Abstract
Acinetobacter baumannii is known for its multidrug antibiotic resistance. New approaches to treating drug-resistant bacterial infections are urgently required. Cathelicidin-related antimicrobial peptide (CRAMP) is a murine antimicrobial peptide that exerts diverse immune functions, including both direct bacterial cell killing and immunomodulatory effects. In this study, we sought to identify the role of CRAMP in the host immune response to multidrug-resistant Acinetobacter baumannii. Wild-type (WT) and CRAMP knockout mice were infected intranasally with the bacteria. CRAMP−/− mice exhibited increased bacterial colony-forming units (CFUs) in bronchoalveolar lavage (BAL) fluid after A. baumannii infection compared to WT mice. The loss of CRAMP expression resulted in a significant decrease in the recruitment of immune cells, primarily neutrophils. The levels of IL-6 and CXCL1 were lower, whereas the levels of IL-10 were significantly higher in the BAL fluid of CRAMP−/− mice compared to WT mice 1 day after infection. In an in vitro assay using thioglycollate-induced peritoneal neutrophils, the ability of bacterial phagocytosis and killing was impaired in CRAMP−/− neutrophils compared to the WT cells. CRAMP was also essential for the production of cytokines and chemokines in response to A. baumannii in neutrophils. In addition, the A. baumannii-induced inhibitor of κB-α degradation and phosphorylation of p38 MAPK were impaired in CRAMP−/− neutrophils, whereas ERK and JNK phosphorylation was upregulated. Our results indicate that CRAMP plays an important role in the host defense against pulmonary infection with A. baumannii by promoting the antibacterial activity of neutrophils and regulating the innate immune responses.
Keywords: Acinetobacter baumannii, CRAMP, Neutrophils, Innate immune response
INTRODUCTION
Acinetobacter baumannii is a ubiquitous, gram-negative, aerobic and non-fermentative coccobacillus (1,2,3). It causes opportunistic infections in patients with underlying diseases and immunosuppression, leading to various diseases, such as nosocomial pneumonia, septicemia, endocarditis, skin and soft-tissue infections, urinary tract infections, and meningitis (2,4). The treatment of A. baumannii infection is complicated by its multidrug antibiotic resistance and new prevention and therapeutic options for this emerging threat are urgently needed (5,6). Despite its clinical importance, relatively little is known about how the innate immune response mediates the resistance of the host to an A. baumannii infection.
Antimicrobial peptides (AMPs) play a crucial role in defending against bacterial infections, as well as in the initiation of the inflammatory response. Previous studies have reported that AMPs are promising candidates for the treatment of gram-positive and gram-negative bacteria, as well as certain fungi (7,8,9). AMPs are generally produced by epithelial cells and immune cells, such as macrophages, dendritic cells (DCs), and neutrophils (10). AMPs interact with the membranes of susceptible bacteria and form higher-order structures that affect membrane permeability and eliminate bacteria (11). As a family of AMPs, cathelicidins have been found in various mammals, including mice and humans and cathelicidin-related antimicrobial peptide (CRAMP) and LL-37 are the only cathelicidins in mice and humans, respectively. In addition to their direct function of bacterial killing, these peptides can also regulate innate immunity and enhance the host innate immunity by increasing the production of reactive oxygen species (ROS), receptor expression, and chemotaxis in various immune cells (12).
Previous studies have shown that LL-37 inhibits the biofilm formation of A. baumannii and exhibits antibacterial activity against several drug-resistant strains of A. baumannii (13,14). In addition, a marsupial cathelicidin WAM-1 also demonstrated strong bactericidal activity against clinical isolates of A. baumannii (14). However, no studies have reported on the in vivo role of endogenous cathelicidin in host defenses against A. baumannii infections. In the present study, we sought to determine how CRAMP contributes to host defense against pulmonary infection with A. baumannii in vivo.
MATERIALS AND METHODS
Mice
Wild-type (WT) and CRAMP-deficient mice on a C57BL/6J background were purchased from the Jackson Laboratory (Bar Harbor, ME, USA). The animal study protocols were approved by the Institutional Animal Care and Use Committee of Chonnam National University (Gwangju, Korea) (approval number: CNU IACUC-YB-2019-34).
Bacterial preparation
The KCCM 35453 A. baumannii strain (ATCC 15150) was purchased from the Korean Culture Center of Microorganisms (Seoul, Korea). Single colonies were inoculated into 10 ml of Luria-Bertani (LB) broth supplemented with ampicillin (50 μg/ml) and grown overnight at 37°C with 200 rpm shaking. A 1:5 dilution of the culture suspension was allowed to grow in fresh medium at 37°C with shaking at 200 rpm for an additional 2 h. The bacteria were washed and resuspended with sterile PBS to a final concentration of 109 colony-forming units (CFU)/ml. The bacteria were diluted to the desired concentrations for use in the experiments.
In vivo experiments
The mice were anesthetized by the intraperitoneal injection of 10 mg/kg Rompun (Bayer, Seoul, Korea) and 50 mg/kg Zoletil (Virbac, Seoul, Korea). Then, they were intranasally (i.n.) inoculated with 30 µl of an A. baumannii (1×109 CFU/ml) suspension in PBS. Bronchoalveolar lavage (BAL) fluids were collected 6 h and 1 day post-infection to quantify bacterial loads, immune cell populations, cytokines, and chemokine production.
Bacterial counts in BAL fluid
Fifty microliters of serially diluted BAL fluid were spread onto LB agar plates supplemented with ampicillin (50 μg/ml). Following overnight culture at 37°C in an incubator, the bacterial colonies were counted and the number of bacteria was expressed as CFU/ml of BAL fluid.
Cell counts in BAL fluid
The total number of cells in the BAL fluid was counted using a hemocytometer. Differential cell counts based on morphologic criteria were performed using Diff-Quik staining (Sysmex Corporation, Kobe, Japan; Cat. No. 38721).
Preparation of thioglycollate-elicited neutrophils and bone marrow-derived macrophages (BMDMs)
Thioglycollate-elicited neutrophils were isolated from the mice peritoneal cavities as previously described (15). Briefly, the mice were injected intraperitoneally with 2 ml of 4% thioglycollate broth (Sigma-Aldrich, St. Louis, MO, USA; Cat. No. 70157). Four hours later, 5 ml of sterile PBS was injected intraperitoneally and peritoneal lavage was performed. Red blood cells in the lavage were lysed with a buffer containing ammonium chloride and the total cell numbers were counted with a hemocytometer. To confirm the purity of isolated neutrophils, the cells were reacted with FITC-conjugated anti-CD11b (BD Biosciences, San Jose, CA, USA; Cat. No. 552850) and APC-conjugated anti-Gr-1 (BD Biosciences; Cat. No. 553129) and analyzed by flow cytometry (Miltenyi Biotec, Bergisch Gladbach, Germany). Cells showing CD11b+ and Gr-1+ were >90% (Supplementary Fig. 1A). The morphology of isolated neutrophils was also determined by staining with Diff-Quik. The result supported that most isolated cells were composed of neutrophils (Supplementary Fig. 1B). BMDMs from WT and CRAMP−/−-deficiency mice were isolated and differentiated as described previously (16). Briefly, BMDMs were incubated in Iscove's modified Dulbecco's medium (IMDM, Gibco, Grand Island, NY, USA) containing 30% L929 cell culture supernatant, 1% MEM Non-Essential Amino Acids (MEM NEAA, Gibco), 1% penicillin/streptomycin (P/S, Gibco), 1% sodium pyruvate (Gibco), and 10% FBS (Corning Costar, Corning, NY, USA), in a 5% CO2 incubator at 37°C for 6 days. After 3 days, fresh medium was added and the cells were incubated for an additional 2 days and used in this study. To measure cytokine and chemokine levels, the cells were seeded in 48-well plates at a concentration of 2×105 cells/well and incubated in a 5% CO2 incubator at 37°C overnight. Subsequently, the cells were either infected or not infected with A. baumannii at the indicated multiplicity of infection (MOI) by exposure for 60 min and extracellular bacterial growth was inhibited by gentamicin (50 μg/ml) treatment. The culture supernatant was collected indicated time points after infection.
Measurement of cytokines and chemokines
The concentrations of IL-6 (Cat. No. DY406), TNF-α (Cat. No. DY410), IL-10 (Cat. No. DY417), IL-1β (Cat. No. DY401), CXCL1 (Cat. No. DY453), CXCL2 (Cat. No. DY452), CXCL12 (Cat. No. DY460), and CCL2 (Cat. No. DY479) from the BAL fluid of A. baumannii-infected mice or culture supernatant of mouse neutrophils were measured using ELISA kits (R&D System, Minneapolis, MN, USA) according to the manufacturer's instructions.
Phagocytic activity and bacterial killing ability of neutrophils and macrophages
Neutrophils and macrophages were seeded into 48-well plates at a density of 2×105 cells/well and subsequently infected with A. baumannii at an MOI of 1/10. After incubation for 60 min, cell membrane-impermeable antibiotic gentamicin (5 µg/ml) was added to the medium for 30 min to eliminate extracellular bacteria. At 1 (for phagocytosis) or 6 h (for bacterial killing) after infection, the neutrophils were washed with PBS and subsequently lysed with 1% Triton X-100 in PBS. To find out whether CRAMP contributes to the extracellular killing of A. baumannii, an additional experiment was carried out without gentamicin treatment and washing. The cell lysate was plated onto LB agar supplemented with ampicillin (50 μg/ml) to determine the number of living bacteria engulfed by neutrophils and macrophages.
Immunoblotting
For immunoblotting, neutrophils were seeded in 6-well plates at a concentration of 2×106 cells/well and infected with A. baumannii at an MOI of 10 by exposure for varying times. A. baumannii infected-cells were mixed with a lysis buffer containing Nonidet P-40, complete protease inhibitor cocktail (Roche, Mannheim, Germany), phosphatase inhibitor (Phosphatase Inhibitor Cocktail 2, Sigma-Aldrich), and 2 mM dithiothreitol. Protein lysates were separated by 8%–12% SDS-PAGE and transferred to nitrocellulose membranes. The membranes were immunoblotted with primary antibodies against inhibitor of κB (IκB)-α (Cell Signaling Technology, Beverly, MA, USA; Cat. No. 9242S), regular (Santa Cruz Biotechnology, Santa Cruz, CA, USA; Cat No. sc-728) and phosphorylated (Cell Signaling Technology; Cat No. 9211S) forms of p38, regular (Santa Cruz Biotechnology; Cat. No. sc-94) and phosphorylated (Cell Signaling Technology; Cat. No. 9101S) forms of ERK, regular (Cell signaling Technology; Cat. No. 9252S) and phosphorylated (Cell Signaling Technology; Cat. No. 9251S) forms of JNK, and β-actin (Santa Cruz Biotechnology, Cat No. sc-47778). After immunoblotting with HRP-labeled goat anti-mouse IgG (H+L) secondary antibody (Thermo Fisher Scientific, Waltham, MA, USA; Cat No. 31430) or HRP-labeled goat anti-rabbit IgG (H+L) secondary antibody (Thermo Fisher Scientific; Cat. No. 31460), the proteins were detected by chemiluminescence (ECL) reagent (BioRad, Hercules, CA, USA; Cat. No. BR170-5061). The density of protein bands was quantified using Image J software. Optical density for the target protein was normalized to β-actin.
Statistical analysis
Differences between the groups were determined by 2-tailed Student's t-tests or one-way analysis of variance, followed by post hoc analysis (Newman-Keuls multiple comparison test). All statistical analyses were performed using GraphPad Prism 5 (GraphPad Software Inc., La Jolla, CA, USA). Statistical significance was considered at p<0.05.
RESULTS
Deficiency of CRAMP leads to impaired bacterial clearance and recruitment of innate immune cells in the BAL fluids of A. baumannii-infected mice
WT and CRAMP−/− mice were infected i.n. with A. baumannii and the bacterial CFUs in the BAL fluids were evaluated. The results showed that bacterial CFUs in the BAL fluid were comparable between the WT and CRAMP−/− mice 6 h after infection (Fig. 1A). However, the bacterial CFUs in the CRAMP−/− mice were significantly higher than those in the WT mice 1 day after infection (Fig. 1A). Thus, we hypothesized that there may be smaller recruitment of innate immune cells in CRAMP−/− mice than in WT mice. In fact, 6 h and 1 day post-infection, the total cell counts in the BAL fluid were significantly lower in the CRAMP−/− mice than in the WT mice (Fig. 1B). The mean number of macrophages in the BAL fluid was lower in CRAMP−/− mice than in WT mice both 6 h and 1 day after infection, although there was no significant difference (Fig. 1C). Furthermore, the number of neutrophils in the CRAMP−/− mice were significantly lower than in WT mice 1 day after infection, but it is comparable between the 2 groups 6 h after infection (Fig. 1D). There was no difference in the ratio of innate immune cells, such as macrophages and neutrophils, between the 2 groups (Fig. 1E and F), but both groups were largely composed of neutrophils. These results suggest that the contribution of CRAMP to the clearance of A. baumannii from the lungs may be dependent upon the function of the neutrophils recruited.
Figure 1. CRAMP deficiency leads to increased bacterial loads and decreased recruitment of immune cells in BAL fluid of mice infected with A. baumannii. WT and CRAMP-deficient mice were infected i.n. with A. baumannii (3×107 CFU/30 µl) and euthanized at 6 h or 1 day after infection (A-F). Bacterial CFUs in the BAL fluid were counted using standard CFU assay procedures (A). The total number of cells in the BAL fluid was determined by counting cells with a hemocytometer (B). Differential counts of macrophages (C and E) and neutrophils (D and F) were obtained by Diff-Quik staining. The results are presented as mean±SD (B-F).
*p<0.05.
CRAMP influences the production of cytokines and chemokines in the BAL fluid of A. baumannii-infected mice
We measured cytokines and chemokines in the BAL fluid of WT and CRAMP−/− mice infected with A. baumannii. The level of IL-6 was slightly lower in the CRAMP−/− mice than in the WT mice 1 day after infection, although there was no significant difference in the level 6 h after infection (Fig. 2A). The levels of TNF-α and IL-1β in the BAL fluid were comparable between the WT and CRAMP−/− mice both 6 h and 1 day after infection (Fig. 2B and C). Of note, the IL-10 levels were higher in the CRAMP−/− mice 1 day after infection (Fig. 2D), although it was undetectable 6 h after infection. Among the chemokines, the levels of CXCL1 were higher in the CRAMP−/− mice than in the WT mice 6 h after infection (Fig. 2E). However, the level was lower in CRAMP−/− mice at 1 day (Fig. 2E). The levels of CXCL2, CXCL12, and CCL2 in the CRAMP−/− mice were similar to those in the WT mice (Fig. 2F-H).
Figure 2. CRAMP is involved in the production of cytokines and chemokines in BAL fluid of mice infected with A. baumannii. WT and CRAMP-deficient mice were infected i.n. with A. baumannii (3×107 CFU/30 µl) and euthanized at 6 h or 1 day after infection (A-H). The levels of IL-6 (A), TNF-α (B), IL-1β (C), IL-10 (D), CXCL1 (E), CXCL2 (F), CXCL12 (G), and CCL2 (H) in the BAL fluid were determined by ELISA. The results are presented as mean±SD.
*p<0.05 and **p<0.01.
CRAMP−/− neutrophils are defective in the uptake and elimination of ingested A. baumannii
Since the clearance of A. baumannii was impaired in CRAMP−/− mice concomitantly with neutrophil recruitment in the lungs (Fig. 1), we further investigated whether CRAMP influenced the ability of neutrophils to phagocytose and remove the bacteria. Neutrophils from CRAMP−/− mice were deficient in their ability to ingest A. baumannii after 1 h of infection compared to those from WT mice (Fig. 3A). After 6 h of infection, the number of ingested bacteria in the neutrophils from WT and CRAMP−/− mice began to decrease due to bacterial cell killing. Thus, the mean number of CFUs/ml was similar (Fig. 3B). Finally, the percentage of bacterial cell killing by CRAMP−/− neutrophils was significantly lower than that of WT neutrophils (Fig. 3C). In addition, neutrophils contribute to host defense through secretion of neutrophil extracellular traps (NETs), which contain antibacterial molecules such as histone, defensin, and cathelicidin (17). We thus investigated whether CRAMP leads to the extracellular killing of A. baumannii in neutrophils. No gentamicin treatment and wasing were performed to measure extracellular growth of the bacteria. After 6 h of infection, the bacterial CFUs in the CRAMP-deficient neutrophils were significantly higher than those in the WT cells (Fig. 3D). In response to A. baumannii, macrophages are also essential for phagocytosis and bacteria killing by regulating IL-10-STAT3-MARCO pathway (18). Accordingly, we sought to determine whether CRAMP is involved in regulating phagocytosis and bacterial killing in macrophages. Results showed that deficiency of CRAMP did not influenced the phagocytosis and killing of A. baumannii in BMDMs (Fig. 3E-G). These findings indicate that CRAMP may play a role in the removal of A. baumannii from neutrophils by promoting phagocytosis and intracellular killing.
Figure 3. CRAMP deficiency leads to impaired phagocytosis and bacterial killing activity of A. baumannii-infected neutrophils. Peritoneal neutrophils (A-D) and BMDMs (E-G) from WT and CRAMP−/− mice were infected with A. baumannii at an MOI of 10, followed by gentamicin treatment at 60 min after infection to remove extracellular bacteria (A-C and E-G). No gentamicin was treated in D to examine the extracellular growth of the bacteria. The live bacteria were counted by plating onto LB agar containing ampicillin (50 μg/ml) at 1 h (for phagocytosis) (A and E) or 6 h (for bacterial killing) (B, D, and F) after infection. The percentage of bacteria killed was obtained by dividing the CFUs at 6 h by the phagocytosed CFUs at 1 h (C and G). The results are presented as mean±SD.
*p<0.05, **p<0.01, and ***p<0.001.
Production of cytokine and chemokine is differently regulated by CRAMP in A. baumannii-infected neutrophils in vitro
To test how CRAMP contributed to the activation of innate immune responses in neutrophils infected with A. baumannii, cytokines and chemokines were measured in the culture supernatants of WT and CRAMP−/− neutrophils 18 h after infection. The production of IL-6 and TNF-α by CRAMP−/− neutrophils was significantly reduced compared to WT cells at varying A. baumannii MOIs (Fig. 4A and B). However, the IL-1β and IL-10 production in CRAMP−/− neutrophils was significantly increased compared to WT neutrophils (Fig. 4C and D). The levels of all tested chemokines, including CXCL1, CXCL2, and CCL2, were lower in CRAMP−/− neutrophils than in WT cells (Fig. 4E-G). Of note, CRAMP was not required for the production of the cytokines and chemokines in response to A. baumannii in macrophages (Supplementary Fig. 2). Thus, CRAMP likely has the potential to mediate the A. baumannii-induced production of cytokines and chemokines, specifically in neutrophils.
Figure 4. CRAMP regulates the production of cytokines and chemokines in neutrophils in response to A. baumannii. Thioglycollate-elicited peritoneal neutrophils from WT and CRAMP-deficient were infected with A. baumannii at the indicated MOIs and were treated with gentamicin 60 min after infection (A-G). After 18 h of infection, the levels of IL-6 (A), TNF-α (B), IL-1β (C), IL-10 (D), CXCL1 (E), CXCL2 (F), and CCL2 (G) in the culture supernatant were determined by ELISA. The results are presented as mean±SD.
*p<0.05, **p<0.01, and ***p<0.001.
CRAMP differently regulates the activation of NF-κB and MAPKs in response to A. baumannii in mice neutrophils
The results in Figure 4 raised the question of how CRAMP regulates the NF-κB and MAPK pathways in response to A. baumannii in neutrophils, which are major pathways regulating the cell survival and pro-inflammatory activation of neutrophils and mediating the production of proinflammatory cytokines in immune cells (19,20,21,22). The loss of CRAMP attenuated the A. baumannii-induced degradation of IκB-α and the phosphorylation of p38 MAPK in CRAMP−/− neutrophils compared to WT cells (Fig. 5A-C). In contrast, the phosphorylation of ERK and JNK was slightly enhanced in CRAMP−/− neutrophils (Fig. 5A, D and E). These experiments were repeated and verified (Supplementary Fig. 3). These findings suggest that CRAMP mediates the production of cytokines and chemokines in response to A. baumannii in mice neutrophils by differentially regulating the activation of NF-κB and MAPKs.
Figure 5. Activation of NF-κB and MAPK in response to A. baumannii is differently regulated by CRAMP in mouse neutrophils. Thioglycollate-elicited peritoneal neutrophils from WT and CRAMP-deficient were infected with A. baumannii at an MOI of 10. After the indicated time points, the cellular proteins were extracted. IκB-α degradation and phosphorylation of p38, ERK, and JNK were examined by Western blotting (A-E). Antibodies against the regular form of p38, ERK, and JNK were used. β-actin was used to confirm the loading doses.
DISCUSSION
The main finding of the current study was that the endogenous loss of CRAMP in mice increased their susceptibility to intranasal infection with A. baumannii, likely due to the deficient intracellular killing of the bacteria by neutrophils. We examined the function of CRAMP in a mouse model of pulmonary infection with A. baumannii because the respiratory tract is rich in innate immune antimicrobials, such as antimicrobial peptides like CRAMP (23,24). Indeed, direct evidence has shown that cathelicidins exerted antibacterial activity against both gram-positive and gram-negative bacteria in the respiratory system (25,26,27,28,29,30). The present study is the first, to our knowledge, to demonstrate that CRAMP serves important roles in defending the host from A. baumannii infection, demonstrated by the failure of neutrophils in CRAMP−/− mice to resolve the infection and upregulate the innate immune response.
In addition to its role in promoting bactericidal activity, CRAMP exerts several immunomodulatory effects, including stimulating the release of inflammatory mediators from immune cells and their chemotaxis (12). A previous study revealed that the recruitment of polymorphonuclear cells (PMNs) in BAL fluid was impaired in CRAMP−/− mice compared to WT mice 24 h after infection with Klebsiella pnuemoniae (30). Thus, we hypothesized that the increased growth of A. baumannii in the lungs of CRAMP−/− mice may be due to differences in the recruited immune cells important for the clearance of bacteria. As expected, at 6 h (when there was no visible bacterial clearance), immune cells were already present in the lungs and the number of cells increased 4-fold after 1 day, suggesting that the observed increase in immune cells might be involved in bacterial clearance. Interestingly, most of the recruited cells were neutrophils and macrophages, which are important for host resistance to A. baumannii infection (31,32), with neutrophils accounting for almost 90% of the cell population. These quantitative results do not necessarily mean that neutrophils are more important than macrophages. In many in vitro experiments, macrophages have been associated with CRAMP and may play an important role in mediating its antibacterial effects (12). In addition, CRAMP is expressed in pulmonary macrophages exposed to gram-negative bacteria (29,30) and acts as a potent chemoattractant for monocytes and macrophages in a dose-dependent manner, inducing ERK1/2 and p38 kinase phosphorylation (33). Nevertheless, CRAMP seems to contribute to the immune activation of neutrophils, rather than macrophages, against A. baumannii infection, demonstrated by the impaired ability to produce cytokines and chemokines in response to A. baumannii in CRAMP-deficient neutrophils, but not in macrophages. Furthermore, CRAMP deficiency impaired the ability to phagocytose and kill the bacteria in neutrophils, but not in macrophages.
Cathelicidins are known to regulate cellular signaling such as NF-κB and MAPKs in various cells. LL-37 treatment led to activation of NF-κB in human monocytes and epithelial cells (34,35). In addition, LL-37 enhanced phosphorylation of MAPKs such as p38 and ERK in microglia and astrocytes as well as neutrophils (36,37). In the present study, we observed that A. baumannii induces activation of NF-κB and MAPKs even in CRAMP-deficient neutrophils. TLRs stimulation mediate cathelicidins expression and secretion (38). A. baumannii induces TLRs-mediated immune responses (39,40) and various TLRs are functionally expressed in neutrophils (41). Accordingly, it is likely that TLRs signaling may participate in regulating NF-κB and MAPKs activation. In addition, A. baumannii-induced IκB-α degradation and p38 phosphorylation were attenuated in CRAMP-deficient neutrophils, as compared with WT cells. Phosphorylation of ERK and JNK was rather slightly increased in CRAMP-deficient neutrophils. It is still uncertain how CRAMP regulates NF-κB and MAPKs activation induced by A. baumannii in neutrophils. It is likely that A. baumannii enhances CRAMP expression in neutrophils, which regulates NF-κB and p38 activation through autocrine loop. Further study is necessary to clarify the precise mechanism by which CRAMP regulates A. baumannii-mediated NF-κB and MAPKs in neutrophils.
In the present study, the production of cytokines and chemokines in response to A. baumannii in vivo and in vitro was controlled differently by CRAMP. IL-6, TNF-α, CXCL1, CXCL2, and CCL2 production was lower in CRAMP-deficient neutrophils as compared with WT cells, which is presumed to be due to reduced NF-κB activity. However, it was inconsistent with in vivo results in BAL fluids. This may be because in vivo system is much more complex. Interestingly, the IL-10 protein levels were significantly higher in CRAMP−/− neutrophils harboring A. baumannii infection, which was consistent with the in vivo result shown in Fig. 2D. This observation appears to be an uncommon response in most innate immune cell types, including macrophages (12). LL-37 upregulates the transcription and production of IL-10 in monocytes, macrophages, and keratinocytes (12). In contrast, Di Nardo et al. showed that pretreatment with LL-37 significantly reduced the LPS-induced IL-10 production in monocyte-derived dendritic cells (42). The upregulation of IL-10 plays a role in the negative regulation of innate immunity in proportion to increasing infection and it is thought to promote a favorable environment for bacterial growth by inhibiting other cytokines. These findings suggest that the loss of CRAMP may contribute to the increased secretion of IL-10, leading to a downregulation of immune response mediators that affect neutrophil phagocytic activity, resulting in poor bacterial cell killing. Further study is required to clarify how CRAMP regulates IL-10 production in neutrophils. In addition, IL-1β level was also slightly higher in CRAMP-deficient neutrophils in response to A. baumannii. IL-1β production is controlled by transcriptional regulation via NF-κB signaling and maturation step by inflammasome activation (43). A. baumannii leads to IL-1β production by activating NLRP3 inflammasome in macrophages (44) and the NLRP3 inflammasome is also functional in neutrophils (45). A previous study revealed that LL-37 inhibits IL-1β production, caspase-1 activation, and pyroptotic cell death induced by LPS/ATP (NLRP3 inflammasome stimulator) (46). Accordingly, it is likely that CRAMP negatively regulates IL-1β production in response to A. baumannii in neutrophils by inhibiting NLRP3 inflammasome.
In conclusion, our study demonstrated that CRAMP was required for the effective bacterial clearance in mice with pulmonary infection of A. baumannii. Although further studies are required to define the precise mechanisms of action, our findings suggest a role for cathelicidins in the immune response that could be exploited as a novel therapeutic target for treating A. baumannii infections in the respiratory tract.
ACKNOWLEDGEMENTS
This work was supported by the Korea Research Institute of Bioscience and Biotechnology Research Initiative Program (KGM4571922) and Mid-Career Researcher Program (Grant No. 2018R1A2B3004143) of the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (Information and Communication Technologies).
Abbreviations
- AMP
antimicrobial peptide
- BAL
bronchoalveolar lavage
- BMDM
bone marrow-derived macrophages
- CFU
colony-forming units
- CRAMP
cathelicidin-related antimicrobial peptide
- DC
dendritic cell
- IκB
inhibitor of κB
- IMDM
Iscove's modified Dulbecco's medium
- i.n.
intranasally
- LB
Luria-Bertan
- MOI
multiplicity of infection
- NET
neutrophil extracellular trap
- P/S
penicillin/streptomycin
- ROS
reactive oxygen species
- PMN
polymorphonuclear cell
- WT
wild-type
Footnotes
Conflicts of Interest: The authors declare no potential conflicts of interest.
- Conceptualization: Park JH.
- Data curation: Kang MJ, Jang AR, Park JY, Ahn JH, Lee TS, Kim DY, Jung DH, Song EJ.
- Project administration: Park JH.
- Supervision: Park JH.
- Writing - original draft: Kang MJ, Jang AR, Hong JJ, Park JH.
SUPPLEMENTARY MATERIALS
Phenotype analysis to determine the purity of isolated peritoneal neutrophils. The isolated peritoneal cells were stained with FITC-conjugated anti-CD11b and APC-conjugated anti-Gr-1 and analyzed using flow cytometry (A). The morphology of isolated cells was determined by staining with Diff-Quik (B).
CRAMP deficiency does not affect the production of cytokines and chemokines by macrophages in response to A. baumannii. WT and CRAMP-deficient BMDMs were infected with A. baumannii at the indicated MOI and were treated with gentamicin 60 min after infection (A-G). After 24 h, the levels of IL-6 (A), TNF-α (B), IL-1β (C), IL-10 (D), CXCL1 (E), CXCL2 (F), and CCL2 (G) in the supernatant were determined by ELISA. The results are presented as mean±SD.
Activation of NF-κB and MAPK in response to A. baumannii is differently regulated by CRAMP in mouse neutrophils. Thioglycollate-elicited peritoneal neutrophils from WT and CRAMP-deficient were infected with A. baumannii at an MOI of 10. After the indicated time points, the cellular proteins were extracted. IκB-α degradation and phosphorylation of p38, ERK, and JNK were examined by Western blotting (A-E). Antibodies against the regular form of p38, ERK, and JNK were used. β-actin was used to confirm the loading doses.
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Associated Data
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Supplementary Materials
Phenotype analysis to determine the purity of isolated peritoneal neutrophils. The isolated peritoneal cells were stained with FITC-conjugated anti-CD11b and APC-conjugated anti-Gr-1 and analyzed using flow cytometry (A). The morphology of isolated cells was determined by staining with Diff-Quik (B).
CRAMP deficiency does not affect the production of cytokines and chemokines by macrophages in response to A. baumannii. WT and CRAMP-deficient BMDMs were infected with A. baumannii at the indicated MOI and were treated with gentamicin 60 min after infection (A-G). After 24 h, the levels of IL-6 (A), TNF-α (B), IL-1β (C), IL-10 (D), CXCL1 (E), CXCL2 (F), and CCL2 (G) in the supernatant were determined by ELISA. The results are presented as mean±SD.
Activation of NF-κB and MAPK in response to A. baumannii is differently regulated by CRAMP in mouse neutrophils. Thioglycollate-elicited peritoneal neutrophils from WT and CRAMP-deficient were infected with A. baumannii at an MOI of 10. After the indicated time points, the cellular proteins were extracted. IκB-α degradation and phosphorylation of p38, ERK, and JNK were examined by Western blotting (A-E). Antibodies against the regular form of p38, ERK, and JNK were used. β-actin was used to confirm the loading doses.