No interplay between gut microbiota composition and the lipopolysaccharide-induced innate immune response in humans in vivo.

OBJECTIVE: Animal studies have demonstrated the extensive interplay between the gut microbiota and immunity. Moreover, in critically ill patients, who almost invariably suffer from a pronounced immune response, a shift in gut microbiota composition is associated with infectious complications and mortality. We examined the relationship between interindividual differences in gut microbiota composition and variation in the in vivo cytokine response induced by bacterial lipopolysaccharide (LPS). Furthermore, we evaluated whether an LPS challenge alters the composition of the gut microbiota. METHODS: Healthy male volunteers received an intravenous bolus of 2 ng kg-1 LPS (n = 70) or placebo (n = 8). Serial plasma concentrations of tumor necrosis factor-α, interleukin (IL)-6, IL-8 and IL-10 were measured, and subjects were divided into high and low cytokine responders. Gut microbiota composition was determined using 16s RNA gene sequencing of faecal samples obtained 1 day before (baseline) and 1 day and 7 days following the LPS challenge. RESULTS: Baseline microbiota composition, analysed by principal coordinate analysis and random forest analysis, did not differ between high and low responders for any of the four measured cytokines. Furthermore, baseline microbiota diversity (Shannon and Chao indices) was similar in high and low responders. No changes in microbiota composition or diversity were observed at 1 and 7 days following the LPS challenge. CONCLUSION: Our results indicate that existing variation in gut microbiota composition does not explain the observed variability in the LPS-induced innate immune response. These findings strongly argue against the interplay between the gut microbiota composition and the innate immune response in humans.

A High-Fat Diet Increases Influenza A Virus-Associated Cardiovascular Damage.

BACKGROUND: Influenza A virus (IAV) causes a wide range of extrarespiratory complications. However, the role of host factors in these complications of influenza virus infection remains to be defined. METHODS: Here, we sought to use transcriptional profiling, virology, histology, and echocardiograms to investigate the role of a high-fat diet in IAV-associated cardiac damage. RESULTS: Transcriptional profiling showed that, compared to their low-fat counterparts (LF mice), mice fed a high-fat diet (HF mice) had impairments in inflammatory signaling in the lung and heart after IAV infection. This was associated with increased viral titers in the heart, increased left ventricular mass, and thickening of the left ventricular wall in IAV-infected HF mice compared to both IAV-infected LF mice and uninfected HF mice. Retrospective analysis of clinical data revealed that cardiac complications were more common in patients with excess weight, an association which was significant in 2 out of 4 studies. CONCLUSIONS: Together, these data provide the first evidence that a high-fat diet may be a risk factor for the development of IAV-associated cardiovascular damage and emphasizes the need for further clinical research in this area.

Frontline Science: Endotoxin-induced immunotolerance is associated with loss of monocyte metabolic plasticity and reduction of oxidative burst.

Secondary infections are a major complication of sepsis and associated with a compromised immune state, called sepsis-induced immunoparalysis. Molecular mechanisms causing immunoparalysis remain unclear; however, changes in cellular metabolism of leukocytes have been linked to immunoparalysis. We investigated the relation of metabolic changes to antimicrobial monocyte functions in endotoxin-induced immunotolerance, as a model for sepsis-induced immunoparalysis. In this study, immunotolerance was induced in healthy males by intravenous endotoxin (2 ng/kg, derived from Escherichia coli O:113) administration. Before and after induction of immunotolerance, circulating CD14+ monocytes were isolated and assessed for antimicrobial functions, including cytokine production, oxidative burst, and microbial (Candida albicans) killing capacity, as well metabolic responses to ex vivo stimulation. Next, the effects of altered cellular metabolism on monocyte functions were validated in vitro. Ex vivo lipopolysaccharide stimulation induced an extensive rewiring of metabolism in naive monocytes. In contrast, endotoxin-induced immunotolerant monocytes showed no metabolic plasticity, as they were unable to adapt their metabolism or mount cytokine and oxidative responses. Validation experiments showed that modulation of metabolic pathways, affected by immunotolerance, influenced monocyte cytokine production, oxidative burst, and microbial (C. albicans) killing in naive monocytes. Collectively, these data demonstrate that immunotolerant monocytes are characterized by a loss of metabolic plasticity and these metabolic defects impact antimicrobial monocyte immune functions. Further, these findings support that the changed cellular metabolism of immunotolerant monocytes might reveal novel therapeutic targets to reverse sepsis-induced immunoparalysis.

The Itaconate Pathway Is a Central Regulatory Node Linking Innate Immune Tolerance and Trained Immunity.

Sepsis involves simultaneous hyperactivation of the immune system and immune paralysis, leading to both organ dysfunction and increased susceptibility to secondary infections. Acute activation of myeloid cells induced itaconate synthesis, which subsequently mediated innate immune tolerance in human monocytes. In contrast, induction of trained immunity by ß-glucan counteracted tolerance induced in a model of human endotoxemia by inhibiting the expression of immune-responsive gene 1 (IRG1), the enzyme that controls itaconate synthesis. ß-Glucan also increased the expression of succinate dehydrogenase (SDH), contributing to the integrity of the TCA cycle and leading to an enhanced innate immune response after secondary stimulation. The role of itaconate was further validated by IRG1 and SDH polymorphisms that modulate induction of tolerance and trained immunity in human monocytes. These data demonstrate the importance of the IRG1-itaconate-SDH axis in the development of immune tolerance and training and highlight the potential of ß-glucan-induced trained immunity to revert immunoparalysis.

Short-term repeated HRV-16 exposure results in an attenuated immune response in vivo in humans.

INTRODUCTION: Naturally, development of adaptive immunity following HRV infection affects the immune response. However, it is currently unclear whether or not HRV re-exposure within a short time frame leads to an altered innate immune response. The "experimental cold model" is used to investigate the pathogenesis of HRV infection and allows us to investigate the effects of repeated exposure on both local and systemic innate immunity. METHODS: 40 healthy male and female (1:1) subjects were nasally inoculated with HRV-16 or placebo. One week later, all subjects received HRV-16. Baseline seronegative subjects (n = 18) were included for further analysis. RESULTS: Infection rate was 82%. Primary HRV infection induced a marked increase in viral load and IP-10 levels in nasal wash, while a similar trend was observed for IL-6 and IL-10. Apart from an increase in IP-10 plasma levels, HRV infection did not induce systemic immune effects nor lower respiratory tract inflammation. With similar viral load present during the second HRV challenge, IP-10 and IL-6 in nasal wash showed no increase, but gradually declined, with a similar trend for IL-10. CONCLUSION: Upon a second HRV challenge one week after the first, a less pronounced response for several innate immune parameters is observed. This could be the result of immunological tolerance and possibly increases vulnerability towards secondary infections.

Effort but not Reward Sensitivity is Altered by Acute Sickness Induced by Experimental Endotoxemia in Humans.

Sickness behavior in humans is characterized by low mood and fatigue, which have been suggested to reflect changes in motivation involving reorganization of priorities. However, it is unclear which specific processes underlying motivation are altered. We tested whether bacterial endotoxin E. coli lipopolysaccharide (LPS) affected two dissociable constructs of motivational behavior, ie, effort and reward sensitivity. After familiarization with 5 effort levels, participants made a series of accept/reject decisions on whether the stake offered (1, 4, 8, 12, or 15 apples) was 'worth the effort' (10%, 27.5%, 45%, 62.5%, and 80% of maximal voluntary contraction in a hand-held dynamometer). Effort and reward levels were parametrically modulated to dissociate their influence on choice. Overall, 29 healthy young males were administered LPS (2 ng/kg; n=14) or placebo (0.9% saline; n=15). The effort-stake task, and self-reported depression and fatigue were assessed prior to LPS/placebo injection, 2 and 5 h post injection. Cytokines and sickness symptoms were assessed hourly till 8 h after LPS injection. LPS transiently increased interleukin-6 and tumor necrosis factor-α, sickness symptoms, body temperature and self-reported fatigue, and depression post injection relative to baseline and placebo. These changes were accompanied by LPS-induced decreases in acceptance rates of high-effort options, without significantly affecting reward sensitivity 2 h post injection, which were partially recovered 5 h post injection. We suggest that LPS-induced changes in motivation may be due to alterations to mesolimbic dopamine. Our behavioral paradigm could be used to further investigate effects of inflammation on motivational behavior in psychiatric and chronic illnesses.

Patterns in Bacterial- and Viral-Induced Immunosuppression and Secondary Infections in the ICU.

Immunosuppression renders the host increased susceptible for secondary infections. It is becoming increasingly clear that not only bacterial sepsis, but also respiratory viruses with both severe and mild disease courses such as influenza, respiratory syncytial virus, and the human rhinovirus may induce immunosuppression. In this review, the current knowledge on (mechanisms of) bacterial- and virus-induced immunosuppression and the accompanying susceptibility toward various secondary infections is described. In addition, the frequently encountered secondary pathogens and their preferred localizations are presented. Finally, future perspectives in the context of the development of diagnostic markers and possibilities for personalized therapy to improve the diagnosis and treatment of immunocompromised patients are discussed.

Development of Endotoxin Tolerance Does Not Influence the Response to a Challenge with the Mucosal Live-Attenuated Influenza Vaccine in Humans In Vivo.

INTRODUCTION: The effects of bacterial infections on the response to subsequent viral infections are largely unknown. This is important to elucidate to increase insight into the pathophysiology of bacterial and viral co-infections, and to assess whether bacterial infections may influence the course of viral infections. METHODS: Healthy male subjects received either bacterial endotoxin [Escherichia coli-derived lipopolysaccharide (LPS), 2 ng/kg, n = 15] or placebo (n = 15) intravenously, followed by intranasal Fluenz (live-attenuated influenza vaccine) 1 week later. RESULTS: LPS administration resulted in increased plasma cytokine levels and development of endotoxin tolerance in vivo and ex vivo, illustrated by attenuated cytokine production upon rechallenge with LPS. Following Fluenz administration, infectivity for the Fluenz A/B strains was similar between the LPS-Fluenz and placebo-Fluenz groups (13/15 subjects in both groups). Also, the Fluenz-induced increase in temperature and IL-6, G-CSF and IP-10 concentrations in nasal wash were similar between both groups. CONCLUSION: While endotoxemia profoundly attenuates the immune response upon a second LPS challenge, it does not influence the Fluenz-induced immune response. These results suggest immune suppression after bacterial infection does not alter the response to a subsequent viral infection.

Effects of serostatus and gender on the HRV-16-induced local immune response.

The "experimental cold model" is widely used to investigate effects of HRV infection. However, effects of serostatus and gender on the HRV-induced immune response have not been clarified. 40 healthy seropositive and seronegative (1:1) male and female (1:1) subjects were inoculated with HRV-16. HRV infection increased viral load in nasal wash, which tended to be more pronounced in seronegative subjects. Furthermore, HRV infection increased levels of IP-10, IL-6, and IL-10 and leukocyte numbers in nasal wash of seronegative, but not of seropositive subjects. No differences in any of the parameters were found between both sexes. The HRV-induced local immune response is diminished in seropositive subjects compared with seronegative subjects, while gender does not influence this response. These results have important implications for the design of future experimental cold studies: seronegative subjects, from both sexes can be included.

Reversal of immunoparalysis in humans in vivo: a double-blind, placebo-controlled, randomized pilot study.

RATIONALE: Reversal of sepsis-induced immunoparalysis may reduce the incidence of secondary infections and improve outcome. Although IFN-γ and granulocyte-macrophage colony-stimulating factor (GM-CSF) restore immune competence of ex vivo stimulated leukocytes of patients with sepsis, effects on immunoparalysis in vivo are not known. OBJECTIVES: To investigate the effects of IFN-γ and GM-CSF on immunoparalysis in vivo in humans. METHODS: We performed a double-blind, placebo-controlled, randomized study in 18 healthy male volunteers that received Escherichia coli endotoxin (LPS; 2 ng/kg, intravenously) on days 1 and 7 (visits 1 and 2). On days 2, 4, and 6, subjects received subcutaneous injections of IFN-γ (100 µg/day; n = 6), GM-CSF (4 µg/kg/day; n = 6), or placebo (NaCl 0.9%; n = 6). MEASUREMENTS AND MAIN RESULTS: In the placebo group, immunoparalysis was illustrated by a 60% (48-71%) reduction of LPS-induced tumor necrosis factor (TNF)-α plasma concentrations during visit 2 (P = 0.03), whereas the antiinflammatory IL-10 response was not significantly attenuated (39% [2-65%]; P = 0.15). In contrast, in the IFN-γ group, TNF-α concentrations during visit 2 were not significantly attenuated (28% [1-47%]; P = 0.09), whereas the IL-10 response was significantly lower (reduction of 54% [47-66%]; P = 0.03). Compared with the placebo group, the reduction in the LPS-induced TNF-α response during visit 2 was significantly less pronounced in the IFN-γ group (P = 0.01). Moreover, compared with placebo, treatment with IFN-γ increased monocyte HLA-DR expression (P = 0.02). The effects of GM-CSF tended in the same direction as IFN-γ, but were not statistically significant compared with placebo. CONCLUSIONS: IFN-γ partially reverses immunoparalysis in vivo in humans. These results suggest that IFN-γ is a promising treatment option to reverse sepsis-induced immunoparalysis.