alpha-Galactosylceramide, a ligand of natural killer T cells, inhibits allergic airway inflammation

Initiating mechanisms of food allergy: Oral tolerance versus allergic sensitization

Immediately after birth the mucosa of the gastrointestinal tract, which represents the greatest body surface area exposed to the outside environment, is confronted with a large variety of foreign antigens. The immune system of the intestine now has to meet the task of discriminating between pathogens and harmless antigens, such as food proteins and commensal bacteria, and to respond accordingly. This important job is fulfilled by cells of the gut-associated lymphoid tissue, the largest immunologic organ in the body. Despite the large extent of food antigen exposure, only a small percentage of individuals experience adverse immunologic reactions to food. This is due to the fact that the normal immune response to dietary proteins is associated with the induction of oral tolerance, which refers to a state of active inhibition of immune responses to an antigen by means of prior exposure to that antigen via the oral route. Abrogation of oral tolerance or failure to induce oral tolerance may result in the development of food hypersensitivity. In the present review, factors that may play a role in the outcome of oral tolerance versus sensitization to food proteins are discussed.

A role for natural killer T cells in asthma

In several mouse models, natural killer T cells have recently been found to be required for the development of airway hyper-reactivity, a cardinal feature of asthma. Moreover, in patients with chronic asthma, natural killer T cells with a T-helper-2-like phenotype (that is, that express CD4 and produce T helper 2 cytokines) are present in the lungs in large numbers. In this Opinion article, we suggest that natural killer T cells, which express a restricted T-cell receptor and respond to glycolipids rather than protein antigens, have a previously unsuspected but crucial role, distinct from that of T helper 2 cells, in the pathogenesis of asthma.

The regulation of allergy and asthma

Allergic diseases and asthma are caused by exaggerated T-helper 2 (Th2)-biased immune responses in genetically susceptible individuals. Tolerance to allergens is a mechanism that normally prevents such responses, but the specific immunological events that mediate tolerance in this setting are poorly understood. A number of recent studies indicate that regulatory T cells (Tregs) play an important role in controlling such Th2-biased responses. Tregs involved in regulating allergy and asthma consist of a family of related types of T cells, including natural CD25+ Tregs as well as inducible forms of antigen-specific adaptive Tregs. Impaired expansion of natural and/or adaptive Tregs is hypothesized to lead to the development of allergy and asthma, and treatment to induce allergen-specific Tregs could provide curative therapies for these problems.

Immune dysregulation in asthma

Allergic diseases and asthma are caused by dysregulated Th2-biased immune responses to environmental allergens in genetically predisposed individuals. Over the past several years there has been much progress in understanding the mechanisms by which Th2 responses are generated and the pathogenic role of natural killer T cells in asthma. In addition, there has been much progress in understanding the mechanisms of tolerance to allergens, the role of natural and adaptive allergen-specific regulatory T cells, and the strategies to prevent or to reverse allergic disease and asthma. Impaired expansion of regulatory T cells is hypothesized to lead to the development of allergy and asthma, and treatment to induce allergen-specific regulatory T cells could provide curative therapies for these problems.

Modulation of murine experimental allergic conjunctivitis by treatment with α-galactosylceramide

When mice are treated with alpha-galactosylceramide (alpha-GalCer), NKT cells are activated and suppress the development of experimental airway inflammation. This suppressive effect is believed to be mediated by the upregulation of IFN-gamma. Here, we investigated whether alpha-GalCer treatment can also modulate the development of experimental allergic conjunctivitis (EC). EC was induced in wild-type and IFN-gamma-deficient Balb/c mice by active immunization with ragweed (RW) followed by challenge with RW in eye drops. The mice were intraperitoneally injected with alpha-GalCer or vehicle at the time of immunization or before RW challenge. Twenty-four hours after RW challenge, conjunctivas, spleens and sera were harvested for histological analysis, flow cytometric, proliferation and cytokine assays, and measurement of immunoglobulin levels, respectively. Treatment with alpha-GalCer at the time of the EC-priming immunization significantly increased Th2 responses and markedly upregulated the severity of the EC. However, treatment with alpha-GalCer just before the Ag challenge that triggers EC in primed animals significantly suppressed the disease. This was associated with an increased frequency of CD4(+)CD25(+) cells, which express Foxp3, in the spleen. alpha-GalCer treatment just prior to Ag challenge also suppressed the development of EC in IFN-gamma-deficient mice, and we found apoptosis and anergy are unlikely to play a major role in the mechanism by which pre-challenge alpha-GalCer treatment suppresses EC. These data suggest that NKT cells can play a downregulatory role in the development of EC and that alpha-GalCer may be useful for treating allergic conjunctivitis.

Graft-versus-host disease in recipients of grafts from natural killer T cell-deficient (Jalpha281(-/-)) donors

Valpha14i natural killer T cells (NKT cells) are a CD1-restricted subset of NKT cells that express an invariable Valpha14+ Jalpha281+ alphabeta T-cell receptor. To determine whether the absence of Valpha14i NKT cells from the graft affects the development of acute GVHD, we induced GVH reactions using Jalpha281(-/-) mice as donors in the C57BL/6-->(C57BL/6 x DBA/2)F1-hybrid strain combination. Recipients of grafts from Jalpha281(-/-) donors were not protected from either the morbidity or the severe wasting syndrome associated with the development of acute GVHD, but the concentrations of some T helper 1 (Th1) and Th2 cytokines were different from those seen in recipients of grafts from wild-type donors. Interferon-gamma was seen earlier (day 4) in recipients of grafts from Jalpha281(-/-) donors but did not reach the concentrations seen in recipients of grafts from wild-type donors on day 8 (P < 0.02). On day 8, the amount of tumour necrosis factor-alpha released into the serum following the injection of a small amount of lipopolysaccharide was lower in recipients of grafts from Jalpha281(-/-) donors (P < 0.02). The amount of interleukin (IL)-5 was also lower in recipients of grafts from Jalpha281(-/-) donors, when compared to the concentration seen in recipients of grafts from wild-type donors (P < 0.002). IL-13 was seen in recipients of grafts from Jalpha281(-/-) donors but not in recipients of grafts from wild-type donors. Our findings show that the absence of donor Valpha14i NKT cells is associated with lower concentrations of some Th1 cytokines. We also observed higher IL-13 concentrations and lower IL-5 concentrations in recipients of grafts from Jalpha281(-/-) donors indicating a variable effect on Th2 cytokine production.

Glycolipid activation of invariant T cell receptor+ NK T cells is sufficient to induce airway hyperreactivity independent of conventional CD4+ T cells

Asthma is an inflammatory lung disease, in which conventional CD4+ T cells producing IL-4/IL-13 appear to play an obligatory pathogenic role. Here we show, in a mouse model of asthma, that activation of pulmonary IL-4/IL-13 producing invariant TCR+ CD1d-restricted natural killer T (NKT) cells is sufficient for the development of airway hyperreactivity (AHR), a cardinal feature of asthma, in the absence of conventional CD4+ T cells and adaptive immunity. Respiratory administration of glycolipid antigens that specifically activate NKT cells (alpha-GalactosylCeramide and a Sphingomonas bacterial glycolipid) rapidly induced AHR and inflammation typically associated with protein allergen administration. Naïve MHC class II-deficient mice, which lack conventional CD4+ T but have NKT cells, showed exaggerated baseline AHR and, when challenged with alpha-GalactosylCeramide, demonstrated even greater AHR. These studies demonstrate an expanded role for NKT cells, in which NKT cells not only produce cytokines that influence adaptive immunity but also function as critical effector cells that can induce AHR. These results suggest that NKT cells responding to glycolipid antigens, as well as conventional CD4+ T cells responding to peptide antigens, may be synergistic in the induction of AHR, although in some cases, each may independently induce AHR.

Exploiting the innate immune system to control allergic asthma

NKT cells are key actors at the interface between innate and acquired immunity. Indeed, upon activation by glycolipid antigens, they rapidly secrete both Th1 and Th2 cytokines, which affects the development of later immune responses. Previous studies have shown that NKT cells are essential for the development of allergic asthma, a prototypical Th2-mediated pathology. By contrast, three papers, two of which are published in this issue of the European Journal of Immunology, demonstrate, in mouse models, that treatment with alpha-galactosylceramide, a specific NKT ligand, inhibits most of the parameters associated with the disease, including airway hyperreactivity, eosinophilia and IgE production. Increased IFN-gamma synthesis, rather than regulatory IL-10, accounts for this paradoxical effect.

α-Galactosylceramide-induced iNKT cells suppress experimental allergic asthma in sensitized mice: Role of IFN-γ

Allergic asthma is a multifaceted syndrome consisting of eosinophil-rich airway inflammation, bronchospasm, and airway hyper-responsiveness (AHR). Using a mouse model of allergic asthma, we previously reported that invariant NKT (iNKT) cells increase the severity of this disease. Herein, we demonstrate that a single i.v. injection of alpha-galactosylceramide (alpha-GalCer), 1 h before the first airway allergen challenge of OVA-sensitized mice, abrogates elicitation of AHR, airway eosinophilia, IL-4 and IL-5 production in bronchoalveolar lavage fluid, and specific anti-OVA IgE antibodies. Further, alpha-GalCer administered intranasally also strongly inhibited the major symptoms of asthma in sensitized and challenged mice. Alpha-GalCer treatment induces iNKT cell accumulation in the lungs, and shifts their cytokine profile from pro-asthmatic IL-4 to a protective IFN-gamma production. The role of IFN-gamma from iNKT cells in protection was shown by adoptive transfer of sorted iNKT cells from OVA-sensitized and alpha-GalCer-treated mice which protected immunized recipients from manifesting asthma by an IFN-gamma-dependent pathway. Our findings demonstrate for the first time that alpha-GalCer administered locally inhibits asthma symptoms, even in predisposed asthmatic mice, through an iNKT cell- and IFN-gamma-dependent pathway.

The diverse functions of CD1d-restricted NKT cells and their potential for immunotherapy

CD1d-restricted NKT cells have been identified as an important component of the immune system that have the capacity both to augment beneficial host immunity and to prevent harmful autoimmunity. These cells have the ability to produce a wide variety of cytokines, including both proinflammatory and antiinflammatory cytokines that can have multiple different effects on the outcome of immune reactions. The discovery that these T cells are activated by specific recognition of glycolipids in the glycosylceramide family has led to new approaches to manipulate the pleiotropic functions of these cells. Here, we review the multiple activities that have been attributed to NKT cells in a variety of different disease models, and the current state of our understanding of the mechanisms that control the functional outcome of NKT cell activation.