By
From the * Division of Rheumatology, Allergy, and Clinical Immunology and the Department of
Medicine; and the Department of Immunology and Duke University Arthritis Center, Duke
University Medical Center, Durham, North Carolina 27710
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Abstract |
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CD7 is an immunoglobulin superfamily molecule involved in T and natural killer (NK) cell activation and cytokine production. CD7-deficient animals develop normally but have antigen-specific defects in interferon (IFN)- production and CD8+ CTL generation. To determine
the in vivo role of CD7 in systems dependent on IFN-
, the response of CD7-deficient mice to
lipopolysaccharide (LPS)-induced shock syndromes was studied. In the high-dose LPS-induced shock model, 67% of CD7-deficient mice survived LPS injection, whereas 19% of control
C57BL/6 mice survived LPS challenge (P < 0.001). CD7-deficient or C57BL/6 control mice
were next injected with low-dose LPS (1 µg plus 8 mg D-galactosamine [D-gal] per mouse)
and monitored for survival. All CD7-deficient mice were alive 72 h after injection of LPS
compared with 20% of C57BL/6 control mice (P < 0.001). After injection of LPS and D-gal,
CD7-deficient mice had decreased serum IFN-
and tumor necrosis factor (TNF)-
levels
compared with control C57BL/6 mice (P < 0.001). Steady-state mRNA levels for IFN-
and
TNF-
in liver tissue were also significantly decreased in CD7-deficient mice compared with
controls (P < 0.05). In contrast, CD7-deficient animals had normal liver interleukin (IL)-12,
IL-18, and interleukin 1 converting enzyme (ICE) mRNA levels, and CD7-deficient splenocytes had normal IFN-
responses when stimulated with IL-12 and IL-18 in vitro. NK1.1+/
CD3+ T cells are known to be key effector cells in the pathogenesis of toxic shock. Phenotypic
analysis of liver mononuclear cells revealed that CD7-deficient mice had fewer numbers of
liver NK1.1+/CD3+ T cells (1.5 ± 0.3 × 105) versus C57BL/6 control mice (3.7 ± 0.8 × 105; P < 0.05), whereas numbers of liver NK1.1+/CD3
NK cells were not different from
controls. Thus, targeted disruption of CD7 leads to a selective deficiency of liver NK1.1+/
CD3+ T cells, and is associated with resistance to LPS shock. These data suggest that CD7 is a
key molecule in the inflammatory response leading to LPS-induced shock.
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Introduction |
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CD7 is a 40-kD member of the Ig gene superfamily that
is expressed on a major subset of human peripheral T
lymphocytes and NK cells (1). CD7 is an early T cell activation antigen in that CD7 mRNA levels rise within 15 min after initiation of a transmembrane calcium ion flux
(8). CD7 can complex with CD3 and CD45 molecules (9),
and CD7 signaling involves both protein kinase C and protein tyrosine kinase (6, 10). CD7 has been shown to be a
functional signal-transducing molecule on resting NK cells
(6, 11). Antibody cross-linking of NK cell CD7 induced increases in free cytoplasmic calcium, secretion of IFN-,
NK cell proliferation, adhesion to fibronectin, and NK cytotoxic activity (6). Although the above studies have demonstrated in vitro roles for CD7 in T and NK cell activation and/or adhesion, relevant functions of CD7 in vivo
remain unknown.
To probe in vivo functional roles of CD7 in the murine
immune system, CD7-deficient mice were generated using
homologous recombination techniques (12). We have previously shown that CD7-deficient animals had decreased in
vitro antigen-specific IFN- production, and had diminished antigen-driven CD8+ CTL activity (12).
Use of genetically deficient mice has been instrumental
in identifying the roles of cytokines in LPS-induced shock.
The low-dose LPS shock model takes advantage of the increased susceptibility of mice to 1 µg of LPS after injection
with 8 mg of D-galactosamine (D-gal) (13, 14). IFN- and
TNF-
are key mediators of hepatocyte necrosis and death
in the low-dose LPS shock model (15). In the high-dose LPS shock model, administration of 100 mg/kg of LPS results in increased levels of serum inflammatory cytokines, neutrophil infiltration, and aggregation in liver and
other tissues, and death within 3-5 d (14). Intracellular adhesion molecule (ICAM)-1, IL-1, and TNF-
are key mediators of the lethal effects of high-dose LPS (19).
In this study, we have determined the sensitivity of CD7-deficient mice to LPS-induced shock syndromes. We found that CD7-deficient mice were completely resistant to death in the low-dose LPS shock model, and they were partially resistant to death in the high-dose LPS shock model.
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Materials and Methods |
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Animals and Treatments.
Homozygous CD7-deficient mice (12) were backcrossed five generations onto the C57BL/6 background. C57BL/6 mice were obtained from The Jackson Laboratory. 10-12-wk-old sex-matched mice were studied. Phenol extracted LPS from Escherichia coli (Sigma Chemical Co.) was administered intraperitoneally at 100 mg/kg for the high-dose LPS shock model. For the low-dose LPS plus D-gal (Sigma Chemical Co.) shock model, animals received 1 µg of LPS and 8 mg D-gal intraperitoneally in 0.5 ml saline. Animals were killed and livers and spleens were excised and studied as indicated below. Mouse handling and experimental procedures were conducted in accordance with the American Association of Accreditation of Laboratory Animal Care guidelines for animal care and use.Cytokine Measurements.
Quantification of murine IFN-In Vitro Splenocyte Cultures.
Splenocytes were stimulated with murine recombinant (r)IL-12 (Genzyme Corp.) and murine rIL-18 (Chemicon) in RPMI 1640 with L-glutamine (GIBCO BRL) supplemented with 10% FCS, 5.5 × 10RNA Isolation and RNase Protection Assays.
Total RNA was isolated from spleen and liver tissue using Trizol (GIBCO BRL) as per the manufacturer's protocol. Steady-state levels of specific cytokine messenger RNA in tissues were determined using the multiprobe RiboQuant RNase Protection Assay (PharMingen).Isolation of Liver Lymphocytes and Flow Cytometry.
Liver lymphocytes were prepared as previously described (23). Phenotypic analysis of liver lymphocytes (106 cells in 50 µl) was performed at 4°C after an initial blocking step with 1 µg of unlabeled anti-FcStatistical Analysis.
Student's t test was used to determine significance of cytokine mRNA and protein levels. Chi-square tests were used to determine P values for mouse survival data. ![]() |
Results |
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To determine the effect of high-dose LPS treatment in CD7-deficient mice, CD7-deficient mice (n = 30) and C57BL/6 control (n = 16) mice were injected intraperitoneally with LPS (100 mg/kg) and survival was assessed daily for 7 d (Fig. 1 A). C57BL/6 mice succumbed to shock between days 1 and 2 after high-dose LPS injection, with only 19% of the animals surviving on day 7. In contrast, 67% of CD7-deficient animals were alive on day 7 (P < 0.001), and demonstrated partial resistance to high-dose LPS shock. As additional controls, saline injected CD7-deficient (n = 3) and C57BL/6 control mice (n = 3), remained alive and healthy throughout the 7-d study (Fig. 1 A).
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Within 12 h of intraperitoneal injection of low-dose LPS (1 µg) and D-gal (8 mg), only 20% of control C57BL/6 animals (n = 10) survived (Fig. 1 B), consistent with previously reported data for this model (13). In contrast, no deaths were observed in CD7-deficient mice (n = 10) up to 72 h after injection with LPS (P < 0.001). Because of the total resistance of CD7-deficient mice to death in the low-dose LPS shock syndrome model, we studied this model further.
Elevated Serum Levels of TNF-Studies using IFN-R-deficient (15) and TNFRI-deficient
(16, 18) mice have clearly demonstrated the importance of
IFN-
and TNF-
as dominant cytokines that induce hepatitis and death in the low-dose LPS shock model (14).
Therefore, serum levels of TNF-
and IFN-
were measured in control C57BL/6 and CD7-deficient mice at multiple time points after injection of LPS and D-gal (Fig. 2).
There was a sharp increase in serum TNF-
levels in C57BL/6 animals 1 h after LPS plus D-gal treatment, that
fell to baseline after 4 h of treatment (Fig. 2 A). In contrast,
a blunted peak in TNF-
secretion was observed in CD7-deficient mice 1 h after LPS plus D-gal treatment, with a
rapid return to baseline 2 h after treatment. Peak TNF-
serum levels in CD7-deficient animals were 55 ± 7% of
that of control animals (P < 0.001) with a decrease in serum TNF-
levels 2 h after LPS plus D-gal treatment.
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In contrast to the early 2 h rise in TNF-, serum IFN-
levels in C57BL/6 mice peaked 6 h after injection of LPS
plus D-gal (Fig. 2 B). Importantly, a near complete absence
of serum IFN-
was observed in CD7-deficient mice at the
same time point (P < 0.001), and throughout the entire
12-h study period. Thus, disruption of the CD7 gene resulted in a partial block in serum TNF-
production, and
in a complete block in serum IFN-
production in the
low-dose LPS-induced shock model.
Next, cytokine gene expression in liver and spleen tissue from C57BL/6 and CD7-deficient mice was quantified by RNase protection assays. C57BL/6 (n = 3) and CD7-deficient (n = 3) animals were treated with low-dose LPS plus D-gal for either 0 or 6 h. The expression of various proinflammatory cytokine genes in liver and spleen are shown in Fig. 3.
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Marked increases in TNF-, IFN-
, and IL-6 steady-state mRNA levels were observed in liver tissues from
C57BL/6 control mice treated with LPS plus D-gal for 6 h
(P < 0.05; Fig. 3 A). As seen in the control mice, expression of TNF-
and IL-6 mRNA were also significantly elevated in liver tissue from CD7-deficient mice exposed to
LPS plus D-gal for 6 h, compared with pretreatment levels
(P < 0.05; Fig. 3 A). However, there was no significant increase in the steady-state level of IFN-
in 6-h-treated
CD7-deficient mice versus the 0 h mice. In addition, liver
from LPS plus D-gal-treated CD7-deficient animals had
lower steady-state mRNA levels of TNF-
and IL-6
mRNA compared with those seen in C57BL/6 liver tissues (P < 0.05). In contrast, spleen of both C57BL/6 and CD7-deficient mice had no significant differences in TNF-
,
IFN-
, or IL-6 steady-state mRNA levels after treatment
with LPS plus D-gal for 6 h (Fig. 3 B).
Deficient IFN- production in the liver could be due to
a lack of induction of the IFN-
inducing factors IL-12 and
IL-18 or ICE. To address these possibilities, IL-12 (p35,
p40), IL-18, and ICE steady-state mRNA expression in
liver was determined by RNase protection assays. IL-18
and ICE mRNA expression in liver (Fig. 3 C) was determined after 6 h of exposure to LPS plus D-gal. There was
no difference between C57BL/6 control mice (n = 3) and CD7-deficient mice (n = 3) in constitutive or LPS-induced
steady-state levels of IL-18 and ICE mRNA expression. At
the 6-h time point no significant induction in p35 or p40
IL-12 mRNA was detected in CD7-deficient or control mice.
In addition, no difference in constitutive or LPS-induced
steady-state levels of either IL-12 mRNA was observed between C57BL/6 control mice and CD7-deficient mice.
Splenocytes from C57BL/6 control
mice (n = 3) and CD7-deficient mice (n = 3) were cultured in vitro for 72 h with a wide dose range of IL-12,
IL-18, and IL-12 plus IL-18 (Fig. 4). As shown in Fig. 4 A,
IL-12 alone did not induce IFN- production by splenocytes from either C57BL/6 control or CD7-deficient mice.
IL-18 alone induced a low level of IFN-
production in
splenocyte cultures from both C57BL/6 control and CD7-deficient mice (P = NS) (Fig. 4 B). When IL-12 and IL-18
were combined to stimulate IFN-
production (Fig. 4 C),
an enhanced induction in IFN-
production was observed for both C57BL/6 splenocytes and CD7-deficient splenocytes. There was no significant difference in the level of
IFN-
stimulated by both cytokines in splenocyte cultures
from C57BL/6 versus CD7-deficient mice.
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NK1.1+/CD3+ T cells have been recently
reported to be major producers of IFN- and key effector
cells in the pathogenesis of lethal shock syndromes (23, 24).
Thus, we isolated liver mononuclear cells from C57BL/6
control and CD7-deficient mice and performed phenotypic analysis with respect to CD3 and NK1.1 expression. Three independent experiments comparing C57BL/6 and
CD7-deficient mice were performed by pooling of liver
mononuclear cells from two to five animals per experiment.
There was no difference in the absolute number of liver
lymphocytes isolated from C57BL/6 versus CD7-deficient
livers (Fig. 5). We observed a significant reduction in the
percentage and number of NK1.1+/CD3+ cells in livers
from untreated CD7-deficient mice (P < 0.05). In contrast, the percentage and absolute number of traditional NK
cells (NK1.1+/CD3
) and T cells (NK1.1
/CD3+) per
liver in C57BL/6 control and CD7-deficient mice were
similar. CD7-deficient livers also had an increase in the
number of B220+ B cells compared with C57BL/6 controls (Fig. 5).
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Discussion |
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In this study we have shown that CD7-deficient mice
are resistant to death in both the high-dose and the low-dose LPS-induced models of shock. Using the low-dose
LPS model we demonstrated decreased induction of serum
TNF- and IFN-
, decreased liver IL-6, TNF-
and IFN-
steady-state mRNA levels, and normal liver IL-12, IL-18,
and ICE mRNA levels in CD7-deficient mice compared with control mice. Moreover, we showed that CD7-deficient lymphocytes responded normally to IL-12 and IL-18
with regard to IFN-
production. Finally, resistance of
CD7-deficient mice to LPS-induced shock was associated
with a low number of resident effector NK1.1+/CD3+ T
cells in liver of CD7-deficient mice.
The CD7-deficient mouse is unique among homologous
recombinant mouse strains with respect to its responses in
low- and high-dose LPS-induced shock syndromes. Similar
to ICAM-1 deficient, ICE-deficient, and TNFRII-deficient mice (19, 20, 22), CD7-deficient mice are partially
resistant to LPS-induced death in the high-dose shock
model. Moreover, CD7-deficient mice are fully resistant to
LPS-induced death in the low-dose shock model, similar to
IFN-R-deficient and TNFRI-deficient mice (14, 18)
(Table I).
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Recent studies have reported that murine liver contains a
unique population of /
T cells that have intermediate
TCR expression and are positive for the NK1.1 antigen
(23). These NK1.1+/CD3+ T cells are phenotypically and
functionally distinct from NK 1.1
T cells, which are high in
TCR expression, and from CD3
NK cells, which are TCR
negative (23). This population of NK1.1+/CD3+ liver T cells
has been suggested to be autoreactive (25, 26), a major
source of IFN-
production, and key effector cells in the
pathogenesis of lethal shock syndromes (23, 24, 27). These
studies further revealed that LPS activates NK1.1+/CD3+
T cells via IL-12 production by Kupffer cells, resulting in the hepatotoxicity observed in septic shock (27). Moreover, depletion of these cells significantly decreased mortality in the
IL-12-primed LPS-induced shock syndrome (24).
We have found a significant reduction in the percentage
and absolute number of NK1.1+/CD3+ cells in livers from
untreated CD7-deficient mice versus age-matched control
mice (Fig. 5). These data suggested that the CD7 molecule may play a critical role in development, function, and/or
migration of liver NK1.1+/CD3+ T cells. The ontogeny of
NK1.1+/CD3+ T cells is currently unknown; however, it
is clear that they are a functionally and phenotypically distinct
subset of cells (23). That CD7-deficient animals have selectively diminished numbers of CD3+/NK1.1+ T cells with
normal numbers of CD3/NK1.1+ NK cells and CD3+/
NK1.1
T cells, also suggests that the CD3+/NK1.1+ T cell
subset may be a distinct cell lineage. We hypothesize that
CD7, which is expressed early in T and NK cell ontogeny
(7), may play a role in development and/or migration of
NK1.1+ T cells in the liver. Decreased numbers of these T
cells in the livers of CD7-deficient mice may result in
blunted cytokine production and be sufficient to protect
mice from both high- and low-dose LPS-induced shock.
Thus, CD7-deficient mice are unique in their resistance to both high-dose and low-dose LPS-induced shock models. Our data suggest that CD7 is an essential molecule that is involved in the lethal cellular and molecular events leading to low-dose LPS-induced hepatocyte necrosis/apoptosis, and to ischemia/reperfusion injury in high-dose LPS-induced shock. Understanding the role of CD7 in LPS shock syndromes and in NK1.1+ T cell maturation and function may spur the development of new treatments for endotoxic shock syndrome in humans.
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Footnotes |
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Address correspondence to Barton F. Haynes, Box 3703, Duke University Medical Center, Durham, NC 27710. Phone: 919-684-5384; Fax: 919-681-8992; E-mail: hayne002{at}mc.duke.edu
Received for publication 18 December 1998 and in revised form 27 January 1999.
We acknowledge the expert technical assistance of Mr. Jonathan L. Baron, helpful discussions with Dr. Charles Dinarello, and expert secretarial assistance from Ms. Kim R. McClammy.
This study was supported by National Institutes of Health grant CA28936.
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