Macrophage-associated pro-inflammatory state in human islets from obese individuals

Abstract

Obesity is associated with inflammatory macrophages in insulin responsive tissues and the resulting inflammatory response is a major contributor to insulin resistance. In insulin-producing pancreatic islets, the intra-islet accumulation of macrophages is observed in patients of type 2 diabetes (T2D), but such has not been investigated in obese individuals. Here, we show that pro-inflammatory cytokines (IL-1β, IL-6, and TNF), anti-inflammatory cytokines (IL-10 and TGF-β) and macrophage polarization markers (CD11c, CD163, and NOS2) were expressed in isolated human islets from non-diabetic donors. Clodronate-mediated depletion of resident macrophages revealed expression of IL1B and IL10 mostly from macrophages, while IL6, TNF, and TGFB1 came largely from a non-macrophage origin in human islets. NOS2 expression came exclusively from non-macrophage cells in non-obese individuals, while it originated also from macrophages in obese donors. Macrophage marker expression of CD68, CD163, and ITGAX was unchanged in islets of non-obese control and obese cohorts. In contrast, IL1B and NOS2 were significantly increased in islets from obese, compared to non-obese individuals, implying a more inflammatory macrophage phenotype in islets in obesity. Our study shows elevated macrophage-associated inflammation in human islets in obesity, which could be an initiating factor to the pro-inflammatory intra-islet milieu and contribute to the higher susceptibility to T2D in obese individuals.

Fig. 1. Macrophage-dependent and -independent inflammation marker expression within isolated human islets.

a mRNA expression levels relative to housekeeping gene (cyclophilin A, PPIA) from human isolated islets. b–d Comparative gene expression analysis of clodronate liposome (1 mg/ml for 48 h) and vehicle liposome (Ctrl)-treated human islets. Separate analysis of NOS2 expression from control (c BMI < 30, n = 4) and obese donors (d, BMI > 30, n = 3). a, b n = 5–7. Data are means ± SEM *p < 0.05 Ctrl vs. clodronate.

Fig. 2. IL1B and NOS2 expression in human macrophages under gluco-/lipotoxicity.

a Comparative gene expression analysis of isolated islets from control (BMI < 30, n = 16) and obese (BMI > 30, n = 12) cohorts. Data are means with whiskers. b Correlation of IL1B and NOS2 expression among all donors (n = 28). c, d IL1B and NOS2 expression in islet-conditioned human macrophages (M_islet) and control non-conditioned macrophages (M₀) treated with 22.2 mM glucose (HG), 0.5 mM palmitate (Pal), combined HG and Pal, or combined 100 ng/ml LPS and 1000 U/ml IFNγ for 24 h. Data are means ± SEM, n = 4. *p < 0.05 control vs. treatment or control vs. obese cohort.