Lymphocytic choriomeningitis virus (LCMV) 1 infection of the murine brain elicits
fatal immunopathology through blood brain barrier (BBB) breakdown 2 and convulsive
seizures 3. While LCMV-specific cytotoxic T lymphocytes (CTL) are essential for disease
4, their mechanism of action is not known. To gain novel insights into disease pathogenesis,
we visualized the dynamics of immune cells in the meninges by two-photon microscopy
(TPM). We observed motile CTL and massive secondary recruitment of pathogenic monocytes
and neutrophils that were required for vascular leakage and acute lethality. CTL expressed
multiple chemoattractants capable of recruiting myelomonocytic cells. We conclude
that a CD8+ T cell dependent disorder can proceed in the absence of direct T cell
effector mechanisms and rely instead on CTL recruited myelomonocytic cells.
To examine the dynamics of LCMV-specific CTL, we transferred 1×105 naïve GFP-tagged
DbGP33–41 T cell receptor (TCR) transgenic (tg) CD8+ T cells (GFP+ P14 cells) into
B6 mice one day prior to intracranial (i.c.) inoculation with LCMV Armstrong (Arm).
TPM was performed through a thinned skull window to visualize the meninges overlying
the visual cortex in asymptomatic (day 5) and symptomatic (day 6) mice (Fig. 1; Movie
1). In contrast to the few P14 cells observed on day 5 (Fig. 1a), the number of GFP+
P14 cells was dramatically increased in the meninges and perivascular regions on day
6 (Fig. 1b, c). To determine if GFP+ P14 cells were engaging in antigen specific interactions,
we analyzed their motion in the presence of control antibody (IgG) or a blocking monoclonal
antibody to Db (anti-class I) introduced into the subarachnoid space through a small
craniotomy (Fig. 1d-i). P14 speed averaged 3.41 ± 0.27 µm/min (mean ± s.e.m.) in the
absence of the craniotomy and 3.04 ± 0.33 µm/min in the presence of the craniotomy
and IgG (Fig. 1j). The anti-class I significantly increased the speed of P14 cells
to 5.16 ± 0.46 µm/min (Fig. 1j,k) and decreased the arrest coefficient (Fig. 1l),
but did not influence the speed of CTL specific for an irrelevant antigen (Fig. S1).
This significant change in P14 cell speed and arrest following anti-class I treatment
was observed in all mice examined and did not depend on CTL abundance (Fig. S2). GFP+
P14 CTL migration appeared random (Fig. S3), with confined motion at longer times
that was reversed by anti-class I. Comparison of the speed distributions showed that
the entire population shifts following anti-class I treatment (Fig. 1k), suggesting
that all GFP+ P14 cells encountered antigen. Despite this high frequency of antigen
encounter, CTL rarely synapsed with any one target for > 10 minutes (Fig. 1g,m). These
intravital observations raised questions about the infected target population and
the CTL effector mechanisms utilized during fatal meningitis.
We identified the LCMV infected cells through immunohistochemical studies (Fig. 2
& S4; Movie 2). The main LCMV infected population in the meninges and around meningeal
vasculature was ER-TR7+ stromal cells. LCMV infection was occasionally observed in
CD45+ infiltrating leukocytes and astrocytic foot processes that comprise the glial
limitans (Fig. S4). Infection of endothelium and smooth muscle cells / pericytes was
never observed (Fig. S4). ER-TR7+ stromal cells support rapid migration of CD8+ and
CD4+ T cells in lymph nodes 5 and may provide strong chemokinetic signals that can
overwhelm synapse forming stop signals 6. This might explain the paucity of antigen
specific arrest (Fig. 1).
Since CD8+ T cells are essential for pathology 4, we evaluated several CTL effector
mechanisms using genetic knockout and mutant mice (Fig. 3a). Surprisingly, mice with
single deficiencies in all major CTL effector pathways – IFNγ receptor, TNFα, Fas,
granzymes, perforin (PFP), and the degranulation pathway (Jinx mutant) -succumbed
to the convulsive seizures observed following LCMV infection of wild type mice. The
delay in disease onset observed in perforin knockout mice was recently attributed
to slower CTL recruitment into CNS 7. These data supported the imaging studies in
suggesting that CTL effector functions might not be responsible for rapid onset disease.
To investigate other effectors, we temporally examined the composition of the CNS
infiltrate following i.c. LCMV infection (Fig. 3b & S5). Baseline populations prevailed
until day 6 at which point monocytes / macrophages were massively recruited into the
CNS. A low number of these cells preceded the arrival of CTL by 2 days. At day 6 a
small increase in the numbers of neutrophils, CD4+ T cells, and B cells was also observed;
however, the latter two populations are not required for disease 8,9. It should be
noted that our methodology accounts primarily for extravasated leukocytes, as cells
arrested in the vasculature (e.g., neutrophils) are expunged during intracardiac saline
perfusions. Nevertheless, our results demonstrate a minimal innate cellular response
to the virus alone and massive recruitment of myelomonocytic cells that coincided
with the arrival of CTL at day 6.
We next asked whether monocytes and/or neutrophils were required for the seizure-induced
death on day 6 (Fig. 3c–f). Neutrophil depletion with low dose anti-Gr-1 antibody
10 (Fig. 3c–d) or monocyte infiltration blockade using CCR2 deficient mice 11 (Fig.
3e–f) had no effect on the nature or kinetics of death. Therefore, we hypothesized
that both populations might have the potential to induce CNS injury. To test this
hypothesis, we depleted monocytes and neutrophils simultaneously by administering
high dose anti-Gr-1 to CCR2 knockout mice (Fig. 3e,f). When both cell populations
were depleted, seizure-induced death at day 6 was averted and survival was extended
by 3 days (Fig. 3e,f), despite a normal frequency of virus-specific CTL on day 6 (data
not shown). These data suggested that myelomonocytic cells were highly pathogenic
and were responsible for the rapid onset seizure-induced death observed at day 6.
During TPM analyses of GFP+ P14 cells, we often noted that the vasculature appeared
ragged and displayed plasma leakage tracked with intravascular injected quantum dots
(Movie 3). We considered that the seizure-induced death at day 6 might be induced
by vascular leakage caused by myelomonocytic cells. To test this possibility, we conducted
TPM in LCMV infected LysM-GFP mice, in which neutrophils and monocytes are labeled
with GFP, to detect the relationship between myelomonocytic extravasation and vascular
leakage. There was a tight correspondence between locally synchronized LysM-GFP+ cell
extravasation and vascular leakage on day 6 (Fig. 4e–h; Movie 4).
To determine the relative contribution of neutrophils versus monocytes to vascular
injury, we imaged LysM-GFP mice injected with low dose anti-Gr-1 antibody, which depletes
only neutrophils (Fig. 3c & S6). Interestingly, synchronous extravasation of LysM-GFP+
cells was not observed in low dose Gr-1-depleted mice (Fig. 4i–l; Movie 5), suggesting
that synchronously extravasating LysM-GFP+ cells are neutrophils. In neutrophil depleted
LysM-GFP mice, we observed perivascular LysM-GFP+ cells (i.e., monocytes / macrophages)
in areas of transient vascular leakage. Unlike neutrophils that display intravascular
accumulation followed by explosive extravasation with vascular leakage, the monocytes
accumulated more gradually in vascular sites that nonetheless displayed leakage. Statistically,
sustained vascular leakage was only correlated (r = 0.99; p < 0.0001) with neutrophils
(Fig. 4m). The presence of intra- or extravascular P14 CTL was not associated with
either pattern of vascular leakage (Fig. 4o). Quantum dot leakage was not observed
on day 5 post-infection (Fig. 4p) despite low numbers of infiltrating monocytes (Fig.
3b). Vascular injury occurred only at day 6 post-infection and extended into the brain
parenchyma (Fig. 4q). Myelomonocytic cells were restricted to the meninges on day
6 (Fig. 4q,r & S10).
The impact of myelomonocytic cells on vascular injury was further assessed by quantifying
leakage of Evans blue dye into the brain (Fig. S7). Only mice depleted of both monocytes
and neutrophils showed significant preservation of vascular integrity at day 6 post-infection
(Fig. S7e,f). Depletion of monocytes (Fig. S7d,f) or neutrophils (Fig. S7c,f) alone
failed to prevent Evans blue leakage. Interestingly, in untreated wild type mice at
day 6 post-infection, we observed substantial leakage of Evans blue from meningeal
blood vessels into the brain parenchyma (Fig. S8), reflecting disrupted BBB integrity,
which has the potential to cause severe seizures 12.
To examine a potential mechanism by which CTL attract myelomonocytic cells, we used
gene arrays to quantify differentially regulated transcripts in the brains of mock-versus
d6 LCMV-infected mice. Our results revealed a statistically significant increase (p
< 0.05) in 6 chemokines (CCL2 - 7.3 fold, CCL3 – 8.1 fold, CCL4 – 1.6 fold, CCL5 –
5.6 fold, CCL7 – 4.2 fold, CXCL2 – 2.8 fold) and 2 chemokine receptors (CCR1 – 2.6
fold, CCR2 – 3.6 fold) that can recruit myelomonocytic cells into the CNS. Because
it was reported that none of these chemokines were observed in the CNS of T cell-deficient
mice infected i.c. with LCMV 13, we next used flow cytometry to examine which of these
chemokines were produced by virus-specific P14 cells (Fig. S9). Our flow cytometric
analyses of CNS and splenic P14 CTL at day 6 revealed that CCL3, CCL4, and CCL5 were
all produced at the protein level, which was confirmed using gene arrays 14,15. Both
CCL3 and CCL4 required GP33–41 peptide stimulation for maximum synthesis, whereas
CCL5 was produced upon differentiation from naïve to effector cells and was not further
upregulated upon peptide stimulation.
It is well known that CD8+ T cells are required for LCMV-induced meningitis 4 and
vascular leakage 16. Our results revealed that P14 CTL could produce three of the
chemokines responsible for attracting the myelomonocytic cells responsible for vascular
injury (Fig. 4 & S7) and rapid onset seizure-induced death (Fig. 3). To establish
a direct link between CD8+ T cells and CNS myelomonocytic cell recruitment, we infected
mice with LCMV and administered anti-CD8 antibody at days 4 and 5 post-infection –
after CTL priming. This treatment, which reduced the number of CD8+ T cells in the
CNS by 94%, prevented the rapid onset of seizures at day 6 post-infection (data not
shown), and significantly reduced the number of monocytes and neutrophils in the CNS
(Fig. S10). These data indicate that virus-specific CTL can contribute to the recruitment
of pathogenic myelomonocytic cells either by directly releasing chemoattractants or
possibly by inducing other cells to release chemoattractants.
The requirement for CD8+ T cells in the pathogenesis of LCMV meningitis led to the
proposal that CTL were directly responsible for tissue injury and death 17. We propose
that CTL activation through transient interactions with infected cells leads to massive
recruitment of myelomonocytic cells, which compromise vascular integrity and initiate
fatal convulsive seizures 3. It is likely that once seizure-induced death is averted,
infected mice ultimately succumb to another pathogenic mechanism possibly mediated
by CTL 18.
It is well established that neutrophil extravasation can be linked to vascular leakage
19–21, and this process usually depends on signaling between leukocytes and endothelial
cells 22. Neutrophil extravasation causes tissue injury in many models including reperfusion
injury and sepsis 23,24. Using TPM we directly visualized this classical process in
LCMV meningitis. Monocytes have been associated with atherosclerosis 25 and facilitating
the trafficking of neutrophils 26. Our results suggest that monocytes also contribute
vascular leakage, possibly through a mechanism linked to adherence to the blood vessels
27 and / or chemokine release 28. Recognizing the complementary pathogenic functions
of neutrophils and monocytes is critical for devising therapeutic approaches in CD8+
T cell-mediated pathology.
It is not clear why CD8+ T cells recruit myelomonocytic cells to a site of viral infection.
CD4+ Th17 cells produce IL-17 to coordinate neutrophil recruitment, but anti-viral
CD8+ T cells express transcription factors that suppress this program 29, and we observed
no IL-17 production by peptide-stimulated P14 cells (data not shown). Therapies directed
at reducing myelomonocytic activation are obvious treatment candidates to prevent
the mode of immunopathology we observed, but are challenging due to their numerous
effector mechanisms, fast turnover, and acute importance in host defense. A more tractable
approach might be to target the chemotactic mechanisms used by CTL to attract myelomonocytic
cells, or, alternatively, to enhance CTL-mediated killing of relevant targets by improving
immunological synapse formation or stability 30. The latter approach might break the
feedback to the pathogenic myelomonocytic arm and improve survival as well as immunity
in viral infections of the CNS.
METHODS SUMMARY
To induce meningitis, mice were infected intracerebrally with the Armstrong strain
of LCMV. Fluorescent protein tagged virus specific P14 CTL and myelomonocytic cells
were imaged through a surgically thinned skull using a Bio-Rad multi-photon microscope.
Skull was detected using second harmonic signal, and meningeal vasculature was visualized
by intravenously injecting quantum dots 10 minutes prior imaging. All imaging data
were processed and analyzed using Volocity software.
For flow cytometric studies, mice received an intracardiac perfusion with saline.
The CNS was then harvested, treated with collagenase D, and mononuclear cells were
isolated using a Percoll gradient. Afterward, mononuclear cells were treated with
Fc block, serum, stained with fluorescently labeled antibodies, and acquired using
a Becton Dickinson digital flow cytometer. Data were analyzed using the FlowJo software.
Myelomonocytic cells were depleted from mice by intraperitoneally injecting 125 ug
(low dose) or 400 ug (high dose) of purified, endotoxin free anti-Gr-1 (RB6-8C5 clone).
Low dose antibody was injected once at day 4 post-infection to deplete neutrophils
only, whereas high dose antibody (to deplete both neutrophils and monocytes) was injected
daily starting at day 3 post-infection. CD8+ T cells were depleted by injecting 1000
ug of purified, endotoxin free anti-CD8 (53-6.72 clone) on day 4 and 500 ug day 5
post-LCMV infection.
CNS vascular leakage was evaluated by retroorbitally injecting mice with Evans Blue.
After 4 hours, mice were perfused with saline and brains were harvested. Evans blue
was extracted using N,N-dimethyl formamide and quantified using a Varioskan Flash
fluorometer (620nm excitation; 695 nm emission). Representative qualitative images
of Evans Blue permeability from were photographed using a digital camera, and fluorescence
images on vibratome brain sections were captured using a BioRad MRC2100 confocal microscope
(637 nm excitation).
All immunohistochemistry was performed on 6-µm frozen sections or 100-µm vibratome
sections. Primary antibodies were detected with fluorescently labeled secondary reagents
and visualized using an Axiovert S100 immunofluorescence microscope or a BioRad MRC2100
confocal microscope.
To quantify gene expression changes in the LCMV-infected CNS, total RNA was isolated
from saline perfused mock- and d6 LCMV infected brains and hybridized overnight to
the Mouse Exon 1.0 ST Array. Chips were scanned using the Affymetrix GeneChip Scanner.
The analysis was run using XRAY (version 2.3) software - the Excel add-in from Biotique
Systems Inc. Myelomonocytic cell chemoattractants with increased expression at day
6 post-infection were identified, and their expression at the protein level by P14
CTL was quantified flow cytometrically.
METHODS
Transgenic mice
C57BL/6 (B6), B6 GFP+DbGP33–41 TCR-tg (GFP+ P14) 31, B6 Thy1.1+DbGP33–41 TCR-tg (Thy1.1+
P14), B6 OT-I TCR-tg, B6 Perforin−/− (PFP−/−), B6 TNFα−/−, and B6 IFNγR−/− mice were
bred and maintained in a closed breeding facility at The Scripps Research Institute.
B6, B6 CCR2−/− and B6 Fas−/− mice were obtained from The Jackson Laboratories. The
following mice were generous gifts from other investigators: B6 LysM-GFP heterozygous
knock in mice (LysM gfp/+) (Dr. Thomas Graf; Albert Einstein) 32, B6 Jinx mice (Dr.
Bruce Beutler; The Scripps Research Institute) 33, and 129 granzyme a × b cluster
knockout mice (deficient in granzymes a, b, c, and f) (Dr. Timothy Ley; Washington
University) 34. All mice were housed in specific pathogen-free conditions and treated
in accordance with Institutional Animal Care and Use Committee protocols of The Scripps
Research Institute and New York University School of Medicine.
Virus
To induce meningitis, adult mice at 6–8 week of age were infected intracerebrally
(i.c.) with 1×103 plaque forming units (PFU) of LCMV Armstrong (Arm) clone 53b. Survival
was monitored daily. Stocks were prepared by a single passage on BHK-21 cells, and
viral titers were determined by plaque formation on Vero cells.
Mononuclear cell isolations and tissue processing
To obtain cell suspensions for flow cytometric analysis, the brain and spinal cord
were harvested from mice after an intracardiac perfusion with 25 ml of 0.9% saline
solution to remove the contaminating blood lymphocytes. The CNS was then incubated
with 1 ml collagenase D (1 mg/ml; Roche) at 37°C for 30 minutes. Single-cell suspensions
were prepared by mechanical disruption through a 100-µm filter. Brain-infiltrating
leukocytes were isolated and counted as described previously 35. For immunohistochemical
analyses, fresh, unfixed brain tissue was either frozen on dry ice in optimal cutting
temperature (OCT; Tissue-Tek) (for frozen sectioning) or incubated overnight with
4% PFA (for vibratome sectioning).
GFP+P14 Cell Transfer
CD8+ T cells were purified from GFP+ P14 mouse splenocytes by negative selection (Stem
Cell Technologies). The purity after enrichment was determined to be greater than
98%. For imaging studies, 1×105 purified GFP+ P14 CD8+ T cells were injected i.v.
into naïve mice. One day later the mice were challenged i.c. with LCMV Arm.
OT-I Cell Transfer
Splenocyes were isolated from OT-I mice and expanded in vitro for 7 days in RPMI 1640
containing 10% FBS, 1% L-glutamine, 1% penicillin/streptomycin, 500 units/ml IL-2,
and 1 µg/mL OVA257–264 (SIINFEKL) peptide. The cells were then labeled with 5 µM CFSE
in PBS (InVitrogen) at 37°C for 5 min and washed 3 times in PBS. 1×107 CFSE-labeled
OT-I cells were transferred into each d6 LCMV-infected B6 mouse and intravital imaging
was performed 6–12 hrs after the transfer.
CD8 T cell Depletion
To deplete CD8+ T cells from LCMV infected B6 mice, 1000 ug of purified, endotoxin
free anti-CD8 (53-6.72 clone) was injected on day 4 and 500 ug was injected on day
5 post-LCMV infection. This resulted in a 94% reduction in CNS-infiltrating CD8+ T
cells. The control group for these experiments was injected with polyclonal rat IgG
(Jackson ImmunoResearch Laboratories).
Neutrophil Depletion
To deplete myelomonocytic cells from B6 and B6 CCR2−/− mice, 125 ug (low dose) or
400 ug (high dose) of purified, endotoxin free anti-Gr-1 (RB6-8C5 clone) antibody
was injected i.p. The RB6-85C hybridoma was generously provided by Dr. Paul Allen
(Washington University). Low dose antibody was injected once at day 4 post-infection
to deplete neutrophils only. This resulted in an 87% reduction of neutrophils in the
CNS and 98% reduction in the blood. High dose antibody was injected daily starting
at day 3 post-infection to deplete both neutrophils and monocytes. Injection of polyclonal
rat IgG (Jackson ImmunoResearch Laboratories) was used as a control.
Gene Array
Total RNA from mock and d6 LCMV infected brains (n=3 mice per group) was isolated
using a using a Qiagen RNeasy Midi prep kit and then quantified using Nanodrop ND-1000.
Sample quality was checked using Agilent 2100 Bioanalyzer. In order to remove most
of the ribosomal RNA from the RNA, 2.5ug of each sample was taken through RiboMinus
(Invitrogen). After RiboMinus treatment samples were amplified and labeled using the
GeneChip Whole Transcript Sense Target Labeling Assay (Affymetrix). Samples were checked
by gel shift assay to assess labeling efficiency as described in the GeneChip Whole
Transcript Sense Target Labeling Assay Manual. Samples were hybridized overnight to
the Mouse Exon 1.0 ST Array. Hybridization and scanning of samples to arrays was performed
using standard Affymetrix protocols and reagents from the GeneChip Hybridization,
Wash, and Stain Kit. Chips were scanned using the Affymetrix GeneChip Scanner 3000
7G with default settings and a target intensity of 250 for scaling. In order to identify
genes with differential gene expression or alternative splicing between the two groups,
we studied 3 hybridizations each on the Mouse Exon 1.0 ST array using mixed model
analysis of variance. The analysis was run using XRAY (version 2.3) software - the
Excel add-in from Biotique Systems Inc.
Analysis of BBB integrity
To quantify BBB permeability, Evans blue leakage in the brains of mock infected or
LCMV i.c. infected mice was assessed. On the indicated day, mice were injected retroorbitally
with 20 mg per kg Evans Blue (Sigma). After 4 hours, the brains were extracted after
a PBS perfusion, which was used to eliminate circulating Evans Blue. The tissue was
homogenized in 600µl of N,N-dimethyl formamide (Sigma). The homogenate was transferred
to new tubes, centrifuged at 14,000 rpm for 20 mins at 4°C and the supernatant was
plated in triplicate wells in a 96 well flat bottom plate. For quantification an Evans
Blue standard was diluted in the supernatant of a PBS-perfused uninfected brain that
received no Evans Blue, but was homogenized in N,N-dimethyl formamide. All samples
plated in triplicate were read using a Varioskan Flash fluorometer (620nm excitation;
695 nm emission) (Thermo Scientific). The excitation and emission wavelengths were
determined by spectral scanning to be optimal for Evans blue. Representative qualitative
images of Evans Blue permeability from PBS perfused brains were taken using a digital
camera.
Flow Cytometry
Brain-infiltrating leukocytes were harvested and blocked with 3.3 µg/ml anti-mouse
CD16/CD32 (Fc Block; BD Biosciences) in PBS containing 1% FBS and 0.1% sodium azide
for 15 minutes on ice. After Fc block, cells were stained with the following conjugated
antibodies: CD45.2 FITC (104), Thy1.2 PE (53-2.1), CD19 PerCP CY5.5 (1D3), Gr1 APC
(RB6-8C5), CD4 APC Cy7 (GK1.5) (BD Biosciences), CD8 Pacific Blue (Caltag), MCA771
PE (7/4; Serotec), and anti-CD11b PE/Cy7 (M1/70; ebioscience). Cells were acquired
using a digital flow cytometer (Digital LSR II; Becton Dickinson) and flow cytometric
data were analyzed with FlowJo software (Tree Star, Inc).
Intracellular chemokine staining
Mice seeded with 104 P14 Thy1.1 cells on day -1 were infected with 103 PFU LCMV Arm
i.c. on day 0. On day 6 post infection, brain-infiltrating leukocytes and splenocytes
from infected mice and splenocytes from naïve P14 Thy1.1 mice were harvested and stimulated
with 50U/ml IL-2 (Roche), 1ug/ml Brefeldin A (Sigma-Aldrich) and 1ug/ml GP33–41 peptide
for 5 hours at 37°C. Cells were centrifuged and then blocked with 3.3 µg/ml anti-mouse
CD16/CD32 (Fc Block; BD Biosciences) in PBS containing 1% FBS and 0.1% sodium azide
for 15 minutes on ice. After Fc block, cells were stained with the following conjugated
antibodies: CD45.2 FITC (104), Thy1.1 PerCP (53-2.1) (BD Biosciences) and CD8 APC/Cy7
(53-6.7) (Biolegend) for 30 minutes on ice. Cells were washed and fixed for 10 minutes
at RT in PBS containing 1% FBS, 0.1% sodium azide, 1% PFA and 0.1% saponin. Intracellular
staining and washes for all intracellular steps were conducted in PBS containing 1%
FBS, 0.1% sodium azide and 0.1% saponin. Cells were stained with PE conjugated antibodies
against CCL-2 (1:100, Biolegend) and CCL3 (1:100, R&D systems), biotinylated antibodies
against CCL5, CXCL2 (1:100, R&D systems) or purified antibodies against CCL4 (1:200,
BD Bioscience) and CCL7 (1:100, R&D systems) for 30 minutes on ice. Secondary and
tertiary incubations with biotinylated anti-rat IgG1 (1:100, Biolegend), PE conjugated
donkey anti-goat (1:100, Jackson Immunoresearch Laboratories) or SA-APC (1:100 Invitrogen)
were used in subsequent steps. After the final wash, cells were resuspended in PBS
containing in 1% FBS and 0.1% sodium azide and acquired using a digital flow cytometer.
Immunohistochemistry
To examine LCMV localization, 6-µm frozen sections were cut, fixed with 4% paraformaldehyde
(PFA) for 15 min, blocked with an avidin/biotin-blocking kit (Vector Laboratories),
and stained for 1 h at room temperature with primary guinea pig antibodies against
LCMV (1:500). Secondary and tertiary incubations with biotinylated donkey anti-guinea
pig (1:200; Jackson ImmunoResearch Laboratories) and streptavidin Rhod-X (1:250; Jackson
ImmunoResearch Laboratories), respectively, were done to detect LCMV antigen. Co-labeling
of fibroblasts (anti-ER-TR7; 1:100; Abcam), astrocytes (anti-GFAP; 1:800, DakoCytomation),
infiltrates (anti-CD45.2; 1:100, BD Biosciences), endothelium (anti-CD31; 1:200; Chemicon)
or smooth muscle actin (anti-SMA; 1:100; Abcam) was also conducted in conjunction
with anti-LCMV staining. The cell marker specific antibodies were detected with secondaries
conjugated to FITC (1:200; Jackson ImmunoResearch Laboratories). All working stocks
of primary and secondary reagents were diluted in PBS containing 2% fetal bovine serum
(FBS). To generate 3D renderings of LCMV-infected fibroblasts, 100-µm vibratome sections
were cut using a Leica VT1000S (Leica) and blocked with PBS containing 10% FBS and
0.1% saponin for 1 h at room temperature. Staining for LCMV and fibroblasts was conducted
as indicated above, with the exception that the antibodies were diluted in PBS containing
2% FBS supplemented with 0.1% saponin. To obtain images of Evans Blue leakage, 100-µm
vibratome sections from PBS perfused mice injected with Evans Blue (as described above)
were stained with anti-CD31 (1:200; Chemicon) diluted in PBS containing 2% FBS supplemented
with 0.5% triton-x (Sigma-Aldrich). CD31 was detected and amplified with a FITC conjugated
goat anti-hamster antibody (1:200; Jackson ImmunoResearch Laboratories), a rabbit
anti-FITC antibody (1:200; Zymed) and a FITC conjugated anti-rabbit antibody (1:200;
Jackson ImmunoResearch Laboratories). All sections described above were further stained
with 1 µg/ml DAPI (Sigma-Aldrich) for 3 minutes at room temperature to visualize cell
nuclei.
One-photon microscopy
2D co-localization images to determine whether LCMV infected fibroblasts, leukocytes,
astrocytes, endothelium, or smooth muscle cells / pericytes (Fig. 2 & S3) were captured
from 6-µm frozen using a MRC2100 confocal microscope (Bio-Rad Laboratories) fitted
with 40x, 63x, and 100x oil objectives and seven laser lines that excite at 405nm,
457nm, 477nm, 488nm, 514nm, 543nm, and 637nm (Carl Zeiss MicroImaging, Inc). 3D z-stacks
were captured with from 100-µm vibratome sections using a step size of 0.1 µm. Maximal
projections and 3D reconstructions (Fig. 2) were generated using Volocity software
(Improvision).
Intravital two-photon microscopy
Mice were anaesthetized and maintained at core temp of 37°C. Thinned skull and open
skull surgery were performed and imaged using Bio-Rad Radiance multi-photon microscope
(Zeiss, Thornwood, NY) powered by Tsunami pulsed laser (Spectraphysics) tuned to 920
nm as described previously 36. Bone (second harmonic signal), GFP-labeled cells (GFP
P14 or LysM-GFP), and intravascular quantum dots (Qdots) were visualized using band-pass
filters 400/10, 480/30 and 540/30, respectively. To visualize meningeal vasculature,
mice were injected i.v. 10 min prior to imaging with 50 µl Qtracker 655nm non-targeted
quantum dots (0.2 uM) (Invitrogen). For MHC I blocking studies, H-2Db monoclonal antibody
and isotype control (10 µg/ml) in 200 µl of artificial cerebral spinal fluid (119
mM NaCl, 26.2 mM NaHCO3, 2.5 mm KCl, 1 mM NaH2PO4, 1.3 mM MgCl2, 1.2 mM CaCl2, 0.4
% glucose, pH 7.4) were administered through a partial open skull adjacent to the
thinned skull viewing area. Antibodies were incubated for 15–30 min to permit adequate
local tissue diffusion. CTL dynamics were imaged through the adjacent thinned skull.
For all imaging studies, stacks of images were acquired using step sizes of 1–3 µm
to a depth of 200 µm below the skull using 20x, 40x or 60x water dipping objectives.
Time-lapse movies were acquired with 1- to 1.5-min intervals between 3D stacks. Image
analysis was performed and cell movements were tracked using Volocity software and
graphs were determined using Graph Prism4. The average speed (µm/min) of CTL and myelomonocytic
cells was quantified manually from 30 min time lapses with 20 intervals. Arrest duration
(min) is the total time that a CTL slowed to < 2 µm/min interval / instantaneous speed
during a 30 min time lapse. The arrest coefficient is the percentage of total elapsed
time that CTL spent moving < 2 µm/min. The confinement index was calculated by dividing
the displacement (or distance a cell traveled) by the speed. The motility coefficient
was calculated as (mean displacement)2 / (4 × time). Histograms showing the relative
frequency of CTL velocities under different conditions were generated using a bin
size of 2 µm/min and Gaussian curve fitting. The fluorescence ratio (FR) of extravascular
(ev) to intravascular (iv) Qdot and GFP signal was calculated by first quantifying
the mean fluorescence intensity in defined extra- and intravascular regions. Mean
extravascular fluorescence was then divided by mean intravascular fluorescence for
each channel (FR ev/iv) and normalized to the ratio at time 0 (FR t0) by division
(FRev/iv / FRt0). All imaging data are representative of at least three independent
experiments.
Sagittal Brain Reconstructions
Two-color organ reconstructions to visualize the distribution of LysM-GFP+ cells on
6-µm frozen sections (Fig. S10) were obtained using an immunofluorescence microscope
(Axiovert S100; Carl Zeiss MicroImaging, Inc.) fitted with an automated xy stage,
a color digital camera (Axiocam, Carl Zeiss MicroImaging, Inc.), and a 5× objective.
Registered images were captured for each field on the tissue section, and reconstructions
were performed using the MosaiX function in KS300 image analysis software (Carl Zeiss
MicroImaging, Inc.).
Statistical Analysis
Statistical significance (p < 0.05) was determined using a Student’s t test, a Mann-Whitney
rank sum test for populations with non-Gaussian distributions, or a one way ANOVA
for experiments containing more than two groups. Correlations were evaluated using
a Pearson Product Moment Correlation test.
Supplementary Material
1
Supplementary Figure 1. OT-I cells do not change speed following MHC I inhibition
CFSE labeled OT-I T cells expanded in vitro were adoptively transferred i.v. into
B6 mice at day 6 post-infection. These cells were tracked in the meninges 6 hours
later. Only a few OT-I cells were seen and many of these cells appeared to be trapped
in perivascular spaces (n=3 mice). Following injection of control IgG or anti-MHC
I antibody, no change in average speed measured over 30 min timelapses (a) or instantaneous
speed measured at 1.5 min intervals (b) was detected.
9
Supplementary Figure 9. Chemokine production by LCMV-specific CTL in the spleen and
CNS at day 6 post-infection
Production of myelomonocytic cell-recruiting chemokines was quantified flow cytometrically
in naïve and effector Thy1.1+ P14 cells with and without GP33–41 peptide stimulation
(n=4 mice per group). Fold increases in chemokine synthesis were calculated by using
unstimulated naïve Thy1.1+ P14 cells as a baseline (a). In panel A data are represented
as the mean + SD, and the dotted blue line denotes a 1-fold induction (or no increase
from baseline). Representative histograms depicting CCL3, CCL4, and CCL5 expression
in Thy1.1+ P14 cells are shown in panel B. CCL2, CCL7, and CXCL2 levels were not increased
over isotype control antibodies (data not shown). Plots are gated on CD8+Thy1.1+ cells.
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Supplementary Figure 10. Myelomonocytic cells localize to the meninges and CD8+ T
cell depletion reduces their influx into the LCMV-infected CNS
The distribution of LysM-GFP+ myelomonocytic cells on sagittal brain reconstructions
is shown for the following groups of mice (n=4–5 mice per group): mock infected (a)
and d6 LCMV infected (b). Note that LysM-GFP+ cells localize primarily to the meninges
and ependyma in LCMV-infected mice. Monocyte / macrophages and neutrophil cells were
enumerated flow cytometrically in the CNS of d6 LCMV infected mice treated with control
rat IgG and anti-CD8 antibody (c). A statistically significant reduction in both cell
populations was observed in mice treated with anti-CD8 antibody. Data are represented
as the mean + SD (n=4 mice per group). Asterisks denote statistical significance (*p
= 0.015, **p = 0.004).
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Supplementary Figure 2. Analysis of field heterogeneity in mice treated with anti-MHC
I or isotype control antibody at day 6
This figure represents a break down of the pooled results shown in Figure 1. a–f,
Representative xy plane thinned skull images of GFP+ P14 cells at day 6 post-infection
(gray scale) (a–c) and corresponding 30 minute time lapse cell tracks (colored lines)
(d–f) below each image are shown for fields containing a low (a, d), medium (b, e),
and high (c, f) density of GFP+ P14 CTL. (Grid scale = 19.7 µm) g–h, The effect of
isotype control IgG or anti-MHC I antibodies on P14 CTL speed (g) and arrest coefficient
(h) are shown for individual fields where 1 to 5 represent results obtained from separate
mice and fields. Statistically significant increases in CTL speed and decreases in
arrest coefficients from baseline (BL) were only noted following treatment with anti-MHC
I. The results did not depend on P14 densities in the field. Asterisks denote statistical
significance (p < 0.05).
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Supplementary Figure 3. P14 CTL trajectories and displacement in the meninges at day
6
a–f, Representative xz plane thinned skull images of GFP+ P14 cells at day 6 post-infection
(gray scale) (a–c) and corresponding 30 minute time lapse cell tracks (colored lines)
(d–f) below each image are shown. Note the confined motion of GFP+ P14 cells in all
groups within the z plane. The motion is confined by the meningeal space. (Grid scale
= 29.6 µm) g–i, Relative GFP+ P14 CTL trajectories in the BL, IgG, and class I groups
(n=3 mice per group) showed little difference in the range of dynamic movement and
no preferential directional bias in movement. (See accompanying Movie 1) j, Random
walk analysis of GFP+ P14 CTL revealed a 2-fold increase in the motility coefficient
for class I block compared with the isotype IgG and baseline controls. Data are represented
as mean ± SEM.
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Supplementary Figure 4. Tropism of LCMV in the meninges
To define the tropism of LCMV in the meninges, 6-µm frozen sections from day 6-infected
mice were co-stained with anti-LCMV antibody and antibodies directed against one of
the denoted cell markers: fibroblasts (ER-TR7; a–c), leukocytes (CD45; d–f), astrocytes
(GFAP; g–i), endothelium (CD31; j–l), and smooth muscle cells / pericytes (SMA; m–o).
Note that LCMV infects fibroblasts, some infiltrating leukocytes (white arrows in
panel F), and occasionally astrocytic foot processes that comprise the glial limitans
(white arrows panel J), but not endothelium or smooth muscle cells / pericytes. Overlapping
fluorescence appears in yellow. All representative 2D images were captured using a
one-photon microscope.
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Supplementary Figure 5. Flow cytometric analysis of the major leukocyte subsets found
in the CNS at day 6 post-infection
a, Representative flow cytometric plots are shown for a mock-infected and day 6 LCMV-infected
mice. Note the marked increase in presence of CD8+ T cells and CD11b+ myelomonocytic
cells in the CNS at day 6 post-infection. b, Microglia (CD45loThy1.2−CD11b+) were
the predominant myeloid cell subset found in the CNS of mock-infected mice. Two additional
subsets were recruited to the CNS of day 6 LCMV-infected mice: neutrophils (CD45intThy1.2−CD11bhi)
and monocytes / macrophages (CD45hiThy1.2−CD11bint / hi). c, To confirm that the gating
strategy shown in panel B defined three distinct myeloid cell subsets, we analyzed
the expression of the neutrophil marker Gr-1. CNS resident microglia (black, gate
1) expressed no Gr-1, monocytes / macrophages (blue, gate 2) expressed intermediate
levels of Gr-1, and neutrophils (red, gate 3) expressed high levels of Gr-1.
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Supplementary Figure 6. Depletion of neutrophils in LysM-GFP mice
a, The diagram illustrates gating strategy and antibodies (anti-Gr-1 and 7/4 antibody)
used to subdivide CD11b+ myelomonocytic cells extracted from the CNS into three distinct
populations: microglia (Gr-1− 7/4−), neutrophils (Gr-1hi 7/4int), and macrophages
/ monocytes (Gr-1int 7/4hi). b, All three subpopulations are clearly observed in representative
FACs plots from d6 LCMV-infected B6 and LysM-GFP mice treated with control IgG. Note
that only neutrophils (gate 2) are depleted in d6 LysM-GFP mice treated with low dose
anti-Gr-1 antibody (n=3 mice per group).c, Representative histograms depicting GFP
expression on microglia, neutrophils, and macrophages / monocytes are shown for d6
LCMV-infected B6, LysM-GFP + control IgG, and LysM-GFP + low dose anti-Gr-1 (n=3 mice
per group). Comparable levels of GFP expression was detected in neutrophils and macrophages
/ monocytes extracted from LysM-GFP mice, whereas no GFP was detected in microglia.
Wild type B6 mice were used as a negative control for this experiment.
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Supplementary Figure 7. Analysis of vascular leakage using Evans blue dye
Evans blue dye was injected intravenously into mice to examine CNS vascular integrity.
Representative brains (n=4 mice per group) are shown for B6 mock infected (a) as well
as B6 and CCR2 knockout mice at day 6 post-infection treated with control IgG (b,
c) or low dose anti-Gr-1 (d, e) antibodies. Note the heavy leakage of Evans blue dye
(blue coloration) into all brains except those harvested from the mock infected controls
(a) and CCR2 knockout mice at day 6 treated with high dose anti-Gr-1 antibody (e).
Quantification of Evans blue leakage using a fluorometer revealed a statistically
significant reduction (p < 0.001) in CCR2 knockout mice treated with high dose anti-Gr-1
when compared to all other infected groups (f).
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Supplementary Figure 8. Distribution of Evans blue leakage in the brains of LCMV infected
mice
The distribution of Evans blue (red) in relation to CD31+ meningeal blood vessels
(green; white arrows) was examined by confocal microscopy in the brains of mock (a–d)
and LCMV infected mice (e–l) at day 6. Minimal to no Evans blue signal was detected
in the brains of mocked infected control mice (a–d). In contrast, in LCMV infected
mice, we observed severe Evans blue leakage from meningeal blood vessels into brain
parenchyma. Two representative meningeal blood vessels from infected mice are shown
(e–l). Cell nuclei are shown in blue.
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