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Journal of Virology, August 2006, p. 8279-8282, Vol. 80, No. 16
0022-538X/06/$08.00+0 doi:10.1128/JVI.00742-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Loss of Cell Membrane Integrity in Puumala Hantavirus-Infected Patients Correlates with Levels of Epithelial Cell Apoptosis and Perforin
Jonas Klingström,1,2*
Jonas Hardestam,1,2
Malin Stoltz,1
Bartek Zuber,3
Åke Lundkvist,1,2
Stig Linder,4 and
Clas Ahlm5
Center
for Microbiological Preparedness, Swedish Institute for Infectious
Disease Control, S-171 82 Solna, Sweden,1
Department of Microbiology,
Tumor and Cell Biology, Karolinska Institutet, S-171 77 Stockholm,
Sweden,2
Mabtech AB, Box
1233, S-131 52 Nacka Strand, Sweden,3
Cancer Center Karolinska,
Department of Oncology-Pathology, Karolinska Institute and
Hospital, S-171 76 Stockholm, Sweden,4
Division of
Infectious Diseases, Department of Clinical Microbiology, Umeå
University, S-901 85 Umeå,
Sweden5
Received 12 April 2006/
Accepted 16 May 2006

ABSTRACT
Hemorrhagic
fever with renal syndrome and hantavirus pulmonary
syndrome are two
diseases caused by hantaviruses. Capillary
leakage is a hallmark of
hantavirus infection. Pathogenic hantaviruses
are not cytotoxic, but
elevated levels of serum lactate dehydrogenase
(LDH), indicative of
cellular damage, are observed in patients.
We report increased levels
of serum perforin, granzyme B, and
the epithelial cell apoptosis marker
caspase-cleaved cytokeratin-18
during Puumala hantavirus infection.
Significant correlation
was observed between the levels of LDH and
perforin and the
levels of LDH and caspase-cleaved cytokeratin-18,
suggesting
that tissue damage is due to an immune reaction and that
epithelial
apoptosis contributed significantly to the
damage.

TEXT
Hantaviruses are the causative agents
of hemorrhagic fever with
renal syndrome (HFRS) and hantavirus
pulmonary syndrome (HPS).
Capillary leak syndrome, causing edema and
hemorrhage, is a
hallmark of HFRS/HPS. Endothelial cells and monocytes
are the
primary cell types infected by hantaviruses, but infection has
no
direct cytopathic effect on these, or other, cells
(
8,
12,
25,
26,
30,
33,
35,
38,
39),
indicating that
direct viral cytotoxicity is not responsible
for the pathology observed
in humans. However, increased levels
of serum lactate dehydrogenase
(LDH), aspartate aminotransferase,
and alanine aminotransferase are
observed in patients (
3,
5,
37),
showing that the
cellular membrane integrity is disturbed during
infection. It has been
argued that pathogenesis is due to cellular
immune responses rather
than to the infection (reviewed in reference
11).
Special
attention has recently been given to the CD8+-T-cell
responses induced during hantavirus infection
(10,
13,
21,
22,
23,
24,
34,
36). For example, the
magnitude of the Sin Nombre hantavirus-specific
CD8+-T-cell responses correlate with the severity of
HPS, implying that Sin Nombre hantavirus-specific
CD8+ T cells contribute to HPS disease outcome
(13).
Antigen-specific
CD8+ T cells can induce target cell apoptosis by the
release of cytolytic granules containing perforin and granzymes, and
virus-infected cells are eliminated mainly via this granule exocytosis
pathway (18). During this
process, some of the perforin and granzymes find their way into the
circulation (29).
Elevated levels of extracellular granzyme B, indicative of the
activation of CD8+ T cells and natural killer (NK)
cells, have been detected in viral, bacterial, and parasitic infections
(9,
17,
32), and we have
previously detected elevated levels of extracellular perforin in human
immunodeficiency virus type 1-infected individuals
(15).
In this
study, serum samples were collected from patients hospitalized with
laboratory-verified Puumala hantavirus infection and with typical
clinical symptoms of acute nephropathia epidemica, a milder form of
HFRS (31). The levels of
perforin (Mabtech, Nacka, Sweden), granzyme B (Euroclone, Pero,
Italy), caspase-cleaved cytokeratin-18 (CK18) (Peviva,
Bromma, Sweden), and total CK18 (Peviva) were analyzed using
enzyme-linked immunosorbent assays according to the manufacturer's
instructions. Three individuals out of the 21 Puumala
hantavirus-infected patients were found to be positive for human
anti-mouse antibodies and were excluded from the study to avoid
false-positive results in the specific enzyme-linked immunosorbent
assays (15). Acute-phase
samples were drawn at the time of hospitalization. The individuals
arrived at the hospital 2 to 12 days after initial onset of fever.
Convalescent-phase serum was drawn 10 days, 1 month, 2 months, or 3
months after recovery from 1, 2, 1, and 14 individuals, respectively.
The project was approved by the Research Ethics Committee of
Umeå University.
The mean concentrations of serum
perforin (Fig.
1A) and granzyme B (Fig.
1B) were significantly
higher in the acute phase than in the convalescent phase (P
< 0.0001 for perforin and granzyme B by Wilcoxon signed-rank
test). All patients except one showed higher levels of extracellular
perforin and all patients showed higher levels of granzyme B during the
acute phase than during the convalescent phase.
A
correlation was observed between the time after onset of fever
and the
concentrations of perforin (Spearman
R = 0.64;
P = 0.0045) and granzyme B (Spearman
R = 0.84;
P =
0.000016),
showing that the highest levels of perforin and
granzyme B were
detected early after the onset of fever (Fig.
1C). The levels
of
perforin and granzyme B in the individual patients during
the acute
phase also correlated (Spearman
R = 0.56;
P
= 0.015)
(Fig.
1D).
Consistent with
previous observations (3,
5,
37), all patients in this
study showed increased levels of serum LDH during the acute phase of
infection. The LDH levels did not, however, correlate significantly
(Spearman R = 0.30; P =
0.24) with time after onset of fever (Fig.
2). The level of LDH correlated significantly with that of perforin
(Spearman R = 0.50; P = 0.039) (Fig.
3) but not with the levels of granzyme B (Spearman R =
0.27; P = 0.30).
To quantify possible epithelial
cell apoptosis in hantavirus-infected
patients, we determined the serum
levels of a caspase cleavage
product of CK18
(
1). CK18 is a type I
intermediate filament
protein that is exclusively expressed by simple
epithelial cells,
like those of the endothelia. CK18 is cleaved by
caspases during
apoptosis and is released into serum
(
1,
2). Increased serum
levels
of caspase-cleaved CK18 have previously been detected
in patients
during septic shock (
28),
in patients with hepatitis
(
16),
and in patients
with various carcinomas
(
19). Interestingly,
significantly
higher levels of caspase-cleaved CK18 were observed
during the
acute phase of infection than during the convalescent phase
(
P = 0.0028 by Wilcoxon signed-rank test) (Fig.
4A). Fourteen of
the 18 patients showed higher levels of caspase-cleaved
CK18
during the acute phase than during the convalescent phase. Similar
to
serum LDH, the level of epithelial cell apoptosis showed no
significant
correlation (Spearman
R = 0.45;
P = 0.059) with time
after onset of fever.
The
level of caspase-cleaved CK18 correlated significantly with
that of LDH
(Spearman
R = 0.74;
P = 0.00067)
(Fig.
4B). No significant
correlation
was observed between the levels of perforin (Spearman
R = 0.40;
P = 0.10) or granzyme B
(Spearman
R = 0.29;
P = 0.24) with
those
of caspase-cleaved CK18.
The observed elevated levels of
caspase-cleaved CK18 show that apoptosis is induced in cells of the
epithelial cell lineage during the acute phase of HFRS. Furthermore,
the very strong correlation between the levels of epithelial apoptosis
and LDH suggests that most of the cell damage observed during
hantavirus infections is caused by apoptosis. To our knowledge, this is
the first report showing apoptosis of epithelial cells during
hantavirus infection in humans.
Significantly higher levels of
perforin and granzyme B were observed during the acute phase than
during the convalescent phase of infection. Furthermore, the levels of
perforin and granzyme B correlated during the acute phase. It could be
speculated that the levels of perforin and granzyme B in released
cytotoxic granules are roughly proportional to each other during acute
infection.
The levels of perforin and LDH correlated
significantly, suggesting that hantavirus-specific
CD8+ T cells and/or NK cells might be involved in
causing the observed cell damage during HFRS/HPS. Interestingly, the
levels of granzyme B and LDH did not correlate. This is in line with
the proposed functions of perforin and granzyme B during the killing of
target cells by cytotoxic cells: although granzyme B induces the
apoptosis, perforin is needed for granzyme B to enter the cell
(18).
The levels of
perforin and caspase-cleaved CK18 did not correlate significantly, and
although it could be speculated that the increased vascular
permeability observed during HFRS/HPS is due to apoptosis caused by
hantavirus-specific CD8+ T cells, this remains to be
clearly shown. Apoptosis of other epithelial cells might also
contribute to the increased levels of caspase-cleaved CK18. The
interaction between the virus and the receptor
vß3 integrin, tumor necrosis
factor, and/or reactive oxygen species might also induce permeability
of endothelial cells (4,
6,
7,
14,
20,
27). The levels of
perforin and granzyme B observed during the convalescent phase is
similar to those previously reported for healthy individuals
(15,
32), but the levels of
caspase-cleaved CK18 were clearly higher, indicating that epithelial
cell apoptosis might be increased for a prolonged time after
infection.
We have shown that elevated levels of extracellular
perforin, granzyme B, and epithelial cell apoptosis are induced during
acute hantavirus infection. The capillary leakage during
HFRS/HPS might be due to apoptosis, and the strong
hantavirus-specific CD8+-T-cell responses observed
might be responsible for the damage.

ACKNOWLEDGMENTS
We thank Mats Linderholm for help with collecting
patient samples.
This project was supported by grants from the
Swedish Medical Research Council (projects 12177 and 12642), the
Swedish Society of Medicine, the Medical Faculty of Umeå
University, Cancerföreningen i Stockholm, and the European
Community (contract no. QLK2-CT-1999-01119 and QLK2-CT-2002-01358).
This publication has been partially funded under the EU 6th Framework
Program (GOCE-CT-2003-010284 EDEN) and is officially catalogued by the
EDEN Steering Committee as
EDEN0018.

FOOTNOTES
* Corresponding
author. Mailing address: Swedish Institute for Infectious Disease
Control, S-171 82 Solna, Sweden. Phone: 46 8 457 25 07. Fax: 46 8 30 79
57. E-mail:
jonas.klingstrom{at}smi.ki.se.


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Journal of Virology, August 2006, p. 8279-8282, Vol. 80, No. 16
0022-538X/06/$08.00+0 doi:10.1128/JVI.00742-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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