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Inflammatory monocytes damage the hippocampus during acute picornavirus infection of the brain.

Howe CL, Lafrance-Corey RG, Sundsbak RS, Lafrance SJ - J Neuroinflammation (2012)

Bottom Line: Identification of the immune effectors responsible for injuring the brain during acute infection is necessary for the development of therapeutic strategies that reduce neuropathology but maintain immune control of the virus.Specific depletion of neutrophils with the 1A8 antibody failed to preserve neurons, suggesting that inflammatory monocytes are the key effectors of brain injury during acute picornavirus infection of the brain.These effector cells may be important therapeutic targets for immunomodulatory or immunosuppressive therapies aimed at reducing or preventing central nervous system pathology associated with acute viral infection.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Neurology, Mayo Clinic College of Medicine, Rochester, MN 55905, USA. howe@mayo.edu

ABSTRACT

Background: Neuropathology caused by acute viral infection of the brain is associated with the development of persistent neurological deficits. Identification of the immune effectors responsible for injuring the brain during acute infection is necessary for the development of therapeutic strategies that reduce neuropathology but maintain immune control of the virus.

Methods: The identity of brain-infiltrating leukocytes was determined using microscopy and flow cytometry at several acute time points following intracranial infection of mice with the Theiler's murine encephalomyelitis virus. Behavioral consequences of immune cell depletion were assessed by Morris water maze.

Results: Inflammatory monocytes, defined as CD45hiCD11b++F4/80+Gr1+1A8-, and neutrophils, defined as CD45hiCD11b+++F4/80-Gr1+1A8+, were found in the brain at 12 h after infection. Flow cytometry of brain-infiltrating leukocytes collected from LysM: GFP reporter mice confirmed the identification of neutrophils and inflammatory monocytes in the brain. Microscopy of sections from infected LysM:GFP mice showed that infiltrating cells were concentrated in the hippocampal formation. Immunostaining confirmed that neutrophils and inflammatory monocytes were localized to the hippocampal formation at 12 h after infection. Immunodepletion of inflammatory monocytes and neutrophils but not of neutrophils only resulted in preservation of hippocampal neurons. Immunodepletion of inflammatory monocytes also preserved cognitive function as assessed by the Morris water maze.

Conclusions: Neutrophils and inflammatory monocytes rapidly and robustly responded to Theiler's virus infection by infiltrating the brain. Inflammatory monocytes preceded neutrophils, but both cell types were present in the hippocampal formation at a timepoint that is consistent with a role in triggering hippocampal pathology. Depletion of inflammatory monocytes and neutrophils with the Gr1 antibody resulted in hippocampal neuroprotection and preservation of cognitive function. Specific depletion of neutrophils with the 1A8 antibody failed to preserve neurons, suggesting that inflammatory monocytes are the key effectors of brain injury during acute picornavirus infection of the brain. These effector cells may be important therapeutic targets for immunomodulatory or immunosuppressive therapies aimed at reducing or preventing central nervous system pathology associated with acute viral infection.

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Histological evidence of rapid immune cell infiltration into the hippocampus of mice acutely infected with Theiler's murine encephalomyelitis virus (TMEV). C57BL/6J mice were injected intracerebrally with 2 × 105 PFU TMEV in 10 μL of RPMI (B, D, H-J) or with 10 μL RPMI (SHAM) (A, C, E-G). The animals were killed by intracardiac perfusion of 4% paraformaldehyde 24 h after infection. Brain was processed for paraffin embedding and sectioning, and stained with hematoxylin and eosin. Robust inflammatory infiltrate is present above the hippocampus and throughout the corpus callosum in the TMEV-infected mice (B, D). Sham-infected mice exhibit no signs of infiltrate (A, C). Higher magnification confirms the presence of infiltrate throughout the corpus callosum and alveus (H), within the CA1 pyramidal neuron cell layer (I), and within the perivascular space along the hippocampal fissure (J) in virus-infected mice but not in sham-infected mice (E-G). Scale bar in B is 500 μm and refers to (A); scale bar in (D) is 100 μm and refers to (C); scale bar in (J) is 20 μm and refers to (E-I). Abbreviations: alv = alveus; CA1 and CA3 = cornu ammonis fields of the hippocampus; cc = corpus callosum; DG = dentate gyrus; fi = fimbria; hf = hippocampal fissure. Findings are representative of more than 20 animals per group.
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Figure 1: Histological evidence of rapid immune cell infiltration into the hippocampus of mice acutely infected with Theiler's murine encephalomyelitis virus (TMEV). C57BL/6J mice were injected intracerebrally with 2 × 105 PFU TMEV in 10 μL of RPMI (B, D, H-J) or with 10 μL RPMI (SHAM) (A, C, E-G). The animals were killed by intracardiac perfusion of 4% paraformaldehyde 24 h after infection. Brain was processed for paraffin embedding and sectioning, and stained with hematoxylin and eosin. Robust inflammatory infiltrate is present above the hippocampus and throughout the corpus callosum in the TMEV-infected mice (B, D). Sham-infected mice exhibit no signs of infiltrate (A, C). Higher magnification confirms the presence of infiltrate throughout the corpus callosum and alveus (H), within the CA1 pyramidal neuron cell layer (I), and within the perivascular space along the hippocampal fissure (J) in virus-infected mice but not in sham-infected mice (E-G). Scale bar in B is 500 μm and refers to (A); scale bar in (D) is 100 μm and refers to (C); scale bar in (J) is 20 μm and refers to (E-I). Abbreviations: alv = alveus; CA1 and CA3 = cornu ammonis fields of the hippocampus; cc = corpus callosum; DG = dentate gyrus; fi = fimbria; hf = hippocampal fissure. Findings are representative of more than 20 animals per group.

Mentions: C57BL/6J mice at 5 to 8 weeks of age were infected by intracranial injection of 2 × 105 PFU of the Daniel's strain of TMEV in a volume of 10 μL. Sham-infected mice received identical injections of vehicle lacking virus. Histological analysis of the brain at 24 h post infection (hpi) revealed marked inflammation and large numbers of infiltrating cells in the infected mice (Figure 1). Inflammation was most concentrated in the hippocampal region (Figure 1B), with high density in the corpus callosum and alveus and at the hippocampal fissure (Figure 1B, D). Infiltrating cells were also observed within the pyramidal neuron layer of cornu ammonis 1 (CA1) in the hippocampus (Figure 1D, I). In sham-infected mice, despite receiving a needle stick through the calvarium, no evidence of inflammation was ever observed in the brain. We also never observed an inflammatory response in mice injected with UV-inactivated TMEV (data not shown). We conclude that productive infection of the brain with TMEV results in the rapid induction of immune cell infiltration that is concentrated in the white matter above the hippocampus and at the hippocampal fissure.


Inflammatory monocytes damage the hippocampus during acute picornavirus infection of the brain.

Howe CL, Lafrance-Corey RG, Sundsbak RS, Lafrance SJ - J Neuroinflammation (2012)

Histological evidence of rapid immune cell infiltration into the hippocampus of mice acutely infected with Theiler's murine encephalomyelitis virus (TMEV). C57BL/6J mice were injected intracerebrally with 2 × 105 PFU TMEV in 10 μL of RPMI (B, D, H-J) or with 10 μL RPMI (SHAM) (A, C, E-G). The animals were killed by intracardiac perfusion of 4% paraformaldehyde 24 h after infection. Brain was processed for paraffin embedding and sectioning, and stained with hematoxylin and eosin. Robust inflammatory infiltrate is present above the hippocampus and throughout the corpus callosum in the TMEV-infected mice (B, D). Sham-infected mice exhibit no signs of infiltrate (A, C). Higher magnification confirms the presence of infiltrate throughout the corpus callosum and alveus (H), within the CA1 pyramidal neuron cell layer (I), and within the perivascular space along the hippocampal fissure (J) in virus-infected mice but not in sham-infected mice (E-G). Scale bar in B is 500 μm and refers to (A); scale bar in (D) is 100 μm and refers to (C); scale bar in (J) is 20 μm and refers to (E-I). Abbreviations: alv = alveus; CA1 and CA3 = cornu ammonis fields of the hippocampus; cc = corpus callosum; DG = dentate gyrus; fi = fimbria; hf = hippocampal fissure. Findings are representative of more than 20 animals per group.
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Figure 1: Histological evidence of rapid immune cell infiltration into the hippocampus of mice acutely infected with Theiler's murine encephalomyelitis virus (TMEV). C57BL/6J mice were injected intracerebrally with 2 × 105 PFU TMEV in 10 μL of RPMI (B, D, H-J) or with 10 μL RPMI (SHAM) (A, C, E-G). The animals were killed by intracardiac perfusion of 4% paraformaldehyde 24 h after infection. Brain was processed for paraffin embedding and sectioning, and stained with hematoxylin and eosin. Robust inflammatory infiltrate is present above the hippocampus and throughout the corpus callosum in the TMEV-infected mice (B, D). Sham-infected mice exhibit no signs of infiltrate (A, C). Higher magnification confirms the presence of infiltrate throughout the corpus callosum and alveus (H), within the CA1 pyramidal neuron cell layer (I), and within the perivascular space along the hippocampal fissure (J) in virus-infected mice but not in sham-infected mice (E-G). Scale bar in B is 500 μm and refers to (A); scale bar in (D) is 100 μm and refers to (C); scale bar in (J) is 20 μm and refers to (E-I). Abbreviations: alv = alveus; CA1 and CA3 = cornu ammonis fields of the hippocampus; cc = corpus callosum; DG = dentate gyrus; fi = fimbria; hf = hippocampal fissure. Findings are representative of more than 20 animals per group.
Mentions: C57BL/6J mice at 5 to 8 weeks of age were infected by intracranial injection of 2 × 105 PFU of the Daniel's strain of TMEV in a volume of 10 μL. Sham-infected mice received identical injections of vehicle lacking virus. Histological analysis of the brain at 24 h post infection (hpi) revealed marked inflammation and large numbers of infiltrating cells in the infected mice (Figure 1). Inflammation was most concentrated in the hippocampal region (Figure 1B), with high density in the corpus callosum and alveus and at the hippocampal fissure (Figure 1B, D). Infiltrating cells were also observed within the pyramidal neuron layer of cornu ammonis 1 (CA1) in the hippocampus (Figure 1D, I). In sham-infected mice, despite receiving a needle stick through the calvarium, no evidence of inflammation was ever observed in the brain. We also never observed an inflammatory response in mice injected with UV-inactivated TMEV (data not shown). We conclude that productive infection of the brain with TMEV results in the rapid induction of immune cell infiltration that is concentrated in the white matter above the hippocampus and at the hippocampal fissure.

Bottom Line: Identification of the immune effectors responsible for injuring the brain during acute infection is necessary for the development of therapeutic strategies that reduce neuropathology but maintain immune control of the virus.Specific depletion of neutrophils with the 1A8 antibody failed to preserve neurons, suggesting that inflammatory monocytes are the key effectors of brain injury during acute picornavirus infection of the brain.These effector cells may be important therapeutic targets for immunomodulatory or immunosuppressive therapies aimed at reducing or preventing central nervous system pathology associated with acute viral infection.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Neurology, Mayo Clinic College of Medicine, Rochester, MN 55905, USA. howe@mayo.edu

ABSTRACT

Background: Neuropathology caused by acute viral infection of the brain is associated with the development of persistent neurological deficits. Identification of the immune effectors responsible for injuring the brain during acute infection is necessary for the development of therapeutic strategies that reduce neuropathology but maintain immune control of the virus.

Methods: The identity of brain-infiltrating leukocytes was determined using microscopy and flow cytometry at several acute time points following intracranial infection of mice with the Theiler's murine encephalomyelitis virus. Behavioral consequences of immune cell depletion were assessed by Morris water maze.

Results: Inflammatory monocytes, defined as CD45hiCD11b++F4/80+Gr1+1A8-, and neutrophils, defined as CD45hiCD11b+++F4/80-Gr1+1A8+, were found in the brain at 12 h after infection. Flow cytometry of brain-infiltrating leukocytes collected from LysM: GFP reporter mice confirmed the identification of neutrophils and inflammatory monocytes in the brain. Microscopy of sections from infected LysM:GFP mice showed that infiltrating cells were concentrated in the hippocampal formation. Immunostaining confirmed that neutrophils and inflammatory monocytes were localized to the hippocampal formation at 12 h after infection. Immunodepletion of inflammatory monocytes and neutrophils but not of neutrophils only resulted in preservation of hippocampal neurons. Immunodepletion of inflammatory monocytes also preserved cognitive function as assessed by the Morris water maze.

Conclusions: Neutrophils and inflammatory monocytes rapidly and robustly responded to Theiler's virus infection by infiltrating the brain. Inflammatory monocytes preceded neutrophils, but both cell types were present in the hippocampal formation at a timepoint that is consistent with a role in triggering hippocampal pathology. Depletion of inflammatory monocytes and neutrophils with the Gr1 antibody resulted in hippocampal neuroprotection and preservation of cognitive function. Specific depletion of neutrophils with the 1A8 antibody failed to preserve neurons, suggesting that inflammatory monocytes are the key effectors of brain injury during acute picornavirus infection of the brain. These effector cells may be important therapeutic targets for immunomodulatory or immunosuppressive therapies aimed at reducing or preventing central nervous system pathology associated with acute viral infection.

Show MeSH
Related in: MedlinePlus