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Severe respiratory syncytial virus (RSV) infections are characterized by airway epithelial cell damage, mucus hypersecretion, and Th2 cytokine production. Less is known about the role of IL-17. We observed increased IL-6 and IL-17 levels in tracheal aspirate samples from severely ill infants with RSV infection. In a mouse model of RSV infection, time-dependent increases in pulmonary IL-6, IL-23, and IL-17 expression were observed. Neutralization of IL-17 during infection and observations from IL-17−/− knockout mice resulted in significant inhibition of mucus production during RSV infection. RSV-infected animals treated with anti-IL-17 had reduced inflammation and decreased viral load, compared with control antibody-treated mice. Blocking IL-17 during infection resulted in significantly increased RSV-specific CD8 T cells. Factors associated with CD8 cytotoxic T lymphocytes, T-bet, IFN-γ, eomesodermin, and granzyme B were significantly up-regulated after IL-17 blockade. Additionally, in vitro analyses suggest that IL-17 directly inhibits T-bet, eomesodermin, and IFN-γ in CD8 T cells. The role of IL-17 was also investigated in RSV-induced exacerbation of allergic airway responses, in which neutralization of IL-17 led to a significant decrease in the exacerbated disease, including reduced mucus production and Th2 cytokines, with decreased viral proteins. Taken together, our data demonstrate that IL-17 plays a pathogenic role during RSV infections.
Nearly 98% of all infants become infected with respiratory syncytial virus (RSV) by the age of 2 years and experience severe bronchiolitis because their small airways easily become occluded.
It is estimated by the U.S. Centers for Disease Control that up to 125,000 pediatric hospitalizations in the United States each year are due to RSV. In addition, RSV is pathogenic for elderly patients and for those with chronic lung disease and asthma, and further is associated with a mortality rate of 30% to 100% in immunosuppressed individuals.
RSV also is associated with acute exacerbations of chronic obstructive pulmonary disease, causing prolonged episodes of illness. Recurrent infections with RSV are common, and the pulmonary pathology is known to persist long after the virus has been cleared efficiently. RSV disease pathology is clinically characterized by airway hyperreactivity (AHR), increased mucus production, and inflammation.
An altered immune environment due to an imbalance in the CD4 helper Th1 and Th2 responses is thought to underlie this disease phenotype. Recently it was reported that IL-17, produced by a subset of CD4 helper T cells (Th17 cells), was regulated by STAT-1 during RSV infections in rodents.
The exact role of IL-17 in RSV disease pathogenicity is not known.
Interleukin-17 belongs to a family of cytokines that has six members: IL-17 (also called IL-17A, the prototype) and IL-17B through IL-17F; IL-17E is also known as IL-25. IL-17F shares the strongest homology to IL-17.
Both IL-17 and IL-17F are proinflammatory and have overlapping roles in the development of various autoimmune disorders, such as rheumatoid arthritis, multiple sclerosis, and inflammatory bowel disease. However, IL-17 plays a critical role in host defense during bacterial and fungal infection, whereas IL-17F is largely involved in the development of asthma and airway inflammation.
The receptors for IL-17 and IL-17F are IL-17RA and IL-17RC, respectively; because these receptors have different tissue expression, the isoforms of IL-17 are tissue-specific. Expression of IL-17RC is limited to nonhematopoietic cells, but IL-17RA is expressed ubiquitously. IL-17 may promote Th2 responses in the lung through IL-17RA, whereas IL-17F has a regulatory role in limiting allergic asthma development.
The combination of AHR and mucus production in the airways is a significant clinical outcome during viral infections. RSV infections induce significant AHR and mucus production in the airways of mice
its specific role in pathogenic responses during RSV infection is not known.
Here, we report increased IL-17 production in infants with RSV infection and identify a role of IL-17 in a mouse model of primary RSV infection, as well as during viral exacerbation of allergic lung disease. Using either IL-17-deficient mice or neutralization of IL-17 significantly inhibited mucus production during RSV infection. In addition, blocking IL-17 significantly decreased viral load and altered cytotoxic CD8 T-cell marker expression. These responses were also observed in RSV-induced allergic airway exacerbation, suggesting that IL-17 plays an important role in the pathogenesis of RSV-induced disease.
Materials and Methods
Mice
Female BALB/c mice, 6 to 8 weeks old, were purchased from the Jackson Laboratory (Bar Harbor, ME). The IL-17−/− mice, derived from breeder animals from the Jackson Laboratory, were a kind gift of Dr. Kathryn Eaton (University of Michigan).
All mice were maintained in specific-pathogen-free facilities in the Unit for Laboratory Animal Medicine at the University of Michigan. The University Committee of Use and Care of Animals (UCUCA), University of Michigan, Ann Arbor, approved all animal experimental protocols, and experiments were conducted according to the guidelines provided by the UCUCA review committee.
Human Specimens
All human studies were performed in accordance with an approved University of Michigan institutional review board protocol after legal consent. The tracheal aspirate samples were diluted 50:50 with PBS containing complete anti-protease cocktail (Sigma-Aldrich, St. Louis, MO) and 0.5% Triton X-100 nonionic detergent to dissociate the mucus. Samples were aliquoted in 75 μL and stored at −80°C until analysis. IL-17 was analyzed using a Bio-Plex 200 System (Bio-Rad Laboratories, Hercules, CA) and IL-6 was analyzed using an enzyme-linked immunosorbent assay (human Quantikine ELISA kit; R&D Systems, Minneapolis, MN). Tracheal aspirate samples were obtained from RSV-infected infants who were hospitalized and given mechanical ventilation in a pediatric intensive care unit. Control uninfected tracheal infant samples were obtained from children undergoing surgery for reasons unrelated to RSV infection, after parental consent.
RSV Infection and Cockroach Sensitization
Female BALB/c mice, 6 to 8 weeks old, were anesthetized and infected with RSV on day 0. Lungs were harvested on days 1, 2, 4, 6, 8, 10, and 12 for the time-course studies and on day 8 for all other studies. Cockroach (CRA) sensitization was performed as described previously.
Monocyte chemoattractant protein-1 mediates cockroach allergen-induced bronchial hyperreactivity in normal but not CCR2-/- mice: the role of mast cells.
Briefly, mice were sensitized with a 1:1 mixture of clinical-grade CRA extract (Hollister-Stier Laboratories, Spokane, WA) and incomplete Freund's adjuvant (Sigma-Aldrich), administered subcutaneously and intraperitoneally on day 0. This cockroach allergen is a skin test/immunotherapy-grade preparation that has very little endotoxin contamination (<10 ng/mL). At day 14, mice were sensitized by intranasal challenges of CRA, which was followed by RSV infection on day 16. In animals treated with antibodies, control or anti-IL-17 purified IgG was given 2 hours before RSV infection on day 16. Two doses of control or anti-IL-17 purified IgG were also given on days 18 and 20. After a final intranasal challenge of CRA on day 21, the mice were euthanized and AHR studies were conducted on day 22. The lungs and draining lymph nodes (DLNs) of these mice were isolated for further characterization.
RSV and Viral Plaque Assay
RSV A strain (line 19) was derived from a clinical isolate at the University of Michigan.
A chimeric A2 strain respiratory syncytial virus (RSV) with the fusion protein of RSV strain line 19 exhibits enhanced viral load, mucus, and airway dysfunction.
and in the present study UV-inactivated virus failed to induce IL-17 in mice (data not shown). Plaque assays were performed on RSV-infected lungs. Whole lungs were harvested and ground with sand using a mortar and pestle. Supernatants were serially diluted and incubated with Vero cells for 4 days. Viral plaques were determined using a RSV-specific polyclonal antibody (Millipore, Billerica, MA).
Generation of Rabbit Anti-Mouse Polyclonal IL-17–Specific Antibody
Rabbit anti-mouse IL-17 antibodies were prepared by multiple-site immunization of New Zealand White rabbits with recombinant mouse IL-17 (R&D Systems) in complete Freund's adjuvant and boosted with IL-17 in incomplete Freund's adjuvant, according to procedures from our laboratory as described previously (both purchased from Sigma-Aldrich).
Polyclonal antibodies were titered by direct ELISA against IL-17 coated onto 96-well plates. Specificity of the antibody was tested by direct ELISA against IL-17, IL-17E, and IL-17F, as described previously,
and was found to react only with IL-17. The polyclonal antibody was further purified from the serum using Protein A columns (Pierce; Thermo Fisher Scientific, Rockford, IL) according to the manufacturer's protocol. Control antibody (Cab) was purified from serum of unimmunized mice also using Protein A columns.
Neutralization of IL-17
Mice were pretreated intraperitoneally with 2.5 mg of purified polyclonal anti-mouse IL-17 antibody 2.5 hours before RSV infection on day 0 and then every other day until day 6. The control group similarly received 2.5 mg of anti-mouse IgG antibody (Cab).
Airway Response
Airway hyperreactivity was assessed as described previously.
Monocyte chemoattractant protein-1 mediates cockroach allergen-induced bronchial hyperreactivity in normal but not CCR2-/- mice: the role of mast cells.
Briefly, mice were anesthetized with sodium pentobarbital, intubated via cannulation of the trachea, and ventilated with a Harvard pump ventilator (0.3 mL tidal volume; 120 breaths/minute). Airway hyperreactivity was measured using a direct ventilation methodology with a sensitive mouse plethysmograph and software for calculation of the measurements (Buxco Research Systems, Wilmington, NC). After baseline measurements, mice were injected intravenously with 7.5 μg of methacholine (Sigma-Aldrich), and the peak airway resistance was recorded as a measure of AHR.
Determination of RSV Antigens in Lungs of Infected Mice
Total RSV antigen levels in lung homogenates were measured by a specific ELISA as described previously.
Briefly, the lung samples (100 μL) were incubated for 1 hour in 96-well plates coated with goat anti-RSV antibody (Millipore). After three washes, the plates were incubated with a mouse anti-RSV polyclonal secondary antibody (Vector Laboratories, Burlingame, CA). After a final incubation with a peroxidase-labeled goat anti-mouse IgG antibody (Millipore), positive wells were assessed by o-phenylenediamine dihydrochloride (Dako, Carpinteria, CA) substrate development. Optical density readings were at 450 nm.
Histology and RT-PCR
For histology studies, right-lung lobes from infected mice were removed, fixed in 10% formalin, and stained with H&E or PAS to detect mucus production. For RT-PCR, the lower left lobe of the freshly harvested lung was snap-frozen and stored at −80°C. Total RNA was extracted from the frozen tissue using TRIzol reagent (Invitrogen, Carlsbad, CA) and was reverse transcribed to cDNA. Real-time PCR was performed with the cDNA using gene-specific primers. Murine primers for IL-4, IL-5, IL-13, IFN-γ, IL-17, IL-17F, IL-23p19, IL-6, RANTES, KC, eotaxin, MDC, MIP1a, MIP2a, T-bet, eomesodermin (Eomes), and GAPDH were purchased from Applied Biosystems (Carlsbad, CA). Primers and probes for Muc5ac, Gob5, RSV-F, RSV-N, and RSV-G were determined using primer/probe detection sets designed by PE Biosystems (Foster City, CA) and purchased from Sigma-Aldrich. All results were normalized to GAPDH expression.
Protein Assays
Single-cell suspensions of lymph nodes were seeded at a concentration of 5 × 106 cells/mL in a 96-well plate and were restimulated with RSV. At 48 hours, the supernatants were harvested and analyzed for cytokines, which were quantified using a Bio-Plex bead-based (Luminex) cytokine assay purchased from Bio-Rad Laboratories.
Flow Cytometry
Single-cell suspensions of lung (after collagenase dispersion) and lymph node cells were prepared as described previously.
To detect various surface markers (CD3, CD8, CD4, and CD69 from eBioscience, San Diego, CA; pan NK and γδ T cell from BioLegend, San Diego, CA; and IL-17RA from R&D Systems), cells were stained with the indicated antibodies after 10 minutes of preincubation with Fc block (BD Biosciences, San Jose, CA). For detection of RSV M protein-specific T-cell receptor-positive CD8 T cells (RSV tetramer, M82-90; H2-kd), staining antibody was added 30 minutes before the addition of any other antibody and the reaction took place at room temperature. The cells were fixed overnight with 4% formalin. For intracellular staining, the cells were fixed and permeabilized using a staining buffer kit (eBioscience) and then stained with antibodies against T-bet, granzyme B, IFN-γ, and Eomes (all from eBioscience). The samples were processed through a flow cytometer (LSRII; Becton-Dickinson, Franklin Lakes, NJ). The data were analyzed using FlowJo software version 8.8.4 (TreeStar, Ashland, OR).
In Vitro CD8 T-Cell Culture
CD8 T cells were sorted from the spleen using magnetic beads and plated in 96-well plates at a concentration of 2 × 105 cells per well in RPMI-1640 complete medium (Lonza, Walkersville, MD). The cells were stimulated with plate-coated anti-CD3 and soluble anti-CD28 (eBioscience) with or without recombinant IL-17 (R&D Systems) at 100 ng/mL. RNA was extracted from the cells at 6 hours after stimulation and was quantified by real-time RT-PCR.
Bronchoalveolar Lavage Fluid Microscopy
After AHR measurement and cervical dislocation, bronchoalveolar lavage fluid was collected from each mouse. One milliliter sterile 0.9 N saline was instilled intratracheally and was suctioned out after a few seconds. Samples were placed in Eppendorf tubes and were centrifuged at 1500 rpm for 5 minutes (240 × g). The cells were centrifuged in a shandon cytospin 2, fixed, and a differential cell count was done after Diff-Quik staining (Dade Behring, Newark, DE).
Statistical Analysis
Data are reported as means ± SE. Statistical significance was determined by Student's t-test or one-way analysis of variance with Newman-Keuls post hoc test. P < 0.05 was considered significant.
Results
RSV Infection Induces IL-17 Production in Mice and Humans
To investigate whether RSV infection induces IL-17 production in vivo, BALB/c mice were infected with RSV and a time-course analysis was performed of mRNA expression of two different IL-17 transcripts (IL-17 and IL-17F) and the cytokines involved in their production (IL-6) and maintenance (IL-23p19). RSV infection significantly induced IL-17 but not IL-17F in the lungs of infected mice compared with control mice, with expression levels peaking at day 8 after infection (Figure 1A). IL-6 and IL-23p19 mRNA expression increased threefold and twofold, respectively, on day 8 after RSV infection (Figure 1A). RSV infection also increased IL-17 protein production in the lungs, and peak IL-17 levels correlated with the transcript levels (Figure 1B). Furthermore, we observed a significant increase in antigen-specific IL-17 production in the DLNs taken at day 8 from infected animals when they were restimulated ex vivo with RSV (Figure 1C).
Figure 1RSV infection induces IL-17 production in mice and humans, and CD4 T cells contribute significantly to peak IL-17 production. A: Total RNA was extracted from the lungs of RSV-infected BALB/c mice on days 1, 2, 4, 6, 8, and 12 after infection, and the relative expression of IL-17, IL-17F, IL-6, and IL-23p19 was analyzed by real-time PCR. The expression levels were normalized to the housekeeping gene GAPDH and fold inductions were compared with naïve mice. B: IL-17 production in the lungs on days 2, 4, 6, 8, and 12 after RSV infection. The data were analyzed by a Bio-Plex system. C: Draining lymph nodes from naïve and RSV-infected mice were harvested on day 8 after infection and were rechallenged with RSV. Supernatants were collected 48 hours later, and IL-17 production was analyzed by a Bio-Plex system. D: Intracellular IL-17 levels were measured by flow cytometry in different IL-17 producing cells in the lungs of mice at day 8 after RSV infection and were compared with naïve lungs. Each experiment was repeated at least twice, with four to five mice per group. Human IL-6 (E) and IL-17 (F) were measured in tracheal aspirate samples from infants with or without RSV infection. Data are reported as means ± SE. *P < 0.04, **P = 0.036.
To determine which lung cells were expressing IL-17 during peak IL-17 production, CD4 T cells, pan natural killer (NK) cells, and γδ T cells taken at day 8 from RSV-infected lungs were sorted by flow cytometry and intracellular IL-17 levels were measured. We observed significant increase in IL-17+CD4 T cells in the lungs of RSV-infected mice, compared with uninfected mice (Figure 1D). For NK and γδ T cells, however, no significant difference was observed between infected and uninfected groups. These data demonstrate that RSV infection induces IL-17 expression by CD4 T cells in the lungs during peak IL-17 production. To validate our data, we further analyzed IL-6 and IL-17 levels in tracheal aspirate samples from infants who had been hospitalized and given mechanical ventilation because of RSV infection. Both cytokines were significantly increased in infected samples (Figure 1, E and F). Thus, IL-17 appears to be induced during RSV infection in severely ill infants and correspondingly in our animal model of RSV-induced disease.
Neutralization of IL-17 Regulates Mucus Production and Neutrophilic Inflammation and Increases Viral Load in the Lung
We next investigated the contribution of IL-17 to the increased pathogenesis in our mouse model of primary RSV infection. Mice were infected with RSV and treated intraperitoneally with anti-IL-17 or Cab every other day (Figure 2A). On day 8 after infection, we measured AHR and mucus production. Although we observed no change in AHR with anti-IL-17 antibody, compared with the Cab group (Figure 2B), neutralization of IL-17 significantly down-regulated the mucus-associated genes Muc5ac and Gob5 (Figure 2C). The decrease in the mucus gene expression was also reflected in the lung histology, with marked reduction of mucus and goblet cell staining in the large airways after neutralization of IL-17 (Figure 2D). Correspondingly, RSV infection of the IL-17−/− knockout mice (on a C57BL6 background) further confirmed our observations with IL-17 neutralization. Absence of IL-17 in the knockout mice, even in the relatively resistant C57BL6 background mice, significantly down-regulated mucus genes, a finding that was also reflected in the lung histology, with marked reduction in mucus staining (Figure 2, E and F).
Figure 2Neutralization of IL-17 inhibits mucus-associated genes and mucus accumulation in the lungs after RSV infection. A: Schematic representation of the primary RSV infection model. B: Airway resistance was measured at 8 days after RSV infection in mice treated with control or anti-IL-17 antibody (aIL17) after a single dose of methacholine. Data are represented as mean AHR in cmH2O/mL per second ± SE. Relative expression of mucus-associated genes in the lungs of mice treated with Cab versus anti-IL-17 antibody (C) or of WT versus IL-17−/− mice (E) was analyzed by real-time PCR. Data are reported as means ± SE. *P < 0.01. Lungs from Cab versus aIL17-treated mice (D) or WT versus IL-17−/− mice (F) were harvested and stained with PAS. Arrows indicate PAS positive staining. Original magnification, ×200. Each experiment was repeated three times, with five mice in each group.
prompted us to investigate the role of IL-17 in neutrophilic infiltration. Although we observed no significant differences in eosinophil, macrophage, or lymphocyte numbers, there was a significant decrease in the number of neutrophils in the bronchoalveolar lavage fluid from mice treated with anti-IL-17 antibody, compared with the Cab-treated mice (Figure 3A). The decrease in neutrophils in the anti-IL-17 antibody-treated animals correlated with the decreased expression of the neutrophil-associated chemokine CXCL1 (mouse GRO-α) in the lungs of RSV-infected mice (Figure 3B). Although the extent of the decrease was not great (5% to 2%), increased neutrophilic inflammation may be a component of enhanced pathological responses to virus infection associated with IL-17.
Figure 3Blockade of IL-17 inhibits neutrophilic infiltration in bronchoalveolar lavage fluid (BALF). A: Differential count of BALF cellularity for RSV-infected mice treated with anti-IL-17 antibody (aIL17) or Cab. B: Relative expression of chemokines in the lungs analyzed by real-time PCR from RSV-infected mice treated with anti-IL-17 antibody (aIL-17) or Cab. The experiment was repeated twice with five mice in each group. Data are reported as means ± SE. *P < 0.05.
An important aspect of infectious disease-associated pathology is the persistence (viral load) and/or clearance of virus. Therefore, we next investigated the kinetics of viral persistence after RSV infection in mice treated with Cab or anti-IL-17 by analyzing the changes in viral mRNA expression of the RSV nuclear protein (RSV-N). Blockade of IL-17 decreased RSV-N with time, with levels significantly lower on days 8 and 10 after infection, compared with Cab-treated mice (Figure 4A). The decrease in RSV-N expression in anti-IL-17 antibody-treated mice started as early as day 4 after infection, and to further confirm this finding we measured the viral load by assessing plaque-forming units from lungs of the virus RSV-infected mice. We found a marked decrease in viral titers in anti-IL-17 antibody-treated animals, compared with those given Cab (Figure 4B). Furthermore, given that we detected significantly lower levels of RSV-N on day 8 after infection, we next examined RSV-specific antigen levels in the lungs by ELISA on day 8 after infection, the time of peak IL-17 production. Anti-IL-17 antibody-treated animals demonstrated significantly lower levels of viral antigen in their lungs (used as one measure of viral clearance as described previously
) (Figure 4C). The results confirmed our RSV-N data and suggest that neutralization of IL-17 lowers viral load at 4 and 8 days after infection.
Figure 4RSV-induced IL-17 reduces viral clearance but does not alter Th2 cytokines. A: RSV nuclear protein N expression in the lungs was measured by real-time PCR on days 2, 4, 8, 10, and 12 after RSV infection and treatment with Cab or anti-IL-17 antibody (aIL-17). The relative expression of RSV-N in Cab and aIL-17 treated groups was normalized to RSV-infected mice at day 1. B: Viral plaque-forming units (PFU) per wet weight of lung were measured 4 days after RSV infection and after treatment with anti-IL-17 antibody or Cab. Data are reported as means ± SE. *P < 0.05. The experiment was repeated twice with similar results. C: RSV antigens were measured by ELISA. O.D., optical density. D: Relative expression of cytokines IL-4, IL-13, IFN-γ, and IL-17A was analyzed by real-time PCR in the lungs of RSV-infected mice treated with Cab or aIL-17 on day 8 after infection. Cytokine mRNA expression levels were normalized to those of naïve mice. E: Lung DLNs from the same mice were restimulated with RSV for 48 hours and the supernatants were analyzed for cytokine production by a Bio-Plex system. Cytokine responses to RSV in WT and IL-17−/− mice were compared in the lungs (F) and lymph nodes (G). The experiment was repeated twice, with four to five mice per group. Data are reported as means ± SE. *P < 0.05.
Although viral load is a critical determinant in virus-induced pathology, the inflammation observed during the disease progression is also associated with altered helper T cell-induced cytokine environment. To address this aspect, we examined the role of IL-17 in specifically altering the immune cell responses. BALB/c mice were treated with anti-IL-17 antibody or Cab (intraperitoneally) 2.5 hours before RSV infection. At 8 days after infection, we assessed the cytokine expression in whole lungs by quantitative PCR and rechallenged the DLNs ex vivo with RSV for an additional 48 hours before analyzing antigen-specific Th1 and Th2 cytokine production. Neutralization of IL-17 significantly reduced the expression of IL-17 mRNA in the lungs (Figure 4D), but did not alter Th1 or Th2 cytokine responses. Similarly, the levels of the CD4 effector cytokines remained unaltered in the DLNs (Figure 4E). In IL-17−/− mice, no significant alteration of the Th2 cytokines was observed in either the lungs or the lymph nodes, compared with wild-type mice (Figure 4, F and G). Altogether, the data suggest that IL-17 has a limited role in altering the production of other key cytokines during primary RSV infection.
IL-17 Alters CD8 T-Cell Response and Cytotoxic Function in the Lung during RSV Infection
Because our studies suggested an alteration in viral clearance and an increase in CD4 T cells producing IL-17, we next investigated the lymphocyte compartment. We analyzed total CD4 T cell and CD8 T cell, as well as activated CD4 T cell (CD4CD69) and CD8 T cell (CD8CD69) populations, in both DLNs and lungs. Although there was no significant change in the total and activated CD4 and CD8 T-cell population in the DLNs of mice treated with anti-IL-17, compared with the Cab treatment (Figure 5A), there was a significant difference in both CD4 and CD8 T -cell numbers in the lungs (Figure 5B). Given the increased viral load in the presence of IL-17, we next examined the expression of IL-17 receptor (IL-17RA) on CD8 T cells in the lungs of RSV-infected mice and found that RSV infection significantly increased IL-17RA expression on CD8 T cells (Figure 5C). To further investigate the role of IL-17 in RSV-specific responses of CD8 T cells in the lungs during viral infections, we assayed CD8 T cells expressing the RSV-specific immunodominant T-cell receptor using tetramer-specific staining in mice treated with Cab or neutralizing antibody to IL-17. There was a significant increase in immunodominant RSV-specific tetramer positive CD8 T cells after neutralization of IL-17, both as percentage of CD3+ cells and total cells in the lung, suggesting that IL-17 is critical in mounting a potent antiviral response (Figure 5, D and E).
Figure 5IL-17 inhibits CD8 T cell effector responses during RSV infection. Flow cytometry in RSV-infected mice treated with Cab or with anti-IL-17 antibody was performed at 8 days after infection using five dispersed lung DLNs (A) or whole-lung digests (B) The results from one experiment of three repeats are shown with total number of CD4 (*P < 0.04), CD4CD69 (*P < 0.02), CD8 (*P < 0.05), and CD8CD69 T cells (*P < 0.05). C: The number of CD8 T cells positive for IL-17 receptor A chain (IL-17RA) in naïve or RSV-infected mice. D: RSV-specific immunodominant T-cell receptor tetramer surface staining (left), expressed as a percentage of CD3+ cells in the lungs (right). E–I: Total tetramer-positive cells (E) IFN-γ (F), Eomes (G), T-bet (H), and granzyme B (I) CD8 T cells in the lungs of RSV-infected mice (*P < 0.05). J: Relative expression of T-bet, Eomes, and IFN-γ was analyzed by real-time PCR in magnetic-cell-sorting–purified CD8 T cells from the spleen of naïve BALB/c mice that were stimulated with anti-CD3 (2 μg/mL) and anti-CD28 (2 μg/mL) in the presence (+rIL-17) or absence (−rIL-17) of recombinant IL-17 for 6 hours in vitro. The data were normalized to GAPDH expression levels and the fold induction was compared with unstimulated CD8 T cell values. N.D. indicates not detected.
Functional activation of cytotoxic T lymphocytes (CTLs) can be assessed using a number of associated markers, including IFN-γ production and its related transcription factors T-bet and Eomes. In addition, granzyme B can be used as a marker for CTL activation. Our data indicate that there was a significant increase in the IFN-γ positive CD8 T cells (Figure 5F), as well as a significant increase of both Eomes (Figure 5G) and T-bet (Figure 5H) levels, in the lungs of mice treated with anti-IL-17 antibody, compared with Cab. Mice treated with anti-IL-17 antibody had significantly higher levels of granzyme B, compared with the Cab-treated mice (Figure 5I), indicating that, in addition to regulating the IFN-γ production by CD8 T cells, IL-17 also decreases cytotoxic T cell development during RSV infections, which in turn could inhibit proper viral clearance. To confirm that alteration of CD8 T cell activation is a direct effect of IL-17, we quantitatively measured the relative expression of T-bet, Eomes, and IFN-γ in isolated naïve splenic CD8 T cells that were stimulated in vitro with anti-CD3 and anti-CD28 antibodies in the presence or absence of recombinant IL-17. Not only did the presence of recombinant IL-17 inhibit the expression of both the T-bet and Eomes transcription factors, it also completely abolished the expression of IFN-γ in the in vitro cultures (Figure 5J).
IL-17 Increases Mucus Production, Alters CD8 T-Cell Activation, and Inhibits Viral Clearance during RSV-Exacerbated Allergic Lung Disease
Respiratory viral infections are known to exacerbate asthma. Given the results this far in the study, we were interested in whether RSV-induced IL-17 production was involved in exacerbation of our previously developed model of allergic asthma exacerbation.
In this model, BALB/c mice sensitized with CRA are given an RSV infection challenge before a final allergen challenge (Figure 6A). To determine whether RSV increased IL-17 production during allergen challenges in the lungs of mice, lung DLNs from allergen-sensitized mice were isolated and challenged with either allergen or RSV. IL-17 was not detected in cells from any of the three groups of sensitized animals after CRA restimulation (Figure 6B). In contrast, cells from RSV-infected mice restimulated with RSV produced significant amounts of IL-17 (Figure 6B). An even greater and significant increase in IL-17 expression was observed in RSV restimulated cells from mice that were originally sensitized with allergen and then infected with RSV (Figure 6B). These data indicate that RSV infection increases the amount of IL-17 induced in this allergic, Th2 immune environment.
Figure 6RSV exacerbation of allergic airway disease is mediated by IL-17 by altering mucus production, activating CD8 T cell effector functions, and decreasing viral clearance. A: Schematic representation of RSV exacerbation of existing CRA-induced allergic airway disease. B: Lung DLN cells were isolated from animals sensitized to CRA, RSV, or both CRA and RSV and then were restimulated with either CRA or RSV for 48 hours. The supernatants were analyzed for IL-17 by a Bio-Plex system. C: Lungs from CRA sensitized, RSV-infected mice treated with Cab or anti-IL-17 antibody with H&E or PAS staining. Original magnification, ×400. Insets: Higher-magnification view of boxed area. Original magnification, ×1000. D: Relative expression of mucus-associated genes (Muc5ac and Gob5) in the lungs was analyzed by real-time PCR. *P < 0.05, **P < 0.01. E: IFN-γ, IL-4, IL-5, and IL-13 production was analyzed by a Bio-Plex system. F: Flow cytometry was performed using whole-lung digests. Data are reported as total number of CD4, CD4CD69, CD8, and CD8CD69 T cells per lobe of the lung. *P < 0.05. G: RSV-specific immunodominant T-cell receptor tetramer surface staining and analysis by flow cytometry. H: Relative expression of RSV proteins (G, F, and N) in the lungs was analyzed by real-time PCR. Data are reported as means ± SE of five mice per group, representative of one of the two independent experiments. N.D. indicates not detected.
To better understand the contribution of IL-17 in this exacerbation model response, we again used IL-17 neutralization. On day 16, 2 hours before RSV infection, the mice were treated with control or neutralizing antibody to IL-17. The antibodies were administered again on days 18 and 20 and, after a final challenge of CRA on day 21, AHR was measured on day 22. We observed no difference in AHR between control mice and anti-IL-17 antibody-treated mice, indicating that IL-17 does not modulate AHR during exacerbated responses. However, neutralization of IL-17 significantly inhibited mucus production, compared with control mice, as observed from PAS-stained lung sections (Figure 6C). The decrease in mucus production was consistent with a significant decrease in Muc5ac and Gob5 gene expression in the lungs (Figure 6D).
Next, we wanted to determine whether IL-17 mediated regulation of effector CD4 T cell cytokine responses in the lymph nodes. RSV exacerbation of CRA-sensitized mice significantly increased IL-17 production (Figure 6B), and neutralization of IL-17 significantly reduced IL-13 levels, both in response to the individual allergen and to RSV restimulation (Figure 6E) in lymph node cell cultures. These data suggest that in a Th2-skewed environment the RSV-induced IL-17 augments Th2 cytokine production. When T cell accumulation was examined in the lungs of RSV exacerbated animals, a significant increase in CD8CD69 T cells was observed, with no change in CD4 T cell numbers (Figure 6F). Furthermore, there was a marked increase (more than threefold) in RSV antigen-specific CD8 T cells in the lungs of mice treated with neutralizing antibody for IL-17, compared with the Cab-treated mice (Figure 6G). Finally, in mice treated with the anti-IL-17 antibody, a beneficial antiviral response was observed, with a significant decrease in expression of RSV proteins (RSV-G, RSV-F, and RSV-N) (Figure 6H). Thus, these data confirm that IL-17 directly decreases the antigen-specific CD8 T cells in the lung, increases viral load, and causes mucus production, a combination of effects that suggest a pathogenic role of IL-17 during viral exacerbation of allergic asthma.
Discussion
Severe RSV infection in infants is characterized by mucus plugging of the airways, airway hyperreactivity, and inflammation.
less is known about the role of IL-17. In the present study, using a mouse model of RSV infection, we investigated the role of IL-17 during primary RSV infection. We found that IL-17 modulates three important underlying mechanisms: i) hypermucus production in the airways, ii) alteration of effector CD8 T cell responses, and iii) viral clearance. Each of these mechanisms may contribute to RSV-induced pulmonary pathology. In addition, IL-17 produced during RSV infection appears to participate in RSV-induced exacerbation of allergic lung disease, as previously suggested.
The relevance of these observations was supported by the detection of IL-17 in tracheal samples from severely ill infants infected by RSV.
The ability of IL-17 produced during RSV infection to regulate mucus production was identified by neutralization of IL-17. Given that IL-17 neutralization had no effect on AHR, it may be that other cytokines (especially Th2 cytokines) influence the airway function: whereas IL-17 alters the epithelial cell response that controls mucus production. This pathogenic effect of IL-17 on mucus production may be important in several diseases, including chronic obstructive pulmonary disease, cystic fibrosis, and asthma. IL-17 did not alter the CD4 effector responses and Th2 cytokine production profile during RSV infection, suggesting that IL-17 may be functioning primarily as an effector cytokine leading to increased mucus production.
Of note, when we assessed Th2 cytokine production in restimulated lymph node cells from RSV-exacerbated allergic mice, the neutralization of IL-17 significantly reduced IL-13 levels. Thus, the role of IL-17 in a Th2-skewed environment appears to enhance Th2 cytokine production. The latter observation is consistent with the reduced expression of Th2 cytokines observed in IL-17 knockout mice in the ovalbumin-alum-induced asthma model.
There are differences between IL-17 and IL-17F in regulating Th2 responses. Whereas IL-17 appears to promote Th2 responses, IL-17F has a regulatory role in restricting Th2 responses.
We speculate that the relative lower expression of IL-17F during RSV infection may contribute to the heightened IL-13 response during an allergic condition; however, further experimental evidence is required. More recently, the IL-13 receptor was found on Th17 cells and was shown to regulate IL-17 production.
The relationship of IL-17 and IL-13 in regulating mucus production during RSV exacerbated disease still remains to be established, but may demonstrate that the two cytokines play synergistic roles in mucus regulation. This would be particularly important during disease exacerbation in allergic asthma, in which virus-induced IL-17 in a Th2 immune environment may present the most severe disease phenotype.
IL-17 has also been associated with increased neutrophilic infiltration at the site of infection during host responses to bacterial and fungal infections through induction of CXC chemokines.
Requirement of interleukin 17 receptor signaling for lung CXC chemokine and granulocyte colony-stimulating factor expression, neutrophil recruitment, and host defense.
Current evidence indicates that viral infection of epithelial cells increases the production of neutrophil chemoattractants, which induce neutrophil migration into the inflammatory sites.
Consistent with our present findings, it was reported that IL-17 enhances human rhinovirus-16-induced epithelial production of IL-8 (human homolog of mouse KC).
Neutrophils augment epithelial cell damage by the release of granule enzymes during viral infection and thus contribute to the pathophysiology of viral disease.
Allergic disease may therefore present the most severe phenotype when intense neutrophil recruitment to the lung occurs, as has been observed in clinical disease.
CD8 T cell antiviral responses confer protective immunity during viral infections. Recently, it was reported that IL-17 impaired CD8 T cell antiviral responses during Theiler's murine encephalomyelitis virus (TMEV) infections,
Accordingly, an aspect of the regulation of CD8 T cell response in the present study was linked to CD4 T cell help, given that CD4 T cells were the primary source of IL-17 during RSV infection. Two T-box-containing transcription factors, T-bet and Eomes, regulate the CD8 cytotoxic T cell effector functions.
Of note, an aberrant Th17 response developed in studies with mice that were deficient in both Eomes and T-bet, suggesting that there may be an antagonistic relationship between the expression of these molecules.
We observed negative regulation of T-bet and Eomes by IL-17, indicating an inhibitory role for IL-17 in cytotoxic CD8 T cell responses. Furthermore, in the presence of IL-17 using in vitro cultures, expression levels of IFN-γ, Eomes, and T-bet in CD8 T cells were significantly decreased, suggesting that IL-17 directly regulates CTL development. Thus, the negative regulatory effects of IL-17 on cytotoxic CD8 T cells during both RSV infection and the exacerbated allergic response contribute to the viral associated pathology.
In the present study, we found that virus-induced IL-17 augments primary RSV infection, as well as existing airway disease, by increasing mucus production, inhibiting CD8 T cell activation, and reducing viral clearance. Conversely, removing IL-17 led to a reduction of the exacerbated disease, possibly in part by increasing viral clearance. Our observation of increased IL-17 in airway samples of severely ill RSV-infected infants and the role of IL-17 in mucus hypersecretion can be implicated in chronic lung conditions such as chronic obstructive pulmonary disease and asthma during exacerbation by RSV.
Monocyte chemoattractant protein-1 mediates cockroach allergen-induced bronchial hyperreactivity in normal but not CCR2-/- mice: the role of mast cells.
A chimeric A2 strain respiratory syncytial virus (RSV) with the fusion protein of RSV strain line 19 exhibits enhanced viral load, mucus, and airway dysfunction.
Requirement of interleukin 17 receptor signaling for lung CXC chemokine and granulocyte colony-stimulating factor expression, neutrophil recruitment, and host defense.
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