Dendritic cells and anergic type I NKT cells play a crucial role in sulfatide-mediated immune regulation in experimental autoimmune encephalomyelitis

Potential therapeutic and diagnostic approaches of exosomes in multiple sclerosis pathophysiology

Exosomes are bilayer lipid vesicles that are released by cells and contain proteins, nucleic acids, and lipids. They can be internalized by other cells, inducing inflammatory responses and instigating toxicities in the recipient cells. Exosomes can also serve as therapeutic vehicles by transporting protective cargo to maintain homeostasis. Multiple studies have shown that exosomes can initiate and participate in the regulation of neuroinflammation, improve neurogenesis, and are closely related to the pathogenesis of central nervous system (CNS) diseases, including multiple sclerosis (MS). Exosomes can be secreted by both neurons and glial cells in the CNS, and their contents change with disease occurrence. Due to their ability to penetrate the blood-brain barrier and their stability in peripheral fluids, exosomes are attractive biomarkers of CNS diseases. In recent years, exosomes have emerged as potential therapeutic agents for CNS diseases, including MS. However, the molecular pathways in the pathogenesis of MS are still unknown, and further research is needed to fully understand the role of exosomes in the occurrence or improvement of MS disease. Thereby, in this review, we intend to provide a more complete understanding of the pathways in which exosomes are involved and affect the occurrence or improvement of MS disease.

Role of sulfatide-reactive vNKT cells in promoting lung Treg cells via dendritic cell modulation in asthma models

Our previous studies have showed that sulfatide-reactive type II NKT (i.e. variant NKT, vNKT) cells inhibit the immunogenic maturation during the development of mature lung dendritic cells (LDCs), leading todeclined allergic airway inflammation in asthma. Nonetheless, the specific immunoregulatory roles of vNKT cells in LDC-mediated Th2 cell responses remain incompletely understood. Herein, we found that administration of sulfatide facilitated the generation of CD4+FoxP3+ regulatory T (Treg) cells in the lungs of wild-type mice, but not in CD1d-/- and Jα18-/- mice, after ovalbumin or house dust mite exposure. This finding implies that the enhancement of lung Treg cells by sulfatide requires vNKT cells, which dependent on invariant NKT (iNKT) cells. Furthermore, the CD4+FoxP3+ Treg cells induced by sulfatide-reactive vNKT cells were found to be associated with PD-L1 molecules expressed on LDCs, and this association was dependent on iNKT cells. Collectively, our findings suggest that in asthma-mimicking murine models, sulfatide-reactive vNKT cells facilitate the generation of lung Treg cells through inducing tolerogenic properties in LDCs, and this process is dependent on the presence of lung iNKT cells. These results may provide a potential therapeutic approach to treat allergic asthma.

The immune system in neurological diseases: What innate-like T cells have to say

The immune system classically consists of 2 lines of defense, innate and adaptive, both of which interact with one another effectively to protect us against any pathogenic threats. Importantly, there is a diverse subset of cells known as innate-like T cells that act as a bridge between the innate and adaptive immune systems and are pivotal players in eliciting inflammatory immune responses. A growing body of evidence has demonstrated the regulatory impact of these innate-like T cells in central nervous system (CNS) diseases and that such immune cells can traffic into the brain in multiple pathological conditions, which can be typically attributed to the breakdown of the blood-brain barrier. However, until now, it has been poorly understood whether innate-like T cells have direct protective or causative properties, particularly in CNS diseases. Therefore, in this review, our attention is focused on discussing the critical roles of 3 unique subsets of unconventional T cells, namely, natural killer T cells, γδ T cells, and mucosal-associated invariant T cells, in the context of CNS diseases, disorders, and injuries and how the interplay of these immune cells modulates CNS pathology, in an attempt to gain a better understanding of their complex functions.

The Plasticity of Immune Cell Response Complicates Dissecting the Underlying Pathology of Multiple Sclerosis

Multiple sclerosis (MS) is a neurodegenerative autoimmune disease characterized by the destruction of the myelin sheath of the neuronal axon in the central nervous system. Many risk factors, including environmental, epigenetic, genetic, and lifestyle factors, are responsible for the development of MS. It has long been thought that only adaptive immune cells, especially autoreactive T cells, are responsible for the pathophysiology; however, recent evidence has indicated that innate immune cells are also highly involved in disease initiation and progression. Here, we compile the available data regarding the role immune cells play in MS, drawn from both human and animal research. While T and B lymphocytes, chiefly enhance MS pathology, regulatory T cells (Tregs) may serve a more protective role, as can B cells, depending on context and location. Cells chiefly involved in innate immunity, including macrophages, microglia, astrocytes, dendritic cells, natural killer (NK) cells, eosinophils, and mast cells, play varied roles. In addition, there is evidence regarding the involvement of innate-like immune cells, such as γδ T cells, NKT cells, MAIT cells, and innate-like B cells as crucial contributors to MS pathophysiology. It is unclear which of these cell subsets are involved in the onset or progression of disease or in protective mechanisms due to their plastic nature, which can change their properties and functions depending on microenvironmental exposure and the response of neural networks in damage control. This highlights the need for a multipronged approach, combining stringently designed clinical data with carefully controlled in vitro and in vivo research findings, to identify the underlying mechanisms so that more effective therapeutics can be developed.

Lysosomal processing of sulfatide analogs alters target NKT cell specificity and immune responses in cancer

In a structure-function study of sulfatides that typically stimulate type II NKT cells, we made an unexpected discovery. We compared analogs with sphingosine or phytosphingosine chains and 24-carbon acyl chains with 0-1-2 double bonds (C or pC24:0, 24:1, or 24:2). C24:1 and C24:2 sulfatide presented by the CD1d monomer on plastic stimulated type II, not type I, NKT cell hybridomas, as expected. Unexpectedly, when presented by bone marrow-derived DCs (BMDCs), C24:2 reversed specificity to stimulate type I, not type II, NKT cell hybridomas, mimicking the corresponding β-galactosylceramide (βGalCer) without sulfate. C24:2 induced IFN-γ-dependent immunoprotection against CT26 colon cancer lung metastases, skewed the cytokine profile, and activated conventional DC subset 1 cells (cDC1s). This was abrogated by blocking lysosomal processing with bafilomycin A1, or by sulfite blocking of arylsulfatase or deletion of this enyzme that cleaves off sulfate. Thus, C24:2 was unexpectedly processed in BMDCs from a type II to a type I NKT cell-stimulating ligand, promoting tumor immunity. We believe this is the first discovery showing that antigen processing of glycosylceramides alters the specificity for the target cell, reversing the glycolipid's function from stimulating type II NKT cells to stimulating type I NKT cells, thereby introducing protective functional activity in cancer. We also believe our study uncovers a new role for antigen processing that does not involve MHC loading but rather alteration of which type of cell is responding.

Lipidomics based on liquid chromatography-high resolution mass spectrometry reveals the protective role of peroxisome proliferator-activated receptor alpha on kidney stone formation in mice treated with glyoxylate

Few studies have examined the relationship between lipid metabolism and kidney stone formation, particularly the role of key lipid regulatory factors in kidney stone formation. We evaluated the effect of the lipid regulatory factor-peroxisome proliferator-activated receptor alpha on the formation of renal stones in mice by injecting them with glyoxylate followed by treatment with either a peroxisome proliferator-activated receptor alpha agonist fenofibrate or an antagonist GW6471 (GW). Liquid chromatography coupled with trapped ion mobility spectrometry-quadrupole-time-of-flight mass spectrometry-based lipidomics was used to determine the lipid profile in the mouse kidneys. Histological and biochemical analyses showed that the mice injected with glyoxylate exhibited crystal precipitation and renal dysfunction. Crystallization decreased significantly in the fenofibrate group, whereas it increased significantly in the GW group. A total of 184 lipids, including fatty acyls, glycerolipids, glycerophospholipids, and sphingolipids differed significantly between the mice in the model and control groups. Peroxisome proliferator-activated receptor alpha activity negatively correlated with glyoxylate-induced kidney stone formation in mice, which may be related to improved fatty acid oxidation, maintenance of ceramide/complex sphingolipids cycle balance, and alleviation of disorder in phospholipid metabolism.

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.

Cerebrospinal fluid sulfatide isoforms lack diagnostic utility in separating progressive from relapsing-remitting multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is an immune-mediated demyelinating disorder of the central nervous system. The glycosphingolipid sulfatide, a lipid particularly enriched in the myelin sheath, has been shown to be involved the maintenance of this specific membrane structure. Sulfatide in cerebrospinal fluid (CSF) may reflect demyelination, a dominating feature of MS. We investigated the diagnostic utility of CSF sulfatide isoform levels to separate different courses or phenotypes of MS disease.

MATERIAL AND METHODS

This was a mono-center, cross-sectional study of relapsing-remitting MS (RRMS) (n = 45) and progressive MS (PMS) (n = 42) patients (consisting of primary PMS (n = 17) and secondary PMS (n = 25)) and healthy controls (n = 19). In total, 20 sulfatide isoforms were measured in CSF by liquid chromatography-mass spectrometry.

RESULTS

CSF total sulfatide concentrations, as well as CSF sulfatide isoform distribution, did not differ across the study groups, and their levels were independent of disease course/phenotype, disease duration, time to conversion to secondary PMS, age, and disability in MS patients.

CONCLUSION

CSF sulfatide isoforms lack diagnostic and prognostic utility as a biomarker for progressive MS.

T cells from MS Patients with High Disease Severity Are Insensitive to an Immune-Suppressive Effect of Sulfatide

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS). Its early phase is characterized by a relapse-remitting disease course, followed by disability progression in the later stage. While chronic inflammation accompanied with degeneration is well-established as the key pathological feature, the pathogenesis of MS, particularly progressive MS, remains elusive. Sulfatide is a major glycolipid component of myelin, and previous studies in experimental autoimmune encephalomyelitis mouse models have demonstrated it to have immune-protective functions. Notably, sulfatide concentration is increased in the serum and cerebrospinal fluid of patients with MS, particularly those in a progressive disease course. Here, we show that the myelin-glycolipid sulfatide displays an ability to suppress the proliferation of polyclonally activated human T cells. Importantly, this suppressive effect was impaired in T cells obtained from MS patients having higher disability status. Therefore, it is plausible that progression of MS is associated with an escape from the immune-regulatory effect of sulfatide. Our study suggests that, although the precise mechanisms remain unrevealed, an escape of T cells from immunosuppression by sulfatide is associated with disease progression in the advanced stage. Further studies will provide novel insights into the pathogenesis of MS, particularly regarding disease progression, and help develop novel treatment strategies for this challenging disease.

Cross Talk Between Natural Killer T and Dendritic Cells and Its Impact on T Cell Responses in Infections

Both innate and adaptive immunity is vital for host defense against infections. Dendritic cells (DCs) are critical for initiating and modulating adaptive immunity, especially for T-cell responses. Natural killer T (NKT) cells are a small population of innate-like T cells distributed in multiple organs. Many studies have suggested that the cross-talk between these two immune cells is critical for immunobiology and host defense mechanisms. Not only can DCs influence the activation/function of NKT cells, but NKT cells can feedback on DCs also, thus modulating the phenotype and function of DCs and DC subsets. This functional feedback of NKT cells on DCs, especially the preferential promoting effect on CD8α+ and CD103+ DC subsets in lymphoid and non-lymphoid tissues, significantly impacts the systemic and local adaptive CD4 and CD8 T cell responses in infections. This review focuses on the two-way interaction between NKT cells and DCs, emphasizing the importance of NKT cell feedback on DCs in bridging innate and adaptive immune responses for host defense purposes.

The role of NK and NKT cells in the pathogenesis and improvement of multiple sclerosis following disease-modifying therapies

BACKGROUND

Multiple sclerosis (MS) is an autoimmune inflammatory disease of the central nervous system (CNS) that T cells become autoreactive by recognizing CNS antigens. Both innate and adaptive immune systems are involved in the pathogenesis of MS. In recent years, the impact of innate immune cells on MS pathogenesis has received more attention. CD56bright NK cells, as an immunoregulatory subset of NK cells, can increase the production of cytokines that modulate adaptive immune responses, whereas CD56dim NK cells are more active in cytolysis functions. These two main subsets of NK cells may have different effects on the onset or progression of MS. Invariant NKT (iNKT) cells are other immune cells involved in the control of autoimmune diseases; however, variant NKT (vNKT) cells, despite limited information, could play a role in MS remission via an immunoregulatory pathway.

AIM

We aimed to evaluate the influence of MS therapeutic agents on NK and NKT cells and NK cell subtypes.

MATERIALS AND METHODS

The possible mechanism of each MS therapeutic agent has been presented here, focusing on the effects of different disease-modifying therapies on the number of NK and NKT subtypes.

RESULTS

Expansion of CD56bright NK cells, reduction in the CD56dim cells, and enhancement in NKT cells are the more important innate immune cells alterations following the disease-modifying therapies.

CONCLUSION

Expansion of CD56bright NK cells or reduction in the CD56dim cells has been associated with a successful response to different treatments in MS. iNKT and vNKT cells could have beneficial effects on MS improving. It seems that they are enhanced due to some of MS drugs, leading to disease improvement. However, a reduction in the number of NKT cells could be due to the adverse effects of some of MS drugs on the bone marrow.

Sulfatide in health and disease. The evaluation of sulfatide in cerebrospinal fluid as a possible biomarker for neurodegeneration

Sulfatide (3-O-sulfogalactosylceramide, SM4) is a glycosphingolipid, highly multifunctional and particularly enriched in the myelin sheath of neurons. The role of sulfatide has been implicated in various biological fields such as the nervous system, immune system, host-pathogen recognition and infection, beta cell function and haemostasis/thrombosis. Thus, alterations in sulfatide metabolism and production are associated with several human diseases such as neurological and immunological disorders and cancers. The unique lipid-rich composition of myelin reflects the importance of lipids in this specific membrane structure. Sulfatide has been shown to be involved in the regulation of oligodendrocyte differentiation and in the maintenance of the myelin sheath by influencing membrane dynamics involving sorting and lateral assembly of myelin proteins as well as ion channels. Sulfatide is furthermore essential for proper formation of the axo-glial junctions at the paranode together with axonal glycosphingolipids. Alterations in sulfatide metabolism are suggested to contribute to myelin deterioration as well as synaptic dysfunction, neurological decline and inflammation observed in different conditions associated with myelin pathology (mouse models and human disorders). Body fluid biomarkers are of importance for clinical diagnostics as well as for patient stratification in clinical trials and treatment monitoring. Cerebrospinal fluid (CSF) is commonly used as an indirect measure of brain metabolism and analysis of CSF sulfatide might provide information regarding whether the lipid disruption observed in neurodegenerative disorders is reflected in this body fluid. In this review, we evaluate the diagnostic utility of CSF sulfatide as a biomarker for neurodegenerative disorders associated with dysmyelination/demyelination by summarising the current literature on this topic. We can conclude that neither CSF sulfatide levels nor individual sulfatide species consistently reflect the lipid disruption observed in many of the demyelinating disorders. One exception is the lysosomal storage disorder metachromatic leukodystrophy, possibly due to the genetically determined accumulation of non-metabolised sulfatide. We also discuss possible explanations as to why myelin pathology in brain tissue is poorly reflected by the CSF sulfatide concentration. The previous suggestion that CSF sulfatide is a marker of myelin damage has thereby been challenged by more recent studies using more sophisticated laboratory techniques for sulfatide analysis as well as improved sample selection criteria due to increased knowledge on disease pathology.

Increased Tc17 cell levels and imbalance of naïve/effector immune response in Parkinson's disease patients in a two-year follow-up: a case control study

BACKGROUND

Neuroinflammation has been proved to play a role in dopaminergic neuronal death in Parkinson's disease (PD). This link highlights the relevance of the immune response in the progression of the disease. However, little is known about the impact of peripheral immune response on the disease. This study is aimed to evaluate how immune cell populations change in untreated PD patients followed-up for 2 years.

METHODS

Thirty-two patients with no previous treatment (PD-0 yr) and twenty-two healthy subjects (controls) were included in the study. PD patients were sampled 1 and 2 years after the start of the treatment. CD4 T cells (naïve/central memory, effector, and activated), CD8 T cells (activated, central memory, effector memory, NKT, Tc1, Tc2, and Tc17), and B cells (activated, plasma, and Lip-AP) were characterized by flow cytometry.

RESULTS

We observed decreased levels of naïve/central memory CD4 and CD8 T cells, Tc1, Tc2, NKT, and plasma cells, and increased levels of effector T cells, activated T cells, and Tc17.

CONCLUSIONS

PD patients treated for 2 years showed an imbalance in the naive/effector immune response. Naïve and effector cell levels were associated with clinical deterioration. These populations are also correlated to aging. On the other hand, higher Tc17 levels suggest an increased inflammatory response, which may impact the progression of the disease. Our results highlight the relevant effect of treatment on the immune response, which could improve our management of the disease.

NKT and NKT-like Cells in Autoimmune Neuroinflammatory Diseases-Multiple Sclerosis, Myasthenia Gravis and Guillain-Barre Syndrome

NKT cells comprise three subsets—type I (invariant, iNKT), type II, and NKT-like cells, of which iNKT cells are the most studied subset. They are capable of rapid cytokine production after the initial stimulus, thus they may be important for polarisation of Th cells. Due to this, they may be an important cell subset in autoimmune diseases. In the current review, we are summarising results of NKT-oriented studies in major neurological autoimmune diseases—multiple sclerosis, myasthenia gravis, and Guillain-Barre syndrome and their corresponding animal models.

Sulfatides are endogenous ligands for the TLR4-MD-2 complex

Many endogenous molecules, mostly proteins, purportedly activate the Toll-like receptor 4 (TLR4)-myeloid differentiation factor-2 (MD-2) complex, the innate immune receptor for lipopolysaccharide (LPS) derived from gram-negative bacteria. However, there is no structural evidence supporting direct TLR4-MD-2 activation by endogenous ligands. Sulfatides (3-O-sulfogalactosylceramides) are natural, abundant sulfated glycolipids that have variously been shown to initiate or suppress inflammatory responses. We show here that short fatty acid (FA) chain sulfatides directly activate mouse TLR4-MD-2 independent of CD14, trigger MyD88- and TRIF-dependent signaling, and stimulate tumor necrosis factor α (TNFα) and type I interferon (IFN) production in mouse macrophages. In contrast to the agonist activity toward the mouse receptor, the tested sulfatides antagonize TLR4-MD-2 activation by LPS in human macrophage-like cells. The agonistic and antagonistic activities of sulfatides require the presence of the sulfate group and are inversely related to the FA chain length. The crystal structure of mouse TLR4-MD-2 in complex with C16-sulfatide revealed that three C16-sulfatide molecules bound to the MD-2 hydrophobic pocket and induced an active dimer conformation of the receptor complex similar to that induced by LPS or lipid A. The three C16-sulfatide molecules partially mimicked the detailed interactions of lipid A to achieve receptor activation. Our results suggest that sulfatides may mediate sterile inflammation or suppress LPS-stimulated inflammation, and that additional endogenous negatively charged lipids with up to six lipid chains of limited length might also bind to TLR4-MD-2 and activate or inhibit this complex.

The Role of NKT Cells in Glioblastoma

Glioblastoma is an aggressive and deadly cancer, but to date, immunotherapies have failed to make significant strides in improving prognoses for glioblastoma patients. One of the current challenges to developing immunological interventions for glioblastoma is our incomplete understanding of the numerous immunoregulatory mechanisms at play in the glioblastoma tumor microenvironment. We propose that Natural Killer T (NKT) cells, which are unconventional T lymphocytes that recognize lipid antigens presented by CD1d molecules, may play a key immunoregulatory role in glioblastoma. For example, evidence suggests that the activation of type I NKT cells can facilitate anti-glioblastoma immune responses. On the other hand, type II NKT cells are known to play an immunosuppressive role in other cancers, as well as to cross-regulate type I NKT cell activity, although their specific role in glioblastoma remains largely unclear. This review provides a summary of our current understanding of NKT cells in the immunoregulation of glioblastoma as well as highlights the involvement of NKT cells in other cancers and central nervous system diseases.

The role of Sphingolipids in myelination and myelin stability and their involvement in childhood and adult demyelinating disorders

Multiple sclerosis (MS) represents the most common demyelinating disease affecting the central nervous system (CNS) in adults as well as in children. Furthermore, in children, in addition to acquired diseases such as MS, genetically inherited diseases significantly contribute to the incidence of demyelinating disorders. Some genetic defects lead to sphingolipid alterations that are able to elicit neurological symptoms. Sphingolipids are essential for brain development, and their aberrant functionality may thus contribute to demyelinating diseases such as MS. In particular, sphingolipidoses caused by deficits of sphingolipid-metabolizing enzymes, are often associated with demyelination. Sphingolipids are not only structural molecules but also bioactive molecules involved in the regulation of cellular events such as development of the nervous system, myelination and maintenance of myelin stability. Changes in the sphingolipid metabolism deeply affect plasma membrane organization. Thus, changes in myelin sphingolipid composition might crucially contribute to the phenotype of diseases characterized by demyelinalization. Here, we review key features of several sphingolipids such as ceramide/dihydroceramide, sphingosine/dihydrosphingosine, glucosylceramide and, galactosylceramide which act in myelin formation during rat brain development and in human brain demyelination during the pathogenesis of MS, suggesting that this knowledge could be useful in identifying targets for possible therapies.

Sulfatide-activated type II NKT cells suppress immunogenic maturation of lung dendritic cells in murine models of asthma

Our previous study showed that sulfatide-activated type II natural killer T (NKT) cells can prevent allergic airway inflammation in an ovalbumin (OVA)-induced murine model of asthma, but the underlying mechanism is unclear. Recently, sulfatide-activated type II NKT cells were shown to modulate the function of dendritic cells in experimental autoimmune encephalomyelitis and nonobese diabetic mice. Thus, it was hypothesized that sulfatide-activated type II NKT cells may modulate the function of lung dendritic cells (LDCs) in asthmatic mice. Our data showed that, in our mouse models, activation of type II NKT cells by sulfatide administration and adoptive transfer of sulfatide-activated type II NKT cells resulted in reduced expression of surface maturation markers and proinflammatory cytokine production of LDCs. LDCs from sulfatide-treated asthmatic mice, in contrast to LDCs from PBS-treated asthmatic mice, significantly reduced allergic airway inflammation in vivo. However, we found no influence of sulfatide-activated type II NKT cells on the phenotypic and functional maturation of bone marrow-derived dendritic cells in vitro. In addition, adoptive transfer of sulfatide-activated type II NKT cells did not influence the phenotypic and functional maturation of LDCs in CD1d^-/- mice, which lack both type I and II NKT cells, immunized and challenged with OVA. Our data reveal that sulfatide-activated type II NKT cells can suppress immunogenic maturation of LDCs to reduce allergic airway inflammation in mouse models of asthma, and it is possible that the immunomodulatory effect needs type I NKT cells.

Expression of CD1d by astrocytes corresponds with relative activity in multiple sclerosis lesions

The CD1 protein family present lipid antigens to the immune system. CD1d has been observed in the CNS of MS patients, yet no studies have quantitatively characterized this expression and related it to inflammatory demyelinative activity in MS plaques. In this study, we set out to localize and quantify the presence of CD1d expression by astrocytes in MS brain tissue lesions. Formalin‐fixed, paraffin‐embedded MS and control brain tissues were examined. Lesions were classified as active, chronic active or chronic silent. Using immunofluorescence, the density of CD1d‐positive cells was determined in active lesions, chronic active lesion edges and chronic active lesion centers. The percentage of CD1d‐positive cells that were GFAP‐positive was also determined in each of these regions. CD1d immunoreactivity was significantly increased in MS compared to control tissue, was significantly more prevalent in areas of active demyelination, and colocalized with GFAP‐positive reactive astrocytes. Increases of CD1d immunoreactivity in the CNS of MS patients being greatest in areas of active demyelination and localized to GFAP‐positive astrocytes lend support to the hypothesis of a lipid‐targeted autoimmune process contributing to the pathogenesis of MS.

Innate, innate-like and adaptive lymphocytes in the pathogenesis of MS and EAE

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) in which the immune system damages the protective insulation surrounding the nerve fibers that project from neurons. A hallmark of MS and its animal model, experimental autoimmune encephalomyelitis (EAE), is autoimmunity against proteins of the myelin sheath. Most studies in this field have focused on the roles of CD4⁺ T lymphocytes, which form part of the adaptive immune system as both mediators and regulators in disease pathogenesis. Consequently, the treatments for MS often target the inflammatory CD4⁺ T-cell responses. However, many other lymphocyte subsets contribute to the pathophysiology of MS and EAE, and these subsets include CD8⁺ T cells and B cells of the adaptive immune system, lymphocytes of the innate immune system such as natural killer cells, and subsets of innate-like T and B lymphocytes such as γδ T cells, natural killer T cells, and mucosal-associated invariant T cells. Several of these lymphocyte subsets can act as mediators of CNS inflammation, whereas others exhibit immunoregulatory functions in disease. Importantly, the efficacy of some MS treatments might be mediated in part by effects on lymphocytes other than CD4⁺ T cells. Here we review the contributions of distinct subsets of lymphocytes on the pathogenesis of MS and EAE, with an emphasis on lymphocytes other than CD4⁺ T cells. A better understanding of the distinct lymphocyte subsets that contribute to the pathophysiology of MS and its experimental models will inform the development of novel therapeutic approaches.

Sulfatides ameliorate experimental autoimmune neuritis by suppressing Th1/Th17 cells

Sulfatides have immunomodulatory functions, and play protective roles in multiple autoimmune diseases. In the present study, we showed that sulfatides ameliorated experimental autoimmune neuritis in Lewis rats induced with bovine peripheral myelin, which was associated with decreased proportions of Th1 and Th17 cells. Furthermore, compared control group, cells from sulfatide-treated rats exhibited lower potential in proliferation and IL-17 secretion in the presence of BPM or ConA in vitro. Moreover, sulfatides also reduced the proportions of NK and NKT cells. In summary, our study indicated that sulfatides might become a new therapeutic agent in Guillain-Barré syndrome in the future.

Complex Network of NKT Cell Subsets Controls Immune Homeostasis in Liver and Gut

The liver-gut immune axis is enriched in several innate immune cells, including innate-like unconventional and adaptive T cells that are thought to be involved in the maintenance of tolerance to gut-derived antigens and, at the same time, enable effective immunity against microbes. Two subsets of lipid-reactive CD1d-restricted natural killer T (NKT) cells, invariant NKT (iNKT) and type II NKT cells present in both mice and humans. NKT cells play an important role in regulation of inflammation in the liver and gut due to their innate-like properties of rapid secretion of a myriad of pro-inflammatory and anti-inflammatory cytokines and their ability to influence other innate cells as well as adaptive T and B cells. Notably, a bi-directional interactive network between NKT cells and gut commensal microbiota plays a crucial role in this process. Here, we briefly review recent studies related to the cross-regulation of both NKT cell subsets and how their interactions with other immune cells and parenchymal cells, including hepatocytes and enterocytes, control inflammatory diseases in the liver, such as alcoholic and non-alcoholic steatohepatitis, as well as inflammation in the gut. Overwhelming experimental data suggest that while iNKT cells are pathogenic, type II NKT cells are protective in the liver. Since CD1d-dependent pathways are highly conserved from mice to humans, a detailed cellular and molecular understanding of these immune regulatory pathways will have major implications for the development of novel therapeutics against inflammatory diseases of liver and gut.

Type II NKT Cells: An Elusive Population With Immunoregulatory Properties

Natural killer T (NKT) cells are unique unconventional T cells that are reactive to lipid antigens presented on the non-polymorphic major histocompatibility class (MHC) I-like molecule CD1d. They have characteristics of both innate and adaptive immune cells, and have potent immunoregulatory roles in tumor immunity, autoimmunity, and infectious diseases. Based on their T cell receptor (TCR) expression, NKT cells are divided into two subsets, type I NKT cells with an invariant TCRα-chain (Vα24 in humans, Vα14 in mice) and type II NKT cells with diverse TCRs. While type I NKT cells are well-studied, knowledge about type II NKT cells is still limited, and it is to date only possible to identify subsets of this population. However, recent advances have shown that both type I and type II NKT cells play important roles in many inflammatory situations, and can sometimes regulate the functions of each other. Type II NKT cells can be both protective and pathogenic. Here, we review current knowledge on type II NKT cells and their functions in different disease settings and how these cells can influence immunological outcomes.

CD1d-Restricted Type II NKT Cells Reactive With Endogenous Hydrophobic Peptides

NKT cells belong to a distinct subset of T cells that recognize hydrophobic antigens presented by major histocompatibility complex class I-like molecules, such as CD1d. Because NKT cells stimulated by antigens can activate or suppress other immunocompetent cells through an immediate production of a large amount of cytokines, they are regarded as immunological modulators. CD1d-restricted NKT cells are classified into two subsets, namely, type I and type II. CD1d-restricted type I NKT cells express invariant T cell receptors (TCRs) and react with lipid antigens, including the marine sponge-derived glycolipid α-galactosylceramide. On the contrary, CD1d-restricted type II NKT cells recognize a wide variety of antigens, including glycolipids, phospholipids, and hydrophobic peptides, by their diverse TCRs. In this review, we focus particularly on CD1d-restricted type II NKT cells that recognize endogenous hydrophobic peptides presented by CD1d. Previous studies have demonstrated that CD1d-restricted type I NKT cells usually act as pro-inflammatory cells but sometimes behave as anti-inflammatory cells. It has been also demonstrated that CD1d-restricted type II NKT cells play opposite roles to CD1d-restricted type I NKT cells; thus, they function as anti-inflammatory or pro-inflammatory cells depending on the situation. In line with this, CD1d-restricted type II NKT cells that recognize type II collagen peptide have been demonstrated to act as anti-inflammatory cells in diverse inflammation-induction models in mice, whereas pro-inflammatory CD1d-restricted type II NKT cells reactive with sterol carrier protein 2 peptide have been demonstrated to be involved in the development of small vessel vasculitis in rats.

The Janus Face of NKT Cell Function in Autoimmunity and Infectious Diseases

Natural killer T cells (NKT) are a subset of T lymphocytes bridging innate and adaptive immunity. These cells recognize self and microbial glycolipids bound to non-polymorphic and highly conserved CD1d molecules. Three NKT cell subsets, type I, II, and NKT-like expressing different antigen receptors (TCR) were described and TCR activation promotes intracellular events leading to specific functional activities. NKT can exhibit different functions depending on the secretion of soluble molecules and the interaction with other cell types. NKT cells act as regulatory cells in the defense against infections but, on the other hand, their effector functions can be involved in the pathogenesis of several inflammatory disorders due to their exposure to different microbial or self-antigens, respectively. A deep understanding of the biology and functions of type I, II, and NKT-like cells as well as their interplay with cell types acting in innate (neuthrophils, innate lymphoid cells, machrophages, and dendritic cells) and adaptive immunity (CD4⁺,CD8⁺, and double negative T cells) should be important to design potential immunotherapies for infectious and autoimmune diseases.

Type II NKT Cells and Their Emerging Role in Health and Disease

NKT cells recognize lipid Ags presented by a class I MHC-like molecule CD1d, a member of the CD1 family. Although most initial studies on NKT cells focused on a subset with semi-invariant TCR termed invariant NKT cells, the majority of CD1d-restricted lipid-reactive human T cells express diverse TCRs and are termed type II NKT cells. These cells constitute a distinct population of circulating and tissue-resident effector T cells with immune-regulatory properties. They react to a growing list of self- as well as non-self-lipid ligands, and share some properties with both invariant NKT and conventional T cells. An emerging body of evidence points to their role in the regulation of immunity to pathogens/tumors and in autoimmune/metabolic disorders. An improved understanding of the biology of these cells and the ability to manipulate their function may be of therapeutic benefit in diverse disease conditions.

Type II NKT cells: a distinct CD1d-restricted immune regulatory NKT cell subset

Type II natural killer T cells (NKT) are a subset of the innate-like CD1d-restricted lymphocytes that are reactive to lipid antigens. Unlike the type I NKT cells, which express a semi-invariant TCR, type II NKT cells express a broader TCR repertoire. Additionally, other features, such as their predominance over type I cells in humans versus mice, the nature of their ligands, CD1d/lipid/TCR binding, and modulation of immune responses, distinguish type II NKT cells from type I NKT cells. Interestingly, it is the self-lipid-reactivity of type II NKT cells that has helped define their physiological role in health and in disease. The discovery of sulfatide as one of the major antigens for CD1d-restricted type II NKT cells in mice has been instrumental in the characterization of these cells, including the TCR repertoire, the crystal structure of the CD1d/lipid/TCR complex, and their function. Subsequently, several other glycolipids and phospholipids from both endogenous and microbial sources have been shown to activate type II NKT cells. The activation of a specific subset of type II NKT cells following administration with sulfatide or lysophosphatidylcholine (LPC) leads to engagement of a dominant immunoregulatory pathway associated with the inactivation of type I NKT cells, conventional dendritic cells, and inhibition of the proinflammatory Th1/Th17 cells. Thus, type II NKT cells have been shown to be immunosuppressive in autoimmune diseases, inflammatory liver diseases, and in cancer. Knowing their relatively higher prevalence in human than type I NKT cells, understanding their biology is imperative for health and disease.

NKT cell subsets as key participants in liver physiology and pathology

Natural killer T (NKT) cells are innate-like lymphocytes that generally recognize lipid antigens and are enriched in microvascular compartments of the liver. NKT cells can be activated by self- or microbial-lipid antigens and by signaling through toll-like receptors. Following activation, NKT cells rapidly secrete pro-inflammatory or anti-inflammatory cytokines and chemokines, and thereby determine the milieu for subsequent immunity or tolerance. It is becoming clear that two different subsets of NKT cells-type I and type II-have different modes of antigen recognition and have opposing roles in inflammatory liver diseases. Here we focus mainly on the roles of both NKT cell subsets in the maintenance of immune tolerance and inflammatory diseases in liver. Furthermore, how the differential activation of type I and type II NKT cells influences other innate cells and adaptive immune cells to result in important consequences for tissue integrity is discussed. It is crucial that better reagents, including CD1d tetramers, be used in clinical studies to define the roles of NKT cells in liver diseases in patients.

IL-10 Production Is Critical for Sustaining the Expansion of CD5+ B and NKT Cells and Restraining Autoantibody Production in Congenic Lupus-Prone Mice

The development and progression of systemic lupus erythematosus is mediated by the complex interaction of genetic and environmental factors. To decipher the genetics that contribute to pathogenesis and the production of pathogenic autoantibodies, our lab has focused on the generation of congenic lupus-prone mice derived from the New Zealand Black (NZB) strain. Previous work has shown that an NZB-derived chromosome 4 interval spanning 32 to 151 Mb led to expansion of CD5⁺ B and Natural Killer T (NKT) cells, and could suppress autoimmunity when crossed with a lupus-prone mouse strain. Subsequently, it was shown that CD5⁺ B cells but not NKT cells derived from these mice could suppress the development of pro-inflammatory T cells. In this paper, we aimed to further resolve the genetics that leads to expansion of these two innate-like populations through the creation of additional sub-congenic mice and to characterize the role of IL-10 in the suppression of autoimmunity through the generation of IL-10 knockout mice. We show that expansion of CD5⁺ B cells and NKT cells localizes to a chromosome 4 interval spanning 91 to 123 Mb, which is distinct from the region that mediates the majority of the suppressive phenotype. We also demonstrate that IL-10 is critical to restraining autoantibody production and surprisingly plays a vital role in supporting the expansion of innate-like populations.

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.

Natural killer T cells in multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a chronic inflammatory disease that causes demyelination of neurons in the central nervous system. Traditional therapies for MS have involved anti-inflammatory and immunosuppressive drugs with significant side effects that often only provide short-term relief. A more desirable outcome of immunotherapy would be to protect against disease before its clinical manifestation or to halt disease after its initiation. One attractive approach to accomplish this goal would be to restore tolerance by targeting immunoregulatory cell networks. Although much of the work in this area has focused on CD4(+) Foxp3(+) regulatory T cells, other studies have investigated natural killer T (NKT) cells, a subset of T cells that recognizes glycolipid antigens in the context of the CD1d glycoprotein. Studies with human MS patients have revealed alterations in the numbers and functions of NKT cells, which have been partially supported by studies with the experimental autoimmune encephalomyelitis model of MS. Additional studies have shown that activation of NKT cells with synthetic lipid antigens can, at least under certain experimental conditions, protect mice against the development of MS-like disease. Although mechanisms of this protection remain to be fully investigated, current evidence suggests that it involves interactions with other immunoregulatory cell types such as regulatory T cells and immunosuppressive myeloid cells. These studies have provided a strong foundation for the rational design of NKT-cell-based immunotherapies for MS that induce tolerance while sparing overall immune function. Nevertheless, additional pre-clinical and clinical studies will be required to bring this goal to fruition.

Type II NKT Cells in Inflammation, Autoimmunity, Microbial Immunity, and Cancer

Natural killer T cells (NKT) recognize self and microbial lipid antigens presented by non-polymorphic CD1d molecules. Two major NKT cell subsets, type I and II, express different types of antigen receptors (TCR) with distinct mode of CD1d/lipid recognition. Though type II NKT cells are less frequent in mice and difficult to study, they are predominant in human. One of the major subsets of type II NKT cells reactive to the self-glycolipid sulfatide is the best characterized and has been shown to induce a dominant immune regulatory mechanism that controls inflammation in autoimmunity and in anti-cancer immunity. Recently, type II NKT cells reactive to other self-glycolipids and phospholipids have been identified suggesting both promiscuous and specific TCR recognition in microbial immunity as well. Since the CD1d pathway is highly conserved, a detailed understanding of the biology and function of type II NKT cells as well as their interplay with type I NKT cells or other innate and adaptive T cells will have major implications for potential novel interventions in inflammatory and autoimmune diseases, microbial immunity, and cancer.

Inhibition of type I natural killer T cells by retinoids or following sulfatide-mediated activation of type II natural killer T cells attenuates alcoholic liver disease in mice

Innate immune mechanisms leading to liver injury subsequent to chronic alcohol ingestion are poorly understood. Natural killer T (NKT) cells, enriched in the liver and comprised of at least two distinct subsets, type I and II, recognize different lipid antigens presented by CD1d molecules. We have investigated whether differential activation of NKT cell subsets orchestrates inflammatory events leading to alcoholic liver disease (ALD). We found that after chronic plus binge feeding of Lieber-DeCarli liquid diet in male C57BL/6 mice, type I, but not type II, NKT cells are activated, leading to recruitment of inflammatory Gr-1(high) CD11b(+) cells into the liver. A central finding is that liver injury after alcohol feeding is dependent upon type I NKT cells. Thus, liver injury is significantly inhibited in Jα18(-/-) mice deficient in type I NKT cells as well as after their inactivation by sulfatide-mediated activation of type II NKT cells. Furthermore, we have identified a novel pathway involving all-trans retinoic acid (ATRA) and its receptor (RARγ) signaling that inhibits type I NKT cells and, consequently, ALD. A semiquantitative polymerase chain reaction analysis of hepatic gene expression of some of the key proinflammatory molecules shared in human disease indicated that their up-regulation in ALD is dependent upon type I NKT cells.

CONCLUSIONS

Type I, but not type II, NKT cells become activated after alcohol feeding. Type I NKT cell-induced inflammation and neutrophil recruitment results in liver tissue damage whereas type II NKT cells protect from injury in ALD. Inhibition of type I NKT cells by retinoids or by sulfatide prevents ALD. Given that the CD1d pathway is highly conserved between mice and humans, NKT cell subsets might be targeted for potential therapeutic intervention in ALD.

Recognition of lysophosphatidylcholine by type II NKT cells and protection from an inflammatory liver disease

Lipids presented by the MHC class I-like molecule, CD1d, are recognized by NK T (NKT) cells, which can be broadly categorized into two subsets. The well-characterized type I NKT cells express a semi-invariant TCR and can recognize both α- and β-linked glycolipids, whereas type II NKT cells are less well studied, express a relatively diverse TCR repertoire, and recognize β-linked lipids. Recent structural studies have shown a distinct mode of recognition of a self-glycolipid sulfatide bound to CD1d by a type II NKT TCR. To further characterize Ag recognition by these cells, we have used the structural data and screened other small molecules able to bind to CD1d and activate type II NKT cells. Using plate-bound CD1d and APC-based Ag presentation assay, we found that phospholipids such as lysophosphatidylcholine (LPC) can stimulate the sulfatide-reactive type II NKT hybridoma Hy19.3 in a CD1d-dependent manner. Using plasmon resonance studies, we found that this type II NKT TCR binds with CD1d-bound LPC with micromolar affinities similar to that for sulfatide. Furthermore, LPC-mediated activation of type II NKT cells leads to anergy induction in type I NKT cells and affords protection from Con A-induced hepatitis. These data indicate that, in addition to self-glycolipids, self-lysophospholipids are also recognized by type II NKT cells. Because lysophospholipids are involved during inflammation, our findings have implications for not only understanding activation of type II NKT cells in physiological settings, but also for the development of immune intervention in inflammatory diseases.