Dectin-2-dependent host defense in mice infected with serotype 3 Streptococcus pneumoniae

Background Streptococcus pneumoniae, a major causative bacterial pathogen of community-acquired pneumonia, possesses a thick polysaccharide capsule. Host defense against this bacterium is mediated by activation of innate immune cells that sense bacterial components. Recently, C-type lectin receptors (CLRs) have garnered much attention in elucidating the recognition mechanism of pathogen-derived polysaccharides. Methods In the present study, we first compared the clinical course and neutrophil accumulation in the lungs of Dectin-2 knock-out (KO) and wild type (WT) mice. Mice were infected intratracheally with a serotype 3 strain of S. pneumoniae, and S. pneumoniae bacterial engulfment by neutrophils and inflammatory cytokine and anti-pneumococcal polysaccharide-specific IgG levels were evaluated in bronchoalveolar lavage fluid (BALF). We also examined the effect of Dectin-2 deficiency on interleukin (IL)-12 production by bone marrow-derived dendritic cells (BM-DCs) stimulated with the bacterial components. Results S. pneumonia-infected Dectin-2KO mice had a shorter survival time, larger bacterial burden and lower interferon gamma (IFN-γ) production in the lungs than WT mice. Although neutrophilic infiltration in the lungs was equivalent between Dectin-2KO mice and WT mice, S. pneumonia engulfment by neutrophils was attenuated in Dectin-2KO mice compared to WT mice. The anti-pneumococcal polysaccharide-specific IgG and IgG3 levels in BALF were lower in Dectin-2KO mice than in WT mice. When BM-DCs were stimulated with S. pneumoniae culture supernatant or its Concanavalin A (ConA)-bound fraction, IL-12 production was abrogated in Dectin-2KO mice compared to WT mice. Conclusions We demonstrated that Dectin-2 is intimately involved in the host defense against infection with a serotype 3 strain of S. pneumoniae. Dectin-2-dependent IL-12 production may contribute to IFN-γ synthesis and subsequent production of serotype-specific anti-capsular polysaccharide IgG after S. pneumoniae infection, which may promote S. pneumoniae bacterial opsonization for engulfment.


Background
Streptococcus pneumoniae is recognized as a major bacterial agent that causes community-acquired pneumonia and other invasive diseases, such as bacteremia and meningitis [1,2]. Upon infection with S. pneumoniae, the innate mechanism of the early phase of immunity plays an important role in host defense, which is largely mediated by neutrophil-dependent immune responses. The infiltration of many neutrophils into the alveolar spaces eradicates the infection via an immunoglobulin G (IgG)-mediated opsonophagocytic killing (OPK) mechanism and the production of reactive oxygen species (ROS) [3]. IgG3 that is specific for pneumococcal capsular polysaccharide, a thymus-independent type 2 (TI-2) antigen, is highly protective against infection with S. pneumoniae [4]. Recently, we reported similar findings that pneumococcal polysaccharide vaccine (PPV) immunization increases serotype 3-specific IgG3 serum levels, which facilitates survival after pneumococcal infection [5]. CXC chemokines, including macrophage inflammatory protein (MIP)-2 and keratinocyte-derived chemokine (KC), a homologue of human interleukin (IL)-8, were involved in neutrophil accumulation at the inflammatory sites. Previous studies showed that death in mice challenged with S. pneumoniae was preceded by bacterial growth within 2 days after infection and was associated with a delayed increase in pulmonary MIP-2 levels and neutrophil recruitment [6].
In the present study, we aimed to determine the role of Dectin-2 in the neutrophil-mediated host defense to S. pneumoniae infection using mice with a genetic disruption of Dectin-2. We found that Dectin-2 knock out (KO) mice were more susceptible to this infection than wild type (WT) mice, and our results suggest that Dectin-2dependent IL-12 production may contribute to IFN-γ synthesis and subsequent production of serotype-specific anti-capsular polysaccharide IgG after S. pneumoniae infection, which may promote opsonization of this bacterium for engulfment.

Role of Dectin-2 in the host defense to pneumococcal infection
To clarify whether Dectin-2 deficiency affects earlyphase host protection against pneumococcal infection, we initially examined the susceptibility of Dectin-2KO mice to S. pneumoniae infection and compared it with WT mice by recording the survival rate of the infected mice and also the bacterial load in their lungs. Dectin-2KO mice had a lower survival rate (17 % by day 4 after intratracheal S. pneumoniae infection), whereas 67 % of WT mice survived throughout the observation period (Fig. 1a). The difference in the survival rate was statistically significant. In addition, the number of live bacterial colonies was significantly lower in the lungs of WT mice than in Dectin-2KO mice on day 3 post-infection (Fig. 1b). These data indicate that Dectin-2 plays a critical role in early-phase host defense against pneumococcal infection.

Role of Dectin-2 in the neutrophil-mediated host defense to pneumococcal infection
Neutrophils rapidly accumulate at the most infected sites and they play a central role in eradicating bacteria after pulmonary S. pneumoniae infection [16]. Therefore, to address the role of Dectin-2 in neutrophil-mediated host defense against this bacterial pathogen, we first evaluated neutrophil recruitment in the infected lungs. A histological analysis showed no apparent difference in inflammatory cell infiltration in the lungs between WT and Dectin-2KO mice 12 h after infection with S. pneumoniae. When observed at a higher magnification, neutrophil accumulation in the alveolar spaces was shown to be comparable between these mice (Fig. 2a). In addition, the number of neutrophils in bronchoalveolar lavage fluid (BALF) was almost equivalent between WT and Dectin-2KO mice 12 h and 24 h after infection (Fig. 2b).
To further clarify the role of Dectin-2 in the neutrophil-mediated host defense, we evaluated the phagocytosis rate and phagocytosis index of these cells in BALF 12 h and 24 h after infection. As shown in Fig. 3a, the rate of neutrophils engulfing pneumococcus was significantly lower in Dectin-2KO mice than in WT mice at both time points, although the average number of pneumococcus per engulfed neutrophil did not differ largely between these mice (Fig. 3b).

Role of Dectin-2 in the production of proinflammatory cytokines and chemokines after pneumococcal infection
To clarify the role of Dectin-2 in the host response to S. pneumoniae infection, we compared the production of proinflammatory cytokines and chemokines, such as IL-1β, TNF-α, IL-6, IFN-γ, IL-17A, and MIP-2, in BALF between WT and Dectin-2KO mice 12 h after infection. As shown in Fig. 4, the production of IFN-γ was significantly attenuated in Dectin-2KO mice compared with WT mice, although there was no significant difference in the production of other cytokines and chemokines.
To define the cellular source of IFN-γ production, we used flow cytometry to examine the intracellular expression of this cytokine in various cells in the lungs of WT mice 12 h after infection. IFN-γ was expressed only in exudate macrophages, but not in alveolar macrophages, neutrophils, natural killer (NK) cells, NKT cells, γδT cells, CD4 + T cells or CD8 + T cells (Fig. 5).

Reduced production of PPS3-specific Ab in Dectin-2KO mice
Serotype-specific IgG against capsular polysaccharides plays a critical role as an opsonin in the phagocytosis of S. pneumoniae by neutrophils, which strongly promotes its eradication [17]. IgG3 is a major subclass of IgG produced under the stimulation of TI-2 Ags such as pneumococcal capsular polysaccharides [18]. We next measured PPS3-specific IgG and IgG3 levels in BALF 24 h after infection. As shown in Fig. 6a and b, the levels of PPS3-specific IgG and IgG3 in BALF were significantly increased 24 h after infection with S. pneumoniae in both WT and Dectin-2KO mice, and their levels were significantly lower in Dectin-2KO mice compared with WT mice. By contrast, PPS3-specific IgM levels in BALF were low, and there was no difference in the levels between WT and Dectin-2KO mice (Fig. 6c).

Role of Dectin-2 in the activation of dendritic cells upon stimulation with S. pneumoniae
To address the role of Dectin-2 in the cellular response to S. pneumoniae, we examined how the lack of Dectin-2 affected the production of IL-12p40 by bone marrowderived dendritic cells (BM-DCs) upon stimulation with viable S. pneumoniae, lysates or culture supernatant from this bacterium. As shown in Fig. 7a, IL-12p40 production by BM-DCs was almost comparable between WT and Dectin-2 KO mice, when stimulated with viable   pneumococcus, whereas IL-12p40 was not produced by BM-DCs stimulated with heat-killed bacteria (data not shown). By contrast, IL-12p40 synthesis by BM-DCs was significantly reduced in Dectin-2KO mice compared with WT mice, when stimulated with S. pneumoniae lysates at lower doses (Fig. 7b). Similar results were obtained when BM-DCs were stimulated with S. pneumoniae culture supernatant (Fig. 7c).
To determine which S. pneumoniae molecule is recognized by Dectin-2, we examined how depletion of the Concanavalin A (ConA)-bound fraction in culture supernatants affects IL-12p40 synthesis by BM-DCs. IL-12p40 synthesis by BM-DCs from WT mice was abolished when stimulated with the culture supernatant that was depleted of the ConA-bound fraction (Fig. 8a). In further experiments, we examined whether the ConA-bound fraction in culture supernatant stimulated BM-DCs and whether this activity was dependent on Dectin-2. As shown in Fig. 8b, the ConA-bound fraction induced IL-12p40 production by BM-DCs from WT mice and this activity was completely abrogated in BM-DCs from Dectin-2KO mice, similar to the response caused by mannan.

Discussion
In the present study, we evaluated the role of Dectin-2 in the neutrophil-mediated host defense to pneumococcal infection. Our data indicate that a defect in Dectin-2 rendered mice highly susceptible to a serotype 3 strain of S. pneumoniae, as shown by elevated mortality and an increased bacterial burden in the lungs. It is well documented that neutrophil-predominant inflammatory responses play a pivotal role in eradicating this bacterium [3]. Therefore, we predicted that a defect of Dectin-2 expression impaired the recruitment of neutrophils and clearance of S. pneumoniae in the lungs after infection. Dessing and co-workers previously demonstrated that TLR2-and TLR4-mediated recognition of pneumococcal components induced the production of proinflammatory cytokines, such as IL-6 and IL-1β, and chemokines, such as MIP-2 and KC, which are critical for neutrophil accumulation in inflamed tissues [19,20]. They also found that a lack of TLR2 led to earlier death from pneumococcal meningitis [21]. In the present study, however, neutrophil accumulation and TNF-α, IL-1β, IL-17A and MIP-2 synthesis in the lungs after infection with S. pneumoniae were not significantly different between WT and Dectin-2KO in the host defense against S. pneumoniae infection [22]. TLR9KO mice were significantly more susceptible to this infection than WT mice, probably as a result of the impaired phagocytic killing of this bacterium by macrophages. Earlier observations demonstrated that the prognosis of bacteremia following pneumococcal pneumonia was improved in the early phase of infection, which might be related to the development of specific anti-capsular Ab [23]. Opsonization of this bacterium by IgG specific to the capsular polysaccharides is a key step in the neutrophil-mediated host defense against this infection [17,24,25]. TI-2 Ag, including pneumococcal capsular polysaccharides, typically elicits a rapid extrafollicular IgG response with limited isotype class switching from IgM to IgG, affinity maturation of immunoglobulin and memory B cell response [26]. However, our recent study demonstrated that the serum levels of serotype-specific IgG against pneumococcal polysaccharide Ag were increased in mice immunized with PPV [5]. Similar observations were reported by other investigators [27,28]. Pneumococcal capsular polysaccharides have been shown to produce serotype-specific IgG, irrespective of their features as TI-2 Ag. In a clinical study, Verbinnen and co-workers demonstrated that B-1-like B cells producing serotype-specific IgG were significantly increased in the peripheral blood of healthy volunteers as early as day 5 after immunization with PPV [29]. In addition, Haas and co-workers demonstrated that B-1 B cells were involved in the protection during early responses against S. pneumoniae infection through the synthesis of IgG3 to TI-2 Ag [30]. Although B cell subsets producing IgG3 remain to be examined, the present data demonstrated that Dectin-2 was involved in the production of serotype-specific IgG3 in BALF as early as 24 h after infection was significantly reduced in Dectin-2KO mice compared with WT mice.
Earlier investigations reported the involvement of IFNγ in isotype class switching of IgM to IgG3 in B cells upon stimulation with TI-2 Ag [31], although in the current study, it remains unclear whether IgG3 was produced as a result of class-switching from IgM because of equivalent anti-PPS3 IgM in BALF between WT and Dectin-2KO mice. In addition, Marchi and co-workers demonstrated that IFN-γ enhanced opsonized zymosan phagocytosis and ROS release by neutrophils [32]. Recently, we reported that a defect in Dectin-2 led to reduced IFN-γ production by NKT cells during immunization with PPV  [5]. Similarly, in the present study, IFN-γ synthesis in the lungs was significantly attenuated in Dectin-2KO mice compared with WT mice after infection with S. pneumoniae. Thus, IFN-γ may play a pivotal role in regulating the phagocytic killing by neutrophils as a downstream event in Dectin-2-mediated recognition of pneumococcal capsular polysaccharides. Previously, we demonstrated that IL-12 plays an important role in host defense against pneumococcal infection by promoting the production of IFN-γ [13]. In the present study, Dectin-2 was essential for triggering IL-12p40 production by DCs upon simulation with S. pneumoniae culture supernatant, which supports the above hypothesis. The ConA-bound fraction in S. pneumoniae culture supernatant induced IL-12p40 production by BM-DCs, and this activity was completely abrogated when BM-DCs were derived from Dectin-2KO mice. These results suggest that certain ConA-bound moieties of the capsular polysaccharides may be involved in Dectin-2-mediated recognition of S. pneumoniae. In earlier studies, Lee and co-workers demonstrated that Dectin-2 had an ability to bind Glc-, Gal-, GlcNAc-and GalNAc-BSA, in addition to the usual Man-and Fuc-BSA [33]. Additionally, ConA has been known to bind molecules that contain α-Dmannose, α-D-glucose and sterically related residues with available C-3, C-4 or C-5 hydroxyl groups [34]. These previous findings suggest that some glucosyl residue in the capsular polysaccharides might be recognized by Dectin-2. Further investigations are necessary to define the precise polysaccharide structure that contributes to S. pneumoniae recognition through Dectin-2.
Among different S. pneumoniae serotypes, capsular polysaccharide structures are not identical [35]. McGreal and co-workers previously reported that serotype 3 capsular polysaccharide inhibited the interaction between Dectin-2 and mannans, whereas other serotypes, such as 2, 9 V, 14, 18C and 19 F, did not show such an effect [36], suggesting a distinct role for Dectin-2 in the recognition of different S. pneumoniae serotypes. In the present study, we used only a serotype 3 strain of S. pneumoniae, and therefore, these findings may not be generalizable to all pneumococcal infections.

Conclusions
In the present study, we demonstrate that Dectin-2KO mice were more susceptible to infection with a serotype 3 strain of S. pneumoniae than WT mice, as shown by a shorter survival time, larger bacterial burden and lower IFN-γ production in the lungs of Dectin-2KO mice. Our results suggest that Dectin-2-dependent IL-12 production may contribute to IFN-γ synthesis and subsequent production of serotype-specific anti-capsular polysaccharide IgG after S. pneumoniae serotype 3 infection, which may promote opsonization of this bacterium for engulfment. Thus, the present study may provide important implications for better understanding in the host defense mechanism against S. pneumoniae serotype 3 and for developing more effective vaccine strategies against this infection.

Mice
Dectin-2KO mice were generated by homologous recombination of the Clec4n gene as described previously [7]. WT littermate mice of the Dectin-2KO mice were used as controls. Male or female mice at 6 to 8 weeks of age were used for the experiments. The mice were bred under specific pathogen-free conditions at the Animal Facility, Tohoku University Graduate School of Medicine (Sendai, Japan). All experimental procedures involving animals followed the Regulations for Animal Experiments and Related Activities at Tohoku University and were approved by the ethics committees of Tohoku University.

Bacteria
A serotype-3 clinical strain of S. pneumoniae, designated as URF918, was established from a patient with pneumococcal pneumonia [37]. The bacteria were cultured in Todd-Hewitt broth (Difco, Detroit, MI, USA) at 37°C in a 5 % CO 2 incubator, harvested at the mid-log phase of growth and then washed twice in phosphate buffered saline (PBS). The inoculum was stored at −80°C until use.

S. pneumoniae infection
WT or Dectin-2KO mice were anaesthetized by an intraperitoneal injection of 70 mg/kg pentobarbital (Abbott Laboratory, North Chicago, IL, USA) and restrained on a small board. Live S. pneumoniae (0.75-3 × 10 5 colony forming units (CFU)) at 50 μl per mouse were inoculated by insertion of a 24G intravenous (IV) catheter (Terumo, Tokyo, Japan) into the trachea. Colony counts were performed to confirm the accuracy of inoculum CFU as a means to determine CFU/ml for S. pneumoniae using a 5 % sheep blood tryptic soy agar plate (Nissui Pharmaceutical Co., Ltd., Tokyo, Japan).

Enumeration of viable S. pneumoniae
WT and Dectin-2KO mice were sacrificed on day 3 postinfection, and the lungs were dissected carefully and excised. They were then homogenized in 5 ml of PBS by teasing with stainless mesh at room temperature. The homogenates (100 μl) were diluted in twofold series using sterile half saline and inoculated onto a 5 % sheep blood tryptic soy agar plate (Nissui Pharmaceutical Co., Ltd.). The homogenate was then cultured for 24 h at 37°C in 5 % CO 2 , and the number of colonies was counted.

Lung histology
Lungs were isolated from WT or Dectin-2KO mice 12 h after pneumococcal infection, and fixed in 10 % buffer formalin, dehydrated and embedded in paraffin. Sections were cut and stained with hematoxylin-eosin (H-E) or Gram stain at the Biomedical Research Core, Animal Pathology Platform of Tohoku University Graduate School of Medicine (Sendai, Japan).

Preparation of BALF
BALF samples from WT or Dectin-2KO mice were collected as described below. Briefly, after bleeding under anesthesia with isoflurane, the chest was opened and the trachea was cannulated with the outer sheath of a 22G IV catheter/needle unit (Terumo), followed by lung lavage three times with 1 ml of chilled PBS. Then, 1 × 10 5 cells were centrifuged onto a glass slide using StatSpin Cytofuge 2 (Iris Sample Processing, Franklin, MA, USA), and then stained with Diff-Quick or Gram stain. After centrifugation of BALF, supernatants were stored at −80°C for cytokine assay. To analyze the leukocyte fraction and S. pneumoniae phagocytosis by neutrophils, at least 500 cells were examined using light microscopy.

In vivo neutrophil phagocytosis
Twelve hours after pneumococcal infection, WT and Dectin-2KO mice were sacrificed and BALF was collected. The cells were spun onto glass slides, and the phagocytic index and phagocytic rate were examined by light microscopy following Gram staining. The phagocytic rate was calculated as follows: (number of neutrophils engulfing pneumococcus/total number of neutrophils) × 100. The phagocytic index was calculated as follows: (number of phagocytized pneumococcus/total number of neutrophils engulfing pneumococcus).

Measurement of serotype-specific Antibodies
BALF samples were collected before infection or 24 h post-pneumococcal infection. The quantities of serotypespecific antibodies (Ab) against pneumococcal polysaccharide type 3 (PPS3) in BALF were measured by ELISA. Microtiter plates (Nunc A/S, Roskilde, Denmark) were coated with 3 μg/ml of PPS3 (American Type Culture Collection, Manassas, VA, USA) in PBS for 1 h at 37°C. Before testing, serum samples were diluted with 0.05 % skim milk PBS. HRP-conjugated goat anti-mouse IgG, IgG3 or IgM antibodies (Southern Biotechnology Associates, Birmingham, AL, USA) diluted with 1:4000 were used as detection Ab. The concentrations of IgG, IgG3 and IgM were determined based on the absorbance at 450 nm.
Preparation of S. pneumoniae homogenates S. pneumoniae were grown on 5 % sheep blood tryptic soy agar plate (Nissui Pharmaceutical Co., Ltd., Tokyo, Japan) at 37°C in a 5 % CO 2 incubator for 24 h. Harvested colonies were crushed in PBS using 0.1 mm glass beads and a Multi-Beads Shocker (Yasuikikai, Osaka, Japan) at 2500 rpm and 4°C for 40 cycles (30 s on/30 s off), passed through a 40 μm nylon mesh filter (Becton, Dickinson and Company, Franklin Lakes, NJ, USA), and then stored at −80°C until use. Sham-operated PBS were treated identically without S. pneumoniae and used as controls.
Preparation of S. pneumoniae culture supernatants S. pneumoniae was diluted with half saline until the turbidity reached 0.5 using the McFarland standard, inoculated in 19 volumes of Todd-Hewitt broth (Difco) and then incubated on an orbital shaker (150 rpm) at 37°C in a 5 % CO 2 incubator for 24 h. The culture supernatants were centrifuged, passed through a 0.45 μm membrane filter (Sartorius, Göttingen, Germany) and stored at −80°C until use. Todd-Hewitt broth incubated without S. pneumoniae was used as a control.

ConA-affinity chromatography of S. pneumoniae culture supernatants
ConA-Sepharose4B (GE Healthcare Bio-Sciences AB, Uppsala, Sweden) was prepared according to the manufacturer's instructions. Briefly, a polystyrene column (0.6 × 18 cm; bed volume, 5 ml) was packed with ConA-Sepharose4B. The column was washed with 50 ml of binding buffer (20 mM Tris-HCl, 0.5 M NaCl, pH 7.4) for regeneration and re-equilibration. S. pneumoniae culture supernatants were applied to the column continuously at a flow rate of 3 ml/h. The bound-fractions were eluted with an elution buffer of 0.4 M methyl-α-D-mannopyranoside (Sigma-Aldrich) in 20 mM Tris-HCl at pH 7.4 and with 0.5 M NaCl, and dialyzed against PBS using a membrane with a 2-kDa molecular weight cutoff (Thermo Fisher Scientific Inc., IL, USA). All the chromatographic operations and dialysis were performed at 4°C. To collect the ConA-unbound fraction, S. pneumoniae culture supernatants were incubated with ConA-Sepharose4B for 15 min at room temperature, and then the ConA-Sepharose4B beads were removed using centrifugation. Sham treatment was performed using Sepharose4B beads. Similarly, mannan received ConA-Sepharose4B or sham treatment.

Statistical analysis
Statistical analysis was conducted using GraphPad Prism 5 software (GraphPad Software, La Jolla,CA,USA). Data are presented as the mean ± standard deviation (SD). Differences between the two groups were tested using a two-tailed analysis in an unpaired Student's t-test.
Survival data was analyzed using the Kaplan-Meier log rank test. A p value less than 0.05 was considered significant.