Both Cerebral and Hematopoietic Deficiencies in CCR2 Result in Uncontrolled Herpes Simplex Virus Infection of the Central Nervous System in Mice

C-C motif chemokine receptor 2 and 7 synergistically control inflammatory monocyte recruitment but the infecting virus dictates monocyte function in the brain

Inflammatory monocytes (iMO) are recruited from the bone marrow to the brain during viral encephalitis. C-C motif chemokine receptor (CCR) 2 deficiency substantially reduces iMO recruitment for most, but not all encephalitic viruses. Here we show CCR7 acts synergistically with CCR2 to control this process. Following Herpes simplex virus type-1 (HSV-1), or La Crosse virus (LACV) infection, we find iMO proportions are reduced by approximately half in either Ccr2 or Ccr7 knockout mice compared to control mice. However, Ccr2/Ccr7 double knockouts eliminate iMO recruitment following infection with either virus, indicating these receptors together control iMO recruitment. We also find that LACV induces a more robust iMO recruitment than HSV-1. However, unlike iMOs in HSV-1 infection, LACV-recruited iMOs do not influence neurological disease development. LACV-induced iMOs have higher expression of proinflammatory and proapoptotic but reduced mitotic, phagocytic and phagolysosomal transcripts compared to HSV-1-induced iMOs. Thus, virus-specific activation of iMOs affects their recruitment, activation, and function.

CCR2 Deficiency Impairs Ly6Clo and Ly6Chi Monocyte Responses in Orientia tsutsugamushi Infection

Orientia (O.) tsutsugamushi, the causative agent of scrub typhus, is a neglected, obligate intracellular bacterium that has a prominent tropism for monocytes and macrophages. Complications often involve the lung, where interstitial pneumonia is a typical finding. The severity of scrub typhus in humans has been linked to altered plasma concentrations of chemokines which are known to act as chemoattractants for myeloid cells. The trafficking and function of monocyte responses is critically regulated by interaction of the CC chemokine ligand 2 (CCL2) and its CC chemokine receptor CCR2. In a self-healing mouse model of intradermal infection with the human-pathogenic Karp strain of O. tsutsugamushi, we investigated the role of CCR2 on bacterial dissemination, development of symptoms, lung histology and monocyte subsets in blood and lungs. CCR2-deficient mice showed a delayed onset of disease and resolution of symptoms, higher concentrations and impaired clearance of bacteria in the lung and the liver, accompanied by a slow infiltration of interstitial macrophages into the lungs. In the blood, we found an induction of circulating monocytes that depended on CCR2, while only a small increase in Ly6Chi monocytes was observed in CCR2-/- mice. In the lung, significantly higher numbers of Ly6Chi and Ly6Clo monocytes were found in the C57BL/6 mice compared to CCR2-/- mice. Both wildtype and CCR2-deficient mice developed an inflammatory milieu as shown by cytokine and inos/arg1 mRNA induction in the lung, but with delayed kinetics in CCR2-deficient mice. Histopathology revealed that infiltration of macrophages to the parenchyma, but not into the peribronchial tissue, depended on CCR2. In sum, our data suggest that in Orientia infection, CCR2 drives blood monocytosis and the influx and activation of Ly6Chi and Ly6Clo monocytes into the lung, thereby accelerating bacterial replication and development of interstitial pulmonary inflammation.

An Early Microglial Response Is Needed To Efficiently Control Herpes Simplex Virus Encephalitis

The role of a signaling pathway through macrophage colony-stimulating factor (MCSF) and its receptor, macrophage colony-stimulating factor 1 receptor (CSF1R), during experimental herpes simplex virus 1 (HSV-1) encephalitis (HSE) was studied by two different approaches. First, we evaluated the effect of stimulation of the MCSF/CSF1R axis before infection. Exogenous MCSF (40 μg/kg of body weight intraperitoneally [i.p.]) was administered once daily to BALB/c mice on days 4 and 2 before intranasal infection with 2,500 PFU of HSV-1. MCSF treatment significantly increased mouse survival compared to saline (50% versus 10%; P = 0.0169). On day 6 postinfection (p.i.), brain viral titers were significantly decreased, whereas beta interferon (IFN-β) was significantly increased in mice treated with MCSF compared to mice treated with saline. The number of CD68⁺ (a phagocytosis marker) microglial cells was significantly increased in MCSF-treated mice compared to the saline-treated group. Secondly, we conditionally depleted CSF1R on microglial cells of CSF1R-loxP-CX3CR1-cre/ERT2 mice (in a C57BL/6 background) through induction with tamoxifen. The mice were then infected intranasally with 600,000 PFU of HSV-1. The survival rate of mice depleted of CSF1R (knockout [KO] mice) was significantly lower than that of wild-type (WT) mice (0% versus 67%). Brain viral titers and cytokine/chemokine levels were significantly higher in KO than in WT animals on day 6 p.i. Furthermore, increased infiltration of monocytes into the brains of WT mice was seen on day 6 p.i., but not in KO mice. Our results suggest that microglial cells are essential to control HSE at early stages of the disease and that the MCSF/CSF1R axis could be a therapeutic target to regulate their response to infection.IMPORTANCE Microglia appear to be one of the principal regulators of neuroinflammation in the central nervous system (CNS). An increasing number of studies have demonstrated that the activation of microglia could result in either beneficial or detrimental effects in different CNS disorders. Hence, the role of microglia during herpes simplex virus encephalitis (HSE) has not been fully characterized. Using experimental mouse models, we showed that an early activation of the MCSF/CSF1R axis improved the outcome of the disease, possibly by inducing a proliferation of microglia. In contrast, depletion of microglia before HSV-1 infection worsened the prognosis of HSE. Thus, an early microglial response followed by sustained infiltration of monocytes and T cells into the brain seem to be key components for a better clinical outcome. These data suggest that microglia could be a potential target for immunomodulatory strategies combined with antiviral therapy to better control the outcome of this devastating disease.

Astrocyte- and Neuron-Derived CXCL1 Drives Neutrophil Transmigration and Blood-Brain Barrier Permeability in Viral Encephalitis

Herpes simplex virus (HSV)-1 encephalitis has significant morbidity partly because of an over-exuberant immune response characterized by leukocyte infiltration into the brain and increased blood-brain barrier (BBB) permeability. Determining the role of specific leukocyte subsets and the factors that mediate their recruitment into the brain is critical to developing targeted immune therapies. In a murine model, we find that the chemokines CXCL1 and CCL2 are induced in the brain following HSV-1 infection. Ccr2 (CCL2 receptor)-deficient mice have reduced monocyte recruitment, uncontrolled viral replication, and increased morbidity. Contrastingly, Cxcr2 (CXCL1 receptor)-deficient mice exhibit markedly reduced neutrophil recruitment, BBB permeability, and morbidity, without influencing viral load. CXCL1 is produced by astrocytes in response to HSV-1 and by astrocytes and neurons in response to IL-1α, and it is the critical ligand required for neutrophil transendothelial migration, which correlates with BBB breakdown. Thus, the CXCL1-CXCR2 axis represents an attractive therapeutic target to limit neutrophil-mediated morbidity in HSV-1 encephalitis.

CCR2 Plays a Protective Role in Rocio Virus-Induced Encephalitis by Promoting Macrophage Infiltration Into the Brain

Rocio virus (ROCV) is a highly neuropathogenic mosquito-transmitted flavivirus responsible for an unprecedented outbreak of human encephalitis during 1975–1976 in Sao Paulo State, Brazil. Previous studies have shown an increased number of inflammatory macrophages in the central nervous system (CNS) of ROCV-infected mice, implying a role for macrophages in the pathogenesis of ROCV. Here, we show that ROCV infection results in increased expression of CCL2 in the blood and in infiltration of macrophages into the brain. Moreover, we show, using CCR2 knockout mice, that CCR2 expression is essential for macrophage infiltration in the brain during ROCV infection and that the lack of CCR2 results in increased disease severity and mortality. Thus, our findings show the protective role of CCR2-mediated infiltration of macrophages in the brain during ROCV infection.

The recruitment of peripheral blood leukocytes to the brain is delayed in susceptible BALB/c compared to resistant C57BL/6 mice during herpes simplex virus encephalitis

The cerebral immune response induced by herpes simplex virus (HSV) encephalitis (HSE) was evaluated in susceptible BALB/c and resistant C57BL/6 mice. BALB/c and C57BL/6 (named C57BL/6-high) mice were respectively infected intranasally with 1 × 10³ and 5 × 10⁵ plaque-forming units (PFUs) of HSV-1. C57BL/6 mice (named C57BL/6-low) infected with a low inoculum (1 × 10³ PFUs) of HSV-1 were tested in parallel. Mice were monitored for weight loss, sickness signs, and survival for 21 days. The viral load, infectious titers, cytokine/chemokine levels, and peripheral leukocyte infiltration were determined in brain homogenates on days 0 (non-infected), 4, 6, and 8 post-infection (p.i.) by qPCR, plaque assay, ELISA/Luminex™, and flow cytometry, respectively. Our results showed that the mortality of BALB/c mice (67%) was higher compared to those of C57BL/6-low (0%; P ≤ 0.01) and C57BL/6-high (20%; P ≤ 0.05) animals. This higher mortality was associated with increased infectious titers and cytokine/chemokine levels in the brains of BALB/c compared to C57BL/6 mice. Recruitment of inflammatory monocytes, dendritic cells, natural killer, and natural killer T cells to the brain was higher in C57BL/6-high compared to BALB/c animals on day 4 p.i. Infiltration of inflammatory monocytes and T cells in the brain of BALB/c mice was seen on day 6 p.i. Our data suggest that a rapid, sustained, and coordinated recruitment of peripheral leukocytes to the brain of C57BL/6-high mice results in an effective control of viral replication and inflammation whereas the delayed infiltration of immune cells in the brain of BALB/c mice was associated with an exacerbated inflammatory response during HSE.

Inflammatory Events Following Subarachnoid Hemorrhage (SAH)

Acute SAH from a ruptured intracranial aneurysm contributes for 30% of all hemorrhagic strokes. The bleeding itself occurs in the subarachnoid space. Nevertheless, injury to the brain parenchyma occurs as a consequence of the bleeding, directly, via several well-defined mechanisms and pathways, but also indirectly, or secondarily. This secondary brain injury following SAH has a variety of causes and possible mechanisms. Amongst others, inflammatory events have been shown to occur in parallel to, contribute to, or even to initiate programmed cell death (PCD) within the central nervous system (CNS) in human and animal studies alike. Mechanisms of secondary brain injury are of utmost interest not only to scientists, but also to clinicians, as they often provide possibilities for translational approaches as well as distinct time windows for tailored treatment options. In this article, we review secondary brain injury due to inflammatory changes, that occur on cellular, as well as on molecular level in the various different compartments of the CNS: the brain vessels, the subarachnoid space, and the brain parenchyma itself and hypothesize about possible signaling mechanisms between these compartments.

Neuronal CCL2 expression drives inflammatory monocyte infiltration into the brain during acute virus infection

Background

Viral encephalitis is a dangerous compromise between the need to robustly clear pathogen from the brain and the need to protect neurons from bystander injury. Theiler’s murine encephalomyelitis virus (TMEV) infection of C57Bl/6 mice is a model of viral encephalitis in which the compromise results in hippocampal damage and permanent neurological sequelae. We previously identified brain-infiltrating inflammatory monocytes as the primary driver of this hippocampal pathology, but the mechanisms involved in recruiting these cells to the brain were unclear.

Methods

Chemokine expression levels in the hippocampus were assessed by microarray, ELISA, RT-PCR, and immunofluorescence. Monocyte infiltration during acute TMEV infection was measured by flow cytometry. CCL2 levels were manipulated by immunodepletion and by specific removal from neurons in mice generated by crossing a line expressing the Cre recombinase behind the synapsin promoter to animals with floxed CCL2.

Results

Inoculation of the brain with TMEV induced hippocampal production of the proinflammatory chemokine CCL2 that peaked at 6 h postinfection, whereas inoculation with UV-inactivated TMEV did not elicit this response. Immunofluorescence revealed that hippocampal neurons expressed high levels of CCL2 at this timepoint. Genetic deletion of CCR2 and systemic immunodepletion of CCL2 abrogated or blunted the infiltration of inflammatory monocytes into the brain during acute infection. Specific genetic deletion of CCL2 from neurons reduced serum and hippocampal CCL2 levels and inhibited inflammatory monocyte infiltration into the brain.

Conclusions

We conclude that intracranial inoculation with infectious TMEV rapidly induces the expression of CCL2 in neurons, and this cellular source is necessary for CCR2-dependent infiltration of inflammatory monocytes into the brain during the most acute stage of encephalitis. These findings highlight a unique role for neuronal production of chemokines in the initiation of leukocytic infiltration into the infected central nervous system.