Chlamydial Lung Infection Induces Transient IL-9 Production Which Is Redundant for Host Defense against Primary Infection

IL-9/Th9 responses are recently found to be important for innate and adaptive immunity particularly in parasitic infections. To date, the study on the role of IL-9 in bacterial infections is limited and the reported data are contradictory. One reported function of IL-9/Th9 is to modulate Th1/Th17 responses. Since our and others’ previous work has shown a critical role of Th1 and Th17 cells in host defense against chlamydial lung infection, we here examined the role of IL-9 responses in Chlamydia muridarum (Cm) lung infection, particularly its effect on Th1 and Th17 responses and outcome infection. Our data showed quick but transient IL-9 production in the lung following infection, peaking at day 3 and back to baseline around day 7. CD4+ T cell was the major source of IL-9 production in the lung infection. Blockade of endogenous IL-9 using neutralizing antibody failed to change Interferon-γ (IFN-γ) and IL-17 production by cultured spleen mononuclear cells isolated from Cm infected mice. Similarly, in vivo neutralization of IL-9 failed to show significant effect on T cell (Th1 and Th17) and antibody responses (IgA, IgG1 and IgG2a). Consistently, the neutralization of IL-9 had no significant effect on disease process, including body weight change, bacterial burden and histopathological score. The data suggest that IL-9 production following chlamydial lung infection is redundant for host defense against the intracellular bacteria.     

Epstein Barr Virus-Induced 3 (EBI3) Together with IL-12 Negatively Regulates T Helper 17-Mediated Immunity to Listeria monocytogenes Infection

Although the protective functions by T helper 17 (Th17) cytokines against extracellular bacterial and fungal infection have been well documented, their importance against intracellular bacterial infection remains unclear. Here, we investigated the contribution of Th17 responses to host defense against intracellular bacteria Listeria monocytogenes and found that Th17 cell generation was suppressed in this model. Unexpectedly, mice lacking both p35 and EBI3 cleared L. monocytogenes as efficiently as wild-type mice, whereas p35-deficient mice failed to do so. Furthermore, both innate cells and pathogen-specific T cells from double-deficient mice produced significantly higher IL-17 and IL-22 compared to wild-type mice. The bacterial burden in the liver of double-deficient mice treated with anti-IL-17 was significantly increased compared to those receiving a control Ab. Transfer of Th17 cells specific for listeriolysin O as well as administration of IL-17 and IL-22 significantly suppressed bacterial growth in p35-deficient mice, indicating the critical contribution of Th17 responses to host defense against the intracellular pathogen in the absence of IL-12 and proper Th1 responses. Our findings unveil a novel immune evasion mechanism whereby the intracellular bacteria exploit IL-27EBI3 to suppress Th17-mediated protective immunity.     

SND1 promotes Th1/17 immunity against chlamydial lung infection through enhancing dendritic cell function

To date, no reports have linked the multifunctional protein, staphylococcal nuclease domain-containing protein 1 (SND1), to host defense against intracellular infections. In this study, we investigated the role and mechanisms of SND1, by using SND1 knockout (SND1^-/-) mice, in host defense against the lung infection of Chlamydia muridarum, an obligate intracellular bacterium. Our data showed that SND1^-/- mice exhibited significantly greater body weight loss, higher organism growth, and more severe pathological changes compared with wild-type mice following the infection. Further analysis showed significantly reduced Chlamydia-specific Th1/17 immune responses in SND1^-/- mice after infection. Interestingly, the dendritic cells (DCs) isolated from SND1^-/- mice showed lower costimulatory molecules expression and IL-12 production, but higher IL-10 production compared with those from wild-type control mice. In the DC-T cell co-culture system, DCs isolated from SND1^-/- infected mice showed significantly reduced ability to promote Chlamydia-specific IFN-γ producing Th1 cells but enhanced capacity to induce CD4⁺T cells into Foxp3⁺ Treg cells. Adoptive transfer of DCs isolated from SND1^-/- mice, unlike those from wild-type control mice, failed to protect the recipients against challenge infection. These findings provide in vivo evidence that SND1 plays an important role in host defense against intracellular bacterial infection, and suggest that SND1 can promote Th1/17 immunity and inhibit the expansion of Treg cells through modulation of the function of DCs.     

Th1-Mediated Immunity against Helicobacter pylori Can Compensate for Lack of Th17 Cells and Can Protect Mice in the Absence of Immunization

Helicobacter pylori (H. pylori) infection can be significantly reduced by immunization in mice. Th17 cells play an essential role in the protective immune response. Th1 immunity has also been demonstrated to play a role in the protective immune response and can compensate in the absence of IL-17. To further address the potential of Th1 immunity, we investigated the efficacy of immunization in mice deficient in IL-23p19, a cytokine that promotes Th17 cell development. We also examined the course of Helicobacter infection in unimmunized mice treated with Th1 promoting cytokine IL-12. C57BL/6, IL-12 p35 KO, and IL-23 p19 KO mice were immunized and challenged with H. pylori. Protective immunity was evaluated by CFU determination and QPCR on gastric biopsies. Gastric and splenic IL-17 and IFNγ levels were determined by PCR or by ELISA. Balb/c mice were infected with H. felis and treated with IL-12 therapy and the resulting gastric bacterial load and inflammatory response were assessed by histologic evaluation. Vaccine induced reductions in bacterial load that were comparable to wild type mice were observed in both IL-12 p35 and IL-23 p19 KO mice. In the absence of IL-23 p19, IL-17 levels remained low but IFNγ levels increased significantly in both immunized challenged and unimmunized/challenged mice. Additionally, treatment of H. felis-infected Balb/c mice with IL-12 resulted in increased gastric inflammation and the eradication of bacteria in most mice. These data suggest that Th1 immunity can compensate for the lack of IL-23 mediated Th17 responses, and that protective Th1 immunity can be induced in the absence of immunization through cytokine therapy of the infected host.     

Pertussis Toxin Stimulates IL-17 Production in Response to Bordetella pertussis Infection in Mice

In a mouse model of respiratory tract infection by Bordetella pertussis, bacteria multiply in the airways over the first week and are then cleared over the next 3–4 weeks by the host immune response. Pertussis toxin (PT), a virulence factor secreted exclusively by B. pertussis, promotes bacterial growth in the airways by suppression and modulation of host immune responses. By comparison of wild type and PT-deficient strains, we examined the role of PT in modulating airway cytokine and chemokine responses affecting neutrophil recruitment during B. pertussis infection in mice. We found that, despite early inhibition of neutrophil recruitment by PT, high numbers of neutrophils were recruited to the airways by 4 days post-infection with the wild type strain, but not with the PT-deficient strain, and that this correlated with upregulation of neutrophil-attracting chemokine gene expression. In addition, there was similar upregulation of genes expressing the cytokines IL-17A (IL-17), TNF-α and IFN-γ, indicating a mixed Th1/Th17 response. Expression of IL-6, a cytokine involved in Th17 induction, was upregulated earlier than the IL-17 response. We showed that PT, rather than bacterial numbers, was important for induction of these responses. Flow cytometric analysis revealed that the IL-17-producing cells were macrophages and neutrophils as well as T cells, and were present predominantly in the airways rather than the lung tissue. Antibody neutralization of IL-17 significantly reduced chemokine gene expression and neutrophil recruitment to the airways, but only modestly increased peak bacterial loads. These data indicate that PT stimulates inflammatory responses by induction of Th1- and Th17-associated cytokines, including IL-17, during B. pertussis infection in mice, but a role for IL-17 in protection against the infection remains to be established.     

Anti-Chlamydial Th17 Responses Are Controlled by the Inducible Costimulator Partially through Phosphoinositide 3-Kinase Signaling

We previously showed that mice deficient in the Inducible Costimulator ligand (ICOSL-KO) develop more severe disease and lung pathology with delayed bacterial clearance upon respiratory infection of Chlamydia muridarum. Importantly, the exacerbation of disease in ICOSL-KO mice was seen despite heightened IFN-γ/Th1 responses, the major defense mechanisms against Chlamydia. To gain insight into the mechanism of ICOS function in this model, we presently analyzed anti-Chlamydia immune responses in mice lacking the entire ICOS (ICOS-KO) versus knock-in mice expressing a mutant ICOS (ICOS-Y181F) that has selectively lost the ability to activate phosphoinositide 3-kinase (PI3K). Like ICOSL-KO mice, ICOS-KO mice showed worse disease with elevated IFN-γ/Th1 responses compared to wild-type (WT) mice. ICOS-Y181F mice developed much milder disease compared to ICOS-KO mice, yet they were still not fully protected to the WT level. This partial protection in ICOS-Y181F mice could not be explained by the magnitude of IFN-γ/Th1 responses since these mice developed a similar level of IFN-γ response compared to WT mice. It was rather IL-17/Th17 responses that reflected disease severity: IL-17/Th17 response was partially impaired in ICOS-Y181F mice compared to WT, but was substantially stronger than that of ICOS-KO mice. Consistently, we found that both polarization and expansion of Th17 cells were partially impaired in ICOS-Y181F CD4 T cells, and was further reduced in ICOS-KO CD4 T cells in vitro. Our results indicate that once the IFN-γ/Th1 response is above a threshold level, the IL-17/Th17 response becomes a limiting factor in controlling Chlamydia lung infection, and that ICOS plays an important role in promoting Th17 responses in part through the activation of PI3K.     

Osteopontin-dependent regulation of Th1 and Th17 cytokine responses in Trypanosoma cruzi-infected C57BL/6 mice

Osteopontin (OPN) is a multifunctional protein participating in the regulation of different Th cell lineages and critically involved in the initiation of immune responses to diverse pathogens. Our study goal was to verify whether OPN helps modulate the protective Th1 and Th17 cytokine responses in C57BL/6 mice infected with Trypanosoma cruzi, the etiological agent of Chagas disease. Parasite infection induced OPN release from murine macrophages in vitro and acute Chagas mice displayed enhanced serum levels of this cytokine at the peak of parasitemia. Upon administration of a neutralizing anti-OPN antibody, recently infected mice presented lower Th1 and Th17 responses, increased parasitemia and succumbed earlier and at higher rates to infection than non-immune IgG-receiving controls. The anti-OPN therapy also resulted in reduced circulating levels of IL-12 p70, IFN-γ, IL-17A and specific IgG_2a antibodies. Furthermore, antibody-mediated blockade of OPN activity abrogated the ex vivo production of IL-12 p70, IFN-γ and IL-17A, while promoting IL-10 secretion, by spleen macrophages and CD4⁺ T cells from T. cruzi-infected mice. Th1 and Th17 cytokine release induced by OPN preferentially involved the α_vβ₃ integrin OPN receptor, whereas concomitant down-modulation of IL-10 production would mostly depend on OPN interaction with CD44. Our findings suggest that, in resistant C57BL/6 mice, elicitation of protective Th1 and Th17 cytokine responses to T. cruzi infection is likely to be regulated by endogenous OPN.     

Dynamics of Th17 cells and their role in Schistosoma japonicum infection in C57BL/6 mice

Background

The current knowledge of immunological responses to schistosomiasis, a major tropical helminthic disease, is insufficient, and a better understanding of these responses would support vaccine development or therapies to control granuloma-associated immunopathology. CD4⁺ T cells play critical roles in both host immune responses against parasitic infection and immunopathology in schistosomiasis. The induction of T helper (Th)1, Th2 and T regulatory (Treg) cells and their roles in schistosome infections are well-illustrated. However, little in vivo data are available on the dynamics of Th17 cells, another important CD4⁺ T cell subset, after Schistosoma japonicum infection or whether these cells and their defining IL-17 cytokine mediate host protective responses early in infection.

Methodology

Levels of Th17 and the other three CD4⁺ T cell subpopulations and the cytokines related to induction or repression of Th17 cell generation in different stages of S. japonicum infection were observed. Contrary to reported in vitro studies, our results showed that the Th17 cells were induced along with the Th1, Th2, Treg cells and the IFN-γ and IL-4 cytokines in S. japonicum infected mice. The results also suggested that S. japonicum egg antigens but not adult worm antigens preferentially induced Th17 cell generation. Furthermore, decreasing IL-17 with a neutralizing anti-IL-17 monoclonal antibody (mAb) increased schistosome-specific antibody levels and partial protection against S. japonicum infection in mice.

Conclusions

Our study is the first to report the dynamics of Th17 cells during S. japonicum infection and indicate that Th17 cell differentiation results from the integrated impact of inducing and suppressive factors promoted by the parasite. Importantly, our findings suggest that lower IL-17 levels may result in favorable host protective responses. This study significantly contributes to the understanding of immunity to schistosomiasis and may aid in developing interventions to protect hosts from infection or restrain immunopathology.     

Th17 Down-regulation Is Involved in Reduced Progression of Schistosomiasis Fibrosis in ICOSL KO Mice

Background

Granulomatous and fibrosing inflammation in response to parasite eggs is the main pathology that occurs during infection with Schistosoma spp. CD4+ T cells play critical roles in both host immune responses against parasitic infection and immunopathology in schistosomiasis,and coordinate many types of immune cells that contribute to fibrosis. ICOSL plays an important role in controlling specific aspects of T cell activation, differentiation, and function. Previous work has suggested that ICOS is essential for Th17 cell development. However, the immunopathogenesis of this pathway in schistosomiasis fibrosisis still unclear.

Methodology/Principal Findings

Using models of schistosomiasis in ICOSL KO and the C57BL/6 WT mice, we studied the role of the ICOSL/ICOS interaction in the mediation of the Th17 response in host granulomatous inflammation, particularly in liver fibrosis during S. japonicum infection, and investigated the immune responses and pathology of ICOSL KO mice in these models. The results showed that ICOSL KO mice exhibited improved survival, reduced liver granulomatous inflammation around parasite eggs, markedly inhibited hepatic fibrosis development, lower levels of Th17-related cytokines (IL-17/IL-21), Th2-related cytokines (IL-4/IL-6/IL-10), a pro-fibrotic cytokine (IL-13), and TGF-β1, but higher level of Th1-related cytokine (IFN-γ) compared to wild-type (WT) mice. The reduced progression of fibrogenesis was correlated with the down-regulation of Th17 and Th2 and the elimination of ICOSL/ICOS interactions.

Conclusions/Significance

Our findings suggest that IL-17-producing cells contribute to the hepatic granulomatous inflammation and subsequent fibrosis. Importantly, there was a clearly positive correlation between the presence of IL-17-producing cells and ICOS expression in ICOSL KO mice, and additional results indicated that Th17 was involved in the pathological tissue remodeling in liver fibrosis induced by schistosomiasis.     

Arming Th17 Cells for Antifungal Host Defense

Interleukin 17 (IL-17)-mediated immunity has emerged as a crucial host defense mechanism against fungal infections. The family of IL-17 cytokines is phylogenetically ancient, but remains the least understood of all cytokine subclasses. The effects mediated by IL-17 are pleiotropic and include the induction of antimicrobial peptides as well as cytokines and chemokines that lead to the recruitment and activation of neutrophils. Neutrophils in turn are key effector cells of the antifungal defense. CD4⁺ T cells act as a major source of IL-17 and a lot has been learned about these cells since their discovery a decade ago. This review highlights key aspects of the underlying mechanisms regulating the development of Th17 responses during fungal infections. We discuss the impact of different subsets of antigen-presenting cells, innate cytokine signals and tissue-specific factors on Th17 differentiation, and we highlight the prerequisites for the mediation by Th17 cells of vaccine immunity against fungi.     

Experimental Persistent Infection of BALB/c Mice with Small-Colony Variants of Burkholderia pseudomallei Leads to Concurrent Upregulation of PD-1 on T Cells and Skewed Th1 and Th17 Responses

BackgroundBurkholderia pseudomallei (B. pseudomallei), the causative agent of melioidosis, is a deadly pathogen endemic across parts of tropical South East Asia and Northern Australia. B. pseudomallei can remain latent within the intracellular compartment of the host cell over prolonged periods of time, and cause persistent disease leading to treatment difficulties. Understanding the immunological mechanisms behind persistent infection can result in improved treatment strategies in clinical melioidosis.MethodsTen-day LD₅₀ was determined for the small-colony variant (SCV) and its parental wild-type (WT) via intranasal route in experimental BALB/c mice. Persistent B. pseudomallei infection was generated by administrating sub-lethal dose of the two strains based on previously determined LD₅₀. After two months, peripheral blood mononuclear cells (PBMCs) and plasma were obtained to investigate host immune responses against persistent B. pseudomallei infection. Lungs, livers, and spleens were harvested and bacterial loads in these organs were determined.ResultsBased on the ten-day LD₅₀, the SCV was ~20-fold less virulent than the WT. The SCV caused higher bacterial loads in spleens compared to its WT counterparts with persistent B. pseudomallei infection. We found that the CD4+ T-cell frequencies were decreased, and the expressions of PD-1, but not CTLA-4 were significantly increased on the CD4+ and CD8+ T cells of these mice. Notably, persistent infection with the SCV led to significantly higher levels of PD-1 than the WT B. pseudomallei. Plasma IFN-γ, IL-6, and IL-17A levels were elevated only in SCV-infected mice. In addition, skewed plasma Th1 and Th17 responses were observed in SCV-infected mice relative to WT-infected and uninfected mice.ConclusionB. pseudomallei appears to upregulate the expression of PD-1 on T cells to evade host immune responses, which likely facilitates bacterial persistence in the host. SCVs cause distinct pathology and immune responses in the host as compared to WT B. pseudomallei.     

Relative Contribution of Th1 and Th17 Cells in Adaptive Immunity to Bordetella pertussis: Towards the Rational Design of an Improved Acellular Pertussis Vaccine

Whooping cough caused by Bordetella pertussis is a re-emerging infectious disease despite the introduction of safer acellular pertussis vaccines (Pa). One explanation for this is that Pa are less protective than the more reactogenic whole cell pertussis vaccines (Pw) that they replaced. Although Pa induce potent antibody responses, and protection has been found to be associated with high concentrations of circulating IgG against vaccine antigens, it has not been firmly established that host protection induced with this vaccine is mediated solely by humoral immunity. The aim of this study was to examine the relative contribution of Th1 and Th17 cells in host immunity to infection with B. pertussis and in immunity induced by immunization with Pw and Pa and to use this information to help rationally design a more effective Pa. Our findings demonstrate that Th1 and Th17 both function in protective immunity induced by infection with B. pertussis or immunization with Pw. In contrast, a current licensed Pa, administered with alum as the adjuvant, induced Th2 and Th17 cells, but weak Th1 responses. We found that IL-1 signalling played a central role in protective immunity induced with alum-adsorbed Pa and this was associated with the induction of Th17 cells. Pa generated strong antibody and Th2 responses, but was fully protective in IL-4-defective mice, suggesting that Th2 cells were dispensable. In contrast, Pa failed to confer protective immunity in IL-17A-defective mice. Bacterial clearance mediated by Pa-induced Th17 cells was associated with cell recruitment to the lungs after challenge. Finally, protective immunity induced by an experimental Pa could be enhanced by substituting alum with a TLR agonist that induces Th1 cells. Our findings demonstrate that alum promotes protective immunity through IL-1β-induced IL-17A production, but also reveal that optimum protection against B. pertussis requires induction of Th1, but not Th2 cells.     

IL-25, IL-33 and TSLP receptor are not critical for development of experimental murine malaria

IL-25, IL-33 and TSLP, which are produced predominantly by epithelial cells, can induce production of Th2-type cytokines such as IL-4, IL-5 and/or IL-13 by various types of cells, suggesting their involvement in induction of Th2-type cytokine-associated immune responses. It is known that Th2-type cytokines contribute to host defense against malaria parasite infection in mice. However, the roles of IL-25, IL-33 and TSLP in malaria parasite infection remain unclear. Thus, to elucidate this, we infected wild-type, IL-25^?/?, IL-33^?/? and TSLP receptor (TSLPR)^?/? mice with Plasmodium berghei (P. berghei) ANKA, a murine malaria strain. The expression levels of IL-25, IL-33 and TSLP mRNA were changed in the brain, liver, lung and spleen of wild-type mice after infection, suggesting that these cytokines are involved in host defense against P. berghei ANKA. However, the incidence of parasitemia and survival in the mutant mice were comparable to in the wild-type mice. These findings indicate that IL-25, IL-33 and TSLP are not critical for host defense against P. berghei ANKA.     

IL-23 and IL-17A, but not IL-12 and IL-22, are required for optimal skin host defense against Candida albicans

IL-23 and Th17 cells play important roles in host defense against systemic infections with extracellular bacteria and fungi, although their roles in immunity against localized skin infections are less well defined. Here, the contributions of IL-23 and Th17 cytokines in host defense against cutaneous Candida albicans infection were evaluated. Mice deficient in IL-23 or IL-17A demonstrated delayed healing and decreased IL-17A production after skin infection with C. albicans compared with wild-type mice or mice deficient in IL-12 or IL-22. Histologic examination revealed epidermal hyperplasia overlying infected dermis four days postinoculation in wild-type mice. In IL-23-deficient mice, fungal burden was greater in skin, neither IL-17A nor IL-22 mRNAs were expressed postinfection, and these mice demonstrated only minimal epidermal hyperplasia. Exogenous recombinant IL-17A injected at the site of skin infection promoted more rapid healing of candidiasis in both wild-type mice and mice deficient in IL-23 and IL-12. Taken together, these results demonstrate that IL-23 and IL-17A, but not IL-12 and IL-22, are required for optimal host defense against cutaneous candidiasis. In addition, recombinant IL-17A may serve as a potential therapy to enhance healing in individuals with chronic cutaneous candidiasis.     

Chlamydia trachomatis mouse pneumonitis lung infection in IL-18 and IL-12 knockout mice: IL-12 is dominant over IL-18 for protective immunity

BACKGROUND: Interferon (IFN)-gamma is a key to protective immunity against a variety of intracellular bacterial infections, including Chlamydia trachomatis. Interleukin (IL)-18, a recently identified Th1 cytokine, together with IL-12 is a strong stimulator for IFN-gamma production. We investigated the relative roles of IL-18 and IL- 12 in protective immunity to C. trachomatis mouse pneumonitis (MoPn) infection using gene knockout (KO) and wild-type (WT) mice. MATERIALS AND METHODS: Mice were intranasally infected with C. trachomatis MoPn and protective immunity was assessed among groups of mice by daily body weight changes, lung growth of MoPn, and histopathological appearances at day 10 postinfection. The corresponding immune responses for each group of mice at the same postinfection time point were evaluated by measuring antigen-specific antibody isotype responses and cytokine profiles. RESULTS: Our results showed that IL-18 deficiency had little or no influence on clearance of MoPn from the lung, although KO mice exhibited slightly more severe inflammatory reactions in lung tissues, as well as reduced systemic and local IFN-gamma production, compared with WT mice. Results with IL-18 KO mice were in sharp contrast to those observed with IL-12 KO mice that showed substantially reduced clearance of MoPn from the lungs, substantial reductions of antigen-specific systemic and lung IFN-gamma production, decreased ratio of MoPn-specific immunoglobulin G (IgG)2a/IgG1, and severe pathological changes in the lung with extensive polymorphonuclear, instead of mononuclear, cell infiltration. Exogenous IL-12 or IL-18 was able to increase IFN-gamma production in IL-18 KO mice; whereas, only exogenous IL-12, but not IL-18, enhanced IFN-gamma production in IL-12 KO mice. Caspase-1 is the key protease for activation of IL-18 precursor into the bioactive form, and caspase-1 KO mice also displayed similar bacterial clearance and body weight loss to that in WT mice at early stages of MoPn infection. This further confirmed that IL-18 was not essential for host defense against chlamydia infection. CONCLUSIONS: These results suggest that IL-12, rather than IL-18, plays the dominant role in the development of protective immunity against chlamydia lung infection, although both cytokines are involved in the in vivo regulation of IFN-gamma production.     

Plasmacytoid Dendritic Cells Mediate the Regulation of Inflammatory Type T Cell Response for Optimal Immunity against Respiratory Chlamydia Pneumoniae Infection

Chlamydia pneumoniae (Cpn) infection is a leading cause for a variety of respiratory diseases and has been implicated in the pathogenesis of chronic inflammatory diseases. The regulatory mechanisms in host defense against Cpn infection are less understood. In this study, we investigated the role of plasmacytoid dendritic cells (pDCs) in immune regulation in Cpn respiratory tract infection. We found that in vivo depletion of pDCs increased the severity of infection and lung pathology. Mice depleted of pDC had greater body weight loss, higher lung bacterial burden and excessive tissue inflammation compared to the control mice. Analysis of specific T cell cytokine production pattern in the lung following Cpn infection revealed that pDC depleted mice produced significantly higher amounts of inflammatory cytokines, especially TNF-α, but lower IL-10 compared to the controls. In particular, pDC depleted mice showed pathogenic T cell responses characterized by inflammatory type-1 (CD8 and CD4) and inflammatory Th2 cell responses. Moreover, pDC depletion dramatically reduced CD4 regulatory T cells (Tregs) in the lungs and draining lymph nodes. Furthermore, pDC-T cell co-culture experiments showed that pDCs isolated from Cpn infected mice were potent in inducing IL-10 producing CD4 Tregs. Together, these findings provide in vivo evidence for a critical role of pDCs in homeostatic regulation of immunity during Cpn infection. Our findings highlight the importance of a ‘balanced’ immune response for host protective immunity and preventing detrimental immunopathology during microbial infections.     

Essential role for the p40 subunit of interleukin-12 in neutrophil-mediated early host defense against pulmonary infection with Streptococcus pneumoniae: involvement of interferon-gamma

Interleukin (IL)-12 is a critical cytokine in the T helper (Th)1 response and host defense against intracellular microorganisms, while its role in host resistance to extracellular bacteria remains elusive. In the present study, we elucidated the role of IL-12 in the early-phase host defense against acute pulmonary infection with Streptococcus pneumoniae, a typical extracellular bacterium, using IL-12p40 gene-disrupted (IL-12p40KO) mice. IL-12p40KO mice were highly susceptible to S. pneumoniae infection, as indicated by the shortened survival time, which was completely restored by the replacement therapy with recombinant (r) IL-12, and increased bacterial counts in the lung. In these mice, recruitment of neutrophils in the lung was significantly attenuated when compared to that in wild-type (WT) mice, which correlated well with the reduced production of macrophage inflammatory protein (MIP-2) and tumor necrosis factor (TNF)-alpha in the infected tissues at the early phase of infection. In vitro synthesis of both cytokines by S. pneumoniae-stimulated lung leukocytes was significantly lower in IL-12p40KO mice than in WT mice, and addition of rIL-12 or interferon (IFN)-gamma restored the reduced production of MIP-2 and TNF-alpha in IL-12p40KO mice. Neutralizing anti-IFN-gamma monoclonal antibody (mAb) significantly decreased the effect of rIL-12. Anti-IFN-gamma mAb shortened the survival time of infected mice and reduced the recruitment of neutrophils and production of MIP-2 and TNF-alpha in the lungs. Our results indicated that IL-12p40 plays a critical role in the early-phase host defense against S. pneumoniae infection by promoting the recruitment of neutrophils to the infected tissues.     

Resident Vdelta1+ gammadelta T cells control early infiltration of neutrophils after Escherichia coli infection via IL-17 production

Neutrophils infiltrate the site of infection and play critical roles in host defense, especially against extracellular bacteria. In the present study, we found a rapid and transient production of IL-17 after i.p. infection with Escherichia coli, preceding the influx of neutrophils. Neutralization of IL-17 resulted in a reduced infiltration of neutrophils and an impaired bacterial clearance. Ex vivo intracellular cytokine flow cytometric analysis revealed that gammadelta T cell population was the major source of IL-17. Mice depleted of gammadelta T cells by mAb treatment or mice genetically lacking Vdelta1 showed diminished IL-17 production and reduced neutrophil infiltration after E. coli infection, indicating an importance of Vdelta1(+) gammadelta T cells as the source of IL-17. It was further revealed that gammadelta T cells in the peritoneal cavity of naive mice produced IL-17 in response to IL-23, which was induced rapidly after E. coli infection in a TLR4 signaling-dependent manner. Thus, although gammadelta T cells are generally regarded as a part of early induced immune responses, which bridge innate and adaptive immune responses, our study demonstrated a novel role of gammadelta T cells as a first line of host defense controlling neutrophil-mediated innate immune responses.