Baseline characteristics

Clinical and biochemical baseline characteristics of the study subjects have been previously presented [12, 13, 16] and relevant variables are shown in Table 1. Patients randomized to liraglutide treatment and lifestyle intervention were similar on most parameters, except for triglycerides (TG), waist circumference and visceral adipose tissue (VAT) being higher in the liraglutide arm. Compared to healthy controls, both patient groups had a lower age, higher BMI, lower CRP higher total- and LDL cholesterol, and lower HDL cholesterol. In the liraglutide group, 10 (50%) subjects had IFG or IGT and in the lifestyle group 13 (65%) subjects had IFG or IGT.

Table 1 Clinical and biochemical baseline characteristics of study participantsBaseline comparisons of soluble immune cell markers between participants with obesity and healthy controls

At baseline, plasma levels of the macrophage marker sCD163, but not sCD14, was higher in patients as compared to healthy subjects (Table 2). The neutrophil marker MPO was higher in the lifestyle group, compared to the liraglutide group, but when compared to controls they were not statistically different (Table 2). There were no significant differences in plasma levels of the neutrophil marker NGAL, or of the T-cell activation markers, sTIM-3 and sCD25, when comparing cases and controls at baseline (Table 2).

Table 2 Baseline concentrations of soluble immune cell activity markersBaseline sCD163 and MPO correlate with markers of metabolic dysfunction and inflammation

In the study group as a whole, baseline levels of sCD163 correlated positively with several metabolic parameters such as BMI (rho = 0.432, p = 0.006), C-peptide (rho = 0.410, p = 0.009), insulin (rho = 0.340, p = 0.034), total cholesterol (rho = 0.358, p = 0.025), leptin (rho = 0.481, p = 0.002) and HOMA-IR (rho = 0.389, p = 0.014), and negatively with Matsuda index (rho = − 0.337, p = 0.042). Baseline levels of sCD163 also correlated positively with other markers related to inflammation, i.e., C-reactive protein (CRP, rho = 0.337, p = 0.042), IL-10 (rho = 0.559, p = 0.002), and total leukocyte counts (rho = 0.493, p = 0.001) as well as with non-alcoholic fatty liver disease (NAFLD) prior to intervention (rho = 0.356, p = 0.026, Additional file 1: Table S1).

Baseline MPO showed a negative correlation with waist-to-hip ratio (WHR, rho = − 0.345, p = 0.031) and beta-index (rho = − 0.356, p = 0.024, Additional file 1: Table S2).

Liraglutide treatment improved metabolic parameters compared to lifestyle intervention

All study participants except one attained the 1.8 mg dose level through-out the study period. The amount of weight loss was prespecified by the protocol to 7% of initial body weight and did not differ between the groups. Median time to predefined weight loss was 4.8 months and did not differ between the two treatment arms [13]. Concomitant therapy was unchanged during the follow-up. At the end of the intervention period (i.e. after achievement of the weight loss target) both groups experienced a reduction in several metabolic parameters, including BMI, HbA1c, fasting plasma insulin, disposition index, as well as CRP as a reliable marker of systemic inflammation, with the decrease in VAT and beta-index being more pronounced in the liraglutide arm (Table 3) [13]. The liraglutide arm showed decreased systolic blood pressure and total cholesterol, and improved beta cell function, as assessed by beta-index, and glucose tolerance, indicated by reduced fasting glucose and 1- and 2-h post load glucose levels (Table 3). On the other hand, the lifestyle intervention group showed a significant decrease in 2-h post load insulin levels, which was not significant in the liraglutide group (Table 3). There were, however, no associations between changes in the inflammatory markers (sCD163 and MPO) and changes in any of metabolic parameters (Additional file 1: Table S4).

Table 3 Clinical and laboratory characteristics of obese patients before and after liraglutide or lifestyle-induced weight loss interventionLiraglutide, but not lifestyle changes reduce levels of sCD163 and MPO

At the end of the intervention period, we observed a significant reduction in sCD163 levels in the liraglutide arm (∆ = 87, SD = 115, p = 0.001), but not in the lifestyle arm (∆ = 24, SD = 85), with a significant between-group difference in ∆sCD163 also when adjusted for basal VAT and basal triglycerides values (p = 0.026) (Fig. 1A). In contrast, levels of sCD14 as an additional marker of monocyte/macrophage activation, were not affected by intervention in any of the two arms (Fig. 1B). Weight loss induced a significant decrease in MPO levels in the group receiving liraglutide (p = 0.048) (Fig. 1C), and not in the lifestyle intervention group, but the difference in decreases between arms was not statistically significant. For the other neutrophil markers (i.e., NGAL) and the T-cell markers (i.e., sTIM-3 and sCD25) no significant between arm difference was observed during the intervention. (Fig. 1D–F).

Fig. 1

Changes in plasma concentration of soluble immune markers during a liraglutide-(green bar) or lifestyle-induced (blue bar) weight loss intervention. A sCD163, (B) sCD14, (C) MPO, (D) NGAL, (E) sTIM-3 and (F) sCD25. LIFE: lifestyle intervention group, LIRA: Liraglutide intervention group, ns: not significant, NGAL: Neutrophil gelatinase‐associated lipocalin, MPO: Myeloperoxidase, sTIM-3: T-cell immunoglobulin mucin domain-3, ∆: Change from baseline (pre) to post-intervention

sCD163 levels were regulated during an oral glucose tolerance test, after weight loss intervention

A 75 g oral glucose load given before the intervention, did not show any change in levels of sCD163 over a period of 120 min (p = 0.835) and there were, as expected, no differences in response between the treatment groups at baseline (p = 0.860) (Fig. 2A). After intervention, however, we observed a reduction in the levels of sCD163 in the whole study population, after the glucose load over time (p = 0.001). This effect was more pronounced in patients randomized to liraglutide compared to the lifestyle arm, although the difference between arms was not statistically significant (p = 0.081, Fig. 2B). Interestingly, and in concordance with the baseline correlations between plasma glucose and sCD163, during the OGTT post intervention, the sCD163 AUC after glucose-load was significantly and directly related to the glucose AUC only in the lifestyle arm (data not shown), suggesting that liraglutide directly affects sCD163, independently of blood glucose rise. No change in any of the other variables studied was observed during OGTT neither before nor after weight loss.

Fig. 2

Percentage change from baseline concentrations of sCD163 during an oral glucose tolerance test, comparing lifestyle and liraglutide intervention groups. (A) pre, and (B) post intervention

Liraglutide, but not lifestyle changes reduce levels of LIGHT

We have previously shown that the inflammatory cytokine LIGHT/TNFSF14 is increased in patients with T2DM and can induce beta cell death and impair insulin secretion [17]. Monocyte/macrophages are important cellular sources of LIGHT and based on the regulation of sCD163 in the liraglutide arm, we therefore analyzed LIGHT levels in the study group. At baseline, LIGHT was significantly increased in the patient cohort, as compared to controls (Fig. 3A), and indeed, LIGHT showed a positive correlation with sCD163 (rho = 0.417, p = 0.001) in the study group as a whole. Further, LIGHT correlated with total leukocyte counts (rho = 0.395, p = 0.017) and with ISSI-2 at baseline (rho = − 0.321, p = 0.044) (Additional file 1: Table S3). After intervention, LIGHT was reduced in the liraglutide group (p = 0.003), but not in the lifestyle group, the difference between treatment arms where however not statistically significant (Fig. 3B, C).

Fig. 3

Plasma concentrations of LIGHT in participants before and after liraglutide or lifestyle-induced weight loss intervention. (A) Comparing concentrations pre- and post-intervention of all participants, (B) Comparing pre- and post-intervention concentrations in liraglutide and lifestyle treatment groups, (C) Comparing change in concentrations from baseline between treatment groups. LIGHT: TNF superfamily (TNFSF) member 14, LIFE: Lifestyle treatment group, LIRA: Liraglutide treatment group, ∆: Change from baseline (pre) to post-intervention