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Short Communication |


From the Department of Molecular Pathology,*
The
Research Institute of Tuberculosis, Kiyose, Tokyo; and the Department
of Host Defense,
Research Institute for
Microbial Diseases, Osaka University, Osaka, Japan
| Abstract |
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B, PU.1,
interferon-regulatory factor 1, Egr-1, and Stat-1).
NF-IL-6 and NF-
B are expressed in macrophages and is induced by
bacterial lipopolysaccharides. To evaluate whether NF-IL-6 is
required for the inflammatory immune response to mycobacterial
infection, in which epithelioid macrophages comprise the
leading cell population, we generated NF-IL-6 knockout (KO)
mutant mice. Airborne infection of these mice with Mycobacterium
tuberculosis strains induced disseminated tuberculosis lacking
granuloma formation, although interferon-
,
tumor necrosis factor-
, and interleukin-12 mRNA expression
levels were within the normal range compared with those of wild-type
mice. Generation of O2- and mycobacterial
killing by neutrophils from these mice were impaired severely compared
with wild-type mice. We conclude that NF-IL-6 is a critical
transcription factor in mycobacterial control as well as in
granulocyte-colony stimulating factor induction resulting in neutrophil
activation.
| Introduction |
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B, PU.1, interferon-regulatory
factor 1, Egr-1, and Stat-1). Especially, NF-IL-6 and NF-
B are
expressed in macrophages and induced by bacterial
lipopolysaccharides.1,2
NF-IL-6 was originally
identified as a nuclear factor that binds to the IL-1 response element
of the human IL-6 gene.3
It is reported that NF-IL-6
expression level is dramatically increased during macrophage
differentiation.4,5
Thus, NF-IL-6 may be closely related
to macrophage functions. NF-IL-6-deficient mice displayed a high susceptibility to Salmonella, Listeria and Brucella abortus infections, indicating that NF-IL-6 plays a role in controlling intracellualr parasite proliferation by an as yet unknown mechanism. Macrophages acquire enhanced bactericidal capacity when fully activated after their interaction with Mycobacterium tuberculosis. Tuberculosis is a chronic disease induced by M. tuberculosis, one of typical intracellular pathogens, and characterized by clusters of epithelioid macrophages with central caseous necrosis.6 Thus, it is of great interest to examine the relationship between NF-IL-6 expression and tuberculosis, because the direct role of NF-IL-6 in mycobacterial infection has not been established fully. To evaluate whether NF-IL-6 is required for the inflammatory immune response to mycobacterial infection, in which epithelioid macrophages comprise the leading cell population, we generated NF-IL-6 knockout (KO) mutant mice7 and demonstrated that NF-IL-6 is an essential transcription factor in defense against mycobacterial infection.
| Materials and Methods |
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An 11-kb genomic fragment spanning from 8.5 kb 5' of the NK-IL6 transcription initiation site to 2.3 kb 3' of the end of the exon was subcloned into pUC18 plasmid vector. The MC1-herpes simplex virus thymidine kinase was inserted into the unique HindIII site in the 5' end of the homologous region. E141 ES cells were electroporated in 800 µl of phosphate-buffered saline (PBS) with 32 µg of SalI-linearized targeting vector DNA in a Bio-Rad Gene Pulser. G418- and Gancyclovir-resistant colonies were picked up 10 to 12 days later. Homologous recombination was screened by polymerase chain reaction (PCR) and subsequently confirmed by genomic Southern blot hybridization.7 These NF-IL-6 mice with C57BL/6 background were back-crossed over six generations. After birth, the NF-IL-6 KO mice developed normally and they have normal number of lymphocytes, leukocytes, and leukocyte subsets. They were kept under specific pathogen-free conditions in an environmentally controlled clean room at the laboratory animal facility at the Research Institute of Tuberculosis.
Bacterial Strains and Mouse Infections
Virulent M. tuberculosis H37Rv (ATCC358121) and more
virulent Kurono (ATCC25618) strains were passed through mice and grown
in 7H9 liquid medium once before storing in aliquots at -85°C and
titered. The cultured strains were filtered with a membrane filter of 4
µm pore size (Millipore, Bedford, MA) before use so that they were
dispersed evenly. The NF-IL-6 KO and wild-type (WT) mice were infected
by an airborne route by placing them in the exposure chamber of an
airborne infection apparatus (Model 099CA4241; Glas-Col, Inc., Terre
Haute, IN). The nebulizer compartment was filled with 5 ml of a
suspension of 106
CFU of H37Rv or Kurono
strain.8
The concentration was calculated to result in
uptake of
200 viable bacilli by the lungs just after inhalation
exposure for 90 minutes under the experimental conditions for this
study.19
The survival of groups of mice for 50 days
after infection with Kurono strain was recorded, and survival curves
were plotted. The lungs and spleens from NF-IL-6 KO and WT mice
infected with H37Rv were retrieved 1, 2, 3, 4, 5, 6, and 7 weeks after
infection, homogenized, diluted with physiological saline, plated on
Ogawa slant agar, and incubated at 37°C for 21 days. The resulting
CFU were counted. Permission (no. 991) to experiments on animals
was given by the Animal Experiment Committee of The Research Institute
of Tuberculosis.
Histology
Some mice were sacrificed just before death after infection. Tissue sections from paraffin blocks containing lung, liver, and spleen tissue were stained with hematoxylin and eosin or the Ziehl-Neelsen method for acid-fast bacilli.
Reverse Transcriptase (RT)-PCR
Pulmonary tissue samples were taken from infected NF-IL-6 KO and
WT mice just before death after infection (usually 35 days), frozen in
liquid nitrogen, and stored at -80°C until required for use, when
RNA was extracted as described previously.8
RT-PCR was
carried out with gene-specific primer sets for interferon-
(IFN-
), IL-12 p40, tumor necrosis factor-
(TNF-
), and IL-1{beta}
(CLP Inc., San Diego, CA). The granulocyte-colony stimulating factor
(G-CSF) cDNA was amplified by primer sets specific for
G-CSF.7
The size of {beta}-actin used as a positive control
was 514 bp. The same amounts of {beta}-actin RNA from the lung tissues
used as an internal control were used in RT-PCR analysis.
Macrophage Nitric Oxide (NO) Assay
Alveolar macrophages were obtained by bronchoalveolar lavage. Briefly, the murine trachea was cannulated and 1 ml of physiological saline was poured in. The saline was recovered using a 1-ml disposable syringe. The cells thus obtained contained more than 99% macrophages as assessed by phagocytosis of BCG and anti-Mac-1 immunostaining. The alveolar macrophages were cultured with H37Rv overnight at the multiplicity of infection of 5:1. Nitrite concentrations were determined by the Griess assay as described previously.9
Enzyme-Linked Immunosorbent Assay (ELISA)
The spleens were collected from the NF-IL-6 KO and WT mice and
pooled, and a single-cell suspension was prepared. The cells were
treated for 5 minutes with a 0.155 mol/L ammonium chloride/0.01
mol/L potassium bicarbonate solution for hemolysis of RBC, washed, and
resuspended in RPMI 1640 medium with heat-inactivated 10% fetal calf
serum. The cell suspensions were plated at 5 x
105
cells/well in 96-well flat-bottomed culture
plates and incubated for 3 days at 37°C in 5%
CO2 in air. The cells were stimulated with either
medium alone or live H37Rv (105
CFU). The
concentrations of IFN-
, TNF-
, IL-1{beta}, and IL-12 in cell culture
supernatants from the cells incubated in the presence of H37Rv were
measured by sandwich ELISA (Biosource International, Camarillo, CA).
The values were expressed as the mean and SE (in picograms per
milliliter) of triplicate determinations for pooled cells from five
animals.
O2- and Neutrophil CFU Assays
The 10% peptone broth-elicited peritoneal exudate cells were plated and cultured on Petri dishes for 6 hours. Non-adherent cells were collected after gentle shaking. The neutrophils (105; more than 98% pure as evaluated by morphology) from NF-IL-6 KO and WT mice were stimulated with H37Rv strain (5 x 106 CFU) in vitro overnight. Their O2- production was examined using a nitroblue tetrazolium (NBT) reagent.10 Briefly, 0.1 ml of 0.1% HEPES-saline-glucose buffer containing NBT was added to 0.1 ml neutrophil cell suspension (1 x 106/ml PBS) and the cell suspension was incubated at 37°C for 7 minutes. Then, 10 µl of 0.2 mol/L EDTA solution was added. The cell suspension was placed on a hemocytometer. The 200 cells were counted, and cells possessing NBT formazan in cytoplasm (blue-stained neutrophils) were judged as positive. At the same time, the supernatants containing neutrophils subjected to lysis with 0.05% sodium dodecyl sulfate were cultured in Lowenstein-Jensen media for 4 weeks before counting mycobacterial colonies.
Reconstitution of NF-IL-6 KO Mice with Exogenous G-CSF
The mice were injected subcutaneously with 2 µg of recombinant murine G-CSF (PeproTech, Rocky Hill, NJ) in PBS or PBS alone three times at weekly intervals to see whether or not recombinant G-CSF affects development of mycobacterial infection. The lungs, spleens, and livers from NF-IL-6 KO mice treated subcutaneously with recombinant G-CSF were retrieved from the infected mice 7 weeks after infection, and their histopathology was evaluated.
Statistical Method
The values were compared by Students t-test. For all statistical analyses, a P value <0.01 was considered significant.
| Results |
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We infected WT and NF-IL-6 KO mice with either H37Rv (ATCC358121)
or Kurono (ATCC25618) M. tuberculosis strains
(106
each). All of the mice infected with the
Kurono strain (each group of 10) died of mycobacteremia within 40 days,
whereas WT mice survived until the date of sacrifice at 50 days (Figure 1)
. When NF-IL-6 KO mice were infected
with H37Rv, the CFU number was much higher than that from WT mice at
weeks 2 through 7 after infection (Figure 2)
. Figure 3
shows that there were extensive
necroses in the lungs, spleen, and liver of NF-IL-6 KO mice infected
with Kurono strain. On the contrary, pulmonary granulomas were observed
in WT mice. Histopathologically, multiple foci of abscesses consisting
of neutrophils and macrophages were noted in the lungs, spleen, and
liver, whereas WT mice developed granulomatous lesions lacking
neutrophil infiltration (Figure 4)
. When
infected with H37Rv by an airborne route, NF-IL-6 KO mice survived but
developed severe lobar pneumonia with pulmonary edema. Necrotic lesions
induced in NF-IL-6 KO mice were inhibited significantly by treatment
with exogenous recombinant murine G-CSF, as shown in Figure 4c
. No
neutrophil infiltration was recognized in the lung treated with
recombinant G-CSF. The NF-IL-6 KO mice treated with recombinant murine
G-CSF lived longer than the WT mice.
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As shown in Figure 5
, RT-PCR showed
that NF-IL-6 KO mice expressed IFN-
, TNF-
, and IL-12 mRNA as
strongly as did those from WT mice. G-CSF mRNA expression (704 bp) was
lower in the lungs of NF-IL-6 KO mice than in the lungs of WT mice.
Fibroblasts other than alveolar macrophages expressed G-CSF mRNA. To
determine whether the absence of NF-IL-6 might influence the induction
and control of various cytokines, the levels of several cytokines in
the splenocyte cultures were determined. Table 1
shows that H37Rv-driven release of
IFN-
, TNF-
, IL-1{beta}, and IL-12 was similar for the two types of
mice.
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We examined the production of NO in culture supernatants of alveolar macrophages stimulated with H37Rv overnight from WT and NF-IL-6 KO mice. NO production was detectable in NF-IL-6 KO mouse macrophages to the same extent as in WT mouse macrophages (63 ± 5 vs. 60 ± 3 µmol/L).
Neutrophil Function Assay
Because induction of G-CSF production was impaired in NF-IL-6 KO
mice, we next examined neutrophil functions by two methods. First, 10%
peptone broth-elicited peritoneal neutrophils
(105) from either NF-IL-6 KO or WT mice were
cultured in vitro overnight in the presence of M.
tuberculosis H37Rv strain (5 x 106
CFU). The supernatants containing neutrophils subject to lysis with
0.05% sodium dodecyl sulfate were cultured in Lowenstein-Jensen media
for 4 weeks before counting of mycobacterial colonies. The numbers of
mycobacterial colonies in these supernatants from NF-IL-6 KO mice were
significantly higher than those from WT mice (54,3215 ± 2850
vs. 4356 ± 248 CFU; P < 0.01) (Figure 6)
.
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| Discussion |
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, IL-12,
and TNF-
, which are important cytokines in defense against
mycobacterial infection, were lower than those of WT mice. However, the
expression levels of these cytokines were within normal range as
evaluated by RT-PCR and ELISA. NO production was detectable significantly in NF-IL-6 KO mouse macrophages. There was no significant difference in NO production by alveolar macrophages between WT and NF-IL-6 mice (P < 0.01). Taken together with RT-PCR and ELISA data, these results suggest that macrophages of NF-IL-6 KO mice function normally to exert normal mycobactericidal activity.
What is the mechanism by which NF-IL-6 KO mice succumb readily to mycobacterial infection? Killing ability of mycobacteria and O2- producing ability by neutrophils was significantly lower in NF-IL-6 KO mice compared with those of WT mice. Since induction of G-CSF production by alveolar macrophages was impaired severely in NF-IL-6 KO mice,7 we examined neutrophil functions in terms of mycobacterial killing ability by neutrophils, O2- generation, and reconstitution of NF-IL-6 KO mice with recombinant murine G-CSF. Consequently, neutrophils of NF-IL-6 KO mice did not function properly and granulomatous formation in NF-IL-6 KO mice was abrogated significantly with exogenous murine G-CSF. Since no neutrophils were recognized in the granulomatous lesions, recombinant murine G-CSF reversed neutrophil function significantly. It is also reported that G-CSF induces proliferation and differentiation of neutrophil precursors.11-13 Electron microscopy revealed tubercle bacilli in phagosomes of macrophages from NF-IL-6 KO mice, but we saw no evidence of M. tuberculosis having escaped from phagosomes to the cytoplasm, although this has been demonstrated in infection with L. monocytogenes (data not shown).7 Therefore, it is concluded that bactericidal activity by neutrophils may be impaired severely in these NF-IL-6 mice.
Alveolar macrophages are targets of mycobacterial infection and
activated macrophages, as well as IFN-
, TNF-
, IL-12, IL-18, and
IL-1, play major roles in protective immunity against
tuberculosis.13-19
Our results demonstrate that
neutrophils play an important role in the early phase of defense
against murine tuberculosis when aerosol infection was used. Since
G-CSF is impaired in NF-IL-6 KO mice,4
NF-IL-6 appears to
be essential for inducing G-CSF production in neutrophil proliferation
and differentiation.14-18
It has been very recently reported that addition of G-CSF, whose induction is impaired in NF-IL-6 KO mouse macrophages, restores both endocytosis and morphology of endosomes, together with bactericidal activity.20 Taken together with our data, both bactericidal activity by neutrophils and modulation of endocytosis by macrophages may be impaired in these NF-IL-6 KO mice. We showed that neutrophils played an important role in preventing development of granulomatous lesions. Tuberculosis is of global importance. This NF-IL-6 KO mouse model may offer a strategy for developing novel anti-tuberculosis drugs that augment the NF-IL-6 expression that is closely associated with neutrophil functions.
| Acknowledgements |
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| Footnotes |
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Supported in part by an International Collaborative Study Grant (to I. S.) from the Ministry of Health and Welfare, Japan.
Accepted for publication October 6, 2000.
| References |
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B synergistically activate transcription of the inflammatory cytokines, IL-6 and IL-8. Proc Natl Acad Sci USA 1993, 90:10193-10197
B and C/EBP family members: a Rel domain-bZip interaction. Mol Cell Biol 1993, 13:3964-3974
. J Immunol 1990, 145:4290-4297[Abstract]
in resistance to Mycobacterium tuberculosis infection. J Exp Med 1993, 178:2249-2254
gene-disrupted mice are inducible by avirulent Mycobacterium, but not by virulent Mycobacterium. J Med Microbiol 1998, 47:871-877
/{beta} double-knockout mice. Lab Invest 2000, 80:759-767[Medline]
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