NKT cell self-reactivity: evolutionary master key of immune homeostasis?

Transcriptome and Function of Novel Immunosuppressive Autoreactive Invariant Natural Killer T Cells That Are Absent in Progressive Multiple Sclerosis

BACKGROUND AND OBJECTIVE

The aim of this study was to determine whether natural killer T (NKT) cells, including invariant (i) NKT cells, have clinical value in preventing the progression of multiple sclerosis (MS) by examining the mechanisms by which a distinct self-peptide induces a novel, protective invariant natural killer T cell (iNKT cell) subset.

METHODS

We performed a transcriptomic and functional analysis of iNKT cells that were reactive to a human collagen type II self-peptide, hCII707-721, measuring differentially induced genes, cytokines, and suppressive capacity.

RESULTS

We report the first transcriptomic profile of human conventional vs novel hCII707-721-reactive iNKT cells. We determined that hCII707-721 induces protective iNKT cells that are found in the blood of healthy individuals but not progressive patients with MS (PMS). By transcriptomic analysis, we observed that hCII707-721 promotes their development and proliferation, favoring the splicing of full-length AKT serine/threonine kinase 1 (AKT1) and effector function of this unique lineage by upregulating tumor necrosis factor (TNF)-related genes. Furthermore, hCII707-721-reactive iNKT cells did not upregulate interferon (IFN)-γ, interleukin (IL)-4, IL-10, IL-13, or IL-17 by RNA-seq or at the protein level, unlike the response to the glycolipid alpha-galactosylceramide. hCII707-721-reactive iNKT cells increased TNFα only at the protein level and suppressed autologous-activated T cells through FAS-FAS ligand (FAS-FASL) and TNFα-TNF receptor I signaling but not TNF receptor II.

DISCUSSION

Based on their immunomodulatory properties, NKT cells have a potential value in the treatment of autoimmune diseases, such as MS. These significant findings suggest that endogenous peptide ligands can be used to expand iNKT cells, without causing a cytokine storm, constituting a potential immunotherapy for autoimmune conditions, including PMS.

Endogenous IFN-β signaling exerts anti-inflammatory actions in experimentally induced focal cerebral ischemia

Background

Interferon (IFN)-β exerts anti-inflammatory effects, coupled to remarkable neurological improvements in multiple sclerosis, a neuroinflammatory condition of the central nervous system. Analogously, it has been hypothesized that IFN-β, by limiting inflammation, decreases neuronal death and promotes functional recovery after stroke. However, the core actions of endogenous IFN-β signaling in stroke are unclear.

Methods

To address this question, we used two clinically relevant models of focal cerebral ischemia, transient and permanent middle cerebral artery occlusion, and two genetically modified mouse lines, lacking either IFN-β or its receptor, the IFN-α/β receptor. Subsets of inflammatory and immune cells isolated from the brain, blood, and spleen were studied using flow cytometry. Sensorimotor deficits were assessed by a modified composite neuroscore, the rotating pole and grip strength tests, and cerebral infarct volumes were given by lack of neuronal nuclei immunoreactivity.

Results

Here, we report alterations in local and systemic inflammation in IFN-β knockout (IFN-βKO) mice over 8 days after induction of focal cerebral ischemia. Notably, IFN-βKO mice showed a higher number of infiltrating leukocytes in the brain 2 days after stroke. Concomitantly, in the blood of IFN-βKO mice, we found a higher percentage of total B cells but a similar percentage of mature and activated B cells, collectively indicating a higher proliferation rate. The additional differential regulation of circulating cytokines and splenic immune cell populations in wild-type and IFN-βKO mice further supports an important immunoregulatory function of IFN-β in stroke. Moreover, we observed a significant weight loss 2–3 days and a reduction in grip strength 2 days after stroke in the IFN-βKO group, while endogenous IFN-β signaling did not affect the infarct volume.

Conclusions

We conclude that endogenous IFN-β signaling attenuates local inflammation, regulates peripheral immune cells, and, thereby, may contribute positively to stroke outcome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0427-0) contains supplementary material, which is available to authorized users.

Invariant natural killer T cells and their ligands: focus on multiple sclerosis

Invariant natural killer T (iNKT) cells are an innate population of T cells identified by the expression of an invariant T-cell receptor and reactivity to lipid-based antigens complexed with CD1d. They account for a small percentage of lymphocytes, but are extremely potent and play central roles in immunity to infection, in some cancers, and in autoimmunity. The list of relevant stimulatory lipids and glycolipid antigens now includes a range of endogenous self-antigens including the myelin-derived acetylated galactosylceramides. Recent progress in studies to identify the nature of lipid recognition for iNKT cells in autoimmune diseases like multiple sclerosis is likely to foster the development of therapeutic strategies aimed at harnessing iNKT cell activity.

The Alpha and Omega of Galactosylceramides in T Cell Immune Function

Glycosphingolipids are a subgroup of glycolipids that contain an amino alcohol sphingoid base linked to sugars. They are found in the membranes of cells ranging from bacteria to vertebrates. This group of lipids is known to stimulate the immune system through activation of a type of white blood cell known as natural killer T cell (NKT cell). Here we summarize the extensive research that has been done to identify the structures of natural glycolipids that stimulate NKT cells and to determine how these antigens are recognized. We also review studies designed to understand how glycolipid variants, both natural and synthetic, can alter the responses of NKT cells, leading to dramatic changes in the global immune response.

Regulated Expression of miR-155 is Required for iNKT Cell Development

Invariant natural killer T cells (iNKT cells) are CD1d-restricted, lipid antigen-reactive T lymphocytes with immunoregulatory functions. iNKT cell development in the thymus proceeds through subsequent stages, defined by the expression of CD44 and NK1.1, and is dictated by a unique gene expression program, including microRNAs. Here, we investigated whether miR-155, a microRNA involved in differentiation of most hematopoietic cells, played any role in iNKT cell development. To this end, we assessed the expression of miR-155 along iNKT cell maturation in the thymus, and studied the effects of miR-155 on iNKT cell development using Lck-miR-155 transgenic mice, which over express miR-155 in T cell lineage under the lymphocyte-specific protein tyrosine kinase (Lck) promoter. We show that miR-155 is expressed by newly selected immature wild-type iNKT cells and turned off along iNKT cells differentiation. In transgenic mice, miR-155 over-expression resulted in a substantial block of iNKT cell maturation at Stage 2, in the thymus toward an overall reduction of peripheral iNKT cells, unlike mainstream T cells. Furthermore, the effects of miR-155 over-expression on iNKT cell differentiation were cell autonomous. Finally, we identified Ets1 and ITK transcripts as relevant targets of miR-155 in iNKT cell differentiation. Altogether, these results demonstrate that a tight control of miR-155 expression is required for the development of iNKT cells.

PyNTTTTGT and CpG immunostimulatory oligonucleotides: effect on granulocyte/monocyte colony-stimulating factor (GM-CSF) secretion by human CD56+ (NK and NKT) cells

CD56+ cells have been recognized as being involved in bridging the innate and acquired immune systems. Herein, we assessed the effect of two major classes of immunostimulatory oligonucleotides (ODNs), PyNTTTTGT and CpG, on CD56+ cells. Incubation of human peripheral blood mononuclear cells (hPBMC) with some of these ODNs led to secretion of significant amounts of interferon gamma (IFN-γ), tumor necrosis factor alpha (TNF-α) and granulocyte/monocyte colony-stimulating factor (GM-CSF), but only if interleukin 2 (IL2) was present. IMT504, the prototype of the PyNTTTTGT ODN class, was the most active. GM-CSF secretion was very efficient when non-CpG ODNs with high T content and PyNTTTTGT motifs lacking CpGs were used. On the other hand, CpG ODNs and IFNα inhibited this GM-CSF secretion. Selective cell type removal from hPBMC indicated that CD56+ cells were responsible for GM-CSF secretion and that plasmacytoid dendritic cells (PDCs) regulate this process. In addition, PyNTTTTGT ODNs inhibited the IFNα secretion induced by CpG ODNs in PDCs by interference with the TLR9 signaling pathway. Since IFNα is essential for CD56+ stimulation by CpG ODNs, there is a reciprocal interference of CpG and PyNTTTTGT ODNs when acting on this cell population. This suggests that these synthetic ODNs mimic different natural alarm signals for activation of the immune system.

NKT cell activation by local α-galactosylceramide administration decreases susceptibility to HSV-2 infection

NKT cells are a subgroup of T cells, which express a restricted TCR repertoire and are critical for the innate immune responses to viral infections. Activation of NKT cells depends on the major histocompatibility complex-related molecule CD1d, which presents bioactive lipids to NKT cells. The marine sponge derived lipid αGalCer has recently been demonstrated as a specific agonist for activation of human and murine NKT cells. In the present study we investigated the applicability of αGalCer pre-treatment for immune protection against intra-vaginal HSV-2 infection. We found that C57BL/6 WT mice that received local pre-treatment with αGalCer prior to intra-vaginal HSV-2 infection had a lower mean disease score, mortality and viral load in the vagina following infection, compared to mice that did not receive αGalCer pre-treatment. Further, we found increased numbers of CD45 and NK1.1 positive cells in vaginal tissue and elevated levels of IFN-γ in the vaginal tissue and in vaginal fluids 24h after αGalCer pre-treatment. Collectively our data demonstrate a protective effect of αGalCer induced activation of NKT cells in the innate immune protection against viral infection.

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.

Invariant Natural Killer T-cell anergy to endogenous myelin acetyl-glycolipids in multiple sclerosis

To extend our studies on glycolipid-reactive invariant Natural Killer T-cell (iNKT-cell) function in multiple sclerosis (MS), we investigated the stimulatory activities of two myelin-derived glycolipids that are poly-acetylated derivatives of β-galactosylceramide designated as fast-migrating cerebrosides (FMC) by thin-layer chromatography. In healthy subjects, FMC stimulation of peripheral blood cells significantly expanded iNKT-cells similar to α-GalCer and induced significant increases in Th1, Th2 and Th17 cytokines. In marked contrast, MS patients failed to respond to FMCs or to α-GalCer stimulation indicating an anergic response. We propose that myelin-derived FMC glycolipids stimulate iNKT-cell responses in vivo and this is blocked in MS.

Role for lysosomal phospholipase A2 in iNKT cell-mediated CD1d recognition

Invariant natural killer T (iNKT) cells recognize self lipid antigens presented by CD1d molecules. The nature of the self-antigens involved in the development and maturation of iNKT cells is poorly defined. Lysophospholipids are self-antigens presented by CD1d that are generated through the action of phospholipases A1 and A2. Lysosomal phospholipase A2 (LPLA2, group XV phospholipase A2) resides in the endocytic system, the main site where CD1d antigen acquisition occurs, suggesting that it could be particularly important in CD1d function. We find that Lpla2(-/-) mice show a decrease in iNKT cell numbers that is neither the result of a general effect on the development of lymphocyte populations nor of effects on CD1d expression. However, endogenous lipid antigen presentation by CD1d is reduced in the absence of LPLA2. Our data suggest that LPLA2 plays a role in the generation of CD1d complexes with thymic lipids required for the normal selection and maturation of iNKT cells.