Neuronal CCL2 expression drives inflammatory monocyte infiltration into the brain during acute virus infection
Charles L Howe1,2,3,4,5,6,7, Reghann G LaFrance-Corey8,9, Emma N Goddery8,10,11
1Translational Neuroimmunology Lab, Mayo Clinic, Rochester, USA. howe@mayo.edu.
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.
Insights
Infectious Theiler's murine encephalomyelitis virus (TMEV) rapidly induces chemokine CCL2 in neurons, driving inflammatory monocyte brain infiltration and hippocampal damage in viral encephalitis.
Area of Science:
- Neuroscience
- Immunology
- Virology
Background:
- Viral encephalitis presents a challenge in balancing pathogen clearance and neuronal protection.
- Theiler's murine encephalomyelitis virus (TMEV) infection in mice models encephalitis, causing hippocampal damage via inflammatory monocytes.
- Mechanisms of inflammatory monocyte recruitment to the brain in TMEV infection were previously unclear.
Purpose of the Study:
- To elucidate the mechanisms of inflammatory monocyte recruitment to the brain during acute TMEV infection.
- To identify the source of chemokines responsible for monocyte infiltration in the hippocampus.
Main Methods:
- Assessed chemokine expression (CCL2) in the hippocampus using microarray, ELISA, RT-PCR, and immunofluorescence.
- Measured monocyte infiltration via flow cytometry.
- Manipulated CCL2 levels through immunodepletion and genetic deletion in neurons.
Main Results:
- TMEV infection induced rapid hippocampal CCL2 production, primarily from neurons.
- Genetic deletion of CCR2 and CCL2 immunodepletion reduced monocyte infiltration.
- Neuronal-specific CCL2 deletion inhibited monocyte infiltration into the brain.
Conclusions:
- Infectious TMEV rapidly induces neuronal CCL2 expression, essential for CCR2-dependent monocyte infiltration.
- Neuronal chemokines play a critical role in initiating leukocytic infiltration in the infected central nervous system.
- This study highlights a novel mechanism in the early stages of viral encephalitis.
