Invariant NKT cells limit activation of autoreactive CD1d-positive B cells

B cell-expressed CD1d promotes MPL/TDCM lipid emulsion adjuvant effects in polysaccharide vaccines

T cell-independent type 2 antigens (TI-2 Ags), such as pneumococcal polysaccharides, elicit weak immunoglobulin G (IgG) responses and are refractive to boosting. Overcoming this challenge is critical for improving vaccines. Previously, we demonstrated a lipid-based adjuvant composed of monophosphoryl lipid A, synthetic cord factor, and squalene significantly boosts primary and secondary IgM and IgG production against polysaccharide Ags. Herein, we show beta-2 microglobulin, but not MHC class II, is essential for adjuvant-induced increases in polysaccharide-specific IgG levels. Furthermore, we demonstrate CD1d expression is essential for optimal adjuvant-induced increases in IgG, but is not required for IgG responses to TI-2 Ags administered without adjuvant, with the exception of the bacterial cell wall polysaccharide component, phosphorylcholine. Adoptive transfer of splenic and peritoneal cells from VHB1-8 transgenic mice into CD1d-/- mice revealed adjuvant-induced increases in NP-Ficoll-specific IgG production by CD1d+/+ transgenic B cells, but not recipient B cells, suggesting B cell-expressed CD1d is critical for adjuvant-induced effects on TI-2 antibody responses. Consistent with this, bone marrow chimera mice with selective CD1d deficiency in B cells demonstrated B cell-expressed CD1d was dispensable for iNKT cell development and maintenance but was required for adjuvant-induced increases in protective levels of polysaccharide- and phosphorylcholine-specific IgG. Notably, both iNKT cells and CD1d crosslinking significantly increased IgG production by B cells coactivated with TI-2 Ag and adjuvant, suggesting iNKT-induced CD1d signaling may promote increased IgG production. In summary, our study reveals B cell-dependent CD1d expression is critical for effectiveness of a potent lipid-based adjuvant in augmenting polysaccharide- and phosphorylcholine-specific IgG responses.

The balance between conventional and unconventional T follicular helper cells influences autoreactive B cell responses

Invariant natural killer T (iNKT) cells are activated by glycolipids presented on CD1d. When iNKT cells interact with and activate B cells, they can differentiate into iNKT follicular helper (iNKTfh) cells, and here, we investigate how this, in turn, regulates conventional T follicular helper (Tfh) cells. This is done in an autoimmune model where antibodies are produced against self-antigens relevant to the autoimmune disease systemic lupus erythematosus (SLE). We find a balance between iNKTfh and Tfh cells that directs the B cell response and influences Tfh cell generation. This altered balance also affects the specificities and increases the autoantibody response. We also show that CD1d expression by B cells is essential for iNKTfh cell generation. In conclusion, our data shed light on how T cell help for B cells is divided between conventional and unconventional helper cell populations and how this has an impact on autoreactive B cell responses.

Pla2g2a promotes innate Th2-type immunity lymphocytes to increase B1a cells

Newborns require early generation of effective innate immunity as a primary physiological mechanism for survival. The neonatal Lin28⁺Let7^– developmental pathway allows increased generation of Th2-type cells and B1a (B-1 B) cells compared to adult cells and long-term maintenance of these initially generated innate cells. For initial B1a cell growth from the neonatal to adult stage, Th2-type IL-5 production from ILC2s and NKT2 cells is important to increase B1a cells. The Th17 increase is dependent on extracellular bacteria, and increased bacteria leads to lower Th2-type generation. Secreted group IIA-phospholipase A2 (sPLA2-IIA) from the Pla2g2a gene can bind to gram-positive bacteria and degrade bacterial membranes, controlling microbiota in the intestine. BALB/c mice are Pla2g2a⁺, and express high numbers of Th2-type cells and B1a cells. C57BL/6 mice are Pla2g2a-deficient and distinct from the SLAM family, and exhibit fewer NKT2 cells and fewer B1a cells from the neonatal to adult stage. We found that loss of Pla2g2a in the BALB/c background decreased IL-5 from Th2-type ILC2s and NKT2s but increased bacterial-reactive NKT17 cells and MAIT cells, and decreased the number of early-generated B1a cells and MZ B cells and the CD4/CD8 T cell ratio. Low IL-5 by decreased Th2-type cells in Pla2g2a loss led to low early-generated B1a cell growth from the neonatal to adult stage. In anti-thymocyte/Thy-1 autoreactive μκ transgenic (ATAμκ Tg) Pla2g2a⁺ BALB/c background C.B17 mice generated NKT2 cells that continuously control CD1d⁺ B1 B cells through old aging and lost CD1d in B1 B cells generating strong B1 ATA B cell leukemia/lymphoma. Pla2g2a-deficient ATAμκTg C57BL/6 mice suppressed the initial B1a cell increase, with low/negative spontaneous leukemia/lymphoma generation. These data confirmed that the presence of Pla2g2a to control bacteria is important to allow the neonatal to adult stage. Pla2g2a promotes innate Th2-type immunity lymphocytes to increase early generated B1a cells.

iNKT cells can effectively inhibit IL-6 production by B cells in systemic sclerosis

OBJECTIVE

Systemic sclerosis (SSc) is a connective tissue disease with poorly understood pathogenesis and limited treatment options. Patient mortality is rooted predominantly in the development of pulmonary and cardiac complications. The overactivated immune system is assumed to sustain the inflammatory signature of this autoimmune disease. Here, we investigate the potential of immunoregulatory invariant natural killer T (iNKT) cells to inhibit proinflammatory B cell responses in an in vitro model of inflammation.

METHODS

B cells from healthy volunteers (n = 17) and patients with SSc (n = 15) were used for functional testing upon lipopolysaccharide (LPS) stimulation in a co-culture system with third-party iNKT cells. Cytokine production was measured with antibody-based immunoassays (ELISA) and intracellular cytokine staining.

RESULTS

iNKT cells strongly inhibited the production of proinflammatory interleukin-6 by B cells upon stimulation with LPS in both healthy volunteers and patients with SSc. In a Transwell assay, cell contact between B cells and iNKT cells proved necessary for this inhibitory effect. Similarly, blocking of CD1d on the surface of B cells abolished the immunoregulatory effect of iNKT cells on B cells. B cell subsets with higher expression of CD1d, namely unswitched memory B cells, were more susceptible to iNKT cell inhibition.

CONCLUSION

Our in vitro data underline the potential of iNKT cells in the control of SSc and provide a rationale for the use of novel iNKT cell-based therapeutic strategies in the context of autoimmune diseases.

Soluble and Exosome-Bound α-Galactosylceramide Mediate Preferential Proliferation of Educated NK Cells with Increased Anti-Tumor Capacity

Natural killer (NK) cells can kill target cells via the recognition of stress molecules and down-regulation of major histocompatibility complex class I (MHC-I). Some NK cells are educated to recognize and kill cells that have lost their MHC-I expression, e.g., tumor or virus-infected cells. A desired property of cancer immunotherapy is, therefore, to activate educated NK cells during anti-tumor responses in vivo. We here analyze NK cell responses to α-galactosylceramide (αGC), a potent activator of invariant NKT (iNKT) cells, or to exosomes loaded with αGC. In mouse strains which express different MHC-I alleles using an extended NK cell flow cytometry panel, we show that αGC induces a biased NK cell proliferation of educated NK cells. Importantly, iNKT cell-induced activation of NK cells selectively increased in vivo missing self-responses, leading to more effective rejection of tumor cells. Exosomes from antigen-presenting cells are attractive anti-cancer therapy tools as they may induce both innate and adaptive immune responses, thereby addressing the hurdle of tumor heterogeneity. Adding αGC to antigen-loaded dendritic-cell-derived exosomes also led to an increase in missing self-responses in addition to boosted T and B cell responses. This study manifests αGC as an attractive adjuvant in cancer immunotherapy, as it increases the functional capacity of educated NK cells and enhances the innate, missing self-based antitumor response.

Invariant natural killer T cells balance B cell immunity

Invariant natural killer T (iNKT) cells mediate rapid immune responses which bridge the gap between innate and adaptive responses to pathogens while also providing key regulation to maintain immune homeostasis. Both types of important iNKT immune responses are mediated through interactions with innate and adaptive B cells. As such, iNKT cells sit at the decision-making fulcrum between regulating inflammatory or autoreactive B cells and supporting protective or regulatory B cell populations. iNKT cells interpret the signals in their environment to set the tone for subsequent adaptive responses, with outcomes ranging from getting licensed to maintain homeostasis as an iNKT regulatory cell (iNKT_reg ) or being activated to become an iNKT follicular helper (iNKT_FH ) cell supporting pathogen-specific effector B cells. Here we review iNKT and B cell cooperation across the spectrum of immune outcomes, including during allergy and autoimmune disease, tumor surveillance and immunotherapy, or pathogen defense and vaccine responses. Because of their key role as influencers, iNKT cells provide a valuable target for therapeutic interventions. Understanding the nature of the interactions between iNKT and B cells will enable the development of clinical interventions to strategically target regulatory iNKT and B cell populations or inflammatory ones, depending on the circumstance.

T Lymphocytes and Testicular Immunity: A New Insight into Immune Regulation in Testes

The immune privilege of the testes is necessary to prevent immune attacks to gamete-specific antigens and paternal major histocompatibility complex (MHC) antigens, allowing for normal spermatogenesis. However, infection and inflammation of the male genital tract can break the immune tolerance and represent a significant cause of male infertility. Different T cell subsets have been identified in mammalian testes, which may be involved in the maintenance of immune tolerance and pathogenic immune responses in testicular infection and inflammation. We reviewed the evidence in the published literature on different T subtypes (regulatory T cells, helper T cells, cytotoxic T cells, γδ T cells, and natural killer T cells) in human and animal testes that support their regulatory roles in infertility and the orchitis pathology. While many in vitro studies have indicated the regulation potential of functional T cell subsets and their possible interaction with Sertoli cells, Leydig cells, and spermatogenesis, both under physiological and pathological processes, there have been no in situ studies to date. Nevertheless, the normal distribution and function of T cell subsets are essential for the immune privilege of the testes and intact spermatogenesis, and T cell-mediated immune response drives testicular inflammation. The distinct function of different T cell subsets in testicular homeostasis and the orchitis pathology suggests a considerable potential of targeting specific T cell subsets for therapies targeting chronic orchitis and immune infertility.

Reading the room: iNKT cells influence B cell responses

Rapid immune responses regulated by invariant Natural Killer T (iNKT) cells bridge the gap between innate and adaptive responses to pathogens, while also providing key regulation to maintain immune homeostasis. iNKT immune protection and immune regulation are both mediated through interactions with innate and adaptive B cell populations that express CD1d. Recent studies have expanded our understanding of the position of iNKT cells at the fulcrum between regulating inflammatory and autoreactive B cells. Environmental signals influence iNKT cells to set the tone for subsequent adaptive responses, ranging from maintaining homeostasis as an iNKT regulatory cell (iNKT_reg) or supporting pathogen-specific effector B cells as an iNKT follicular helper (iNKT_FH). Here we review recent advances in iNKT and B cell cooperation during autoimmunity and sterile inflammation. Understanding the nature of the interactions between iNKT and B cells will enable the development of clinical interventions to strategically target regulatory iNKT and B cell populations or inflammatory ones, across a range of indications.

The B cell response to both protein and nucleic acid antigens displayed on apoptotic cells are dependent on endosomal pattern recognition receptors

In systemic autoimmune diseases such as systemic lupus erythematosus (SLE), B cell tolerance is lost and there is a production of autoantibodies that drive pathology. The specificities of these antibodies are towards a wide range of autoantigens including proteins such as serum factors including cytokines as well as towards nucleic acids and modified glycolipids. It is known that endosomal pattern recognition receptors are involved in specific responses but if they drive specificity towards a specific group of autoantigens is not known. Here, we used syngeneic apoptotic cells alone to break B cell tolerance and investigated the antibody response in Unc93b1 mutant mice that lack signalling from the TLR3, TLR7 and TLR9 receptors. We found that specific B cell responses known from patients with SLE including antibodies towards Ro-52/60, La, cardiolipin as well as DNA were all significantly lower in the knockout mice. Thus, we found that endosomal TLR receptors were involved in break of tolerance and drive B cell responses for protein, nucleic acid and modified lipid antigens. This pinpoints these receptors as key drivers for the full range of antibody driven pathology in SLE and suggests that targeting of endosomal TLR driven responses will quench all B cell driven autoreactivity.

CD1d Selectively Down Regulates the Expression of the Oxidized Phospholipid-Specific E06 IgM Natural Antibody in Ldlr-/- Mice

Natural antibodies (NAbs) are important regulators of tissue homeostasis and inflammation and are thought to have diverse protective roles in a variety of pathological states. E06 is a T15 idiotype IgM NAb exclusively produced by B-1 cells, which recognizes the phosphocholine (PC) head group in oxidized phospholipids on the surface of apoptotic cells and in oxidized LDL (OxLDL), and the PC present on the cell wall of Streptococcus pneumoniae. Here we report that titers of the E06 NAb are selectively increased several-fold in Cd1d-deficient mice, whereas total IgM and IgM antibodies recognizing other oxidation specific epitopes such as in malondialdehyde-modified LDL (MDA-LDL) and OxLDL were not increased. The high titers of E06 in Cd1d-deficient mice are not due to a global increase in IgM-secreting B-1 cells, but they are specifically due to an expansion of E06-secreting splenic B-1 cells. Thus, CD1d-mediated regulation appeared to be suppressive in nature and specific for E06 IgM-secreting cells. The CD1d-mediated regulation of the E06 NAb generation is a novel mechanism that regulates the production of this specific oxidation epitope recognizing NAb.

Combined proinflammatory cytokine and cognate activation of invariant natural killer T cells enhances anti-DNA antibody responses

iNKT cells can both provide help and inhibit B cell responses. Our data show that when iNKT cells are activated with the glycolipid agonist αGalCer together with the inflammatory cytokine IL-18, they switch from regulating autoreactive B cells to promoting their expansion. As a consequence, autoreactive B cell responses remain unchecked by iNKT cells. The glycolipid αGalCer has been shown to have promising effects when administered as an adjuvant to achieve a better response to vaccines, as an antitumor agent, as well as in the regulation of autoimmunity. Our results highlight a facet of αGalCer-mediated iNKT cell activation in the context of inflammation and have broad implications for understanding the regulation of autoimmunity and use of αGalCer in therapy.

Invariant natural killer T (iNKT) cells serve as early rapid responders in the innate immune response to self-derived autoantigens and pathogen-derived danger signals and antigens. iNKT cells can serve both as helpers for effector B cells and negatively regulate autoreactive B cells. Specifically, iNKT cells drive B cell proliferation, class switch, and antibody production to induce primary antigen-specific immune responses. On the other hand, inflammasome-mediated activation drives accumulation of neutrophils, which license iNKT cells to negatively regulate autoreactive B cells via Fas ligand (FasL). This positions iNKT cells at an apex to support or inhibit B cell responses in inflammation. However, it is unknown which effector mechanism dominates in the face of cognate glycolipid activation during chronic inflammation, as might result from glycolipid vaccination or infection during chronic autoimmune disease. We stimulated iNKT cells by cognate glycolipid antigen α-galactosylceramide (αGalCer) and measured B cell activation during interleukin 18 (IL-18)-induced chronic inflammation. Moreover, glycolipid-activated iNKT cells increased the serum concentration of autoantibodies, frequency of germinal center (GC) B cells, and antigen-specific plasma cells induced during chronic IL-18–mediated inflammation, as compared with IL-18 alone. Further, activation of iNKT cells via cognate glycolipid during IL-18–mediated inflammation overrides the licensing function of neutrophils, instead inducing iNKT follicular helper (iNKTfh) cells that in turn promote autoimmunity. Thus, our data demonstrate that glycolipids which engage iNKT cells support antigen-specific B cell help during inflammasome-mediated inflammation.

Invariant NKT Cells and Rheumatic Disease: Focus on Primary Sjogren Syndrome

Primary Sjogren syndrome (pSS) is a complex autoimmune disease mainly affecting salivary and lacrimal glands. Several factors contribute to pSS pathogenesis; in particular, innate immunity seems to play a key role in disease etiology. Invariant natural killer (NK) T cells (iNKT) are a T-cell subset able to recognize glycolipid antigens. Their function remains unclear, but studies have pointed out their ability to modulate the immune system through the promotion of specific cytokine milieu. In this review, we discussed the possible role of iNKT in pSS development, as well as their implications as future markers of disease activity.

NKT Cells in Neurological Diseases

Natural killer T (NKT) cells are a unique subset of T lymphocytes with the expression of T cell receptor (TCR) and NK cell lineage receptors. These cells can rapidly release large quantities of cytokines and function as a bridge between innate and adaptive immunity. To date, multiple reports have investigated the role of NKT cells under various pathological conditions, such as cancer, autoimmune disease, and infection. Knowledge about NKT cells in neurological diseases is increasing, albeit limited. Here, we review evidence for the involvement of NKT cells in neurological diseases, and discuss immunotherapeutic potential and future study goals. As the development and function of NKT cells become increasingly well understood, the next few years should yield many new insights into NKT cell function, and mechanistic regulation in neurological disorders.

Antiphospholipid Antibodies Overlapping in Isolated Neurological Syndrome and Multiple Sclerosis: Neurobiological Insights and Diagnostic Challenges

Antiphospholipid syndrome (APS) is characterized by arterial and venous thrombosis, pregnancy morbidity and fetal loss caused by pathogenic autoantibodies directed against phospholipids (PL) and PL-cofactors. Isolated neurological APS may represent a significant diagnostic challenge, as epidemiological, clinical and neuroimaging features may overlap with those of multiple sclerosis (MS). In an open view, MS could be considered as an organ-specific anti-lipid (phospholipid and glycosphingolipid associated proteins) disease, in which autoreactive B cells and CD8+ T cells play a dominant role in its pathophysiology. In MS, diverse autoantibodies against the lipid-protein cofactors of the myelin sheath have been described, whose pathophysiologic role has not been fully elucidated. We carried out a review to select clinical studies addressing the prevalence of antiphospholipid (aPL) autoantibodies in the so-called MS-like syndrome. The reported prevalence ranged between 2% and 88%, particularly aCL and aβ2GPI, with predominant IgM isotype and suggesting worse MS prognosis. Secondarily, an updated summary of current knowledge on the pathophysiological mechanisms and events responsible for these conditions is presented. We draw attention to the clinical relevance of diagnosing isolated neurological APS. Prompt and accurate diagnosis and antiaggregant and anticoagulant treatment of APS could be vital to prevent or at least reduce APS-related morbidity and mortality.

Differential ACPA Binding to Nuclear Antigens Reveals a PAD-Independent Pathway and a Distinct Subset of Acetylation Cross-Reactive Autoantibodies in Rheumatoid Arthritis

Rheumatoid arthritis (RA) associated anti-citrullinated protein autoantibodies (ACPA) target a wide range of modified proteins. Citrullination occurs during physiological processes such as apoptosis, yet little is known about the interaction of ACPA with nuclear antigens or apoptotic cells. Since uncleared apoptotic cells and neutrophil extracellular trap (NET) products have been postulated to be central sources of autoantigen and immunostimulation in autoimmune disease, we sought to characterize the anti-nuclear and anti-neutrophil reactivities of ACPA. Serology showed that a subset of anti-CCP2 seropositive RA patients had high reactivity to full-length citrullinated histones. In contrast, seronegative RA patients displayed elevated IgG reactivity to native histone compared to controls, but no citrulline-specific reactivity. Screening of 10 single B-cell derived monoclonal ACPA from RA patients revealed that four ACPA exhibited strong binding to apoptotic cells and three of these had anti-nuclear (ANA) autoantibody reactivity. Modified histones were confirmed to be the primary targets of this anti-nuclear ACPA subset following immunoprecipitation from apoptotic cell lysates. Monoclonal ACPA were also screened for reactivities against stimulated murine and human neutrophils, and all the nuclear-reactive monoclonal ACPA bound to NETs. Intriguingly, one ACPA mAb displayed a contrasting cytoplasmic perinuclear neutrophil binding and may represent a different NET-reactive ACPA subset. Notably, studies of CRISPR-Cas9 PAD4 KO cells and cells from PAD KO mice showed that the cytoplasmic NET-binding was fully dependent on PAD4, whilst nuclear- and histone-mediated NET reactivity was largely PAD-independent. Our further analysis revealed that the nuclear binding could be explained by consensus-motif driven ACPA cross-reactivity to acetylated histones. Specific acetylated histone peptides targeted by the monoclonal antibodies were identified and the anti-modified protein autoantibody (AMPA) profile of the ACPA was found to correlate with the functional activity of the antibodies. In conclusion, when investigating monoclonal ACPA, we could group ACPA into distinct subsets based on their nuclear binding-patterns and acetylation-mediated binding to apoptotic cells, neutrophils, and NETs. Differential anti-modified protein reactivities of RA-autoantibody subsets could have an important functional impact and provide insights in RA pathogenesis.

B cell responses to apoptotic cells in MFG-E8-/- mice

Defective clearance of apoptotic cells in MFG-E8 deficient mice results in lupus-like disease in the mixed B6x129, but not pure B6 background. The lack of overt autoimmunity in MFG-E8-/- B6 mice suggests that accumulation of apoptotic cells is not sufficient to break central tolerance. However, the delayed clearance of apoptotic cells in the follicles of MFG-E8-/- B6 mice provides an excellent opportunity to investigate how B cells respond to excessive apoptotic cells in the periphery under relatively non-inflammatory conditions. In MFG-E8-/- B6 mice, we found increased IgG2c production against apoptotic cells and oxidized LDL. Apoptotic cell induced antibody responses depended on MyD88 signal and T cell help. In addition, MFG-E8-/- B6 mice had enlarged MZ B cell compartments as well as an enhanced antibody response to NP-Ficoll. Moreover, a significant percentage of MZ B cells in aged MFG-E8-/- B6 mice migrated into follicles. Injecting apoptotic cells or oxidized LDL into wild type mice as well as physiological accumulation of LDL in ApoE-/- mice recapitulated the translocation of MZ B cells. To determine how MFG-E8 deficiency affects the functions of autoreactive B cells specific for nucleic acids in the periphery under non-inflammatory conditions, we utilized BCR transgenic mice to bypass central selection and compared the differentiation of TLR9 dependent anti-dsDNA 56R B cells and TLR7 dependent anti-ssRNA H564 B cells in MFG-E8-/- mice. In MFG-E8-/- 56R mice, anti-dsDNA specific 56R/Vκ38c B cells differentiated into MZ B cells but not AFCs. On the contrary, in MFG-E8-/-H564 mice, anti-ssRNA specific H564 B cells further differentiated into GC B cells and AFCs. Adoptive transfer of activated autoreactive B cells confirmed that H564 B cells were more sensitive to apoptotic cell antigens than 56R B cells. Our observations provide new insights about the MZ B cell translocation in lupus patients as well as the dichotomy of TLR9 and TLR7 signals in the pathogenesis of lupus.

Linking CD1-Restricted T Cells With Autoimmunity and Dyslipidemia: Lipid Levels Matter

Dyslipidemia, or altered blood lipid content, is a risk factor for developing cardiovascular disease. Furthermore, several autoimmune diseases, including systemic lupus erythematosus, psoriasis, diabetes, and rheumatoid arthritis, are correlated highly with dyslipidemia. One common thread between both autoimmune diseases and altered lipid levels is the presence of inflammation, suggesting that the immune system might act as the link between these related pathologies. Deciphering the role of innate and adaptive immune responses in autoimmune diseases and, more recently, obesity-related inflammation, have been active areas of research. The broad picture suggests that antigen-presenting molecules, which present self-peptides to autoreactive T cells, can result in either aggravation or amelioration of inflammation. However, very little is known about the role of self-lipid reactive T cells in dyslipidemia-associated autoimmune events. Given that a range of autoimmune diseases are linked to aberrant lipid profiles and a majority of lipid-specific T cells are reactive to self-antigens, it is important to examine the role of these T cells in dyslipidemia-related autoimmune ailments and determine if dysregulation of these T cells can be drivers of autoimmune conditions. CD1 molecules present lipids to T cells and are divided into two groups based on sequence homology. To date, most of the information available on lipid-reactive T cells comes from the study of group 2 CD1d-restricted natural killer T (NKT) cells while T cells reactive to group 1 CD1 molecules remain understudied, despite their higher abundance in humans compared to NKT cells. This review evaluates the mechanisms by which CD1-reactive, self-lipid specific T cells contribute to dyslipidemia-associated autoimmune disease progression or amelioration by examining available literature on NKT cells and highlighting recent progress made on the study of group 1 CD1-restricted T cells.

The Role of Invariant NKT in Autoimmune Liver Disease: Can Vitamin D Act as an Immunomodulator?

Natural killer T (NKT) cells are a distinct lineage of T cells which express both the T cell receptor (TCR) and natural killer (NK) cell markers. Invariant NKT (iNKT) cells bear an invariant TCR and recognize a small variety of glycolipid antigens presented by CD1d (nonclassical MHC-I). CD1d-restricted iNKT cells are regulators of immune responses and produce cytokines that may be proinflammatory (such as interferon-gamma (IFN-γ)) or anti-inflammatory (such as IL-4). iNKT cells also appear to play a role in B cell regulation and antibody production. Alpha-galactosylceramide (α-GalCer), a derivative of the marine sponge, is a potent stimulator of iNKT cells and has been proposed as a therapeutic iNKT cell activator. Invariant NKT cells have been implicated in the development and perpetuation of several autoimmune diseases such as multiple sclerosis and systemic lupus erythematosus (SLE). Animal models of SLE have shown abnormalities in iNKT cells numbers and function, and an inverse correlation between the frequency of NKT cells and IgG levels has also been observed. The role of iNKT cells in autoimmune liver disease (AiLD) has not been extensively studied. This review discusses the current data with regard to iNKT cells function in AiLD, in addition to providing an overview of iNKT cells function in other autoimmune conditions and animal models. We also discuss data regarding the immunomodulatory effects of vitamin D on iNKT cells, which may serve as a potential therapeutic target, given that deficiencies in vitamin D have been reported in various autoimmune disorders.

Activation and Regulation of B Cell Responses by Invariant Natural Killer T Cells

CD1d-restricted invariant natural killer T (iNKT) cells play central roles in the activation and regulation of innate and adaptive immunity. Cytokine-mediated and CD1d-dependent interactions between iNKT cells and myeloid and lymphoid cells enable iNKT cells to contribute to the activation of multiple cell types, with important impacts on host immunity to infection and tumors and on the prevention of autoimmunity. Here, we review the mechanisms by which iNKT cells contribute to B cell maturation, antibody and cytokine production, and antigen presentation. Cognate interactions with B cells contribute to the rapid production of antibodies directed against conserved non-protein antigens resulting in rapid but short-lived innate humoral immunity. iNKT cells can also provide non-cognate help for the generation of antibodies directed against protein antigens, by promoting the activation of follicular helper T cells, resulting in long-lasting adaptive humoral immunity and B cell memory. iNKT cells can also regulate humoral immunity by promoting the development of autoreactive B cells into regulatory B cells. Depletions and functional impairments of iNKT cells are found in patients with infectious, autoimmune and malignant diseases associated with altered B cell function and in murine models of these conditions. The adjuvant and regulatory activities that iNKT cells have for B cells makes them attractive therapeutic targets for these diseases.

CD1d-dependent immune suppression mediated by regulatory B cells through modulations of iNKT cells

Regulatory B cells (Breg) express high levels of CD1d that presents lipid antigens to invariant natural killer T (iNKT) cells. The function of CD1d in Breg biology and iNKT cell activity during inflammation remains unclear. Here we show, using chimeric mice, cell depletion and adoptive cell transfer, that CD1d-lipid presentation by Bregs induces iNKT cells to secrete interferon (IFN)-γ to contribute, partially, to the downregulation of T helper (Th)1 and Th17-adaptive immune responses and ameliorate experimental arthritis. Mice lacking CD1d-expressing B cells develop exacerbated disease compared to wild-type mice, and fail to respond to treatment with the prototypical iNKT cell agonist α-galactosylceramide. The absence of lipid presentation by B cells alters iNKT cell activation with disruption of metabolism regulation and cytokine responses. Thus, we identify a mechanism by which Bregs restrain excessive inflammation via lipid presentation.

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.

Invariant natural killer T cells promote immunogenic maturation of lung dendritic cells in mouse models of asthma

Our previous study showed that invariant natural killer T (iNKT) cells might act as an adjuvant to promote Th2 inflammatory responses in an OVA-induced mouse model of allergic asthma, but the mechanism remains unknown. To clarify the underlying mechanism through which iNKT cells promote Th2 inflammatory responses, we investigated the modulatory influence of iNKT cells on phenotypic and functional maturation of lung dendritic cells (LDCs) using iNKT cell-knockout mice, specific iNKT cell activation, coculture experiments, and adoptive transfer of iNKT cells in mouse models of asthma. Our data showed that iNKT cell deficiency could downregulate surface maturation markers and proinflammatory cytokine secretion of LDCs from a mouse model of asthma. However, elevated activation of iNKT cells by α-galactosylceramide and adoptive transfer of iNKT cells could upregulate surface maturation markers and proinflammatory cytokine secretion of LDCs from mouse models of asthma. Meanwhile, iNKT cells significantly influenced the function of LDCs, markedly enhancing Th2 responses in vivo and in vitro. In addition, iNKT cell can induce LDCs expression of CD206 and RELM-α, reflecting alternative activation of LDCs in a mouse model of asthma. α-Galactosylceramide treatment significantly enhanced expression of CD40L of lung iNKT cells from a mouse model of asthma, and the coculture experiment of LDCs with iNKT cells showed that the blockade of CD40L strongly suppressed surface maturation markers and proinflammatory cytokine production by LDCs. Our data suggest that iNKT cells can promote immunogenic maturation of LDCs to enhance Th2 responses in mouse models of asthma.

In Vitro and In Vivo Assessment of T, B and Myeloid Cells Suppressive Activity and Humoral Responses from Transplant Recipients

The main concern in transplantation is to achieve specific tolerance through induction of regulatory cells. The understanding of tolerance mechanisms requires reliable models. Here, we describe models of tolerance to cardiac allograft in rat, induced by blockade of costimulation signals or by upregulation of immunoregulatory molecules through gene transfer. Each of these models allowed in vivo generation of regulatory cells such as regulatory T cells (Tregs), regulatory B cells (Bregs) or regulatory myeloid cells (RegMCs). In this manuscript, we describe two complementary protocols that have been used to identify and define in vitro and in vivo regulatory cell activity to determine their responsibility in tolerance induction and maintenance. First, an in vitro suppressive assay allowed rapid identification of cells with suppressive capacity on effector immune responses in a dose dependent manner, and can be used for further analysis such as cytokine measurement or cytotoxicity. Second, the adoptive transfer of cells from a tolerant treated recipient to a newly irradiated grafted recipient, highlighted the tolerogenic properties of these cells in controlling graft directed immune responses and/or converting new regulatory cells (termed infectious tolerance). These methods are not restricted to cells with known phenotypic markers and can be extended to any cell population. Furthermore, donor directed allospecificity of regulatory cells (an important goal in the field) can be assessed by using third party donor cells or graft either in vitro or in vivo. Finally, to determine the specific tolerogenic capacity of these regulatory cells, we provide protocols to assess the humoral anti-donor antibody responses and the capacity of the recipient to develop humoral responses against new or former known antigens. The models of tolerance described can be used to further characterize regulatory cells, to identify new biomarkers, and immunoregulatory molecules, and are adaptable to other transplantation models or autoimmune diseases in rodent or human.

Human regulatory B cells in health and disease: therapeutic potential

Regulatory B cells (Bregs) modulate immune responses predominantly, although not exclusively, via the release of IL-10. The importance of human Bregs in the maintenance of immune homeostasis comes from a variety of immune-related pathologies, such as autoimmune diseases, cancers, and chronic infections that are often associated with abnormalities in Breg numbers or function. A continuous effort toward understanding Breg biology in healthy individuals will provide new opportunities to develop Breg immunotherapy that could prove beneficial in treating various immune-mediated pathologies. In this Review, we discuss findings regarding human Bregs, including their mechanisms of suppression and role in different disease settings. We also propose several therapeutic strategies targeting Bregs for better management of immune disorders.

The Clonal Invariant NKT Cell Repertoire in People with Type 1 Diabetes Is Characterized by a Loss of Clones Expressing High-Affinity TCRs

Invariant NKT (iNKT) cells in healthy people express iNKT-TCRs with widely varying affinities for CD1d, suggesting different roles for high- and low-affinity iNKT clones in immune regulation. However, the functional implications of this heterogeneity have not yet been determined. Functionally aberrant iNKT responses have been previously demonstrated in different autoimmune diseases, including human type 1 diabetes, but their relationship to changes in the iNKT clonal repertoire have not been addressed. In this study, we directly compared the clonal iNKT repertoire of people with recent onset type 1 diabetes and age- and gender-matched healthy controls with regard to iNKT-TCR affinity and cytokine production. Our results demonstrate a selective loss of clones expressing high-affinity iNKT-TCRs from the iNKT repertoire of people with type 1 diabetes. Furthermore, this bias in the clonal iNKT repertoire in type 1 diabetes was associated with increased GM-CSF, IL-4, and IL-13 cytokine secretion among Ag-stimulated low-affinity iNKT clones. Thus, qualitative changes of the clonal iNKT repertoire with the potential to affect the regulatory function of this highly conserved T cell population are already established at the early stages in type 1 diabetes. These findings may inform future rationales for the development of iNKT-based therapies aiming to restore immune tolerance in type 1 diabetes.

Mechanisms and New Strategies for Primary Sjögren's Syndrome

Primary Sjögren's syndrome (SS) is a common chronic autoimmune disease characterized by lymphocytic infiltration of exocrine glands, mainly salivary and lacrimal, resulting in oral and ocular dryness, although virtually any organ system can be affected. SS-related systemic manifestations are classified as either related to the presence of periepithelial infiltrates in exocrine and parenchymal organs or resulting from immunocomplex deposition due to B cell hyperactivity with increased risk for B cell lymphoma development. Activation of both innate and adaptive immune pathways contributes to disease pathogenesis, with prominent interferon (IFN) signatures identified in both peripheral blood and affected salivary gland tissues. Recently, LINE-1 genomic repeat elements have been proposed as potential triggers of type I IFN pathway activation in SS through activation of Toll-like receptor-dependent and -independent pathways. In view of the increasingly appreciated variability of SS, elucidation of distinct operating pathways in relation to diverse clinical phenotypes and selection of the optimal therapeutic intervention remain major challenges. Inhibition of cathepsin S molecules, blockade of costimulation through administration of abatacept and inhibitors of B7-related molecules and CD40, blockade of B cell function and B cell survival factors, and disruption of the formation of ectopic germinal centers are considered the main therapeutic targets. Well-controlled multicenter clinical trials are ongoing and data are awaited.

Evaluation of TLR9 expression on PBMCs and CpG ODN-TLR9 ligation on IFN-α production in SLE patients

CONTEXT

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease characterized by autoreactive antibodies. Recent findings revealed the importance of innate immune responses, especially Toll-like receptors (TLRs) in the pathogenesis of SLE.

OBJECTIVE

In this study, the level of TLR9 expression on peripheral blood mononuclear cells (PBMCs) was analyzed. The levels of produced IFN-α were also measured in supernatant of PBMCs from SLE patients and healthy controls after stimulation with CpG ODN2216 which is a plasmocytoid dendritic cell (pDC)-specific TLR9 ligand.

MATERIALS AND METHODS

TLR9 expression was analyzed by real-time polymerase chain reaction (PCR) and flow cytometry in 35 SLE patients and 38 healthy controls and IFN-α concentration was measured in supernatants using enzyme-linked immunosorbent assay (ELISA).

RESULTS

The results showed that the TLR9 expression in the mRNA and the protein level was significantly higher in PBMCs from SLE patients. However, IFN-α concentration in patients and controls significantly increased in response to CpG stimulation but this increase was significantly higher in healthy controls compared with SLE patients. Our results do not show any association between taking hydroxychloroquine and reduction in IFN-α production in SLE patients.

DISCUSSION AND CONCLUSIONS

Regarding the findings of the study, there is the possibility that TLR9 has played a role in SLE pathogenesis, and consequently it implies that TLRs can be considered to be the therapeutic targets for systemic autoimmunity. We may conclude that PBMCs in patients are functionally impaired in response to TLR ligation via innate response stimulating pathogen-associated molecular patterns (PAMPs).

Neutrophils license iNKT cells to regulate self-reactive mouse B cell responses

The innate responsiveness of the immune system is important not only for quick responses to pathogens but also for the initiation and shaping of the subsequent adaptive immune response. Activation via the cytokine IL-18, a product of inflammasomes, gives rise to a rapid response that includes the production of self-reactive antibodies. As increased concentrations of this cytokine are found in inflammatory diseases, we investigated the origin of the B cell response and its regulation. We identified an accumulation of B cell-helper neutrophils in the spleen that interacted with innate-type invariant natural killer T cells (iNKT cells) to regulate B cell responses. We found that neutrophil-dependent expression of the death-receptor ligand FasL by iNKT cells was needed to restrict autoantibody production. Neutrophils can thus license iNKT cells to regulate potentially harmful autoreactive B cell responses during inflammasome-driven inflammation.

Apoptotic cell responses in the splenic marginal zone: a paradigm for immunologic reactions to apoptotic antigens with implications for autoimmunity

Apoptotic cells drive innate regulatory responses that result in tolerogenic immunity. This is a critical aspect of cell physiology as apoptotic cells expose potentially dangerous nuclear antigens on the surface in apoptotic blebs, and failure in their recognition, phagocytosis, or destruction can cause dramatic autoimmunity in experimental models and is linked to development and progression of systemic pathology in human. The marginal zone is a specialized splenic environment that serves as a transitional site from circulation to peripheral lymphoid structures. The marginal zone serves a key role in trapping of particulates and initiation of innate responses against systemic microbial pathogens. However in recent years, it has become clear the marginal zone is also important for initiation of immune tolerance to apoptotic cells, driving a coordinated response involving multiple phagocyte and lymphocyte subsets. Recent reports linking defects in splenic macrophage function to systemic lupus erythematosus in a manner analogous to marginal zone macrophages in lupus-prone mice provide an impetus to better understand the mechanistic basis of the apoptotic cell response in the marginal zone and its general applicability to apoptotic cell-driven tolerance at other tissue sites. In this review, we discuss immune responses to apoptotic cells in the spleen in general and the marginal zone in particular, the relationship of these responses to autoimmune disease, and comparisons to apoptotic cell immunity in humans.

Coordination between T helper cells, iNKT cells, and their follicular helper subsets in the humoral immune response against Clostridium difficile toxin B

Activation of iNKT cells with the CD1d-binding glycolipid adjuvant α-galactosylceramide (α-GC) enhances humoral immunity specific for coadministered T-dependent Ag. However, the relationship between the iNKT cell and the classic T helper (Th) or T follicular helper (Tfh) function following this immunization modality remains unclear. We show that immunization with the C-terminal domain (CTD) of Clostridium difficile toxin B (TcdB), accompanied by activation of iNKT cells with α-GC, led to enhanced production of CTD-specific IgG, which was CD1d- and iNKT cell-dependent and associated with increased neutralization of active TcdB. Immunization with CTD plus α-GC followed by NP hapten-linked CTD increased NP-specific IgG1 titers in an NKT-dependent manner, suggesting that iNKT activation could enhance Th or Tfh function or that iNKT and iNKTfh cells could provide supplemental, yet independent, B cell help. Th, Tfh, iNKT, and iNKTfh cells were, therefore, examined quantitatively, phenotypically, and functionally following immunization with CTD or with CTD plus α-GC. Our results demonstrated that α-GC-activated iNKT cells had no direct effect on the numbers, phenotype, or function of Th or Tfh cells. However, CD4⁺ T cell-specific ablation of the Bcl6 transcription factor demonstrated that Tfh and iNKTfh cells both contributed to B cell help. This work extends our understanding of the immune response to vaccination and demonstrates an important contribution by NKTfh cells to humoral immunity.

Dysregulated osteoclastogenesis is related to natural killer T cell dysfunction in rheumatoid arthritis

OBJECTIVE

To investigate the role played by natural killer T (NKT) cells in osteoclastogenesis and their effects on inflammatory bone destruction.

METHODS

Patients with rheumatoid arthritis (RA) (n = 25) and healthy controls (n = 12) were enrolled in this study. In vitro osteoclastogenesis experiments were performed using peripheral blood mononuclear cells (PBMCs) in the presence of macrophage colony-stimulating factor and RANKL. PBMCs were cultured in vitro with α-galactosylceramide (αGalCer), and proliferation indices of NKT cells were estimated by flow cytometry. In vivo effects of αGalCer-stimulated NKT cells on inflammation and bone destruction were determined in mice with collagen-induced arthritis.

RESULTS

In vitro osteoclastogenesis was found to be significantly inhibited by αGalCer in healthy controls but not in RA patients. Proliferative responses of NKT cells and STAT-1 phosphorylation in monocytes in response to αGalCer were impaired in RA patients. Notably, αGalCer-stimulated NKT cells inhibited osteoclastogenesis mainly via interferon-γ production in a cytokine-dependent manner (not by cell-cell contact) and down-regulated osteoclast-associated genes. Mice treated with αGalCer showed less severe arthritis and reduced bone destruction. Moreover, proinflammatory cytokine expression in arthritic joints was found to be reduced by αGalCer treatment.

CONCLUSION

This study primarily demonstrates that αGalCer-stimulated NKT cells have a regulatory effect on osteoclastogenesis and a protective effect against inflammatory bone destruction. However, it also shows that these effects of αGalCer are diminished in RA patients and that this is related to NKT cell dysfunction. These findings provide important information for those searching for novel therapeutic strategies to prevent bone destruction in RA.

Immunoregulation of NKT Cells in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease with different variety of clinical manifestations. Natural killer T (NKT) cells are innate lymphocytes that play a regulatory role during broad range of immune responses. A number of studies demonstrated that the quantity and quality of invariant NKT (iNKT) cells showed marked defects in SLE patients in comparison to healthy controls. This finding suggests that iNKT cells may play a regulatory role in the occurrence and development of this disease. In this review, we mainly summarized the most recent findings about the behavior of NKT cells in SLE patients and mouse models, as well as how NKT cells affect the proportion of T helper cells and the production of autoreactive antibodies in the progress of SLE. This will help people better understand the role of NKT cells in the development of SLE and improve the therapy strategy.

Type I IFN Contributes to the Phenotype of Unc93b1D34A/D34A Mice by Regulating TLR7 Expression in B Cells and Dendritic Cells

TLR7 recognizes pathogen-derived and self-derived RNA, and thus a regulatory system for control of the TLR7 response is required to avoid excessive activation. Unc93 homolog B1 (Unc93B1) is a regulator of TLR7 that controls the TLR7 response by transporting TLR7 from the endoplasmic reticulum to endolysosomes. We have previously shown that a D34A mutation in Unc93B1 induces hyperactivation of TLR7, and that Unc93b1(D34A/D34A) mice (D34A mice) have systemic inflammation spontaneously. In this study, we examined the roles of inflammatory cytokines such as IFN-γ, IL-17A, and type I IFNs to understand the mechanism underlying the phenotype in D34A mice. mRNAs for IFN-γ and IL-I7A in CD4(+) T cells increased, but inflammatory phenotype manifesting as thrombocytopenia and splenomegaly was still observed in Ifng(-/-) or Il17a(-/-) D34A mice. In contrast to T cell-derived cytokines, Ifnar1(-/-) D34A mice showed an ameliorated phenotype with lower expression of TLR7 in B cells and conventional dendritic cells (cDCs). The amount of TLR7 decreased in B cells from Ifnar1(-/-) D34A mice, but the percentage of TLR7(+) cells decreased among CD8α(-) cDCs. In conclusion, type I IFNs maintain expression of TLR7 in B cells and cDCs in different ways; total amount of TLR7 is kept in B cells and TLR7(+) population is retained among cDCs. Our results suggested that these TLR7-expressing cells are activated initially and influence TLR7-dependent systemic inflammation.

Cd1d regulates B cell development but not B cell accumulation and IL10 production in mice with pathologic CD5(+) B cell expansion

Background

CD1d is a widely expressed lipid antigen presenting molecule required for CD1d-restricted invariant natural killer T (iNKT) cell development. Elevated CD1d expression is detected in CD5⁺ IL10-producing B cells, called B10 B cells, and is correlated with poorer prognosis in chronic lymphocytic leukemia (CLL), a CD5⁺ B cell malignancy with B10-like functional properties. Whether CD1d expression regulates CD5⁺ B cell accumulation, IL10 competence, and antibody production in naïve mice with pathologic CD5⁺ B cell expansion remains untested.

Results

Using three different transgenic mouse models of benign or leukemic CD5⁺ B cell expansion, we found that CD1d was differentially expressed on CD5⁺ B cells between the three models, but loss of CD1d expression had no effect on CD5⁺ B cell abundance or inducible IL10 expression in any of the models. Interestingly, in the CLL-prone Eμ-TCL1 model, loss of CD1d expression suppressed spontaneous IgG (but not IgM) production, whereas in the dnRAG1xEμ-TCL1 (DTG) model of accelerated CLL, loss of CD1d expression was associated with elevated numbers of splenic CD4⁺ and CD8⁺ T cells and an inverted CD4⁺:CD8⁺ T cell ratio. Unexpectedly, before leukemia onset, all three transgenic CD1d-deficient mouse strains had fewer splenic transitional B cells than their CD1d-proficient counterparts.

Conclusions

The results show that CD1d expression and iNKT cells are dispensable for the development, accumulation, or IL10 competence of CD5⁺ B cells in mice prone to benign or leukemic CLL-like B cell expansion, but reveal a novel role for iNKT cells in supporting B cell progression through the transitional stage of development in these animals. These results suggest CD1d-directed therapies to target CLL could be evaded by downregulating CD1d expression with little effect on continued leukemic CD5⁺ B cell survival. The data also imply that iNKT cells help restrain pro-leukemic CD8⁺ T cell expansion in CLL, potentially explaining a reported correlation in human CLL between disease progression, the loss of NKT cells, and a paradoxical increase in CD8⁺ T cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12865-015-0130-z) contains supplementary material, which is available to authorized users.

Cognate interaction with iNKT cells expands IL-10-producing B regulatory cells

Successful induction of B-cell activation and memory depends on help from CD4+ T cells. Invariant natural killer T (iNKT) cells (glycolipid-specific, CD1d-restricted innate lymphocytes) provide both cognate (direct) and noncognate (indirect) helper signals to enhance B-cell responses. Both forms of iNKT-cell help induce primary humoral immune responses, but only noncognate iNKT-cell help drives humoral memory and plasma cells. Here, we show that iNKT cognate help for B cells is fundamentally different from the help provided by conventional CD4+ T cells. Cognate iNKT-cell help drives an early, unsustained germinal center B-cell expansion, less reduction of T follicular regulatory cells, an expansion of marginal zone B cells, and early increases in regulatory IL-10-producing B-cell numbers compared with noncognate activation. These results are consistent with a mechanism whereby iNKT cells preferentially provide an innate form of help that does not generate humoral memory and has important implications for the application of glycolipid molecules as vaccine adjuvants.

Suppression of autoimmunity by CD5(+) IL-10-producing B cells in lupus-prone mice

Systemic lupus erythematosus is a complex autoimmune disorder characterized by the production of pathogenic anti-nuclear antibodies. Previous work from our laboratory has shown that the introgression of a New Zealand Black-derived chromosome 4 interval onto a lupus-prone background suppresses the disease. Interestingly, the same genetic interval promoted the expansion of both Natural Killer T- and CD5(+) B cells in suppressed mice. In this study, we show that ablation of NKT cells with a CD1d knockout had no impact on either the suppression of lupus or the expansion of CD5(+) B cells. On the other hand, suppressed mice had an expanded population of IL-10-producing B cells that predominantly localized to the CD5(+)CD1d(low) compartment. The expansion of CD5(+) B cells negatively correlated with the frequency of pro-inflammatory IL-17 A-producing T-cells and kidney damage. Adoptive transfer with a single injection of total B cells with an enriched CD5(+) compartment reduced the frequency of memory/activated, IFNγ-producing, and IL-17 A-producing CD4 T-cells but did not significantly reduce autoantibody levels. Taken together, these data suggest that the expansion of CD5(+) IL-10-producing B cells and not NKT cells protects against lupus in these mice, by limiting the expansion of pro-inflammatory IL-17 A- and IFNγ-producing CD4 T-cells.

Invariant natural killer T cells in lupus patients promote IgG and IgG autoantibody production

IgG autoantibodies, including antibodies to double-stranded DNA (dsDNA), are pathogenic in systemic lupus erythematosus (SLE), but the mechanisms controlling their production are not understood. To assess the role of invariant natural killer T (iNKT) cells in this process, we studied 44 lupus patients. We took advantage of the propensity of PBMCs from patients with active disease to spontaneously secrete IgG in vitro. Despite the rarity of iNKT cells in lupus blood (0.002-0.05% of CD3-positive T cells), antibody blockade of the conserved iNKT TCR or its ligand, CD1d, or selective depletion of iNKT cells, inhibited spontaneous secretion of total IgG and anti-dsDNA IgG by lupus PBMCs. Addition of anti-iNKT or anti-CD1d antibody to PBMC cultures also reduced the frequency of plasma cells, suggesting that lupus iNKT cells induce B-cell maturation. Like fresh iNKT cells, expanded iNKT-cell lines from lupus patients, but not healthy subjects, induced autologous B cells to secrete antibodies, including IgG anti-dsDNA. This activity was inhibited by anti-CD40L antibody, as well as anti-CD1d antibody, confirming a role for CD40L-CD40 and TCR-CD1d interactions in lupus iNKT-cell-mediated help. These results reveal a critical role for iNKT cells in B-cell maturation and autoantibody production in patients with lupus.

T helper type 2-polarized invariant natural killer T cells reduce disease severity in acute intra-abdominal sepsis

Sepsis is characterized by a severe systemic inflammatory response to infection that is associated with high morbidity and mortality despite optimal care. Invariant natural killer T (iNK T) cells are potent regulatory lymphocytes that can produce pro- and/or anti-inflammatory cytokines, thus shaping the course and nature of immune responses; however, little is known about their role in sepsis. We demonstrate here that patients with sepsis/severe sepsis have significantly elevated proportions of iNK T cells in their peripheral blood (as a percentage of their circulating T cells) compared to non-septic patients. We therefore investigated the role of iNK T cells in a mouse model of intra-abdominal sepsis (IAS). Our data show that iNK T cells are pathogenic in IAS, and that T helper type 2 (Th2) polarization of iNK T cells using the synthetic glycolipid OCH significantly reduces mortality from IAS. This reduction in mortality is associated with the systemic elevation of the anti-inflammatory cytokine interleukin (IL)-13 and reduction of several proinflammatory cytokines within the spleen, notably interleukin (IL)-17. Finally, we show that treatment of sepsis with OCH in mice is accompanied by significantly reduced apoptosis of splenic T and B lymphocytes and macrophages, but not natural killer cells. We propose that modulation of iNK T cell responses towards a Th2 phenotype may be an effective therapeutic strategy in early sepsis.

Different subsets of natural killer T cells may vary in their roles in health and disease

Natural killer T cells (NKT) can regulate innate and adaptive immune responses. Type I and type II NKT cell subsets recognize different lipid antigens presented by CD1d, an MHC class-I-like molecule. Most type I NKT cells express a semi-invariant T-cell receptor (TCR), but a major subset of type II NKT cells reactive to a self antigen sulphatide use an oligoclonal TCR. Whereas TCR-α dominates CD1d-lipid recognition by type I NKT cells, TCR-α and TCR-β contribute equally to CD1d-lipid recognition by type II NKT cells. These variable modes of NKT cell recognition of lipid-CD1d complexes activate a host of cytokine-dependent responses that can either exacerbate or protect from disease. Recent studies of chronic inflammatory and autoimmune diseases have led to a hypothesis that: (i) although type I NKT cells can promote pathogenic and regulatory responses, they are more frequently pathogenic, and (ii) type II NKT cells are predominantly inhibitory and protective from such responses and diseases. This review focuses on a further test of this hypothesis by the use of recently developed techniques, intravital imaging and mass cytometry, to analyse the molecular and cellular dynamics of type I and type II NKT cell antigen-presenting cell motility, interaction, activation and immunoregulation that promote immune responses leading to health versus disease outcomes.

Aberrant presentation of self-lipids by autoimmune B cells depletes peripheral iNKT cells

Invariant natural killer T (iNKT) cells provide cognate help via CD1d to lipid antigen-presenting B cells for antibody production, but whether B cells reciprocally regulate iNKT cells remains largely unexplored. Here, we found peripheral, but not thymic, iNKT cells to be numerically reduced in autoimmune mice lacking Fas specifically in B cells. The residual iNKT cells were antigenically overstimulated, had altered cytokine production, and manifested enhanced proliferation and apoptosis. B cell-specific ablation of CD1d ameliorated these iNKT defects, suggesting that inappropriate presentation of CD1d-restricted self-lipids by autoimmune B cell-depleted peripheral iNKT cells. CD1d(+) autoimmune B cells have reduced α-galactosidase A expression and, as revealed by lipidomic profiling, altered lipidome with aberrant accumulation of certain self-lipids and reduction of others. These findings unveil a critical link between autoimmunity, B cell lipidome, and the maintenance of peripheral iNKT cells and highlight an essential homeostatic function of B cells beyond antibody production.

Mycobacterium tuberculosis-specific memory NKT cells in patients with tuberculous pleurisy

Natural killer T (NKT) cells from mouse and human play a protective role in the immune responses against the infection of Mycobacterium tuberculosis. However, the characteristic of CD3(+)TCRvβ11(+) NKT cells at the local site of M. tuberculosis infection remains poorly defined. In the present study, we found that the numbers of CD3(+)TCRvβ11(+) NKT cells in pleural fluid mononuclear cells (PFMCs) were significantly lower than those in peripheral blood mononuclear cells (PBMCs). However, CD3(+)TCRvβ11(+) NKT cells from PFMCs spontaneously expressed high levels of CD69 and CD25 and effector memory phenotypes of CD45RO(high)CD62L(low)CCR7(low). After stimulation with the antigens of M. tuberculosis, CD3(+)TCRvβ11(+) NKT cells from PFMCs produced high levels of IFN-γ. Sorted CD3(+)TCRvβ11(+) NKT cells from PFMCs cultured with antigen presenting cells (APCs) produced IFN-γ protein and mRNA. The production of IFN-γ could be completely inhibited by AG490 and Wortmannin. In addition, CD3(+)TCRvβ11(+) NKT cells from PFMCs expressed higher levels of Fas (CD95), FasL (CD178) and perforin but lower levels of granzyme B compared with those from PBMCs. Taken together, our data demonstrated for the first time that M. tuberculosis-specific CD3(+)TCRvβ11(+) NKT cells participated in the local immune responses against M. tuberculosis through the production of IFN-γ and the secretion of cytolytic molecules.

Administration of sulfatide to ameliorate type I diabetes in non-obese diabetic mice

The endogenous glycosphingolipid sulfatide is a ligand for CD1d-restricted type II natural killer T (NKT) lymphocytes. Through the action of these cells,sulfatide treatment has been shown to modulate the immune response in mouse models for autoimmune diseases, infections and tumour immunity. Sulfatide exists naturally in different organs including the pancreas, where sulfatide colocalizes with insulin within the Langerhans islet b-cells, targets for the immune destruction in type 1 diabetes (T1D). Human T1D patients, but not patients with type 2 diabetes nor healthy individuals, have autoantibodies against sulfatide in serum, suggesting that sulfatide induces an immune response in the natural course of T1D in humans. Here, we investigate sulfatide as an autoantigen and a modulator of autoimmune disease in the murine model forT1D, the non-obese diabetic (NOD) mice. We demonstrate that aged NOD mice displayed serum autoantibody reactivity to sulfatide; however, this reactivity did not correlate with onset of T1D. Repeated administration of sulfatide did not result in an increase in serum reactivity to sulfatide. Moreover, a multidose sulfatide treatment of female NOD mice initiated at an early (5 weeks of age),intermediate (8 weeks of age) or late (12 weeks of age) phase of T1D progression did not influence the incidence of disease. Thus, we demonstrate that a fraction of NOD mice develop autoantibody reactivity to sulfatide; however, we fail to demonstrate that sulfatide treatment reduces the incidence of T1D in this mouse strain.

Ly108 expression distinguishes subsets of invariant NKT cells that help autoantibody production and secrete IL-21 from those that secrete IL-17 in lupus prone NZB/W mice

Lupus is a systemic autoimmune disease characterized by anti-nuclear antibodies in humans and genetically susceptible NZB/W mice that can cause immune complex glomerulonephritis. T cells contribute to lupus pathogenesis by secreting pro-inflammatory cytokines such as IL-17, and by interacting with B cells and secreting helper factors such as IL-21 that promote production of IgG autoantibodies. In the current study, we determined whether purified NKT cells or far more numerous conventional non-NKT cells in the spleen of NZB/W female mice secrete IL-17 and/or IL-21 after TCR activation in vitro, and provide help for spontaneous IgG autoantibody production by purified splenic CD19(+) B cells. Whereas invariant NKT cells secreted large amounts of IL-17 and IL-21, and helped B cells, non-NKT cells did not. The subset of IL-17 secreting NZB/W NKT cells expressed the Ly108(lo)CD4(-)NK1.1(-) phenotype, whereas the IL-21 secreting subset expressed the Ly108(hi)CD4(+)NK1.1(-) phenotype and helped B cells secrete a variety of IgG anti-nuclear antibodies. α-galactocylceramide enhanced the helper activity of NZB/W and B6.Sle1b NKT cells for IgG autoantibody secretion by syngeneic B cells. In conclusion, different subsets of iNKT cells from mice with genetic susceptibility to lupus can contribute to pathogenesis by secreting pro-inflammatory cytokines and helping autoantibody production.

Signaling lymphocytic activation molecule (SLAM)/SLAM-associated protein pathway regulates human B-cell tolerance

BACKGROUND

Signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) can mediate the function of SLAM molecules, which have been proposed to be involved in the development of autoimmunity in mice.

OBJECTIVE

We sought to determine whether the SLAM/SAP pathway regulates the establishment of human B-cell tolerance and what mechanisms of B-cell tolerance could be affected by SAP deficiency.

METHODS

We tested the reactivity of antibodies isolated from single B cells from SAP-deficient patients with X-linked lymphoproliferative disease (XLP). The expressions of SAP and SLAM family members were assessed in human bone marrow-developing B cells. We also analyzed regulatory T (Treg) cell function in patients with XLP and healthy control subjects.

RESULTS

We found that new emigrant/transitional B cells from patients with XLP were enriched in autoreactive clones, revealing a defective central B-cell tolerance checkpoint in the absence of functional SAP. In agreement with a B cell-intrinsic regulation of central tolerance, we identified SAP expression in a discrete subset of bone marrow immature B cells. SAP colocalized with SLAMF6 only in association with clustered B-cell receptors likely recognizing self-antigens, suggesting that SLAM/SAP regulate B-cell receptor-mediated central tolerance. In addition, patients with XLP displayed defective peripheral B-cell tolerance, which is normally controlled by Treg cells. Treg cells in patients with XLP seem functional, but SAP-deficient T cells were resistant to Treg cell-mediated suppression. Indeed, SAP-deficient T cells were hyperresponsive to T-cell receptor stimulation, which resulted in increased secretion of IL-2, IFN-γ, and TNF-α.

CONCLUSIONS

SAP expression is required for the counterselection of developing autoreactive B cells and prevents their T cell-dependent accumulation in the periphery.

Intrinsic hyporesponsiveness of invariant natural killer T cells precedes the onset of lupus

Patients with systemic lupus erythematosus (SLE) display reduced numbers and functions of invariant natural killer T (iNK T) cells, which are restored upon treatment with corticosteroids and rituximab. It is unclear whether the iNK T cell insufficiency is a consequence of disease or is a primary abnormality that precedes the onset of disease. To address this, we analysed iNK T cell function at different stages of disease development using the genetically lupus-susceptible NZB × NZW F1 (BWF(1)) model. We found that iNK T cell in-vivo cytokine responses to an iNK T cell ligand α-galactosylceramide (α-GalCer) were lower in BWF(1) mice than in non-autoimmune BALB/c and major histocompatibility complex (MHC)-matched NZB × N/B10.PL F1 mice, although iNK T cell numbers in the periphery were unchanged in BWF(1) mice compared to control mice. Such iNK T cell hyporesponsiveness in BWF(1) mice was detected at a young age long before the animals exhibited any sign of autoimmunity. In-vivo activation of iNK T cells is known to transactivate other immune cells. Such transactivated T and B cell activation markers and/or cytokine responses were also lower in BWF(1) mice than in BALB/c controls. Finally, we show that iNK T cell responses were markedly deficient in the NZB parent but not in NZW parent of BWF(1) mice, suggesting that BWF(1) might inherit the iNK T cell defect from NZB mice. Thus, iNK T cells are functionally insufficient in lupus-prone BWF(1) mice. Such iNK T cell insufficiency precedes the onset of disease and may play a pathogenic role during early stages of disease development in SLE.

Fine tuning a well-oiled machine: Influence of NK1.1 and NKG2D on NKT cell development and function

Natural killer T cells (NKT) represent a group of CD1d-restricted T-lineage cells that provide a functional interface between innate and adaptive immune responses in infectious disease, cancer, allergy and autoimmunity. There have been remarkable advances in understanding the molecular events that underpin NKT development in the thymus and in the complex array of functions in the periphery. Most functional studies have focused on activation of T cell antigen receptors expressed by NKT cells and their responses to CD1d presentation of glycolipid and related antigens. Receiving less attention has been several molecules that are hallmarks of Natural Killer (NK) cells, but nonetheless expressed by NKT cells. These include several activating and inhibitory receptors that may fine-tune NKT development and survival, as well as activation via antigen receptors. Herein, we review the possible roles of the NK1.1 and NKG2D receptors in regulating development and function of NKT cells in health and disease. We suggest that pharmacological alteration of NKT activity should consider the potential complexities commensurate with NK1.1 and NKG2D expression.