Pharmacological modulation of TSPO in microglia/macrophages and neurons in a chronic neurodegenerative model of prion disease.

Neuroinflammation is an important component of many neurodegenerative diseases, whether as a primary cause or a secondary outcome. For that reason, either as diagnostic tools or to monitor progression and/or pharmacological interventions, there is a need for robust biomarkers of neuroinflammation in the brain. Mitochondrial TSPO (18 kDa Translocator protein) is one of few available biomarkers of neuroinflammation for which there are clinically available PET imaging agents. In this study, we further characterised neuroinflammation in a mouse model of prion-induced chronic neurodegeneration (ME7) including a pharmacological intervention via a CSF1R inhibitor. This was achieved by autoradiographic binding of the second-generation TSPO tracer, [3H]PBR28, along with a more comprehensive examination of the cellular contributors to the TSPO signal changes by immunohistochemistry. We observed regional increases of TSPO in the ME7 mouse brains, particularly in the hippocampus, cortex and thalamus. This increased TSPO signal was detected in the cells of microglia/macrophage lineage as well as in astrocytes, endothelial cells and neurons. Importantly, we show that the selective CSF1R inhibitor, JNJ-40346527 (JNJ527), attenuated the disease-dependent increase in TSPO signal, particularly in the dentate gyrus of the hippocampus, where JNJ527 attenuated the number of Iba1+ microglia and neurons, but not GFAP+ astrocytes or endothelial cells. These findings suggest that [3H]PBR28 quantitative autoradiography in combination with immunohistochemistry are important translational tools for detecting and quantifying neuroinflammation, and its treatments, in neurodegenerative disease. Furthermore, we demonstrate that although TSPO overexpression in the ME7 brains was driven by various cell types, the therapeutic effect of the CSF1R inhibitor was primarily to modulate TSPO expression in microglia and neurons, which identifies an important route of biological action of this particular CSF1R inhibitor and provides an example of a cell-specific effect of this type of therapeutic agent on the neuroinflammatory process.

CSF1R inhibitor JNJ-40346527 attenuates microglial proliferation and neurodegeneration in P301S mice.

Neuroinflammation and microglial activation are significant processes in Alzheimer's disease pathology. Recent genome-wide association studies have highlighted multiple immune-related genes in association with Alzheimer's disease, and experimental data have demonstrated microglial proliferation as a significant component of the neuropathology. In this study, we tested the efficacy of the selective CSF1R inhibitor JNJ-40346527 (JNJ-527) in the P301S mouse tauopathy model. We first demonstrated the anti-proliferative effects of JNJ-527 on microglia in the ME7 prion model, and its impact on the inflammatory profile, and provided potential CNS biomarkers for clinical investigation with the compound, including pharmacokinetic/pharmacodynamics and efficacy assessment by TSPO autoradiography and CSF proteomics. Then, we showed for the first time that blockade of microglial proliferation and modification of microglial phenotype leads to an attenuation of tau-induced neurodegeneration and results in functional improvement in P301S mice. Overall, this work strongly supports the potential for inhibition of CSF1R as a target for the treatment of Alzheimer's disease and other tau-mediated neurodegenerative diseases.

Distribution of Misfolded Prion Protein Seeding Activity Alone Does Not Predict Regions of Neurodegeneration.

Protein misfolding is common across many neurodegenerative diseases, with misfolded proteins acting as seeds for "prion-like" conversion of normally folded protein to abnormal conformations. A central hypothesis is that misfolded protein accumulation, spread, and distribution are restricted to specific neuronal populations of the central nervous system and thus predict regions of neurodegeneration. We examined this hypothesis using a highly sensitive assay system for detection of misfolded protein seeds in a murine model of prion disease. Misfolded prion protein (PrP) seeds were observed widespread throughout the brain, accumulating in all brain regions examined irrespective of neurodegeneration. Importantly, neither time of exposure nor amount of misfolded protein seeds present determined regions of neurodegeneration. We further demonstrate two distinct microglia responses in prion-infected brains: a novel homeostatic response in all regions and an innate immune response restricted to sites of neurodegeneration. Therefore, accumulation of misfolded prion protein alone does not define targeting of neurodegeneration, which instead results only when misfolded prion protein accompanies a specific innate immune response.

The blood-brain interface: a culture change.

The blood-brain interface (BBI) is the subject of a new named series at Brain, Behavior, and Immunity. It is timely to reflect on a number of advances in the field within the last ten years, which may lead to an increased understanding of human behaviour and a wide range of psychiatric and neurological conditions. We cover discoveries made in solute and cell trafficking, endothelial cell and pericyte biology, extracellular matrix and emerging tools, especially those which will enable study of the human BBI. We now recognize the central role of the BBI in a number of immunopsychiatric syndromes, including sickness behaviour, delirium, septic encephalopathy, cognitive side effects of cytokine-based therapies and the frank psychosis observed in neuronal surface antibody syndromes. In addition, we find ourselves interrogating and modulating the brain across the BBI, during diagnostic investigation and treatment of brain disease. The past ten years of BBI research have been exciting but there is more to come.

Systemic infections and inflammation affect chronic neurodegeneration.

It is well known that systemic infections cause flare-ups of disease in individuals with asthma and rheumatoid arthritis, and that relapses in multiple sclerosis can often be associated with upper respiratory-tract infections. Here we review evidence to support our hypothesis that in chronic neurodegenerative diseases such as Alzheimer's disease, with an ongoing innate immune response in the brain, systemic infections and inflammation can cause acute exacerbations of symptoms and drive the progression of neurodegeneration.

The Chemokine CCL2 Mediates the Seizure-enhancing Effects of Systemic Inflammation.

Epilepsy is a chronic disorder characterized by spontaneous recurrent seizures. Brain inflammation is increasingly recognized as a critical factor for seizure precipitation, but the molecular mediators of such proconvulsant effects are only partly understood. The chemokine CCL2 is one of the most elevated inflammatory mediators in patients with pharmacoresistent epilepsy, but its contribution to seizure generation remains unexplored. Here, we show, for the first time, a crucial role for CCL2 and its receptor CCR2 in seizure control. We imposed a systemic inflammatory challenge via lipopolysaccharide (LPS) administration in mice with mesial temporal lobe epilepsy. We found that LPS dramatically increased seizure frequency and upregulated the expression of many inflammatory proteins, including CCL2. To test the proconvulsant role of CCL2, we administered systemically either a CCL2 transcription inhibitor (bindarit) or a selective antagonist of the CCR2 receptor (RS102895). We found that interference with CCL2 signaling potently suppressed LPS-induced seizures. Intracerebral administration of anti-CCL2 antibodies also abrogated LPS-mediated seizure enhancement in chronically epileptic animals. Our results reveal that CCL2 is a key mediator in the molecular pathways that link peripheral inflammation with neuronal hyperexcitability. SIGNIFICANCE STATEMENT: Substantial evidence points to a role for inflammation in epilepsy, but currently there is little insight as to how inflammatory pathways impact on seizure generation. Here, we examine the molecular mediators linking peripheral inflammation with seizure susceptibility in mice with mesial temporal lobe epilepsy. We show that a systemic inflammatory challenge via lipopolysaccharide administration potently enhances seizure frequency and upregulates the expression of the chemokine CCL2. Remarkably, selective pharmacological interference with CCL2 or its receptor CCR2 suppresses lipopolysaccharide-induced seizure enhancement. Thus, CCL2/CCR2 signaling plays a key role in linking systemic inflammation with seizure susceptibility.

Pharmacological targeting of CSF1R inhibits microglial proliferation and prevents the progression of Alzheimer's-like pathology.

The proliferation and activation of microglial cells is a hallmark of several neurodegenerative conditions. This mechanism is regulated by the activation of the colony-stimulating factor 1 receptor (CSF1R), thus providing a target that may prevent the progression of conditions such as Alzheimer's disease. However, the study of microglial proliferation in Alzheimer's disease and validation of the efficacy of CSF1R-inhibiting strategies have not yet been reported. In this study we found increased proliferation of microglial cells in human Alzheimer's disease, in line with an increased upregulation of the CSF1R-dependent pro-mitogenic cascade, correlating with disease severity. Using a transgenic model of Alzheimer's-like pathology (APPswe, PSEN1dE9; APP/PS1 mice) we define a CSF1R-dependent progressive increase in microglial proliferation, in the proximity of amyloid-ß plaques. Prolonged inhibition of CSF1R in APP/PS1 mice by an orally available tyrosine kinase inhibitor (GW2580) resulted in the blockade of microglial proliferation and the shifting of the microglial inflammatory profile to an anti-inflammatory phenotype. Pharmacological targeting of CSF1R in APP/PS1 mice resulted in an improved performance in memory and behavioural tasks and a prevention of synaptic degeneration, although these changes were not correlated with a change in the number of amyloid-ß plaques. Our results provide the first proof of the efficacy of CSF1R inhibition in models of Alzheimer's disease, and validate the application of a therapeutic strategy aimed at modifying CSF1R activation as a promising approach to tackle microglial activation and the progression of Alzheimer's disease.

Microglia regulate hippocampal neurogenesis during chronic neurodegeneration.

Neurogenesis is altered in neurodegenerative disorders, partly regulated by inflammatory factors. We have investigated whether microglia, the innate immune brain cells, regulate hippocampal neurogenesis in neurodegeneration. Using the ME7 model of prion disease we applied gain- or loss-of CSF1R function, as means to stimulate or inhibit microglial proliferation, respectively, to dissect the contribution of these cells to neurogenesis. We found that increased hippocampal neurogenesis correlates with the expansion of the microglia population. The selective inhibition of microglial proliferation caused a reduction in neurogenesis and a restoration of normal neuronal differentiation, supporting a pro-neurogenic role for microglia. Using a gene screening strategy, we identified TGFß as a molecule controlling the microglial pro-neurogenic response in chronic neurodegeneration, supported by loss-of-function mechanistic experiments. By the selective targeting of microglial proliferation we have been able to uncover a pro-neurogenic role for microglia in chronic neurodegeneration, suggesting promising therapeutic targets to normalise the neurogenic niche during neurodegeneration.

Analysis of the hippocampal proteome in ME7 prion disease reveals a predominant astrocytic signature and highlights the brain-restricted production of clusterin in chronic neurodegeneration.

Prion diseases are characterized by accumulation of misfolded protein, gliosis, synaptic dysfunction, and ultimately neuronal loss. This sequence, mirroring key features of Alzheimer disease, is modeled well in ME7 prion disease. We used iTRAQ(TM)/mass spectrometry to compare the hippocampal proteome in control and late-stage ME7 animals. The observed changes associated with reactive glia highlighted some specific proteins that dominate the proteome in late-stage disease. Four of the up-regulated proteins (GFAP, high affinity glutamate transporter (EAAT-2), apo-J (Clusterin), and peroxiredoxin-6) are selectively expressed in astrocytes, but astrocyte proliferation does not contribute to their up-regulation. The known functional role of these proteins suggests this response acts against protein misfolding, excitotoxicity, and neurotoxic reactive oxygen species. A recent convergence of genome-wide association studies and the peripheral measurement of circulating levels of acute phase proteins have focused attention on Clusterin as a modifier of late-stage Alzheimer disease and a biomarker for advanced neurodegeneration. Since ME7 animals allow independent measurement of acute phase proteins in the brain and circulation, we extended our investigation to address whether changes in the brain proteome are detectable in blood. We found no difference in the circulating levels of Clusterin in late-stage prion disease when animals will show behavioral decline, accumulation of misfolded protein, and dramatic synaptic and neuronal loss. This does not preclude an important role of Clusterin in late-stage disease, but it cautions against the assumption that brain levels provide a surrogate peripheral measure for the progression of brain degeneration.

Temporal dynamics of hippocampal neurogenesis in chronic neurodegeneration.

The study of neurogenesis during chronic neurodegeneration is crucial in order to understand the intrinsic repair mechanisms of the brain, and key to designing therapeutic strategies. In this study, using an experimental model of progressive chronic neurodegeneration, murine prion disease, we define the temporal dynamics of the generation, maturation and integration of new neurons in the hippocampal dentate gyrus, using dual pulse-chase, multicolour γ-retroviral tracing, transmission electron microscopy and patch-clamp. We found increased neurogenesis during the progression of prion disease, which partially counteracts the effects of chronic neurodegeneration, as evidenced by blocking neurogenesis with cytosine arabinoside, and helps to preserve the hippocampal function. Evidence obtained from human post-mortem samples, of both variant Creutzfeldt-Jakob disease and Alzheimer's disease patients, also suggests increased neurogenic activity. These results open a new avenue into the exploration of the effects and regulation of neurogenesis during chronic neurodegeneration, and offer a new model to reproduce the changes observed in human neurodegenerative diseases.

Regulation of microglial proliferation during chronic neurodegeneration.

An important component of chronic neurodegenerative diseases is the generation of an innate inflammatory response within the CNS. Microglial and astroglial cells play a key role in the development and maintenance of this inflammatory response, showing enhanced proliferation and activation. We studied the time course and regulation of microglial proliferation, using a mouse model of prion disease. Our results show that the proliferation of resident microglial cells accounts for the expansion of the population during the development of the disease. We identify the pathway regulated by the activation of CSF1R and the transcription factors PU.1 and C/EBPα as the molecular regulators of the proliferative response, correlating with the chronic human neurodegenerative conditions variant Creutzfeldt-Jakob disease and Alzheimer's disease. We show that targeting the activity of CSF1R inhibits microglial proliferation and slows neuronal damage and disease progression. Our results demonstrate that microglial proliferation is a major component in the evolution of chronic neurodegeneration, with direct implications for understanding the contribution of the CNS innate immune response to disease progression.

Differential role of CCR2 in the dynamics of microglia and perivascular macrophages during prion disease.

The expansion of the microglial population is one of the hallmarks of numerous brain disorders. The addition of circulating progenitors to the pool of brain macrophages can contribute to the progression of brain disease and needs to be precisely defined to better understand the evolution of the glial and inflammatory reactions in the brain. We have analyzed the degree of infiltration/recruitment of circulating monocytes to the microglial pool, in a prion disease model of chronic neurodegeneration. Our results indicate a minimal/absent level of CCR2-dependent recruitment of circulating monocytes, local proliferation of microglia is the main driving force maintaining the amplification of the population. A deficiency in CCR2, and thus the absence of recruitment of circulating monocytes, does not impact microglial dynamics, the inflammatory profile or the temporal behavioral course of prion disease. However, the lack of CCR2 has unexpected effects including the failure to recruit perivascular macrophages in diseased but not healthy CNS and a small reduction in microglia proliferation. These data define the composition of the CNS-resident macrophage populations in prion disease and will help to understand the dynamics of the CNS innate immune response during chronic neurodegeneration.

Systemic inflammation associates with and precedes cord atrophy in progressive multiple sclerosis.

In preclinical models of multiple sclerosis, systemic inflammation has an impact on the compartmentalized inflammatory process within the central nervous system and results in axonal loss. It remains to be shown whether this is the case in humans, specifically whether systemic inflammation contributes to spinal cord or brain atrophy in multiple sclerosis. Hence, an observational longitudinal study was conducted to delineate the relationship between systemic inflammation and atrophy using magnetic resonance imaging: the SIMS (Systemic Inflammation in Multiple Sclerosis) study. Systemic inflammation and progression were assessed in people with progressive multiple sclerosis (n = 50) over two and a half years. Eligibility criteria included: (i) primary or secondary progressive multiple sclerosis; (ii) age ≤ 70; and (iii) Expanded Disability Status Scale ≤ 6.5. First morning urine was collected weekly to quantify systemic inflammation by measuring the urinary neopterin-to-creatinine ratio using a validated ultra-performance liquid chromatography mass spectrometry technique. The urinary neopterin-to-creatinine ratio temporal profile was characterized by short-term responses overlaid on a background level of inflammation, so these two distinct processes were considered as separate variables: background inflammation and inflammatory response. Participants underwent MRI at the start and end of the study, to measure cervical spinal cord and brain atrophy. Brain and cervical cord atrophy occurred on the study, but the most striking change was seen in the cervical spinal cord, in keeping with the corticospinal tract involvement that is typical of progressive disease. Systemic inflammation predicted cervical cord atrophy. An association with brain atrophy was not observed in this cohort. A time lag between systemic inflammation and cord atrophy was evident, suggesting but not proving causation. The association of the inflammatory response with cord atrophy depended on the level of background inflammation, in keeping with experimental data in preclinical models where the effects of a systemic inflammatory challenge on tissue injury depended on prior exposure to inflammation. A higher inflammatory response was associated with accelerated cord atrophy in the presence of background systemic inflammation below the median for the study population. Higher background inflammation, while associated with cervical cord atrophy itself, subdued the association of the inflammatory response with cord atrophy. Findings were robust to sensitivity analyses adjusting for potential confounders and excluding cases with new lesion formation. In conclusion, systemic inflammation associates with, and precedes, multiple sclerosis progression. Further work is needed to prove causation since targeting systemic inflammation may offer novel treatment strategies for slowing neurodegeneration in multiple sclerosis.

An antigen-specific pathway for CD8 T cells across the blood-brain barrier.

CD8 T cells are nature's foremost defense in encephalitis and brain tumors. Antigen-specific CD8 T cells need to enter the brain to exert their beneficial effects. On the other hand, traffic of CD8 T cells specific for neural antigen may trigger autoimmune diseases like multiple sclerosis. T cell traffic into the central nervous system is thought to occur when activated T cells cross the blood-brain barrier (BBB) regardless of their antigen specificity, but studies have focused on CD4 T cells. Here, we show that selective traffic of antigen-specific CD8 T cells into the brain occurs in vivo and is dependent on luminal expression of major histocompatibility complex (MHC) class I by cerebral endothelium. After intracerebral antigen injection, using a minimally invasive technique, transgenic CD8 T cells only infiltrated the brain when and where their cognate antigen was present. This was independent of antigen presentation by perivascular macrophages. Marked reduction of antigen-specific CD8 T cell infiltration was observed after intravenous injection of blocking anti-MHC class I antibody. These results expose a hitherto unappreciated route by which CD8 T cells home onto their cognate antigen behind the BBB: luminal MHC class I antigen presentation by cerebral endothelium to circulating CD8 T cells. This has implications for a variety of diseases in which antigen-specific CD8 T cell traffic into the brain is a beneficial or deleterious feature.

C1q: the perfect complement for a synaptic feast?

The efficient and selective removal of apoptotic cells is an important feature of tissue development, homeostasis and pathology. In the nervous system, synapses and distal axons are selectively eliminated as part of the remodelling that underpins development and pathology, through a process that has some features in common with apoptotic cell removal. Components of the complement cascade are implicated in the efficient removal of apoptotic cells outside the nervous system, and recent evidence suggests that the complement components C1q and C3 have a role in the selective tagging of supernumerary synapses in the developing visual system and in their efficient removal by as yet unidentified cells.

Impact of stroke-associated infection on long-term survival: a cohort study.

BACKGROUND AND OBJECTIVE: The effects of stroke-associated infection (SAI) on long-term survival are unclear. We performed a prospective evaluation to explore risk factors of SAI, and compared survival status over the 3 years following stroke onset between those who experienced SAI and those who did not. METHODS: Consecutive patients with acute stroke admitted to a stroke unit between April 2005 and December 2006 were invited to participate. We prospectively collected data on demographics, pathological and clinical stroke subtype, stroke severity, and neurological and functional consequences, and abstracted additional data on occurrence and timing of SAI in hospital from medical notes. Survival status 3 years after stroke onset was obtained. RESULTS: We recruited 413 acute stroke patients, 161 (39%) experienced SAI. After excluding patients with infection at onset, patients with intracerebral haemorrhage (p=0.014), dysphagia (p=0.003) and urinary incontinence/catheterisation (p=0.000) were at higher risk of infection after controlling for case mix. The risk of death in hospital was greater following an SAI (HR 3.56; 95% CI 1.94 to 6.53; p=0.000), as was risk of death calculated over the whole 3-year follow-up period among those acquiring SAI within 2 weeks of onset (HR 1.66; 95% CI 1.14 to 2.40; p=0.031). CONCLUSIONS: SAIs have long-lasting effects on patient survival. This serves to emphasise the importance of immediate access to organised stroke unit care for people with acute stroke, with active physiological monitoring and protocols for early detection and treatment of SAIs.

Systemic inflammation modulates Fc receptor expression on microglia during chronic neurodegeneration.

Chronic neurodegeneration is a major worldwide health problem, and it has been suggested that systemic inflammation can accelerate the onset and progression of clinical symptoms. A possible explanation is that systemic inflammation "switches" the phenotype of microglia from a relatively benign to a highly aggressive and tissue-damaging phenotype. The current study investigated the molecular mechanism underlying this microglia phenotype "switching." We show in mice with chronic neurodegeneration (ME7 prion model) that there is increased expression of receptors that have a key role in macrophage activation and associated signaling pathways, including TREM-2, Siglec-F, CD200R, and FcγRs. Systemic inflammation induced by LPS further increased protein levels of the activating FcγRIII and FcγRIV, but not of other microglial receptors, including the inhibitory FcγRII. In addition to these changes in receptor expression, IgG levels in the brain parenchyma were increased during chronic neurodegeneration, and these IgG levels further increased after systemic inflammation. γ-Chain-deficient mice show modified proinflammatory cytokine expression in the brain after systemic inflammation. We conclude that systemic inflammation during chronic neurodegeneration increases the expression levels of activating FcγR on microglia and thereby lowers the signaling threshold for Ab-mediated cell activation. At the same time, IgG influx into the brain could provide a cross-linking ligand resulting in excessive microglia activation that is detrimental to neurons already under threat by misfolded protein.

Docosahexaenoic acid, but not eicosapentaenoic acid, reduces the early inflammatory response following compression spinal cord injury in the rat.

Docosahexaenoic acid (DHA, 22 : 6) and eicosapentaenoic acid (EPA, 20 : 5) are omega-3 polyunsaturated fatty acids (n-3 PUFAs) with distinct anti-inflammatory properties. Both have neuroprotective effects acutely following spinal cord injury (SCI). We examined the effect of intravenous DHA and EPA on early inflammatory events after SCI. Saline, DHA or EPA (both 250 nmol/kg) were administered 30 min after T12 compression SCI, to female Sprague-Dawley rats. DHA significantly reduced the number of neutrophils to some areas of the injured epicentre at 4 h and 24 h. DHA also reduced C-reactive protein plasma levels, whereas EPA did not significantly reduce neutrophils or C-reactive protein. Laminectomy and SCI elicited a sustained inflammatory response in the liver, which was not reversed by the PUFAs. The chemokine KC/GRO/CINC and the cytokine IL-6 provide gradients for chemotaxis of neutrophils to the epicentre. At 4 h after injury, there was a significant increase in IL-6, KC/GRO/CINC, IL-1ß and tumour necrosis factor-α in the epicentre, with a return to baseline at 24 h. Neither DHA nor EPA returned their levels to control values. These results indicate that the acute neuroprotective effects of n-3 PUFAs in rat compression SCI may be only partly attributed to reduction of some of the early inflammatory events occurring after injury.

The intrathecal CD163-haptoglobin-hemoglobin scavenging system in subarachnoid hemorrhage.

Delayed cerebral ischemia resulting from extracellular hemoglobin is an important determinant of outcome in subarachnoid hemorrhage. Hemoglobin is scavenged by the CD163-haptoglobin system in the circulation, but little is known about this scavenging pathway in the human CNS. The components of this system were analyzed in normal cerebrospinal fluid and after subarachnoid hemorrhage. The intrathecal presence of the CD163-haptoglobin-hemoglobin scavenging system was unequivocally demonstrated. The resting capacity of the CD163-haptoglobin-hemoglobin system in the normal CNS was 50 000-fold lower than that of the circulation. After subarachnoid hemorrhage, the intrathecal CD163-haptoglobin-hemoglobin system was saturated, as shown by the presence of extracellular hemoglobin despite detectable haptoglobin. Hemoglobin efflux from the CNS was evident, enabling rescue hemoglobin scavenging by the systemic circulation. Therefore, the CNS is not capable of dealing with significant intrathecal hemolysis. Potential therapeutic options to prevent delayed cerebral ischemia ought to concentrate on augmenting the capacity of the intrathecal CD163-haptoglobin-hemoglobin scavenging system and strategies to encourage hemoglobin efflux from the brain.

Long-term impact of systemic bacterial infection on the cerebral vasculature and microglia.

BACKGROUND: Systemic infection leads to generation of inflammatory mediators that result in metabolic and behavioural changes. Repeated or chronic systemic inflammation leads to a state of innate immune tolerance: a protective mechanism against overactivity of the immune system. In this study, we investigated the immune adaptation of microglia and brain vascular endothelial cells in response to systemic inflammation or bacterial infection. METHODS: Mice were given repeated doses of lipopolysaccharide (LPS) or a single injection of live Salmonella typhimurium. Inflammatory cytokines were measured in serum, spleen and brain, and microglial phenotype studied by immunohistochemistry. To assess priming of the innate immune response in the brain, mice were infected with Salmonella typhimurium and subsequently challenged with a focal unilateral intracerebral injection of LPS. RESULTS: Repeated systemic LPS challenges resulted in increased brain IL-1ß, TNF-α and IL-12 levels, despite attenuated systemic cytokine production. Each LPS challenge induced significant changes in burrowing behaviour. In contrast, brain IL-1ß and IL-12 levels in Salmonella typhimurium-infected mice increased over three weeks, with high interferon-γ levels in the circulation. Behavioural changes were only observed during the acute phase of the infection. Microglia and cerebral vasculature display an activated phenotype, and focal intracerebral injection of LPS four weeks after infection results in an exaggerated local inflammatory response when compared to non-infected mice. CONCLUSIONS: These studies reveal that the innate immune cells in the brain do not become tolerant to systemic infection, but are primed instead. This may lead to prolonged and damaging cytokine production that may have a profound effect on the onset and/or progression of pre-existing neurodegenerative disease.