hou et al-2012-human psychopharmacology- clinical and experimental

REVIEW ARTICLE

A neuroimmunological perspective on anxiety disorders

Ruihua Hou* and David S. Baldwin

University Department of Psychiatry, Faculty of Medicine, University of Southampton, Southampton, UK

Objective Research into psychoneuroimmunology has led to substantial advances in our understanding of the reciprocal interactionsbetween the central nervous system and the immune system in neuropsychiatric disorders. To date, the presence of inflammatory responsesand the crucial role of cytokines in major depression have been addressed in numerous studies. However, neuroinflammatory hypotheses inanxiety disorders have been studied less extensively than in major depression. There is a high research need for better understanding of boththe heterogeneous role of specific cytokines in the control of anxious states and in different anxiety disorders and of the immunomodulatingeffects of antidepressants on anxiety.Methods Relevant literature was identified through a search of MEDLINE via PubMed. We discuss recent research on neuroimmunologyin anxiety and make methodological recommendations for future investigation of neuroinflammatory hypotheses in anxiety disorders.Results Some accumulating evidence has indicated modulatory effects of cytokines on neuronal communication and anxiety; however,research has not revealed consistent reproducible findings.Conclusions The availability of inflammatory biomarkers may provide an opportunity to identify patients via specific pathophysiologicalprocesses and to monitor therapeutic responses within relevant pathways. Further understanding of the neuroimmunological mechanisms tountangle the reciprocal associations between inflammation and anxiety is warranted. Copyright © 2011 John Wiley & Sons, Ltd.

key words—neuroimmunology; anxiety disorders; cytokines

INTRODUCTION

Over the past 30 years, research into psychoneuroim-munology, the study of neural-endocrine-immune sys-tem interactions, has led to substantial advances in ourunderstanding of the reciprocal interactions betweenthe central nervous system (CNS) and the immunesystem in neuropsychiatric disorders (Ader et al.,1995; Raison et al., 2006; Leonard and Myint, 2009;Miller et al., 2009). Experimental and clinical researchreveals the pivotal roles of cytokines signalling thebrain to produce neurochemical, neuroendocrine, neu-roimmune and behavioural changes (Kronfol andRemick, 2000; Maier, 2003; Dantzer et al., 2008b;Loftis et al., 2010; Capuron and Miller, 2011). Greaterunderstanding of the role of cytokines in the bidi-rectional communications between the nervous andimmune systems has inspired integrative, explanatorymodels for neuropsychiatric disorders.

METHODS

The source of literature was the electronic databaseMEDLINE (1950–2011). The initial search strategyincluded the use of a combination of the followingthesaurus terms: ‘generalised anxiety disorder’, ‘ panicdisorder’, ‘phobias’, ‘obsessive-compulsive disorder’,‘post-traumatic stress disorder’ and ‘inflammation’/‘cytokine’; however, as there is only limited researchin this field, we expanded our search by addingthe terms ‘anxiety’, ‘anxious state’ and ‘depression’.The inclusion criteria included: (i) studies examininginflammatory mechanisms underlying anxious stateor anxiety in patients with physical conditions and/orpatients with anxiety disorders; (ii) review articles oninflammation and anxiety; (iii) adult participants only;and (iv) articles written in English. The exclusioncriteria included: (i) letters to editors and editorialswithout data; and (ii) studies outside the timescale(1950–2011), as these were not available electroni-cally. On the basis of the inclusion and exclusion cri-teria, we reviewed the titles of all citations andretrieved relevant abstracts for more detailed evalua-tion. Where there was uncertainty, we studied the full

*Correspondence to: Dr. R. Hou, University Department of Psychiatry,Clinical and Experimental Sciences, Faculty of Medicine, University ofSouthampton, Academic Centre, College Keep, 4–12 Terminus Terrace,Southampton SO14 3DT, UK. Tel: +44 (0)23 8071 8537; Fax: +44 (0)238071 8532. E-mail: [email protected]

Received 15 August 2011Accepted 18 November 2011Copyright © 2011 John Wiley & Sons, Ltd.

human psychopharmacologyHum. Psychopharmacol Clin Exp 2012; 27: 6–14.Published online 26 December 2011 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hup.1259

paper. We also hand-searched the reference list of rele-vant studies and reviews to aid identification of furtherstudies. We identified 584 references and 103 wereincluded in this review.

CYTOKINES ANDPSYCHONEUROIMMUNOLOGY

Just as the nervous and endocrine systems conveyinformation to the immune system via neurotrans-mitters and hormones, the immune system conveysinformation to the nervous and endocrine systems viacytokines and chemokines (Leonard and Myint,2009). Cytokines are soluble bioactive mediatorsreleased by various cell types both at the periphery(such as monocytes and macrophages) and in the brain(such as microglia, astrocytes, oligodendroglia andneurons), which operate within a complex networkand act either synergistically or antagonistically. Theyare generally associated with inflammation, immuneactivation and cell differentiation or death and includeinterleukins (ILs), tumour necrosis factors (TNFs),interferons (IFNs), chemokines and growth factors(such as brain-derived neurotrophic factor) (Allan andRothwell, 2003). Based on the functional profile ofan immune response, cytokine production is orche-strated by type 1 helper cells (Th1) that generally med-iate a pro-inflammatory cellular immune response, andtype 2 helper cells (Th2) that enhance humoralimmune reactions. Pro-inflammatory cytokines, suchas IL-1, IL-6, INF-g and TNFa, enhance the immuneresponse to help speed the elimination of pathogensand the resolution of the inflammatory challenge;anti-inflammatory cytokines, such as IL-4, IL-10 andIL-13, serve to dampen the immune response viadecreasing cell function and synthesis of pro-inflammatory cytokines (Kronfol and Remick, 2000).The balance between Th1 and Th2 is an essentialdeterminant in containing the inflammatory response(Dantzer et al., 2008a), and a delicate balance ofpro-inflammatory and anti-inflammatory cytokines isrequired for normal regulation of neuropsychiatricfunctioning (Loftis et al., 2010). In addition to theTh1 and Th2 cytokines, there is evidence that T helpertype 3 cells exert their action primarily by secretingtransforming growth factor beta-1 that facilitates abalance between the Th1 and Th2 arms of cellularimmunity(Myint et al., 2005).Cytokines have been implicated in the modulation of

neuronal activity in regions such as the amygdala,hippocampus, hypothalamus and cerebral cortex(Besedovsky and del Rey, 1996; Elenkov et al.,2000). Peripheral cytokine signals can reach the brain

through humoral, neural and cellular pathways via fivepossible mechanisms: (i) passage of cytokines through‘leaky’ regions of the blood–brain barrier; (ii) activetransport via saturable cytokine-specific transportmolecules on brain endothelium; (iii) activation ofendothelial cells and inducing the release of secondmessengers such as prostaglandins and nitric oxide;(iv) transmission via afferent nerve fibres such as thevagus nerve; and (v) entry into the brain parenchymavia peripherally activated monocytes (Capuron andMiller, 2011).Physical or psychological stress and infection or

inflammation within the brain or the periphery canmodulate cytokine expression in the CNS (Lucaset al., 2006). An acute immune challenge triggers anadaptive, temporary and controlled reaction of theCNS. However, when immune challenge becomeschronic and/or dysregulated, because of chronic medi-cal illness, chronic stress, or cytokine treatments, theresultant chronic inflammatory response contributesto the development of maladaptive behavioural symp-toms and neuropsychiatric disorders. Cytokines maylead to behavioural changes through their effects on:(i) neurotransmitter function: cytokines can alter themetabolism of serotonin, dopamine and glutamate(Moron et al., 2003; Cai et al., 2005; Ida et al.,2008); (ii) neuroendocrine activity: cytokines can alterthe function of hypothalamic–pituitary–adrenal axisvia stimulant effects on the expression and release ofcorticotropin-releasing hormone, adrenocorticotropichormone and cortisol (Pariante and Miller, 2001;Raison et al., 2010); (iii) neurogenesis: cytokinesmay affect neurogenesis via activation of nuclear fac-tor kappa-light-chain-enhancer of activated B cells(NF-kB) (Ben Menachem-Zidon et al., 2008); and(iv) neurocircuitry: the basal ganglia and subgenualand dorsal aspects of the anterior cingulate cortex aretarget regions of cytokines (Brydon et al., 2008;Miller, 2009). Behavioural consequences of theseeffects include depression, anxiety, fatigue, psycho-motor slowing, anorexia, cognitive dysfunction andsleep impairment; all symptoms that overlap withthose which characterize a range of neuropsychiatricdisorders (Capuron and Miller, 2011). Therefore,increased recognition of the role of cytokines in theCNS has opened important new areas for investigatingthe origin and treatment of neuropsychiatric disorders.

ANXIETY DISORDERS AND AFFECTIVESPECTRUM DISORDERS

In general terms, an anxiety disorder involves anexcessive or inappropriate state of arousal characterized

7neuroimmunology of anxiety disorders

Copyright © 2011 John Wiley & Sons, Ltd. Hum. Psychopharmacol Clin Exp 2012; 27: 6–14.DOI: 10.1002/hup

by feelings of apprehension, uncertainty, or fear. Inclinical practice, anxiety disorders are classifiedaccording to the severity and duration of theirsymptoms and specific behavioural characteristics.Common categories include generalized anxiety disor-der (GAD), panic disorder (PD), phobias, obsessive-compulsive disorder (OCD) and post-traumatic stressdisorder (PTSD). GAD, the most common impairinganxiety disorder, is a prevalent and disabling chroniccondition characterized by excessive, uncontrollableand often irrational worry about everyday things.GAD affects approximately 1.9–5.1% of the generalpopulation and 8% of patients in primary care(Wittchen, 2002). The chronic exaggerated worryingtypically interferes with daily functioning and reducesquality of life. Clinical research suggests that impair-ments of GAD are similar in magnitude to those ofmajor depression (Hoffman et al., 2008). Because ofhigh degrees of overlapping comorbidities and com-mon drug efficacies, anxiety disorders are consideredone of a family of related conditions referred to asthe ‘affective spectrum disorders’ that also includemajor depressive disorder, fibromyalgia, irritable bowelsyndrome and chronic fatigue syndrome (Hudsonet al,. 2004). Accumulating research evidence stronglysuggests that inflammation is an important componentin the pathogenesis of affective spectrum disorders,particularly in major depression.

NEUROIMMUNOLOGY IN MAJOR DEPRESSION

The most studied and documented area for investigat-ing neuroimmunological factors in neuropsychiatricdisorders is major depression. Because signs ofimmune disturbances in depression were first reported20 years ago (Maes et al., 1990; Maes et al., 1991;Maes et al., 1992a, 1992b), the presence of inflamma-tory responses and the crucial role of cytokines inmajor depression have been addressed in numerousstudies, and our understanding of depression hasmoved far beyond the ‘monoamine hypothesis’. Arecent meta-analysis of 24 studies reports significantlyhigher concentrations of the pro-inflammatory cyto-kines (such as TNF-a and IL-6) in depressed subjectscompared with control subjects and stresses thatdepression is accompanied by activation of theimmune system (Dowlati et al., 2010). In addition,the immune dysregulation hypothesis of major depres-sion, which considers an altered balance of Th1 pro-inflammatory cytokines and Th2 anti-inflammatorycytokines with Th1 pathway being predominant overTh2 pathway, has also been proposed (Brietzkeet al., 2009; Gabbay et al., 2009; Leonard and Myint,

2009). Both external and internal stressors may triggerdepression via acquired (e.g. T and B cell) and/orinnate (e.g. macrophage) immune responses (Mossneret al., 2007), whereas the hypothalamic–pituitary–adrenal axis and serotonin play pivotal role in theconnexion between stress and depression (Leonardand Myint 2009).

IMMUNOMODULATING EFFECTS OFANTIDEPRESSANTS

Antidepressants have demonstrated efficacy in theaffective spectrum disorders, and their marked effectson the production of cytokines have been highlighted(Maes et al., 1999; Kubera et al., 2000a, 2000b;Kubera et al., 2001; Kenis and Maes, 2002; Kuberaet al., 2004; Kubera et al., 2005). Both preclinicaland clinical data demonstrate that antidepressant treat-ments are associated with decreases in inflammatorymarkers (Miller et al., 2009). Many antidepressantshave specific anti-inflammatory effects (Carvalho andPariante, 2008; Lim et al., 2009) and significant immu-noregulatory activities, such as reducing the number ofTh1 cells secreting IFN-g, altering the production ofIL-6 and IL-10 and inhibiting IFN-g-induced micro-glial production of IL-6 and nitric oxide (Hashiokaet al., 2007; Loftis et al., 2010). The immunomodula-tory effects vary depending on the antidepressant usedand the period of administration. Clinical data alsoindicate that immune activation in patients withdepression is associated with resistance to treatmentwith traditional antidepressants; therefore inflamma-tory biomarkers may provide an indicator of treatmentresponse (O’Brien et al., 2007; Eller et al. 2008, 2009).

NEUROIMMUNOLOGY IN ANXIETY

Apart from the high co-morbidity of anxiety disordersand major depression, similar treatment effects of anti-depressants in anxiety and depression suggest thatsimilar neurobiological substrates may be affected inboth conditions. In addition, the pronounced responseof central and peripheral cytokines to stress hasprompted further interest in the role of cytokines inthe pathogenesis of anxiety disorders. Both experimentaland clinical evidence shows that stress can produce aninflammatory reaction, indicated by a rise in circulatingconcentrations of pro-inflammatory cytokines. Chronicstress, by initiating changes in the hypothalamic–pituitary–adrenal axis and the immune system, cantherefore act as a trigger for anxiety and depression(Leonard and Myint, 2009).

8 r. hou and d. baldwin


Inflammatory responses and anxious state

Experimental animal studies reveal that increasedexpression of cytokines in the periphery is associatedwith enhanced anxiety in autoimmune mice (Blutheet al., 1992; Sakic et al., 1994; Schrott and Crnic,1996) that mice over-expressing IL-6 or TNF likewisereveal an anxiogenic phenotype (Connor and Leonard,1998; Fiore et al., 1998) and that deletion of the geneencoding IFN-g leads to heightened anxiety (Fioreet al., 1998; Kustova et al., 1998; Lesch, 2001).Human studies indicate that a chronic anxious state hasa deleterious impact on immune function (Boscarino,2004; Schneiderman et al., 2005; Zhou et al., 2005;Godbout and Glaser, 2006), leading to damaged cellularand humoral immune responses (Koh and Lee, 2004;Zhou et al., 2005; Arranz et al., 2007) and increasedincidence of viral and bacterial infections (Takkoucheet al., 2001; Aviles et al., 2004). Anxiety is also relatedto an impaired immune response to several antiviral/bacterial vaccines, such as hepatitis B virus (Jabaaijet al., 1996), pneumococcal bacteria (Glaser et al.,2000), rubella virus (Morag et al., 1999), meningitisvirus (Burns et al., 2002) and influenza virus (Vedharaet al., 1999; Vedhara et al., 2002; Miller et al., 2004).The associations between inflammatory responses

and anxious state have been evaluated in physicallyill patients with known activation of inflammatoryresponses. Thus, an early report showed an associationbetween IFN-g levels and anxiety in systemic lupuserythematosus (Figueiredo-Braga et al., 2009), andboth depressed and anxious states are associated withsignificantly lower ratios of IFN-g (TH1 cells) versusIL-4 (TH2 cells) among ovarian cancer patients(Lutgendorf et al., 2008). Patients receiving IFN-afor hepatitis C exhibit significantly greater activa-tion in the dorsal anterior cingulate cortex that is asso-ciated with increased anxiety and arousal comparedwith control subjects (Capuron et al., 2005; Harrisonet al., 2009).Whereas studies of subclinical psychoneuroimmu-

nological variations in healthy populations demon-strate that a positive correlation between anxiousstate and inflammation and coagulation markers, suchas TNFa, IL-6 and C-reactive protein (CRP) (Maeset al., 1998; Pitsavos et al., 2006; Arranz et al.,2007), the study by Zorrilla found that male subjectswho were characteristically more anxious had signifi-cantly lower levels of circulating IL-1b compared withless anxious subjects, indicating a negative correlate(Zorrilla et al., 1994). The inconsistent results maybe due to small sample size and gender difference,indicating these results need to be confirmed in a larger

sample exploring also gender differences. When nor-mal volunteers were injected with lipopolysaccharide,a well-known immune activator, they exhibited acuteincreases in symptoms of anxiety (Reichenberg et al.,2001). When compared with non-anxious participants,clinically anxious participants exhibited significantlyhigher levels of IL-6, independent of depressive symp-toms that indicate an anxiety-specific effect on inflam-matory activity and highlights a pathway by whichanxiety may increase risk for inflammatory diseases(O’Donovan et al., 2010).

Inflammatory responses in anxiety disorders

Studies in clinical populations have shown that a highlevel of anxiety is associated with impaired cellularimmunity. Anxiety disorders of special immunologicalinterest are PTSD, PD, OCD and GAD.

Post-traumatic stress disorder. Anxiety is associatedwith increased circulating levels of CRP, IL-6,TNF-a, IL-1b and IL-8 in patients with PTSD, whichmay be due to an insufficient regulation of immunefunction (Rohleder et al., 2004; Gill et al., 2008; Gillet al., 2009; von Kanel et al., 2010; Pace and Heim,2011). The strong evidence for systemic inflamma-tion and deleterious health consequences in PTSDhas been most well-documented disorders, and arecent review has suggested that cellular immunityis implicated in PTSD risk and resilience (Bakeret al., 2011).

Panic disorder. Alterations in circulating levels ofIL-1 have previously been demonstrated in patientswith PD (Brambilla et al., 1994). Reduced anxiety levelby therapeutic interventions (cognitive behavioural ther-apy and the anxiolytic ethyl loflazepate) has been foundto be associated with decreased cell-mediated immunity(Koh and Lee, 2004). A prospective experimentallyinduced stress study demonstrates that 35% CO2 inhala-tion induced significantly higher levels of anxiety in PDpatients as compared with the control subjects, but nodifferences in immune parameters were found, either inbasal conditions or after experimental panic induction(van Duinen et al., 2008).

Obsessive-compulsive disorder. Immunological find-ings from OCD studies are equivocal. Whereas severalreports found no IL-6 differences in OCD, either inplasma or in cerebrospinal fluid (CSF) (Monteleoneet al., 1998; Carpenter et al., 2002), a decrease in pro-duction of TNF-a and the low levels of lipopolysac-charide-stimulated IL-6 have been reported (Denyset al., 2004; Fluitman et al., 2010), which is in contrast



with the study by Konuk et al. (Konuk et al., 2007)in which their findings demonstrated higher levelsof TNF-a and IL-6 in OCD patients, which maybe due to a high co-morbidity of depressive disor-der. Some research has suggested that the pathophy-siology of OCD differs from that of other anxietydisorders, and different cytokine profiles betweenOCD and other anxiety-related disorders are in lightof the ongoing debate on the position of OCDwithin the group of anxiety disorders (Bartz andHollander, 2006).

Generalized anxiety disorder. An evaluation of majordepressive disorder and GAD in stable coronary heartdisease patients indicates a significant associationbetween CRP and GAD (Bankier et al., 2008) andimplies that different inflammatory responses mayoccur in these two conditions. Two recent studiesconducted in 20 patients with GAD and 20 healthycontrols demonstrated a T cell functional dysregulationin individuals with GAD via examining the T cellprofile following in vitro activation in cultures. Thecytokine profile in GAD revealed that Th1 and Th2deficiencies were associated with dominant Th17phenotype, which was enhanced by substance P(Vieira et al., 2010; Barros et al., 2011). The pro-foundly altered composition of the peripheral T cellcompartment might cause a state of compromisedimmune responsiveness, which may explain why someanxious patients show an increased susceptibility toinfections, inflammatory and autoimmune diseases.Whereas important, the significance of these findingsis uncertain, partly because of the small sample sizeof most studies.

Limitations of current research

To date, neuroinflammatory hypotheses in anxiety dis-orders have been studied less extensively than in majordepression. Because of a reliance on cross-sectionalstudy design, small sample sizes (an average of20–40), the lack of standardized measurements andhigh co-morbidity with depression, findings are notconsistently observed across studies. There is a highresearch need for better understanding of both theheterogeneous role of specific cytokines in the controlof anxious states and in different anxiety disordersand of the immunomodulating effects of antidepres-sants on anxiety. Moreover, whether anxiety is asso-ciated with inflammatory activity either through aspecific anxiety pathway or through a more generalnegative emotionality pathway remains an interestingarea to be explored.

RECOMMENDATIONS FOR FUTURE RESEARCH

Suggestions for future investigation of neuroinflamma-tory hypotheses in anxiety disorders are:

(1) Longitudinal study design

Given well-known fluctuations in behavioural symp-toms as well as immunologic status, research intoneuroinflammation in anxiety should employ a longi-tudinal study design in which repeated samples shouldbe taken from the same subjects over a period ideallyfree of inflammatory clinical events. So far, moststudies rely on cross-sectional design with immuneparameters measured by a single evaluation at somegiven time point.

(2) Standardized measures of inflammatory biomarkers

Inflammatory biomarkers should be standardized toallow comparison between studies:

(a) Assays based both on immunological detection ofpeptides (immunoassays) and biological responses(bioassays) are recommended to measure cytokineprofile. Results with multiplex assays should beconfirmed with standard ELISA assays. However,multiplex assays are considered to be ideal forassessing relevant inflammatory molecules insupernatants of stimulated cells in the context ofhypothesis generation.

(b) Inflammation can fluctuate rapidly according toenvironmental and internal factors, such as circa-dian rhythm (Coogan and Wyse, 2008). Thus, thetiming of blood or tissue sampling may affect thecytokine profile. Ambulatory monitoring techni-ques, also known as the Experience SamplingMethod and Ecological Momentary Assessment,are recommended for studying acute inflammatoryfluctuations (Tournier et al., 2003).

(c) Cytokine expression can vary depending onwhethermeasurements are obtained from plasma, serum,CSF or stimulated peripheral blood samples, thusthe source of cytokines should be clearly defined.

(d) Proper sample handling and storage is important forreliable measurement of circulating ‘cytokines’. Ithas been suggested that serum and plasma shouldbe ‘separated’ immediately after sampling is takenand frozen at �80 �C after blood draw. A delay ofsample processing may cause degradation, absorp-tion or cellular production of cytokine, leading tovariable cytokine expression. Cytokines are stablefor a period of 2 years of storage at�80 �C (de Jageret al., 2009).

(e) As indicated earlier, a delicate balance betweenpro-inflammatory and anti-inflammatory cytokines



is required for normal regulation of neuropsychia-tric functioning (Loftis et al., 2010). It would beuseful to test the pattern of immune dysregula-tion, such as whether an anxious state is character-ized by a shift in the pro/anti-inflammatory or Th1/Th2 cytokine ratio.

(3) Standardized behavioural assessments and recordof inflammatory events

Clinical tools for assessing anxious state andrecording inflammatory events should be standar-dized and made available for clinical research.Appropriate monitoring techniques should be devel-oped for assessing short-term fluctuations in clinicalsymptoms.

(4) Vulnerability factors

A variety of vulnerability factors and demographiccharacteristics may potentially affect the directionand magnitude of cytokine changes in response tostress/anxious state including:

(a) Body mass index (BMI): BMI has been shown tocorrelate with increased peripheral markers ofinflammation, in part related to the capacity ofadipose tissue to produce IL-6 and other cytokines(Vgontzas et al., 2000; Kern et al., 2001). Obesityis associated with low-grade inflammatory pro-cesses, with increased circulating levels ofacute phase proteins (CRP in particular) andpro-inflammatory cytokines.

(b) Ageing: Epidemiological studies indicate that anxi-ety disorders are more common among older ageindividuals (Wolitzky-Taylor et al., 2010). Normalageing is characterized by chronic low-gradeinflammatory factors, with an over-expression ofperiphery pro-inflammatory cytokines and impairedpro-inflammatory versus anti-inflammatory balance(Capuron and Miller, 2011).

(c) Medical conditions: The presence of medicalconditions, in particular, the presence of acute orchronic inflammatory challenge.

(d) Childhood history: Adverse childhood experienceshave been described as major environmental riskfactors and the extent of prior stress exposureshould be considered.

(e) Exercise: Skeletal muscle has now been viewed asan immunogenic organ that by contraction stimu-lates the production of cytokines, such as IL-6.Exercise affects circulating cytokine levels, andthis impact has been a remarkably consistentresearch finding (Suzuki et al., 2002; Woods et al.,2006; Pedersen and Febbraio, 2008).

(5) Effects of medications

Anti-inflammatory effects of many antidepressantmedications have been indicated in numerous studies(Hashioka et al., 2007; Carvalho and Pariante, 2008;Lim et al., 2009; Loftis et al., 2010), controlling forthe use of antidepressants and for any other medica-tions known to affect inflammation, such as steroids,should be considered when assessing the cytokine pro-file. In particular, the immunomodulatory effects mayvary depending on different antidepressants used andthe treatment duration.

(6) Development of novel measurement strategies

Novel approaches to measure inflammatory biomar-kers are needed to improve the efficiency and ease oftesting of relevant clinical populations. Such techni-ques would include development of blood spottingapproaches, measurement of inflammatory markers inother body compartments and the use of in vitrochallenge strategies to reveal altered inflammatoryresponse that might appear within normal limits with-out perturbation. In addition, application of computer-ized approaches to data collection and the use ofmultilevel statistical techniques for processing timeseries data are recommended. Sampling and biomarkercharacterization of other bodily compartments includ-ing CSF, joint spaces and amniotic fluid would be ofrelevance. As ‘peripheral’ cytokines can reach thebrain, further development of strategies should includethe measure of inflammatory biomarkers in the CSF.To gain productive insights into interactions between

the immune system and the central nervous system andhow these affect behaviour in anxiety disorders, withthe goal of identifying new potential therapeutic tar-gets, a potential research programme within a neuro-immunology facility could include investigationswhich: (i) identify modifiable inflammatory processesor molecules in patients with anxiety disorders; (ii)assess the effects of conventional medications (suchas antidepressants) on these processes or molecules;and (iii) based on experimental medicine paradigms,develop behavioural and/or psychopharmacologicalintervention approach to alter neuroimmune mechan-isms and improve clinical outcomes.

CONFLICT OF INTEREST

The authors have declared no conflict of interest.

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