Letter to the editor

Natural killer T cells in the lungs of patients with asthma

Asthma is a chronic lung disease comprising multiple endotypes and characterized by periodic exacerbations. A diverse array of T cells has been found to contribute to all endotypes of asthma in pathogenic and regulatory roles. Here, we review the contributions of CD4+, CD8+, and unconventional T cells in allergic and nonallergic asthma.

Review of published literature pertaining to conventional and unconventional T-cell types in asthma.

Recent peer-reviewed articles pertaining to T cells in asthma, with additional peer-reviewed studies for context.

Much research in asthma has focused on the roles of CD4+ T_H cells. Roles for T_H2 cells in promoting allergic asthma pathogenesis have been well-described, and the recent description of pathogenic T_H2A cells provides additional insight into these responses. Other T_H types, notably T_H1 and T_H17, have been linked to neutrophilic and steroid-resistant asthma phenotypes. Beyond CD4+ T cells, CD8+ Tc2 cells are also strongly associated with allergic asthma. An emerging area for study is unconventional T-cell types, including γδT, invariant natural killer T, and mucosal-associated invariant T cells. Although data in asthma remain limited for these cells, their ability to bridge innate and adaptive responses likely makes them key players in asthma. A number of asthma therapies target T-cell responses, and, although data are limited, they seem to modulate T-cell populations.

Given the diversity and heterogeneity of asthma and T-cell responses, there remain many rich avenues for research to better understand the pathogenesis of asthma. Despite the breadth of T cells in asthma, approved therapeutics remain limited to T_H2 networks.

Food allergy (FA) affects an increasing proportion of children for reasons that remain obscure. Novel disease biomarkers and curative treatment options are strongly needed.

We sought to apply untargeted metabolomic profiling to identify pathogenic mechanisms and candidate disease biomarkers in patients with FA.

Mass spectrometry–based untargeted metabolomic profiling was performed on serum samples of children with either FA alone, asthma alone, or both FA and asthma, as well as healthy pediatric control subjects.

In this pilot study patients with FA exhibited a disease-specific metabolomic signature compared with both control subjects and asthmatic patients. In particular, FA was uniquely associated with a marked decrease in sphingolipid levels, as well as levels of a number of other lipid metabolites, in the face of normal frequencies of circulating natural killer T cells. Specific comparison of patients with FA and asthmatic patients revealed differences in the microbiota-sensitive aromatic amino acid and secondary bile acid metabolism. Children with both FA and asthma exhibited a metabolomic profile that aligned with that of FA alone but not asthma. Among children with FA, the history of severe systemic reactions and the presence of multiple FAs were associated with changes in levels of tryptophan metabolites, eicosanoids, plasmalogens, and fatty acids.

Children with FA have a disease-specific metabolomic profile that is informative of disease mechanisms and severity and that dominates in the presence of asthma. Lower levels of sphingolipids and ceramides and other metabolomic alterations observed in children with FA might reflect the interplay between an altered microbiota and immune cell subsets in the gut.

Natural killer T (NKT) cells express a T-cell receptor that recognizes endogenous and environmental glycolipid antigens. Several subsets of NKT cells have been identified, including IFN-γ–producing NKT1 cells, IL-4–producing NKT2 cells, and IL-17–producing NKT17 cells. However, little is known about the factors that regulate their differentiation and respective functions within the immune system.

We sought to determine whether the polycomb repressive complex 2 protein enhancer of zeste homolog 2 (Ezh2) restrains pathogenicity of NKT cells in the context of asthma-like lung disease.

Numbers of invariant natural killer T (iNKT) 1, iNKT2, and iNKT17 cells and tissue distribution, cytokine production, lymphoid tissue localization, and transcriptional profiles of iNKT cells from wild-type and Ezh2 knockout (KO) iNKT mice were determined. The contribution of NKT cells to development of spontaneous and house dust mite–induced airways pathology, including airways hyperreactivity (AHR) to methacholine, was also assessed in wild-type, Ezh2 KO, and Ezh2 KO mice lacking NKT cells.

Ezh2 restrains development of pathogenic NKT cells, which induce spontaneous asthma-like disease in mice. Deletion of Ezh2 increased production of IL-4 and IL-13 and induced spontaneous AHR, lung inflammation, mucus production, and IgE. Increased IL-4 and IL-13 levels, AHR, lung inflammation, and IgE levels were all dependent on iNKT cells. In house dust mite–exposed animals Ezh2 KO resulted in enhanced AHR that was also dependent on iNKT cells.

Ezh2 is a central regulator of iNKT pathogenicity and suppresses the ability of iNKT cells to induce asthma-like pathology.

Invariant natural killer T (iNKT) cells are a unique subset of T lymphocytes characterized by the expression of an invariant T-cell antigen receptor rearrangement, and the recognition of glycolipid antigens presented by CD1d, a nonpolymorphic major histocompatibility complex class I–like antigen-presenting molecule. After activation iNKT cells rapidly produce copious amounts of Th1, Th2, and Th17 cytokines. As a consequence of their vigorous and early response, iNKT cells have been implicated in a dazzling variety of different immune reactions, ranging from responses to pathogens and tumors to autoimmune diseases. In this review we will focus on the diverse roles of iNKT cells in autoimmune diseases and will consider the proposed mechanisms whereby they exert their influence.

Asthmatics sensitized to fungi are reported to have more severe asthma, yet the immunopathogenic pathways contributing to this severity have not been identified. In a pilot assessment of human asthmatics, those subjects sensitized to fungi demonstrated elevated levels of the common γ-chain cytokine IL-7 in lung lavage fluid, which negatively correlated with the lung function measurement PC20. Subsequently, we show that IL-7 administration during experimental fungal asthma worsened lung function and increased the levels of type 2 cytokines (IL-4, IL-5, IL-13), proallergic chemokines (CCL17, CCL22) and proinflammatory cytokines (IL-1α, IL-1β). Intriguingly, IL-7 administration also increased IL-22, which we have previously reported to drive immunopathogenic responses in experimental fungal asthma. Employing IL22^CreR26R^eYFP reporter mice, we identified γδ T cells, iNKT cells, CD4 T cells and ILC3s as sources of IL-22 during fungal asthma; however, only iNKT cells were significantly increased after IL-7 administration. IL-7-induced immunopathogenesis required both type 2 and IL-22 responses. Blockade of IL-7Rα in vivo resulted in attenuated IL-22 production, lower CCL22 levels, decreased iNKT cell, CD4 T-cell and eosinophil recruitment, yet paradoxically increased dynamic lung resistance. Collectively, these results suggest a complex role for IL-7 signaling in allergic fungal asthma.

Invariant natural killer T (iNKT) cells play complex functions in the immune system, releasing both Th1 and Th2 cytokines. The role of iNKT cells in human asthma is still controversial and never described in severe therapy-resistant asthma in children. The objective of this work was to analyse iNKT frequency in peripheral blood of children with severe therapy-resistant asthma (STRA), compared to children with milder asthma and healthy controls.

Children with asthma (n = 136) (non-severe and STRA) from a referral centre and healthy controls (n = 40) were recruited. Peripheral blood mononuclear cells were isolated, stained with anti-CD3 and anti-iNKT (Vα24Jα18), and analysed through flow cytometry. Atopic status was defined by measuring specific IgE in serum. Airway inflammation was assessed by induced sputum.

Children with asthma presented an increased frequency of CD3⁺iNKT⁺ cells (median 0.38% IQR 0.18–1.9), compared to healthy controls (median 0.26% IQR 0.10–0.43) (p = 0.025). Children with STRA also showed an increased frequency of iNKT cells (1.5% IQR 1.05–2.73) compared to healthy controls and non-severe asthmatic children (0.35% IQR 0.15–1.6; p = 0.002). The frequency of iNKT cells was not different between atopic and non-atopic children. In addition, iNKT cells were not associated with any inflammatory pattern of induced sputum studied.

Our data suggests that iNKT cells play a role in paediatric asthma, which is also associated with the severity of disease, but independent of the atopic status.