Polymorphisms in CD1d affect antigen presentation and the activation of CD1d-restricted T cells

CD1d-iNKT Axis in Infectious Diseases: Lessons Learned From the Past

CD1d is an antigen-presenting molecule that presents lipid or glycolipid antigens to iNKT cells, a distinct subset of T lymphocytes characterised by their innate-like properties and restricted use of Vα, Jα and Vβ segments. The CD1d-iNKT axis represents an interesting aspect of the immune system with significant potential for therapeutic interventions against infectious diseases. Upon recognition of lipid antigens, iNKT cells initiate rapid and potent immune responses, releasing a diverse array of cytokines such as IL-4, IL-13, IFN-γ etc. that profoundly influence immune reactions against various pathogens, including bacteria and parasites, bridging innate and adaptive immunity. We identify and describe the key features of lipidic antigens and their derivatives that determine the nature of their antigenicity. Furthermore, modulating CD1d-driven iNKT cell responses by an array of lipid and glycolipid antigens holds promise as adjunctive therapy to existing antimicrobial treatments. Understanding the complexities of the CD1d-iNKT axis and exploiting its therapeutic potential in the case of infectious diseases could lead to innovative immunotherapeutic strategies, ushering in a new era of immunotherapy against pathogenic insults.

Natural Killer T Cell Diversity and Immunotherapy

Invariant natural killer T cells (iNKTs), a type of unconventional T cells, share features with NK cells and have an invariant T cell receptor (TCR), which recognizes lipid antigens loaded on CD1d molecules, a major histocompatibility complex class I (MHC-I)-like protein. This interaction produces the secretion of a wide array of cytokines by these cells, including interferon gamma (IFN-γ) and interleukin 4 (IL-4), allowing iNKTs to link innate with adaptive responses. Interestingly, molecules that bind CD1d have been identified that enable the modulation of these cells, highlighting their potential pro-inflammatory and immunosuppressive capacities, as required in different clinical settings. In this review, we summarize key features of iNKTs and current understandings of modulatory α-galactosylceramide (α-GalCer) variants, a model iNKT cell activator that can shift the outcome of adaptive immune responses. Furthermore, we discuss advances in the development of strategies that modulate these cells to target pathologies that are considerable healthcare burdens. Finally, we recapitulate findings supporting a role for iNKTs in infectious diseases and tumor immunotherapy.

CD1d-dependent neuroinflammation impairs tissue repair and functional recovery following a spinal cord injury

Tissue damage resulting from a spinal cord injury (SCI) is primarily driven by a robust neuroimmune/neuroinflammatory response. This intricate process is mainly governed by a multitude of cytokines and cell surface proteins in the central nervous system (CNS). However, the critical components of the neuroimmune/neuroinflammatory response during SCI are still not well-defined. In this study, we investigated the impact of CD1d, an MHC class I-like molecule mostly known for presenting lipid antigens to natural killer T (NKT) cells and regulating immune/inflammatory responses, on neuroimmune/neuroinflammatory responses induced by SCI. We observed an increased expression of CD1d on various cell types within the spinal cord, including microglia/macrophages, oligodendrocytes (ODCs), and endothelial cells (DCs), but not on neurons or astrocytes post-SCI. In comparison to wildtype (WT) mice, a T10 contusive SCI in CD1d knockout (CD1dKO or Cd1d -/- ) mice resulted in markedly reduced proinflammatory cytokine release, microglia/macrophage activation and proliferation. Following SCI, the levels of inflammatory cytokines and activation/proliferation of microglia/macrophages were dramatically reduced, while anti-inflammatory cytokines such as IL-4 and growth factors like VEGF were substantially increased in the spinal cord tissues of CD1dKO mice when compared to WT mice. In the post-acute phase of SCI (day 7 post-SCI), CD1dKO mice had a significantly higher frequency of tissue-repairing macrophages, but not other types of immune cells, in the injured spinal cord tissues compared to WT mice. Moreover, CD1d-deficiency protected spinal cord neuronal cells and tissue, promoting functional recovery after a SCI. However, the neuroinflammation in WT mouse spinal cords was independent of the canonical CD1d/NKT cell axis. Finally, treatment of injured mice with a CD1d-specific monoclonal antibody significantly enhanced neuroprotection and improved functional recovery. Therefore, CD1d promotes the proinflammatory response following a SCI and represents a potential therapeutic target for spinal cord repair.

Significance Statement

The cell surface molecule, CD1d, is known to be recognized by cells of the immune system. To our knowledge, this is the first observation that the CD1d molecule significantly contributes to neuroinflammation following a spinal cord injury (SCI) in a manner independent of the CD1d/NKT cell axis. This is important, because this work reveals CD1d as a potential therapeutic target following an acute SCI for which there are currently no effective treatments.

Adipocyte CD1d Gene Transfer Induces T Cell Expansion and Adipocyte Inflammation in CD1d Knockout Mice

CD1d, a lipid Ag-presenting molecule for invariant NKT (iNKT) cells, is abundantly expressed on adipocytes and regulates adipose homeostasis through iNKT cells. CD1d gene expression was restored in visceral adipose tissue adipocytes of CD1d knockout (KO) mice to investigate the interactions between adipocytes and immune cells within adipose tissue. We developed an adipocyte-specific targeting recombinant adeno-associated viral vector, with minimal off-target transgene expression in the liver, to rescue CD1d gene expression in visceral adipose tissue adipocytes of CD1d KO mice, followed by assessment of immune cell alternations in adipose tissue and elucidation of the underlying mechanisms of alteration. We report that adeno-associated virus-mediated gene transfer of CD1d to adipocytes in CD1d KO mice fails to rescue iNKT cells but leads to massive and selective expansion of T cells within adipose tissue, particularly CD8+ T effector cells, that is associated with adipocyte NLRP3 inflammasome activation, dysregulation of adipocyte functional genes, and upregulation of apoptotic pathway proteins. An NLRP3 inhibitor has no effect on T cell phenotypes whereas depletion of CD8+ T cells significantly attenuates inflammasome activation and abolishes the dysregulation of adipocyte functional genes induced by adipocyte CD1d. In contrast, adipocyte overexpression of CD1d fails to induce T cell activation in wild-type mice or in invariant TCR α-chain Jα18 KO mice that have a normal lymphocyte repertoire except for iNKT cells. Our studies uncover an adipocyte CD1d → CD8+ T cell → adipocyte inflammasome cascade, in which CD8+ T cells function as a key mediator of adipocyte inflammation likely induced by an allogeneic response against the CD1d molecule.

CD1 gene polymorphism and susceptibility to celiac disease: Association of CD1E*02/02 in Moroccans

CD1 glycoproteins are a class of antigen presenting molecules that bind and present non-peptidic antigens (lipids and glycolipids) for immune recognition. CD1 polymorphisms, although limited, could have a critical role in antimicrobial, anticancer, and autoimmune responses and disease susceptibility. Ethnic differences and interactions between genetic and environmental factors make it attractive the study of these molecules in autoimmune inflammatory disorders, such as celiac disease (CD), in which a strong genetic predisposition (HLA-DQ2/DQ8) and pressure of environmental factors have a central role. CD1A, CD1D and CD1E polymorphisms in exon 2 were assessed in patients from Morocco affected by CD, using direct sequencing analysis, in order to investigate possible associations with the disease in a North African population. Differences in genotype and haplotype distribution of CD1E between celiac patients and controls were found: in particular, an increase of CD1E*02/02 homozygous (OR 2.93, CI 1.30-6.59, p = 0.007) and CD1A*02-E*02 estimated haplotypes in CD, compared with controls. Frequencies of CD1A and CD1D genotypes/alleles were not different between groups. CD1E*02/02, previously suggested as a potential immune protective genotype to malaria susceptibility, could be an additional gene involved in celiac risk in this geographic area.

Invariant Natural Killer T-Cells and Total CD1d Restricted Cells Differentially Influence Lipid Metabolism and Atherosclerosis in Low Density Receptor Deficient Mice

Natural killer T (NKT) cells are a distinct subset of lymphocytes that bridge the innate and adaptive immune response and can be divided into type I invariant NKT cells (iNKT) and type II NKT cells. The objective of this study is to examine the effects of NKT cell on lipid metabolism and the initiation and progression of atherosclerosis in LDL receptor deficient (LDLR^−/−) mice. Mice were fed an atherogenic diet for 4 or 8 weeks and plasma lipids, lipoproteins, and atherosclerosis were measured. The selective absence of iNKT cells in Jα18^−/−LDLR^−/− mice led to an increase in plasma cholesterol levels in female mice. Transgenic Vα14tg/LDLR^−/− mice with elevated numbers of iNKT cells had increased late atherosclerosis of the innominate artery, though absence of either iNKT cells or all NKT cells and other CD1d expressing cells had varying effects on atherosclerotic lesion burden in the ascending aortic arch and aortic root. These studies not only highlight the potential modulatory role played by NKT cells in atherosclerosis and lipid metabolism, but also raise the possibility that divergent roles may be played by iNKT and CD1d restricted cells such as type II NKT cells or other CD1d expressing cells.

Lipid Antigen Presentation by CD1b and CD1d in Lysosomal Storage Disease Patients

The lysosome has a key role in the presentation of lipid antigens by CD1 molecules. While defects in lipid antigen presentation and in invariant Natural Killer T (iNKT) cell response were detected in several mouse models of lysosomal storage diseases (LSD), the impact of lysosomal engorgement in human lipid antigen presentation is poorly characterized. Here, we analyzed the capacity of monocyte-derived dendritic cells (Mo-DCs) from Fabry, Gaucher, Niemann Pick type C and Mucopolysaccharidosis type VI disease patients to present exogenous antigens to lipid-specific T cells. The CD1b- and CD1d-restricted presentation of lipid antigens by Mo-DCs revealed an ability of LSD patients to induce CD1-restricted T cell responses within the control range. Similarly, freshly isolated monocytes from Fabry and Gaucher disease patients had a normal ability to present α-Galactosylceramide (α-GalCer) antigen by CD1d. Gaucher disease patients' monocytes had an increased capacity to present α-Gal-(1-2)-αGalCer, an antigen that needs internalization and processing to become antigenic. In summary, our results show that Fabry, Gaucher, Niemann Pick type C, and Mucopolysaccharidosis type VI disease patients do not present a decreased capacity to present CD1d-restricted lipid antigens. These observations are in contrast to what was observed in mouse models of LSD. The percentage of total iNKT cells in the peripheral blood of these patients is also similar to control individuals. In addition, we show that the presentation of exogenous lipids that directly bind CD1b, the human CD1 isoform with an intracellular trafficking to the lysosome, is normal in these patients.

The processing and presentation of lipids and glycolipids to the immune system

The recognition of CD1-lipid complexes by T cells was discovered 20 years ago and has since been an emerging and expanding field of investigation. Unlike protein antigens, which are presented on MHC class I and II molecules, lipids can only be presented by CD1 molecules, a unique family of MHC-like proteins whose singularity is a hydrophobic antigen-binding groove. The processing and loading of lipid antigens inside this groove of CD1 molecules require localization to endosomal and lysosomal subcellular compartments and their acidic pHs. This particular environment provides the necessary glycolytic enzymes and lipases that process lipid and glycolipid antigens, as well as a set of lipid transfer proteins that load the final version of the antigen inside the groove of CD1. The overall sequence of events needed for efficient presentation of lipid antigens is now understood and presented in this review. However, a large number of important details have been elusive. This elusiveness is linked to the inherent technical difficulties of studying lipids and the lipid-protein interface in vitro and in vivo. Here, we will expose some of those limitations and describe new approaches to address them during the characterization of lipids and glycolipids antigen presentation.

CD1A, D and E gene polymorphisms in a North African population from Morocco

CD1 molecules are specialized in capturing and presenting lipids and glycolipids to distinct subsets of T and NKT cells. Glycolipid presentation could play a significant role in the immune response against microbial infections. There are five closely linked CD1 genes in humans, named CD1A, B, C, D, and E, which all show a limited polymorphism. In this study, exon 2 polymorphisms of CD1A, CD1D and CD1E were investigated and allele, genotype and haplotype frequencies of these loci were reported in a Moroccan population. A comparison with allele, genotype and haplotype frequencies observed in other geographic areas was also performed. Results confirmed the presence of ethnic differences in CD1 polymorphism, mainly in CD1D (in this population two additional CD1D variant alleles, CD1D(∗)03 and CD1D(∗)04, were described) and E genes. These data could be useful to evaluate a possible pathogenetic role of CD1 in diseases. Increasing the knowledge in this field may offer possibilities for the development of new immunotherapeutic approaches.

Role of Group 1 CD1-Restricted T Cells in Infectious Disease

The evolutionarily conserved CD1 family of antigen-presenting molecules presents lipid antigens rather than peptide antigens to T cells. CD1 molecules, unlike classical MHC molecules, display limited polymorphism, making CD1-restricted lipid antigens attractive vaccine targets that could be recognized in a genetically diverse human population. Group 1 CD1 (CD1a, CD1b, and CD1c)-restricted T cells have been implicated to play critical roles in a variety of autoimmune and infectious diseases. In this review, we summarize current knowledge and recent discoveries on the development of group 1 CD1-restricted T cells and their function in different infection models. In particular, we focus on (1) newly identified microbial and self-lipid antigens, (2) kinetics, phenotype, and unique properties of group 1 CD1-restricted T cells during infection, and (3) the similarities of group 1 CD1-restricted T cells to the closely related group 2 CD1-restricted T cells.

Increased CD1D polymorphism: identification of two novel alleles, CD1D*03 and *04, in individuals from Morocco

Two novel CD1D alleles were identified in unrelated individuals from Morocco. They differ each from the common CD1D*01 allele by one nucleotide substitution in exon 2 resulting in one amino acid change in the G-ALPHA1-LIKE domain. According to the IMGT unique numbering for G domain, CD1D*03 has one nucleotide transition c136 > t in codon 46, with an arginine-to-cysteine amino acid change (R46 > C) in the D-STRAND, whereas CD1D*04 has one transition c98 > t in codon 33, with a threonine-to-methionine amino acid change (T33 > M) in the C-STRAND. This suggests that CD1D is more polymorphic than previously assumed.

Nonclassical T cells and their antigens in tuberculosis

T cells that recognize nonpeptidic antigens, and thereby are identified as nonclassical, represent important yet poorly characterized effectors of the immune response. They are present in large numbers in circulating blood and tissues and are as abundant as T cells recognizing peptide antigens. Nonclassical T cells exert multiple functions including immunoregulation, tumor control, and protection against infections. They recognize complexes of nonpeptidic antigens such as lipid and glycolipid molecules, vitamin B2 precursors, and phosphorylated metabolites of the mevalonate pathway. Each of these antigens is presented by antigen-presenting molecules other than major histocompatibility complex (MHC), including CD1, MHC class I-related molecule 1 (MR1), and butyrophilin 3A1 (BTN3A1) molecules. Here, we discuss how nonclassical T cells participate in the recognition of mycobacterial antigens and in the mycobacterial-specific immune response.

Type II natural killer T cells foster the antitumor activity of CpG-oligodeoxynucleotides

Type II natural killer T (NKT) cells in cancer immunity are typically associated with suppression of tumor immunosurveillance through secretion of IL-13. We previously demonstrated that CpG oligonucleotide therapy activated Type II NKT cells to produce T helper type 1 (Th1) rather than T helper type 2 (Th2) cytokines. This cytokine skewing may manifest in Type II NKT cell antitumor properties in an immunotherapeutic setting.

Lipid antigens in immunity

Lipids are not only a central part of human metabolism but also play diverse and critical roles in the immune system. As such, they can act as ligands of lipid-activated nuclear receptors, control inflammatory signaling through bioactive lipids such as prostaglandins, leukotrienes, lipoxins, resolvins, and protectins, and modulate immunity as intracellular phospholipid- or sphingolipid-derived signaling mediators. In addition, lipids can serve as antigens and regulate immunity through the activation of lipid-reactive T cells, which is the topic of this review. We will provide an overview of the mechanisms of lipid antigen presentation, the biology of lipid-reactive T cells, and their contribution to immunity.

Polymorphisms in the CD1d promoter that regulate CD1d gene expression are associated with impaired NKT cell development

CD1d-restricted NKT cells comprise an innate-like T cell population that exerts significant influence over early events in the developing immune response. The frequency of NKT cells is highly variable in humans and in mice, but the basis for this variability remains unclear. In this study, we report a striking deficiency of type I NKT cells in the wild-derived inbred strains PWD/PhJ, SPRET/EiJ, and CAST/EiJ. Investigation of the underlying basis for the lack of type I NKT cells revealed that one strain, PWD/PhJ, exhibited a significant impairment in thymocyte and splenocyte CD1d gene and protein expression. Accordingly, both thymocytes and bone marrow-derived dendritic cells from PWD mice exhibited a significant impairment in the ability to present α-galactosylceramide to NKT cells. The impaired PWD CD1d gene expression was due to impaired CD1d promoter activity. Fine-mapping of the promoter activity revealed that two single nucleotide substitutions at positions -331 and -164 in the proximal promoter were each sufficient to account for the diminished PWD CD1d promoter activity. Examination of the strain distribution pattern of these polymorphisms revealed that, of 19 strains analyzed, only PWD and PWK mice possessed both CD1d promoter polymorphisms. A subsequent examination of the PWK strain revealed that it also exhibited impaired thymocyte CD1d expression and very low numbers of NKT cells. Taken together, these results provide new insight into the control of CD1d gene expression, and they have implications for the evolution of CD1d and type I NKT cells.

The functions of type I and type II natural killer T cells in inflammatory bowel diseases

CD1d-restricted natural killer T (NKT) cells are a distinct subset of T cells that rapidly produce an array of cytokines on activation and play a critical role in regulating various immune responses. NKT cells are classified into 2 groups based on differences in T-cell receptor usage. Type I NKT cells have an invariant T-cell receptor α-chain and are readily detectable by α-galactosylceramide (α-GalCer)-loaded CD1d tetramers. Type II NKT cells have a more diverse T-cell receptor repertoire and cannot be directly identified. Both types of NKT cells and multiple CD1d-expressing cell types are present in the intestine, and their interactions are likely to be modulated by pathogenic and commensal microbes, which in turn contribute to the intestinal immune responses in health and disease. Indeed, in several animal models of inflammatory bowel disease, type I NKT cells have been shown to make both protective and pathogenic contributions to disease. In contrast, in patients with ulcerative colitis, and a mouse model in which both CD1d expression and the frequency of type II NKT cells are increased, type II NKT cells seem to promote intestinal inflammation. In this review, we summarize the present knowledge on the antigen recognition, activation, and function of NKT cells with a particular focus on their role in inflammatory bowel disease and discuss factors that may influence the functional outcome of NKT cell responses in intestinal inflammation.

Type II natural killer T cells use features of both innate-like and conventional T cells to recognize sulfatide self antigens

Glycolipids presented by the major histocompatibility complex (MHC) class I homolog CD1d are recognized by natural killer T cells (NKT cells) characterized by either a semi-invariant T cell antigen receptor (TCR) repertoire (type I NKT cells or iNKT cells) or a relatively variable TCR repertoire (type II NKT cells). Here we describe the structure of a type II NKT cell TCR in complex with CD1d-lysosulfatide. Both TCR α-chains and TCR β-chains made contact with the CD1d molecule with a diagonal footprint, typical of MHC-TCR interactions, whereas the antigen was recognized exclusively with a single TCR chain, similar to the iNKT cell TCR. Type II NKT cell TCRs, therefore, recognize CD1d-sulfatide complexes by a distinct recognition mechanism characterized by the TCR-binding features of both iNKT cells and conventional peptide-reactive T cells.

Natural killer T cells in lipoprotein metabolism and atherosclerosis

Cells of both the innate and adaptive immune system participate in the development of atherosclerosis, a chronic inflammatory disorder of medium and large arteries. Natural killer T (NKT) cells express surface markers characteristic of natural killer cells and conventional T cells and bridge the innate and adaptive immune systems. The development and activation of NKT cells is dependent upon CD1d, a MHC-class I-type molecule that presents lipids, especially glycolipids to the T cell receptors on NKT cells. There are two classes of NKT cells; invariant NKT cells that express a semi-invariant T cell receptor and variant NKT cells. This review summarises studies in murine models in which the effect of the activation, overexpression or deletion of NKT cells or only invariant NKT cells on atherosclerosis has been examined.

Genetic control of murine invariant natural killer T-cell development dynamically differs dependent on the examined tissue type

Previous studies using gene-targeted mutant mice revealed several molecules important for the development or function of invariant natural killer T (iNKT) cells. However, these gene knockout mice represent cases that are rare in humans. Thus, it remains unclear how naturally occurring allelic variants of these genes or others regulate the numerical and functional diversity of iNKT cells in both mice and humans. Studies in humans are mostly limited to iNKT cells in peripheral blood (PB). It is not known if the relative distribution of iNKT cells between PB and other lymphoid organs is correlated or under common genetic control. To initially address these questions, we analyzed iNKT cells in the spleen, thymus and PB of 38 inbred mouse strains. Percentages of iNKT cells in these three anatomical sites varied significantly in a strain-dependent manner. The correlation between PB and spleen was moderate, and none was observed between PB and thymus. Similarly, proportions of the CD4-expressing subset of iNKT cells differed significantly among inbred strains. The percentages of CD4-positive iNKT cells displayed a strong correlation between PB and spleen, although it remained poor between PB and thymus. Genome-wide association studies across strains identified only partially overlapping loci associated with variability of iNKT cell frequencies within and between differing anatomical sites.

Natural killer T cells in health and disease

Natural killer T (NKT) cells are a subset of T lymphocytes that share surface markers and functional characteristics with both conventional T lymphocytes and natural killer cells. Most NKT cells express a semi-invariant T cell receptor that reacts with glycolipid antigens presented by the major histocompatibility complex class I-related protein CD1d on the surface of antigen-presenting cells. NKT cells become activated during a variety of infections and inflammatory conditions, rapidly producing large amounts of immunomodulatory cytokines. NKT cells can influence the activation state and functional properties of multiple other cell types in the immune system and, thus, modulate immune responses against infectious agents, autoantigens, tumors, tissue grafts and allergens. One attractive aspect of NKT cells is that their immunomodulatory activities can be readily harnessed with cognate glycolipid antigens, such as the marine sponge-derived glycosphingolipid alpha-galactosylceramide. These properties of NKT cells are being exploited for therapeutic intervention to prevent or treat cancer, infections, and autoimmune and inflammatory diseases.

On the use of donor-derived iNKT cells for adoptive immunotherapy to prevent leukemia recurrence in pediatric recipients of HLA haploidentical HSCT for hematological malignancies

T-cell-depleted hematopoietic stem cell transplantation from an HLA haploidentical relative (hHSCT) is a useful therapy for children with high-risk leukemia lacking suitable HLA-matched donors. The immune deficiency ensuing hHSCT renders patients susceptible to life-threatening infections and disease recurrence. Adoptive immunotherapy can restore/enhance early post-transplantation immunocompetence of hHSCT recipients. Efforts are directed to identify strategies for inducing graft-versus-leukemia (GVL) response, while avoiding graft-versus-host disease (GVHD) occurrence. CD1d-restricted invariant iNKT cells are innate-like, lipid-reactive T lymphocytes implicated in the control of innate and adaptive immunity. Preclinical data suggest that iNKT cells positively modulate both GVL response and GVHD. Our recent findings in a cohort of 22 children given hHSCT for different hematological malignancies show that failure to reconstitute iNKT cells after transplantation correlates with leukemia relapse. In this review, we will discuss potential new options for adoptively transferring donor-derived iNKT cells into hHSCT recipients in the early post-transplantation period to prevent disease recurrence.

CD1d expression in paneth cells and rat exocrine pancreas revealed by novel monoclonal antibodies which differentially affect NKT cell activation

Background

CD1d is a nonpolymorphic MHC class I-like molecule which presents nonpeptide ligands, e.g. glycolipids, to NKT cells. These cells are known to have multiple effects on innate and adaptive immune responses and on the development of pathological conditions. In order to analyze CD1d expression and function in the rat, the first rat CD1d-specific monoclonal antibodies (mAbs) were generated.

Methodology/Principal Findings

Two mAbs, WTH-1 and WTH-2, were generated which bound equally well to cell surface-expressed rat and mouse CD1d. Their non-overlapping epitopes were mapped to the CD1d heavy chain. Flow cytometry and immunohistological analyses revealed a nearly identical degree and pattern of CD1d expression for hematopoieitic cells of both species. Notable is also the detection of CD1d protein in mouse and rat Paneth cells as well as the extremely high CD1d expression in acinar exocrine cells of the rat pancreas and the expression of CD4 on rat marginal zone B cells. Both mAbs blocked α-galactosylceramide recognition by primary rat and mouse NKT cells. Interestingly, the two mAbs differed in their impact on the activation of various autoreactive T cell hybridomas, including the XV19.2 hybridoma whose activation was enhanced by the WTH-1 mAb.

Conclusions/Significance

The two novel monoclonal antibodies described in this study, allowed the analysis of CD1d expression and CD1d-restricted T cell responses in the rat for the first time. Moreover, they provided new insights into mechanisms of CD1d-restricted antigen recognition. While CD1d expression by hematopoietic cells of mice and rats was extremely similar, CD1d protein was detected at not yet described sites of non-lymphatic tissues such as the rat exocrine pancreas and Paneth cells. The latter is of special relevance given the recently reported defects of Paneth cells in CD1d^−/− mice, which resulted in an altered composition of the gut flora.

Structural basis for lipid-antigen recognition in avian immunity

CD1 proteins present self- and foreign lipid Ags to activate specific T cells in the mammalian immune system. These T cells play an important role in controlling autoimmune diseases, suppression of tumor growth, and host defense against invading pathogens. Humans use five CD1 isoforms, whereas only two exist in birds. Unlike mammals' CD1, the structure of chicken CD1-2 showed a primitive lipid-binding groove, suggesting that chicken may only recognize single-chain lipids. In contrast, the crystal structure of the second chicken CD1 isoform, chCD1-1, reported in this study at 2.2 A resolution, reveals an elaborated binding groove with a dual-pocket, dual-cleft architecture. The A' and F' deep pockets are separated from each other, but each is connected to a hydrophobic surface cleft, which may participate in lipid binding. The long endogenous ligand found inside the binding groove of chCD1-1, together with binding data on various glycolipids and mycolic acid, strongly suggest that the unique avian CD1 family could bind long dual- and possibly triacyl-chain lipids.

How do natural killer T cells help B cells?

CD1d-restricted natural killer (NKT) cells are important contributors to antigen-specific antibody responses. There is, therefore, considerable interest in the design and implementation of strategies to appropriately activate NKT cells and boost vaccine-induced protective antibody responses. In order to achieve these goals, investigators are examining the mechanisms by which NKT cells enhance antibody responses. Although information is limited, it is now appreciated that both cognate and noncognate interactions between CD1d-expressing B cells and NKT cells drive enhanced antibody responses. NKT cells may provide B-cell help in the form of direct receptor-mediated interactions as well as by secretion of soluble effectors, including cytokines. In this article, we review the evidence in support of these mechanisms and discuss how they likely take place in the context of interactions of NKT cells with other cell types, such as dendritic cells and helper T cells. We also discuss the evidence that NKT cells affect discrete differentiation events in the multistep process by which a naive B cell experiences antigen and develops into a memory B cell or an antibody-secreting plasma cell. Since most information on NKT cells and humoral immunity has been derived from murine studies, we discuss what is known about human NKT cells and humoral immunity. We offer thoughts on whether the findings in murine systems will translate to humans.