Central Activation of Alpha7 Nicotinic Signaling Attenuates LPS-Induced Neuroinflammation and Sickness Behavior in Adult but Not in Aged Animals.

We previously reported that lipopolysaccharide (LPS) challenge caused microglial-mediated neuroinflammation and sickness behavior that was amplified in aged mice. As α7 nAChRs are implicated in the "Cholinergic anti-inflammatory pathway", we aimed to determine how α7 nAChR stimulation modulates microglial phenotype in an LPS-induced neuroinflammation model in adult and aged mice. For this, BALB/c mice were injected intraperitoneally with LPS (0.33 mg/kg) and treated with the α7 nAChR agonist PNU282987, using different administration protocols. LPS challenge reduced body weight and induced lethargy and social withdrawal in adult mice. Peripheral (intraperitoneal) co-administration of the α7 nAChR agonist PNU282987 with LPS, attenuated body weight loss and sickness behavior associated with LPS challenge in adult mice, and reduced microglial activation with suppression of IL-1ß and TNFα mRNA levels. Furthermore, central (intracerebroventricular) administration of the α7 nAChR agonist, even 2 h after LPS injection, attenuated the decrease in social exploratory behavior and microglial activation induced by peripheral administration of LPS, although this recovery was not achieved if activation of α7 nAChRs was performed peripherally. Finally, we observed that the positive results of central activation of α7 nAChRs were lost in aged mice. In conclusion, we provide evidence that stimulation of α7 nAChR signaling reduces microglial activation in an in vivo LPS-based model, but this cholinergic-dependent regulation seems to be dysfunctional in microglia of aged mice.

Chronic spinal cord injury impairs primary CD8 T cell antiviral immunity but does not affect generation or function of memory CD8 T cells.

Antiviral immunity is severely compromised following trauma to the central nervous system. In mice with chronic spinal cord injury (SCI), primary infection with influenza virus leads to high mortality rates due to impaired expansion of virus-specific CD8 T cells. One strategy to increase resistance to viral infections is to generate memory immune cells that protect from recurrent infections. However, it is unknown if chronic SCI also impairs secondary immune responses to influenza challenge as it does primary responses. Here, we used a mouse model of chronic SCI and a clinically relevant influenza A infection to investigate CD8 T cell response. As shown previously, chronic SCI mice had impaired primary antiviral responses with high mortality rates and decreased expansion of virus-specific CD8 T cells following intranasal infection. To investigate CD8 T cell memory, we used two strains of influenza A virus [PR8(H1N1) and X31(H3N2)] that share internal proteins but differ in surface antigens. Chronic SCI mice immunized with live X31 were able to generate memory CD8 T cells that secreted IFNγ upon stimulation with viral peptides ex vivo, which was comparable to immunized uninjured mice. Importantly, immunization prior to challenge with a lethal dose of PR8 resulted in no mortality and significant CD8 T cell recall responses in both uninjured and chronic SCI mice. In addition, memory CD8 T cells generated before SCI remained functional up to 8 weeks after injury. These pre-existing memory CD8 T cells provided full protection from lethal PR8 challenge given at the chronic timepoint following injury. Overall, this study shows that memory CD8 T cells generated either before or after chronic SCI still remain functional. These results highlight the need for proper immunization of SCI patients and show the potential of memory T cells to confer protection against not only influenza, but other viral infections as well.

Bone Marrow-Derived Monocytes Drive the Inflammatory Microenvironment in Local and Remote Regions after Thoracic Spinal Cord Injury.

Spinal cord injury (SCI) produces a toxic inflammatory microenvironment that negatively affects plasticity and recovery. Recently, we showed glial activation and peripheral myeloid cell infiltration extending beyond the epicenter through the remote lumbar cord after thoracic SCI. The presence and role of infiltrating monocytes is important, especially in the lumbar cord where locomotor central pattern generators are housed. Therefore, we compared the inflammatory profile of resident microglia and peripheral myeloid cells after SCI. Bone marrow chimeras received midthoracic contusive SCI, and trafficking was determined 1-7 days later. Fluorescence-activated cell (FAC) sorting showed similar infiltration timing of both neutrophils and macrophages in epicenter and lumbar regions. While neutrophil numbers were attenuated by day 3, macrophages remained unchanged at day 7, suggesting that macrophages have important long-term influence on the microenvironment. Nanostring gene array identified a strong proinflammatory profile of infiltrating macrophages relative to microglia at both epicenter and lumbar sites. Macrophages had elevated expression of inflammatory cytokines (IL-1ß, IFNγ), chemokines (CCL2, CXCL2), mediators (COX-1, MMP-9), and receptors (CCR2, Ly6C), and decreased expression of growth promoting genes (GDNF, BDNF). Importantly, lumbar macrophages had elevated expression of active trafficking genes (CCR2, l-selectin, MMP-9) compared with epicenter macrophages. Further, acute rehabilitation exacerbated the inflammatory profile of infiltrated macrophages in the lumbar cord. Such high inflammatory potential and negative response to rehabilitation of infiltrating macrophages within lumbar locomotor central pattern generators likely impedes activity-dependent recovery. Therefore, limiting active trafficking of macrophages into the lumbar cord identifies a novel target for SCI therapies to improve locomotion.

Kistimonas alittae sp. nov., a gammaproteobacterium isolated from the marine annelid Alitta succinea.

A novel Gram-stain-negative, rod-shaped, motile, non-spore-forming, facultatively anaerobic marine bacterium was isolated from the gastrointestinal tract of the sandworm Alitta succinea collected from Grice Cove, South Carolina, USA. The strain was arginine dihydrolase-positive, and oxidase- and catalase-positive. Growth occurred between 10 and 37 °C, with optimal growth occurring between 30 and 32 °C. Comparative 16S rRNA gene sequence analysis showed its nearest neighbours are members of the genus Kistimonas of the family Hahellaceae, which is found in the order Oceanospirillales, class Gammaproteobacteria. The closest related species was Kistimonas asteriae KMD 001T with 16S rRNA gene sequence similarity of 99.0 %. However, DNA-DNA hybridization between these strains revealed less than 70 % DNA-DNA relatedness, supporting the novel species status of the strain. The major fatty acids were C16 : 0, C18 : 0, C18 : 1ω7c and a summed feature that contained C16 : 1ω6c/C16 : 1ω7c. The major respiratory quinone was ubiquinone-9 and the predominant polar lipids were phosphatidylserine, phosphoethanolamine, phosphatidylglycerol and diphosphatidylglycerol. The genomic DNA G+C content was 52.5 mol%. Based on the data presented, strain BGP-2T is considered to represent a novel member of the genus Kistimonas, for which the name Kistimonas alittae sp. nov. is proposed. The type strain is BGP-2T (=CCUG 65711T=JCM 30010T).

Impaired CD8 T cell antiviral immunity following acute spinal cord injury.

BACKGROUND: Spinal cord injury (SCI) disrupts essential neuroimmune communication, leading to severe immune depression. Previous studies confirmed immune dysfunction in mice with chronic SCI and following high thoracic level injury where sympathetic innervation of the spleen is disrupted. Here, we induced a mid-thoracic injury where integrity of the sympathetic response is maintained and investigated the antiviral T cell response to influenza virus after acute SCI. METHODS: One week following a contusion SCI at thoracic level T9, mice were infected intranasally with influenza virus. Profiles of immune cell populations were analyzed before infection, and virus-specific CD8 T cell response was analyzed 7 days post-infection. RESULTS: Following intranasal infection, injured mice had prolonged recovery and significant weight loss. Importantly, expansion and effector functions of virus-specific CD8 T cells were decreased in injured mice. The compromised CD8 T cell response was associated with inflammation and stress responses initiated after injury. Regulatory mechanisms, including increased regulatory T cells (Tregs) and upregulated PD-1/PD-L1, were induced following SCI. Furthermore, we show that increased corticosterone (CORT) levels can inhibit CD8 T cells and that blocking CORT in vivo following SCI enhances CD8 T cell antiviral responses. CONCLUSIONS: Our results show that mice with mid-thoracic SCI have impaired CD8 T cell function during the acute stage of injury, indicating that impaired antiviral responses occur rapidly following SCI and is not dependent on injury level.

The Alarmin HMGB1 Mediates Age-Induced Neuroinflammatory Priming.

UNLABELLED: Amplified neuroinflammatory responses following an immune challenge occur with normal aging and can elicit or exacerbate neuropathology. The mechanisms mediating this sensitized or "primed" immune response in the aged brain are not fully understood. The alarmin high mobility group box 1 (HMGB1) can be released under chronic pathological conditions and initiate inflammatory cascades. This led us to investigate whether HMGB1 regulates age-related priming of the neuroinflammatory response. Here, we show that HMGB1 protein and mRNA were elevated in the hippocampus of unmanipulated aged rats (24-month-old F344XBN rats). Furthermore, aged rats had increased HMGB1 in the CSF, suggesting increased HMGB1 release. We demonstrate that blocking HMGB1 signaling with an intracisterna magna (ICM) injection of the competitive antagonist to HMGB1, Box-A, downregulates basal expression of several inflammatory pathway genes in the hippocampus of aged rats. This indicates that blocking the actions of HMGB1 might reduce age-associated inflammatory priming. To test this hypothesis, we evaluated whether HMGB1 antagonism blocks the protracted neuroinflammatory and sickness response to peripheral Escherichia coli (E. coli) infection in aged rats. ICM pretreatment of aged rats with Box-A 24 h before E. coli infection prevented the extended hippocampal cytokine response and associated cognitive and affective behavioral changes. ICM pretreatment with Box-A also inhibited aging-induced potentiation of the microglial proinflammatory response to lipopolysaccharide ex vivo Together, these results suggest that HMGB1 mediates neuroinflammatory priming in the aged brain. Blocking the actions of HMGB1 appears to "desensitize" aged microglia to an immune challenge, thereby preventing exaggerated behavioral and neuroinflammatory responses following infection. SIGNIFICANCE STATEMENT: The world's population is aging, highlighting a need to develop treatments that promote quality of life in aged individuals. Normal aging is associated with precipitous drops in cognition, typically following events that induce peripheral inflammation (e.g., infection, surgery, heart attack). Peripheral immune stimuli cause exaggerated immune responses in the aged brain, which likely underlie these behavioral deficits. Here, we investigated whether the alarmin high mobility group box 1 (HMGB1) mediates age-associated "priming" of the neuroinflammatory response. HMGB1 is elevated in aged rodent brain and CSF. Blocking HMGB1 signaling downregulated expression of inflammatory pathway genes in aged rat brain. Further, HMGB1 antagonism prevented prolonged infection-induced neuroinflammatory and sickness responses in aged rats. Overall, blocking HMGB1 "desensitized" microglia in the aged brain, thereby preventing pathological infection-elicited neuroinflammatory responses.

Insensitivity of astrocytes to interleukin 10 signaling following peripheral immune challenge results in prolonged microglial activation in the aged brain.

Immune-activated microglia from aged mice produce exaggerated levels of cytokines. Despite high levels of microglial interleukin (IL)-10 in the aged brain, neuroinflammation was prolonged and associated with depressive-like deficits. Because astrocytes respond to IL-10 and, in turn, attenuate microglial activation, we investigated if astrocyte-mediated resolution of microglial activation was impaired with age. Here, aged astrocytes had a dysfunctional profile with higher glial fibrillary acidic protein, lower glutamate transporter expression, and significant cytoskeletal re-arrangement. Moreover, aged astrocytes had reduced expression of growth factors and IL-10 receptor-1 (IL-10R1). After in vivo lipopolysaccharide immune challenge, aged astrocytes had a molecular signature associated with reduced responsiveness to IL-10. This IL-10 insensitivity of aged astrocytes resulted in a failure to induce IL-10R1 and transforming growth factor ß and resolve microglial activation. In addition, adult astrocytes reduced microglial activation when co-cultured ex vivo, whereas aged astrocytes did not. Consistent with the aging studies, IL-10R(KO) astrocytes did not augment transforming growth factor ß after immune challenge and failed to resolve microglial activation. Collectively, a major cytokine-regulatory loop between activated microglia and astrocytes is impaired in the aged brain.

Lumbar Myeloid Cell Trafficking into Locomotor Networks after Thoracic Spinal Cord Injury.

Spinal cord injury (SCI) promotes inflammation along the neuroaxis that jeopardizes plasticity, intrinsic repair and recovery. While inflammation at the injury site is well-established, less is known within remote spinal networks. The presence of bone marrow-derived immune (myeloid) cells in these areas may further impede functional recovery. Previously, high levels of the gelatinase, matrix metalloproteinase-9 (MMP-9) occurred within the lumbar enlargement after thoracic SCI and impeded activity-dependent recovery. Since SCI-induced MMP-9 potentially increases vascular permeability, myeloid cell infiltration may drive inflammatory toxicity in locomotor networks. Therefore, we examined neurovascular reactivity and myeloid cell infiltration in the lumbar cord after thoracic SCI. We show evidence of region-specific recruitment of myeloid cells into the lumbar but not cervical region. Myeloid infiltration occurred with concomitant increases in chemoattractants (CCL2) and cell adhesion molecules (ICAM-1) around lumbar vasculature 24h and 7days post injury. Bone marrow GFP chimeric mice established robust infiltration of bone marrow-derived myeloid cells into the lumbar gray matter 24h after SCI. This cell infiltration occurred when the blood-spinal cord barrier was intact, suggesting active recruitment across the endothelium. Myeloid cells persisted as ramified macrophages at 7days post injury in parallel with increased inhibitory GAD67 labeling. Importantly, macrophage infiltration required MMP-9.

Sequential activation of microglia and astrocyte cytokine expression precedes increased Iba-1 or GFAP immunoreactivity following systemic immune challenge.

Activation of the peripheral immune system elicits a coordinated response from the central nervous system. Key to this immune to brain communication is that glia, microglia, and astrocytes, interpret and propagate inflammatory signals in the brain that influence physiological and behavioral responses. One issue in glial biology is that morphological analysis alone is used to report on glial activation state. Therefore, our objective was to compare behavioral responses after in vivo immune (lipopolysaccharide, LPS) challenge to glial specific mRNA and morphological profiles. Here, LPS challenge induced an immediate but transient sickness response with decreased locomotion and social interaction. Corresponding with active sickness behavior (2-12 h), inflammatory cytokine mRNA expression was elevated in enriched microglia and astrocytes. Although proinflammatory cytokine expression in microglia peaked 2-4 h after LPS, astrocyte cytokine, and chemokine induction was delayed and peaked at 12 h. Morphological alterations in microglia (Iba-1(+)) and astrocytes (GFAP(+)), however, were undetected during this 2-12 h timeframe. Increased Iba-1 immunoreactivity and de-ramified microglia were evident 24 and 48 h after LPS but corresponded to the resolution phase of activation. Morphological alterations in astrocytes were undetected after LPS. Additionally, glial cytokine expression did not correlate with morphology after four repeated LPS injections. In fact, repeated LPS challenge was associated with immune and behavioral tolerance and a less inflammatory microglial profile compared with acute LPS challenge. Overall, induction of glial cytokine expression was sequential, aligned with active sickness behavior, and preceded increased Iba-1 or GFAP immunoreactivity after LPS challenge.

Ibuprofen ameliorates fatigue- and depressive-like behavior in tumor-bearing mice.

AIMS: Cancer-related fatigue (CRF) is often accompanied by depressed mood, both of which reduce functional status and quality of life. Research suggests that increased expression of pro-inflammatory cytokines is associated with skeletal muscle wasting and depressive- and fatigue-like behaviors in rodents and cancer patients. We have previously shown that treatment with ibuprofen, a nonsteroidal anti-inflammatory drug, preserved muscle mass in tumor-bearing mice. Therefore, the purpose of the present study was to determine the behavioral effects of ibuprofen in a mouse model of CRF. MAIN METHODS: Mice were injected with colon-26 adenocarcinoma cells and treated with ibuprofen (10mg/kg) in the drinking water. Depressive-like behavior was determined using the forced swim test (FST). Fatigue-like behaviors were determined using voluntary wheel running activity (VWRA) and grip strength. The hippocampus, gastrocnemius muscle, and serum were collected for cytokine analysis. KEY FINDINGS: Tumor-bearing mice showed depressive-like behavior in the FST, which was not observed in mice treated with ibuprofen. VWRA and grip strength declined in tumor-bearing mice, and ibuprofen attenuated this decline. Tumor-bearing mice had decreased gastrocnemius muscle mass and increased expression of IL-6, MAFBx and MuRF mRNA, biomarkers of protein degradation, in the muscle. Expression of IL-1ß and IL-6 was also increased in the hippocampus. Treatment with ibuprofen improved muscle mass and reduced cytokine expression in both the muscle and hippocampus of tumor-bearing mice. SIGNIFICANCE: Ibuprofen treatment reduced skeletal muscle wasting, inflammation in the brain, and fatigue- and depressive-like behavior in tumor-bearing mice. Therefore, ibuprofen warrants evaluation as an adjuvant treatment for CRF.

Storage Conditions and Passages Alter IL-6 secretion in C26 adenocarcinoma cell lines.

The C26 adenocarcinoma tumor line is frequently used to establish peripheral tumors in mice for the study of cancer cachexia and cancer-related fatigue. Recently, we have noticed a progressive decline in the effects of tumor growth on our biological and behavioral measures in the tumor-bearing mice. Therefore, we compared effects of three aliquots of the C26 tumor cell line that differed in storage condition and number of passages on cytokine secretion, tumor growth, weight loss and fatigue behavior. Three aliquots of the C26 tumor line were selected as alpha (α), beta (ß), and gamma (γ). Aliquot α was an original C26 stock line that had been stored at -80°C. Aliquot ß was stored in liquid nitrogen. Aliquot γ was taken from aliquot ß and passaged three times. The three aliquots of the C26 tumor line showed differences in IL-6 mRNA and protein secretion in vitro, with aliquot ß showing the greatest IL-6 secretion. These differences were mirrored in vivo. Plasma IL-6 levels were elevated in all tumor bearing mice but was greatest in group ß mice. Carcass weight was decreased in all three tumor groups. Brain expression of IL-1ß mRNA was greatest in group ß and group ß demonstrated the greatest decline in running activity at day 19. Storage conditions and number of passages influence C26 tumor cell secretion of cytokines.Variations in C26 aliquots may explain differences observed between laboratories using the same cell line.We recommend always storing cell lines in liquid nitrogen and limiting the number of passages before use in experiments.

Fluoxetine prevents the development of depressive-like behavior in a mouse model of cancer related fatigue.

Cancer patients frequently suffer from fatigue, a complex syndrome associated with tiredness and depressed mood. Cancer-related fatigue (CRF) can be present at the time of diagnosis, escalates during treatment, and can persist for years after treatment. CRF negatively influences quality of life, limits functional independence, and is associated with decreased survival in patients with incurable disease. We have previously shown that increased pro-inflammatory cytokine expression in the brain contributes to depressive- and fatigue-like behaviors in a mouse model of CRF. Inflammatory cytokines increase the activity of indoleamine 2,3-dioxygenase (IDO) and kynurenine 3-monooxygenase (KMO), which competitively reduce serotonin synthesis. Reduced serotonin availability in the brain and increased production of alternative neuroactive metabolites of tryptophan are thought to contribute to the development of depression and fatigue. The purpose of this study was to determine the effects of fluoxetine, a selective serotonin reuptake inhibitor (SSRI), on brain cytokines and behavioral measures of fatigue and depression in tumor-bearing mice. Here we show that tumor growth increased brain expression of pro-inflammatory cytokines and KMO. Treatment with fluoxetine had no effect on tumor growth, muscle wasting, fatigue behavior, or cytokine expression in the brain. Fluoxetine, however, reduced depressive-like behaviors in tumor bearing mice. In conclusion, our data confirm that increased brain expression of pro-inflammatory cytokines is associated with tumor-induced fatigue- and depressive-like behaviors. However, it is possible to separate the effects of tumor growth on mood and fatigue-like behaviors using SSRIs such as fluoxetine.

Tumor growth increases neuroinflammation, fatigue and depressive-like behavior prior to alterations in muscle function.

Cancer patients frequently suffer from fatigue, a complex syndrome associated with loss of muscle mass, weakness, and depressed mood. Cancer-related fatigue (CRF) can be present at the time of diagnosis, during treatment, and persists for years after treatment. CRF negatively influences quality of life, limits functional independence, and is associated with decreased survival in patients with incurable disease. Currently there are no effective treatments to reduce CRF. The aim of this study was to use a mouse model of tumor growth and discriminate between two main components of fatigue: loss of muscle mass/function and altered mood/motivation. Here we show that tumor growth increased fatigue- and depressive-like behaviors, and reduced body and muscle mass. Decreased voluntary wheel running activity (VWRA) and increased depressive-like behavior in the forced swim and sucrose preference tests were evident in tumor-bearing mice within the first two weeks of tumor growth and preceded the loss of body and muscle mass. At three weeks, tumor-bearing mice had reduced grip strength but this was not associated with altered expression of myosin isoforms or impaired contractile properties of muscles. These increases in fatigue and depressive-like behaviors were paralleled by increased expression of IL-1ß mRNA in the cortex and hippocampus. Minocycline administration reduced tumor-induced expression of IL-1ß in the brain, reduced depressive-like behavior, and improved grip strength without altering muscle mass. Taken together, these results indicate that neuroinflammation and depressed mood, rather than muscle wasting, contribute to decreased voluntary activity and precede major changes in muscle contractile properties with tumor growth.

Microglial priming and enhanced reactivity to secondary insult in aging, and traumatic CNS injury, and neurodegenerative disease.

Glia of the central nervous system (CNS) help to maintain homeostasis in the brain and support efficient neuronal function. Microglia are innate immune cells of the brain that mediate responses to pathogens and injury. They have key roles in phagocytic clearing, surveying the local microenvironment and propagating inflammatory signals. An interruption in homeostasis induces a cascade of conserved adaptive responses in glia. This response involves biochemical, physiological and morphological changes and is associated with the production of cytokines and secondary mediators that influence synaptic plasticity, cognition and behavior. This reorganization of host priorities represents a beneficial response that is normally adaptive but may become maladaptive when the profile of microglia is compromised. For instance, microglia can develop a primed or pro-inflammatory mRNA, protein and morphological profile with aging, traumatic brain injury and neurodegenerative disease. As a result, primed microglia exhibit an exaggerated inflammatory response to secondary and sub-threshold challenges. Consequences of exaggerated inflammatory responses by microglia include the development of cognitive deficits, impaired synaptic plasticity and accelerated neurodegeneration. Moreover, impairments in regulatory systems in these circumstances may make microglia more resistant to negative feedback and important functions of glia can become compromised and dysfunctional. Overall, the purpose of this review is to discuss key concepts of microglial priming and immune-reactivity in the context of aging, traumatic CNS injury and neurodegenerative disease. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

TGFß produced by IL-10 redirected astrocytes attenuates microglial activation.

While there clearly is an intimate relationship between astrocytes and microglia, few studies have examined these potentially dynamic interactions. In this study, cytokine-mediated communication between microglia and astrocytes under inflammatory conditions was investigated. We have previously shown that activated microglia produce Interleukin (IL)-10, a regulatory cytokine that plays an important role in resolving neuroinflammation. Nonetheless, the mechanism by which IL-10 attenuates pro-inflammatory cytokine expression in the brain is unclear. Here, we show that IL-10 redirected astrocytes regulate the activation of microglia in a transforming growth factor (TGF)-ß dependent manner. In support of this concept, astrocytes in the brain maintained higher IL-10 receptor (IL-10R1) expression and primary astrocytes in culture were markedly more sensitive to the anti-inflammatory effects of IL-10 compared with microglia. Moreover, studies using primary cultures and an astrocyte-microglia coculture system revealed that astrocytes mediated the anti-inflammatory effects of IL-10 on microglia through the production of TGFß. For instance, only when astrocytes were present did IL-10 stimulation reduce the expression of IL-1ß and increase expression of anti-inflammatory mediators fractalkine receptor (CX3 CR1) and interleukin 4 receptor-α (IL-4Rα) in microglia. Importantly, these IL-10-astrocyte dependent effects on microglia were blocked by a TGFß inhibitor. Furthermore, inhibition of TGFß signaling in the brain resulted in prolonged sickness behavior and amplified pro-inflammatory cytokine expression in mice challenged with lipopolysaccharide. Taken together, IL-10 stimulated the production of TGFß by astrocytes, which in turn, attenuated microglial activation. Overall, these findings provide novel insight into the mechanisms by which astrocytes modulate microglia under inflammatory conditions.

Indoleamine 2,3-dioxygenase inhibition attenuates lipopolysaccharide induced persistent microglial activation and depressive-like complications in fractalkine receptor (CX(3)CR1)-deficient mice.

An impaired ability to regulate the activation of microglia by fractalkine (CX3CL1) leads to persistent neuroinflammation and behavioral alterations following lipopolysaccharide (LPS) challenge. While these responses are usually transient, LPS injection caused prolonged depressive-like behavior in fractalkine receptor deficient mice (CX3CR1(-/-)) that was associated with exaggerated microglial activation and induction of the tryptophan (TRP) degrading enzyme indoleamine 2,3-dioxygenase (IDO). IDO activation and subsequent generation of neuroactive kynurenine metabolites may have a pivotal role in the development of depression. Therefore, the purpose of this study was to determine the extent to which LPS-induced depressive-like behavior in CX3CR1(-/-) mice was dependent on IDO activation. CX3CR1(-/-) mice were implanted prior to LPS challenge with a slow release pellet of 1-methyl-tryptophan (1-MT), a competitive inhibitor of IDO. Here we show that the depressive-like behavior evident in CX3CR1(-/-) mice 72 h after LPS injection was abrogated by inhibition of IDO. LPS also decreased body weight and locomotor activity in CX3CR1(-/-) mice, but these effects were independent of 1-MT. Consistent with the increased metabolism of TRP by IDO, the ratio of 3-hydroxykynurenine (3-HK) to TRP was increased in the brain 72 h after LPS. Increased serotonin (5-HT) turnover was also evident in the brain. The LPS-associated increases in both 3-HK:TRP and 5-HIAA:5-HT ratios were prevented by the inhibition of IDO. Last, IDO blockade attenuated microglial activation in the prefrontal cortex and hippocampus 72 h after LPS. Collectively these data indicate that LPS-induced IDO activation contributes to persistent microglial activation and depressive-like behavior in CX3CR1(-/-) mice.