Network beyond IDO in psychiatric disorders: revisiting neurodegeneration hypothesis

(13)C-tryptophan breath test detects increased catabolic turnover of tryptophan along the kynurenine pathway in patients with major depressive disorder

Altered tryptophan-kynurenine (KYN) metabolism has been implicated in major depressive disorder (MDD). The L-[1-(13)C]tryptophan breath test ((13)C-TBT) is a noninvasive, stable-isotope tracer method in which exhaled (13)CO2 is attributable to tryptophan catabolism via the KYN pathway. We included 18 patients with MDD (DSM-IV) and 24 age- and sex-matched controls. (13)C-tryptophan (150 mg) was orally administered and the (13)CO2/(12)CO2 ratio in the breath was monitored for 180 min. The cumulative recovery rate during the 180-min test (CRR0-180; %), area under the Δ(13)CO2-time curve (AUC; %*min), and the maximal Δ(13)CO2 (Cmax; %) were significantly higher in patients with MDD than in the controls (p = 0.004, p = 0.008, and p = 0.002, respectively). Plasma tryptophan concentrations correlated negatively with Cmax in both the patients and controls (p = 0.020 and p = 0.034, respectively). Our results suggest that the (13)C-TBT could be a novel biomarker for detecting a subgroup of MDD with increased tryptophan-KYN metabolism.

Beyond Urinary Tract Infections (UTIs) and Delirium: A Systematic Review of UTIs and Neuropsychiatric Disorders

Urinary tract infections (UTIs) are among the most common bacterial infections. Although comorbid UTI in geriatric patients with delirium or dementia is well known, the prevalence and scope of the association with other neuropsychiatric disorders is unclear. We performed a systematic review of the association between UTIs and delirium, dementia, psychotic disorders, and mood disorders in hospitalized patients. We identified studies by searching PubMed, PsycInfo, and Web of Knowledge, and the reference lists of identified studies and review papers. Seventeen publications met the inclusion criteria. The primary findings were: (1) 88% of publications reported a positive association between UTIs and neuropsychiatric disorders; (2) 47% reported that the clinical course of a neuropsychiatric disorder may be precipitated or exacerbated by a UTI; (3) the mean weighted prevalence of UTIs in subjects was 19.4% for delirium, 11.2% for dementia, 21.7% for nonaffective psychotic disorders, and 17.8% for mood disorders. Our findings, which must be interpreted carefully given the heterogeneity among the studies, suggest that UTIs are highly comorbid in hospitalized patients and may precipitate or exacerbate some neuropsychiatric disorders. The association extends beyond geriatric patients with delirium, affects males and females, and includes adults with psychotic and mood disorders. These findings underscore the important interface between physical and mental health. Potential underlying mechanisms are also reviewed, including complex interactions between the immune system and the brain.

Molecular Targets of Cannabidiol in Neurological Disorders

Cannabis has a long history of anecdotal medicinal use and limited licensed medicinal use. Until recently, alleged clinical effects from anecdotal reports and the use of licensed cannabinoid medicines are most likely mediated by tetrahydrocannabinol by virtue of: 1) this cannabinoid being present in the most significant quantities in these preparations; and b) the proportion:potency relationship between tetrahydrocannabinol and other plant cannabinoids derived from cannabis. However, there has recently been considerable interest in the therapeutic potential for the plant cannabinoid, cannabidiol (CBD), in neurological disorders but the current evidence suggests that CBD does not directly interact with the endocannabinoid system except in vitro at supraphysiological concentrations. Thus, as further evidence for CBD's beneficial effects in neurological disease emerges, there remains an urgent need to establish the molecular targets through which it exerts its therapeutic effects. Here, we conducted a systematic search of the extant literature for original articles describing the molecular pharmacology of CBD. We critically appraised the results for the validity of the molecular targets proposed. Thereafter, we considered whether the molecular targets of CBD identified hold therapeutic potential in relevant neurological diseases. The molecular targets identified include numerous classical ion channels, receptors, transporters, and enzymes. Some CBD effects at these targets in in vitro assays only manifest at high concentrations, which may be difficult to achieve in vivo, particularly given CBD's relatively poor bioavailability. Moreover, several targets were asserted through experimental designs that demonstrate only correlation with a given target rather than a causal proof. When the molecular targets of CBD that were physiologically plausible were considered for their potential for exploitation in neurological therapeutics, the results were variable. In some cases, the targets identified had little or no established link to the diseases considered. In others, molecular targets of CBD were entirely consistent with those already actively exploited in relevant, clinically used, neurological treatments. Finally, CBD was found to act upon a number of targets that are linked to neurological therapeutics but that its actions were not consistent withmodulation of such targets that would derive a therapeutically beneficial outcome. Overall, we find that while >65 discrete molecular targets have been reported in the literature for CBD, a relatively limited number represent plausible targets for the drug's action in neurological disorders when judged by the criteria we set. We conclude that CBD is very unlikely to exert effects in neurological diseases through modulation of the endocannabinoid system. Moreover, a number of other molecular targets of CBD reported in the literature are unlikely to be of relevance owing to effects only being observed at supraphysiological concentrations. Of interest and after excluding unlikely and implausible targets, the remaining molecular targets of CBD with plausible evidence for involvement in therapeutic effects in neurological disorders (e.g., voltage-dependent anion channel 1, G protein-coupled receptor 55, CaV3.x, etc.) are associated with either the regulation of, or responses to changes in, intracellular calcium levels. While no causal proof yet exists for CBD's effects at these targets, they represent the most probable for such investigations and should be prioritized in further studies of CBD's therapeutic mechanism of action.

Cerebrospinal fluid monoamine metabolite concentrations as intermediate phenotypes between glutamate-related genes and psychosis

Glutamate-related genes have been associated with schizophrenia, but the results have been ambiguous and difficult to replicate. Homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA) and 3-methoxy-4-hydroxyphenylglycol (MHPG) are the major degradation products of the monoamines dopamine, serotonin and noradrenaline, respectively, and their concentrations in the cerebrospinal fluid (CSF), mainly HVA, have been associated with schizophrenia. In the present study, we hypothesized that CSF HVA, 5-HIAA and MHPG concentrations represent intermediate phenotypes in the association between glutamate-related genes and psychosis. To test this hypothesis, we searched for association between 238 single nucleotide polymorphisms (SNPs) in ten genes shown to be directly or indirectly implicated in glutamate transmission and CSF HVA, 5-HIAA and MHPG concentrations in 74 patients with psychotic disease. Thirty-eight nominally significant associations were found. Further analyses in 111 healthy controls showed that 87% of the nominal associations were restricted to the patients with psychosis. Some of the psychosis-only-associated SNPs found in the d-amino acid oxidase activator (DAOA) and the kynurenine 3-monooxygenase (KMO) genes have previously been reported to be associated with schizophrenia. The present results suggest that CSF monoamine metabolite concentrations may represent intermediate phenotypes in the association between glutamate-related genes and psychosis.

Increased IL-3 serum levels in chronic patients with schizophrenia: Associated with psychopathology

Schizophrenia is associated with the inflammation-related pathways, including aberrant cytokines levels. In this study, we examined the association of serum IL-3 levels with psychopahological symtoms in chronic schizophrenia. Serum IL-3 levels were assessed in 42 patients diagnosed with schizophrenia and compared to 43 healthy control subjects matched for age and gender. Schizophrenia symptomatology was assessed with the Positive and Negative Syndrome Scale (PANSS), and serum IL-3 levels were measured by sandwich enzyme-linked immunosorbent assay (ELISA). Our results showed that IL-3 levels were significantly increased in patients with chronic schizophrenia compared to healthy control subjects. Correlation analysis revealed a significant positive correlation between the IL-3 levels and the PANSS general subscore. Moreover, IL-3 levels were significantly positively correlated with depressive subscore. Our results suggested that IL-3 related pathway is associated with psychopathology of schizophrenia patients.

The transcription factor nuclear factor interleukin 6 mediates pro- and anti-inflammatory responses during LPS-induced systemic inflammation in mice

The transcription factor nuclear factor interleukin 6 (NF-IL6) plays a pivotal role in neuroinflammation and, as we previously suggested, hypothalamus-pituitary-adrenal-axis-activation. Here, we investigated its contribution to immune-to-brain communication and brain controlled sickness symptoms during lipopolysaccharide (LPS)-induced (50 or 2500 μg/kg i.p.) systemic inflammation in NF-IL6-deficient (KO) or wildtype mice (WT). In WT LPS induced a dose-dependent febrile response and reduction of locomotor activity. While KO developed a normal fever after low-dose LPS-injection the febrile response was almost abolished 3-7 h after a high LPS-dose. High-dose LPS-stimulation was accompanied by decreased (8 h) followed by enhanced (24 h) inflammation in KO compared to WT e.g. hypothalamic mRNA-expression including microsomal prostaglandin E synthase, inducible nitric oxide synthase and further inflammatory mediators, neutrophil recruitment to the brain as well as plasma levels of inflammatory markers such as IL-6 and IL-10. Interestingly, KO showed reduced locomotor activity even under basal conditions, but enhanced locomotor activity to novel environment stress. Hypothalamic-pituitary-adrenal-axis-activity of KO was intact, but tryptophan-metabolizing enzymes were shifted to enhanced serotonin production and reuptake. Overall, we showed for the first time that NF-IL6 plays a dual role for sickness response and immune-to-brain communication: acting pro-inflammatory at 8h but anti-inflammatory at 24 h after onset of the inflammatory response reflecting active natural programming of inflammation. Moreover, reduced locomotor activity observed in KO might be due to altered tryptophan metabolism and serotonin reuptake suggesting some role for NF-IL6 as therapeutic target for depressive disorders.

Neurodevelopmental Animal Models Reveal the Convergent Role of Neurotransmitter Systems, Inflammation, and Oxidative Stress as Biomarkers of Schizophrenia: Implications for Novel Drug Development

Schizophrenia is a life altering disease with a complex etiology and pathophysiology, and although antipsychotics are valuable in treating the disorder, certain symptoms and/or sufferers remain resistant to treatment. Our poor understanding of the underlying neuropathological mechanisms of schizophrenia hinders the discovery and development of improved pharmacological treatment, so that filling these gaps is of utmost importance for an improved outcome. A vast amount of clinical data has strongly implicated the role of inflammation and oxidative insults in the pathophysiology of schizophrenia. Preclinical studies using animal models are fundamental in our understanding of disease development and pathology as well as the discovery and development of novel treatment options. In particular, social isolation rearing (SIR) and pre- or postnatal inflammation (PPNI) have shown great promise in mimicking the biobehavioral manifestations of schizophrenia. Furthermore, the "dual-hit" hypothesis of schizophrenia states that a first adverse event such as genetic predisposition or a prenatal insult renders an individual susceptible to develop the disease, while a second insult (e.g., postnatal inflammation, environmental adversity, or drug abuse) may be necessary to precipitate the full-blown syndrome. Animal models that emphasize the "dual-hit" hypothesis therefore provide valuable insight into understanding disease progression. In this Review, we will discuss SIR, PPNI, as well as possible "dual-hit" animal models within the context of the redox-immune-inflammatory hypothesis of schizophrenia, correlating such changes with the recognized monoamine and behavioral alterations of schizophrenia. Finally, based on these models, we will review new therapeutic options, especially those targeting immune-inflammatory and redox pathways.

Does escitalopram reduce neurotoxicity in major depression?

A pro-inflammatory state and a dysregulation in the tryptophan/kynurenine pathway have been documented in depression. This study examined whether treatment with the SSRI, escitalopram (ESC), could suppress inflammation and favorably shift metabolites of the kynurenine pathway in patients with major depressive disorder (MDD) within the utilized treatment period. Twenty seven healthy control subjects were included for comparison. Thirty patients were enrolled after completing baseline assessments. They received a 12-week ESC monotherapy. Twenty subjects were completers. Clinical assessments were carried out at each visit using the HAM-D, HAM-A, CGI and BDI rating scales. Blood samples were collected at each assessment and stored until analyzed. Cytokines were analyzed with Randox multiplex assay and tryptophan and kynurenine metabolites were analyzed using HPLC/GCMS. Baseline plasma concentrations of hsCRP, TNFα, IL6 and MCP-1 were significantly higher in patients compared to healthy controls. IL10 trended toward an increase. Baseline plasma IL1β correlated significantly with IL1α, and IL4. Patients showed significant improvement in all outcome measures with a high remission rate. Significant correlations were obtained between specific symptoms and certain biomarkers at baseline but these correlations must be viewed as very preliminary. During ESC treatment concentrations of inflammatory biomarkers did not change except for TNFα that trended lower. Metabolites and ratios of the tryptophan/kynurenine pathway showed reductions of the neurotoxic metabolites, 3-hydroxykynurenine and quinolinic acid, 3-hydroxykynurenine/kynurenine, quinolinic acid/tryptophan, kynurenic acid/quinolinic acid and quinolinic acid/3-hydroxykynurenine. The results indicate that ESC may exert its antidepressant effect in part through inhibition of synthesis of certain neurotoxic kynurenine metabolites and possibly also through reduction of the inflammatory response, although there was no concordance in the time course of changes between antidepressant efficacy and reversal of the pro-inflammatory status.

Ongoing episode of major depressive disorder is not associated with elevated plasma levels of kynurenine pathway markers

BACKGROUND

It has been suggested that the development of depressive symptoms as a result of cytokine therapy is attributable to cytokine-induced elevated activity of the kynurenine pathway. The few studies of this mechanism in patients with common major depressive disorder (MDD) have yielded inconsistent results. The aim of the present study was to identify markers of the kynurenine pathway in a clinical MDD sample with increased cytokine levels.

METHODS

Fifty medication-free MDD patients with a depressive episode and 34 healthy controls were included at baseline; the patients were followed for 12 weeks. Before initiating treatment, the patients were diagnosed and assessed for depressive symptoms and their blood was analyzed for tryptophan and its metabolites in the kynurenine pathway. The clinical assessments and metabolite measurements were repeated after 12 weeks of "treatment as usual".

RESULTS

We did not find significant elevation of kynurenine plasma markers in patients with a depressive episode compared to healthy controls, despite elevated cytokine levels in the patients. Clinical depression scores were significantly reduced after 12 weeks, but no significant change in the plasma kynurenine pathway plasma markers was observed.

CONCLUSION

The obtained results do not support the hypothesis that MDD depressive episodes are associated with elevated activity in the kynurenine pathway. This suggests that the pathophysiology underlying depressive episodes in common MDD differs from that of interferon induced depression. Our results warrant further study of the interplay between the kynurenine pathway and the cytokine activation patterns in these conditions.

Decreased quinolinic acid in the hippocampus of depressive patients: evidence for local anti-inflammatory and neuroprotective responses?

Disturbances of glutamatergic neurotransmission and mononuclear phagocyte system activation have been described uni- and bipolar depression (UD/BD). Linking the glutamate and immune hypotheses of depression, quinolinic acid (QUIN) is synthesized by activated microglia and acts as an endogenous N-methyl-D-aspartate glutamate receptor (NMDA-R) agonist with neurotoxic properties. Recently, we observed an increased microglial QUIN expression in the subgenual and supracallosal, but not in the pregenual part of the anterior cingulate cortex in postmortem brains of suicide cases with severe depression. Since several hints point to a role of the hippocampus in depression, we extended our study and addressed the question whether microglial QUIN is also changed in subregions of the hippocampus (CA1 and CA2/3 areas) in these patients. Postmortem brains of 12 acutely depressed patients (UD, n = 6; BD, n = 6) and 10 neuropsychiatric healthy age- and gender-matched control subjects were analyzed using QUIN-immunohistochemistry. Hippocampal volumes were determined in order to assess possible neurotoxic or neurodegenerative aspects. Microglial QUIN expression in the whole group of depressed patients was either comparable (left CA1, right CA2/3) or decreased (right CA1: p = 0.004, left CA2/3: p = 0.044) relative to controls. Post hoc tests showed that QUIN was reduced both in UD and BD in the right CA1 field (UD, p = 0.048; BD, p = 0.031). No loss of hippocampal volume was detected. Our data indicate that UD and BD are associated with a local reduction in QUIN-immunoreactive microglia in the hippocampus and underline the importance of the NMDA-R signaling in depressive disorders.

Tryptophan-kynurenine pathway is dysregulated in inflammation, and immune activation

The kynurenine (Kyn) pathway is the major route for tryptophan (Trp) metabolism, and it contributes to several fundamental biological processes. Trp is constitutively oxidized by tryptophan 2, 3-dioxygenase in liver cells. In other cell types, it is catalyzed by an alternative inducible indoleamine-pyrrole 2, 3-dioxygenase (IDO) under certain pathophysiological conditions, which consequently increases the formation of Kyn metabolites. IDO is up-regulated in response to inflammatory conditions as a novel marker of immune activation in early atherosclerosis. Besides, IDO and the IDO-related pathway are important mediators of the immunoinflammatory responses in advanced atherosclerosis. In particular, Kyn, 3-hydroxykynurenine, and quinolinic acid are positively associated with inflammation, oxidative stress (SOX), endothelial dysfunction, and carotid artery intima-media thickness values in end-stage renal disease patients. Moreover, IDO is a potential novel contributor to vessel relaxation and metabolism in systemic infections, which is also activated in acute severe heart attacks. The Kyn pathway plays a key role in the increased prevalence of cardiovascular disease by regulating inflammation, SOX, and immune activation.

Inflammation and neuronal plasticity: a link between childhood trauma and depression pathogenesis

During the past two decades, there has been increasing interest in understanding and characterizing the role of inflammation in major depressive disorder (MDD). Indeed, several are the evidences linking alterations in the inflammatory system to Major Depression, including the presence of elevated levels of pro-inflammatory cytokines, together with other mediators of inflammation. However, it is still not clear whether inflammation represents a cause or whether other factors related to depression result in these immunological effects. Regardless, exposure to early life stressful events, which represent a vulnerability factor for the development of psychiatric disorders, act through the modulation of inflammatory responses, but also of neuroplastic mechanisms over the entire life span. Indeed, early life stressful events can cause, possibly through epigenetic changes that persist over time, up to adulthood. Such alterations may concur to increase the vulnerability to develop psychopathologies. In this review we will discuss the role of inflammation and neuronal plasticity as relevant processes underlying depression development. Moreover, we will discuss the role of epigenetics in inducing alterations in inflammation-immune systems as well as dysfunction in neuronal plasticity, thus contributing to the long-lasting negative effects of stressful life events early in life and the consequent enhanced risk for depression. Finally we will provide an overview on the potential role of inflammatory system to aid diagnosis, predict treatment response, enhance treatment matching, and prevent the onset or relapse of Major Depression.

Distinct plasma immune signatures in ME/CFS are present early in the course of illness

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is an unexplained incapacitating illness that may affect up to 4 million people in the United States alone. There are no validated laboratory tests for diagnosis or management despite global efforts to find biomarkers of disease. We considered the possibility that inability to identify such biomarkers reflected variations in diagnostic criteria and laboratory methods as well as the timing of sample collection during the course of the illness. Accordingly, we leveraged two large, multicenter cohort studies of ME/CFS to assess the relationship of immune signatures with diagnosis, illness duration, and other clinical variables. Controls were frequency-matched on key variables known to affect immune status, including season of sampling and geographic site, in addition to age and sex. We report here distinct alterations in plasma immune signatures early in the course of ME/CFS (n = 52) relative to healthy controls (n = 348) that are not present in subjects with longer duration of illness (n = 246). Analyses based on disease duration revealed that early ME/CFS cases had a prominent activation of both pro- and anti-inflammatory cytokines as well as dissociation of intercytokine regulatory networks. We found a stronger correlation of cytokine alterations with illness duration than with measures of illness severity, suggesting that the immunopathology of ME/CFS is not static. These findings have critical implications for discovery of interventional strategies and early diagnosis of ME/CFS.

Glial cells as key players in schizophrenia pathology: recent insights and concepts of therapy

The past decade has witnessed an explosion of knowledge on the impact of glia for the neurobiological foundation of schizophrenia. A plethora of studies have shown structural and functional abnormalities in all three types of glial cells. There is convincing evidence of reduced numbers of oligodendrocytes, impaired cell maturation and altered gene expression of myelin/oligodendrocyte-related genes that may in part explain white matter abnormalities and disturbed inter- and intra-hemispheric connectivity, which are characteristic signs of schizophrenia. Earlier reports of astrogliosis could not be confirmed by later studies, although the expression of a variety of astrocyte-related genes is abnormal in psychosis. Since astrocytes play a key role in the synaptic metabolism of glutamate, GABA, monoamines and purines, astrocyte dysfunction may contribute to certain aspects of disturbed neurotransmission in schizophrenia. Finally, increased densities of microglial cells and aberrant expression of microglia-related surface markers in schizophrenia suggest that immunological/inflammatory factors are of considerable relevance for the pathophysiology of psychosis. This review describes current evidence for the multifaceted role of glial cells in schizophrenia and discusses efforts to develop glia-directed therapies for the treatment of the disease.

Aldosterone Signals the Onset of Depressive Behaviour in a Female Rat Model of Depression along with SSRI Treatment Resistance

Depression is a serious condition that occurs more frequently in women and is often associated with treatment resistance. The main hypotheses of this study are that (a) aldosterone is an early marker of depression onset and (b) a tryptophan (TRP) depletion model of depression previously validated in male rats is treatment resistant in females. To explore possible underlying mechanisms, we have focused on factors shown to be altered in patients with treatment-resistant depression. Female Sprague-Dawley rats were treated with a control or low-TRP-containing diet for various time periods up to 21 days. The results show that aldosterone secretion increased after 4 days of TRP depletion and prior to corticosterone. Optimal effects of TRP depletion occurred at 14 days. In addition to neurochemical and behavioural changes observed previously in males, TRP depletion in females was associated with a significant decline in serum magnesium concentrations, increased serum interleukin-6, enhanced gene expression of orexin A in the frontal cortex and induced a rise in N-methyl-D-aspartate (NMDA) receptor Bmax in the amygdala. Depression-like behaviour, NMDA receptor upregulation, enhancement of the kynurenine-to-kynurenic acid ratio and magnesium were resistant to paroxetine treatment (10 mg/kg/day in drinking water for 14 days). In conclusion, aldosterone may represent an important early marker for the onset of depression-like behaviour. With respect to treatment resistance, the underlying mechanisms may involve pro-inflammatory cytokines, the kynurenine pathway, magnesium, glutamate neurotransmission and the orexin pathway. This model of treatment-resistant depression may be useful for the future development of new compounds with novel antidepressant properties.

Comorbidity between Type 2 Diabetes and Depression in the Adult Population: Directions of the Association and Its Possible Pathophysiological Mechanisms

Type 2 diabetes and depression are regarded as comorbid conditions, and three possible directions of the association between the diseases can underlie this observation of comorbidity. First, common etiology can increase a person's risk of both diseases; second, persons with type 2 diabetes have increased prevalence or risk of future development of depression; or third, persons with depression have increased prevalence or risk of development of type 2 diabetes. This review gives an overview over possible pathophysiological mechanisms for each of the directions of the association between type 2 diabetes and depression and further discusses epigenetics as an additional, direction independent approach. We argue that unspecific pathophysiological mechanisms involved in the stress response might, at least to some extent, explain each of the directions of the association between type 2 diabetes and depression, while changes in brain structure and function among persons with diabetes and possible increased risk of development of type 2 diabetes after use of antidepressant agents could represent more disease specific mechanisms underlying the comorbidity.

Microglial intracellular Ca(2+) signaling as a target of antipsychotic actions for the treatment of schizophrenia

Microglia are resident innate immune cells which release many factors including proinflammatory cytokines, nitric oxide (NO) and neurotrophic factors when they are activated in response to immunological stimuli. Recent reports show that pathophysiology of schizophrenia is related to the inflammatory responses mediated by microglia. Intracellular Ca²⁺ signaling, which is mainly controlled by the endoplasmic reticulum (ER), is important for microglial functions such as release of NO and cytokines, migration, ramification and deramification. In addition, alteration of intracellular Ca²⁺ signaling underlies the pathophysiology of schizophrenia, while it remains unclear how typical or atypical antipsychotics affect intracellular Ca²⁺ mobilization in microglial cells. This mini-review article summarizes recent findings on cellular mechanisms underlying the characteristic differences in the actions of antipsychotics on microglial intracellular Ca²⁺ signaling and reinforces the importance of the ER of microglial cells as a target of antipsychotics for the treatment of schizophrenia.

A phase-specific neuroimmune model of clinical depression

Immune dysfunction and pro-inflammatory states in particular have been implicated in the aetiology and pathogenesis of depression. Whilst the onset of an episode and certain symptoms of depression appear well explained by this inflammatory model, the underpinnings of the episodic and progressive nature, as well as relapse and remission status in depression require attention. In this review it is suggested that additional immune factors beyond pro- and anti-inflammatory cytokines may effectively contribute to the understanding of the neurobiology of clinical depression. Considering neurobiological effects of immunomodulatory factors such as T cells, macrophages, microglia and astrocytes relevant to depression, we suggest a neuroimmune model of depression underpinned by dynamic immunomodulatory processes. This perspective paper then outlines a neuroimmune model of clinical phases of depression in an attempt to more adequately explain depression-like behaviours in pre-clinical models and the dynamic nature of depression in clinical populations. Finally, the implications for immunomodulatory treatments of depression are considered.