Role of CD1d-restricted NKT cells in microbial immunity

Presumed guilty: natural killer T cell defects and human disease

Natural killer T (NKT) cells are important regulatory lymphocytes that have been shown in mouse studies, to have a crucial role in promoting immunity to tumours, bacteria and viruses, and in suppressing cell-mediated autoimmunity. Many clinical studies have indicated that NKT cell deficiencies and functional defects might also contribute to similar human diseases, although there is no real consensus about the nature of the NKT cell defects or whether NKT cells could be important for the diagnosis and/or treatment of these conditions. In this Review, we describe the approaches that have been used to analyse the NKT cell populations of various patient groups, suggest new strategies to determine how (or indeed, if) NKT cell defects contribute to human disease, and discuss the prospects for using NKT cells for therapeutic benefit.

Cross-talk between cd1d-restricted nkt cells and γδ cells in t regulatory cell response

CD1d is a non-classical major histocompatibility class 1-like molecule which primarily presents either microbial or endogenous glycolipid antigens to T cells involved in innate immunity. Natural killer T (NKT) cells and a subpopulation of γδ T cells expressing the Vγ4 T cell receptor (TCR) recognize CD1d. NKT and Vγ4 T cells function in the innate immune response via rapid activation subsequent to infection and secrete large quantities of cytokines that both help control infection and modulate the developing adaptive immune response. T regulatory cells represent one cell population impacted by both NKT and Vγ4 T cells. This review discusses the evidence that NKT cells promote T regulatory cell activation both through direct interaction of NKT cell and dendritic cells and through NKT cell secretion of large amounts of TGFβ, IL-10 and IL-2. Recent studies have shown that CD1d-restricted Vγ4 T cells, in contrast to NKT cells, selectively kill T regulatory cells through a caspase-dependent mechanism. Vγ4 T cell elimination of the T regulatory cell population allows activation of autoimmune CD8+ effector cells leading to severe cardiac injury in a coxsackievirus B3 (CVB3) myocarditis model in mice. CD1d-restricted immunity can therefore lead to either immunosuppression or autoimmunity depending upon the type of innate effector dominating during the infection.

NKT cells: the culprits of sepsis?

Sepsis is currently a leading cause of death in hospital intensive care units. Previous studies suggest that the pathophysiology of sepsis involves the hyperactivation of complex pro-inflammatory cascades that include the activation of various immune cells and the exuberant secretion of pro-inflammatory cytokines by these cells. Natural killer T-cells (NKT) are a sub-lineage of T cells that share characteristics of conventional T cells and NK cells, and bridge innate and adaptive immunity. More recently, NKT cells have been implicated in microbial immunity, including the onset of sepsis. Moreover, apolipoprotein E (apoE), a component of triglyceride-rich lipoproteins, has been shown to be protective in endotoxemia and gram-negative infections in addition to its well-known role in lipid metabolism. Here, we will review the role of NKT cells in sepsis and septic shock, the immunoregulatory role of apoE in the host immune response to infection, and propose a mechanism for this immunoregulation.

NKT cells in sepsis

Sepsis is currently a leading cause of death in hospital intensive care units. Previous studies suggest that the pathophysiology of sepsis involves the hyperactivation of complex proinflammatory cascades that include the activation of various immune cells and the exuberant secretion of proinflammatory cytokines by these cells. Natural killer T-cells (NKTs) are a sublineage of T cells that share characteristics of conventional T cells and NK cells and bridge innate and adaptive immunity. More recently, NKT cells have been implicated in microbial immunity, including the onset of sepsis. Moreover, apolipoprotein E (apoE), a component of triglyceride-rich lipoproteins, has been shown to be protective in endotoxemia and gram-negative infections in addition to its well-known role in lipid metabolism. Here, we will review the role of NKT cells in sepsis and septic shock, the immunoregulatory role of apoE in the host immune response to infection, and propose a mechanism for this immunoregulation.

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.

CD1d, a sentinel molecule bridging innate and adaptive immunity, is downregulated by the human papillomavirus (HPV) E5 protein: a possible mechanism for immune evasion by HPV

CD1d and CD1d-restricted natural killer T (NKT) cells serve as a natural bridge between innate and adaptive immune responses to microbes. CD1d downregulation is utilized by a variety of microbes to evade immune detection. We demonstrate here that CD1d is downregulated in human papillomavirus (HPV)-positive cells in vivo and in vitro. CD1d immunoreactivity was strong in HPV-negative normal cervical epithelium but absent in HPV16-positive CIN1 and HPV6-positive condyloma lesions. We used two cell lines for in vitro assay; one was stably CD1d-transfected cells established from an HPV-negative cervical cancer cell line, C33A (C33A/CD1d), and the other was normal human vaginal keratinocyte bearing endogenous CD1d (Vag). Flow cytometry revealed that cell surface CD1d was downregulated in both C33A/CD1d and Vag cells stably transfected with HPV6 E5 and HPV16 E5. Although the steady-state levels of CD1d protein decreased in both E5-expressing cell lines compared to empty retrovirus-infected cells, CD1d mRNA levels were not affected. Confocal microscopy demonstrated that residual CD1d was not trafficked to the E5-expressing cell surface but colocalized with E5 near the endoplasmic reticulum (ER). In the ER, E5 interacted with calnexin, an ER chaperone known to mediate folding of CD1d. CD1d protein levels were rescued by the proteasome inhibitor, MG132, indicating a role for proteasome-mediated degradation in HPV-associated CD1d downregulation. Taken together, our data suggest that E5 targets CD1d to the cytosolic proteolytic pathway by inhibiting calnexin-related CD1d trafficking. Finally, CD1d-mediated production of interleukin-12 from the C33A/CD1d cells was abrogated in both E5-expressing cell lines. Decreased CD1d expression in the presence of HPV E5 may help HPV-infected cells evade protective immunological surveillance.

Plasmodium vivax: comparison of the immune responses between oral and parenteral immunization of rPv54 in BALB/c mice

The merozoite surface protein-1 (MSP-1) from Plasmodium vivax was evaluated as an oral vaccine candidate by cloning and expressing the interspecies conserved block 10 (ICB10) of the MSP-1 from a Korean isolate in Escherichia coli. The expressed fusion protein contained ICB10 and a maltose-binding protein (MBP), rPv54, has a molecular weight of approximately 54 kDa as determined by SDS-PAGE analysis. IgG against rPv54 was successfully produced in BALB/c mice by oral immunization and sustained for more than 4 months. IgG2b was dominantly produced in both oral and parenteral immunizations. The rPv54 increased the frequency of NK, NKT, CD4+ T, CD8+ T, and B cells in both immunizations. IL-5 and TNF-alpha were increased in both significantly. In conclusion, rPv54 might be a valuable potential vaccine candidate for the oral and parenteral immunization against vivax malaria.

Eradication of Helicobacter spp. by using medicated diet in mice deficient in functional natural killer cells and complement factor D

A commercial 4-drug diet has shown promise in eradicating Helicobacter spp. from rodents; however, its effectiveness in immunocompromised mice is unknown. This study evaluated the efficacy of this treatment in eradicating Helicobacter spp. from mice deficient in functional natural killer cells (Cd1(-/-)) or complement factor D (Df(-/-)). Cd1(-/-) mice naturally infected with H. hepaticus with or without H. rodentium were fed either control or medicated diet for 8 wk followed by 4 wk on control diet. Fecal samples were PCR-evaluated for Helicobacter spp. before mice began treatment and then every 2 wk thereafter for 12 wk. The same experimental design was repeated for eighteen 9- to 21-wk-old Df(-/-) mice naturally infected with H. bilis with or without H. rodentium. All Df(-/-) mice and 8- to 21-wk-old Cd1(-/-) mice ceased shedding Helicobacter spp. after 2 wk of treatment and remained negative throughout the study. In contrast, the Cd1(-/-) mice that were 24 wk or older shed Helicobacter spp. for the first 8 wk but tested negative at 10 and 12 wk. All treated animals had enlarged ceca and gained less weight than control untreated mice, and 6 of 7 treated Cd1(-/-) male mice developed mild portal fibrosis. These findings show that within 2 wk of treatment, the 4-drug diet eradicated H. hepaticus and H. rodentium from young Cd1(-/-) mice and H. bilis and H. rodentium from Df(-/-) mice, but eradication of established infections in Cd1(-/-) mice required 8 wk of treatment.

Kupffer cell and interleukin-12-dependent loss of natural killer T cells in hepatosteatosis

Hepatosteatosis is associated with increased expression of tumor necrosis factor alpha (TNF-alpha) and interleukin (IL)-12, major T helper (Th) 1 cytokines, and reduced hepatic natural killer T (NKT) cell numbers. The relationship between lipid accumulation, cytokine expression, and hepatic NKT cells is not known. This study was conducted to assess the role of IL-12 in the development of hepatic steatosis and its potential impact on liver NKT cells. Male C57Bl/6 wildtype (WT) and IL-12-deficient (IL-12(-/-)) mice were fed a choline-deficient diet (CDD) for 0, 10, or 20 weeks. CDD led to marked hepatosteatosis, reduced hepatic but not splenic NKT cell numbers and function, and increased hepatic expression of the T(h)1-type cytokines IL-12, interferon gamma (IFN-gamma), and TNF-alpha in WT mice. The absence of IL-12 resulted in similar CDD-induced hepatosteatosis, but preserved hepatic NKT cells and significantly reduced hepatic IFN-gamma and TNF-alpha expression. Treatment of CDD-fed mice with lipopolysaccharide led to a significant increase in hepatic IL-12 expression, and Kupffer cell (KC) depletion reduced liver IL-12 expression and restored NKT cells in CDD-induced fatty liver. Interestingly, KCs from CDD-fed mice failed to produce increased quantities of IL-12 upon activation in vitro when compared to similarly treated KCs from control fed mice, suggesting that secondary factors in vivo promote heightened IL-12 production. Finally, human livers with severe steatosis showed a substantial decrease in NKT cells.

CONCLUSION

Hepatosteatosis reduces the numbers of hepatic NKT cells in a KC-and IL-12-dependent manner. Our results suggest a pivotal and multifunctional role of KC-derived IL-12 in the altered immune response in steatotic liver, a process that is likely active within human nonalcoholic fatty liver disease.

Alpha versus beta: are we on the way to resolve the mystery as to which is the endogenous ligand for natural killer T cells?

Natural killer T (NKT) lymphocytes are a unique subset of cells that play a role in regulating the immune system. For the past decade, studies have focused upon attempts to define these cells and to determine the ligand(s) that are required for their development and peripheral activation. Many research groups have focused upon determining the mechanisms for activating or inhibiting NKT cells in an attempt to control immune-mediated disorders as well as infectious and malignant conditions by using different ligand structures. Alpha-anomeric glycolipids and phospholipids derived from mammalian, bacterial, protozoan and plant species have been suggested as potential ligands for these lymphocytes. Some of these ligands were structured in forms that can bind to CD1d molecules. The lack of alpha-anomeric glycosphingolipids in mammals and the modest effect of these ligands in human studies, along with recent data from animal models and humans on the NKT-dependent immunomodulatory effect of beta-glycosphingolipids, suggest that the beta-anomeric ligands have the potential to be the endogenous NKT ligand.

Natural killer T cells and the regulation of asthma

A crucial role has been suggested for invariant natural killer T cells (iNKT) in regulating the development of asthma, a complex and heterogeneous disease characterized by airway inflammation and airway hyperreactivity (AHR). iNKT cells constitute a unique subset of T cells responding to endogenous and exogenous lipid antigens, rapidly secreting a large amount of cytokines, which amplify both innate and adaptive immunity. Herein, we review recent studies showing a requirement for iNKT cells in various models of asthma in mice and monkeys as well as studies in human patients. Surprisingly, in several different murine models of asthma, distinct subsets of iNKT cells were required, suggesting that iNKT cells serve as a common critical pathogenic element for many different forms of asthma. The importance of iNKT cells in both allergic and non-allergic forms of asthma, which are independent of adaptive immunity and associated with airway neutrophils, may explain situations previously found to be incompatible with the Th2 paradigm of asthma.

Immune pathways for translating viral infection into chronic airway disease

To better understand the immune basis for chronic inflammatory lung disease, we analyzed a mouse model of lung disease that develops after respiratory viral infection. The disease that develops in this model is similar to asthma and chronic obstructive pulmonary disease (COPD) in humans and is manifested after the inciting virus has been cleared to trace levels. The model thereby mimics the relationship of paramyxoviral infection to the development of childhood asthma in humans. When the acute lung disease appears in this model (at 3 weeks after viral inoculation), it depends on an immune axis that is initiated by expression and activation of the high-affinity IgE receptor (FcvarepsilonRI) on conventional lung dendritic cells (cDCs) to recruit interleukin (IL)-13-producing CD4(+) T cells to the lower airways. However, when the chronic lung disease develops fully (at 7 weeks after inoculation), it is driven instead by an innate immune axis that relies on invariant natural killer T (iNKT) cells that are programmed to activate macrophages to produce IL-13. The interaction between iNKT cells and macrophages depends on contact between the semi-invariant Valpha14Jalpha18-TCR on lung iNKT cells and the oligomorphic MHC-like protein CD1d on macrophages as well as NKT cell production of IL-13 that binds to the IL-13 receptor (IL-13R) on the macrophage. This innate immune axis is also activated in the lungs of humans with severe asthma or COPD based on detection of increased numbers of iNKT cells and alternatively activated IL-13-producing macrophages in the lung. Together, the findings identify an adaptive immune response that mediates acute disease and an innate immune response that drives chronic inflammatory lung disease in experimental and clinical settings.

Natural killer T cells: an unconventional T-cell subset with diverse effector and regulatory functions

Natural killer T (NKT) cells are a unique subset of lymphocytes that express NK cell markers such as CD161 and CD94, as well as a T-cell receptor (TCR) alpha/beta, with a restricted repertoire, which distinguishes them from NK cells, which lack a TCR. In contrast to conventional T-lymphocytes, the TCR of NKT cells does not interact with that of peptide antigens presented by classical major histocompatibility complex-encoded class I or II molecules. Instead, this TCR recognizes glycolipids presented by CD1d, a non-classical antigen-presenting molecule. The rapid response of NKT cells to their cognate antigens is characteristic of an innate immune response, and allows the polarizing cytokines (IFN-gamma and/or IL-4) to regulate adaptive immunity. NKT cells have been found to be critical in the immune response against viral infections and malaria, as well as in tumor immunity, and certain autoimmune diseases. NKT cells have been assessed to represent the "trait d'union" between innate and adaptive immunity. They play an active role in skin diseases, such as contact sensitivity, which have been implicated in UV-induced immunosuppression and psoriasis. Thus, NKT-cells are emerging as an important subset of lymphocytes, with a protective role in host defense and a pathogenic role in certain immune-mediated disease states.

V alpha14 i NKT cells are innate lymphocytes that participate in the immune response to diverse microbes

Natural Killer T (NKT) cells constitute a conserved T lymphocyte sublineage that has been implicated in the regulation of various immune responses, including the responses to viruses, bacteria, and parasites. NKT cells recognize self and foreign glycolipids presented by CD1d, a non-classical antigen-presenting molecule, and they rapidly produce various cytokines. Many studies have shown that NKT cells have protective roles following microbial infection through the amplification of innate and adaptive immunity, although NKT cells have detrimental roles in some cases. Recent studies have shed light on the natural antigens recognized by NKT cells and the mechanisms whereby they contribute to host defense, and they suggest that these unique T cells have evolved to jump start the immune response to microbes.

Capture of heat-killed Mycobacterium bovis bacillus Calmette-Guérin by intelectin-1 deposited on cell surfaces

Intelectin is an extracellular animal lectin found in chordata. Although human and mouse intelectin-1 recognize galactofuranosyl residues included in cell walls of various microorganisms, the physiological function of mammalian intelectin had been unclear. In this study, we found that human intelectin-1 was a serum protein and bound to Mycobacterium bovis bacillus Calmette-Guérin (BCG). Human intelectin-1-binding to BCG was inhibited by Ca(2+)-depletion, galactofuranosyl disaccharide, ribose, or xylose, and was dependent on the trimeric structure of human intelectin-1. Although monomeric, mouse intelectin-1 bound to BCG, with its C-terminal region contributing to efficient binding. Human intelectin-1-transfected cells not only secreted intelectin-1 into culture supernatant but also expressed intelectin-1 on the cell surface. The cell surface intelectin-1 was not a glycosylphosphatidylinositol-anchored membrane protein. Intelectin-1-transfected cells captured BCG more than untransfected cells, and the BCG adherence was inhibited by an inhibitory saccharide of intelectin-1. Intelectin-1-preincubated cells took up BCG more than untreated cells, but the adhesion of intelectin-1-bound BCG was the same as that of untreated BCG. Mouse macrophages phagocytosed BCG more efficiently in medium containing mouse intelectin-1 than in control medium. These results indicate that intelectin is a host defense lectin that assists phagocytic clearance of microorganisms.

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.

beta-Glycosphingolipids as immune modulators

beta-Glycosphingolipids have emerged as a family of potential ligands for natural killer T (NKT)-regulatory lymphocytes. This subset of regulatory lymphocytes has been implicated in the regulation of autoimmune processes. The major histocompatibility complex (MHC) Class I-like CD1d glycoprotein is a member of the CD1 family of antigen-presenting molecules and is responsible for selection of NKT cells. beta-Glycolipids have been shown to alter immune responses in the opposing settings of autoimmune diseases or cancer. In this review, we discuss the potential use of beta-glycoshpingolipids for NKT-based immunotherapy.

Prevention of injury-induced suppression of T-cell immunity by the CD1d/NKT cell-specific ligand alpha-galactosylceramide

Infection, sepsis, and multiple organ failure continue to be significant factors leading to morbidity and mortality after severe injury. Studies by our laboratory and others have identified injury-induced defects in both innate and adaptive components of host defense. We previously reported that CD1d-restricted natural killer T (NKT) cells actively suppress effector T-cell immunity after burn injury via production of excess IL-4 and failure to produce IFN-gamma. alpha-Galactosylceramide (alpha-GalCer) is a synthetic NKT cell-specific ligand presented exclusively to invariant NKT cells and is known to improve immunity against tumors and infection by promoting IFN-gamma production. Here, we confirmed the role of Valpha14-Jalpha281 invariant NKT cells in mouse model of burn injury-induced suppression of T-cell immunity and further asked whether alpha-GalCer can improve immunity after injury via similar mechanisms. We observed that systemic treatment with alpha-GalCer prevented the injury-induced suppression of Ag-specific T-cell responsiveness both in vitro and in vivo and restored the ability of splenic lymphocytes to produce both IL-2 and IFN-gamma. Moreover, burn injury was associated with diminished expression of major histocompatibility complex II and CD40 on antigen presenting cells that were both restored by alpha-GalCer treatment to levels seen in sham-treated mice. Collectively, these data suggest that, via manipulation of the NKT cell population, we may be able to maintain T-cell function and improve host defense after burn injury.

Alpha-galactosylceramide modulates the induction of indoleamine 2,3-dioxygenase in antigen presenting cells

The glycolipid alpha-galactosylceramide (alpha-GalCer), when presented on CD1 molecules by antigen presenting cells (APCs) to invariant NKT (iNKT cells), is a potent immunomodulator. Indoleamine 2,3-dioxygenase (IDO), an enzyme catalyzing the catabolism of L-tryptophan along the kynurenine pathway, is inducible in APC and represents one of the main endogenous mechanisms of T cell homeostasis, peripheral tolerance and immunosuppression. No data have been published yet on the effect of alpha-GalCer on IDO in APC. We aimed to determine if: (1) alpha-GalCer modulates IDO in APC; (2) the alpha-GalCer-induced effect on IDO correlates with the production by APC of active compounds; (3) the medium from alpha-GalCer-treated APC is able to stimulate iNKT cells. From our results alpha-GalCer alone did not modify IDO expression (RT-PCR) in APC, but when human peripheral blood mononuclear cells (PBMC), monocytes, and monocytic cell lines (THP-1), expressing high levels of CD1d, were treated with interferon-gamma (IFN-gamma) plus alpha-GalCer a significant potentiation of IDO transcription was measured. This effect was not induced by increased IFN-gamma release by APC, and it was functionally correlated with increased L-kynurenine (L-KYN) release by alpha-GalCer-treated CD1d-transfected THP-1 cells. The medium of these cells stimulated iNKT hybridoma cells to release interleukin (IL)-2, while alpha-GalCer alone resulted ineffective. The data demonstrate that alpha-GalCer: (1) does not induce IFN-gamma release by APC; (2) potentiates IFN-gamma-induced IDO expression and function in APC; (2) requires CD1d molecules for inducing these effects; (3) induces the release by APC of compounds active in stimulating iNKT cells.

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.

Persistent activation of an innate immune response translates respiratory viral infection into chronic lung disease

To understand the pathogenesis of chronic inflammatory disease, we analyzed an experimental mouse model of chronic lung disease with pathology that resembles asthma and chronic obstructive pulmonary disease (COPD) in humans. In this model, chronic lung disease develops after an infection with a common type of respiratory virus is cleared to only trace levels of noninfectious virus. Chronic inflammatory disease is generally thought to depend on an altered adaptive immune response. However, here we find that this type of disease arises independently of an adaptive immune response and is driven instead by interleukin-13 produced by macrophages that have been stimulated by CD1d-dependent T cell receptor-invariant natural killer T (NKT) cells. This innate immune axis is also activated in the lungs of humans with chronic airway disease due to asthma or COPD. These findings provide new insight into the pathogenesis of chronic inflammatory disease with the discovery that the transition from respiratory viral infection into chronic lung disease requires persistent activation of a previously undescribed NKT cell-macrophage innate immune axis.

Expression of CD1d and ligand-induced cytokine production are tissue specific in mucosal epithelia of the human lower reproductive tract

Mucosal epithelia of the human lower reproductive tract (vagina, cervix, and penile urethra) are exposed to sexually transmitted microbes, including Chlamydia trachomatis. The in vivo susceptibility of each tissue type to infection with C. trachomatis is quite distinct. CD1d is expressed on the surface of antigen-presenting cells, including mucosal epithelial cells, and interacts specifically with invariant NKT cells. Invariant NKT cells play a role in both innate and adaptive immune responses to microbes. Here we assessed CD1d expression in normal reproductive tissues by using immunohistochemistry. Immortalized epithelial cell lines from the human lower reproductive tract (vagina, endocervix, and penile urethra) were examined for CD1d expression and for ligand-induced cytokine production induced by CD1d cross-linking. CD1d expression in normal tissue was strong in the vagina but weak in the endocervix and penile urethra. Gamma interferon exposure induced CD1d transcription in all of the cell types studied, with the strongest induction in vaginal cells. Flow cytometry revealed cell surface expression of CD1d in vaginal and penile urethral epithelial cells but not in endocervical cells. Ligation of surface-expressed CD1d by monoclonal antibody cross-linking promoted interleukin-12 (IL-12) and IL-15, but not IL-10, production in vaginal and penile urethral cells. No induction was demonstrated in endocervical cells. CD1d-mediated cytokine production in penile urethral cells was abrogated by C. trachomatis infection. Basal deficiency in CD1d-mediated immune responsiveness may result in susceptibility to sexually transmitted agents. Decreased CD1d-mediated signaling may help C. trachomatis evade detection by innate immune cells.

Killer cells in chronic obstructive pulmonary disease

COPD (chronic obstructive pulmonary disease) is a treatable and preventable disease state, characterized by progressive airflow limitation that is not fully reversible. It is a current and growing cause of mortality and morbidity worldwide, with the WHO (World Health Organization) projecting that total deaths attributed to COPD will increase by more than 30% in the next 10 years. The pathological hallmarks of COPD are destruction of the lung parenchyma (pulmonary emphysema), inflammation of the central airways (chronic bronchitis) and inflammation of the peripheral airways (respiratory bronchiolitis). The destructive changes and tissue remodelling observed in COPD are a result of complex interactions between cells of the innate and adaptive immune systems. The focus of the present review is directed towards the role of CD8(+) T-lymphocytes, NK (natural killer) cells and NKT cells (NK T-cells). These three classes of killer cell could all play an important part in the pathogenesis of COPD. The observed damage to the pulmonary tissue could be caused in three ways: (i) direct cytotoxic effect against the lung epithelium mediated by the activities of perforin and granzymes, (ii) FasL (Fas ligand)-induced apoptosis and/or (iii) cytokine and chemokine release. The present review considers the role of these killer cells in COPD.

Conventional, regulatory, and unconventional T cells in the immunologic response to Helicobacter pylori

Infection by the gastroduodenal pathogen Helicobacter pylori elicits a complex immunologic response in the mucosa involving neutrophils, plasma cells, eosinophils, and lymphocytes, of which T cells are the principal orchestrators of immunity. While so-called classical T cells (e.g. T-helper cells) that are activated by peptide fragments presented on antigen-presenting cells have received much attention in H. pylori infection, there exists a diverse array of other T cell populations that are potentially important for the outcome of the ensuing immune response, some of which have not been extensively studied in H. pylori infection. Pathogen-specific regulatory T cells that control and prevent the development of immunopathology associated with H. pylori infection have been investigated, but these cells can also benefit the bacterium in helping to prolong the chronicity of the infection by suppressing protective immune responses. An overlooked T cell population, the more recently described Th17 cells, may play a role in H. pylori-induced inflammation, due to triggering responses that ultimately lead to the recruitment of polymorphs, including neutrophils. The so-called innate or unconventional T cells, that include two conserved T cell subsets expressing invariant antigen-specific receptors, the CD1d-restricted natural killer T cells which are activated by glycolipids, and the mucosal-associated invariant T cells which play a role in defense against orally acquired pathogens in the intestinal mucosa, have only begun to receive attention. A greater knowledge of the range of T cell responses induced by H. pylori is required for a deeper understanding of the pathogenesis of this bacterium and its ability to perpetuate chronic infection, and could reveal new strategies for therapeutic exploitation.

CXCL16 is a novel mediator of the innate immunity of epidermal keratinocytes

The epidermis is constantly exposed to a variety of microbial pathogens and plays a vital role in resisting them. Soluble CXC chemokine ligand (CXCL) 16, which is one of the ELR- CXC chemokines, acts as a mediator of innate immunity by attracting CXC chemokine receptor (CXCR) 6-expressing cells, such as activated T cells and NKT cells. However, the production of CXCL16 by non-immune cells remains unclear. We found that cultured keratinocytes produced a significant amount of CXCL16 (2-3 ng per 10(6) cells per 24 h). Stimulation with tumor necrosis factor alpha, IL-1alpha, IFN-gamma, peptidoglycan and polyinosinic-polycytidylic acid [poly(I:C)] enhanced CXCL16 production. The forms of CXCL16 in the culture supernatants had molecular weights of 14, 28 and 50 kDa. Immunohistochemical analysis revealed that the normal human epidermis expressed CXCL16. As several chemokines have anti-microbial activities, we studied the anti-microbial activity of CXCL16. The chemokine domain of CXCL16 at concentrations >5 microg ml(-1) had significant anti-microbial activity against Staphylococcus aureus and Escherichia coli. Killing activity was retained at the physiological salt concentration in the presence of carbonate. In conclusion, CXCL16 is a novel mediator of the innate immune reactivities of epidermal keratinocytes.

Glycolipids as immunostimulating agents

The processing and presentation of lipid antigens by antigen presenting cells (APC) is important for defense against infection, tumor immunosurveillance, and autoimmunity. CD1, a family of cell surface glycoproteins, is responsible for the binding and presentation of lipid antigens to receptors expressed on the surface of T lymphocytes. Among the several (glyco)lipids identified to cause T-cell stimulation in complex with CD1, alpha-galactosyl ceramide (alpha-GalCer) is one of the most well studied. A combination of structure-activity relationship (SAR), crystallographic studies, and discovery of new 'natural' antigens has led to greater understanding of the structural requirements for optimal natural killer T-cell activation.

NKT cells are critical to initiate an inflammatory response after Pseudomonas aeruginosa ocular infection in susceptible mice

CD4(+) T cells produce IFN-gamma contributing to corneal perforation in C57BL/6 (B6) mice after Pseudomonas aeruginosa infection. To determine the role of NK and NKT cells, infected corneas of B6 mice were dual immunolabeled. Initially, more NKT than NK cells were detected, but as disease progressed, NK cells increased, while NKT cells decreased. Therefore, B6 mice were depleted of NK/NKT cells with anti-asialo GM1 or anti-NK1.1 Ab. Either treatment accelerated time to perforation, increased bacterial load and polymorphonuclear neutrophils, but decreased IFN-gamma and IL-12p40 mRNA expression vs controls. Next, RAG-1 knockout (-/-; no T/NKT cells), B6.TCR Jalpha281(-/-) (NKT cell deficient), alpha-galactosylceramide (alphaGalCer) (anergized NKT cells) injected and IL-12p40(-/-) vs B6 controls were tested. IFN-gamma mRNA was undetectable in RAG-1(-/-)- and alphaGalCer-treated mice at 5 h and was significantly reduced vs controls at 1 day postinfection. It also was reduced significantly in B6.TCR Jalpha281(-/-), alphaGalCer-treated, and IL-12p40(-/-) (activated CD4(+) T cells also reduced) vs control mice at 5 days postinfection. In vitro studies tested whether endotoxin (LPS) stimulated Langerhans cells and macrophages (Mphi; from B6 mice) provided signals to activate NKT cells. LPS up-regulated mRNA expression for IL-12p40, costimulatory molecules CD80 and CD86, NF-kappaB, and CD1d, and addition of rIFN-gamma potentiated Mphi CD1d levels. Together, these data suggest that Langerhans cell/Mphi recognition of microbial LPS regulates IL-12p40 (and CD1d) driven IFN-gamma production by NKT cells, that IFN-gamma is required to optimally activate NK cells to produce IFN-gamma, and that depletion of both NKT/NK cells results in earlier corneal perforation.

CD1-restricted T cells in host defense to infectious diseases

CD1 has been clearly shown to function as a microbial recognition system for activation of T cell responses, but its importance for mammalian protective responses against infections is still uncertain. The function of the group 1 CD1 isoforms, including human CD1a, CDlb, and CDLc, seems closely linked to adaptive immunity. These CD1 molecules control the responses of T cells that are highly specific for particular lipid antigens, the best known of which are abundantly expressed by pathogenic mycobacteria such as Mycobacterium tuberculosis and Mycobacterium leprae. Studies done mainly on human circulating T cells ex vivo support a significant role for group I CD1-restricted T cells in protective immunity to mycobacteria and potentially other pathogens, although supportive data from animal models is currently limited. In contrast, group 2 CD1 molecules, which include human CD1d and its orthologs, have been predominantly associated with the activation of CD1d-restricted NKT cells, which appear to be more appropriately viewed as a facet of the innate immune system. Whereas the recognition of certain self-lipid ligands by CD d-restricted NKT cells is well accepted, the importance of these T cells in mediating adaptive immune recognition of specific microbial lipid antigens remains controversial. Despite continuing uncertainty about the role of CD 1d-restricted NKT cells in natural infections, studies in mouse models demonstrate the potential of these T cells to exert various effects on a wide spectrum of infectious diseases, most likely by serving as a bridge between innate and adaptive immune responses.

Upregulation of CD4+ NKT Cells Is Important for Allograft Survival in Staphylococcal-Enterotoxin-B-Treated Rats after High-Risk Corneal Transplantation

OBJECTIVE

To study the role of CD4+ natural killer T (NKT) cells in staphylococcal-enterotoxin-B (SEB)-treated rats after high-risk corneal transplantation.

METHOD

Fisher 344 donor corneas were transplanted into Lewis recipients. Corneal neovascularization was induced by sutures. All the recipients were randomly divided into 3 groups. The SEB group was intraperitoneally injected with SEB at a concentration of 75 microg/kg. The drug combination group received SEB and dexamethasone at a concentration of 5 mg/ml. The control group received saline buffer. All transplants were evaluated for 30 days. Ten days after transplantation, 3 recipients in each group were sacrificed for immunological study.

RESULT

The survival time of the allografts in the SEB group was 12.50 +/- 1.41 days, much longer than in the control group (7.30 +/- 0.67 days) and the drug combination group (10.38 +/- 3.07 days). The lymphocyte proliferation ability was the weakest and the percentage of CD4+ NKT cells in both the spleen and the mandibular lymph nodes was the highest in the SEB group, while the percentage of CD4+ and CD8+ cells was the lowest in the drug combination group. IL-2 in the aqueous humor and the serum was lower while IL-10 was higher in the SEB group than in the other 2 groups.

CONCLUSION

SEB prolongs allograft survival in rat high-risk corneal transplantation. This effect seems to be mediated by the upregulation of CD4+ NKT cells.

The unique role of natural killer T cells in the response to microorganisms

Natural killer T (NKT) cells combine features of the innate and adaptive immune systems. Recently, it has become evident that these T cells have crucial roles in the response to infectious agents. The antigen receptor expressed by NKT cells directly recognizes unusual glycolipids that are part of the membrane of certain Gram-negative bacteria and spirochetes. Moreover, even in the absence of microbial glycolipid antigens, these T cells respond to innate cytokines produced by dendritic cells that have been activated by microbes. This indirect sensing of infection, by responding to cytokines from activated dendritic cells, allows NKT cells to react to a broad range of infectious agents.

Innate Lymphocyte Subsets and Their Immunoregulatory Roles in Burn Injury and Sepsis

The vast majority of clinical and basic science research on the immune consequences of burn injury and sepsis conducted during the last three decades has focused mainly on the roles of macrophages, neutrophils and, to a lesser extent, conventional T lymphocytes. During recent years, however, it has become increasingly clear that minor subsets of innate immune cells, innate regulatory lymphocytes in particular, are central to processes involved in both protective immunity and immunopathology. Recent reports by our laboratory and others have just begun to shed light on the critical roles of innate lymphocyte subsets, including natural killer T cells, natural killer cells, gamma-delta T cells, and naturally occurring CD4+CD25+ regulatory T cells during the immune response to burn injury and sepsis. Given their emerging importance and documented upstream regulatory capacities over macrophage, dendritic cell, and T lymphocyte functions, innate regulatory lymphocytes represent attractive new targets for therapeutic intervention for the overall immune paralysis that occurs with injury and sepsis. Here, we provide an overview of the current state of knowledge of these particular cell subsets in the immune response to burn injury and sepsis.

Retinoic acid regulates CD1d gene expression at the transcriptional level in human and rodent monocytic cells

CD1d belongs to a group of nonclassical antigen-presenting molecules that present glycolipid antigens and thereby activate natural killer T (NKT) cells, a subset of bifunctional T cells. Little is known so far regarding the expression and physiologic regulation of CD1d. Here we show that all-trans-retinoic acid (RA), the active metabolite of vitamin A, rapidly (1 hr after treatment) increases CD1d mRNA in human and rodent monocytic cells at a physiologic dose (10 nM). The induction is RA specific and RA receptor (RAR) dependent-RA and an RARalpha agonist, Am580, both had a pronounced positive effect, whereas the addition of RARalpha antagonist partially blocked the increase in CD1d mRNA induced by RA and Am580. The induction was also completely blocked by the presence of actinomycin D. A putative RA-response element was identified in the distal 5' flanking region of the CD1d gene, which binds nuclear retinoid receptors and was responsive to RA in both gel mobility shift assay and transient transfection assay in THP-1 cells. These results further confirmed the transcriptional regulation of RA in CD1d gene expression. Moreover, RA significantly increased alpha-galactosylceramide-induced spleen cell proliferation. These studies together provide evidence for a previously unknown mechanism of CD1d gene expression regulation by RA and suggest that RA is a significant modulator of NKT cell activation.

Invariant and noninvariant natural killer T cells exert opposite regulatory functions on the immune response during murine schistosomiasis

CD1d-restricted natural killer T (NKT) cells represent a heterogeneous population of innate memory immune cells expressing both NK and T-cell markers distributed into two major subsets, i.e., invariant NKT (iNKT) cells, which express exclusively an invariant T-cell receptor (TCR) alpha chain (Valpha14Jalpha18 in mice), and non-iNKT cells, which express more diverse TCRs. NKT cells quickly produce Th1- and/or Th2-type cytokines following stimulation with glycolipid antigen (Ag) and, through this property, play potent immunoregulatory roles in autoimmune diseases, cancer, and infection. No study has addressed the role of NKT cells in metazoan parasite infections so far. We show that during murine schistosomiasis, the apparent frequency of both iNKT cells and non-iNKT cells decreased in the spleen as early as 3 weeks postinfection (p.i.) and that both populations expressed a greater amount of the activation marker CD69 at 6 weeks p.i., suggesting an activated phenotype. Two different NKT-cell-deficient mouse models, namely, TCR Jalpha18-/- (exclusively deficient in iNKT cells) and CD1d-/- (deficient in both iNKT and non-iNKT cells) mice, were used to explore the implication of these subsets in infection. We show that whereas both iNKT and non-iNKT cells do not have a major impact on the immune response during the early phase (1 and 4 weeks) of infection, they exert important, although opposite, effects on the immune response during the acute phase of the disease (7 and 12 weeks), after schistosome egg production. Indeed, iNKT cells contribute to Th1 cell differentiation whereas non-iNKT cells might be mostly implicated in Th2 cell differentiation in response to parasite Ag. Our findings suggest, for the first time, that helminths activate both iNKT and non-iNKT cells in vivo, enabling them to differentially influence the Th1/Th2 balance of the immune response.

Global transcriptional analysis delineates the differential inflammatory response interleukin-15 elicits from cultured human T cells

OBJECTIVE

Interleukin 15 (IL)-15 controls proliferation and survival of T cells, but its effects and the underlying cellular regulation are not well understood. Previous studies have focused on its effects on short-term T-cell cultures. In view of the potential problems associated with using IL-2 alone in adoptive immunotherapy protocols, we investigated the impact of IL-15 on T-cell cultures and the global transcriptional effects it elicits in such cultures.

MATERIALS AND METHODS

DNA microarrays and flow cytometry were used to examine the differential effect of 20 ng/mL IL-15 on primary serum-free T-cell cultures activated and cultured in the presence of IL-2. Quantitative reverse transcriptase polymerase chain reaction confirmed select microarray data.

RESULTS

IL-15 significantly increased ex vivo expansion of primary human T cells over the entire 11-day expansions without affecting viability. The 1133 genes were consistently differentially expressed among three donor samples. Ontological analysis demonstrated that IL-15 increases expression of genes involved in inflammatory response (e.g., tumor necrosis factor [TNF]-alpha, Oncostatin M, CD40L, and CD33) and apoptosis (e.g., TNF-related apoptosis-inducing ligand). IL-15 also induced expression of four suppressors of cytokine signaling (SOCS) family genes (SOCS1-3, cytokine-inducible SH2-containing protein), which are classical negative regulators of cytokine signaling. IL-15 strongly suppressed the expression of inhibitory natural killer cell receptor genes, including three C-type lectins (KLRB1, KLRC1, and KLRD1), as well as IL-7Ra and Granzyme H. Finally, IL-15 induced differential expression of TNF receptor superfamily members (CD27 and CD30).

CONCLUSION

These findings suggest that exogenous IL-15 may have a potential role in adoptive immunotherapy by both enhancing proliferation and modulating functionality during ex vivo T-cell expansion.

Invariant NKT cells sustain specific B cell responses and memory

Invariant natural killer T (iNKT) cells are innate-like lymphocytes recognizing CD1d-restricted glycolipid antigens, such as alpha-galactosylceramide (alphaGC). We assessed whether iNKT cells help B lymphocyte responses and found that mice immunized with proteins and alphaGC develop antibody titers 1-2 logs higher than those induced by proteins alone. Activation of iNKT cells enhances protection against infections such as influenza and elicits higher frequencies of memory B cells and higher antibody responses to booster immunizations. Protein vaccination with alphaGC, but not with conventional adjuvants, elicits IgG responses in mice lacking MHC class II molecules, demonstrating that iNKT cells can substitute for CD4(+) T cell help to B cells. Interestingly, the decay of circulating antibodies is faster in mice lacking iNKT cells. These findings point to a homeostatic role for iNKT cells on critical features of the antibody response such as immunity and B cell memory.

The effect of treating infected skin grafts with Acticoat™ on immune cells

A study was conducted to determine the effect of Acticoat placed on an infected skin graft on parameters of immunity. Two partial thickness wounds (2 cm x 4 cm) were created on the dorsal midline of Hartley guinea pigs (n=28). Wounds were covered with autologous skin graft and maintained either aseptically (Noninoculated, n=8), inoculated with Staphylococcus aureus (Surgery-Inoculated, n=8) with or without Acticoat bandage (Surgery-Inoculated-Acticoat, n=6). Five days later, splenocytes and blood were collected to estimate natural killer cell (NK) cytotoxicity, proliferative response to T and B cell mitogens and neutrophil oxidative burst. Animals that did not undergo surgery were included as a nonsurgery control group. [(3)H]-thymidine incorporation in response to a variety of T and B cell mitogens was significantly lower for all groups undergoing surgery compared to the nonsurgery control group (p<0.0001) and no additional effect was observed on this immune measure by applying the Acticoat bandage. The Surgery-Inoculated-Acticoat group exhibited greater NK cytotoxic activity (as assessed as the ability to lyse K562 tumor cells) compared to the Surgery-Inoculated group (p<0.006). The Surgery-Inoculated-Acticoat group had higher neutrophil oxidative burst at 5 min post stimulation, but was not different from controls after 15 min. In conclusion, the application of an Acticoat bandage to an inoculated surgery wound did not alter the low cell-mediated immune response that followed surgery, but appeared to increase parameters (NK cytotoxic activity and neutrophil function) of innate immunity.

CD1d degradation in Chlamydia trachomatis-infected epithelial cells is the result of both cellular and chlamydial proteasomal activity

Chlamydia trachomatis is an obligate intracellular pathogen that can persist in the urogenital tract. Mechanisms by which C. trachomatis evades clearance by host innate immune responses are poorly described. CD1d is MHC-like, is expressed by epithelial cells, and can signal innate immune responses by NK and NKT cells. Here we demonstrate that C. trachomatis infection down-regulates surface-expressed CD1d in human penile urethral epithelial cells through proteasomal degradation. A chlamydial proteasome-like activity factor (CPAF) interacts with the CD1d heavy chain, and CPAF-associated CD1d heavy chain is then ubiquitinated and directed along two distinct proteolytic pathways. The degradation of immature glycosylated CD1d was blocked by the proteasome inhibitor lactacystin but not by MG132, indicating that degradation was not via the conventional proteasome. In contrast, the degradation of non-glycosylated CD1d was blocked by lactacystin and MG132, consistent with conventional cellular cytosolic degradation of N-linked glycoproteins. Immunofluorescent microscopy confirmed the interruption of CD1d trafficking to the cell surface, and the dislocation of CD1d heavy chains into both the cellular cytosol and the chlamydial inclusion along with cytosolic CPAF. C. trachomatis targeted CD1d toward two distinct proteolytic pathways. Decreased CD1d surface expression may help C. trachomatis evade detection by innate immune cells and may promote C. trachomatis persistence.

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

Previously, we demonstrated that Valpha14+ NKT cells and IFN-gamma are important upstream components in neutrophil-mediated host defense against infection with Streptococcus pneumoniae. In the present study, we extended these findings by elucidating the role of IFN-gamma in this Valpha14+ NKT cell-promoted process. Administration of recombinant IFN-gamma to Jalpha18KO mice prolonged the shortened survival, promoted the attenuated clearance of bacteria and improved the reduced accumulation of neutrophils and synthesis of MIP-2 and TNF-alpha in the lungs, in comparison to wild-type (WT) mice. In addition, intravenous transfer of liver mononuclear cells (LMNC) from WT mice into Jalpha18KO mice resulted in complete recovery of the depleted responses listed above, whereas such effects were not detected when LMNC were obtained from IFN-gammaKO or Jalpha18KO mice. Activation of Valpha14+ NKT cells by alpha-galactosylceramide (alpha-GalCer) significantly enhanced the clearance of bacteria, accumulation of neutrophils and synthesis of MIP-2 and TNF-alpha in the infected lungs; this effect was significantly inhibited by a neutralizing anti-IFN-gamma antibody. Finally, in a flow cytometric analysis, TNF-alpha synthesis was detected largely by CD11b(bright+) cells in the infected lungs. Our results demonstrated that IFN-gamma plays an important role in the neutrophil-mediated host protective responses against pneumococcal infection promoted by Valpha14+ NKT cells.

Experimental African trypanosomiasis: lack of effective CD1d-restricted antigen presentation

BALB/c mice are highly susceptible to African trypanosomiasis, whereas C57BL/6 mice are relatively resistant. Other investigators have reported that the synthesis of IgG antibodies to purified membrane form of variant surface glycoprotein (mfVSG) of Trypanosoma brucei is CD1 restricted. In this study, we examine the role of the CD1d/NKT cell pathway in susceptibility and resistance of mice to infection by African trypanosomes. Administration of anti-CD1d antibodies to Trypanosoma congolense-infected BALB/c mice neither affects the parasitemia nor the survival time. Correspondingly, CD1d(-/-) and CD1d(+/+) BALB/c mice infected with T. congolense or T. brucei show no differences in either parasitaemia or survival time. The course of disease in relative resistant C57BL/6 mice infected with T. congolense is also not affected by the absence of CD1d. Parasitaemia, survival time, and plasma levels of IgG2a and IgG3 parasite-specific antibodies in infected CD1d(-/-) C57BL/6 are not different from those of infected CD1d(+/+) C57BL/6 mice. We conclude that CD1d-restricted immune responses do not play an important role in susceptibility/resistance of mice infected with virulent African trypanosomes. We speculate that virulent trypanosomes have an evasion mechanism that prevents the induction of a parasite-specific, CD1d-restricted immune response by the host.

Invariant natural killer T cells in the response to bacteria: the advent of specific antigens

Invariant natural killer T (iNKT) cells are a unique subset of T lymphocytes that have been implicated in diverse immune reactions, ranging from self-tolerance and development of autoimmunity to responses to pathogens and tumors. Although some degree of autoreactivity of iNKT cells has been shown, it remained controversial whether the T-cell antigen receptor expressed by these cells could recognize microbial antigens, hampering the investigation of their physiological role during tolerance and immunity. Several recent publications have now defined natural antigens for the majority of iNKT cells in some Proteobacteria and in Borrelia burgdorferi, demonstrating specificity of these cells for microbes in addition to self-reactivity. The characterization of natural antigens from bacteria, and the iNKT cell response to bacteria containing them, are decisive steps toward the clarification of the natural role of iNKT cells in host defense against pathogens, and will likely spur numerous findings in the near future.

CD1d mediates T-cell-dependent resistance to secondary infection with encephalomyocarditis virus (EMCV) in vitro and immune response to EMCV infection in vivo

The innate and adaptive immune responses have evolved distinct strategies for controlling different viral pathogens. Encephalomyocarditis virus (EMCV) is a picornavirus that can cause paralysis, diabetes, and myocarditis within days of infection. The optimal innate immune response against EMCV in vivo requires CD1d. Interaction of antigen-presenting cell CD1d with distinct natural killer T-cell ("NKT") populations can induce rapid gamma interferon (IFN-gamma) production and NK-cell activation. The T-cell response of CD1d-deficient mice (lacking all NKT cells) against acute EMCV infection was further studied in vitro and in vivo. EMCV persisted at higher levels in CD1d-knockout (KO) splenocyte cultures infected in vitro. Furthermore, optimal resistance to repeat cycles of EMCV infection in vitro was also shown to depend on CD1d. However, this was not reflected in the relative levels of NK-cell activation but rather by the responses of both CD4(+) and CD8(+) T-cell populations. Repeated EMCV infection in vitro induced less IFN-gamma and alpha interferon (IFN-alpha) from CD1d-deficient splenocytes than with the wild type. Furthermore, the level of EMCV replication in wild-type splenocytes was markedly and specifically increased by addition of blocking anti-CD1d antibody. Depletion experiments demonstrated that dendritic cells contributed less than the combination of NK and NKT cells to anti-EMCV responses and that none of these cell types was the main source of IFN-alpha. Finally, EMCV infection in vivo produced higher levels of viremia in CD1d-KO mice than in wild-type animals, coupled with significantly less lymphocyte activation and IFN-alpha production. These results point to the existence of a previously unrecognized mechanism of rapid CD1d-dependent stimulation of the antiviral adaptive cellular immune response.

Natural killer T cells recognize diacylglycerol antigens from pathogenic bacteria

Natural killer T (NKT) cells recognize glycosphingolipids presented by CD1d molecules and have been linked to defense against microbial infections. Previously defined foreign glycosphingolipids recognized by NKT cells are uniquely found in nonpathogenic sphingomonas bacteria. Here we show that mouse and human NKT cells also recognized glycolipids, specifically a diacylglycerol, from Borrelia burgdorferi, which causes Lyme disease. The B. burgdorferi-derived, glycolipid-induced NKT cell proliferation and cytokine production and the antigenic potency of this glycolipid was dependent on acyl chain length and saturation. These data indicate that NKT cells recognize categories of glycolipids beyond those in sphingomonas and suggest that NKT cell responses driven by T cell receptor-mediated glycolipid recognition may provide protection against diverse pathogens.