Deleterious Effect of the IL-23/IL-17A Axis and γδT Cells on Left Ventricular Remodeling After Myocardial Infarction

Background Left ventricular (LV) remodeling leads to chronic heart failure and is a main determinant of morbidity and mortality after myocardial infarction (MI). At the present time, therapeutic options to prevent LV remodeling are limited. Methods and Results We created a large MI by permanent ligation of the coronary artery and identified a potential link between the interleukin (IL)–23/IL-17A axis and γδT cells that affects late-stage LV remodeling after MI. Despite the finsinf that infarct size 24 hours after surgery was similar to that in wild-type mice, a deficiency in IL-23, IL-17A, or γδT cells improved survival after 7 days, limiting infarct expansion and fibrosis in noninfarcted myocardium and alleviating LV dilatation and systolic dysfunction on day 28 post-MI. M1 macrophages and neutrophils were the major cellular source of IL-23, whereas >90% of IL-17A-producing T cells in infarcted heart were CD4− TCRγδ+ (γδT) cells. Toll-like receptor signaling and IL-1β worked in concert with IL-23 to drive expansion and IL-17A production in cardiac γδT cells, whereas the sphingosine-1-phosphate receptor and CCL20/CCR6 signaling pathways mediated γδT cell recruitment into infarcted heart. IL-17A was not involved in the acute inflammatory response, but it functioned specifically in the late remodeling stages by promoting sustained infiltration of neutrophils and macrophages, stimulating macrophages to produce proinflammatory cytokines, aggravating cardiomyocyte death, and enhancing fibroblast proliferation and profibrotic gene expression. Conclusions The IL-23/IL-17A immune axis and γδT cells are potentially promising therapeutic targets after MI to prevent progression to end-stage dilated cardiomyopathy.

L eft ventricular (LV) remodeling leads to chronic heart failure and is a main determinant of morbidity and mortality after myocardial infarction (MI). 1 Myocardial infarct volume and subsequent adverse LV remodeling (dilation and fibrosis) determine the degree of LV dysfunction 2 ; however, current therapeutic options to prevent LV remodeling are limited. Timely revascularization of ischemic myocardium followed by standard therapy with renin-angiotensin-aldosterone inhibitors and b-blockers can alleviate post-MI remodel-ing. [2][3][4][5] However, the last decade has witnessed a paradoxical increase in the incidence of heart failure within 30 days post-MI. 6 More therefore needs to be done to prevent progressive LV dysfunction during hospitalization following MI.
Activation of the immune system is critically involved in adverse LV remodeling after MI. [7][8][9][10] The immune response after tissue damage is primarily responsible for wound healing, although this response can also exacerbate tissue injury if the inflammatory cascades are inappropriately or excessively activated. [9][10][11] Attempts at introducing nonselective anti-inflammatory glucocorticoids for the treatment of acute MI produced conflicting results. 12 There is a concern that impairment of the wound healing process by glucocorticoids may facilitate wall thinning and ventricular rupture. 12 To develop successful immunomodulatory therapies, we need to delineate a distinct subset of cells and cytokines that have only minor effects on the acute wound healing process but that are strongly involved in the smoldering inflammation responsible for adverse LV geometric and functional remodeling.

Methods
Mice IL-17A-knockout (KO) 24 and IL-23p19-KO (IL-23-KO) 20 mice were described previously. TCRcd-KO mice were purchased from Jackson Laboratories. The TLR2-KO, TLR4-KO, and TLR2/4 double-knockout mice were a generous gift from Dr Shizuo Akira (Osaka University) and described previously. 25 All mice were bred on the C57BL/6 background, and 10-to 16-week-old male mice were used in this study. All animal experiments were reviewed and approved by the Institutional Animal Care and Use Committee at the Keio University School of Medicine.

Induction of MI and Infarct Size Evaluation
Mice were subjected to a permanent (MI) ligation of the left anterior descending artery or to a sham operation without ligation as described previously. 26 In brief, mice were lightly anesthetized with diethyl ether, intubated, and then fully anesthetized with 1.0% to 1.5% isoflurane gas while being mechanically ventilated with a rodent respirator. The chest cavity was opened via left thoracotomy to expose the heart such that the left anterior descending coronary could be visualized by microscopy and permanently ligated with a 7-0 silk suture at the site of its emergence from the left atrium. Complete occlusion of the vessel was confirmed by the presence of myocardial blanching in the perfusion bed. Mice that died during recovery from anesthesia were excluded from the analysis. Sham-operated animals underwent the same procedure without coronary artery ligation. In the functional experiments, the sham operation was done and waited for 28 days; however, in the immune cells infiltration experiments and qPCR experiments, the sham operation was done and sacrifice was on day 2 or day 7. In addition, we separately verified that the numbers of macrophages, T cells, and neutrophils in the heart remained constant on days 1, 4, 7, and 14 after the sham operation. To evaluate the infarct size on day 1 after MI, hearts were weighed and frozen at À80°C. The frozen hearts were cut transversely into 1-mm-thick slices using a Mouse Heart Slicer Matrix and stained with 2% triphenyltetrazolium chloride (TTC) in PBS (pH 7.4) for 20 minutes in a 37°C water bath. After fixation for 4 to 6 hours in 10% neutral buffered formaldehyde, both sides of each slice were photographed. Viable myocardium stained brick red, and infarct tissues appeared pale white. Infarct and LV area were measured by automated planimetry using Image J software (version 1.43u, National Institutes of Health), with the infarct size expressed as a percentage of the total LV area.

Cell Preparation for Flow Cytometry
At each time, mice were deeply anesthetized and intracardially perfused with 40 mL of ice-cold PBS to exclude blood cells. The heart was dissected, minced with fine scissors, and then enzymatically digested with a cocktail of type II collagenase (Worthington Biochemical Corporation, Lakewood, NJ), elastase (Worthington Biochemical Corporation), and DNaseI (Sigma, St. Louis, MO) for 1.5 hours at 37°C with gentle agitation. After digestion, the tissue was triturated and passed through a 70-lm cell strainer. Leukocyte-enriched fractions were isolated by a 37% to 70% Percoll (GE Healthcare) density gradient centrifugation as described elsewhere. 27 Cells were removed from the interface and washed with RPMI-1640 cell culture medium for further analysis. Spleens were removed, homogenized, and then passed through a 70-lm nylon mesh in PBS. After the addition of red blood cell lysis buffer (eBioscience) to exclude erythrocytes, the single-cell suspension in PBS was refiltered through a 70-lm nylon mesh to remove connective tissue.

Intracellular Cytokine Staining
For surface and intracellular cytokine staining, single cells prepared from spleen and heart were restimulated for 4.5 hours with 50 ng/mL phorbol 12-myristate 13-acetate (PMA; Sigma-Aldrich) and 1 lg/mL ionomycin (Sigma-Aldrich) in the presence of Golgistop (Cytofix/Cytoperm Plus Kit with Golgistop, BD Biosciences). Surface staining was performed for 20 minutes with the corresponding mixture of fluorescently labeled antibodies. After fixation and permeabilization, the cells were incubated for 30 minutes at 4°C with anti-IL-17A-APC and anti-IFN-c-APC (eBioscience).

In Vitro Cardiac Cell Stimulation
Mouse heart cells were prepared from infarcted heart, and CD45 MicroBeads were used to enrich for heart-derived leukocytes. Cells were stimulated with 10 ng/mL rmIL-23, 10 ng/mL rmIL-1b (R&D Systems), 100 ng/mL LPS (Sigma-Aldrich), and 1 lg/mL Pam3CSK4 (InvivoGen) for 3 days in the presence or absence of 3 lg/mL IL-1RI (IL-1 receptor I) antibody. The supernatants were harvested and assayed for IL-17A levels by ELISA (R&D Systems). After 3 days stimulation, cells were also restimulated for 4.5 hours with PMA and ionomycin in the presence of Golgistop for intracellular IL-17A staining. For the cdT cell proliferation assay, cardiac cells were stained with 5(6)-carboxyfluorescein diacetate N-succinimidyl ester (CFSE; Sigma-Aldrich) according to the manufacturer's recommendations. Briefly, cells were stained in 10 lmol/L CFSE in PBS at 37°C for 5 minutes and then washed 3 times with cold PBS. Labeled cells were stimulated for 3 days, and then stained for surface markers CD3 and TCRcd prior to flow cytometry.

Isolation of Neonatal and Adult Cardiomyocytes and Nonmyocytes
Neonatal ventricles from 1-day-old C57BL6/J mice were minced and digested with collagenase type II (Worthington) solution. 28 To enrich for cardiomyocytes, the cells were preplated for 2 hours to remove nonmyocytes. The unattached viable cells, which were rich in cardiomyocytes, were plated on gelatin-coated plastic dishes and treated with Ara C (Sigma) to inhibit nonmyocyte proliferation. Using this protocol, we consistently obtained cell populations containing ≥90% to 95% cardiomyocytes. Nonmyocyte cells that attached to the dishes were cultured in DMEM supplemented with 10% FBS and allowed to grow to confluence; these were then trypsinized and passaged at 1 in 4. This procedure yielded cell cultures that were almost exclusively fibroblasts by the first passage. Experiments were carried out after 2 passages.

Quantitative Real-Time PCR
Total RNA samples from sorted cells, cultured cells, and heart tissue were prepared using using an RNeasy Mini Kit (Qiagen) or Trizol reagent (Invitrogen), according to the manufacturer's instructions. A First-strand cDNA synthesis kit (Invitrogen) was used for cDNA synthesis. Quantitative real-time PCR was performed using the ABI Prism 7700 sequence detection system (Applied Biosystems). Predesigned gene-specific primer and probe sets (Taqman Gene Expression Assays, Applied Biosystems) were used. The 18S ribosomal RNA was amplified as an internal control.

Measurement of Cytokines by ELISA
LVs were homogenized in PBS containing protease inhibitors (Sigma-Aldrich). Supernatants were collected after centrifugation and stored at À80°C. The concentrations of IL-23, IL-1b, and IL-17A in LV lysates and culture supernatants were measured by Quantikine ELISA kits (R&D Systems).

Gelatin Zymography
To evaluate the activity of gelatinase, matrix metalloproteinase 9 (MMP9), and MMP2, gelatin zymography was performed. Equal volumes containing 35 lg of protein were loaded into each lane of 10% gelatin zymogram gels (Novex, Invitrogen). After running at 125 V for 90 minutes, the gels were incubated in zymogram renaturing buffer for 30 minutes at room temperature with gentle agitation, equilibrated with zymogram developing buffer for 30 minutes, and then further incubated in zymogram developing buffer at 37°C overnight with gentle agitation. After washing with deionized water, the gels were stained with Coomassie blue for 90 minutes followed by destaining with deionized water. The presence of different MMPs was identified on the basis of their molecular weight. The gels were photographed using MF-ChemiBIS (DNR Bio-Imaging Systems) and analyzed using Image J software (version 1.43u, National Institutes of Health).

Low Serum Hypoxia and Reoxygenation
An anaerobic jar containing an Anaero Pack (Mitsubishi Gas Chemical) was used to expose the cells to hypoxic stress. 29 Cultured cardiomyocytes were serum starved in DMEM with 0.5% FBS, and then exposed to hypoxic stress and/or IL-17A stimulation (421-ML, R&D systems). Neutralizing anti-IL-17A antibody (AF-421-NA) was administered 2 hours before hypoxic stress. After 12 hours of exposure to hypoxia, the medium was replaced with 10% FBS-containing DMEM (reoxygenation medium). Cell viability was determined by a LIVE/DEAD Viability/Cytotoxicity Assay Kit (Invitrogen) on the basis of the simultaneous determination of live and dead cells with the calcein AM and ethidium homodimer-1 probes, which are specific for intracellular esterase activity and membrane integrity, respectively. The cells were imaged with a fluorescence microscope (BZ-9000; Keyence): live cells were labeled green, whereas nuclei of dead cells were labeled red.

CCK-8 Assay
Mouse fibroblasts were cultivated in DMEM supplemented with 10% fetal bovine serum (FBS), penicillin, and streptomycin in a cell incubator with 5% CO 2 at 37°C. Cell proliferation was analyzed by the CCK-8 assay (Dojindo Molecular Technologies, Japan) as directed by the manufacturer. Fibroblasts were seeded at 4000 cells per well in 96-well culture plates and cultured for 72 hours for cell proliferation analysis.

Morphometric Analysis
Heart tissue was fixed in formalin, embedded in paraffin, and cut into 5-lm-thick sections. Hematoxylin and eosin (H&E) and Azan staining were performed on paraffin-embedded sections to determine the morphological effects, infarct size, and extent of cardiac fibrosis. The infarct size was calculated as total infarct circumference divided by total LV circumfer-ence9100, as described previously. 26 In addition, for each Azan-stained section, 20 microscopic fields (9400 magnification) were randomly chosen (BZ-9000; Keyence, Osaka, Japan), and the area of myocardial fibrosis in noninfarct and infarct areas was measured and analyzed using analysis software (BZ image analyzer II; Keyence).

Echocardiography
Transthoracic echocardiography was performed with a Vevo 2100 instrument (VisualSonics) equipped with an MS-400 imaging transducer. Mice were kept awake without anesthesia during the echocardiographic examination to minimize data deviation, and heart rate was maintained at %550 to 650 bpm in all mice. M-mode tracings were recorded through the anterior and posterior LV walls at the papillary muscle level to measure LV end-diastolic dimension (LVEDD) and LV end-systolic dimension (LVESD). LV fractional shortening (FS) was calculated according to the following formula: LV FS=([LVEDDÀLVESD]/LVEDD)9100.

Hemodynamics
Cardiac catheterization studies were performed using a 1.4 French microtip catheter (SPR-671, Millar Instruments, Houston, TX) under sedation using 1.5% isoflurane inhalation with spontaneous respiration. LV end-systolic pressure (LVESP), maximum rate of isovolumic pressure development, and minimum rate of isovolumic pressure decay were measured using analysis software (PowerLab, AD Instruments). Ten sequential beats were averaged for each measurement.

Immunohistochemistry
For immunostaining of cdT lymphocytes in the murine ischemic heart tissue, we used frozen sections as described elsewhere. 27 Cryostat sections (6 lm) were air-dried and then fixed in cold acetone at room temperature for 10 minutes. Endogenous peroxidase activity was blocked with 0.3% hydrogen peroxide (Sigma-Aldrich) in PBS for 20 minutes. Blocking with normal goat serum was applied for 1 hour at room temperature. Sections were washed with PBS and then incubated overnight at 4°C with hamster-anti-mouse TCRcd antibody (5 lg/mL; clone GL-3, BD Biosciences/Pharmingen). After 3 washes with PBS, the sections were incubated with HRP-conjugated secondary antibody (mouse anti-hamster IgG-HRP, Santa Cruz Biotechnology) at room temperature for 4 hours in the dark. After washing with PBS, staining was developed with 3, 3′-diaminobenzidine tetrahydrochloride (Histofine DAB Substrate Kit, Nichirei). To visualize the cardiomyocytes, a sequential section was stained with antia-actinin antibody (Sigma-Aldrich).
(Sigma-Aldrich) was used to identify cardiomyocytes. TUNELpositive nuclei were counted, and the data were normalized per total nuclei identified by DAPI staining (Invitrogen) in the same sections.

FTY720 Administration
FTY720 was dissolved at 5 mg/mL in DMSO, and then diluted with distilled water. FTY720 (1 mg/kg body weight) was administered intravenously from day 0 to day 5 post-MI, and leukocyte infiltration into the heart was analyzed by flow cytometry on day 6 post-MI.

Statistics
Values are presented as mean±SEM. Comparisons between groups were made using a Mann-Whitney U test, whereas data among multiple groups were compared using either the Kruskall-Wallis test with Dunn's multiple comparisons test or 2-way ANOVA followed by Tukey's post hoc analysis, as appropriate. Survival distributions were estimated by the Kaplan-Meier method and compared by the log-rank test. A value of P<0.05 was considered statistically significant. Statistical analysis was performed with GraphPad Prism 5.0 (Graph Pad Prism Software Inc, San Diego, CA) and SPSS 15.0 for Windows (SPSS, Inc, Chicago, IL).

IL-23 and IL-17A Adversely Affected Post-MI Cardiac Remodeling
We first investigated the dynamics of IL-23 and IL-17A in post-MI heart. Expression of IL-23p19 subunit mRNA increased rapidly in infarcted heart on day 1 post-MI, returning to nearbaseline levels by 3 days post-MI ( Figure 1A). In contrast, expression of IL-23 receptor (IL-23R), IL-12/IL-23p40 subunit, RORct, and IL-17A mRNA increased more gradually, peaking 7 days post-MI. Expression of IL-17 receptor A (IL-17RA) mRNA increased on day 1 post-MI and remained elevated until 14 days post-MI. ELISA analysis showed that the levels of IL-23 protein in post-MI heart increased rapidly to a peak at 24 hours, and then remained substantially elevated until at least day 4 post-MI ( Figure 1B).
We next investigated whether IL-23 acts as an upstream regulator of IL-17A in infarcted heart. The leukocyte-enriched fraction was collected from sham-operated and infarcted hearts on day 6 post-MI, and cells were cultured with either vehicle or IL-23 (10 ng/mL) for 24 hours ( Figure 2).   Severe anterior MI induced by proximal left coronary artery ligation in mice leads to high mortality because of severe LV dysfunction. Some of mice die from cardiac rupture. The protective effect of IL-23 and IL-17A deficiency on survival became obvious after 7 days. The survival rate on day 28 post-MI was 34.7% (25/72) in WT mice, 65.5% (19/29) in IL-17A-KO mice, and 62.5% (15/24) in IL-23-KO mice ( Figure 4A).
Intracellular cytokine staining revealed that 90.1±1.2% of IL-17A-expressing cells in the infarcted hearts on day 7 post-MI were CD3 + T lymphocytes ( Figure 5B). IL-23-KO mice had a significantly smaller proportion of IL-17A-producing cells among CD3 + T lymphocytes compared with WT mice (13.6±0.4% versus 2.2±0.2%, n=4, P<0.001), whereas the proportion of IFN-c-producing cells among CD3 + T lymphocytes was not affected in IL-17A-KO or IL-23-KO mice ( Figure 5C). More than 90% of IL-17A-producing T cells in this study were CD4 À TCRcd + (cdT) cells, but not CD4 + T cells (Th17) ( Figure 5D and 5F). By contrast, both CD4 À TCRcd + cells (65.2±2.6%) and CD4 + T cells (29.6±1.7%) were predominant cellular sources of IL-17A in the spleen ( Figure 5E and 5G). After MI, the number of IL-17A-expressing cells gradually increased to a peak 7 days post-MI and then remained high up to 14 days post-MI ( Figure 5H). The cdT cells started to increase on day 1 and peaked on day 7 after MI ( Figure 5I). Immunohistochemical examination revealed that cdT cells were mainly in the infarct and border areas, but not in the noninfarct area ( Figure 5J and 5K). In addition, TCRcd + cells were not major sources of IFN-c in the infarcted heart, but showed a relatively higher contribution of IFN-c production in spleen (5% in heart versus 10% in spleen), thus splenic cdT cells produced both IL-17A and IFN-c ( Figure 6A through 6E).

IL-23 Enhanced cdT Cell Recruitment, Whereas IL-17A Promoted Neutrophil Infiltration Into Post-MI Heart
We analyzed the impact of deficiency in IL-23 or IL-17A on the cellular infiltrate in the infarcted hearts on day 7 post-MI. The total number of infiltrating CD45 + immune cells was not altered in IL-23-KO and IL-17A-KO mice compared with WT mice ( Figure 7A). However, when we looked at leukocyte subsets, the number of infiltrating neutrophils was significantly reduced in both IL-23-KO and IL-17A-KO mice compared with WT mice. The number of infiltrating macrophages, DCs, NK cells, and NKT cells was not altered in IL-23-KO and IL-17A-KO mice compared with those in WT mice. Unexpectedly, the number of infiltrating CD4 + and CD8 + cells in the infarcted heart was higher in IL-23-KO and IL-17A-KO than in WT mice. Notably, the number of infiltrating cdT cells was markedly reduced in IL-23-KO mice but was not altered in IL-17A-KO mice compared with wild types (Figure 7A and 7B).    Figure 9B). LVESP and maximum and minimum dP/dt were higher, whereas the heart weight/body weight ratio was lower in TCRcd-KO mice compared with WT mice (Table 1). Azan staining revealed a reduced infarct size (infarct circumference/LV circumference) in TCRcd-KO hearts compared with WT mice (46.3±2.3% versus 36.7±2.8%, n=10 to 16) ( Figure 9C and 9D), and the area of myocardial fibrosis in noninfarcted heart was significantly smaller in TCRcd-KO compared with WT mice (0.71±0.21%, n=9, versus 1.72±0.27%, n=10) ( Figure 9E and 9F).
The TCRcd-KO mice showed markedly lower numbers of infiltrating IL-17A-expressing T cells in infarcted hearts on day 7 post-MI compared with WT mice (12.1±0.3% versus 2.5±0.6%, n=4) ( Figure 9G and 9I). In contrast, the number of infiltrating cdT cells in IL-17A-KO mice was comparable to that in WT mice in day 7 post-MI heart ( Figure 9H and 9J), but there were no IL-17A-expressing cells. Furthermore, TCRcd-KO mice had a markedly decreased number of CD45 + -leukocytes including macrophages, T cells, and neutrophils than WT mice ( Figure 10A). MMP1, MMP3, MMP9, collagen 1, periostin, TGF-b, and CCL2 were markedly attenuated on day 7 in TCRcd-KO mice compared with WT mice (Figure 10B), whereas MMP1, MMP3, MMP9, TNF-a, IL-1b, IL-6, and CCL2 expression was   comparable between the groups on day 2 post-MI (Figure 10C). Expression of IFN-c mRNA on day 7 post-MI did not differ between TCRcd-KO mice and WT mice ( Figure 10D).

Sphingosine-1-Phosphate Receptor and CCL20/ CCR6 Signaling Pathways Mediated cdT Cell Recruitment to Infarcted Heart
The immunomodulatory drug FTY720 interferes with sphingosine-1-phosphate (S1P) receptor signaling, leading to sequestration of lymphocytes in lymph nodes but not spleen. 30 Infusion of FTY720 significantly suppressed the number of infiltrating T cells, and particularly cdT cells, in the postinfarct heart ( Figure 11A through 11C). These findings indicated that cardiac cdT cells migrated from lymph nodes, at least in part via S1P receptor signaling.
Chemokine receptor 6 (CCR6) contributes to the migration of Th17 cells to a particular site of inflammation. 31 We found that almost all cardiac cdT cells constitutively expressed CCR6, whereas only 27% of spleen cdT cells expressed CCR6 ( Figure 11D). Expression of the CCR6 ligand CCL20 was significantly increased on day 1 post-MI, before peaking on day 3 post-MI, and then remaining high to day 14 post-MI ( Figure 11E). These findings suggested an important role for the CCL20-CCR6 signaling axis in recruiting cdT cells into infarcted heart.
Toll-Like Receptor (TLR) Signaling and IL-1b Worked in Concert With IL-23 to Drive IL-17A Production by Cardiac cdT Cells Engagement of these receptors with their respective ligands could stimulate NF-jB and IL-23 production. TLR1 and TLR2 were also expressed in cdT cells. 32 To examine whether TLR2 and TLR4 operate upstream of IL-17A production in the infarcted heart, we analyzed the number of IL-17A-expressing T cells in day 7 post-MI heart from WT, TLR2-KO, TLR4-KO, or TLR2/4-DKO mice using flow cytometry. TLR2-KO, TLR4-KO, and TLR2/4-DKO mice had a significantly smaller proportion of IL-17A-producing cells among CD3 + T lymphocytes than did WT mice (15.9±1.4% versus 6.2±0.9%, 5.2±0.5%, and 4.8±1.0%, respectively, n=4 to 7; P<0.001) (Figure 12A and 12B). IL-1b mRNA and protein levels in the heart were also dramatically increased as early as 24 hours after MI and remained significantly elevated above baseline levels thereafter ( Figure 12C and 12D). Therefore, we examined the synergistic effect of TLRs ligands and IL-1b acting with IL-23 on the proliferation of and IL-17A production from cdT cells. Cardiac cells prepared from day 7 post-MI hearts were stimulated using lipopolysaccharide (LPS; TLR4 ligand), Pam3CSK4 (TLR1/2 ligand), and IL-1b, each alone and in combination with IL-23. A synergistic effect on IL-17A production was observed when cells were exposed to the combination of IL-23 and LPS or of Pam3CSK4 and IL-1b Figure 11. Sphingosine 1-phosphate (S1P) signaling and the CCL20/CCR6 axis mediate cdT cell recruitment into the heart. A through C, FTY720 or H 2 O was administered before MI and every 24 hours thereafter. A, Representative figures revealed that proportions of the lymphocyte (CD45 + CD11b À ) and cdT-cell (CD45 + CD11b À TCRcd + ) infiltration into the heart were suppressed by FTY720 on day 6 after MI. B and C, Absolute number of heart-infiltrating inflammatory cells (B) and percentage of cdT cells among total T cells (C) were quantified on day 6 after MI (n=6 for H 2 O group and n=4 for FTY720 group).  ( Figure 12E and 12F). The synergistic effect of TLR ligands and IL-1b on IL-17A production was further examined using CD45 + cells sorted from day 7 post-MI hearts. LPS, Pam3CSK4, or IL-1b alone could not stimulate IL-17A production; however, IL-23 alone was sufficient to drive IL-17A production. A synergistic effect on IL-17A production was observed when CD45 + cells were exposed to the combination of IL-23 and IL-1b or of IL-23 and Pam3CSK4, but not to the combination of IL-23 and LPS ( Figure 12G). Similarly, IL-17A production from cardiac cells or CD45 + cells was significantly suppressed when cells were pretreated with IL-1 receptor I antibody (IL-1RI) ( Figure 12F and 12G), suggesting that the IL-1R signaling pathway is essential for IL-17A production in heart.
Examination of the proliferation-promoting effect of LPS, Pam3CSK4, and IL-1b on cdT cells by the CFSE dilution assay showed that the combination of IL-23 and IL-1b had the most pronounced effect on cardiac cdT cell proliferation ( Figure 12H and 12I).

IL-17A Had Proapoptotic, Profibrotic, or Proinflammatory Properties
Finally, we investigated the mechanisms behind the pathogenic role of IL-17A in post-MI cardiac remodeling. To identify the important cellular targets for IL-17A in vivo, we separated macrophages, lymphocytes, fibroblasts, endothelial cells, and cardiomyocytes from the infarcted hearts on day 7 post-MI. IL-17RA was highly expressed in cardiomyocytes, fibroblasts, and macrophages ( Figure 13A and 13B).
To examine the impact of IL-17A on cardiomyocyte survival, we performed TUNEL staining combined with a-actinin staining on the infarcted hearts of WT and IL-17A-KO mice on both day 2 and day 7 post-MI. Ablation of IL-17A had no effect on the number of TUNEL-positive cardiomyocytes in the border regions on day 2 (9.0±2.1% and 6.8±1.2%, respectively, n=6). Although there were markedly fewer TUNEL-positive cardiomyocytes on day 7 post-MI than on day 2 in WT mice, ablation of IL-17A further decreased the number of survivors (WT, 1.2±0.2%, versus IL-17A-KO, 0.5±0.1%, n=6) ( Figure 13C). Consistent with previous reports, 22,23 IL-17A significantly augmented low serum/ hypoxia-induced cell death in cultured murine cardiomyocytes, and this cytotoxic effect of IL-17A could be completely rescued by neutralizing anti-IL-17A antibody ( Figure 14A).
We also examined the gene expression profiles in macrophages isolated from the infarcted hearts on day 7 post-MI ( Figure 13E). Expression of M 1 macrophage signature genes such as TNF-a, IL-6, IL-1b, CCL2, and CXCL1 in the sorted macrophages were dramatically suppressed in the IL-17A-KO mice compared with WT mice, while those of M 2 signature gene YM-1 was not altered. The mRNA levels of MMP1, MMP9, and TGF-b in the sorted macrophages were also lower in the IL-17A-KO mice than in WT cells. In the cultured monocyte/macrophage cell line RAW264.7, IL-17A significantly increased mRNA expression of M 1 macrophage signature genes, such as TNF-a, IL-6, IL-1b, CCL2, MMP9, and CXCL1, but did not alter those of the M 2 signature gene arginase-1 ( Figure 14D), and these gene inductions in RAW264.7 cells were blocked by neutralizing anti-IL-17A antibody. The above results strongly suggested that IL-17A has proapoptotic, profibrotic, and proinflammatory properties that combine to promote LV remodeling.

Discussion
Two recent articles demonstrated that IL-17A is involved in the early cardiomyocyte death evident in ischemia-reperfusion injury, with functional involvement demonstrated 2 to 3 hours after reperfusion. 22,23 Here, we created a substantially sized MI by permanent ligation of the coronary artery, and identified a functional link between the IL-23/IL-17A signaling axis and cdT cells in late-stage LV remodeling after MI. Despite the finding that infarct size 24 hours after surgery was comparable among all mice including WT, ablation of IL-23, IL-17A, or cdT cells improved survival after 7 days, limited infarct expansion, and reduced fibrosis in the noninfarcted myocardium, alleviating LV dilatation and systolic dysfunction on day 28 post-MI. IL-17A was not involved in the acute inflammatory response on day 2 post-MI.
The number of IL-17A-expressing cells gradually increased after MI to a peak 7 days post-MI and then remained high to 14 days post-MI. This trend was consistent with the time course change in the numbers of infiltrating cdT cells in infarcted heart. The cdT cells constituted approximately 90% of IL-17A-producing cells in the heart on day 7 post-MI. Furthermore, the infiltrating IL-17A-expressing T cells on day  7 post-MI were almost completely ablated in TCRcd-KO mice. These results implicate the functional significance of cardiac cdT cells as a source of IL-17A after MI. Defining cdT cells as the major producer of IL-17A in infarcted heart coupled with the decreased number of neutrophils, macrophages, and T cells in the infarcted hearts of TCRcd-KO mice, but not IL-17A-KO mice, on day 7 post-MI suggested that factors other than IL-17A also mediated the cdT cell-stimulated adverse cardiac remodeling.
We further provided mechanistic insight about how signals converge on the cardiac cdT cells to produce IL-17A. Dangerassociated molecular patterns (DAMPs) such as mitochondrial DNA, adenosine triphosphate (ATP), high-mobility group box-1 (HMGB1), heat-shock protein (HSP), peroxiredoxins, and F-actin are detected via cell-surface TLRs or DNGR-1 and promote nuclear factor (NF)-jB nuclear translocation. 25,33-37 NF-jB mediates expression of cytokines including IL-23 and the proform of IL-1b, and IL-23 is an indispensable upstream regulator of IL-17A production in cdT cells. 32 DAMPs released from injured myocardium could also drive the formation of inflammasomes in cardiomyocytes, fibroblasts, and macrophages, leading to the processing of caspase-1 into its enzymatically active form. This active caspase-1 cleaves pro-IL-1b to release active IL-1b, which can then work in concert with IL-23 to drive IL-17A production in cdT cells. 14,15,34,38 The cdT cells themselves can directly respond to TLR stimulation in synergy with IL-23. Consistent with our findings, previous studies have revealed that TLR2 or TLR4 deficiency and antibodies targeting IL-1 signaling alleviated cardiac remodeling. [39][40][41][42] Notably, treatment of rats even 24 hours after MI with an IL-1 receptor blocker alleviated LV remodeling without affecting early infarct size. 39 These findings suggested that the cardioprotective effect of an IL-1 receptor blocker is at least in part mediated by inhibiting the IL-23/IL-17 axis in post-MI heart. In terms of cell migration, FTY720-sensitive S1P receptor signaling helps to mediate cdT cell egress from lymph nodes, 30,43 whereas CCR6-expressing cdT cells are recruited to the sites of injury that express CCL20 ( Figure 15).
Notably, resident cardiac fibroblasts were the major target of IL-17A in stimulating the expression of CXCL1 and CCL2, and cardiac neutrophils constitutively expressed the CXCL1 receptor CXCR2 ( Figure 13F). IL-17A is known to promote the expression of CXCL1 by inducing gene transcription and posttranscriptional stabilization of mRNA in fibroblasts. 44,45 Neutrophils accumulating Figure 15. Mechanism of signal convergence on cardiac cdT cells for recruitment into infarcted heart and production of IL-17A. Damaged cells release DAMPs, which are recognized by TLRs. The subsequent TLR signaling promotes nuclear translocation of NF-jB, leading to expression of cytokines including pro-IL-1b and IL-23, which are indispensable for IL-17A production in cdT cells. Inflammasome activation leads to processing of caspase-1 into its enzymatically active form. Caspase-1 in turn cleaves pro-IL-1b, releasing active IL-1b, which can work in concert with IL-23 to drive IL-17A production in cdT cells, which also directly respond to TLR stimulation in synergy with IL-23. FTY720-sensitive S1P receptor signaling helps to mediate cdT-cell egress from lymph nodes, and CCR6-expressing cdT cells are recruited to the sites of injury that express CCL20. DAMPs indicates Danger associated molecular patterns; TLR, toll-like receptor. at injured sites might therefore release proteolytic enzymes or reactive oxygen species to damage surrounding myocytes, which play a crucial role in neutrophil-mediated cardiac injury. 11,46 Furthermore, IL-17A enhances M1 macrophage polarization and directly activates macrophages to express TNF-a, IL-6, IL-1b, and MMP-9, further exacerbating myocardial damage. [47][48][49][50] Early scar formation at the site of the infarct is important to prevent cardiac rupture, whereas subsequent interstitial fibrosis of adjacent myocardium and increased MMP production associated with a sustained inflammatory response contributes to adverse remodeling later on. 51 We found that IL-17A plays a crucial role in the expression of MMPs and fibrosis-related genes in infarcted heart only in the late stage of myocardial injury, not earlier on. Consistent with this, we did not see an increased prevalence of cardiac death from cardiac rupture in mice lacking IL-23, IL-17A, or TCRcd, despite our relatively small sample sizes. In addition, survival benefit in mice lacking IL-23, IL-17A, or TCRcd became apparent 7 days after MI.
We could not detect cdT cells in the post-MI tissues of patients because no antibody against human TCRcd was available. However, we localized some IL-17A-expressing T cells in the border and infarct areas of heart sampled from patients after MI (data not shown). Thus, it seems highly likely that infiltrating IL-17A-producing T lymphocytes have a substantial role in post-MI cardiac remodeling in patients.
IL-17A had no effect on the early post-MI inflammatory process, which is part of the healing process. Instead, it seemed to act at a later stage (%7 days) as effector cytokineinducing cardiac remodeling and leading to end-stage dilated cardiomyopathy. This finding supports a recent study showing an essential such role for IL-17A in remodeling, but not in the development of autoimmune myocarditis. 19 This study, together with ours, indicated that the late remodeling-specific effect of IL-17A is a potential therapeutic target for preventing the development of chronic heart failure from postmyocarditis and post-MI cardiac remodeling. To translate these interesting but preliminary findings into the clinic, further human studies with larger sample sizes are needed.