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Division of Immunology and Cell Biology, John Curtin School of Medical Research, Australian National University, Canberra, Australia; and
Centenary Institute of Cancer Medicine and Cell Biology, Royal Prince Alfred Hospital, Sydney, Australia
| Abstract |
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. Only mice lacking both TNFRI and TNFRII were protected
from the lethal affects of the mutant adenovirus. Mice deficient in TNF
or TNF and lymphotoxin-
displayed the fatal pathology. This outcome
is consistent with the existence of another related ligand that binds
TNFRI/II to mediate liver damage during infection with this
mutant. | Introduction |
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TNF is a multifunctional cytokine produced by activated macrophages and
lymphocytes, as well as other cell types (12). It is
expressed in two forms, the 17-kDa soluble form, and its precursor, the
26-kDa membrane form, both of which are bioactive (13).
TNF and its close relative, secreted homotrimeric lymphotoxin (LT)-
(LT-
3), bind to two receptors, TNFRI (p55) and TNFRII (p75) that
have distinct molecular masses of 5560 kDa and 7580 kDa,
respectively (12). Most cells express both receptor types,
with TNFRI being constitutively expressed at a low level, whereas
TNFRII is expressed in response to external stimuli (14, 15). There is
30% homology between the extracellular
domains of both TNF receptors; however, there is no homology between
the intracellular domains, suggesting that they interact with different
signaling pathways (16, 17). Generally, activation of
TNFRI is associated with cytotoxicity and induction of gene expression,
whereas TNFRII expression is associated with thymocyte proliferation
and T cell activation (18). Furthermore, TNFRII seems to
be the dominant receptor for the 26-kDa membrane form of TNF
(19).
Human Ad does not replicate in mouse cells; however, proteins of the
early regions of the viral genome, including E1A and E3, are expressed.
Previously, we reported the unexpected finding that dl327, an Ad5
mutant lacking the E3 region, caused fatal disease in mice
(20). The present study explores the pathogenesis of
dl327-induced disease in mice, concentrating in particular on the role
of TNF, LT-
3, and their receptors.
| Materials and Methods |
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The Ad5 wild-type (wt) and the deletion mutant dl327, which lacks most of the E3 region, were propagated in HeLa cells, as described previously (21). Virus stocks were titrated on HeLa cells using the immunofluorescent method, in which rabbit polyclonal anti-Ad5 serum and FITC-conjugated anti-rabbit Ig (Silenus Laboratory, Hawthorn, Australia) were used as primary and secondary Ab, respectively (21). Virus titer was expressed in infectious units per ml (IU/ml). In addition, titration of dl327 in vivo determined the lethal dose in mice. E1A and E3 19-kDa product expression of the two viruses used was confirmed by immunoprecipitation, as has been described (22). The E3 region, represented by E3 19K, was expressed in Ad5 wt-infected cells, but not in dl327-infected cells, and E1A was overexpressed in dl327-infected cells (23).
Ad5 wt was heat inactivated at 60°C for 1 h. Titration as described above confirmed the loss of infectivity. Live and heat-inactivated virus were used to determine distribution of viral DNA in mouse tissue.
Cells
HeLa cells and the mouse methylcholanthrane-induced fibrosarcoma cell line, HTG, were cultured in DMEM supplemented with 10% FBS in a 37°C humidified incubator with 5% CO2.
Mice
Female C57BL/6J (H-2b) and (B6 x
129)F1 (H-2b) mice were
obtained from the specific pathogen-free breeding facilities of the
Animal Services Division, the John Curtin School of Medical Research
(Canberra, Australia). TNFRI-deficient mice (24) (obtained
from H. Blüthmann, Hoffmann-La Roche, Basel, Switzerland) and
TNFRII and TNFRI/RII double-deficient mice (generously
provided by J. Peschon, Immunex Research and Development, Seattle, WA)
(25) were used as indicated. All TNFR-deficient mice used
in these studies were hybrids between C57BL/6J and 129/Sv. TNF and
TNF/LT-
gene-targeted mice were both generated directly on the
C57BL/6J background and have been described (26). Mice in
which the gene for LT-
has been inactivated produce neither the
secreted LT-
3 homotrimer, nor a functional LT-
1
2 membrane
heterotrimer (27). Thus, the TNF/LT-
gene-targeted mice
are broadly TNF as well as LT deficient. Unless otherwise specified,
all mice used in these experiments were 816 wk of age.
Mouse infections
For assessment of tissue tropism of the virus, mice were inoculated i.p. with 1 x 108 IU Ad5 wt, live or heat-inactivated, and various tissues were removed and used for DNA preparation. For assessment of the lethality of infection and histopathology, mice were inoculated i.p. with 18 x 108 IU of dl327 or up to 8 x 109 IU Ad5 wt. The dose variation between experiments was caused by variation in the dose of virus stock needed to cause death in normal control mice, which was determined before each experiment.
Histopathologic studies
At the prescribed times postinoculation, necropsies were performed, in which segments of the liver were removed and immediately immersed in 10% neutral buffered Formalin. Samples of liver tissue were then embedded in paraffin for sectioning and staining with hematoxylin and eosin. Evaluation of histologic sections was performed in a blinded fashion.
Detection of viral DNA sequences in mouse tissue
Total DNA was purified from freshly collected or frozen organs of Ad-infected mice. Mouse lung, liver, kidney, spleen, heart, and trachea were homogenized individually and DNA purified using standard methods. A 1012-bp fragment was amplified from the E1 region by nested PCR, initially using the primer pair 5'-CCG GGA GGA TCC ATG AGA CAT ATT ATC TGC C and 3'-CAG CCA CCT CTA GAA CAT TCA TTC CCG, and then for the second pair using the same 5' primer as above with 3': CAC AGG TTG AAT TCT TAT GGC CTG GGG CGT T. The annealing temperature was 60°C for both reactions.
| Results and Discussion |
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lyse dl327-infected mouse cells in vitro
(31, 32, 33), we suspected that the liver pathology was caused
by TNF. Normally resistant cells become sensitive to TNF when infected with Ad E3 deletion mutants. This sensitivity is dependent on E1A (4, 5). E1A is expressed in Ad-infected mouse cells in vivo because E1A-specific cytotoxic T cells can be generated (28). Although we have no evidence for elevated E1A expression in dl327-infected compared with Ad5-infected mice in vivo, such elevation has been shown to occur in mouse cells in vitro infected with E3 deletion mutants, including dl327 (22, 23). Elevated levels of E1A in vivo could possibly be responsible for high sensitivity of hepatocytes of dl327-infected mice to TNF. However, normal hepatocytes in vivo do not divide and results in vitro (5), showing a need for DNA synthesis for increased sensitivity to TNF in E1A-expressing cells may not be applicable. No evidence of mitosis in hepatocytes was seen in liver of dl327-infected mice before development of liver pathology. The lack of expression of E3 proteins that inhibit TNF-mediated cytolysis may be the key issue (9).
Mice deficient in one or both of the TNF receptors or their ligands,
TNF and LT-
3, were infected, and Ad-mediated liver pathology was
assessed. Table I
shows that mice
deficient in TNFRI alone incurred the same level of morbidity and
mortality as wt mice after infection with dl327. Similarly, a majority
of mice deficient in TNFRII alone were not protected from the increased
virulence of infection by dl327 (Table II
). Both TNFRI-deficient and
TNFRII-deficient mice had liver pathology similar to that shown in Fig. 2
, B and C. However, when TNFR double knockout
mice (TNFRI/II) were used, no deaths occurred under the same conditions
(Table II
). This result indicates that signaling via either TNFRI or
TNFRII is sufficient to mediate severe liver damage in dl327-infected
mice, and that protection is afforded only when mice are deficient for
both receptors.
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3, is responsible for liver damage. To determine whether TNF, LT,
or both are responsible for the Ad-induced liver damage, C57BL/6J
strain TNF- and TNF/LT-
-deficient mice (26)
were infected with dl327 and examined. dl327 was titrated in normal
C57BL/6J before experimentation. Results with TNF-deficient and
TNF/LT-
-deficient mice are shown in Table III
, the animals were not protected and behaved like normal
C57BL/6J animals. Comparable severe liver pathology occurred in both
groups of deficient mice and was similar in nature and magnitude to
that in normal C57BL/6J mice (Fig. 2
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were susceptible to the lethal effects of dl327 was unexpected. We have
discussed the unlikely possibility that elevated E1A levels directly
mediate liver pathology. An interesting alternative explanation is that
a ligand other than TNF or LT-
released during Ad infection is
responsible for liver damage and death in these mice. However, the
conundrum is that liver pathology failed to occur in the absence of
TNFRI and TNFRII, and yet no ligands other than TNF and the LT-
3
homotrimer are known to bind to either of these receptors (
(12). Therefore, the use of the Ad dl327 mutant may have
revealed the existence of an alternative ligand for these receptors.
This possibility is strengthened by the recent demonstration of LIGHT
(34), a cell membrane-associated ligand produced by
activated T cells that has strongest homology to LT-
, although it is
distinct from this as well as LT-
. Most interesting in the context
of the current study is that LIGHT binds not only to the LT-
previously thought to bind predominantly the membrane-associated
LT-
1
2 ligand, but also to the TNFR family member, herpes virus
entry mediator, present on T lymphocytes. Thus, it seems that viruses
such as Ad and herpes may stimulate infected tissue cells or cells of
the immune system to produce additional ligands within the TNF
superfamily. The role that TNF family ligands may play in virus-induced
tissue damage was highlighted in a recent study by Orange and
colleagues, who demonstrated a TNF-mediated component to viral
pathogenesis during murine CMV infection in mice (35) by
neutralization of TNF directly using anti-TNF Ab. Their results
indicated a requirement for TNF in development of necrotic but not
inflammatory foci in the liver.
| Acknowledgments |
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| Footnotes |
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2 Current address: DNAX Research Institute, 901 California Avenue, Palo Alto, CA 94304-1104. ![]()
3 Address correspondence and reprint requests to Dr. Arno Müllbacher, Division of Immunology and Cell Biology, John Curtin School of Medical Research, Australian National University, P. O. Box 334, Canberra, ACT 2601, Australia. E-mail address: ![]()
4 Abbreviations used in this paper: AD, adenovirus; E, early; LT, lymphotoxin; Tc, cytotoxic T; wt, wild type. ![]()
Received for publication December 23, 1998. Accepted for publication May 10, 1999.
| References |
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and tumor necrosis factor in organogenesis and spatial organization of lymphoid tissue. Eur. J. Immunol. 27:2600.[Medline]
(LT
) subunit is essential for the assembly, but not for the receptor specificity, of the membrane-anchored LT
1
2 heterotrimeric ligand. J. Biol. Chem. 272:19451.
and
inhibit virus replication and synergize with interferons. Nature 323:819.[Medline]
are ligands for herpesvirus entry mediator. Immunity 8:21.[Medline]
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