Interleukin-23 (IL-23)-IL-17 cytokine axis in murine Pneumocystis carinii infection

Trypanosoma cruzi trans-sialidase initiates a program independent of the transcription factors RORγt and Ahr that leads to IL-17 production by activated B cells

We identified B cells as a major source for rapid, innate-like interleukin 17 (IL-17) production in vivo in response to Trypanosoma cruzi infection. IL-17⁺ B cells exhibited a plasmablast phenotype, outnumbered T_H17 cells and were required for optimal response to this pathogen. Using both murine and human primary B cells, we demonstrate that exposure to parasite-derived trans-sialidase in vitro was sufficient to trigger modification of the cell surface mucin, CD45, leading to Btk-dependent signaling and IL-17A or IL-17F production via an ROR-γt and AHR-independent transcriptional program. Our combined data suggest that generation of IL-17⁺ B cells may be an unappreciated feature of innate immune responses required for pathogen control or IL-17-mediated autoimmunity.

Dried plasma spots in the diagnosis of tuberculosis: IP-10 release assay on filter paper

Interferon (IFN)-γ release assays (IGRAs) are probably the most accurate tests for the detection of latent Mycobacterium tuberculosis infection, but IGRAs are labour intensive and the transport of samples over longer distances is difficult. IFN-γ-induced protein (IP)-10 is expressed at 100-fold higher levels than IFN-γ, and IP-10 release assays have comparable performance to IGRAs. The aim of this study was to explore the diagnostic potential of a novel IP-10 release assay based on dried plasma spots (DPS). The presence of IP-10 and IFN-γ was determined in plasma and in DPS by ELISA. Diagnostic algorithms for plasma and DPS tests for IP-10 were developed on a training cohort comprising 60 tuberculosis (TB) patients and 59 healthy controls. Diagnostic accuracy was assessed in a validation cohort comprising 78 TB patients and 98 healthy controls. Plasma was measured in Spain and DPS samples were sent to Denmark using the conventional postal service for analysis. IP-10 was readily detectable in both plasma and DPS, and correlation was excellent (r(2) = 0.95). QuantiFERON-TB Gold In-Tube (QFT-TB) and IP-10 in DPS and plasma rendered comparable sensitivity (78%, 82% and 84%, respectively), specificity (100%, 97% and 97%, respectively) and indeterminate rates (p>0.55). The DPS-based IP-10 test has comparable diagnostic accuracy to the QFT-TB and samples can be sent via conventional mail over long distances for analysis without affecting the results.

A Th17- and Th2-skewed cytokine profile in cystic fibrosis lungs represents a potential risk factor for Pseudomonas aeruginosa infection

RATIONALE

Cystic fibrosis (CF) is characterized by progressive pulmonary inflammation that is infection-triggered. Pseudomonas aeruginosa represents a risk factor for deterioration of lung function and reduced life expectancy.

OBJECTIVES

To assess T-cell cytokine/chemokine production in clinically stable children with CF and evaluate the association between T-cell subtypes and susceptibility for infection with P. aeruginosa.

METHODS

T-cell cytokine/chemokine profiles were measured in bronchoalveolar lavage fluid (BALF) from children with CF (n = 57; 6.1 ± 5.9 yr) and non-CF control subjects (n = 18; 5.9 ± 4.3 yr). Memory responses to Aspergillus fumigatus and P. aeruginosa were monitored. High-resolution computed tomography-based Helbich score was assessed. In a prospective observational trial the association between BALF cytokine/chemokine profiles and subsequent infection with P. aeruginosa was studied.

MEASUREMENTS AND MAIN RESULTS

Th1- (INF-γ), Th2- (IL-5, IL-13), Th17- (IL-17A), and Th17-related cytokines (IL-1β, IL-6) were significantly up-regulated in airways of patients with CF. IL-17A, IL-13, and IL-5 were significantly higher in BALF of symptomatic as compared with clinically asymptomatic patients with CF. IL-17A and IL-5 correlated with the percentage of neutrophils in BALF (r = 0.41, P < 0.05 and r = 0.46, P < 0.05, respectively). Th17- (IL-17A, IL-6, IL-1β, IL-8) and Th2-associated cytokines and chemokines (IL-5, IL-13, TARC/CCL17), but not IFN-γ levels, significantly correlated with high-resolution computed tomography changes (Helbich score; P < 0.05). P. aeruginosa- and A. fumigatus-specific T cells from patients with CF displayed significantly higher IL-5 and IL-17A mRNA expression. IL-17A and TARC/CCL17 were significantly augmented in patients that developed P. aeruginosa infection within 24 months.

CONCLUSIONS

We propose a role for Th17 and Th2 T cells in chronic inflammation in lungs of patients with CF. High concentrations of these cytokines/chemokines in CF airways precede infection with P. aeruginosa.

Targeting IL-17 and TH17 cells in chronic inflammation

The key role of interleukin-17 (IL-17) and T helper 17 (T(H)17) cells in tissue inflammation, autoimmunity and host defence led to the experimental targeting of these molecules in mouse models of diseases as well as in clinical settings. Moreover, the demonstration that IL-17 and T(H)17 cells contribute to local and systemic aspects of disease pathogenesis, as well as the finding that the IL-17-T(H)17 cell pathway is regulated by IL-23, prompted the identification of inhibitors. These inhibitors include biotechnology products that target IL-23 as well as the leading member of the IL-17 family, IL-17A, and one of its receptors, IL-17 receptor A. Several clinical trials of these inhibitors are underway, and positive results have been obtained in psoriasis, rheumatoid arthritis and ankylosing spondylitis. This Review focuses on the current knowledge of the IL-17-T(H)17 cell pathway to better understand the positive as well as potential negative consequences of targeting them.

Depletion of neutrophils in a protective model of pulmonary cryptococcosis results in increased IL-17A production by γδ T cells

Protective responses in mice immunized with an interferon-gamma producing strain of Cryptococcus neoformans, H99γ, are associated with IL-17A production by neutrophils. Neutrophil depletion in H99γ-immunized mice did not affect pulmonary fungal burden, indicating that neutrophils are not required for clearance. However, we observed an increase in IL-17A in the lungs of neutrophil-depleted H99γ infected mice, which corresponded to an increase in IL-17A+ γδ+ T cells. Moreover, we observed increased IL-17A+/ CD3+ cells and IL-17A+/γδ+ cells, but decreased IL-17A+/Ly6G+ neutrophils in the lungs of IL-17 receptor (R)A deficient mice compared to wild-type mice. Increased production of IL-17A in neutropenic mice coincided with increased IL-6 and CXCL1, but not Th17 inducing cytokines TGF-β, IL-21 and IL-23. Concurrent depletion of neutrophils and γδ+ T cells reduced IL-17A levels. Our results suggest that γδ+ T cells mediate significant IL-17A production in neutropenic mice during the protective response to C. neoformans infection.

Memory CD4+ T cells are required for optimal NK cell effector functions against the opportunistic fungal pathogen Pneumocystis murina

Little is known about the role of NK cells or their interplay with other immune cells during opportunistic infections. Using our murine model of Pneumocystis pneumonia, we found that loss of NK cells during immunosuppression results in substantial Pneumocystis lung burden. During early infection of C57B/6 CD4(+) T cell-depleted mice, there were significantly fewer NK cells in the lung tissue compared with CD4(+) T cell-intact animals, and the NK cells present demonstrated decreased upregulation of the activation marker NKp46 and production of the effector cytokine, IFN-γ. Furthermore, coincubation studies revealed a significant increase in fungal killing when NK cells were combined with CD4(+) T cells compared with either cell alone, which was coincident with a significant increase in perforin production by NK cells. Finally, however, we found through adoptive transfer that memory CD4(+) T cells are required for significant NK cell upregulation of the activation marker NK group 2D and production of IFN-γ, granzyme B, and perforin during Pneumocystis infection. To the best of our knowledge, this study is the first to demonstrate a role for NK cells in immunity to Pneumocystis pneumonia, as well as to establish a functional relationship between CD4(+) T cells and NK cells in the host response to an opportunistic fungal pathogen.

Eimeria tenella: interleukin 17 contributes to host immunopathology in the gut during experimental infection

Although IL-17 is a key factor in Th17 lineage host responses and plays critical roles in immunological control of a variety of infectious diseases, the contribution of IL-17 to immune function during Eimeria tenella infection is unknown. In the present study, we used an experimental E. tenella infection model to clarify the role of Th17-associated response in the resulting immune response by quantitative real-time PCR assays. We observed robust production of STAT-3 (the transcription factors), IL-1β, IL-6 and IL-17 in cecal intraepithelial lymphocytes during the early infection, peaking at 6h p.i. and declining thereafter. The expression of TGF-β was moderately upregulated and had 2 peaks at 6 and 72h p.i. during the early infection. To further investigate the role of chIL-17 during the infection, we treated the infected chickens with IL-17 and its neutralized antibody. As a result, the reduced fecal oocyst shedding and cecal lesion scores, but enhanced body weight gains were observed in IL-17 neutralized chickens. The results of histopathology showed that the neutrophils recruitment diminished and the parasite burden in IL-17 neutralized chickens decreased. These results may be due to the significant decrease in the production of IL-17, IL-6 and TGF-β, but enhanced IL-12 and IFN-γ expression in IL-17 neutralized chickens. The converse results were shown in IL-17 treated infected-chickens in which chickens showed increased fecal oocyst shedding, exacerbated lesion scores, and reduced body weight gains. These results suggested that chicken IL-17 might mediate E. tenella - induced immunopathology during the infection.

Colonization by Pneumocystis jirovecii and its role in disease

Although the incidence of Pneumocystis pneumonia (PCP) has decreased since the introduction of combination antiretroviral therapy, it remains an important cause of disease in both HIV-infected and non-HIV-infected immunosuppressed populations. The epidemiology of PCP has shifted over the course of the HIV epidemic both from changes in HIV and PCP treatment and prevention and from changes in critical care medicine. Although less common in non-HIV-infected immunosuppressed patients, PCP is now more frequently seen due to the increasing numbers of organ transplants and development of novel immunotherapies. New diagnostic and treatment modalities are under investigation. The immune response is critical in preventing this disease but also results in lung damage, and future work may offer potential areas for vaccine development or immunomodulatory therapy. Colonization with Pneumocystis is an area of increasing clinical and research interest and may be important in development of lung diseases such as chronic obstructive pulmonary disease. In this review, we discuss current clinical and research topics in the study of Pneumocystis and highlight areas for future research.

Tc17 cells mediate vaccine immunity against lethal fungal pneumonia in immune deficient hosts lacking CD4+ T cells

Vaccines may help reduce the growing incidence of fungal infections in immune-suppressed patients. We have found that, even in the absence of CD4⁺ T-cell help, vaccine-induced CD8⁺ T cells persist and confer resistance against Blastomyces dermatitidis and Histoplasma capsulatum. Type 1 cytokines contribute to that resistance, but they also are dispensable. Although the role of T helper 17 cells in immunity to fungi is debated, IL-17 producing CD8⁺ T cells (Tc17 cells) have not been investigated. Here, we show that Tc17 cells are indispensable in antifungal vaccine immunity in hosts lacking CD4⁺ T cells. Tc17 cells are induced upon vaccination, recruited to the lung on pulmonary infection, and act non-redundantly in mediating protection in a manner that requires neutrophils. Tc17 cells did not influence type I immunity, nor did the lack of IL-12 signaling augment Tc17 cells, indicating a distinct lineage and function. IL-6 was required for Tc17 differentiation and immunity, but IL-1R1 and Dectin-1 signaling was unexpectedly dispensable. Tc17 cells expressed surface CXCR3 and CCR6, but only the latter was essential in recruitment to the lung. Although IL-17 producing T cells are believed to be short-lived, effector Tc17 cells expressed low levels of KLRG1 and high levels of the transcription factor TCF-1, predicting their long-term survival and stem-cell like behavior. Our work has implications for designing vaccines against fungal infections in immune suppressed patients.

Systemic fungal infections have emerged as a public health problem, especially for patients with suppressed immunity. At present, there are no vaccines against fungi, partly because it is hard to elicit strong immunity in immune suppressed patients. We have found however that residual elements of T cell immunity can be harnessed by vaccination even in immune suppressed hosts. We show here that immune suppressed mice lacking T helper cells can still be vaccinated successfully against lethal fungal pneumonia. A population of T cytotoxic IL-17-producing cells (Tc17 cells) is instrumental and indispensible in vaccine protection. We describe here mechanisms that explain how these cells are induced to mature in Tc17 cells, persist for long periods in the body providing “immune memory”, recruit to the site of infection, and clear the tissue of fungi. Our work sheds new light on potent T cells that can be harnessed by vaccine strategies against fungal infection in vulnerable patients.

Immunology of fungal infections: lessons learned from animal models

The continuing AIDS epidemic coupled with increased usage of immunosuppressive drugs to prevent organ rejection or treat autoimmune diseases has resulted in an increase in individuals at risk for acquiring fungal diseases. These concerns highlight the need to elucidate mechanisms of inducing protective immune responses against fungal pathogens. Consequently, several experimental models of human mycoses have been developed to study these diseases. The availability of transgenic animal models allows for in-depth analysis of specific components, receptors, and signaling pathways that elicit protection against fungal diseases. This review focuses on recent advances in our understanding of immune responses to fungal infections gained using animal models.

Glycosphingolipids mediate pneumocystis cell wall β-glucan activation of the IL-23/IL-17 axis in human dendritic cells

Pneumocystis species are opportunistic fungal organisms that cause severe pneumonia in immune-compromised hosts, with resultant high morbidity and mortality. Recent work indicates that IL-17 responses are important components of host defense against fungal pathogens. In the present study, we demonstrate that cell-surface β-glucan components of Pneumocystis (PCBG) stimulate human dendritic cells (DCs) to secrete IL-23 and IL-6. These cytokines are well established to stimulate a T helper-17 (Th17) phenotype. Accordingly, we further observe that PCBG-stimulated human DCs interact with lymphocytes to drive the secretion of IL-17 and IL-22, both Th17-produced cytokines. The activation of DCs was shown to involve the dectin-1 receptor with a downstream activation of the Syk kinase and subsequent translocation of both the canonical and noncanonical components of the NF-κB transcription factor family. Finally, we demonstrate that glycosphingolipid-rich microdomains of the plasma membrane participate in the activation of DCs by PCBG through the accumulation of lactosylceramide at the cell surface during stimulation with PCBG. These data strongly support the idea that the β-glucan surface components of Pneumocystis drive the activation of the IL-23/IL-17 axis during this infection, through a glycosphingolipid-initiated mechanism.

A simple method to quantitate IP-10 in dried blood and plasma spots

BACKGROUND

Antigen specific release of IP-10 is an established marker for infection with M.tuberculosis. Compared to IFN-γ, IP-10 is released in 100-fold higher concentrations enabling the development of novel assays for detection. Dried blood spots are a convenient sample for high throughput newborn screening.

AIM

To develop a robust and sensitive ELISA-based assay for IP-10 detection in plasma, dried blood spots (DBS) and dried plasma spots (DPS); to validate the ELISA in clinically relevant samples; and to assess the performance of the assay for detection of Cytomegalovirus (CMV) and M.tuberculosis specific immune responses.

METHOD

We raised mice and rat monoclonal antibodies against human IP-10 and developed an ELISA. The assay was validated and applied to the detection of CMV and M.tuberculosis specific responses in 18 patients with immune reactivity towards M.tuberculosis and 32 healthy controls of which 22 had immune reactivity towards CMV and none towards M.tuberculosis. We compared the performance of this new assay to IFN-γ.

RESULTS

The ELISA was reliable for IP-10 detection in both plasma and filter paper samples. The linear range of the ELISA was 2.5-600 pg/ml. IFN-γ was not readily detectable in DPS samples. IP-10 was stabile in filter paper samples for at least 4 weeks at 37 °C. The correlation between IP-10 detected in plasma, DPS and DBS samples was excellent (r(2)>0.97).

CONCLUSIONS

This newly developed assay is reliable for IP-10 quantification in plasma, DBS and DPS samples from antigen stimulated and non-stimulated whole blood. The filter paper assays enable easy sample acquisition and transport at ambient temperature e.g. via the postal system. The system can potentially simplify diagnostic assays for M.tuberculosis and CMV infection.

IP-10 release assays in the diagnosis of tuberculosis infection: current status and future directions

The current state-of-the-art tests for infection with Mycobacterium tuberculosis - the IFN-γ release assays - rely on accurate measurement of the cytokine IFN-γ. Many other potential biomarkers are expressed in concert with IFN-γ, and IP-10 in particular has shown promising results. IP-10 is produced in large amounts, allowing for the development of new and simplified test platforms, such as lateral flow. In this review, we summarize the results of 22 clinical studies exploring the use of IP-10 as an alternative marker to IFN-γ. The studies report that diagnostic accuracy of IP-10 is on par with IFN-γ, but also that IP-10 may be more robust in young children and in HIV-infected individuals with low CD4 cell counts. We conclude the review by presenting limitations of the published works and outline recent developments and future directions.

[HIV infection and chronic inflammation: is the bacterial translocation the underlying cause?]

Currently, non-AIDS comorbidities (cardiovascular disease, non-AIDS-related cancers, liver disease, osteoporosis, etc.) have become an important cause of morbimortality in patients with human immunodeficiency virus type 1 (HIV-1) infection. The elevation of plasma markers of inflammation has been associated with the development of cardiovascular disease and death from all causes. Therefore, there is great interest in elucidating the underlying causes responsible for this persistent inflammatory status. The intestinal barrier disruption associated with HIV-1 infection may favor the passage of gut microbial products into the blood, resulting in immune stimulation. In this article we review the pathogenesis of bacterial translocation and its relevance to HIV-1 infection.

Immunity to fungi

The global increase in fungal disease burden, the emergence of novel pathogenic fungi, and the lack of fungal vaccines have focused intense interest in elucidating immune defense mechanisms against fungi. Recent studies in animal models and in humans identify an integrated role for C-type lectin and Toll-like receptor signaling in activating innate and adaptive responses that control medically relevant fungi. Beyond the critical role of phagocytes in host defense, the generation and balance of specific T helper subsets contributes to sterilizing immunity. These advances form a basis for the development of fungal vaccines and immune-based therapeutic adjuncts.

Prostaglandin E2 suppresses antifungal immunity by inhibiting interferon regulatory factor 4 function and interleukin-17 expression in T cells

T helper 17 (Th17) cells play an important role in mucosal host defense through production of the signature cytokines IL-17 and IL-22. Prostaglandin E2 (PGE2) has been shown to enhance IL-17 production by mature Th17 cells. However, when present during Th17 cell differentiation, we found that PGE2 inhibited the transcription factor IRF4 and suppressed production of IL-17 but not IL-22. We show that IRF4 was required for IL-17 expression but inhibited IL-22 expression, highlighting the potential for discordant regulation of these two cytokines in Th17 cells. The pathogenic fungus Cryptococcus neoformans produces PGE2, and we found that it uses PGE2- and IRF4-dependent mechanisms to specifically inhibit induction of IL-17 during Th17 cell differentiation. Blockade of host PGE2 during infection led to increased IL-17 production from CD4(+) T cells and increased survival of mice. These findings suggest that host- or pathogen-derived PGE2 can act directly on Th17 cells during differentiation to inhibit IL-17-dependent antimicrobial responses.

Coexpression of CCR6 and CD146 (MCAM) is a marker of effector memory T-helper 17 cells

Effector memory T (T(EM)) cells are a subpopulation of memory T cells that express receptors mediating migration to inflamed tissues and produce various cytokines. Effector memory T-helper (Th)17 (Th17(EM)) cells are thought to be essential for inflammation in Th17-mediated diseases, but have not been studied in detail. To identify superior surface markers to isolate a homogeneous population of Th17(EM) cells from peripheral blood, CD4(+) T cells were isolated from the peripheral blood of healthy donors based on the expression of CCR7, CCR6 and CD146 using six-color flow cytometry. After 4days of culture in the presence of anti-CD3/28 beads, intracellular cytokines were determined by flow cytometric analysis. To investigate the relevance of Th17(EM) cells in Th17-mediated disease, the frequencies of T(EM) -cell subsets in psoriasis were quantified using six-color flow cytometry. An enzyme-linked immunosorbent assay was performed to confirm the interleukin (IL)-17-producing capacity of T(EM) -cell subsets from the peripheral blood of a patient with psoriasis. CCR6(+) CD146(+) T(EM) (CD4(+) CD45RA(-) CCR7(-)) cells had a greater capacity to produce IL-17 than CCR6(+) CD146(-) or CCR6(-) CD146(+) T(EM) cells. Although the percentage of CCR6(+) CD146(+) cells in T(EM) cells was not significantly different between patients with psoriasis and controls, three of eight patients had a higher percentage of CCR6(+) CD146(+) T(EM) cells than the mean +5 standard deviations of the controls. Coexpression of CCR6 and CD146 is a useful marker for Th17(EM) cells. Increasing the number of CCR6(+) CD146(+) Th17(EM) cells in peripheral blood may facilitate estimation of systemic Th17-cell activity in Th17-mediated diseases.

Current investigational drugs in psoriasis

INTRODUCTION

The advent of biologic therapies has revolutionized the treatment of psoriasis. Increased understanding of immunogenetic pathways has allowed for the development of more selective targeted biologic therapies. Multiple new treatments are currently in development for the treatment of psoriasis. Preliminary data for many of these agents, particularly with regard to agents targeting the IL-23/Th17 pathway, are promising. Proven long-term safety, however, is an absolute necessity with newly developed drugs, and should, therefore, still be considered second-line agents to current established treatments with long-term safety data.

AREAS COVERED

This review details the mechanisms of action of drugs currently in development or in clinical trials for the treatment of psoriasis, using clinical trial registries and associated publications. Readers will gain a comprehensive overview about the mechanism of action of emerging treatments targeting various immune pathways deeply involved in psoriasis. Pathogenesis, clinical efficacy and safety data for these treatments are discussed where available.

EXPERT OPINION

Psoriasis remains a heavily undertreated systemic immune-mediated disease despite increased understanding of immunopathogenesis of the disease and advent of a multitude of novel therapeutic agents with potentially improved bioavailability and safety profiles. Limitations, however, remain in the realm of topical agents for treatment of mild to moderate psoriasis, which has seen little progress over the years. A concerted effort will need to be made among researchers, clinicians and patient advocacy groups to ensure new therapeutic agents are developed and gain proper exposure.

Role of osteoclasts and interleukin-17 in the pathogenesis of rheumatoid arthritis: crucial 'human osteoclastology'

Many papers have reported that osteoclasts play an important role in the pathogenesis of rheumatoid arthritis (RA); however, when we started to investigate the pathogenesis of RA, the roles of osteoclasts were not highlighted in RA bone resorption. In recent years, the number of articles on the roles of osteoclasts and interleukin (IL)-17 in the pathogenesis of RA has increased exponentially. In this review article, we describe our articles on the roles of osteoclasts and IL-17 in joint destruction in RA, from 1990 to 2011, and highlight a novel term, 'human osteoclastology', which we have used since 2008.

IL-23 dampens the allergic response to Cryptococcus neoformans through IL-17-independent and -dependent mechanisms

The cytokines IL-23 and IL-17 have been implicated in resistance to cryptococcal disease, but it is not clear whether IL-23-mediated production of IL-17 promotes fungal containment following pulmonary challenge with Cryptococcus neoformans. We used mice lacking IL-23 (IL-23p19(-/-)) or IL-17RA (IL-17RA(-/-)), and wild type (WT) C57BL/6 mice to examine the IL-23/IL-17 axis after intranasal infection with the C. neoformans strain 52D. The absence of IL-23 or IL-17RA had no effect on pulmonary or brain fungal burden at 1 or 6 weeks after infection. However, survival of IL-23p19(-/-) mice was reduced compared to IL-17RA(-/-) mice. IL-I7 production by CD4 T cells and natural killer T (NKT) cells was impaired in IL-23p19(-/-) lungs, but was not completely abolished. Both IL-23p19(-/-) and IL-17RA(-/-) mice exhibited impaired neutrophil recruitment, increased serum levels of IgE and IgG2b, and increased deposition of YM1/YM2 crystals in the lung, but only IL-23p19(-/-) mice developed persistent lung eosinophilia. Although survival of IL-17RA(-/-) and WT mice was similar after 17 weeks of infection, only surviving IL-17RA(-/-) mice exhibited cryptococcal dissemination to the blood. These data demonstrate that IL-23 dampens the allergic response to cryptococcal infection through IL-17-independent suppression of eosinophil recruitment and IL-17-dependent regulation of antibody production and crystal deposition. Furthermore, IL-23, and to a lesser extent IL-17, contribute to disease resistance.

Adaptive immunity to fungi

Only a handful of the more than 100,000 fungal species on our planet cause disease in humans, yet the number of life-threatening fungal infections in patients has recently skyrocketed as a result of advances in medical care that often suppress immunity intensely. This emerging crisis has created pressing needs to clarify immune defense mechanisms against fungi, with the ultimate goal of therapeutic applications. Herein, we describe recent insights in understanding the mammalian immune defenses deployed against pathogenic fungi. The review focuses on adaptive immune responses to the major medically important fungi and emphasizes how dendritic cells and subsets in various anatomic compartments respond to fungi, recognize their molecular patterns, and signal responses that nurture and shape the differentiation of T cell subsets and B cells. Also emphasized is how the latter deploy effector and regulatory mechanisms that eliminate these nasty invaders while also constraining collateral damage to vital tissue.

Cytokines and the regulation of fungus-specific CD4 T cell differentiation

CD4 T cells play important and non-redundant roles in protection against infection with diverse fungi. Distinct CD4 T cell subsets can mediate protection against fungal disease where Th1 and Th17 CD4 T cell subsets have been found to promote fungal clearance and protective immunity against diverse fungal pathogens. The differentiation of naïve CD4 T cells into Th1 or Th17 cells is crucially controlled by their interaction with dendritic cells and instructed by cytokines. IL-12 and IFN-γ promote Th1 differentiation while TGF-β, IL-6, IL-1, IL-21 and IL-23 promote Th17 differentiation and maintenance. The production of these cytokines by DCs is in turn regulated by innate receptors triggered in response to fungal infection. In this review we will discuss the contributions of cytokines found to influence fungus-specific CD4 T cell differentiation and their role in defense against fungal disease. We will also highlight the contributions of innate receptors involved in recognition of fungi and how they shape cytokine secretion and CD4 T cell differentiation.

Case report of a young child with disseminated histoplasmosis and review of hyper immunoglobulin e syndrome (HIES)

Type 1 hyper IgE syndrome (HIES), also known as Job's Syndrome, is an autosomal dominant disorder due to defects in STAT3 signaling and Th17 differentiation. Symptoms may present during infancy but diagnosis is often made in childhood or later. HIES is characterized by immunologic and non-immunologic findings such as recurrent sinopulmonary infections, recurrent skin infections, multiple fractures, atopic dermatitis and characteristic facies. These manifestations are accompanied by elevated IgE levels and reduced IL-17 producing CD3+CD4+ T cells. Diagnosis in young children can be challenging as symptoms accumulate over time along with confounding clinical dilemmas. A NIH clinical HIES scoring system was developed in 1999, and a more recent scoring system with fewer but more pathogonomonic clinical findings was reported in 2010. These scoring systems can be used as tools to help in grading the likelihood of HIES diagnosis. We report a young child ultimately presenting with disseminated histoplasmosis and a novel STAT3 variant in the SH2 domain.

Targeting IL-23: insights into the pathogenesis and the treatment of psoriasis

Therapeutic experience strongly supports the use of TNF antagonists as important modalities in the treatment of psoriatic arthritis and plaque psoriasis. Studies with anti-IL-12/23 therapeutic agents, which act in different steps of the psoriatic inflammatory cascade, have also shown demonstrable efficacy. Here, we discuss this approach and its potential within the armamentarium for the treatment of psoriasis. Evidences that the selective blocking of IL-23 may be effective and safe therapy are also addressed.

The interleukin-17 cytokine family: critical players in host defence and inflammatory diseases

The interleukin-17 (IL-17) cytokines, IL-17A to IL-17F, are emerging as critical players in host defence responses and inflammatory diseases. Substantial data support the role of these proteins in innate and adaptive immunity. Of these family members, IL-17A, IL-17F and IL-17E have been the best studied. Both IL-17A and IL-17F contribute to the host response to extracellular bacteria and fungi, and IL-17E has been shown to play a role in parasitic infections. In addition, numerous pre-clinical and clinical studies link these proteins to the pathogenesis of inflammatory diseases, and a number of therapeutic programmes targeting these family members are in clinical development. This review will highlight the cellular sources, receptors/target cells, and role in inflammation of these and the less-characterized family members, IL-17B, IL-17C and IL-17D.

A role for TLR4 in Clostridium difficile infection and the recognition of surface layer proteins

Clostridium difficile is the etiological agent of antibiotic-associated diarrhoea (AAD) and pseudomembranous colitis in humans. The role of the surface layer proteins (SLPs) in this disease has not yet been fully explored. The aim of this study was to investigate a role for SLPs in the recognition of C. difficile and the subsequent activation of the immune system. Bone marrow derived dendritic cells (DCs) exposed to SLPs were assessed for production of inflammatory cytokines, expression of cell surface markers and their ability to generate T helper (Th) cell responses. DCs isolated from C3H/HeN and C3H/HeJ mice were used in order to examine whether SLPs are recognised by TLR4. The role of TLR4 in infection was examined in TLR4-deficient mice. SLPs induced maturation of DCs characterised by production of IL-12, TNFα and IL-10 and expression of MHC class II, CD40, CD80 and CD86. Furthermore, SLP-activated DCs generated Th cells producing IFNγ and IL-17. SLPs were unable to activate DCs isolated from TLR4-mutant C3H/HeJ mice and failed to induce a subsequent Th cell response. TLR4⁻/⁻ and Myd88⁻/⁻, but not TRIF⁻/⁻ mice were more susceptible than wild-type mice to C. difficile infection. Furthermore, SLPs activated NFκB, but not IRF3, downstream of TLR4. Our results indicate that SLPs isolated from C. difficile can activate innate and adaptive immunity and that these effects are mediated by TLR4, with TLR4 having a functional role in experimental C. difficile infection. This suggests an important role for SLPs in the recognition of C. difficile by the immune system.

CXCL10/IP-10 in infectious diseases pathogenesis and potential therapeutic implications

C-X-C motif chemokine 10 (CXCL10) also known as interferon γ-induced protein 10 kDa (IP-10) or small-inducible cytokine B10 is a cytokine belonging to the CXC chemokine family. CXCL10 binds CXCR3 receptor to induce chemotaxis, apoptosis, cell growth and angiostasis. Alterations in CXCL10 expression levels have been associated with inflammatory diseases including infectious diseases, immune dysfunction and tumor development. CXCL10 is also recognized as a biomarker that predicts severity of various diseases. A review of the emerging role of CXCL10 in pathogenesis of infectious diseases revealed diverse roles of CXCL10 in disease initiation and progression. The potential utilization of CXCL10 as a therapeutic target for infectious diseases is discussed.

Innate immune responses to systemic Acinetobacter baumannii infection in mice: neutrophils, but not interleukin-17, mediate host resistance

Acinetobacter baumannii is a nosocomial pathogen with a high prevalence of multiple-drug-resistant strains, causing pneumonia and sepsis. The current studies further develop a systemic mouse model of this infection and characterize selected innate immune responses to the organism. Five clinical isolates, with various degrees of antibiotic resistance, were assessed for virulence in two mouse strains, and between male and female mice, using intraperitoneal infection. A nearly 1,000-fold difference in virulence was found between bacterial strains, but no significant differences between sexes or mouse strains were observed. It was found that microbes disseminated rapidly from the peritoneal cavity to the lung and spleen, where they replicated. A persistent septic state was observed. The infection progressed rapidly, with mortality between 36 and 48 h. Depletion of neutrophils with antibody to Ly-6G decreased mean time to death and increased mortality. Interleukin-17 (IL-17) promotes the response of neutrophils by inducing production of the chemokine keratinocyte-derived chemoattractant (KC/CXCL1), the mouse homolog of human IL-8. Acinetobacter infection resulted in biphasic increases in both IL-17 and KC/CXCL1. Depletion of neither IL-17 nor KC/CXCL1, using specific antibodies, resulted in a difference in bacterial burdens in organs of infected mice at 10 h postinfection. Comparison of bacterial burdens between IL-17a(-/-) and wild-type mice confirmed that the absence of this cytokine did not sensitize mice to Acinetobacter infection. These studies definitely demonstrate the importance of neutrophils in resistance to systemic Acinetobacter infection. However, neither IL-17 nor KC/CXCL1 alone is required for effective host defense to systemic infection with this organism.

Risk of Pneumocystis jiroveci pneumonia in patients long after renal transplantation

BACKGROUND

Pneumocystis jiroveci pneumonia (PCP) is an important cause of morbidity and mortality in renal transplant recipients (RTRs). Chemoprophylaxis with trimethoprim/sulphamethoxazole is recommended during the early post-transplantation period, but the optimal duration has not been determined and a main drawback of chemoprophylaxis is the development of resistance of the commensal faecal flora. A cluster outbreak of PCP occurred in our outpatient Renal Transplant Unit. We aimed to investigate risk factors for PCP in RTRs to determine who should receive long-term chemoprophylaxis.

METHODS

In a case-control study, we investigated common demographic variables and immunological parameters. Nine PCP cases diagnosed between August 2006 and April 2007 were matched with 18 control patients, who did not develop PCP, received their transplant in the same time-period and had a similar follow-up period with a comparable immunosuppressive drug regimen.

RESULTS

The median time from transplantation to PCP was 19 months. We observed no significant differences in gender, age, donor type or number of rejections. In PCP cases, the median lymphocyte count just before PCP diagnosis was 0.49 (0.26-0.68), which was significantly reduced compared to the control patients after a similar follow-up period (median 1.36, 0.59-3.04, P = 0.002). This lymphocytopaenia was chronic and existed in most patients already for many months. CD4(+) T-cell counts were also significantly reduced in the PCP cases. We found no difference in the Th1, Th2 and Th17 subsets between PCP cases and control patients.

CONCLUSION

Long-term prophylactic therapy for PCP may be indicated for RTR with persistent severe lymphocytopaenia.

Role of IL-17A on resolution of pulmonary C. neoformans infection

The current studies evaluated the role of interleukin (IL)-17A in the induction of protective immunity against pulmonary cryptococcosis in mice. Protection against pulmonary infection with C. neoformans strain H99γ was associated with increased IL-17A production. Signaling through the IFN-γ receptor (R) was required for increased IL-17A production, however, a Th17-type cytokine profile was not observed. Neutrophils were found to be the predominant leukocytic source of IL-17A, rather than T cells, suggesting that the IL-17A produced was not part of a T cell-mediated Th17-type immune response. Depletion of IL-17A in mice during pulmonary infection with C. neoformans strain H99γ resulted in an initial increase in pulmonary fungal burden, but had no effect on cryptococcal burden at later time points. Also, depletion of IL-17A did not affect the local production of other cytokines. IL-17RA⁻/⁻ mice infected with C. neoformans strain H99γ survived the primary infection as well as a secondary challenge with wild-type cryptococci. However, dissemination of the wild-type strain to the brain was noted in the surviving IL-17RA⁻/⁻ mice. Altogether, our results suggested that IL-17A may be important for optimal protective immune responsiveness during pulmonary C. neoformans infection, but protective Th1-type immune responses are sufficient for protection against cryptococcal infection.

Early activation of the interleukin-23-17 axis in a murine model of oropharyngeal candidiasis

Candida albicans is an oral commensal yeast that causes oropharyngeal candidiasis (OPC) in immunocompromised individuals. The immunological pathways involved in OPC have been revisited after the interleukin-17 (IL-17) pathway was implicated in fungal immunity. We studied immediate (<24 h) and adaptive (3-6 day) IL-12 and IL-23-17 pathway activation in naive p40(-/-) mice, which lack IL-12 and IL-23 and develop severe, chronic OPC upon oral inoculation with C. albicans. Macrophages from p40(-/-) mice were less efficient than C57BL/6J controls at killing C. albicans in vitro but very low numbers in the oral mucosae of infected C57BL/6J mice suggest that they are not critical in vivo, at least in this strain. Migration of macrophages to regional lymph nodes of infected p40(-/-) mice was impaired; however, dendritic cell migration was not affected. Recombinant IL-12 therapy provided only temporary relief from OPC, suggesting that IL-23 is required for full protection. In C57BL/6J mice, but not p40(-/-) mice, messenger RNAs encoding IL-23p19 and IL-17 were induced in the oral mucosa within 24 h of infection (6 ± 0.6 and 12 ± 2.7-fold). By day 6 of infection in C57BL/6J mice, IL-17A messenger RNA level had increased 5.1 ± 1.8 and 83 ± 21-fold in regional lymph nodes and oral tissues respectively. Ablation of p40 was associated with delayed or abrogated induction of IL-17A pathway targets (monocyte chemoattractant protein-1, IL-6 and macrophage inflammatory protein-2), and a lack of organized recruitment of neutrophils to the infected oral mucosa. Overall our data show that the IL-23-17A axis is activated early in the oral mucosae of immunologically naive mice with OPC.

Vaccine-induced protection against 3 systemic mycoses endemic to North America requires Th17 cells in mice

Worldwide rates of systemic fungal infections, including three of the major pathogens responsible for such infections in North America (Coccidioides posadasii, Histoplasma capsulatum, and Blastomyces dermatitidis), have soared recently, spurring interest in developing vaccines. The development of Th1 cells is believed to be crucial for protective immunity against pathogenic fungi, whereas the role of Th17 cells is vigorously debated. In models of primary fungal infection, some studies have shown that Th17 cells mediate resistance, while others have shown that they promote disease pathology. Here, we have shown that Th1 immunity is dispensable and that fungus-specific Th17 cells are sufficient for vaccine-induced protection against lethal pulmonary infection with B. dermatitidis in mice. Further, vaccine-induced Th17 cells were necessary and sufficient to protect against the three major systemic mycoses in North America. Mechanistically, Th17 cells engendered protection by recruiting and activating neutrophils and macrophages to the alveolar space, while the induction of Th17 cells and acquisition of vaccine immunity unexpectedly required the adapter molecule Myd88 but not the fungal pathogen recognition receptor Dectin-1. These data suggest that human vaccines against systemic fungal infections should be designed to induce Th17 cells if they are to be effective.

Conserved natural IgM antibodies mediate innate and adaptive immunity against the opportunistic fungus Pneumocystis murina

Natural IgM antibodies in diverse species recognize conserved carbohydrates in fungal cell walls and influence early host defense against Pneumocystis in mice.

Host defense against opportunistic fungi requires coordination between innate and adaptive immunity for resolution of infection. Antibodies generated in mice vaccinated with the fungus Pneumocystis prevent growth of Pneumocystis organisms within the lungs, but the mechanisms whereby antibodies enhance antifungal host defense are poorly defined. Nearly all species of fungi contain the conserved carbohydrates β-glucan and chitin within their cell walls, which may be targets of innate and adaptive immunity. In this study, we show that natural IgM antibodies targeting these fungal cell wall carbohydrates are conserved across many species, including fish and mammals. Natural antibodies bind fungal organisms and enhance host defense against Pneumocystis in early stages of infection. IgM antibodies influence recognition of fungal antigen by dendritic cells, increasing their migration to draining pulmonary lymph nodes. IgM antibodies are required for adaptive T helper type 2 (Th2) and Th17 cell differentiation and guide B cell isotype class-switch recombination during host defense against Pneumocystis. These experiments suggest a novel role for the IgM isotype in shaping the earliest steps in recognition and clearance of this fungus. We outline a mechanism whereby serum IgM, containing ancient specificities against conserved fungal antigens, bridges innate and adaptive immunity against fungal organisms.

T-cell Subsets and Antifungal Host Defenses

It has been long appreciated that protective immunity against fungal pathogens is dependent on activation of cellular adaptive immune responses represented by T lymphocytes. The T-helper (Th)1/Th2 paradigm has proven to be essential for the understanding of protective adaptive host responses. Studies that have examined the significance of regulatory T cells in fungal infection, and the recent discovery of a new T-helper subset called Th17 have provided crucial information for understanding the complementary roles played by the various T-helper lymphocytes in systemic versus mucosal antifungal host defense. This review provides an overview of the role of the various T-cell subsets during fungal infections and the reciprocal regulation between the T-cell subsets contributing to the tailored host response against fungal pathogens.

The role of Th17 cytokines in primary mucosal immunity

The T helper type 17 (Th17) lineage of CD4+ T-cells produce several effector molecules including IL-17A, IL-17F, IL-21, and IL-22. In addition to CD4+, αβ T-cells, these cytokines can be produced by natural killer and γδ T-cells. These effector cytokines can be produced rapidly upon infection at mucosal sites and evidence to date strongly implicates that this arm of the immune system plays a critical role in mucosal immunity to many extracellular pathogens. Moreover these cytokines can also coordinate adaptive immunity to some intracellular pathogens. In this review, we will highlight recent progress in our understanding of these cytokines, and mechanisms of their effector function in the mucosa.

C-type lectins, fungi and Th17 responses

Th17 cells are a recently discovered subset of T helper cells characterised by the release of IL-17, and are thought to be important for mobilization of immune responses against microbial pathogens, but which also contribute to the development of autoimmune diseases. The identification of C-type lectin receptors which are capable of regulating the balance between Th1 and Th17 responses has been of particular recent interest, which they control, in part, though the release of Th17 inducing cytokines. Many of these receptors recognise fungi, and other pathogens, and play key roles in driving the development of protective anti-microbial immunity. Here we will review the C-type lectins that have been linked to Th17 type responses and will briefly examine the role of Th17 responses in murine and human anti-fungal immunity.

IL-17 family member cytokines: regulation and function in innate immunity

Recently, the IL-17 family member cytokines have become prominent subjects of investigation. IL-17 (IL-17A) is the best-described member of this family where its production has been mainly attributed to a specialized T helper subset of the adaptive immune response termed Th17. However, recent research on this and other Th17 cytokines has revealed new sources and functions of IL-17 family members in the innate immune response. This review will highlight recent advances in the field of IL-17 family member cytokines and will predominantly focus on the innate regulation and function of IL-17, IL-17F, and IL-25.

Generation of IL-23 producing dendritic cells (DCs) by airborne fungi regulates fungal pathogenicity via the induction of T(H)-17 responses

Interleukin-17 (IL-17) producing T helper cells (T(H)-17) comprise a newly recognized T cell subset with an emerging role in adaptive immunity to a variety of fungi. Whether different airborne fungi trigger a common signaling pathway for T(H)-17 induction, and whether this ability is related to the inherent pathogenic behavior of each fungus is currently unknown. Here we show that, as opposed to primary pathogenic fungi (Histoplasma capsulatum), opportunistic fungal pathogens (Aspergillus and Rhizopus) trigger a common innate sensing pathway in human dendritic cells (DCs) that results in robust production of IL-23 and drives T(H)-17 responses. This response requires activation of dectin-1 by the fungal cell wall polysaccharide b-glucan that is selectively exposed during the invasive growth of opportunistic fungi. Notably, unmasking of b-glucan in the cell wall of a mutant of Histoplasma not only abrogates the pathogenicity of this fungus, but also triggers the induction of IL-23 producing DCs. Thus, b-glucan exposure in the fungal cell wall is essential for the induction of IL-23/T(H)-17 axis and may represent a key factor that regulates protective immunity to opportunistic but not pathogenic fungi.

Interleukin-17-mediated control of parasitemia in experimental Trypanosoma congolense infection in mice

BALB/c mice are highly susceptible to experimental Trypanosoma congolense infections, whereas C57BL/6 mice are relatively resistant. Infected highly susceptible BALB/c mice die of systemic inflammatory response syndrome. Because interleukin-17 (IL-17) and Th17 cells regulate inflammatory responses, we investigated their role in the pathogenesis of experimental African trypanosomiasis in mice. We show that the production of IL-17 by spleen and liver cells and the serum IL-17 level increased after T. congolense infection in mice. Interestingly, infected highly susceptible BALB/c mice produced more IL-17 and had more Th17 cells than infected relatively resistant C57BL/6 mice. Paradoxically, neutralization of IL-17 with anti-IL-17 monoclonal antibody in vivo induced higher parasitemia in both the susceptible and the relatively resistant mice. Interestingly, anti-IL-17 antibody-treated mice had higher serum levels of alanine aminotransferase and aspartate aminotransferase, and the production of IL-10 and nitric oxide by liver cells was markedly decreased. Moreover, recombinant IL-17-treated mice exhibited significantly faster parasite control and lower peak parasitemia compared to control mice. Collectively, these results suggest that the IL-17/Th17 axis plays a protective role in murine experimental African trypanosomiasis.

Th17 cell dynamics in HIV infection

PURPOSE OF REVIEW

Th17 cells are a newly identified subtype of CD4 T cells that respond to bacterial and fungal antigens and are important in mucosal immunology. Because HIV infection results in loss of CD4 T cells as well as disruption to the gastrointestinal tract that causes microbial translocation and immune activation, Th17 cells potentially play an important role in HIV pathogenesis. Here we examine the relationship between Th17 cells and HIV disease pathogenesis.

RECENT FINDINGS

Th17 cells are preferentially lost from the gastrointestinal tract of HIV-infected individuals, which is not entirely due to direct infection, as Th17 cells can be infected in vivo, but are not preferentially infected. Long-term highly active antiretroviral therapy (HAART) can result in restoration of Th17 cells in the gastrointestinal, which may be associated with better disease prognosis. Furthermore, other cells, such as Vdelta1 T cells, can make IL-17 in vivo during HIV infection and may contribute to antibacterial immunity after loss of Th17 cells.

SUMMARY

Recent studies have improved our understanding of the role for Th17 cells during HIV infection; however, more studies are needed to discern better the detrimental consequences of loss of Th17 cells during HIV infection.

Th17 cells, Job's syndrome and HIV: opportunities for bacterial and fungal infections

PURPOSE OF REVIEW

Patients with hyper IgE syndrome (HIES) share with HIV patients a predisposition to infections, including candidiasis in autosomal dominant HIES (AD-HIES) and molluscum contagiosum and other viral infections in other disorders of elevated IgE with infectious predilections. This review highlights the underlying pathogenesis of these diseases and their relevance to HIV infection.

RECENT FINDINGS

Patients with mutations in STAT3, who lack Th17 cells, develop AD-HIES, whereas other disorders of elevated IgE may be caused by mutations in Tyk2 or DOCK8, the latter of which is associated with decreased expansion of CD8 more so than CD4 T cells. Recent studies on patients with recurrent mucocutaneous candidiasis have led to the discovery of mutations in CARD9 and DECTIN-1, genes key to the production of the Th17-driving cytokines IL-1beta, IL-6, and IL-23. Studies of the peripheral blood of HIV-positive patients have shown a decreased Th17:Th1 ratio, and Th17 cells were preferentially depleted from the gastrointestinal tract within weeks of simian immunodeficiency virus infection in rhesus macaques.

SUMMARY

The consequences of inadequate Th17 production in primary immunodeficiency syndromes illustrate the role of Th17 cells in controlling pathogens to which HIV-positive individuals are susceptible. Further understanding of the pathogenesis of opportunistic disease in HIV infection will probably require exploring the role of Th17 cells.

Th17 cells, HIV and the gut mucosal barrier

PURPOSE OF REVIEW

We will present recent studies on a subset of CD4 T helper cells, Th17 cells, that appears to be critical for regulating gut mucosal immune responses against extracellular microbial pathogens and may serve as a link between innate and adaptive immune responses. Implications of the loss of Th17 CD4 T cells in HIV infection will be discussed in relation to the chronic immune activation and HIV pathogenesis.

RECENT FINDINGS

Severe depletion of CD4 T cells occurs in the gut mucosa during primary HIV and simian immunodeficiency virus infections. A pronounced loss of mucosal Th17 CD4 T cells in the simian immunodeficiency virus-infected rhesus macaque model of AIDS is linked to impaired immune responses in the gut mucosa to an enteric pathogen, Salmonella typhimurium, leading to the lack of local control of the pathogen and its translocation. Recovery of the gut mucosal immune system during highly active antiretroviral therapy is slow and incomplete compared with the peripheral blood compartment. Recent studies suggest that the replenishment of Th17 CD4 T cells in the gut mucosa during highly active antiretroviral therapy, or during nonpathogenic simian immunodeficiency virus infections in the nonhuman primate models, correlates with better restoration and function of the gut mucosal immune system.

SUMMARY

A better understanding of the role of Th17 CD4 cells in the generation of mucosal immune responses to enteric pathogens and maintenance of the intestinal epithelial integrity in HIV-infected patients will help in the development of novel strategies to modulate and enhance mucosal immune system and its function.

Endogenous PGE2 promotes the induction of human Th17 responses by fungal ß-glucan

The interaction of PAMPs with cells of the innate immune system shapes the adaptive host response. Here, we report that β-glucan, a major fungal PAMP purified from Candida albicans, stimulates human DCs to secrete a pro-Th17 cytokine pattern. Notably, β-glucan induces PGE2 production, which has been shown to play a pivotal role in Th17 cell expansion. Inhibition of PGE2 synthesis or blockade of PGE2 receptors EP2 and EP4 drastically reduces IL-23 production by β-glucan-activated DCs, suggesting that endogenous PGE2 amplifies IL-23 synthesis in response to the C. albicans PAMP. Moreover β-glucan promotes the expansion of Th17 cells, which is strongly decreased by EP2 and EP4 receptor blockade on DCs. Our results highlight a novel role for PGE2 in the regulation of innate and adaptive immune response triggered by recognition of a prominent, highly conserved fungal PAMP such as β-glucan.

Cytokine networks in the infected lung

The generation of an innate immune response is essential for rapid clearance of microbes from the respiratory tract, whereas acquired immunity is required for the generation of cellular immunity necessary for the killing of certain intracellular pathogens and the development of immunological memory. Cytokines play an integral role in host defense by serving as leukocyte chemoattractants, leukocyte-activating factors or afferent signals in the induction or regulation of other effector molecules. This review assesses the contribution of cytokine networks to the generation of antimicrobial host defenses in the lung, with an emphasis on cytokines/cytokine networks that are instrumental in innate antibacterial responses, including mucosal immunity, and also introduces networks that instruct the development of adaptive immunity.