Role of interferon-gamma in Valpha14+ natural killer T cell-mediated host defense against Streptococcus pneumoniae infection in murine lungs

CD4(+) T-cell subsets and host defense in the lung

CD4(+) T-helper subsets are lineages of T cells that have effector function in the lung and control critical aspects of lung immunity. Depletion of these cells experimentally or by drugs or human immunodeficiency virus (HIV) infection in humans leads to the development of opportunistic infections as well as increased rates of bacteremia with certain bacterial pneumonias. Recently, it has been proposed that CD4(+) T-cell subsets may also be excellent targets for mucosal vaccination to prevent pulmonary infections in susceptible hosts. Here, we review recent findings that increase our understanding of T-cell subsets and their effector cytokines in the context of pulmonary infection.

Immunology of tuberculosis

Various T cells and macrophages as well as cytokines are involved in the immunopathogenesis of tuberculosis (TB). A better understanding of immunology of TB can not only lead to the discovery of new immunodiagnostic tools, accelerate and facilitate the assessment of new therapeutic methods, but also find new treatment regimens. In this highlight topic we cover the latest developments in the role of T cells, macrophages, Natural killer (NK) cells, invariant NK T (iNKT) cells and γδ T cells with TB infection. Histologically, TB displays exudative inflammation, proliferative inflammation and productive inflammation depending on the time course. T cells first recognize antigen within the mycobacterially-infected lung, and then activate, differentiate, but the first T cell activation occurs in the draining lymph nodes of the lung. When protective T cells reach sufficient numbers, they can stop bacterial growth. Except for T cells, neutrophils also participate actively in defense against early-phase TB. NK cells are innate lymphocytes which are a first line of defense against mycobacterial infection. Human NK cells use the NKp46, NCRs and NKG2D receptors to lyse Mycobacterium TB-infected monocytes and alveolar macrophages. NK cells produce not only interferon-γ, but also interleukin (IL)-22, which is induced by IL-15 and DAP-10. iNKT cells show different phenotypes and functions. Many iNKT cells are CD4+, few iNKT cells are CD8+, while an additional fraction of iNKT cells are negative for both CD4 and CD8. γδ T cells represent an early innate defense in antimycobacterial immunity. Studies done in humans and animal models have demonstrated complex patterns of γδ T cell immune responses during chronic TB. Human alveolar macrophages and monocytes can serve as antigen presentation cells for γδ T cells. Furthermore, the predominance of Vγ9Vδ2 T cells in TB has been confirmed.

Mouse Hobit is a homolog of the transcriptional repressor Blimp-1 that regulates NKT cell effector differentiation

The transcriptional repressor Blimp-1 mediates the terminal differentiation of many cell types, including T cells. Here we identified Hobit (Znf683) as a previously unrecognized homolog of Blimp-1 that was specifically expressed in mouse natural killer T cells (NKT cells). Through studies of Hobit-deficient mice, we found that Hobit was essential for the formation of mature thymic NKT cells. In the periphery, Hobit repressed the accumulation of interferon-γ (IFN-γ)-producing NK1.1(lo) NKT cells at steady state. After antigenic stimulation, Hobit repressed IFN-γ expression, whereas after innate stimulation, Hobit induced granzyme B expression. Thus, reminiscent of the function of Blimp-1 in other lymphocytes, Hobit controlled the maintenance of quiescent, fully differentiated NKT cells and regulated their immediate effector functions.

HVEM signalling at mucosal barriers provides host defence against pathogenic bacteria

The herpes virus entry mediator (HVEM), a member of the tumour-necrosis factor receptor family, has diverse functions, augmenting or inhibiting the immune response. HVEM was recently reported as a colitis risk locus in patients, and in a mouse model of colitis we demonstrated an anti-inflammatory role for HVEM, but its mechanism of action in the mucosal immune system was unknown. Here we report an important role for epithelial HVEM in innate mucosal defence against pathogenic bacteria. HVEM enhances immune responses by NF-κB-inducing kinase-dependent Stat3 activation, which promotes the epithelial expression of genes important for immunity. During intestinal Citrobacter rodentium infection, a mouse model for enteropathogenic Escherichia coli infection, Hvem−/− mice showed decreased Stat3 activation, impaired responses in the colon, higher bacterial burdens and increased mortality. We identified the immunoglobulin superfamily molecule CD160 (refs 7 and 8), expressed predominantly by innate-like intraepithelial lymphocytes, as the ligand engaging epithelial HVEM for host protection. Likewise, in pulmonary Streptococcus pneumoniae infection, HVEM is also required for host defence. Our results pinpoint HVEM as an important orchestrator of mucosal immunity, integrating signals from innate lymphocytes to induce optimal epithelial Stat3 activation, which indicates that targeting HVEM with agonists could improve host defence.

Key role for respiratory CD103(+) dendritic cells, IFN-γ, and IL-17 in protection against Streptococcus pneumoniae infection in response to α-galactosylceramide

BACKGROUND

Exogenous activation of pulmonary invariant natural killer T (iNKT) cells, a population of lipid-reactive αβ T lymphocytes, with use of mucosal α-galactosylceramide (α-GalCer) administration, is a promising approach to control respiratory bacterial infections. We undertook the present study to characterize mechanisms leading to α-GalCer-mediated protection against lethal infection with Streptococcus pneumoniae serotype 1, a major respiratory pathogen in humans.

METHODS AND RESULTS

α-GalCer was administered by the intranasal route before infection with S. pneumoniae. We showed that respiratory dendritic cells (DCs), most likely the CD103(+) subset, play a major role in the activation (IFN-γ and IL-17 release) of pulmonary iNKT cells, whereas alveolar and interstitial macrophages are minor players. After challenge, S. pneumoniae was rapidly (4 hours) eliminated in the alveolar spaces, a phenomenon that depended on respiratory DCs and neutrophils, but not macrophages, and on the early production of both IFN-γ and IL-17. Protection was also associated with the synthesis of various interferon-dependent and IL-17-associated genes as revealed by transcriptomic analysis.

CONCLUSIONS

These data imply a new function for pulmonary CD103(+) DCs in mucosal activation of iNKT cells and establish a critical role for both IFN-γ and IL-17 signalling pathways in mediating the innate immune response to S. pneumoniae.

Interplay between carbohydrate and lipid in recognition of glycolipid antigens by natural killer T cells

Natural killer T (NKT) cells are a T cell subpopulation that were named originally based on coexpression of receptors found on natural killer (NK) cells, cells of the innate immune system, and by T lymphocytes. The maturation and activation of NKT cells requires presentation of glycolipid antigens by CD1d, a cell surface protein distantly related to the major histocompatibility complex (MHC)-encoded antigen presenting molecules. This specificity distinguishes NKT cells from most CD4(+) and CD8(+) T cells that recognize peptides presented by MHC class I and class II molecules. The rapid secretion of a large amount of both Th1 and Th2 cytokines by activated NKT cells endows them with the ability to play a vital role in the host immune defense against various microbial infections. In this review, we summarize progress on identifying the sources of microbe-derived glycolipid antigens recognized by NKT cells and the biochemical basis for their recognition.

Invariant natural killer T cells recognize glycolipids from pathogenic Gram-positive bacteria

Natural killer T cells (NKT cells) recognize glycolipid antigens presented by CD1d. These cells express an evolutionarily conserved, invariant T cell antigen receptor (TCR), but the forces that drive TCR conservation have remained uncertain. Here we show that NKT cells recognized diacylglycerol-containing glycolipids from Streptococcus pneumoniae, the leading cause of community-acquired pneumonia, and group B Streptococcus, which causes neonatal sepsis and meningitis. Furthermore, CD1d-dependent responses by NKT cells were required for activation and host protection. The glycolipid response was dependent on vaccenic acid, which is present in low concentrations in mammalian cells. Our results show how microbial lipids position the sugar for recognition by the invariant TCR and, most notably, extend the range of microbes recognized by this conserved TCR to several clinically important bacteria.

Human monocytes promote Th1 and Th17 responses to Streptococcus pneumoniae

Streptococcus pneumoniae is a leading cause of bacterial pneumonia, meningitis, and sepsis in children. Human immunity to pneumococcal infections has been assumed to depend on anticapsular antibodies. However, recent findings from murine models suggest that alternative mechanisms, dependent on T helper cells, are also involved. Although the immunological events in which T helper cells contribute to acquired immunity have been studied in mice, little is known about how these responses are generated in humans. Therefore, we examined bacterial and host factors involved in the induction of Th1 and Th17 responses, using a coculture model of human monocytes and CD4(+) T cells. We show that monocytes promote effector cytokine production by memory T helper cells, leading to a mixed Th1/Th17 (gamma interferon [IFN-γ]/interleukin-17 [IL-17]) profile. Both T helper cytokines were triggered by purified pneumococcal peptidoglycan; however, the balance between the two immune effector arms depended on bacterial viability. Accordingly, live pneumococci triggered a Th1-biased response via monocyte production of IL-12p40, whereas heat-killed pneumococci triggered a Th17 response through TLR2 signaling. An increased understanding of human T helper responses is essential for the development of novel pneumococcal vaccines designed to elicit cell-mediated immunity.

Innate and cytokine-driven signals, rather than microbial antigens, dominate in natural killer T cell activation during microbial infection

Invariant natural killer T cells (iNKT cells) are critical for host defense against a variety of microbial pathogens. However, the central question of how iNKT cells are activated by microbes has not been fully explained. The example of adaptive MHC-restricted T cells, studies using synthetic pharmacological α-galactosylceramides, and the recent discovery of microbial iNKT cell ligands have all suggested that recognition of foreign lipid antigens is the main driver for iNKT cell activation during infection. However, when we compared the role of microbial antigens versus innate cytokine-driven mechanisms, we found that iNKT cell interferon-γ production after in vitro stimulation or infection with diverse bacteria overwhelmingly depended on toll-like receptor-driven IL-12. Importantly, activation of iNKT cells in vivo during infection with Sphingomonas yanoikuyae or Streptococcus pneumoniae, pathogens which are known to express iNKT cell antigens and which require iNKT cells for effective protection, also predominantly depended on IL-12. Constitutive expression of high levels of IL-12 receptor by iNKT cells enabled instant IL-12-induced STAT4 activation, demonstrating that among T cells, iNKT cells are uniquely equipped for immediate, cytokine-driven activation. These findings reveal that innate and cytokine-driven signals, rather than cognate microbial antigen, dominate in iNKT cell activation during microbial infections.

A case report of a patient with rheumatoid arthritis complicated with Mycobacterium avium during tocilizumab treatment

A female patient with rheumatoid arthritis (RA) suffered from Mycobacterium avium (M. avium) infection during tocilizumab treatment. Tocilizumab was discontinued and she was treated with a recommended chemotherapy, resulting in improvement of M. avium. Tocilizumab retreatment did not aggravate M. avium infection, and radiographic abnormalities improved over 1 year after cessation of the recommended therapy. Tocilizumab may be one candidate for intractable RA patients with M. avium if any biologic is required.

Invariant NK T cells: potential for immunotherapeutic targeting with glycolipid antigens

Invariant NK T (iNKT) cells are a subset of T lymphocytes that recognize glycolipid antigens bound with the antigen-presenting molecule CD1d. iNKT cells have potent immunoregulatory activities that can promote or suppress immune responses during different pathological conditions. These immunoregulatory properties can be harnessed for therapeutic purposes with cognate glycolipid antigens, such as the marine sponge-derived glycosphingolipid α-galactosylceramide. Preclinical studies have shown substantial promise for iNKT cell-based treatments of infections, cancer and autoimmune and inflammatory diseases. Translation of these preclinical studies to the clinic, while faced with some obstacles, has already had some initial success. In this article, we review the immunodulatory activities of iNKT cells and the potential for developing iNKT cell-based prophylactic and curative therapies of human disease.

Activation of pulmonary invariant NKT cells leads to exacerbation of acute lung injury caused by LPS through local production of IFN-γ and TNF-α by Gr-1+ monocytes

Invariant NK T (iNKT) cells are known to play a critical role in the regulation of inflammatory responses in various clinical settings. In the present study, we assessed the contribution of iNKT cells to the development of acute lung injury (ALI), which was caused by intra-tracheal administration of LPS. Jα18 gene-disrupted mice lacking these cells underwent neutrophilic inflammatory responses in lungs at an equivalent level as control mice. Next, mice were sensitized intra-tracheally with α-galactosylceramide, an activator of iNKT cells, followed by challenge with LPS. In this model, mice showed severe lung injury, and all mice were killed within 72 h after LPS injection. IFN-γ and tumor necrosis factor (TNF)-α were strikingly elevated in the lungs of these mice. Administration of neutralizing mAb against IFN-γ and TNF-α attenuated lung injury in a histopathological analysis and improved their survival rate. Flow cytometric analysis revealed that IFN-γ was expressed in NK cells, iNKT cells and also Gr-1(dull+)Ly-6C(+) monocytes and TNF-α was detected mainly in Gr-1(bright+)Ly-6G(+) neutrophils and Gr-1(dull+)Ly-6C(+) monocytes. Otherwise, in mice treated with LPS alone, IFN-γ was not detected in the lungs and Gr-1(bright+)Ly-6G(+) neutrophil was a main cellular source of TNF-α production. Anti-Gr-1 mAb resulted in the attenuation of ALI and decrease in the level of these cytokines. These results indicated that activation of iNKT cells led to striking exacerbation of ALI caused by LPS and that Gr-1(+) monocytes were recruited in the lungs with expressing IFN-γ and TNF-α and played an important role in the development of these responses.

Pneumolysin activates the NLRP3 inflammasome and promotes proinflammatory cytokines independently of TLR4

Pneumolysin (PLY) is a key Streptococcus pneumoniae virulence factor and potential candidate for inclusion in pneumococcal subunit vaccines. Dendritic cells (DC) play a key role in the initiation and instruction of adaptive immunity, but the effects of PLY on DC have not been widely investigated. Endotoxin-free PLY enhanced costimulatory molecule expression on DC but did not induce cytokine secretion. These effects have functional significance as adoptive transfer of DC exposed to PLY and antigen resulted in stronger antigen-specific T cell proliferation than transfer of DC exposed to antigen alone. PLY synergized with TLR agonists to enhance secretion of the proinflammatory cytokines IL-12, IL-23, IL-6, IL-1β, IL-1α and TNF-α by DC and enhanced cytokines including IL-17A and IFN-γ by splenocytes. PLY-induced DC maturation and cytokine secretion by DC and splenocytes was TLR4-independent. Both IL-17A and IFN-γ are required for protective immunity to pneumococcal infection and intranasal infection of mice with PLY-deficient pneumococci induced significantly less IFN-γ and IL-17A in the lungs compared to infection with wild-type bacteria. IL-1β plays a key role in promoting IL-17A and was previously shown to mediate protection against pneumococcal infection. The enhancement of IL-1β secretion by whole live S. pneumoniae and by PLY in DC required NLRP3, identifying PLY as a novel NLRP3 inflammasome activator. Furthermore, NLRP3 was required for protective immunity against respiratory infection with S. pneumoniae. These results add significantly to our understanding of the interactions between PLY and the immune system.

Natural killer T cells: innate lymphocytes positioned as a bridge between acute and chronic inflammation?

Natural killer T cells are an innate population of T lymphocytes that recognize antigens derived from host lipids and glycolipids. In this review, we focus on how these unique T cells are positioned to influence both acute and chronic inflammatory processes through their early recruitment to sites of inflammation, interactions with myeloid antigen presenting cells, and recognition of lipids associated with inflammation.

Alpha-GalCer ameliorates listeriosis by accelerating infiltration of Gr-1+ cells into the liver

Alpha-galactosylceramide (alpha-GalCer) activates invariant (i)NKT cells, which in turn stimulate immunocompetent cells. Although activation of iNKT cells appears critical for regulation of immune responses, it remains elusive whether protection against intracellular bacteria can be induced by alpha-GalCer. Here, we show that alpha-GalCer treatment ameliorates murine listeriosis, and inhibits inflammation following Listeria monocytogenes infection. Liver infiltration of Gr-1+ cells and gamma/delta T cells was accelerated by alpha-GalCer treatment. Gr-1+ cell and gamma/delta T-cell depletion exacerbated listeriosis in alpha-GalCer-treated mice, and this effect was more pronounced after depletion of Gr-1+ cells than that of gamma/delta T cells. Although GM-CSF and IL-17 were secreted by NKT cells after alpha-GalCer treatment, liver infiltration of Gr-1+ cells was not prevented by neutralizing mAb. In parallel to the numerical increase of CD11b+Gr-1+ cells in the liver following alpha-GalCer treatment, CD11b-Gr-1+ cells were numerically reduced in the bone marrow. In addition, respiratory burst in Gr-1+ cells was enhanced by alpha-GalCer treatment. Our results indicate that alpha-GalCer-induced antibacterial immunity is caused, in part, by accelerated infiltration of Gr-1+ cells and to a lesser degree of gamma/delta T cells into the liver. We also suggest that the infiltration of Gr-1+ cells is caused by an accelerated supply from the bone marrow.

Early production of tumor necrosis factor-alpha by Gr-1 cells and its role in the host defense to pneumococcal infection in lungs

In this study, we elucidated the role of tumor necrosis factor (TNF)-alpha in the host defense to pulmonary infection with Streptococcus pneumoniae and defined the cellular source of this cytokine at an early stage of infection. Administration of anti-TNF-alpha monoclonal antibody (mAb) resulted in the reduced accumulation of neutrophils in bronchoalveolar lavage fluids (BALFs) and severe exacerbation of this infection. In a flow cytometric analysis, the intracellular expression of TNF-alpha was detected in Gr-1(bright+) and Gr-1(dull+) cells during the time intervals postinfection, and F4/80(+) cells expressed intracellular TNF-alpha before Gr-1(dull+) cells appeared. The Gr-1(bright+) and Gr-1(dull+) cells sorted from BALF cells at 24 h were identified as neutrophils and macrophage-like cells, respectively, and the Gr-1(dull+) cells expressing CD11c, partially CD11b and a marginal level of F4/80 secreted TNF-alpha in in vitro cultures. Finally, deletion of Gr-1(+) cells by administration of the specific mAb significantly reduced the concentrations of this cytokine in BALF at 6 and 12 h postinfection, but not the expression of TNF-alpha in F4/80(+) cells. Thus, these results demonstrated that neutrophils, F4/80(+) macrophages and Gr-1(dull+) CD11c(+) macrophage-like cells played an important role in the production of TNF-alpha in lungs at an early stage of infection with S. pneumoniae.

Rapid and reliable generation of invariant natural killer T-cell lines in vitro

Several tools have proved useful in the study of invariant natural killer T (iNKT) cells, including CD1d-deficient mice, J alpha281-deficient mice, synthetic lipid antigens and antigen-loaded CD1d tetramers. However, the generation and examination of long-term primary murine iNKT cell lines in vitro has been challenging. Here, we show the rapid generation of iNKT cell lines from splenic iNKT cells of V alpha14 T-cell receptor (TCR) transgenic (Tg) mice. These purified iNKT cells were stimulated by bone marrow-derived dendritic cells (BMDCs) loaded with alpha-galactosylceramide (alphaGalCer) and cultured with interleukin (IL)-2 and IL-7. iNKT cells proliferated dramatically, and the cell number exhibited a 100-fold increase within 2 weeks and a 10(5)-fold increase in 8 weeks after repeated stimulation with alphaGalCer. The iNKT cell lines consisted of iNKT cells expressing V beta chains including V beta8.1/8.2, V beta14, V beta10, V beta6 and V beta7, and responded to stimulation with alphaGalCer presented both by BMDCs and by plate-bound CD1d. In addition, the iNKT cell lines produced interferon (IFN)-gamma when activated by lipopolysaccharide (LPS) or CpG oligodeoxynucleotide (ODN)-stimulated BMDCs. Further, we show that iNKT cell lines produced cytokines in response to microbial antigens. In summary, high-yield iNKT cell lines were generated very rapidly and robustly expanded, and these iNKT cells responded to both TCR and cytokine stimulation in vitro. Given the desire to study primary iNKT cells for many purposes, these iNKT cell lines should provide an important tool for the study of iNKT cell subsets, antigen and TCR specificity, activation, inactivation and effector functions.

Innate invariant NKT cells recognize Mycobacterium tuberculosis-infected macrophages, produce interferon-gamma, and kill intracellular bacteria

Cellular immunity to Mycobacterium tuberculosis (Mtb) requires a coordinated response between the innate and adaptive arms of the immune system, resulting in a type 1 cytokine response, which is associated with control of infection. The contribution of innate lymphocytes to immunity against Mtb remains controversial. We established an in vitro system to study this question. Interferon-γ is produced when splenocytes from uninfected mice are cultured with Mtb-infected macrophages, and, under these conditions, bacterial replication is suppressed. This innate control of bacterial replication is dependent on CD1d-restricted invariant NKT (iNKT) cells, and their activation requires CD1d expression by infected macrophages as well as IL-12 and IL-18. We show that iNKT cells, even in limiting quantities, are sufficient to restrict Mtb replication. To determine whether iNKT cells contribute to host defense against tuberculosis in vivo, we adoptively transferred iNKT cells into mice. Primary splenic iNKT cells obtained from uninfected mice significantly reduce the bacterial burden in the lungs of mice infected with virulent Mtb by the aerosol route. Thus, iNKT cells have a direct bactericidal effect, even in the absence of synthetic ligands such as α-galactosylceramide. Our finding that iNKT cells protect mice against aerosol Mtb infection is the first evidence that CD1d-restricted NKT cells mediate protection against Mtb in vivo.

Host resistance to Mycobacterium tuberculosis (Mtb) requires a coordinated response by the different components of the immune system. We established an in vitro model to study the contribution of innate lymphocytes to immunity against Mtb. When co-cultured with Mtb-infected macrophages, splenocytes from uninfected mice become activated and suppress bacterial replication. By fractionating the different splenocyte cell populations, we discovered that the invariant NKT (iNKT) cell is essential for suppressing intracellular bacterial replication. iNKT cells, which are conserved in rodents and humans, recognize lipids presented by the antigen-presenting molecule CD1d. While we had previously shown that iNKT cell-deficient mice are not more susceptible to tuberculosis, a potential contribution of iNKT cells during the early phase of immunity may have been masked. To address this issue, we showed that highly purified iNKT cells were sufficient to reduce the lung bacterial burden of mice infected with virulent Mtb. This is the first evidence that CD1d-restricted iNKT cells play a physiological role in mediating protection against aerosol Mtb infection in vivo. Thus, by being an early producer of interferon-g and suppressing intracellular bacterial growth, iNKT cells function as an important part of the early immune response against Mtb.

NK cells play a critical protective role in host defense against acute extracellular Staphylococcus aureus bacterial infection in the lung

Staphylococcus aureus remains a common cause of nosocomial bacterial infections and are often antibiotic resistant. The role of NK cells and IL-15 and their relationship in host defense against extracellular bacterial pathogens including S. aureus remain unclear. We have undertaken several approaches to address this issue using wild type (WT), IL-15 gene knock-out (KO), and NK cell-depleted mouse models. Upon pulmonary staphylococcal infection WT mice had markedly increased activated NK cells, but not NKT or gammadelta T cells, in the airway lumen that correlated with IL-15 production in the airway and with alveolar macrophages. In vitro exposure to staphylococcal products and/or coculture with lung macrophages directly activated NK cells. In contrast, lung macrophages better phagocytosed S. aureus in the presence of NK cells. In sharp contrast to WT controls, IL-15 KO mice deficient in NK cells were found to be highly susceptible to pulmonary staphylococcal infection despite markedly increased neutrophils and macrophages in the lung. In further support of these findings, WT mice depleted of NK cells were similarly susceptible to staphylococcal infection while they remained fully capable of IL-15 production in the lung at levels similar to those of NK-competent WT hosts. Our study thus identifies a critical role for NK cells in host defense against pulmonary extracellular bacterial infection and suggests that IL-15 is involved in this process via its indispensable effect on NK cells, but not other innate cells. These findings hold implication for the development of therapeutics in treating antibiotic-resistant S. aureus infection.

Manipulation of acute inflammatory lung disease

Inflammatory lung disease to innocuous antigens or infectious pathogens is a common occurrence and in some cases, life threatening. Often, the inflammatory infiltrate that accompanies these events contributes to pathology by deleterious effects on otherwise healthy tissue and by compromising lung function by consolidating (blocking) the airspaces. A fine balance, therefore, exists between a lung immune response and immune-mediated damage, and in some the "threshold of ignorance" may be set too low. In most cases, the contributing, potentially offending, cell population or immune pathway is known, as are factors that regulate them. Why then are targeted therapeutic strategies to manipulate them not more commonplace in clinical medicine? This review highlights immune homeostasis in the lung, how and why this is lost during acute lung infection, and strategies showing promise as future immune therapeutics.

CD1d-restricted iNKT cells, the 'Swiss-Army knife' of the immune system

Natural Killer T cells are a distinct lymphocyte lineage that regulates a broad range of immune responses. NKT cells recognize glycolipids presented by the non-classical MHC molecule CD1d. Structural insight into the TCR/glycolipid/CD1d tri-complex has revealed an unusual and unexpected mode of recognition. Recent studies have also identified some of the signaling events during NKT cell development that give NKT cells their innate phenotype. Pathogen-derived glycolipid antigens continue to be found, and new mechanisms of NKT cell activation have been described. Finally, NKT cells have been shown to be remarkably versatile in function during various immune responses. Whether these extensive functional capacities can be attributed to a single population sensitive to environmental cues or if functionally distinct NKT cell subpopulations exist remains unresolved.

The role of natural killer T cells in lung inflammation

Invariant NK T cells (iNKT) bridge the innate and adaptive immune response, being characterized by the ability to use invariant T cell receptors to recognize glycolipid antigens presented by CD1d, leading to an explosive cytokine effector response. As such it has been proposed that iNKT cells perform important roles as both effector and regulatory cells in a wide range of disease settings. These roles have been characterized in experiments depending on the use of iNKT-null mice, due to lack of either CD1d expression or Jalpha18 and the use of CD1d tetramers loaded with the model glycolipid antigen, alpha-galactosylceramide (alphaGalCer). Several studies have examined lung disease, infectious and allergic, in humans and mice. While the lung itself does not carry an exceptionally large population of iNKT cells (compared with, say, the liver), it does appear to be a site at which these cells can exert a profound effect. Several models of bacterial, fungal and viral murine lung infection have been investigated that have sometimes produced conflicting results. Abrogation of iNKT cell function in knockouts is often associated with disease exacerbation, indicating a regulatory role in lung infection. Studies in murine asthma models and in patients have similarly probed the role of iNKT cells in these settings. While there are again somewhat contradictory findings, evidence suggests a likely role for iNKT cells in mediating airway hyper-responsiveness (AHR), but probably not in Th2 polarization or lung eosinophilia. In marginally different models, administration of alphaGalCer has either ameliorated or exacerbated AHR. Different studies of BAL from human asthma patients show variously that there is either a very enlarged population of iNKT cells in the asthmatic lung, or that there is no significant difference from controls. Taken together, there are some observations that argue compellingly for an important role of iNKT cells in the lung, but resolution of some of the contradictory findings may await the development of reagents capable of providing alternative readouts of iNKT activation in these diverse disease settings.

Critical involvement of pneumolysin in production of interleukin-1alpha and caspase-1-dependent cytokines in infection with Streptococcus pneumoniae in vitro: a novel function of pneumolysin in caspase-1 activation

Pneumolysin is a pore-forming cytolysin known as a major virulence determinant of Streptococcus pneumoniae. This protein toxin has also been shown to activate the Toll-like receptor 4 (TLR4) signaling pathway. In this study, a mutant S. pneumoniae strain deficient in pneumolysin (Deltaply) and a recombinant pneumolysin protein (rPLY) were constructed. Upon infection of macrophages in vitro, the ability to induce the production of interleukin-1alpha (IL-1alpha), IL-1beta, and IL-18 was severely impaired in the Deltaply mutant, whereas there was no marked difference in the induction of tumor necrosis factor alpha (TNF-alpha) and IL-12p40 between the wild type and the Deltaply mutant of S. pneumoniae. When macrophages were stimulated with rPLY, the production of IL-1alpha, IL-1beta, and IL-18 was strongly induced in a TLR4-dependent manner, whereas lipopolysaccharide, a canonical TLR4 agonist, hardly induced these cytokines. In contrast, lipopolysaccharide was more potent than rPLY in inducing the production of TNF-alpha, IL-6, and IL-12p40, the cytokines requiring no caspase activation. Activation of caspase-1 was observed in macrophages stimulated with rPLY but not in those stimulated with lipopolysaccharide, and the level of activation was higher in macrophages infected with wild-type S. pneumoniae than in those infected with the Deltaply mutant. These results clearly indicate that pneumolysin plays a key role in the host response to S. pneumoniae, particularly in the induction of caspase-1-dependent cytokines.