The role of CD1d in the immune response against Listeria infection

Embryonic macrophages function during early life to determine invariant natural killer T cell levels at barrier surfaces

It is increasingly recognized that immune development within mucosal tissues is under the control of environmental factors during early life. However, the cellular mechanisms that underlie such temporally and regionally restrictive governance of these processes are unclear. Here, we uncover an extrathymic pathway of immune development within the colon that is controlled by embryonic but not bone marrow-derived macrophages, which determines the ability of these organs to receive invariant natural killer T (iNKT) cells and allow them to establish local residency. Consequently, early-life perturbations of fetal-derived macrophages result in persistent decreases of mucosal iNKT cells and is associated with later-life susceptibility or resistance to iNKT cell-associated mucosal disorders. These studies uncover a host developmental program orchestrated by ontogenically distinct macrophages that is regulated by microbiota, and they reveal an important postnatal function of macrophages that emerge in fetal life.

Identification of a Potent Microbial Lipid Antigen for Diverse NKT Cells

Semi-invariant/type I NKT cells are a well-characterized CD1d-restricted T cell subset. The availability of potent Ags and tetramers for semi-invariant/type I NKT cells allowed this population to be extensively studied and revealed their central roles in infection, autoimmunity, and tumor immunity. In contrast, diverse/type II NKT (dNKT) cells are poorly understood because the lipid Ags that they recognize are largely unknown. We sought to identify dNKT cell lipid Ag(s) by interrogating a panel of dNKT mouse cell hybridomas with lipid extracts from the pathogen Listeria monocytogenes. We identified Listeria phosphatidylglycerol as a microbial Ag that was significantly more potent than a previously characterized dNKT cell Ag, mammalian phosphatidylglycerol. Further, although mammalian phosphatidylglycerol-loaded CD1d tetramers did not stain dNKT cells, the Listeria-derived phosphatidylglycerol-loaded tetramers did. The structure of Listeria phosphatidylglycerol was distinct from mammalian phosphatidylglycerol because it contained shorter, fully-saturated anteiso fatty acid lipid tails. CD1d-binding lipid-displacement studies revealed that the microbial phosphatidylglycerol Ag binds significantly better to CD1d than do counterparts with the same headgroup. These data reveal a highly potent microbial lipid Ag for a subset of dNKT cells and provide an explanation for its increased Ag potency compared with the mammalian counterpart.

The Extended Family of CD1d-Restricted NKT Cells: Sifting through a Mixed Bag of TCRs, Antigens, and Functions

Natural killer T (NKT) cells comprise a family of specialized T cells that recognize lipid antigens presented by CD1d. Based on their T cell receptor (TCR) usage and antigen specificities, CD1d-restricted NKT cells have been divided into two main subsets: type I NKT cells that use a canonical invariant TCR α-chain and recognize α-galactosylceramide (α-GalCer), and type II NKT cells that use a more diverse αβ TCR repertoire and do not recognize α-GalCer. In addition, α-GalCer-reactive NKT cells that use non-canonical αβ TCRs and CD1d-restricted T cells that use γδ or δ/αβ TCRs have recently been identified, revealing further diversity among CD1d-restricted T cells. Importantly, in addition to their distinct antigen specificities, functional differences are beginning to emerge between the different members of the CD1d-restricted T cell family. In this review, while using type I NKT cells as comparison, we will focus on type II NKT cells and the other non-invariant CD1d-restricted T cell subsets, and discuss our current understanding of the antigens they recognize, the formation of stimulatory CD1d/antigen complexes, the modes of TCR-mediated antigen recognition, and the mechanisms and consequences of their activation that underlie their function in antimicrobial responses, anti-tumor immunity, and autoimmunity.

The Response of CD1d-Restricted Invariant NKT Cells to Microbial Pathogens and Their Products

Invariant natural killer T (iNKT) cells become activated during a wide variety of infections. This includes organisms lacking cognate CD1d-binding glycolipid antigens recognized by the semi-invariant T cell receptor of iNKT cells. Additional studies have shown that iNKT cells also become activated in vivo in response to microbial products such as bacterial lipopolysaccharide, a potent inducer of cytokine production in antigen-presenting cells (APCs). Other studies have shown that iNKT cells are highly responsive to stimulation by cytokines such as interleukin-12. These findings have led to the concept that microbial pathogens can activate iNKT cells either directly via glycolipids or indirectly by inducing cytokine production in APCs. iNKT cells activated in this manner produce multiple cytokines that can influence the outcome of infection, usually in favor of the host, although potent iNKT cell activation may contribute to an uncontrolled cytokine storm and sepsis. One aspect of the response of iNKT cells to microbial pathogens is that it is short-lived and followed by an extended time period of unresponsiveness to reactivation. This refractory period may represent a means to avoid chronic activation and cytokine production by iNKT cells, thus protecting the host against some of the negative effects of iNKT cell activation, but potentially putting the host at risk for secondary infections. These effects of microbial pathogens and their products on iNKT cells are not only important for understanding the role of these cells in immune responses against infections but also for the development of iNKT cell-based therapies.

The function of the chemokine receptor CXCR6 in the T cell response of mice against Listeria monocytogenes

The chemokine receptor CXCR6 is expressed on different T cell subsets and up-regulated following T cell activation. CXCR6 has been implicated in the localization of cells to the liver due to the constitutive expression of its ligand CXCL16 on liver sinusoidal endothelial cells. Here, we analyzed the role of CXCR6 in CD8+ T cell responses to infection of mice with Listeria monocytogenes. CD8+ T cells responding to listerial antigens acquired high expression levels of CXCR6. However, deficiency of mice in CXCR6 did not impair control of the L. monocytogenes infection. CXCR6-deficient mice were able to generate listeria-specific CD4+ and CD8+ T cell responses and showed accumulation of T cells in the infected liver. In transfer assays, we detected reduced accumulation of listeria-specific CXCR6-deficient CD8+ T cells in the liver at early time points post infection. Though, CXCR6 was dispensable at later time points of the CD8+ T cell response. When transferred CD8+ T cells were followed for extended time periods, we observed a decline in CXCR6-deficient CD8+ T cells. The manifestation of this cell loss depended on the tissue analyzed. In conclusion, our results demonstrate that CXCR6 is not required for the formation of a T cell response to L. monocytogenes and for the accumulation of T cells in the infected liver but CXCR6 appears to influence long-term survival and tissue distribution of activated cells.

Helminth-induced interleukin-4 abrogates invariant natural killer T cell activation-associated clearance of bacterial infection

Helminth infections affect 1 billion people worldwide and render these individuals susceptible to bacterial coinfection through incompletely understood mechanisms. This includes urinary tract coinfection by bacteria and Schistosoma haematobium worms, the etiologic agent of urogenital schistosomiasis. To study the mechanisms of S. haematobium-bacterial urinary tract coinfections, we combined the first tractable model of urogenital schistosomiasis with an established mouse model of bacterial urinary tract infection (UTI). A single bladder exposure to S. haematobium eggs triggers interleukin-4 (IL-4) production and makes BALB/c mice susceptible to bacterial UTI when they are otherwise resistant. Ablation of IL-4 receptor alpha (IL-4Rα) signaling restored the baseline resistance of BALB/c mice to bacterial UTI despite prior exposure to S. haematobium eggs. Interestingly, numbers of NKT cells were decreased in coexposed versus bacterially monoinfected bladders. Given that schistosome-induced, non-natural killer T (NKT) cell leukocyte infiltration may dilute NKT cell numbers in the bladders of coexposed mice without exerting a specific functional effect on these cells, we next examined NKT cell biology on a per-cell basis. Invariant NKT (iNKT) cells from coexposed mice expressed less gamma interferon (IFN-γ) per cell than did those from mice with UTI alone. Moreover, coexposure resulted in lower CD1d expression in bladder antigen-presenting cells (APC) than did bacterial UTI alone in an IL-4Rα-dependent fashion. Finally, coexposed mice were protected from prolonged bacterial infection by administration of α-galactosylceramide, an iNKT cell agonist. Our findings point to a previously unappreciated role for helminth-induced IL-4 in impairment of iNKT cell-mediated clearance of bacterial coexposure.

Saposins modulate human invariant Natural Killer T cells self-reactivity and facilitate lipid exchange with CD1d molecules during antigen presentation

Lipid transfer proteins, such as molecules of the saposin family, facilitate extraction of lipids from biological membranes for their loading onto CD1d molecules. Although it has been shown that prosaposin-deficient mice fail to positively select invariant natural killer T (iNKT) cells, it remains unclear whether saposins can facilitate loading of endogenous iNKT cell agonists in the periphery during inflammatory responses. In addition, it is unclear whether saposins, in addition to loading, also promote dissociation of lipids bound to CD1d molecules. To address these questions, we used a combination of cellular assays and demonstrated that saposins influence CD1d-restricted presentation to human iNKT cells not only of exogenous lipids but also of endogenous ligands, such as the self-glycosphingolipid β-glucopyranosylceramide, up-regulated by antigen-presenting cells following bacterial infection. Furthermore, we demonstrated that in human myeloid cells CD1d-loading of endogenous lipids after bacterial infection, but not at steady state, requires trafficking of CD1d molecules through an endo-lysosomal compartment. Finally, using BIAcore assays we demonstrated that lipid-loaded saposin B increases the off-rate of lipids bound to CD1d molecules, providing important insights into the mechanisms by which it acts as a "lipid editor," capable of fine-tuning loading and unloading of CD1d molecules. These results have important implications in understanding how to optimize lipid-loading onto antigen-presenting cells, to better harness iNKT cells central role at the interface between innate and adaptive immunity.

Role of type 1 natural killer T cells in pulmonary immunity

Mucosal sites are populated by a multitude of innate lymphoid cells and "innate-like" T lymphocytes expressing semiconserved T-cell receptors. Among the latter group, interest in type I natural killer T (NKT) cells has gained considerable momentum over the last decade. Exposure to NKT cell antigens is likely to occur continuously at mucosal sites. For this reason, and as they rapidly respond to stress-induced environmental cytokines, NKT cells are important contributors to immune and inflammatory responses. Here, we review the dual role of mucosal NKT cells during immune responses and pathologies with a particular focus on the lungs. Their role during pulmonary acute and chronic inflammation and respiratory infections is outlined. Whether NKT cells might provide a future attractive therapeutic target for treating human respiratory diseases is discussed.

Targeting the diverse immunological functions expressed by hepatic NKT cells

INTRODUCTION

NKT cells comprise approximately 30% of the hepatic lymphoid population in mice (∼ 50% in humans). Most mouse hepatic NKT cells [invariant (i)NKT cells] express T cell receptors, composed of invariant Vα14Jα18 chains. Unlike conventional T cells, iNKT cells recognize glycolipids presented in association with MHC class Ib (CD1d) molecules. Purportedly, iNKT cells serve key functions in several immunological events; the nature of these is often unclear. The consequences of hepatic iNKT cell activation can be beneficial or detrimental. α-Galactosylceramide stimulates the production of IFN-γ and IL-4. The reciprocal suppression exhibited by these cytokines limits the potential therapeutic value of α-galactosylceramide. Efforts are ongoing to develop α-galactosylceramide analogs that modulate iNKT cell activity and selectively promote IFN-γ or IL-4.

AREAS COVERED

An overview of hepatic iNKT cells and their purported role in liver disease. Efforts to develop therapeutic agents that promote their beneficial contributions.

EXPERT OPINION

While a growing body of literature documents the differential effects of α-GalCer analogs on IFN-γ and IL-4 production, the effects of these analogs on other iNKT cell activities remain to be determined. An exhaustive examination of the effects of these analogs on inflammation and liver injury in animal models remains prior to considering their utility in clinical trials.

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.

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.

Elevated non-specific immunity and normal Listeria clearance in young and old vitamin D receptor knockout mice

1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and the vitamin D receptor (VDR) are important regulators of autoimmunity. The effect of the VDR on the ability of mice to fight a primary or secondary infection has not been determined. Young and old VDR knockout (KO) mice were able to clear both primary and secondary infections with Listeria monocytogenes. However, the kinetics of clearance was somewhat delayed in the absence of the VDR. Memory T cell development was not different in young VDR KO and wild-type (WT) mice; however, old VDR KO mice had significantly less memory T cells than their WT counterparts but still mounted an adequate immune response as determined by the complete clearance of L. monocytogenes. Although the primary and secondary immune responses were largely intact in the VDR KO mice, the old VDR KO mice had increased cytokines and antibody responses compared with the old WT mice. In particular, old VDR KO mice had elevated antigen non-specific antibodies; however, these magnified immune responses did not correspond to more effective Listeria clearance. The increased antibody and cytokine responses in the old VDR KO mice are consistent with the increased susceptibility of these mice to autoimmunity.

Role of NKT cells in the digestive system. IV. The role of canonical natural killer T cells in mucosal immunity and inflammation

Lymphocytes that combine features of T cells and natural killer (NK) cells are named natural killer T (NKT) cells. The majority of NKT cells in mice bear highly conserved invariant Valpha chains, and to date two populations of such canonical NKT cells are known in mice: those that express Valpha14 and those that express Valpha7.2. Both populations are selected by nonpolymorphic major histocompatibility complex class I-like antigen-presenting molecules expressed by hematopoietic cells in the thymus: CD1d for Valpha14-expressing NKT cells and MR1 for those cells expressing Valpha7.2. The more intensely studied Valpha14 NKT cells have been implicated in diverse immune reactions, including immune regulation and inflammation in the intestine; the Valpha7.2 expressing cells are most frequently found in the lamina propria. In humans, populations of canonical NKT cells are found to be highly similar in terms of the expression of homologous, invariant T cell antigen-receptor alpha-chains, specificity, and function, although their frequency differs from those in the mouse. In this review, we will focus on the role of both of these canonical NKT cell populations in the mucosal tissues of the intestine.

Role of NKT cells in the digestive system. III. Role of NKT cells in intestinal immunity

Natural killer T (NKT) cells are a small subset of unconventional T cells that recognize lipid antigens presented by the nonclassical major histocompatibility complex (MHC) class I molecule CD1d. NKT cells are involved in the host response to a variety of microbial pathogens and likely commensals. In the intestine, invariant and noninvariant NKT cells can be found among intraepithelial lymphocytes and in the lamina propria. Activation of intestinal NKT cells by CD1d-expressing intestinal epithelial cells and professional antigen-presenting cells may contribute to induction of oral tolerance and protection from mucosal infections. On the other hand, sustained and uncontrolled activation of NKT cells may play a pivotal role in the pathogenesis of inflammatory bowel disease. Here we review the current literature on intestinal NKT cells and their function in the intestine in health and disease.

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.

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.

Exacerbated susceptibility to infection-stimulated immunopathology in CD1d-deficient mice

Mice lacking functional CD1d genes were used to study mechanisms of resistance to the protozoan parasite Toxoplasma gondii. Wild-type (WT) BALB/c mice, CD1d-deficient BALB/c mice, and WT C57BL/6 mice all survived an acute oral infection with a low dose of mildly virulent strain ME49 T. gondii cysts. In contrast, most CD1d-deficient C57BL/6 mice died within 2 wk of infection. Despite having parasite burdens that were only slightly higher than WT mice, CD1d-deficient C57BL/6 mice displayed greater weight loss and intestinal pathology. In C57BL/6 mice, CD4(+) cells can cause intestinal pathology during T. gondii infection. Compared with WT mice, infected CD1d-deficient C57BL/6 mice had higher frequencies and numbers of activated (CD44(high)) CD4(+) cells in mesenteric lymph nodes. Depletion of CD4(+) cells from CD1d-deficient mice reduced weight loss and prolonged survival, demonstrating a functional role for CD4(+) cells in their increased susceptibility to T. gondii infection. CD1d-deficient mice are deficient in Valpha14(+) T cells, a major population of NKT cells. Involvement of these cells in resistance to T. gondii was investigated using gene-targeted Jalpha18-deficient C57BL/6 mice, which are deficient in Valpha14(+) T cells. These mice did not succumb to acute infection, but experienced greater weight loss and more deaths than B6 mice during chronic infection, indicating that Valpha14(+) cells contribute to resistance to T. gondii. The data identify CD4(+) cells as a significant component of the marked susceptibility to T. gondii infection observed in CD1d-deficient C57BL/6 mice, and establish T. gondii as a valuable tool for deciphering CD1d-dependent protective mechanisms.

Invariant Vα14+NKT Cells Participate in the Early Response to EntericListeria monocytogenesInfection

Invariant Valpha14(+) NKT cells are a specialized CD1-reactive T cell subset implicated in innate and adaptive immunity. We assessed whether Valpha14(+) NKT cells participated in the immune response against enteric Listeria monocytogenes infection in vivo. Using CD1d tetramers loaded with the synthetic lipid alpha-galactosylceramide (CD1d/alphaGC), we found that splenic and hepatic Valpha14(+) NKT cells in C57BL/6 mice were early producers of IFN-gamma (but not IL-4) after L. monocytogenes infection. Adoptive transfer of Valpha14(+) NKT cells derived from TCRalpha degrees Valpha14-Jalpha18 transgenic (TCRalpha degrees Valpha14Tg) mice into alymphoid Rag(null) gamma(c)(null) mice demonstrated that Valpha14(+) NKT cells were capable of providing early protection against enteric L. monocytogenes infection with systemic production of IFN-gamma and reduction of the bacterial burden in the liver and spleen. Rechallenge experiments demonstrated that previously immunized wild-type and Jalpha18null mice, but not TCRalpha(null) or TCRalpha(null) Valpha14Tg mice, were able to mount adaptive responses to L. monocytogenes. These data demonstrate that Valpha14(+) NKT cells are able to participate in the early response against enteric L. monocytogenes through amplification of IFN-gamma production, but are not essential for, nor capable of, mediating memory responses required to sterilize the host.

CD1d-independent regulation of NKT cell migration and cytokine production upon Listeria monocytogenes infection

Natural killer T (NKT) cells are a unique T-cell population that is positively selected by CD1d-expressing cells. In this study, we examined the kinetics of conventional CD4+TCRbeta+ and CD4-TCRbeta+ cells along with various NKT cell populations from WT and CD1d KO mice after oral Listeria monocytogenes (Lm) infection at different time points in tissue compartments. We found that CD4+TCRbeta+ cells expressing NK1.1+ (NKT) were constitutively expressed in the lung of both strains of mice, but disappeared after infection. In contrast, CD4-TCRbeta+ NK1.1+ cells migrated to the spleen. Here, we demonstrated that endogenous IL-12 was predominantly expressed in the spleen of CD1d KO mice 2 days after infection, whereas IL-4 was predominantly expressed in the liver of WT mice. Higher levels of IFN-gamma were expressed in MLN of CD1d KO but not in WT mice on day 5. Thus, tissue-specific ligands orchestrate the localization and activation of NKT cells to control immune response to Listeria, which may explain the difference in disease susceptibility.

To be or not to be NKT: natural killer T cells in the liver

Much of the hepatology literature to date has focused on the adaptive, antigen-specific response mediated by classical T-cell populations in both the protection and pathogenesis of liver disease. However, the liver is selectively enriched for cells representative of innate immunity, including natural killer T (NKT) cells. In particular, certain CD1d-reactive T cells are present at much higher frequencies in the liver than in the peripheral blood. Although these cells have previously been defined mostly on the basis of phenotypic markers, recent emerging literature regarding NKT cell populations has revealed considerable functional complexity. This review summarizes the recent literature regarding NKT cells, which may have important roles in a variety of liver diseases. Although there is an abundance of literature on the phenotype, distribution, and function of these cells in mice, much less is known about them in human health or liver diseases.