Presentation of self and microbial lipids by CD1 molecules

CD1-mediated immune responses in mucosal tissues: molecular mechanisms underlying lipid antigen presentation system

The cluster of differentiation 1 (CD1) molecule differs from major histocompatibility complex class I and II because it presents glycolipid/lipid antigens. Moreover, the CD1-restricted T cells that recognize these self and foreign antigens participate in both innate and adaptive immune responses. CD1s are constitutively expressed by professional and nonprofessional antigen-presenting cells in mucosal tissues, namely, the skin, lung, and intestine. This suggests that CD1-reactive T cells are involved in the immune responses of these tissues. Indeed, evidence suggests that these cells play important roles in diverse diseases, such as inflammation, autoimmune disease, and infection. Recent studies elucidating the molecular mechanisms by which CD1 presents lipid antigens suggest that defects in these mechanisms could contribute to the activities of CD1-reactive T cells. Thus, improving our understanding of these mechanisms could lead to new and effective therapeutic approaches to CD1-associated diseases. In this review, we discuss the CD1-mediated antigen presentation system and its roles in mucosal tissue immunity.

Evidence of CD1d pathway of lipid antigen presentation in mouse primary lung epithelial cells and its up-regulation upon Mycobacterium bovis BCG infection

Presentation of a prototype lipid antigen α-Galactosylceramide (αGC) was examined on primary epithelial cells derived from mouse lungs and on bronchoalveolar lavage (BAL) cells that essentially comprise alveolar macrophages. Presence of CD1d molecules coupled to αGC was demonstrated on both types of cells pre-treated with αGC, suggesting that both cell types are equipped to present lipid antigens. Internalization of Mycobacterium bovis Bacillus Calmette–Guérin (BCG: a prototype pathogen), a pre-requisite to the processing and presentation of protein as well as lipid antigens, was clearly demonstrated in primary lung epithelial (PLE) cells as well as BAL cells. Both PLE and BAL cells expressed CD1d molecule and a significant up-regulation of its expression occurred upon infection of these cells with BCG. Besides CD1d, the expression of other important molecules that participate in lipid antigen presentation pathway (i.e. microsomal triglyceride transfer protein (MTTP), scavenger receptor B1 (SR-B1) and Saposin) was also significantly upregulated in PLE and BAL cells upon BCG infection. In situ up-regulation of CD1d expression on lung epithelial cells was also demonstrated in the lungs of mice exposed intra-tracheally to BCG. Taken together these results suggest that lung epithelial cells may have the ability to present lipid antigens and this pathway seems to get significantly upregulated in response to BCG infection.

B cells interactions in lipid immune responses: implications in atherosclerotic disease

Atherosclerosis is considered as an inflammatory and chronic disorder with an important immunologic component, which underlies the majority of cardiovascular diseases; condition that belongs to a group of noncommunicable diseases that to date and despite of prevention and treatment approaches, they remain as the main cause of death worldwide, with 17.5 million of deaths every year. The impact of lipids in human health and disease is taking center stage in research, due to lipotoxicity explained by elevated concentration of circulating lipids, in addition to altered adipose tissue metabolism, and aberrant intracellular signaling. Immune response and metabolic regulation are highly integrated systems and the proper function of each one is dependent on the other. B lymphocytes express a variety of receptors that can recognize foreign, endogenous or modified self-antigens, among them oxidized low density lipoproteins, which are the main antigens in atherosclerosis. Mechanisms of B cells to recognize, remove and present lipids are not completely clear. However, it has been reported that B cell can recognize/remove lipids through a range of receptors, such as LDLR, CD1d, FcR and SR, which might have an atheroprotector or proatherogenic role during the course of atherosclerotic disease. Pertinent literature related to these receptors was examined to inform the present conclusions.

Lipid antigen presentation through CD1d pathway in mouse lung epithelial cells, macrophages and dendritic cells and its suppression by poly-dispersed single-walled carbon nanotubes

Effect of poly-dispersed acid-functionalized single-walled carbon nanotubes (AF-SWCNTs) was examined on lipid antigen presentation through CD1d pathway on three cell lines, LA4, MHS, and JAWSII used as prototype antigen presenting cells (APCs). CD1d molecule was expressed on 80-90% MHS (prototype macrophages) and JAWSII (prototype dendritic cells) cells whereas <5% LA4 cells (lung epithelial cells, non-classical APCs) expressed CD1d. Treatment with AF-SWCNTs but not with pristine SWCNTs resulted in a significant decline in the level of CD1d mRNA as well as mRNA levels of some other intracellular proteins involved in lipid antigen presentation pathway (MTP, ApoE, prosaposin, SR-BI and LDLr). Lipid antigen presentation was assessed by first incubating the cells with a prototype lipid antigen (α-Glactosylceramide or αGC) and then staining with L363 monoclonal antibody that detects αGC bound to CD1d molecule. While 100% MHS and JAWSII cells presented αGC, only 20% LA4 cells presented the CD1d antigen. Treatment with AF-SWCNTs resulted in a 30-40% decrease in αGC antigen presentation in all three cell lines. These results show that AF-SWCNT treatment down regulated the lipid antigen presentation pathway in all three cell lines and significantly lowered the ability of these cell lines to present αGC antigen.

Disseminated varicella infection caused by varicella vaccine strain in a child with low invariant natural killer T cells and diminished CD1d expression

BACKGROUND

Live attenuated varicella vaccine is considered a safe vaccine with serious adverse effects reported only in immunocompromised children. We describe a severe life-threatening infection with varicella vaccine virus causing rash and pneumonitis in a 6-year-old boy with no apparent immunodeficiency.

METHODS AND RESULTS

Polymerase chain reaction (PCR) analysis of vesicle swab samples demonstrated varicella zoster virus (VZV). Sequencing of the PCR product demonstrated 100% homology with human herpesvirus 3 strain VZV-Oka ORF62 gene. Routine immunologic investigations failed to demonstrate any abnormality. Total leukocyte, lymphocyte, and neutrophil counts and lymphocyte subsets were normal. Immunoglobulins, C3, C4, and CH50 were intact. Specific IgG to protein and polysaccharide antigens and to Epstein-Barr virus and cytomegalovirus were present. Normal lymphocyte proliferation to phytohemagglutinin and VZV antigens was detected. Neutrophil function and natural killer (NK) cell activity were normal. The analysis of invariant NK T (iNKT) cell numbers and function revealed diminished iNKT cells, reported once previously and unique to our patient, deficient expression of the cognate receptor, CD1d.

CONCLUSIONS

This report provides a further link between deficiency of the iNKT/CD1d pathway and increased susceptibility to varicella vaccine virus, suggesting an important role of this innate pathway in host defense against yet another member of the herpesvirus family.

Fine specificity of natural killer T cells against GD3 ganglioside and identification of GM3 as an inhibitory natural killer T-cell ligand

GD3, a ganglioside expressed on melanoma, is the only tumour-associated glycolipid described to date that can induce a CD1d-restricted natural killer T (NKT)-cell response. We analysed the fine specificity of GD3-reactive NKT cells and discovered that immunization with GD3 induced two populations of GD3-reactive NKT cells. One population was CD4+ CD8- and was specific for GD3; the other population was CD4- CD8- and cross-reacted with GM3 in a CD1d-restricted manner, but did not cross-react with GM2, GD2, or lactosylceramide. This indicated that the T-cell receptors reacting with GD3 recognize glucose-galactose linked to at least one N-acetyl-neuraminic acid but will not accommodate a terminal N-acetylgalactosamine. Immunization with GM2, GM3, GD2, or lactosylceramide did not induce an NKT-cell response. Coimmunization of GM3-loaded antigen-presenting cells (APCs) with GD3-loaded APCs suppressed the NKT-cell response to GD3 in a CD1d-restricted manner. This suppressive effect was specific for GM3 and was a local effect lasting 2-4 days. In vitro, GM3-loaded APCs also suppressed the interleukin-4 response, but not the interferon-gamma response, of NKT cells to alpha-galactosylceramide. However, there was no effect on the T helper type 2 responses of conventional T cells. We found that this suppression was not mediated by soluble factors. We hypothesize that GM3 induces changes to the APC that lead to suppression of T helper type 2-like NKT-cell responses.

Efficient activation of Valpha14 invariant NKT cells by foreign lipid antigen is associated with concurrent dendritic cell-specific self recognition

A burst release of cytokines by Valpha14 invariant NKT (iNKT) cells upon their TCR engagement critically regulates innate and adaptive immune responses. However, it remains unclear in vivo why iNKT cells respond efficiently to microbial or intracellular lipid Ags that are at low levels or that possess suboptimal antigenicity. We found that dendritic cells (DCs) potentiated iNKT cells to respond to a minimal amount of ligand alpha-galactosylceramide (alphaGalCer) through CD1d-dependent autoreactive responses that require endosomal processing and CD1d trafficking. The ability of potentiation of NKT cells was DC specific and did not depend on costimulatory signals and IL-12 production by DCs. However, DCs that failed to synthesize a major endogenous lipid Ag isoglobotrihexosylceramide were unable to potentiate NKT cells for efficient activation. Further analysis showed that differences in the level and pattern of endogenous lipid Ag presentation differentiate DCs and B cells for effective potentiation and subsequent activation of iNKT cells in the presence of an exogenous Ag. Thus, CD1d-dependent potentiation by DCs may be crucial for iNKT cell-mediated immunity against infectious agents.

Evolution of mammalian CD1: marsupial CD1 is not orthologous to the eutherian isoforms and is a pseudogene in the opossum Monodelphis domestica

CD1 is a member of the major histocompatibility complex (MHC) class I family of proteins that present lipid antigens to T cells and natural killer (NK) T cells; it is found in both eutherian mammals and birds. In eutherians, duplication of the CD1 gene has resulted in multiple isoforms. A marsupial CD1 homologue was identified in a set of expressed sequence tags from the thymus of the bandicoot Isoodon macrourus. Southern blot and genomic sequence analyses revealed that CD1 is a single copy gene in both I. macrourus and a distantly related marsupial, the opossum Monodelphis domestica, which is currently the only marsupial species for which a whole genome sequence is available. We found that the opossum CD1 is located in a genomic region with a high degree of conserved synteny to the chromosomal regions containing human and mouse CD1. A phylogenetic analysis of mammalian CD1 revealed that marsupial CD1 is not orthologous to the eutherian CD1 isoforms, consistent with the latter having emerged by duplication after the separation of marsupials and eutherians 170-180 million years ago. The I. macrourus CD1 gene is actively transcribed and appears to encode a functional protein. In contrast, transcription of the M. domestica CD1 was not detected in any tissue and the predicted CD1 gene sequence contains a number of deletions that appear to render the locus a pseudogene.

Role of apolipoproteins in gammadelta and NKT cell-mediated innate immunity

Recent findings reveal unanticipated connections between the fields of lipid metabolism and immunology. They concern gammadelta and NKT cells, nonconventional T cell populations that do not recognize protein antigens and are involved in immunity against cancer, defense against infections, or in regulation of classical immune responses. In this review, we summarize data linking perturbations of apolipoprotein levels and nonconventional T cells with inflammatory processes such as autoimmune diseases or atherosclerosis. We integrate and discuss recent findings on the implication of apolipoproteins in antigen recognition by gammadelta and NKT cells, with emphasis on apolipoproteins A-I and E. These findings also provide indications that apolipoproteins influence antitumor immunosurveillance.

Upregulation of group 1 CD1 antigen presenting molecules in guinea pigs with experimental autoimmune encephalomyelitis: an immunohistochemical study

In humans, group 1 CD1 glycoproteins present foreign and self lipid and glycolipid antigens to T-cells. Homologues of these molecules are not found in mice or rats but are present in guinea pigs (GPs). We examined CD1 and MHC class II expression in the central nervous system (CNS) of GPs sensitized for experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. In normal GPs and the uninflamed CNS, low-level MHC class II (MHC II) immunoreactivity occurred on vascular elements, meningeal macrophages and parenchymal microglial cells, whereas immunoreactivity for CD1 was absent. In the inflamed CNS, the majority of infiltrating cells were MHC II+ and microglia showed increased expression. CD1 immunoreactivity was detected on astrocytes and subsets of inflammatory cells Including B cells and macrophages. Minimal CD1 and MHC II co-expression was noted on inflammatory cells or glia. We conclude that group 1 CD1 molecules are strongly upregulated in the inflamed CNS on subsets of cells distinct from the majority of MHC II bearing cells. The expression of CD1 proteins in such lesions broadens the potential repertoire of antigens recognized at these sites and highlights the value of the GP as a model for studies of the relevance of CD1 molecules in host defense and autoimmune diseases.

Differential regulation of cytokine production by CD1d-restricted NKT cells in response to superantigen staphylococcal enterotoxin B exposure

NKT cells are a heterogeneous population characterized by the ability to rapidly produce cytokines, such as interleukin 2 (IL-2), IL-4, and gamma interferon (IFN-gamma) in response to infections by viruses, bacteria, and parasites. The bacterial superantigen staphylococcal enterotoxin B (SEB) interacts with T cells bearing the Vbeta3, -7, or -8 T-cell receptors, inducing their expansion and cytokine secretion, leading to death in some cases due to cytokine poisoning. The majority of NKT cells bear the Vbeta7 or -8 T-cell receptor, suggesting that they may play a role in regulating this response. Using mice lacking NKT cells (CD1d(-/-) and Jalpha18(-/-) mice), we set out to identify the role of these cells in T-cell expansion, cytokine secretion, and toxicity induced by exposure to SEB. We find that Vbeta8(+) CD4(+) T-cell populations similarly expand in wild-type (WT) and NKT cell-null mice and that NKT cells did not regulate the secretion of IL-2. By contrast, these cells positively regulated the secretion of IL-4 and IFN-gamma production and negatively regulated the secretion of tumor necrosis factor alpha (TNF-alpha). However, this negative regulation of TNF-alpha secretion by NKT cells provides only a minor protective effect on SEB-mediated shock in WT mice compared to mice lacking NKT cells. These data suggest that NKT cells may regulate the nature of the cytokine response to exposure to the superantigen SEB and may act as regulatory T cells during exposure to this superantigen.

Increased infiltration of CD1d and natural killer T cells in periodontal disease tissues

BACKGROUND

Natural killer T (NKT) cells are a unique T lymphocyte subset that has been implicated in the regulation of immune responses associated with a broad range of diseases including autoimmunity, infectious diseases, and cancer. In contrast to conventional T cells, NKT cells are reactive to major histocompatibility complex (MHC) class I-like molecule CD1d. Considering the periodontitis having both aspects of infection and autoimmunity in nature, CD1d and reactive NKT cells are of particular importance.

OBJECTIVE

The aim of the present study was to examine whether the expression of CD1 isoforms and Valpha24(+) invariant NKT cells is associated with different disease entities, namely gingivitis and periodontitis.

MATERIAL AND METHODS

Immunohistochemical analysis was performed on cryostat sections of gingival tissues from 19 patients with periodontitis and eight patients with gingivitis using antibodies to CD1a, b, c, d, Valpha24(+) invariant NKT cells, CD83, CD3 and CD19.

RESULTS

Although all four subsets of CD1 molecules were expressed in periodontal lesions, CD1d was most abundant. CD1d expression was more frequent in periodontitis than gingivitis and increased together with increase of invariant NKT cell infiltration. Double immunohistochemical staining showed co-expression of CD1d and CD19 on identical cells and proximate infiltration of CD1d(+) and invariant NKT cells.

CONCLUSION

These findings suggest that CD1d-expressing B cells could activate NKT cells by CD1d-restricted manner and this NKT cell activation may play roles in pathogenesis of periodontal diseases.

Two CD1 genes map to the chicken MHC, indicating that CD1 genes are ancient and likely to have been present in the primordial MHC

CD1 molecules play an important role in the immune system, presenting lipid-containing antigens to T and NKT cells. CD1 genes have long been thought to be as ancient as MHC class I and II genes, based on various arguments, but thus far they have been described only in mammals. Here we describe two CD1 genes in chickens, demonstrating that the CD1 system was present in the last common ancestor of mammals and birds at least 300 million years ago. In phylogenetic analysis, these sequences cluster with CD1 sequences from other species but are not obviously like any particular CD1 isotype. Sequence analysis suggests that the expressed proteins bind hydrophobic molecules and are recycled through intracellular vesicles. RNA expression is strong in lymphoid tissues but weaker to undetectable in some nonlymphoid tissues. Flow cytometry confirms expression from one gene on B cells. Based on Southern blotting and cloning, only two such CD1 genes are detected, located approximately 800 nucleotides apart and in the same transcriptional orientation. The sequence of one gene is nearly identical in six chicken lines. By mapping with a backcross family, this gene could not be separated from the chicken MHC on chromosome 16. Mining the draft chicken genome sequence shows that chicken has only these two CD1 genes located approximately 50 kb from the classical class I genes. The unexpected location of these genes in the chicken MHC suggests the CD1 system was present in the primordial MHC and is thus approximately 600 million years old.

Lipid rafts are required for efficient signal transduction by CD1d

Plasma membranes of eukaryotic cells are not uniform, possessing distinct cholesterol- and sphingolipid-rich lipid raft microdomains which constitute critical sites for signal transduction through various immune cell receptors and their co-receptors. CD1d is a conserved family of major histocompatibility class I-like molecules, which has been established as an important factor in lipid antigen presentation to natural killer T (NKT) cells. Unlike conventional T cells, recognition of CD1d by the T cell receptor (TCR) of NKT cells does not require CD4 or CD8 co-receptors, which are critical for efficient TCR signaling. We found that murine CD1d (mCD1d) was constitutively present in the plasma membrane lipid rafts on antigen presenting cells, and that this restricted localization was critically important for efficient signal transduction to the target NKT cells, at low ligand densities, even without the involvement of co-receptors. Further our results indicate that there may be additional regulatory molecule(s), co-located in the lipid raft with mCD1d for NKT cell signaling.

Dendritic cell maturation by CD11c− T cells and Vα24+ natural killer T-cell activation by α-Galactosylceramide

Human invariant Valpha24+ natural killer T (NKT) cells display potent antitumor activity upon stimulation. Activation of endogenous Valpha24+ NKT cells would be one strategy for the treatment of cancer patients. For example, dendritic cells (DCs) loaded with a glycolipid NKT cell ligand, alpha-galactosylceramide (alphaGalCer, KRN7000), are a possible tool for the activation and expansion of functional Valpha24+ NKT cells in vivo. In this report, we demonstrate that the levels of expansion and the ability to produce IFN-gamma of Valpha24+ NKT cells induced by alphaGalCer-loaded whole PBMCs cultured with IL-2 and GM-CSF (IL-2/GM-CSF-cultured PBMCs) were superior to those of cells induced by monocyte-derived CD11c+ DCs (moDCs) developed with IL-4 and GM-CSF. Interestingly, CD11c+ cells in the IL-2/GM-CSF-cultured PBMCs showed a mature phenotype without further stimulation and exerted potent stimulatory activity on Valpha24+ NKT cells to enable them to produce IFN-gamma preferentially at an extent equivalent to mature moDCs induced by stimulation with LPS or a cytokine cocktail. Cocultivation with CD11c- cells in the IL-2/GM-CSF-cultured PBMCs induced maturation of moDCs. In particular, CD11c-CD3+ T cells appeared to play important roles in DC maturation. In addition, TNF-alpha was preferentially produced by CD11c-CD3+ T cells in IL-2/GM-CSF-cultured PBMCs and was involved in the maturation of moDCs. Thus, the maturation of DCs induced by CD11c- T cells through TNF-alpha production appears to result in the efficient expansion and activation of Valpha24+ NKT cells to produce IFN-gamma preferentially.

Chlamydia trachomatis-specific human CD8+ T cells show two patterns of antigen recognition

Chlamydia trachomatis is an intracellular gram-negative bacteria which causes several clinically important diseases. T-cell-mediated immunity and the production of gamma interferon (IFN-gamma) are known to be essential for the clearance of the bacteria in vivo. Here we have investigated CD8(+)-T-cell responses to C. trachomatis in patients with previous episodes of chlamydia infection. To isolate C. trachomatis-specific CD8(+)-T-cell lines, dendritic cells (DC) were infected with C. trachomatis and cocultured with purified CD8(+) T cells to generate C. trachomatis-specific CD8(+)-T-cell lines which were then cloned. Two patterns of recognition of C. trachomatis-infected cells by CD8(+)-T-cell clones were identified. In the first, C. trachomatis antigens were recognized in association with classical class I HLA antigens, and responses were inhibited by class I HLA-specific monoclonal antibodies. The second set of clones was unrestricted by classical HLA class I, and further studies showed that CD1 molecules were also not the restriction element for those clones. Both types of clones produced IFN-gamma in response to C. trachomatis and were able to lyse C. trachomatis-infected target cells. However, unrestricted clones recognized C. trachomatis-infected cells at much earlier time points postinfection than HLA-restricted clones. Coculture of C. trachomatis-infected DC with the C. trachomatis-specific clones induced DC activation and a rapid enhancement of interleukin-12 (IL-12) production. Early production of IL-12 during C. trachomatis infection, facilitated by unrestricted CD8(+)-T-cell clones, may be important in ensuring a subsequent Th1 T-cell-mediated response by classical major histocompatibility complex-restricted CD4(+) and CD8(+) T cells.

Mycobacterial phosphatidylinositol mannoside is a natural antigen for CD1d-restricted T cells

A group of T cells recognizes glycolipids presented by molecules of the CD1 family. The CD1d-restricted natural killer T cells (NKT cells) are primarily considered to be self-reactive. By employing CD1d-binding and T cell assays, the following structural parameters for presentation by CD1d were defined for a number of mycobacterial and mammalian lipids: two acyl chains facilitated binding, and a polar head group was essential for T cell recognition. Of the mycobacterial lipids tested, only a phosphatidylinositol mannoside (PIM) fulfilled the requirements for CD1d binding and NKT cell stimulation. This PIM activated human and murine NKT cells via CD1d, thereby triggering antigen-specific IFN-gamma production and cell-mediated cytotoxicity, and PIM-loaded CD1d tetramers identified a subpopulation of murine and human NKT cells. This phospholipid, therefore, represents a mycobacterial antigen recognized by T cells in the context of CD1d.

The Guillain-Barré syndrome: a true case of molecular mimicry

Molecular mimicry between microbial antigens and host tissue forms an attractive hypothetical mechanism for the triggering of autoimmune disease by preceding infections. Recent crucial reviews state that molecular mimicry, as the causative mechanism, remains unproven for any human autoimmune disease. However, the peripheral neuropathy Guillain-Barré syndrome (GBS) is largely overseen in this debate. Based on recent evidence, we argue that GBS should be considered as an excellent paradigm and an attractive model for elucidation of both host and microbial aspects of molecular mimicry.

Inhibition of CD1d activation suppresses septic mortality: a role for NK-T cells in septic immune dysfunction

BACKGROUND

Studies indicate that following septic insult there is development of generalized immune dysfunction in T cells, B cells and phagocytes, which is thought to contribute to morbidity and mortality. Specifically, there is a shift in the lymphocytes of septic animals toward an increased release of Th2 cytokines. NK-T cells have been shown to contribute to propagation of the Th2 response. The influence of NK-T cells on the immune response to septic challenge is poorly understood. In this study, we examine whether NK-T cells contribute to the immune dysfunction seen following the onset of polymicrobial sepsis, as produced by cecal ligation and puncture (CLP).

MATERIALS AND METHODS

Male 129S1/SvImJ mice were pretreated with either rat IgG (isotypic control) or monoclonal antibody to CD1d (clone 1B1) (0.5 mg), which blocks signaling/antigen presentation via the CD1d cell surface receptor, thereby, ablating the activation and differentiation of the NK-T cells. Septic survival with and without anti-CD1d (CLP/CD1d) pretreatment was assessed. Mice sacrificed 24 h after CLP were assessed for change in splenic %NK-T cell (via flourescense activated cell sector) and for splenic, hepatic, and lymphoid/macrophage production of pro-inflammatory or anti-inflammatory cytokines (via enzyme-linked immunosorbent assay).

RESULTS

Administration of anti-CD1d reduced septic mortality 35% at 6-10 d (n = 23 mice/group) (P <.05). There was a consistent increase in the %CD3(+) NK1.1(+) cell population (NK-T cells) in septic mice (1.706%), which was markedly suppressed by pretreatment with anti-CD1d (0.592%). IL-6 and IL-10 levels were suppressed by anti-CD1d in the spleen and blood.

CONCLUSIONS

Together these findings imply not only that NK-T cells may play a role in mediating the immune suppression seen in bacterial sepsis, but that inhibition of their activation promotes survival to septic challenge.

Syngeneic pregnancy induces overexpression of natural killer T cells in maternal liver

Conditions such as stress, infection, autoimmune disease, etc. elevate the number and function of extrathymic T cells that are generated mainly in the liver. As primitive, self-reactive clones of T cells that coexpress receptors of the natural killer (NK) lineage, they mediate cytotoxicity against altered self, malignant and infected cells and have the unique potential to rapidly secrete large amount of T helper 1 (Th1) or Th2 cytokines. To elucidate whether some of these changes occur even during the syngeneic pregnancy, we made phenotypic and functional characterization of mononuclear lymphatic cells (MNLCs) isolated from the liver and spleen of pregnant C57BL/6 mice, testing their cytotoxicity against syngeneic thymocytes as well as against NK- and lymphokine-activated killer (LAK)-sensitive targets. The data have shown that on the sixteenth day of syngeneic pregnancy TCRint, NK1.1+ and IL-2Rbeta+ cells were accumulated in the liver, while the quantities of CD4+ and CD8+ T cells and total number classical NK (NK1.1+CD3- or IL-2Rbeta+CD3-) cells were increased in the spleen. Pregnancy-activated hepatic and splenic MNLCs were more cytotoxic against syngeneic thymocytes, YAC-1 and P815 targets, suggesting that the maternal liver is a main producer of autoreactive NKT clones, which subsequently augment NK- and LAK cell-mediated cytotoxicity in the liver and spleen.

Dendritic cells and host resistance to infection

Host defence against infection requires an integrated response of both the innate and adaptive arms of the immune system. Emerging data indicate that dendritic cells contribute an essential part to the initiation and regulation of adaptive immunity. Dendritic cells guard the sites of pathogen entry to the host and are uniquely suited to detect and capture invading microbes. Upon recognition of microbial structures and appropriate activation, a maturation programme is triggered and dendritic cells migrate to lymphoid organs to stimulate a primary cell-mediated immune response. Moreover, dendritic cells play a critical role in shaping the emerging response, thereby controlling the course of infection. They can discriminate between various types of microorganisms and are capable of producing different cytokines in response to different microbial stimuli. On the other hand, pathogens developed numerous strategies to evade and subvert dendritic cell functions. Elucidating the interactions of dendritic cells with microbial pathogens may lead to novel strategies for combating infectious diseases by dendritic cell-based vaccination and immunotherapy. This review highlights recent advances in our knowledge of the unique role of dendritic cells in counteracting microbial infections.

Specific lytic activity against mycobacterial antigens is inversely correlated with the severity of tuberculosis

The ability of peripheral blood mononuclear cells (PBMC) from patients with active tuberculosis to display cytotoxic responses against autologous Mycobacterium tuberculosis (Mtb)-pulsed macrophages was evaluated. Non-MHC restricted cell-dependent lytic activity was observed in ex vivo effector cells from tuberculosis patients and was mediated mainly by CD3(+)gammadelta TCR(+) T (gammadelta T) cells bearing CD56 and/or CD16 molecules. MHC-restricted and non-MHC restricted cytotoxic T cells (CTL) were differentially expanded upon stimulation with Mtb in tuberculosis patients and normal controls (N). Class-I restricted CD8(+) CTL and class-II restricted CD4(+) CTL were generated in PPD(+)N and to a lesser extent in PPD(-)N. Mtb-stimulated effector cells from tuberculosis patients became progressively non-MHC restricted CD4(-)CD8(-)gammadelta T cells, while lytic activity of CD4(+) and CD8(+)CTL decreased gradually as the disease became more severe. On the other hand, target cells were lysed by ex vivo cells from tuberculosis patients through the Fas-FasL and perforin pathways. Mtb-induced CD4(+) CTL from tuberculosis patients and N controls preferentially employed the Fas-FasL mechanism. Mtb-induced CD8(+) CTL effector cells from patients used the perforin-based mechanism while cells from N controls also used the Fas-FasL pathway. While Mtb-induced gammadelta CTL from patients and PPD(-)N employed the latter mechanism cells from PPD(+)N individuals also used the perforin pathway. It can be concluded that shifts in the CTL response and the cytolytic mechanisms take place as the pulmonary involvement becomes more severe.

B cell positive selection by soluble self-antigen

It is well established that autoreactive B cells undergo negative selection. This stands in paradox with the high frequency of so-called natural autoreactive B cells producing low affinity polyreactive autoantibodies with recurrent specificities, suggesting that these B cells are selected on the basis of their autoreactivity. We previously described two transgenic mouse lines (with and without IgD) producing a human natural autoantibody (nAAb) that binds ssDNA and human Fcgamma. In the absence of human IgG, nAAb-transgenic B cells develop normally. By crossing these mice with animals expressing knockin chimeric IgG with the human Fcgamma, we now show that the constitutive expression of chimeric IgG promotes the increase of nAAb-expressing B cells. This positive selection is critically dependent on the presence of IgD, occurs in the spleen, and concerns all mature B cell subsets, with a relative preferential enrichment of marginal zone B cells. These data support the view that soluble self-Ags can result in positive clonal selection.

CD1 tetramers: a powerful tool for the analysis of glycolipid-reactive T cells

CD1 proteins constitute a third class of antigen-presenting molecules. They bind lipids rather than peptides, and the T cells reactive to lipids presented by CD1 have been implicated in the protection against autoimmune diseases and infectious microorganisms and in the immune surveillance for tumors. Thus, the ability to identify, purify, and track the response of CD1-reactive cells is of paramount importance. Previously existing methods for identifying these T cells were not based on TCR specificity, and therefore the data obtained by these methods were in some cases difficult to interpret. The recent generation of tetramers of alpha-galactosyl ceramide (alpha-GalCer) with CD1d has already permitted significant insight into the biology of NKT cells. Tetramers constructed from other CD1 molecules also have been obtained during the previous year. Collectively, these new reagents promise to greatly expand knowledge of the functions of lipid-reactive T cells, with potential use in monitoring the response to lipid-based vaccines and other treatments and in the diagnosis of autoimmune diseases.

Prolonged IFN-gamma-producing NKT response induced with alpha-galactosylceramide-loaded DCs

Natural killer T (NKT) lymphocytes mediate a rapid reaction to the glycolipid drug alpha-galactosylceramide (alpha GalCer), which triggers release of large amounts of cytokines into the serum within 12 h, starting with interleukin 4 (IL-4). When alpha GalCer is administered to mice on dendritic cells (DCs) instead, the response is more prolonged (>4 days) and marked by a large expansion in IFN-gamma-producing NKT cells as well as greater resistance to metastases of the B16 melanoma. Nevertheless, DCs from mice given free alpha GalCer are able to induce strong IFN-gamma-producing NKT responses when transferred to naïve mice, but not when transferred to alpha GalCer-treated recipients. In the latter, the NKT cells are energized and can respond to glycolipid only in the presence of supplemental IL-2. Therefore, when alpha GalCer is selectively targeted to DCs, mice develop a stronger, more prolonged and effector type of NKT response, but this response can be blocked by the induction of anergy after presentation of alpha GalCer on other cells.

The V alpha 14 NKT cell TCR exhibits high-affinity binding to a glycolipid/CD1d complex

Most CD1d-dependent NKT cells in mice have a canonical V alpha 14J alpha 18 TCR rearrangement. However, relatively little is known concerning the molecular basis for their reactivity to glycolipid Ags presented by CD1d. Using glycolipid Ags, soluble forms of a V alpha 14 NKT cell-derived TCR, and mutant and wild-type CD1d molecules, we probed the TCR/CD1d interaction by surface plasmon resonance, tetramer equilibrium staining, and tetramer staining decay experiments. By these methods, several CD1d alpha-helical amino acids could be defined that do not greatly alter lipid binding, but that affect the interaction with the TCR. Binding of the V alpha 14(+) TCR to CD1d requires the agonist alpha-galactosylceramide (alpha-GalCer), as opposed to the nonantigenic beta-galactosylceramide, although both Ags bind to CD1d, indicating that the carbohydrate moiety of the CD1d-bound Ag plays a major role in the TCR interaction. The TCR has a relatively high-affinity binding to the alpha-GalCer/CD1d complex, with a particularly slow off rate. These unique properties are consistent with the coreceptor-independent action of the V alpha 14 TCR and may be related to the intense response to alpha-GalCer by NKT cells in vivo.

Antigen presentation and T cell stimulation by dendritic cells

Dendritic cells take up antigens in peripheral tissues, process them into proteolytic peptides, and load these peptides onto major histocompatibility complex (MHC) class I and II molecules. Dendritic cells then migrate to secondary lymphoid organs and become competent to present antigens to T lymphocytes, thus initiating antigen-specific immune responses, or immunological tolerance. Antigen presentation in dendritic cells is finely regulated: antigen uptake, intracellular transport and degradation, and the traffic of MHC molecules are different in dendritic cells as compared to other antigen-presenting cells. These specializations account for dendritic cells' unique role in the initiation of immune responses and the induction of tolerance.

Presentation of the same glycolipid by different CD1 molecules

Five CD1 molecules are expressed in humans and it is unclear whether they have specialized or redundant functions. We found that sulfatide is a promiscuous CD1-binding ligand and have isolated T cell clones that are specific for sulfatide and restricted by distinct CD1 molecules. These clones have been used to compare the capacity of different CD1 to present the same glycolipid, to induce effector functions, and to form persistent immunogenic complexes. CD1a, CD1b, and CD1c molecules similarly load sulfatide on the cell surface without processing, and prime Th1 and Th2 responses. Stimulation by sulfatide-loaded CD1a persists much longer than that by CD1b and CD1c in living cells. Use of recombinant soluble CD1a confirmed the prolonged capacity to stimulate T cells. Moreover, other glycosphingolipids bind to all CD1, which suggests the presence of additional promiscuous ligands. Thus, group I CD1 molecules present an overlapping set of self-glycolipids, even though they are quite divergent from an evolutionary point of view.

Development and selection of NKT cells

NKT cells utilize a restricted alphabeta TCR repertoire that recognizes glycolipids in association with CD1d. The recent development of fluorescent CD1d tetramers loaded with the synthetic glycolipid alpha-galactosyl-ceramide has led to a clearer definition of NKT-cell subsets as well as important insights into their developmental origin. As many as four subsets may exist, differing in NK1.1 expression, TCR repertoire and dependence on CD1d and various glycolipids for development. Two different lineage-commitment models have been proposed, with most evidence favoring a byproduct of conventional-T-cell development.

Interactions between double positive thymocytes and high affinity ligands presented by cortical epithelial cells generate double negative thymocytes with T cell regulatory activity

Previous studies on thymocyte differentiation by using reaggregate cultures (RC) of double positive T cell receptor (TCR) transgenic thymocytes and the thymic epithelial cell line ANV indicated that low concentrations of high affinity ligands for the TCR were efficient inducers of thymocyte maturation to CD4 single positive (SP) functional cells. In this study, it is demonstrated that, when high concentrations of high affinity ligands are used in this RC system, double positive (DP) cells down-modulate expression of both coreceptors and that, as a result, large numbers of double negative (DN) cells are generated. These DN cells proliferated modestly in response to stimulation by antigen, and this response was considerably augmented by the addition of IL-2 to the cultures. Notably, these antigen-stimulated DN cells produced large amounts of IL-10. When the DN cells generated in RC were cocultured with naive TCR transgenic T cells in the presence of antigen, they suppressed the proliferative response of the naive T cells. Thus, high affinity ligands, when presented to DP thymocytes by cortical thymic epithelial cells in reaggregate cultures, rather than causing deletion of the immature thymocytes, induce their differentiation into immunoregulatory DN cells, suggesting a distinct mechanism by which self tolerance may be maintained.

NKT cells-containing inflammatory lesions induced by Yersinia pseudotuberculosis glycolipids

Valpha14-expressing NKT (invNKT) cells are a population of non-conventional T lymphocytes (TL) that bridge mammalian innate and adaptive immunity. Their role in infectious diseases and inflammatory processes is still largely ununderstood. A previous report has shown that an acute granulomatous-like reaction can be elicited by sub-cutaneous injection of Mycobacterium tuberculosis glycolipids in mice, and that recruitment of invNKT cells at the injection site is instrumental in this process. Here, we describe the mouse response to enterobacterium Yersinia pseudotuberculosis glycolipids extracts during the first week post injection. The cellular reaction is an acute inflammatory infiltrate where TL are abundant from early times on. InvNKT cells are present in the lesions, detectable as early as day 1 post injection. They compose all of the Valpha14-expressing TL, although conventional T cells expressing non-Valpha14 alpha-chains can be detected. The reaction is strictly dependent on ester-linked fatty acids as mild alkaline treatment of the extract prior to injection results in the absence of analysable lesions. Thus, glycolipids from Yersinia induce inflammatory lesions comparable to those induced by mycobacteria glycolipids, in spite of the totally different cell wall composition in the two genera. Moreover, the present findings show that invNKT cell response is not unique to mycobacterial glycolipids.

Natural killer T cells reactive to a single glycolipid exhibit a highly diverse T cell receptor beta repertoire and small clone size

CD1d-restricted natural killer (NK) T cells reactive with the glycolipid alpha-galactosylceramide (alpha-GalCer) are a distinct lymphocyte sublineage. They express an invariant Valpha14-Jalpha18 T cell receptor (TcR), but the role of the beta chain has been controversial. Here, we have used CD1d tetramers to identify and isolate NK T cells based on their antigen specificity. In mice lacking germline Vbeta8, most of the alpha-GalCer-reactive T cells express either Vbeta2 or Vbeta7, strong Vbeta selection being revealed by the lack of an increase in other Vbeta regions. By contrast to the selection for complementarity determining region (CDR) 3beta sequences in some anti-peptide responses, alpha-GalCer-reactive T cells have polyclonal CDR3beta sequences. There is little CDR3beta sequence redundancy between organs or individual mice, and, surprisingly, there also is no evidence for organ-specific CDR3beta sequence motifs. These data argue against a T cell receptor-mediated self-reactivity for tissue-specific CD1d-bound ligands. Each NKT clone is represented by only 5-10 cells. This clone size is similar to naive conventional T cells, and much lower than that reported for memory T cells, although NK T cells have an activated/memory phenotype.

Why isn't the fetus rejected?

The long-standing question of how the fetal allograft avoids immune rejection during pregnancy has lately been generating renewed interest. Recent insights have emerged from studies in mice on uterine NK cells, NKT cells, complement inhibition and the reproductive effects of 1-methyl-tryptophan.

Structural characterization of two CD1A allelic variants

CD1 molecules are specialized in presenting lipidic antigens to T lymphocytes. They are structurally and evolutionary related to MHC molecules and show very limited polymorphism. We have previously described and partially characterized a new human CD1A allele differing from the wild type CD1A by a substitution of Cysteine by Tryptophan at position 52 in the alpha1 domain of the CD1A molecule. The frequency of this allele varies from 10% in individuals of Caucasian origin to 56% in Chinese people. The aim of the present work was to structurally characterize this CD1A allele. To do this we have cloned and sequenced the full-length cDNA encoding the new CD1A allele. The cDNA sequence of this allele encodes a protein differing the wild type in two amino acids at positions 14 (Threonine versus Isoleucine) and 52 (Cysteine versus Tryptophan). The cDNAs encoding both wild type and mutant CD1A were cloned in the expression vector pSRalphaNeo and transfected into C1R and L721.221 cells. Cell surface expression of the protein products in transfected cell lines were analyzed by flow cytometry and immunoprecipitation using CD1a-specific monoclonal antibodies. Our results indicate that both allelic products are efficiently expressed on the cell surface.

Acylation state of the phosphatidylinositol mannosides from Mycobacterium bovis bacillus Calmette Guérin and ability to induce granuloma and recruit natural killer T cells

Previous studies have found that, when injected into mice, glycolipidic fractions of mycobacterial cell walls containing phosphatidylinositol mannosides (PIM) induced a granuloma and recruitment of Natural Killer T cells in the lesions. The dimannoside (PIM(2)) and the hexamannoside (PIM(6)) PIM were isolated from Mycobacterium bovis bacillus Calmette Guérin and shown to act alike, but the activity was found to be dependent on the presence of the lipidic part. The chemical structure of PIM was then re-evaluated, focusing on the characterization of their lipidic part, defining mono- to tetra-acylated PIM(2). The structure of these acyl forms was elucidated using a sophisticated combination of chemical degradations and analytical tools including electrospray ionization/mass spectrometry, electrospray ionization/mass spectrometry/mass spectrometry, and two-dimensional NMR. Finally, the acyl forms were purified by hydrophobic interaction chromatography and tested for their capacity to induce the granuloma and Natural Killer T cell recruitment. We found that there is an absolute requirement for the molecules to possess at least one fatty acyl chain, but the number, location, and size of the acyl chains was without effect. Moreover, increasing the complexity of the carbohydrate moiety did not lead to significant differences in the biological responses.

Towards the rational design of Th1 adjuvants

Finding adjuvants in order to enhance immune responses against target immunogens has been a major and recurrent issue for the vaccine industry. It is yet to be solved, most particularly in the context of a growing interest in designing new types of vaccines capable of eliciting Th1 immune responses. A review of synthetic adjuvants which have been (or are being) tested in clinical studies is presented. Importantly, recent advances in our understanding of the physiology of immune responses offer new avenues to design and test candidate adjuvants, based on either synthetic or natural molecules, with the aim to mimic and recapitulate pro-inflammatory signals initiating both innate and adaptative immune effector mechanisms. Thus, adjuvants of the future might be a mixture of molecules selected singularly for a capacity to attract, target or activate professional antigen presenting cells. Used as a combination, such molecules should facilitate antigen presentation by professional APCs and lead to a potent induction of T cell-mediated effector and immune memory mechanisms.