese subjects, each. Jejunum samples were obtained from a subgroup of 6 obese individuals with a BMI >35 kg/m² at the time of bariatric bypass surgery.

Figure 2

Intestinal macrophages increase along the gastrointestinal tract, particularly the anti-inflammatory/resident subpopulation P5. (A) Flow cytometry gating strategy of human intestinal macrophages (CD14^high (pro-inflammatory P1, P2 and intermediate P3) and CD14^low (anti-inflammatory/resident P4, P5 subpopulations). (B) Frequency of total intestinal macrophages in non-obese (BMI <27 kg/m²) and obese subjects (BMI >32 kg/m²) in the stomach, duodenum, and colon (n=12-13). (C, D) Distribution of intestinal macrophage subpopulations in non-obese (C) or obese subjects (D) in the stomach, duodenum, jejunum, and colon (n = 10-12). One data point represents one subject. Statistical data are expressed as mean ± SEM. * p <0.05. ** p <0.01, *** p <0.001, tested by ordinary one-way ANOVA.

Figure 3

Pro-inflammatory macrophages increase throughout the gastrointestinal tract in obese individuals and are accompanied by higher intermediate blood monocytes. (A) Representative flow cytometry plots identifying intestinal macrophages subpopulations in non-obese (black) and obese (red) individuals. CD14^high represent pro-inflammatory P1, P2 and intermediate P3 subpopulations and CD14^low anti-inflammatory/resident P4, P5 subpopulations. (B) Fold change of intestinal macrophage subpopulations comparing obese (n = 10-12) to non-obese (n = 12-13) subjects along the gastrointestinal tract. (C) Frequency of classical, intermediate, and non-classical blood monocytes in non-obese (black; n = 22) and obese (red; n = 25) subjects. (D) Spearman correlation between intestinal macrophage subpopulation P2 in stomach, duodenum or colon and intermediate blood monocytes. (E) Spearman correlation of intestinal macrophage subpopulation P2 in stomach, duodenum, and colon transversum or intermediate blood monocytes with BMI. One data point represents one subject. Statistical data are expressed as mean ± SEM. * p <0.05. ** p <0.01, unpaired Mann-Whitney U test with two tailed distribution (B, C). For Spearman correlation analysis, values above Mean + 2SD were excluded (D, E).

Figure 4

Pro-inflammatory macrophage subpopulation P2 and intermediate blood monocytes correlate with unhealthy lifestyle habits. (A) Spearman correlation between servings of vegetables, fruits, processed meat, or hours of physical activity and body weight. (B) Spearman correlation between servings of vegetables, processed meat, or hours of physical activity and the portion of intestinal macrophage subpopulation P2. (C) Spearman correlation between servings of soft drinks or dairy products, and CD14^high or intermediate blood monocytes. One data point represents one subject. For statistical analysis, Spearman correlation analysis was performed, and p-values < 0.05 were considered significant.

Figure 5

Cardiovascular risk factors other than obesity do not affect intestinal macrophage subpopulation P2 and intermediate blood monocytes. (A–D) Frequency of P2 intestinal macrophages in the stomach, duodenum, or colon transversum, and intermediate blood monocytes in patients with or without cardiovascular risk factors (cvRF) (blue) (A: Non-obese and obese patients, B: only non-obese patients), hypertension (green) (C), and PPI intake (orange) (D). The right panels are additionally stratified by the number of cvRFs (0, 1, >2) (A, B) or by the non-obese or obese body habitus (C, D). One data point represents one subject. Statistical data are expressed as mean ± SEM. * p <0.05. ** p <0.01, *** p <0.001, unpaired Mann-Whitney U test with two tailed distribution. For Spearman correlation analysis, p-values < 0.05 were considered significant.