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Journal of Virology, July 2004, p. 7828-7832, Vol. 78, No. 14
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.14.7828-7832.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Expression of the Mouse Hepatitis Virus Receptor by Central Nervous System Microglia
Chandran Ramakrishna,1,2 Cornelia C. Bergmann,1,2 Kathryn V. Holmes,3 and Stephen A. Stohlman1,2,4*
Departments of Neurology,1
Pathology,2
Molecular Microbiology and Immunology, Keck School of Medicine, University of Southern California, Los Angeles, California 90033,4
Department of Microbiology, University of Colorado Health Science Center, Denver, Colorado 802623
Received 23 December 2003/
Accepted 9 March 2004

ABSTRACT
Detection of the mouse hepatitis virus receptor within the central
nervous system (CNS) has been elusive. Receptor expression on
microglia was reduced during acute infection and restored following
immune-mediated virus control. Receptor down regulation was
independent of neutrophils, NK cells, gamma interferon, or perforin.
Infection of mice devoid of distinct inflammatory cells revealed
CD4
+ T cells as the major cell type influencing receptor expression
by microglia. In addition to demonstrating receptor expression
on CNS resident cells, these data suggest that transient receptor
down regulation on microglia aids in establishing persistence
in the CNS by assisting virus infection of other glial cell
types.

TEXT
Interactions between the mouse hepatitis virus (MHV) spike (S)
protein and the virus receptor influences tropism, spread, and
pathogenesis (
8,
17). The primary MHV receptor is the four-immunoglobulin
isoform of CEACAM1a (
1,
4). Low-affinity binding to related
molecules and receptor-independent infection in vitro have also
been described (
5,
7,
9,
25-
27). Despite limited in vitro receptor
usage by the neurotropic JHM strain of MHV (JHMV) (
5,
25,
27),
central nervous system (CNS) ependymal cells, microglia, oligodendroglia,
astrocytes, and neurons are infected in vivo (
22). Low levels
of MHV receptor mRNA are expressed by the CNS (
5,
24). However,
no receptor was identified by a direct binding assay (
23), and
only CNS endothelial cells, but not glial cells, expressed the
receptor by immunohistochemistry (
10). The absence of CNS expression
led to proposals of alternate lower-affinity receptors (
5) or
a receptor-independent mechanism of virus infection (
15). To
resolve this issue, CEACAM1a expression on CD45
low microglia
isolated from the CNS of adult mice was examined. CNS mononuclear
cells were isolated from homogenates by using a Percoll step
gradient as described previously (
2,
3). CNS-derived cells were
labeled with phycoerythrin, fluorescein isothiocyanate, or allophycocyanin-labeled
monoclonal antibodies (MAb) specific for CD45 (30-F11), CD4
(RM4-5), CD8 (53-6.7), MHC class II I-A/I-E (2G9), Ly-6G (1A8),
CD11b (M1/70) (BD PharMingen, San Diego, Calif.), and F4/80
(Serotec, Raleigh, N.C.). Receptor expression was detected by
using biotinylated MAb CC-1 (
23) and phycoerythrin-conjugated
avidin. Biotinylated immunoglobulin G1 (BD PharMingen) was used
as an isotype control. Cells were analyzed on a FACS Calibur
flow cytometer (Becton Dickinson, San Jose, Calif.), using Cellquest
software. CEACAM1a was expressed by microglia from naïve
susceptible C57BL/6 and BALB/c mice but not by microglia from
resistant SJL mice (National Cancer Institute, Fredrick, Md.)
(Fig.
1).
Microglia from BALB/c mice infected intracerebrally with 500
PFU of the J.2.2v-1 MAb-derived variant of JHMV (
8) were examined
at various times postinfection (p.i.). Whereas receptor expression
was unaltered at 5 days p.i, expression decreased at 7 days
p.i. based on mean fluorescence intensity (MFI) (Fig.
2A). The
majority of microglia regained CEACAM1a expression by day 14
p.i. (Fig.
2A), and this expression returned to uninfected levels
by day 30 p.i. (data not shown) concomitantly with virus clearance
and partial resolution of inflammation. Similarly, microglia
from C57BL/6 mice infected with 250 PFU of JHMV also exhibited
reduced receptor expression by 7 days p.i. (Fig.
2B). Although
down regulation was protracted in infected C57BL/6 mice compared
to BALB/c mice (Fig.
2A and B), receptor was reexpressed at
naïve levels by day 30 p.i. (Fig.
2B). These results suggest
that either virus or inflammation regulates microglia receptor
expression. Intracellular S protein-receptor interaction (
18)
or extracellular progeny could inhibit receptor detection. Sequestration
appears unlikely, as only a small fraction of microglia are
infected (
22). To distinguish between a viral effect and inflammation,
receptor expression was investigated during uncontrolled virus
replication in mice immunosuppressed by irradiation (850 rad)
24 h prior to infection (
16,
28) and in immunodeficient SCID
mice (
3). At day 7 p.i., receptor expression on microglia from
infected irradiated BALB/c mice (Fig.
3A) and C57BL/6 mice (data
not shown) was equivalent to that on microglia from naïve
mice. No loss in receptor expression was noted in a limited
number of survivors at day 9 p.i. (data not shown). CD45
high inflammatory cells (Fig.
3D) were below the detection level
(
16,
28). Unaltered receptor expression despite uncontrolled
virus replication (
16,
28) indicates that neither intracellular
trapping, extracellular binding of viral particles, nor soluble
S protein masks receptor detection.
CNS infection induces the recruitment of innate and adaptive
immune components (
2,
3,
29). SCID mice were infected with 500
PFU of JHMV to determine whether neutrophils, NK cells, or macrophages
regulate receptor expression (Fig.
3D). Microglia from uninfected
and infected SCID mice exhibited no difference in receptor expression
at 7 days p.i. (Fig.
3B) or at 10 days p.i. (data not shown).
These data confirm that decreased receptor expression is not
associated with virus replication and suggest that neutrophils,
matrix metalloprotease-9 secretion (
29), NK cells, and macrophages
are not sufficient to influence receptor expression. To test
gamma interferon (IFN-

) secretion and perforin-mediated cytolysis
as potential mediators, mice deficient in both functions (PKO/GKO
mice) (
2) were infected with 500 PFU of JHMV. Receptor expression
on microglia from infected PKO/GKO mice was decreased at day
7 p.i. (Fig.
3C), indicating that neither of these anti-viral
effector mechanisms are sufficient to mediate decreased receptor
expression.
The fact that receptor expression in infected SCID mice was distinct from that in infected PKO/GKO mice suggested a link between T-cell infiltration and reduced receptor levels. Mice were therefore depleted of CD4+ T cells (MAb GK1.5) or CD8+ T cells (MAb 2.43) by MAb treatment on days 1, +1, and + 3 relative to infection (Fig. 4A). Controls received anti-ß-galactosidase MAb GL113. The majority of microglia isolated from infected CD4+-T-cell-deficient mice at day 7 p.i. retained receptor expression compared to controls (Fig. 4B). Receptor loss was less severe on microglia from CD8+-T-cell-depleted mice (Fig. 4B), despite CD4+-T-cell accumulation in the CNS similar to that for infected controls (data not shown). CD45high CD11b+/class II+ macrophages from the CNS of CD4+-T-cell-deficient mice also retained receptor expression at 7 days p.i., while a minority of macrophages within the CNS of CD8-deficient mice exhibited decreased expression (Fig. 4C). These data indicate that CD4+ T cells and, to a lesser extent, CD8+ T cells influence the loss of receptor expression on both microglia and infiltrating macrophages.
Regulation of receptor expression by T cells appears to be a
novel method by which immune responses influence the course
of viral infection in an immune-privileged site via inhibition
of attachment and entry. While protective, cytotoxic mechanisms
in the CNS also induce immune-mediated pathology (
3), decreased
microglial susceptibility may thus be a mechanism to avoid overt
immune pathology. The mechanism(s) by which CD4
+ T cells regulate
receptor expression on microglia is unclear but is likely to
involve soluble factors, although the possibility of a reversible
conformational change cannot be ruled out. JHMV infection results
in the release of proinflammatory cytokines, chemokines, and
matrix metalloproteinases (
13,
16,
28). Analysis of IFN-

-deficient
mice excluded the possibility of a role for IFN-

in receptor
regulation in vivo, despite contradicting in vitro studies (
21).
IFN-

/ß mRNA is also increased in the CNS of immunodeficient
mice (unpublished observation), suggesting no correlation with
receptor down regulation. However, unlike many potential proinflammatory
cytokines that are decreased prior to diminished receptor expression
(
16), RANTES is maintained until JHMV infection and inflammation
resolve (
13), correlating with the transient down regulation
of CEACAM1a on microglia. Reduced receptor expression occurs
as infectious virus declines and thus may contribute to the
limiting of virus spread, cytopathology, and potential glial
cell-mediated damage. Viral persistence in vitro is associated
with both reduced cytopathology (
19,
20) and reduced receptor
expression (
6,
19), while increased expression enhances virus-induced
cytopathology (
18). Receptor down regulation in vivo appears
to be widespread, although only a few microglia or macrophages
are infected (
22). Down regulation may be specific for only
microglia and macrophages, thereby facilitating the infection
of other glial cell types not responsive to receptor down regulation.
Alternatively, reduced expression may modulate lymphocyte function.
CEACAM1a signaling increases T-cell chemotaxis and cytokine
secretion (
12,
14) as well as B-cell activation (
11). Thus,
reduced receptor on CNS resident antigen-presenting cells may
reduce T-cell recruitment, cytokine secretion, and possibly
cytolysis in an attempt to limit immune-mediated pathology.

ACKNOWLEDGMENTS
This work was supported by National Institutes of Health grants
NS18146, NS40667, AI47248, and AI25231.

FOOTNOTES
* Corresponding author. Mailing address: Department of Neurology, Keck School of Medicine, University of Southern California, MCH 142, Los Angeles, CA 90033. Phone: (323) 442-1063. Fax: (323) 225-2369. E-mail:
stohlman{at}usc.edu.


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Journal of Virology, July 2004, p. 7828-7832, Vol. 78, No. 14
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.14.7828-7832.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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