Lipopolysaccharide-induced experimental immune activation does not impair memory functions in humans

An overview of the relationship between inflammation and cognitive function in humans, molecular pathways and the impact of nutraceuticals

Inflammation has been associated with cognitive decline, whether in the peripheral or central nervous systems. The primary mechanism involves the response of microglia, an immune cell in the brain, which generates pro-inflammatory mediators such as cytokines, chemokines, and adhesion molecules. The excessive production of pro-inflammatory mediators may accelerate the damage to neurons, contributing to the development of neurodegenerative diseases such as Alzheimer's disease, mild cognitive impairment, and vascular dementia, as well as a general decline in cognitive function. Various studies have supported the correlation between elevated pro-inflammatory mediators and a decline in cognitive function, particularly in aging and age-related neurodegenerative diseases. Moreover, this association has also been observed in other inflammatory-related conditions, including post-operative cognitive impairment, diabetes, stroke, obesity, and cancer. However, the interaction between inflammatory processes and cognitive function in humans remains unclear and varies according to different health conditions. Therefore, this review aims to consolidate and evaluate the available evidence from original studies as well as meta-analyses in order to provide a greater understanding of the inflammatory process in connection with cognitive function in humans. Furthermore, relevant biological cellular processes, putative inflammatory biomarkers, and the role of nutraceuticals on the interaction between cognitive performance and inflammatory status are outlined.

The endotoxin hypothesis of Alzheimer's disease

Lipopolysaccharide (LPS) constitutes much of the surface of Gram-negative bacteria, and if LPS enters the human body or brain can induce inflammation and act as an endotoxin. We outline the hypothesis here that LPS may contribute to the pathophysiology of Alzheimer's disease (AD) via peripheral infections or gut dysfunction elevating LPS levels in blood and brain, which promotes: amyloid pathology, tau pathology and microglial activation, contributing to the neurodegeneration of AD. The evidence supporting this hypothesis includes: i) blood and brain levels of LPS are elevated in AD patients, ii) AD risk factors increase LPS levels or response, iii) LPS induces Aβ expression, aggregation, inflammation and neurotoxicity, iv) LPS induces TAU phosphorylation, aggregation and spreading, v) LPS induces microglial priming, activation and neurotoxicity, and vi) blood LPS induces loss of synapses, neurons and memory in AD mouse models, and cognitive dysfunction in humans. However, to test the hypothesis, it is necessary to test whether reducing blood LPS reduces AD risk or progression. If the LPS endotoxin hypothesis is correct, then treatments might include: reducing infections, changing gut microbiome, reducing leaky gut, decreasing blood LPS, or blocking LPS response.

Inflamed but not impulsive: Acute inflammatory cytokine response does not impact prepotent response inhibition

BACKGROUND

Prior evidence has linked inflammation with impulsivity, but most of this evidence is cross-sectional. In this study, we provoked an acute inflammatory cytokine response to see whether it lowered prepotent response inhibition on three cognitive tasks.

METHOD

This study features secondary analyses from a randomized crossover trial in which 171 postmenopausal breast cancer survivors (Stage I-IIIA) each received a typhoid capsular polysaccharide vaccination and a saline placebo injection in a random sequence at two separate visits at least one month apart. Participants completed the Stroop Color-Discrepant Task, the 2-back, and the Conners Continuous Performance Test (CPT) on the computer between 5 and 7 h after the injections. They had their blood drawn once before and repeatedly after the injection to measure interleukin-1 receptor antagonist and interleukin-6 responses.

RESULTS

Women committed marginally fewer errors on the Stroop color-discrepant trials after the typhoid vaccine (M = 0.36, SE = 0.08), compared to placebo (M = 0.54, SE = 0.09, p = .076). Injection type did not predict 2-back accuracy (p = .80) or CPT commission errors (p = .47). Inflammatory cytokine responses were also unrelated to the outcomes of interest (ps>.16).

CONCLUSION

We found no evidence that an acute inflammatory cytokine response provokes response disinhibition - an important facet of impulsivity. In fact, our only marginally non-significant result suggested that women were better able to inhibit their prepotent responses on the Stroop after receiving the typhoid vaccine, compared to placebo. Further experimental tests of the acute inflammatory cytokine response's effect on other aspects of impulsivity are warranted.

LIMITATIONS

The sample was female, primarily White, highly educated cancer survivors, and recruitment was not premised on impulsive traits or diagnosis with an impulsive-related disorder. Also, there are many facets of impulsivity, and this study only measured response inhibition.

Fatigue during acute systemic inflammation is associated with reduced mental effort expenditure while task accuracy is preserved

BACKGROUND

Earlier work within the physical domain showed that acute inflammation changes motivational prioritization and effort allocation rather than physical abilities. It is currently unclear whether a similar motivational framework accounts for the mental fatigue and cognitive symptoms of acute sickness. Accordingly, this study aimed to assess the relationship between fatigue, cytokines and mental effort-based decision making during acute systemic inflammation.

METHODS

Eighty-five participants (41 males; 18-30 years (M=23.0, SD=2.4)) performed a mental effort-based decision-making task before, 2 hours after, and 5 hours after intravenous administration of 1 ng/kg bacterial lipopolysaccharide (LPS) to induce systemic inflammation. Plasma concentrations of cytokines (interleukin (IL)-6, IL-8 and tumor necrosis factor (TNF)) and fatigue levels were assessed at similar timepoints. In the task, participants decided whether they wanted to perform (i.e., 'accepted') arithmetic calculations of varying difficulty (3 levels: easy, medium, hard) in order to obtain rewards (3 levels: 5, 6 or 7 points). Acceptance rates were analyzed using a binomial generalized estimated equation (GEE) approach with effort, reward and time as independent variables. Arithmetic performance was measured per effort level prior to the decisions and included as a covariate. Associations between acceptance rates, fatigue (self-reported) and cytokine concentrations levels were analyzed using partial correlation analyses.

RESULTS

Plasma cytokine concentrations and fatigue were increased at 2 hours post-LPS compared to baseline and 5 hours post-LPS administration. Acceptance rates decreased for medium, but not for easy or hard effort levels at 2 hours post-LPS versus baseline and 5 hours post-LPS administration, irrespective of reward level. This reduction in acceptance rates occurred despite improved accuracy on the arithmetic calculations itself. Reduced acceptance rates for medium effort were associated with increased fatigue, but not with increased cytokines.

CONCLUSION

Fatigue during acute systemic inflammation is associated with alterations in mental effort allocation, similarly as observed previously for physical effort-based choice. Specifically, willingness to exert mental effort depended on effort and not reward information, while task accuracy was preserved. These results extend the motivational account of inflammation to the mental domain and suggest that inflammation may not necessarily affect domain-specific mental abilities, but rather affects domain-general effort-allocation processes.

The Central Inflammatory Network: A Hypothalamic fMRI Study of Experimental Endotoxemia in Humans

INTRODUCTION

Inflammation is a mechanism of the immune system that is part of the reaction to pathogens or injury. The central nervous system closely regulates inflammation via neuroendocrine or direct neuroimmune mechanisms, but our current knowledge of the underlying circuitry is limited. Therefore, we aimed to identify hypothalamic centres involved in sensing or modulating inflammation and to study their association with known large-scale brain networks.

METHODS

Using high-resolution functional magnetic resonance imaging (fMRI), we recorded brain activity in healthy male subjects undergoing experimental inflammation from intravenous endotoxin. Four fMRI runs covered key phases of the developing inflammation: pre-inflammatory baseline, onset of endotoxemia, onset of pro-inflammatory cytokinemia, and peak of pro-inflammatory cytokinemia. Using masked independent component analysis, we identified functionally homogeneous subregions of the hypothalamus, which were further tested for changes in functional connectivity during inflammation and for temporal correlation with tumour necrosis factor and adrenocorticotropic hormone serum levels. We then studied the connection of these inflammation-associated hypothalamic subregions with known large-scale brain networks.

RESULTS

Our results show that there are at least 6 hypothalamic subregions associated with inflammation in humans including the paraventricular nucleus, supraoptic nucleus, dorsomedial hypothalamus, bed nucleus of the stria terminalis, lateral hypothalamic area, and supramammillary nucleus. They are functionally embedded in at least 3 different large-scale brain networks, namely a medial frontoparietal network, an occipital-pericentral network, and a midcingulo-insular network.

CONCLUSION

Measuring how the hypothalamus detects or modulates systemic inflammation is a first step to understand central nervous immunomodulation.

Increased intestinal permeability with elevated peripheral blood endotoxin and inflammatory indices for e-waste lead exposure in children

Lead (Pb) entering the body through different channels can damage the function of intestinal mucosal barrier and cause the body stressful inflammatory response to enhance. This study conducted a cross-sectional study to investigate the effects of Pb exposure on intestinal permeability in children by measuring the level of bacterial endotoxin and index of inflammatory cell types in peripheral blood. From November to December 2018, we recruited 187 participants aged 3-6 years by stratified randomization, from an electronic-waste-exposed group (n = 82) and a referent group (n = 105). General demographic information, past history of the digestive system in child, and family situation were informed by children's guardians with questionnaires. Children in the exposed group showed lower weight, height, and body mass index while more diarrhea in a month. Blood Pb and plasma endotoxin were elevated in exposed children than referent children and the positive relationship between them was shown in all children [B (95% CI): 0.072 (0.008, 0.137), P = 0.033]. Peripheral monocyte counts and leukotriene B₄ (LTB₄) levels were significantly increased in the exposed group. Endotoxin levels were positively correlated with neutrophils, monocytes, and LTB₄ [B (95% CI): 0.054 (0.015, 0.093), 0.018 (0.005, 0.031), and 0.049 (0.011, 0.087), respectively, P < 0.05]. To sum up, the exposed children showed lower physical growth levels, poorer gut health, and increased intestinal permeability, which was related to high blood Pb and peripheral inflammatory indices. These results suggest the possible adverse impact of environmental Pb exposure on the intestinal health of children.

Inhibition of PAD2 Improves Survival in a Mouse Model of Lethal LPS-Induced Endotoxic Shock

Endotoxemia induced by lipopolysaccharide (LPS) is an extremely severe syndrome identified by global activation of inflammatory responses. Neutrophil extracellular traps (NETs) play an important role in the development of endotoxemia. Histone hypercitrullination catalyzed by peptidylarginine deiminases (PADs) is a key step of NET formation. We have previously demonstrated that simultaneous inhibition of PAD2 and PAD4 with pan-PAD inhibitors can decrease NETosis and improve survival in a mouse model of LPS-induced endotoxic shock. However, the effects of PAD2 specific inhibition during NETosis and endotoxic shock are poorly understood. Therefore, in the present study, we aimed to investigate the effect of the specific PAD2 or PAD4 inhibitor on LPS-induced endotoxic shock in mice. We found that PAD2 inhibition but not PAD4 inhibition improves survival. Also, the levels of proinflammatory cytokines and NETosis were significantly reduced by PAD2 inhibitor. To our knowledge, this study demonstrates for the first time that PAD2 inhibition can reduce NETosis, decrease inflammatory cytokine production, and protect against endotoxin-induced lethality. Our findings provided a novel therapeutic strategy for the treatment of endotoxic shock.

Dendritic Cells: Neglected Modulators of Peripheral Immune Responses and Neuroinflammation in Mood Disorders?

Affective disorders (AD) including major depressive disorder (MDD) and bipolar disorder (BD) are common mood disorders associated with increased disability and poor health outcomes. Altered immune responses characterized by increased serum levels of pro-inflammatory cytokines and neuroinflammation are common findings in patients with AD and in corresponding animal models. Dendritic cells (DCs) represent a heterogeneous population of myeloid cells that orchestrate innate and adaptive immune responses and self-tolerance. Upon sensing exogenous and endogenous danger signals, mature DCs secrete proinflammatory factors, acquire migratory and antigen presenting capacities and thus contribute to neuroinflammation in trauma, autoimmunity, and neurodegenerative diseases. However, little is known about the involvement of DCs in the pathogenesis of AD. In this review, we summarize the current knowledge on DCs in peripheral immune responses and neuroinflammation in MDD and BD. In addition, we consider the impact of DCs on neuroinflammation and behavior in animal models of AD. Finally, we will discuss therapeutic perspectives targeting DCs and their effector molecules in mood disorders.

Glucocorticoids and the Brain after Critical Illness

Treatment for critical illness typically focuses on a patient's short-term physical recovery; however, recent work has broadened our understanding of the long-term implications of illness and treatment strategies. In particular, survivors of critical illness have significantly elevated risk of developing lasting cognitive impairment and psychiatric disorders. In this review, we examine the role of endogenous and exogenous glucocorticoids in neuropsychiatric outcomes following critical illness. Illness is marked by acute elevation of free cortisol and adrenocorticotropic hormone suppression, which typically normalize after recovery; however, prolonged dysregulation can sometimes occur. High glucocorticoid levels can cause lasting alterations to the plasticity and structural integrity of the hippocampus and prefrontal cortex, and this mechanism may plausibly contribute to impaired memory and cognition in critical illness survivors, though specific evidence is lacking. Glucocorticoids may also exacerbate inflammation-associated neural damage. Conversely, current evidence indicates that glucocorticoids during illness may protect against the development of post-traumatic stress disorder. We propose future directions for research in this field, including determining the role of persistent glucocorticoid elevations after illness in neuropsychiatric outcomes, the role of systemic vs neuroinflammation, and probing unexplored lines of investigation on the role of mineralocorticoid receptors and the gut-brain axis. Progress toward personalized medicine in this area has the potential to produce tangible improvements to the lives patients after a critical illness, including Coronavirus Disease 2019.

Circulating inflammatory biomarkers and academic performance in adolescents: DADOS study

OBJECTIVE

The present study aimed (1) to examine the association between circulating inflammatory biomarkers and academic performance in adolescents, and (2) to identify the ability of circulating inflammatory biomarkers to predict low academic performance.

METHODS

A total of 244 adolescents (13.9±0.3 years, 112 girls) from the DADOS study were included in the analysis. Four inflammatory biomarkers were quantified: white blood cell (WBC) count, interleukin-6, tumor necrosis factor-α (TNF-α), and C-reactive protein (CRP). Academic performance was assessed through academic grades and the Spanish version of the Science Research Associates Test of Educational Abilities.

RESULTS

TNF-α was inversely associated with math, Spanish and grade point average (β ranging from -0.166 to -0.124; all p<0.05), while CRP was inversely associated with verbal ability (β = -0.128; p<0.05). Overall, receiver operating characteristic (ROC) curves analyses showed discriminatory ability of WBC and TNF-α in identifying low academic performance (all p<0.05). Moreover, logistic regression analyses indicated that students with levels of WBC and TNF-α above the ROC cut-offs values showed between 78% to 87% increased likelihood of lower academic performance (p<0.05).

CONCLUSIONS

Our findings suggested that some circulating inflammatory biomarkers were associated with academic performance in adolescents. Further larger longitudinal and interventional studies are needed to clarify the short-term and long-term relationship between inflammation and academic performance in youths.

Role of enhanced glucocorticoid receptor sensitivity in inflammation in PTSD: insights from computational model for circadian-neuroendocrine-immune interactions

Although glucocorticoid resistance contributes to increased inflammation, individuals with posttraumatic stress disorder (PTSD) exhibit increased glucocorticoid receptor (GR) sensitivity along with increased inflammation. It is not clear how inflammation coexists with a hyperresponsive hypothalamic-pituitary-adrenal (HPA) axis. To understand this better, we developed and analyzed an integrated mathematical model for the HPA axis and the immune system. We performed mathematical simulations for a dexamethasone (DEX) suppression test and IC₅₀-dexamethasone for cytokine suppression by varying model parameters. The model analysis suggests that increasing the steepness of the dose-response curve for GR activity may reduce anti-inflammatory effects of GRs at the ambient glucocorticoid levels, thereby increasing proinflammatory response. The adaptive response of proinflammatory cytokine-mediated stimulatory effects on the HPA axis is reduced due to dominance of the GR-mediated negative feedback on the HPA axis. To verify these hypotheses, we analyzed the clinical data on neuroendocrine variables and cytokines obtained from war-zone veterans with and without PTSD. We observed significant group differences for cortisol and ACTH suppression tests, proinflammatory cytokines TNFα and IL6, high-sensitivity C-reactive protein, promoter methylation of GR gene, and IC_50-DEX for lysozyme suppression. Causal inference modeling revealed significant associations between cortisol suppression and post-DEX cortisol decline, promoter methylation of human GR gene exon 1F (NR3C1-1F), IC_50-DEX, and proinflammatory cytokines. We noted significant mediation effects of NR3C1-1F promoter methylation on inflammatory cytokines through changes in GR sensitivity. Our findings suggest that increased GR sensitivity may contribute to increased inflammation; therefore, interventions to restore GR sensitivity may normalize inflammation in PTSD.

Inflammation Mediates Body Weight and Ageing Effects on Psychomotor Slowing

Inflammation (immune system activation) affects neuronal function and may have consequences for the efficiency and speed of functional brain processes. Indeed, unusually slow psychomotor speed, a measure predictive of behavioural performance and health outcomes, is found with obesity and ageing, two conditions also associated with chronic inflammation. Yet whether inflammation is the mediating factor remains unclear. Here, we assessed inflammation by indexing interleukin-6 level in blood and measured psychomotor speed as well as indices of selective visual attention in young (mean = 26 years) or old (mean = 71 years) adults (N = 83) who were either lean or currently significantly overweight (mean body mass index = 22.4 and 33.8, respectively). Inflammation was positively and significantly correlated with psychomotor speed, age, and body mass index but not with attention measures. Using mediation analyses we show for the first time that inflammation fully accounts for the significant psychomotor slowing found in those with high BMI. Moreover, we further show that age-related psychomotor slowing is partially mediated by inflammation. These findings support the proposal that reducing inflammation may mitigate weight- and age-related cognitive decline and thereby improve performance on daily tasks and health outcomes more generally.

Inflammatory biomarkers and brain health indicators in children with overweight and obesity: The ActiveBrains project

INTRODUCTION

Chronic inflammation plays an important role on the pathogenesis of several cardiovascular and metabolic diseases, as well as on brain function and behaviour. The aim of the present study was to examine the associations between inflammatory biomarkers and a wide range of brain health indicators (i.e., academic performance, executive function, behavioural and emotional functioning, and brain volume) in children with overweight/obesity.

METHODS

A total of 107 children (10.0 ± 1.1 years, 41% girls) from the ActiveBrains project were included in the analysis. Five inflammatory biomarkers were analysed in plasma: white blood cell (WBC) count, interleukin-6 (IL-6), interleukin-1β, tumor necrosis factor-α (TNF-α), and C-reactive protein (CRP). Academic performance was assessed by Woodcock-Muñoz Tests of Achievement. Executive function was assessed through the Design Fluency Test for cognitive flexibility, the Stroop test for cognitive inhibition, and the Delayed Non-Match-to-Sample task for working memory. Behavioural and emotional functioning was evaluated through the Behavior Assessment System for Children (BASC) questionnaire. Total and regional brain volume was assessed by magnetic resonance imaging.

RESULTS

IL-6 was inversely associated with adaptive skills (β = -0.228; p = 0.030), while TNF-α was related to mathematics (β = -0.198; p = 0.034). In addition, CRP was positively associated with externalizing (β = 0.246; p = 0.046) and internalizing problems (β = 0.234; p = 0.039), as well as the behavioural symptoms index (β = 0.236; p = 0.047). However, these significant associations disappeared after multiple comparisons correction. Inflammatory biomarkers were not associated with executive function and total brain volumes. Regarding regional brain analyses, WBC was positively associated with gray matter volume in the left middle temporal gyrus (β = 0.387; p < 0.001, k = 44), and CRP was positively associated with gray matter volume in the right superior temporal gyrus (β = 0.439; p < 0.001, k = 29). Additionally, when adjusting by total brain volume, CRP was positively associated with gray matter volume in the right supplementary motor cortex (β = 0.453; p < 0.001, k = 51). Moreover, both, IL-6 (β = 0.366; p < 0.001, k = 81) and TNF-α (β = 0.368; p < 0.001, k = 62) were positively associated with white matter volume around the right inferior frontal gyrus pars opercularis, while CRP was inversely associated with white matter volume around the left superior frontal gyrus (β = -0.482; p < 0.001, k = 82). After adjusting by total brain volume, CRP was also inversely associated with white matter volume in 3 additional clusters (β ranging from -0.473 to -0.404; p < 0.001, k = 87).

CONCLUSIONS

Inflammation was slightly associated with brain health (i.e., academic performance, behavioural and emotional functioning and regional brain volume) in children with overweight or obesity. Further larger longitudinal and interventional studies are warranted to elucidate the short-term and long-term effect of systemic low-grade inflammation on children's brain health.

Metabolic Endotoxemia: A Potential Underlying Mechanism of the Relationship between Dietary Fat Intake and Risk for Cognitive Impairments in Humans?

(1) Background: Nutrition is a major lifestyle factor that can prevent the risk of cognitive impairment and dementia. Diet-induced metabolic endotoxemia has been proposed as a major root cause of inflammation and these pathways emerge as detrimental factors of healthy ageing. The aim of this paper was to update research focusing on the relationship between a fat-rich diet and endotoxemia, and to discuss the potential role of endotoxemia in cognitive performances. (2) Methods: We conducted a non-systematic literature review based on the PubMed database related to fat-rich meals, metabolic endotoxemia and cognitive disorders including dementia in humans. A total of 40 articles out of 942 in the first screening met the inclusion criteria. (3) Results: Evidence suggested that a fat-rich diet, depending on its quality, quantity and concomitant healthy food components, could influence metabolic endotoxemia. Since only heterogeneous cross-sectional studies are available, it remains unclear to what extent endotoxemia could be associated or not with cognitive disorders and dementia. (4) Conclusions: A fat-rich diet has the capability to provide significant increases in circulating endotoxins, which highlights nutritional strategies as a promising area for future research on inflammatory-associated diseases. The role of endotoxemia in cognitive disorders and dementia remains unclear and deserves further investigation.

Selective effects of acute low-grade inflammation on human visual attention

Illness is often accompanied by perceived cognitive sluggishness, a symptom that may stem from immune system activation. The current study used electroencephalography (EEG) to assess how inflammation affected three different distinct attentional processes: alerting, orienting and executive control. In a double-blinded placebo-controlled within-subjects design (20 healthy males, mean age = 24.5, SD = 3.4), Salmonella typhoid vaccination (0.025 mg; Typhim Vi, Sanofi Pasteur) was used to induce transient mild inflammation, while a saline injection served as a placebo-control. Participants completed the Attention Network Test with concurrent EEG recorded 6 h post-injection. Analyses focused on behavioral task performance and on modulation of oscillatory EEG activity in the alpha band (9-12 Hz) for alerting as well as orienting attention and frontal theta band (4-8 Hz) for executive control. Vaccination induced mild systemic inflammation, as assessed by interleukin-6 (IL-6) levels. While no behavioral task performance differences between the inflammation and placebo condition were evident, inflammation caused significant alterations to task-related brain activity. Specifically, inflammation produced greater cue-induced suppression of alpha power in the alerting aspect of attention and individual variation in the inflammatory response was significantly correlated with the degree of alpha power suppression. Notably, inflammation did not affect orienting (i.e., alpha lateralization) or executive control (i.e., frontal theta activity). These results reveal a unique neurophysiological sensitivity to acute mild inflammation of the neural network that underpins attentional alerting functions. Observed in the absence of performance decrements, these novel findings suggest that acute inflammation requires individuals to exert greater cognitive effort when preparing for a task in order to maintain adequate behavioral performance.

The role of inflammation in core features of depression: Insights from paradigms using exogenously-induced inflammation

A wealth of evidence has implicated inflammation in the development of depression. Yet, the heterogeneous nature of depression has impeded efforts to understand, prevent, and treat the disease. The purpose of this integrative review is to summarize the connections between inflammation and established core features of depression that exhibit more homogeneity than the syndrome itself: exaggerated reactivity to negative information, altered reward processing, decreased cognitive control, and somatic syndrome. For each core feature, we first provide a brief overview of its relevance to depression and neurobiological underpinnings, and then review evidence investigating a potential role of inflammation. We focus primarily on findings from experimental paradigms of exogenously-induced inflammation. We conclude that inflammation likely plays a role in exaggerated reactivity to negative information, altered reward reactivity, and somatic symptoms. There is less evidence supporting an effect of inflammation on cognitive control as assessed by standard neuropsychological measures. Finally, we discuss implications for future research and recommendationsfor how to test the role of inflammation in the pathogenesis of heterogeneous psychiatric disorders.

Altered temporal variance and functional connectivity of BOLD signal is associated with state anxiety during acute systemic inflammation

Systemic inflammation is accompanied by complex behavioral changes and disturbed emotion regulation that have been related to the pathophysiology of mood disorders including depression and anxiety. However, the causal role of systemic inflammation on mood disorders is still unclear. We herein investigated neural resting state patterns of temporal variance of the amygdala and functional connectivity within the salience network underlying changes in state anxiety during experimentally-induced systemic inflammation. In this randomized, double-blind study, N = 43 healthy men received an intravenous injection of either low-dose lipopolysaccharide (LPS, 0.4 ng/kg body weight) or saline. Resting state functional magnetic resonance imaging was assessed before and 3.5 h after injection. State anxiety, assessed with a standardized questionnaire, and plasma cytokine concentrations were repeatedly measured. LPS administration induced a transient systemic inflammatory response reflected in increases in plasma Interleukin (IL)-6 and Tumor Necrosis Factor (TNF)-α concentration. Compared to placebo, state anxiety and temporal variance in the amygdala significantly increased while functional connectivity in the salience network decreased during LPS-induced systemic inflammation. Together, these data indicate that acute systemic inflammation alters temporal variance of the BOLD signal as well as functional connectivity in brain regions and networks implicated in emotion processing and regulation. These results are of translational importance to encourage further research on the role of inflammatory pathways in the pathophysiology of neuropsychiatric conditions including anxiety disorders.

Sex differences in how inflammation affects behavior: What we can learn from experimental inflammatory models in humans

Human models demonstrate that experimental activation of the innate immune system has profound effects on brain activation and behavior, inducing fatigue, worsened mood and pain sensitivity. It has been proposed that inflammation is a mechanism involved in the etiology and maintenance of depression, chronic pain and long-term fatigue. These diseases show a strong female overrepresentation, suggesting that a better understanding of sex differences in how inflammation drives behavior could help the development of individualized treatment interventions. For this purpose, we here review sex differences in studies using experimental inflammatory models to investigate changes in brain activity and behavior. We suggest a model in which inflammation accentuates sex differences in brain networks and pre-existing vulnerability factors. This effect could render women more vulnerable to the detrimental effects of immune-to-brain communication over time. We call for systematic and large scale investigations of vulnerability factors for women in the behavioral response to inflammation.

Inflammation, Self-Regulation, and Health: An Immunologic Model of Self-Regulatory Failure

Self-regulation is a fundamental human process that refers to multiple complex methods by which individuals pursue goals in the face of distractions. Whereas superior self-regulation predicts better academic achievement, relationship quality, financial and career success, and lifespan health, poor self-regulation increases a person's risk for negative outcomes in each of these domains and can ultimately presage early mortality. Given its centrality to understanding the human condition, a large body of research has examined cognitive, emotional, and behavioral aspects of self-regulation. In contrast, relatively little attention has been paid to specific biologic processes that may underlie self-regulation. We address this latter issue in the present review by examining the growing body of research showing that components of the immune system involved in inflammation can alter neural, cognitive, and motivational processes that lead to impaired self-regulation and poor health. Based on these findings, we propose an integrated, multilevel model that describes how inflammation may cause widespread biobehavioral alterations that promote self-regulatory failure. This immunologic model of self-regulatory failure has implications for understanding how biological and behavioral factors interact to influence self-regulation. The model also suggests new ways of reducing disease risk and enhancing human potential by targeting inflammatory processes that affect self-regulation.

The effects of acute inflammation on cognitive functioning and emotional processing in humans: A systematic review of experimental studies

OBJECTIVE

The cognitive neuropsychological model of depression proposes that negative biases in the processing of emotionally salient information have a central role in the development and maintenance of depression. We have conducted a systematic review to determine whether acute experimental inflammation is associated with changes to cognitive and emotional processing that are thought to cause and maintain depression.

METHODS

We identified experimental studies in which healthy individuals were administered an acute inflammatory challenge (bacterial endotoxin/vaccination) and standardised tests of cognitive function were performed.

RESULTS

Fourteen references were identified, reporting findings from 12 independent studies on 345 participants. Methodological quality was rated strong or moderate for 11 studies. Acute experimental inflammation was triggered using a variety of agents (including endotoxin from E. coli, S. typhi, S. abortus Equi and Hepatitis B vaccine) and cognition was assessed over hours to months, using cognitive tests of i) attention/executive functioning, ii) memory and iii) social/emotional processing. Studies found mixed evidence that acute experimental inflammation caused changes to attention/executive functioning (2 of 6 studies showed improvements in attention executive function compared to control), changes in memory (3 of 5 studies; improved reaction time: reduced memory for object proximity: poorer immediate and delayed memory) and changes to social/emotional processing (4 of 5 studies; reduced perception of emotions, increased avoidance of punishment/loss experiences, and increased social disconnectedness).

CONCLUSIONS

Acute experimental inflammation causes negative biases in social and emotional processing that could explain observed associations between inflammation and depression.

Systemic TNF-α produces acute cognitive dysfunction and exaggerated sickness behavior when superimposed upon progressive neurodegeneration

Inflammation influences chronic neurodegeneration but its precise roles are not yet clear. Systemic inflammation caused by infection, trauma or co-morbidity can alter the brain's inflammatory status, produce acute cognitive impairments, such as delirium, and drive new pathology and accelerated decline. Consistent with this, elevated systemic TNF-α is associated with more rapid cognitive decline over 6months in Alzheimer's disease patients. In the current study we challenged normal animals and those with existing progressive neurodegeneration (ME7 prion disease) with TNF-α (i.p.) to test the hypothesis that this cytokine has differential effects on cognitive function, sickness behavior and features of underlying pathology contingent on the animals' baseline condition. TNF-α (50μg/kg) had no impact on performance of normal animals (normal brain homogenate; NBH) on working memory (T-maze) but produced acute impairments in ME7 animals similarly challenged. Plasma TNF-α and CCL2 levels were equivalent in NBH and ME7 TNF-challenged animals but hippocampal and hypothalamic transcription of IL-1β, TNF-α and CCL2 and translation of IL-1β were higher in ME7+TNF-α than NBH+TNF-α animals. TNF-α produced an exaggerated sickness behavior response (hypothermia, weight loss, inactivity) in ME7 animals compared to that in NBH animals. However a single challenge with this dose was not sufficient to produce de novo neuronal death, synaptic loss or tau hyperphosphorylation that was distinguishable from that arising from ME7 alone. The data indicate that acutely elevated TNF-α has robust acute effects on brain function, selectively in the degenerating brain, but more sustained levels may be required to significantly impact on underlying neurodegeneration.

Alterations in functional connectivity of resting state networks during experimental endotoxemia - An exploratory study in healthy men

Systemic inflammation impairs mood and cognitive functions, and seems to be involved in the pathophysiology of psychiatric disorders. Functional magnetic resonance imaging (fMRI) studies revealed altered task-related blood-oxygen-level-dependent (BOLD) responses during experimental endotoxemia, but little is known about effects of systemic inflammation on resting-state activity of the brain. Thus, we conducted a randomized, placebo-controlled study in healthy men receiving an intravenous injection of either low-dose (0.4 ng/kg) lipopolysaccharide (LPS) (N=20) or placebo (N=25). Resting state activity was measured at baseline and 3.5h post-injection. Based on a two (condition) × two (group) design, we used multi-subject independent component analysis (ICA) to decompose and estimate functional connectivity within resting-state networks (RSNs). Seed-based analyses were applied to investigate the effect of LPS on the functional coupling for a priori-defined regions-of-interest (ROIs). ICA analyses identified 13 out of 35 components displaying common RSNs. Seed based analysis revealed greater functional connectivity between the left thalamus and the cerebellum after LPS compared to placebo administration, while the functional coupling between seeds within the amygdala, insula, and cingulate cortex and various brain regions including parieto-frontal networks was significantly reduced. Within the LPS group, endotoxin-induced increases in Interleukin (IL)-6 were significantly associated with resting-state connectivity between the left thalamus and left precuneus as well as the right posterior cingulate cortex. In summary, this exploratory study provides first evidence that systemic inflammation affects the coupling and regulation of multiple networks within the human brain at rest.

Inflammation-induced pain sensitization in men and women: does sex matter in experimental endotoxemia?

A role of the innate immune system is increasingly recognized as a mechanism contributing to pain sensitization. Experimental administration of the bacterial endotoxin lipopolysaccharide (LPS) constitutes a model to study inflammation-induced pain sensitization, but all existing human evidence comes from male participants. We assessed visceral and musculoskeletal pain sensitivity after low-dose LPS administration in healthy men and women to test the hypothesis that women show greater LPS-induced hyperalgesia compared with men. In this randomized, double-blind, placebo-controlled crossover study, healthy men (n = 20) and healthy women using oral contraceptives (n = 20) received an intravenous injection of 0.4 ng/kg body weight LPS or placebo. Pain sensitivity was assessed with established visceral and musculoskeletal pain models (ie, rectal pain thresholds; pressure pain thresholds for different muscle groups), together with a heartbeat perception (interoceptive accuracy) task. Plasma cytokines (tumor necrosis factor-α and interleukin-6) were measured along with state anxiety at baseline and up to 6-hour postinjection. Lipopolysaccharide application led to significant increases in plasma cytokines and state anxiety and decreased interoceptive awareness in men and women (P < 0.001, condition effects), with more pronounced LPS-induced cytokine increases in women (P < 0.05, interaction effects). Although both rectal and pressure pain thresholds were significantly decreased in the LPS condition (all P < 0.05, condition effect), no sex differences in endotoxin-induced sensitization were observed. In summary, LPS-induced systemic immune activation leads to visceral and musculoskeletal hyperalgesia, irrespective of biological sex. These findings support the broad applicability of experimental endotoxin administration as a translational preclinical model of inflammation-induced pain sensitization in both sexes.

Neural circuitry mediating inflammation-induced central pain amplification in human experimental endotoxemia

BACKGROUND & AIMS

To elucidate the brain mechanisms underlying inflammation-induced visceral hyperalgesia in humans, in this functional magnetic resonance imaging (fMRI) study we tested if intravenous administration of lipopolysaccharide (LPS) involves altered central processing of visceral pain stimuli.

METHODS

In this randomized, double-blind, placebo-controlled fMRI study, 26 healthy male subjects received either an intravenous injection of low-dose LPS (N=14, 0.4 ng/kg body weight) or placebo (N=12, control group). Plasma cytokines (TNF-α, IL-6), body temperature, plasma cortisol and mood were assessed at baseline and up to 6 h post-injection. At baseline and 2 h post-injection (test), rectal pain thresholds and painful rectal distension-induced blood oxygen level-dependent (BOLD) responses in brain regions-of-interest were assessed. To address specificity for visceral pain, BOLD responses to non-painful rectal distensions and painful somatic stimuli (i.e., punctuate mechanical stimulation) were also analyzed as control stimuli.

RESULTS

Compared to the control group, LPS-treated subjects demonstrated significant and transient increases in TNF-α, IL-6, body temperature and cortisol, along with impaired mood. In response to LPS, rectal pain thresholds decreased in trend, along with enhanced up-regulation of rectal pain-induced BOLD responses within the posterior insula, dorsolateral prefrontal (DLPFC), anterior midcingulate (aMCC) and somatosensory cortices (all FWE-corrected p<0.05). Within the LPS group, more pronounced cytokine responses correlated significantly with enhanced rectal pain-induced neural activation in DLPFC and aMCC. No significant LPS effects were observed on neural responses to non-painful rectal distensions or mechanical stimulation.

CONCLUSIONS

These findings support that peripheral inflammatory processes affect visceral pain thresholds and the central processing of sensory-discriminative aspects of visceral pain.

Induced mild systemic inflammation is associated with impaired ability to improve cognitive task performance by practice

Elevated inflammatory levels are linked to poorer cognition, but experimental confirmation is lacking. This report examined associations between cognitive performance and inflammation induced by exercise and vaccination. Thirty-six (exercise N = 18, vaccination N = 18) healthy males completed a paced auditory serial addition test (PASAT), which is a multifaceted measure of cognitive function. The task was completed in placebo and elevated inflammation states. Improvements in PASAT performance were related to inflammation. In the exercise study, IL-6 during the first PASAT negatively correlated with PASAT improvement (p = .022). In the vaccination study, increases in C-reactive protein between PASATs correlated with reduced PASAT improvement (p < .001). Inflammation was linked to reduced improvements in cognitive performance. Further research should identify the specific cognitive functions affects and the underlying mechanisms.

Inflammation-induced hyperalgesia: effects of timing, dosage, and negative affect on somatic pain sensitivity in human experimental endotoxemia

BACKGROUND

Inflammation-induced pain amplification and hypersensitivity play a role in the pathophysiology of numerous clinical conditions. Experimental endotoxemia has recently been implemented as model to analyze immune-mediated processes in human pain. In this study, we aimed to analyze dose- and time-dependent effects of lipopolysaccharide (LPS) on clinically-relevant pain models for musculoskeletal and neuropathic pain as well as the interaction among LPS-induced changes in inflammatory markers, pain sensitivity and negative affect.

METHODS

In this randomized, double-blind, placebo-controlled study, healthy male subjects received an intravenous injection of either a moderate dose of LPS (0.8 ng/kg Escherichiacoli), low-dose LPS (0.4 ng/kg), or saline (placebo control group). Pressure pain thresholds (PPT), mechanical pain sensitivity (MPS), and cold pain sensitivity (CP) were assessed before and 1, 3, and 6h post injection to assess time-dependent LPS effects on pain sensitivity. Plasma cytokines (TNF-α, IL-6, IL-8, IL-10) and state anxiety were repeatedly measured before, and 1, 2, 3, 4, and 6h after injection of LPS or placebo.

RESULTS

LPS administration induced a systemic immune activation, reflected by significant increases in cytokine levels, body temperature, and negative mood with pronounced effects to the higher LPS dose. Significant decreases of PPTs were observed only 3h after injection of the moderate dose of LPS (0.8 ng/kg). MPS and CP were not affected by LPS-induced immune activation. Correlation analyses revealed that decreased PPTs were associated with peak IL-6 increases and negative mood.

CONCLUSIONS

Our results revealed widespread increases in musculoskeletal pain sensitivity in response to a moderate dose of LPS (0.8 ng/kg), which correlate both with changes in IL-6 and negative mood. These data extend and refine existing knowledge about immune mechanisms mediating hyperalgesia with implications for the pathophysiology of chronic pain and neuropsychiatric conditions.

Neurocognitive disturbances associated with acute infectious mononucleosis, Ross River fever and Q fever: a preliminary investigation of inflammatory and genetic correlates

Disturbances in neurocognitive performance are a core feature of the acute sickness response to infection; however the underlying mechanisms remain unclear. The current study used a computerised battery to assess neurocognitive functioning in subjects enrolled in the Dubbo Infection Outcomes Study (n=107) - a prospective cohort of subjects followed from documented acute infection with Epstein Barr virus, Ross River virus, or Coxiella burnetii until recovery. Subjects were assessed when ill, and a subset again after complete recovery. Associations between sickness-related cognitive disturbances and single nucleotide polymorphisms (SNPs) in cytokine (interleukin [IL]-6, IL-10, tumor necrosis factor-α and interferon-γ) and neurobehavioral genes (serotonin transporter and catechol-O-methyltransferase) were explored. During acute infection, subjects exhibited slower matching-to-sample responses (p=0.03), poorer working memory capacity (p=0.014), mental planning (p=0.045), and dual attention task performance (p=0.02), and required longer to complete discordant Stroop trials (p=0.01) compared to recovery. Objective impairments correlated significantly with self-reported symptoms (p<0.05) as well as levels of the inflammation marker, C-reactive protein (p=0.001). Linear regression analysis identified an association between neurocognitive disturbance during acute illness and functional polymorphisms in inflammatory cytokine genes. Specifically, the high cytokine producing G allele of the IL-6-174G/C SNP was associated with poorer neurocognitive performance when subjects were ill (p=0.027). These findings confirm that acute infection impacts on neurocognitive performance, manifesting as slowed responses and impaired performance on complex tasks requiring higher-order functioning which has important real-world implications. The data provide the first preliminary evidence for a role of a genetic predisposition to more intense inflammatory responses in objective neurocognitive disturbances during acute infections. These associations require replication in a larger sample size.

Endotoxin-induced experimental systemic inflammation in humans: a model to disentangle immune-to-brain communication

Systemic inflammation is among the most prominent and most frequently observed responses of the immune system. Over the past decades, it has become clear that inflammatory cytokines not only affect immune and metabolic functions but also cause a wide range of behavioral and mood changes. Based on experimental findings in animals and observations in clinical populations it has been hypothesized that inflammation-induced neurocognitive changes contribute to the pathophysiology of neuropsychiatric diseases. However, since certain aspects of human behavior cannot be modeled in laboratory animals, there is a need for human models of systemic inflammation. In this review, we summarize recent studies employing administration of endotoxin as a model to induce transient systemic inflammation in healthy human subjects.

Experimental human endotoxemia enhances brain activity during social cognition

Acute peripheral inflammation with corresponding increases in peripheral cytokines affects neuropsychological functions and induces depression-like symptoms. However, possible effects of increased immune responses on social cognition remain unknown. Therefore, this study investigated the effects of experimentally induced acute inflammation on performance and neural responses during a social cognition task assessing Theory of Mind (ToM) ability. In this double-blind randomized crossover functional magnetic resonance imaging study, 18 healthy right-handed male volunteers received an injection of bacterial lipopolysaccharide (LPS; 0.4 ng/kg) or saline, respectively. Plasma levels of pro- and anti-inflammatory cytokines as well as mood ratings were analyzed together with brain activation during a validated ToM task (i.e. Reading the Mind in the Eyes Test). LPS administration induced pronounced transient increases in pro- (IL-6, TNF-α) and anti-inflammatory (IL-10, IL-1ra) cytokines as well as decreases in mood. Social cognition performance was not affected by acute inflammation. However, altered neural activity was observed during the ToM task after LPS administration, reflected by increased responses in the fusiform gyrus, temporo-parietal junction, superior temporal gyrus and precuneus. The increased task-related neural responses in the LPS condition may reflect a compensatory strategy or a greater social cognitive processing as a function of sickness.

At the extreme end of the psychoneuroimmunological spectrum: delirium as a maladaptive sickness behaviour response

Delirium is a common and severe neuropsychiatric syndrome characterised by acute deterioration and fluctuations in mental status. It is precipitated mainly by acute illness, trauma, surgery, or drugs. Delirium affects around one in eight hospital inpatients and is associated with multiple adverse consequences, including new institutionalisation, worsening of existing dementia, and death. Patients with delirium show attentional and other cognitive deficits, altered alertness (mostly reduced, but some patients develop agitation and hyperactivity), altered sleep-wake cycle and psychoses. The pathways from the various aetiologies to the heterogeneous clinical presentations are hardly studied and are poorly understood. One of the key questions, which research is only now beginning to address, is how the factors determining susceptibility interact with the stimuli that trigger delirium. Inflammatory signals arising during systemic infection evoke sickness behaviour, a coordinated set of adaptive changes initiated by the host to respond to, and to counteract, infection. It is now clear that the same systemic inflammatory signals can have severe deleterious effects on brain function when occuring in old age or in the presence of neurodegenerative disease. Multiple animal studies now show that even mild acute systemic inflammation can induce exaggerated sickness behaviour responses and cognitive dysfunction in aged animals or those with prior degenerative pathology when compared to young and/or healthy controls. These findings appear highly promising in understanding aspects of delirium. In this review our aim is to describe and assess the parallels between exaggerated sickness behaviour in vulnerable animals and delirium in older humans. We discuss inflammatory and stress-related triggers of delirium in the context of new animal models that allow us to dissect some aspects of the mechanisms underpinning these episodes. We discuss some differences between the sickness behaviour syndrome model and delirium in the context of the complexity in the latter due to other factors such as prior pathology, psychological stress and drug effects. We conclude that, with appropriate caveats, the study of sickness behaviour in the vulnerable brain offers a promising route to uncover the mechanisms of this common and serious unmet medical need.

Cognitive dysfunction with aging and the role of inflammation

As the average lifespan continues to climb because of advances in medical care, there is a greater need to understand the factors that contribute to quality of life in the elderly. The capacity to live independently is highly significant in this regard, but is compromised by cognitive dysfunction. Aging is associated with decreases in cognitive function, including impairments in episodic memory and executive functioning. The prefrontal cortex appears to be particularly vulnerable to the effects of advancing age. Although the mechanism of age-related cognitive decline is not yet known, age-related inflammatory changes are likely to play a role. New insights from preclinical and clinical research may give rise to novel therapeutics which may have efficacy in slowing or preventing cognitive decline with advancing age.

Placebo effects on the immune response in humans: the role of learning and expectation

Placebo responses are primarily mediated via two neuropsychological mechanisms: patients’ expectation towards the benefit of a treatment and associative learning processes. Immune functions, like other physiological responses, can be modulated through behavioral conditioning. However, it is unknown whether learned immune responses are affected by the number of re-expositions to the conditioned stimulus (CS) during evocation. Moreover, it is unclear whether immune functions can also be modulated through mere verbally induced expectation. In the experiments reported here, we investigated in healthy male volunteers with an established model of learned immunosuppression whether a single re-exposition to the CS is able to induce a behaviorally conditioned immunosuppression. This conditioned immunosuppression is reflected through a significantly decreased interleukin (IL)-2 production by anti-CD3 stimulated peripheral blood mononuclear cells. Our data revealed that in contrast to four CS re-expositions (control group n = 15; experimental group n = 17), a single CS re-exposition was not sufficient to significantly suppress IL-2 production (control group n = 9, experimental group n = 10). Furthermore, we could demonstrate that mere expectation of taking an immunosuppressant did not cause an immunosuppressive response (n = 8–9 per expectation condition). Together, these findings extend our knowledge about the kinetics and mechanisms of placebo-induced immunosuppression and provide therewith information for designing conditioning protocols, which might be employed as a supportive therapy in clinical settings.

Cytokines: how important are they in mediating sickness?

Sickness refers to a set of coordinated physiological and behavioral changes in response to systemic inflammation. It is characterized by fever, malaise, social withdrawal, fatigue, and anorexia. While these responses collectively represent a protective mechanism against infection and injury, increasing lines of evidence indicate that over-exaggerated or persistent sickness can damage the brain, and could possibly raise the risk to developing delirium. Therefore, a clear understanding in sickness will be beneficial. It has long been believed that sickness results from increased systemic cytokines occurring during systemic inflammation. However, in recent years more and more conflicting data have suggested that development of sickness following peripheral immune challenge could be independent of cytokines. Hence, it is confusing as to whether cytokines really do act as primary mediators of sickness, or if they are secondary to alternative inducing factor(s). In this review, we will (1) introduce the relationships between systemic inflammation, cytokines, sickness, and delirium, and (2) attempt to interpret the recent controversies.

Experimental endotoxemia as a model to study neuroimmune mechanisms in human visceral pain

The administration of bacterial endotoxin (i.e., lipopolysaccharide, LPS) constitutes a well-established experimental approach to study the effects of an acute and transient immune activation on physiological, behavioral, and emotional aspects of sickness behavior in animals and healthy humans. However, little is known about possible effects of experimental endotoxemia on pain in humans. This knowledge gap is particularly striking in the context of visceral pain in functional as well as chronic-inflammatory gastrointestinal disorders. Although inflammatory processes have been implicated in the pathophysiology of visceral pain, it remains incompletely understood how inflammatory mediators interact with bottom-up (i.e., increased afferent input) and top-down (i.e., altered central pain processing) mechanisms of visceral hyperalgesia. Considering the recent findings of visceral hyperalgesia after LPS application in humans, in this review, we propose that experimental endotoxemia with its complex peripheral and central effects constitutes an experimental model to study neuroimmune communication in human pain research. We summarize and attempt to integrate relevant animal and human studies concerning neuroimmune communication in visceral and somatic pain, discuss putative mechanisms, and conclude with future research directions.

Neural response to emotional stimuli during experimental human endotoxemia

Increases in peripheral cytokines during acute inflammation may affect various neuropsychological functions. The aim of this functional magnetic resonance imaging (fMRI) study was to investigate the effects of acute endotoxemia on mood and the neural response to emotionally aversive visual stimuli in healthy human subjects. In a double-blind, randomized crossover study, 18 healthy males received a bolus injection of bacterial lipopolysaccharide (LPS; 0.4 ng/kg) or saline. Plasma levels of pro- and anti-inflammatory cytokines and cortisol as well as mood ratings were analyzed together with the blood-oxygen-level dependent (BOLD) response during the presentation of aversive versus neutral pictures. Endotoxin administration induced pronounced transient increases in plasma levels of TNF-α, IL-1ra, IL-6, IL-10, and cortisol. Positive mood was decreased and state anxiety increased. In addition, activation of right inferior orbitofrontal cortex (OFC) in response to emotional visual stimuli was significantly increased in the LPS condition. Increased prefrontal activation during the presentation of emotional material may reflect enhanced cognitive regulation of emotions as an adaptive response during an acute inflammation. These findings may have implications for the putative role of inflammatory processes in the pathophysiology of depression.

Single-trial conditioning in a human taste-endotoxin paradigm induces conditioned odor aversion but not cytokine responses

Immunological responses to bacterial endotoxin can be behaviorally conditioned in rodents. However, it is unclear whether an acute systemic inflammatory response can be behaviorally conditioned in humans. Thus, in a double-blind placebo-controlled study, 20 healthy, male subjects received either a single injection of lipopolysaccharide (LPS) or saline together with a novel tasting beverage (conditioned stimulus, CS). Five days later, all subjects received a saline injection and were re-exposed to the CS. Blood was drawn prior to as well as 0.5, 1.5, 3, 4, 6, and 24 h after LPS administration or CS re-exposure. Endotoxin administration led to transient increases in plasma concentrations of interleukin (IL)-6, IL-10, and tumor necrosis factor (TNF)-α and to a significant rise in body temperature. Sole presentation of the CS during evocation did induce neither alterations in body temperature nor changes in plasma cytokine levels. However, subjects in the experimental group rated the smell of the CS significantly more aversive compared to the control group. Employing endotoxin as a US in a single trial taste-immune conditioning paradigm in humans shows a behaviorally conditioned smell aversion but no learned alterations in cytokine levels.

Acute experimental endotoxemia induces visceral hypersensitivity and altered pain evaluation in healthy humans

Growing evidence suggests that systemic immune activation plays a role in the pathophysiology of pain in functional bowel disorders. By implementing a randomized crossover study with an injection of endotoxin or saline, we aimed to test the hypothesis that endotoxin-induced systemic inflammation increases visceral pain sensitivity in humans. Eleven healthy men (mean ± standard error of the mean age 26.6 ± 1.1 years) received an intravenous injection of either lipopolysaccharide (LPS; 0.4 ng/kg) or saline on 2 otherwise identical study days. Blood samples were collected 15 min before and 1, 2, 3, 4, and 6h after injection to characterize changes in immune parameters including proinflammatory cytokines. Rectal sensory and pain thresholds and subjective pain ratings were assessed with barostat rectal distensions 2h after injection. LPS administration induced an acute inflammatory response indicated by transient increases in tumor necrosis factor alpha, interleukin 6, and body temperature (all P<.001). The LPS-induced immune activation increased sensitivity to rectal distensions as reflected by significantly decreased visceral sensory and pain thresholds (both P<.05) compared to saline control. Visceral stimuli were rated as more unpleasant (P<.05) and inducing increased urge to defecate (P<.01). Pain thresholds correlated with interleukin 6 at +1h (r=0.60, P<.05) and +3h (r=0.67, P<.05) within the LPS condition. This report is novel in that it demonstrates that a transient systemic immune activation results in decreased visceral sensory and pain thresholds and altered subjective pain ratings. Our results support the relevance of inflammatory processes in the pathophysiology of visceral hyperalgesia and underscore the need for studies to further elucidate immune-to-brain communication pathways in gastrointestinal disorders.

Aging microglia: relevance to cognition and neural plasticity

Over the years it has become evident that the immune system can affect the function of the central nervous system (CNS), including altering cognitive processes. The impact of immune activation on the CNS is particularly important for aged individuals, as the brain's resident immune cells, microglia, acquire a pro-inflammatory profile. The low-grade chronic neuroinflammation that develops with normal aging likely contributes to the susceptibility to cognitive deficits and a host of age-related pathologies. Understanding why microglia show increased inflammatory activity (i.e., neuroinflammation) and identifying effective treatments to reduce microglia activation is expected to have beneficial effects on cognitive performance and measures of neural plasticity. However, microglia also promote regeneration after injury. Therefore, effective treatments must dampen inflammatory activity while preserving microglia's neuroprotective function. Discovering factors that induce neuroinflammation and investigating potential preventative therapies is expected to uncover the ways of maintaining normal microglia activity in the aged brain.

Plasma noradrenaline and state anxiety levels predict placebo response in learned immunosuppression

Large interindividual differences exist in the presence and extent of placebo responses in both experimental and clinical studies, but little is known about possible predictors of these responses. We employed a behaviorally conditioned immunosuppression paradigm in healthy men to analyze predictors of learned placebo responses. During acquisition, the subjects received either the immunosuppressant cyclosporin A (n = 32) or a placebo (n = 14) (unconditioned stimuli (US)) together with a novel-tasting drink (conditioned stimulus (CS)). During evocation, the subjects were reexposed to the CS alone. In responders (n = 15), the CS alone caused a significant inhibition of interleukin (IL)-2 production by anti-CD3-stimulated peripheral blood T cells, closely mimicking the drug effect. Nonresponders (n = 17) did not show responses different from those of the controls. Multiple-regression analyses showed that baseline IL-2, plasma noradrenaline, and state anxiety predicted nearly 60% of the variance in the conditioned IL-2 response. These data provide first evidence for putative biological and psychological predictors of learned placebo responses.

Dose-dependent effects of endotoxin on neurobehavioral functions in humans

Clinical and experimental evidence document that inflammation and increased peripheral cytokine levels are associated with depression-like symptoms and neuropsychological disturbances in humans. However, it remains unclear whether and to what extent cognitive functions like memory and attention are affected by and related to the dose of the inflammatory stimulus. Thus, in a cross-over, double-blind, experimental approach, healthy male volunteers were administered with either placebo or bacterial lipopolysaccharide (LPS) at doses of 0.4 (n = 18) or 0.8 ng/kg of body weight (n = 16). Pro- and anti-inflammatory cytokines, norephinephrine and cortisol concentrations were analyzed before and 1, 1.75, 3, 4, 6, and 24 h after injection. In addition, changes in mood and anxiety levels were determined together with working memory (n-back task) and long term memory performance (recall of emotional and neutral pictures of the International Affective Picture System). Endotoxin administration caused a profound transient physiological response with dose-related elevations in body temperature and heart rate, increases in plasma interleukin (IL)-6, IL-10, tumor necrosis factor (TNF)-α and IL-1 receptor antagonist (IL-1ra), salivary and plasma cortisol, and plasma norepinephrine. These changes were accompanied by dose-related decreased mood and increased anxiety levels. LPS administration did not affect accuracy in working memory performance but improved reaction time in the high-dose LPS condition compared to the control conditon. In contrast, long-term memory performance was impaired selectively for emotional stimuli after administration of the lower but not of the higher dose of LPS. These data suggest the existence of at least two counter-acting mechanisms, one promoting and one inhibiting cognitive performance during acute systemic inflammation.

Comparison of mouse urinary metabolic profiles after exposure to the inflammatory stressors γ radiation and lipopolysaccharide

Metabolomics on easily accessible biofluids has the potential to provide rapid identification and distinction between stressors and inflammatory states. In the event of a radiological event, individuals with underlying medical conditions could present with similar symptoms to radiation poisoning, prominently nausea, diarrhea, vomiting and fever. Metabolomics of radiation exposure in mice has provided valuable biomarkers, and in this study we aimed to identify biomarkers of lipopolysaccharide (LPS) exposure to compare and contrast with ionizing radiation. LPS treatment leads to a severe inflammatory response and a cytokine storm, events similar to radiation exposure, and LPS exposure can recapitulate many of the responses seen in sepsis. Urine from control mice, LPS-treated mice, and mice irradiated with 3, 8 and 15 Gy of γ rays was analyzed by LCMS, and markers were extracted using SIMCA-P(+) and Random Forests. Markers were validated through tandem mass spectrometry against pure chemicals. Five metabolites, cytosine, cortisol, adenine, O-propanoylcarnitine and isethionic acid, showed increased excretion at 24 h after LPS treatment (P < 0.0001, 0.0393, 0.0393, <0.0001 and 0.0004, respectively). Of these, cytosine, adenine and O-propanoylcarnitine showed specificity to LPS treatment when compared to radiation. On the other hand, increased excretion of cortisol after LPS and radiation treatments indicated a rapid systemic response to inflammatory agents. Isethionic acid excretion, however, showed elevated levels not only after LPS treatment but also after a very high dose of radiation (15 Gy), while additional metabolites showed responsiveness to radiation but not LPS. Metabolomics therefore has the potential to distinguish between different inflammatory responses based on differential ion signatures. It can also provide quick and reliable assessment of medical conditions in a mass casualty radiological scenario and aid in effective triaging.

Dissociation between learning and memory impairment and other sickness behaviours during simulated Mycoplasma infection in rats

To investigate potential consequences for learning and memory, we have simulated the effects of Mycoplasma infection, in rats, by administering fibroblast-stimulating lipopepide-1 (FSL-1), a pyrogenic moiety of Mycoplasma salivarium. We measured the effects on body temperature, cage activity, food intake, and on spatial learning and memory in a Morris Water Maze. Male Sprague-Dawley rats had radio transponders implanted to measure abdominal temperature and cage activity. After recovery, rats were assigned randomly to receive intraperitoneal (I.P.) injections of FSL-1 (500 or 1000 μg kg(-1) in 1 ml kg(-1) phosphate-buffered saline; PBS) or vehicle (PBS, 1 ml kg(-1)). Body mass and food intake were measured daily. Training in the Maze commenced 18 h after injections and continued daily for four days. Spatial memory was assessed on the fifth day. In other rats, we measured concentrations of brain pro-inflammatory cytokines, interleukin (IL)-1β and IL-6, at 3 and 18 h after injections. FSL-1 administration induced a dose-dependent fever (∼1°C) for two days, lethargy (∼78%) for four days, anorexia (∼65%) for three days and body mass stunting (∼6%) for at least four days. Eighteen hours after FSL-1 administration, when concentrations of IL-1β, but not that of IL-6, were elevated in both the hypothalamus and the hippocampus, and when rats were febrile, lethargic and anorexic, learning in the Maze was unaffected. There also was no memory impairment. Our results support emerging evidence that impaired learning and memory is not inevitable during simulated infection.

Repeated recall of learned immunosuppression: evidence from rats and men

Akin to other physiological responses, the immune system can be modified, via Pavlovian or behavioral conditioning. It is unknown, however, whether and to what extent learned immune responses can be repeatedly recalled over time. Here we demonstrate in both rats and humans that repeated contingent pairing of a novel taste (conditioned stimulus, CS) together with the immunosuppressive drug cyclosporine A as unconditioned stimulus (US) leads to the acquisition of a learned immunosuppression. Sole presentation of the CS caused a significant inhibition of interleukin (IL)-2 and interferon (IFN)-γ production by rat splenic T cells and human peripheral T lymphocytes, closely mimicking the effect of the drug. More importantly, a comparable suppression of cytokine production was also observed after a second, unreinforced exposure to the CS that was separated from the first evocation by an interval of 6 (rats) or 11 (humans)days, respectively. Together, our findings demonstrate that a learned immunosuppression can be repeatedly recalled in both animals and humans, which is an important prerequisite for the implementation of conditioning paradigms as supportive therapy.

The evolving experience with therapeutic TNF inhibition in sepsis: considering the potential influence of risk of death

INTRODUCTION

Septic shock is highly lethal and its incidence is increasing. Although TNF-α plays a key role in sepsis pathogenesis, past efforts to therapeutically inhibit it had limited success. However, there is continued interest in such therapies and there are now ongoing Phase II sepsis trials testing the effects of AZD9773, a TNF-directed polyclonal antibody fragment preparation. Experience with anti-inflammatory agents suggested that their efficacy may relate to sepsis-associated risk of death.

AREAS COVERED

An overview of the biology of TNF and experimental data implicating TNF as a key mediator in sepsis pathogenesis; a review of the earlier clinical experience with anti-TNF therapies demonstrating that when examined across 12 trials, these agents had a highly consistent overall effect which although not reaching significance, was on the side of benefit; a review of data showing that sepsis-associated risk of death may influence the efficacy of anti-inflammatory agents like anti-TNF ones and a review of the rational and clinical experience to date with AZD9773 and its precursor, CytoFab.

EXPERT OPINION

Discusses variables that may need to be accounted for to maximize the success of clinical trials in sepsis testing agents that modulate host inflammation.