The effect of lipopolysaccharide-induced obesity and its chronic inflammation on influenza virus-related pathology

https://doi.org/10.1016/j.etap.2015.09.020Get rights and content

Highlights

  • Obesity is characterized by low levels of chronic inflammation.

  • Chronic inflammation is thought to reduce efficacy of vaccines for infectious diseases.

  • Chronic inflammation may aggravate the symptoms of influenza virus infection.

  • These effects seem to occur because of dysfunction of macrophages.

  • Dysfunction of macrophages may influence vaccine efficacy and virus pathogenicity.

Abstract

Obese individuals show increased susceptibility to infection, low vaccine efficacy, and worse pathophysiology. However, it is unclear how obesity affects these events. The aim of this study was to investigate the effect of obesity-triggered chronic inflammation on immune cells after influenza virus infection. Control and lipopolysaccharide mice, in which an osmotic pump continually released Tween saline or lipopolysaccharide, were prepared and 3 weeks later were infected with pandemic H1N1 2009 influenza A virus. In lipopolysaccharide mice, we found a reduction in macrophage activation markers in the steady state, and reduced production of pro-inflammatory cytokines including tumor necrosis factor-α, interleukin-1β, and interleukin-6, in restimulated peritoneal macrophages. Interestingly, lipopolysaccharide-triggered chronic inflammation exacerbated the severity of pathological symptoms in the lungs after challenge with influenza virus. Taken together, the increased severity of virus-induced symptoms in obese individuals with chronic inflammation may be, at least partially, caused by macrophage dysfunction.

Introduction

Obesity has become a major problem worldwide and is considered a risk factor for clinically important infectious diseases (Huttunen and Syrjanen, 2013). Obesity is particularly associated with respiratory disorders such as chronic obstructive pulmonary disease, asthma, obstructive sleep apnea, and pulmonary embolic disease (McClean et al., 2008). Recent cohort studies reported an association between obesity and susceptibility to influenza infection (Karlsson and Beck, 2010, Huttunen and Syrjanen, 2013). Obese individuals (body mass index [BMI]  30 kg/m2) experienced increased hospitalizations and death from influenza infection during the 2009 influenza season (Kwong et al., 2011, Kim et al., 2012a). The Center for Disease Control and Prevention speculated that this was caused by the influence of excess adipose tissue on immune responses in the lung (Centers for Disease Control and Prevention, 2009). Karlsson et al. investigated the impact of obesity on memory cell-mediated defenses against influenza virus infection in a diet-induced obesity (DIO) mouse model (Karlsson et al., 2010). They demonstrated that influenza-specific CD8+ memory T cells and production of interferon (IFN)-γ were decreased in obese mice compared with normal mice. This shows that obesity reduces immune responses against influenza virus. Based on these data, other studies found a correlation between vaccine efficacy and obesity (Kim et al., 2012b, Sheridan et al., 2012, Park et al., 2014). Our previous study showed that the efficacy of influenza vaccine is reduced in a DIO mouse model. In the obese mice, humoral immunity was diminished through a reduction in neutralizing antibodies against influenza virus, indicating that vaccine efficacy was attenuated. In addition, the number of effector memory T cells in the stromal vascular fraction (SVF) of epididymal fat tissue was significantly reduced after infection, whereas inflammation was increased (Kim et al., 2012b). These reports suggest that obesity induces a disorder of immune responses that might contribute to the susceptibility of organisms to pathogen infection, to low vaccine efficacy, and to worse pathophysiology (Kim et al., 2012b, Sheridan et al., 2012, Park et al., 2014). However, the mechanisms by which obesity affects disease outcome are not fully understood.

Obesity, a common metabolic disorder, is associated with accumulated fat and low-grade chronic inflammation in various tissues, which show dysregulation of cytokine production and infiltration of immune cells (Gregor and Hotamisligil, 2011). Several studies have reported that in the obese state, adipose tissue and immune cells, especially macrophages, are altered, with the predominant macrophage type changed from M2 to M1 macrophages, and that this results in chronic inflammation (Fujisaka et al., 2009, Martinez-Santibanez and Lumeng, 2014). Chronic inflammation is defined as prolonged inflammation (from weeks to months) caused by tissue injury or disorders of the immune response (Shacter and Weitzman, 2002). In particular, low-level chronic inflammation was associated with the expansion of adipocytes. However, it is unclear which occurs first in the development of obesity: chronic inflammation or adipocyte expansion (Na and Nam, 2012). Cani et al. demonstrated that metabolic endotoxemia induced by lipopolysaccharide (LPS) triggers obesity in mice (Cani et al., 2012). They used an osmotic minipump, implanted subcutaneously, to infuse LPS. Osmotic minipumps are inserted subcutaneously or intraperitoneally and used to deliver drugs, hormones, or other soluble agents (Theeuwes and Yum, 1976). They produce a constant flow rate over a period from 1 day to 6 weeks using the principle of osmotic pressure difference between components of the device. The advantage of this type of pump is that it can continuously infuse the target reagent. Therefore, Cani used this device to maintain a constant concentration of LPS in vivo. Interestingly, the results show that metabolic endotoxemia induced by LPS can induce an increase in body mass and epididymal fat tissue ratio even without a high fat diet. Moreover, it can trigger insulin resistance, dyslipidemia, and deterioration of glycemic control in vivo. In addition, macrophage infiltration was increased and pro-inflammatory cytokine mRNA levels were upregulated in the adipose tissue (Cani et al., 2012). Thus, experimental chronic metabolic endotoxemia can induce obesity by induction of chronic inflammation. It may also contribute to inactivation of immune cells, as does obesity (Fujisaka et al., 2009, Kim et al., 2009, Strissel et al., 2010). However, the mechanism by which it causes the dysfunction of immune cells is unknown.

In this study, we investigated the effect of chronic inflammation on immune cells. To mimic chronic inflammation, we developed a mouse model of metabolic endotoxemia using an osmotic minipump containing LPS, as described previously (Geurts et al., 2013). In this mouse model, we observed dysregulation of activation markers on peritoneal macrophages and splenic T cells in the steady state. LPS-induced chronic inflammation was able to cause immune tolerance in peritoneal macrophages and splenic T cells, and significantly reduced production of the pro-inflammatory cytokines interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α. Taken together, these results suggest that chronic inflammation in the obese state induces the dysfunction of macrophages and T cells. Accordingly, it might contribute to obesity-induced disorders and the severity of virus infection-induced pathology.

Section snippets

Animals

Twelve-week-old male C57BL/6 mice (Dae-Han Bio link, Eumseong, Korea) were housed in a controlled environment (inverted 12-h daylight cycle) with free access to food and water. All animal experimental procedures were approved by the Animal Care and Use Committee of the Catholic University of Korea (Cho and Seok, 2013). Mice were fed a normal-fat diet (containing 5% fat) and were simultaneously treated with LPS from Escherichia coli 055:B5 (Sigma, St Louis, MO, USA). Body weight and food intake

LPS-triggered chronic inflammation induces obesity in C57BL/6 mice

We mimicked chronic inflammation by implanting an osmotic minipump subcutaneously and infusing a constant rate of LPS over 4 weeks. Body mass and food intake data were measured once a week, and epididymal fat, liver, and pancreas were collected after 4 weeks of implantation to check the histology. The results show no differences in food intake between groups (Fig. 1A). Moreover, the relative spleen, liver, and pancreas weights (organ mass/total body mass) were similar in the NC and LPS groups (

Discussion

Previous studies reported that during the 2009 seasonal influenza period, obesity was associated with an increased susceptibility to infection and a reduction in vaccine efficacy (Kim et al., 2012b, Sheridan et al., 2012, Park et al., 2014). Our group previously showed a consistent decline in neutralizing antibodies and an attenuation of memory responses in DIO mice (Park et al., 2014). Moreover, the level of inflammation in DIO mice was higher than that in mice fed a regular-fat diet (Park et

Conclusion

Overall, we suggest that dysfunction of macrophages induced by chronic inflammation may influence vaccine efficacy and virus pathogenicity. Therefore, there is a need to develop efficient strategies that consider chronic inflammation when giving vaccine and/or antiviral therapy to obese patients, elderly patients, and patients with various inflammatory diseases.

Conflict of interest

The authors have no conflicts of interest and nothing to disclose.

Transparency document

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Acknowledgments

This study was supported by a grant from the Korean Healthcare Technology R&D project of the Ministry of Health & Welfare (A103001 and HI13C0826), Basic Science Research, Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M3A9B5030116), and Gyunggi Regional Research Center (GRRC) of the Catholic University of Korea.

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