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* Second Department of Internal Medicine, Nagasaki University School of Medicine, Nagasaki, Japan;
Department of Immunology, Okayama University Graduate School of Medicine and Dentistry, Okayama, Japan; and
RIKEN BioResource Center, Tsukuba Institute, Tsukuba, Japan
| Abstract |
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| Introduction |
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(4, 5), and platelet-derived growth factor (6, 7) have been suggested to promote infiltration of inflammatory
cells and proliferation of fibroblasts. Because of its
pathophysiological similarity to IPF, bleomycin (BLM)-induced
pneumopathy has been frequently used as an animal model of IPF. Because
treatment with anti-mouse TNF polyclonal Abs (8) or a
TNF antagonist (9) prevents pulmonary inflammation in
BLM-injected mice, TNF has been considered to be the crucial mediator
that promotes lung inflammation triggered by BLM. Using p55p75
TNFR-/- mice, Ortiz et al. (10)
recently showed resistance of these mice to BLM-induced pneumopathy,
confirming the detrimental role of TNF in this condition. TNF was initially recognized as the factor that induced hemorrhagic necrosis of transplanted tumors in mice (11). Cloning of the TNF gene and the subsequent generation of recombinant TNF and TNF-specific mAbs facilitated further analyses of the biological properties of TNF, elucidating that TNF is one of the most potent proinflammatory cytokines (12, 13). However, studies using TNF-/- mice revealed a new biological facet of TNF. Following single injection of heat-inactivated Corynebacterium parvum, delayed and intense inflammatory responses associated with ascites and hepatosplenomegaly were observed in TNF-/- mice at a time when granuloma formation and inflammation were completely resolved in TNF+/+ mice (14). Consistent with this observation was that TNF-/- mice immunized with myelin basic protein exhibited prolonged myelin-specific T cell reactivity with development of late-onset and chronic autoimmune encephalomyelitis (15). These two observations indicated the anti-inflammatory property of TNF. Because TNF-/- mice showed no inflammatory responses or disease signs in the early phase in both experimental systems, TNF may exert its anti-inflammatory function in the late phase of inflammation or the disease course.
In this study, we have examined the effects of TNF deficiency on pulmonary inflammation using TNF-/- mice. In this work, we demonstrate persistent infiltration and failure of elimination of inflammatory cells from the bronchoalveolar space in TNF-/- mice. Challenge via airways with murine, but not human rTNF induced apoptosis of inflammatory cells, indicating that TNF eliminates inflammatory cells by apoptosis through interaction with p75 TNFR. We also evaluated the long-term effects of TNF inhalation on pulmonary inflammation and tissue repair of damaged lungs.
| Materials and Methods |
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TNF-/- mice were generated and maintained at the Ludwig Institute for Cancer Research at Memorial Sloan-Kettering Cancer Center (New York, NY). TNF-/- mice were backcrossed to C57BL/6 mice for at least five generations. C57BL/6 mice were purchased from Charles River (Yokohama, Japan). All mice were maintained under specific pathogen-free conditions in the Laboratory Animal Center for Biomedical Science at Nagasaki University. Eight- to 12-wk-old female mice were used for experiments. Experiments were performed in accordance with the protocol approved by the Ethics Review Committee for Animal Experimentation at Nagasaki University.
Abs and rTNF
The following Abs were used: goat polyclonal Abs specific to murine CD120a (p55 TNFR) or CD120b (p75 TNFR) from R&D Systems (Minneapolis, MN); rabbit anti-goat IgG Abs conjugated with FITC from Jackson ImmunoResearch Laboratories (West Grove, PA); anti-CD3 mAb (17A2) labeled with FITC from BD PharMingen (San Diego, CA); and anti-CD120a (HM104) and anti-CD120b (HM102) mAbs labeled with PE from Caltag Laboratories (Burlingame, CA). rTNF protein was obtained from Peprotech (London, U.K.).
Intratracheal injection of BLM, bronchoalveolar lavage (BAL), and preparation of cells from lung tissue
After anesthesia with sodium pentobarbital, 5 U/kg mouse weight of bleomycin hydrochloride (Nippon Kayaku, Tokyo, Japan) was administered intratracheally through i.v. catheters. For control, 50 µl sterile PBS was comparably administered. For BAL, a tracheal cannula was inserted into the tracheal lumen after sacrificing mice by cervical dislocation. Lungs were lavaged five times with 1-ml aliquots of sterile PBS. After washing twice with PBS, cells were suspended in 0.5 ml PBS. Lung cells were isolated, as described previously (16). Total cell counts were performed using trypan blue and a hemocytometer. Differential cell counts were performed in May-Giemsa-stained cytocentrifuge preparations of recovered cells. At least 200 cells were counted using a microscope.
Histological examination
Excised lungs were fixed with 10% formaldehyde neutral buffer solution for 24 h and embedded in paraffin. Sections (5 µm) were placed on glass slides and deparaffinized, followed by staining with H&E.
Hydroxyproline assay
Lungs were homogenized in PBS and hydrolyzed in concentrated HCL at 100°C for 20 h. The hydroxyproline content of each sample was determined as previously described (17).
Flow cytometry
Cells (1 x 1052 x 105) infiltrating into the bronchoalveolar space were stained with each Ab for 30 min on ice in PBS with 1% FCS. The annexin V-FITC apoptosis detection kit (BD PharMingen) was used to detect apoptotic cells in bronchoalveolar space. A total of 1 x 104 cells were analyzed for flow cytometry with a FACSCalibur using CellQuest software (BD Biosciences, San Jose, CA).
ELISA for TNF
To determine concentrations of TNF, lungs from mice were homogenized in 0.9% NaCl and centrifuged at 15,000 x g for 10 min at 4°C. Supernatants (100 µl) were added onto TNF ELISA plates purchased from TFB (Tokyo, Japan). Assays were performed using reagents supplied by the manufacturer.
Airway challenge with rTNF
TNF-/- mice were put in a closed chamber and challenged via airways with various concentrations of rTNF solution by ultrasonic nebulization.
Statistical analysis
All results were expressed as mean ± SEM. The unpaired two-tailed Mann-Whitney (nonparametric) test was used to analyze the data. Values of p less than 0.05 denoted a significant difference.
| Results |
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We investigated pulmonary inflammation induced by intratracheal
injection of BLM. As shown in Fig. 1
a, the number of inflammatory
cells in BAL fluid (BALF) reached a peak level on day 7 and decreased
thereafter in TNF+/+ mice. In contrast,
persistent infiltration of inflammatory cells was observed in BALF from
TNF-/- mice. Cells in BALF harvested from
TNF+/+ mice after day 14 were predominantly
macrophages, whereas significant number of lymphocytes was observed in
BALF from TNF-/- mice even on day 35 after BLM
(Fig. 1
a). Intratracheal injection of sterile PBS alone
caused no infiltration of inflammatory cells in the bronchoalveolar
space (Fig. 1
b). For further analysis, cells from lung
tissue were isolated on day 21 after BLM instillation. Total cell and
differential cell counts were compared in TNF+/+
and TNF-/- mice. An increase in total cell and
lymphocyte numbers was observed in mice injected with BLM. More total
lung cells and lymphocytes were seen in TNF-/-
mice than TNF+/+ mice (Fig. 2
a). No significant increase
in total cells and lymphocytes was observed in mice injected with
sterile PBS alone, compared with nontreated mice (Fig. 2
b).
Histological examination revealed typical inflammatory changes induced
by BLM, such as infiltration of lymphocytes and neutrophils, thickening
of alveolar septa, and proliferation of fibroblasts in lung specimens
from both TNF+/+ (Fig. 3
a) and
TNF-/- mice (Fig. 3
b) on day 14
after BLM. In TNF+/+ mice, inflammatory responses
gradually subsided with restoration of normal alveolar structures (Fig. 3
c), whereas massive infiltration of lymphocytes and a
honeycomb structure was observed in TNF-/- mice
on day 75 after BLM instillation (Fig. 3
d). Only unilateral
lungs were damaged macroscopically and histologically in
TNF-/- mice that survived >35 days after BLM
instillation. All the data shown were from mice in which bilateral
lungs were affected macroscopically or histologically, except
TNF-/- mice with long survival. The lung
hydroxyproline content was measured in TNF+/+ and
TNF-/- mice on day 21 after BLM. No significant
difference in the amount of hydroxyproline was observed (Fig. 4
).
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TNF production in the lungs of TNF+/+ mice
after BLM instillation showed a biphasic response; TNF production
reached a peak level 12 h after BLM injection, followed by a
transient decline, and then resumed increasing after day 7 (Fig. 5
a). TNF was persistently
produced until day 50. No TNF was detectable in
TNF-/- mice. Flow cytometric analysis revealed
that expression of p55 and p75 TNFRs was up-regulated on inflammatory
cells in BALF from both TNF+/+ and
TNF-/- mice on day 14 (Fig. 5
b).
Inflammatory cells expressing TNFRs were still detected in the
bronchoalveolar space of TNF-/- mice on day 28.
Transient expression of TNFRs was observed 12 h after BLM
instillation. However, no expression was detected during the period
from day 1 to day 12. Most p75 TNFR-positive cells were
CD3+ T cells, whereas p55 TNFR was expressed in
both T and non-T cells (Fig. 5
c).
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We then examined apoptosis of inflammatory cells in the BALF after
BLM injection. Flow cytometric analysis revealed that significant
numbers of inflammatory cells in BALF from TNF+/+
mice were apoptotic, whereas fewer apoptotic cells were observed in
TNF-/- mice (Fig. 6
, a and b). To
confirm the direct effects of TNF on induction of inflammatory cell
apoptosis in BALF, TNF-/- mice were challenged
via airways with rTNF protein. Inflammatory cells in BALF from
TNF-/- mice were counted 48 h after airway
challenge with murine rTNF protein solution at a concentration of 250
ng/ml. Fewer inflammatory cells were observed in
TNF-/- mice challenged with TNF than in those
that inhaled PBS alone on day 14 after BLM instillation (Fig. 7
a). However, no significant
difference was observed in the number of inflammatory cells in the BALF
between TNF-treated and untreated TNF-/- mice
on day 7 (Fig. 7
a). The effect of murine TNF was transient,
with inflammatory cells appearing again in the bronchoalveolar space
96 h after TNF challenge (Fig. 7
b). Airway challenge
with human rTNF did not eliminate inflammatory cells (Fig. 7
b), indicating that signals through p75 TNFR are critical
in inducing apoptosis (Fig. 7
c). The apoptotic effect of
murine TNF on inflammatory cells was observed at a concentration as low
as 2.5 ng/ml (Fig. 7
d).
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| Discussion |
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and TNF share two TNFRs as
the ligands (21), lymphotoxin
, rather than TNF, may be
the crucial factor involved in boosting inflammation. We showed that endogenous TNF was required for resolution of inflammation and subsequent remission of BLM-induced pneumopathy. In the absence of TNF, inflammatory cells accumulated in the bronchoalveolar space, and thus, the repair process of lung injury was impaired, which was often lethal to mice. In contrast to TNF-/- mice, clearance of inflammatory cells and restoration of normal alveolar structure were observed in TNF+/+ mice, in which persistent production of TNF in lungs was observed until day 50 after BLM instillation. More apoptotic cells were observed in BALF harvested from TNF+/+ than from TNF-/- mice, suggesting that TNF is an important mediator for inducing apoptosis of inflammatory cells.
Little is known about the relative contribution of Fas-Fas ligand (FasL) and TNF in mediating apoptosis associated with elimination of inflammatory cells. The involvement of Fas-FasL in BLM-induced pneumopathy has been controversial (22, 23). Fas expression was not detected on inflammatory cells in the BALF in either TNF-/- or TNF+/+ mice, and challenge via airways of TNF-/- mice with murine rTNF effectively eliminated inflammatory cells from the bronchoalveolar space by apoptosis. We therefore conclude that TNF, but not Fas-FasL, plays a critical role in inducing apoptosis of inflammatory cells in BLM-induced pneumopathy. Because of the lack of expression of TNFRs on inflammatory cells in BALF, TNF inhalation was not effective on day 7 after BLM instillation. Long-term airway challenge of TNF-/- mice with murine TNF starting on day 14 was effective in reducing lung inflammation and prolonging survival of those mice, without causing weight loss and ruffled hair in TNF-/- mice. In this regard, local production of endogenous TNF and up-regulation of expression of TNFRs after the acute phase of inflammation may be an important host response for resolution of inflammation. It will be important to clarify the triggers of TNF production in the lungs and of up-regulation of TNFR expression on inflammatory cells. TNF is not required for inducing expression of TNFRs.
Our results showed that murine, but not human rTNF was effective in elimination of inflammatory cells from the bronchoalveolar space by apoptosis. Because human TNF binds only mouse p55 TNFR, but not mouse p75 TNFR (24), our findings in this study revealed the critical role of TNF-p75 TNFR in inducing apoptosis in vivo. In vitro analysis showed that the interaction between TNF and p75 TNFR induced apoptosis of activated CD8+ T cells (25, 26). Receptor interacting protein, a Ser/Thr protein kinase, was suggested to be up-regulated in activated T cells by IL-2 and to mediate death signals through the interaction of TNF and p75 TNFR (27). Receptor interacting protein may be up-regulated in inflammatory cells and may mediate apoptosis of those cells in vivo.
In conclusion, we have demonstrated in the present study that endogenous and persistent production of TNF and up-regulation of p75 TNFR expression were indispensable for clearance of inflammatory cells from the bronchoalveolar space by apoptosis and tissue repair of damaged lungs in BLM-induced pneumopathy. Long-term airway challenge of TNF-/- mice with murine TNF was effective and safe, suggesting that TNF may have a clinical application in IPF.
| Acknowledgments |
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| Footnotes |
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2 Abbreviations used in this paper: IPF, idiopathic pulmonary fibrosis; BAL, bronchoalveolar lavage; BALF, BAL fluid; BLM, bleomycin; FasL, Fas ligand. ![]()
Received for publication May 20, 2002. Accepted for publication October 25, 2002.
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