Are We Meeting the Promise of Endotypes and Precision Medicine in Asthma?

Abstract

While the term asthma has long been known to describe heterogeneous groupings of patients, only recently have data evolved which enable a molecular understanding of the clinical differences. The evolution of transcriptomics (and other 'omics platforms) and improved statistical analyses in combination with large clinical cohorts opened the door for molecular characterization of pathobiologic processes associated with a range of asthma patients. When linked with data from animal models and clinical trials of targeted biologic therapies, emerging distinctions arose between patients with and without elevations in type 2 immune and inflammatory pathways, leading to the confirmation of a broad categorization of type 2-Hi asthma. Differences in the ratios, sources, and location of type 2 cytokines and their relation to additional immune pathway activation appear to distinguish several different (sub)molecular phenotypes, and perhaps endotypes of type 2-Hi asthma, which respond differently to broad and targeted anti-inflammatory therapies. Asthma in the absence of type 2 inflammation is much less well defined, without clear biomarkers, but is generally linked with poor responses to corticosteroids. Integration of "big data" from large cohorts, over time, using machine learning approaches, combined with validation and iterative learning in animal (and human) model systems is needed to identify the biomarkers and tightly defined molecular phenotypes/endotypes required to fulfill the promise of precision medicine.

Keywords: asthma; endotypes; eosinophils; inflammation; phenotypes; precision medicine; type 2.

Conflict of interest statement

S. Wenzel has consulted for and served as an investigator on multicenter clinical trials from the following companies: AstraZeneca, Boehringer Ingelheim, GSK, Novartis, and Sanofi Aventis. She has never personally received more than $10,000 from any particular company in a given year.

Figures

Graphical abstract

FIGURE 1.

Generalized paradigm for the implementation of precision medicine. Asthma presents in distinctly different ways based on integration of genetics and environmental exposures. Granular clinical and molecular phenotyping can generate novel target pathways that can be validated over time and through the use of animal models. These approaches should lead to development of biomarker targeted therapeutic trials which improve the overall efficacy and safety of drugs, bringing the best drug option to the correct patient.

FIGURE 2.

Complex interplay between type 1 and type 2 immune pathways contributes to differing asthma phenotypes. Repeated exposure to allergens in genetically susceptible individuals induces the development of Th2 cells, which produce interleukin (IL)-4, IL-5, and IL-13. These type 2 cytokines can be also produced by alternate mechanisms involving cells of the innate immune system. In this regard, pathogens including bacteria, viruses, and fungi as well as allergens can cause epithelial damage in which proteases encoded by these agents play a prominent role. Epithelial damage leads to increased expression and release of IL-33 and thymic stromal lymphopoietin (TSLP), which stimulate ILC2 cells to produce IL-5 and IL-13. Tissue resident mast cells and basophils recruited from the periphery or generated from in situ differentiation of progenitors can also generate these cytokines. IL-5 released from circulating Th2 cells and other growth factors such as IL-3 and granulocyte-macrophage colony stimulating factor (GM-CSF) stimulate eosinophil differentiation, proliferation in the bone marrow, which are then recruited to the airways under the influence eosinophilic chemokines. Mast cells also produce prostaglandin D2 (PGD2) that binds its cognate receptor CRTH2 on Th2, ILC2 cells, triggering cytokine release. IL-4 and IL-13 produced by these different cell types augment mucus production via expression of mucin genes including MUC5AC. In certain individuals, gene-environment interactions also promote a type 1/interferon (IFN)-γ response with or without IL-17 production from T cells (not shown). IFN-γ in turn stimulates production of the chemokines CXCL9 and CXCL10 from both airway epithelial cells and resident macrophages with the potential to create a positive feedback loop promoting recruitment of Th1 cells and eosinophils via interaction with the receptor CXCR3. Increased expression of the cytokine IL-18 in macrophages can also promote development of Th1 and Th2 cells. Synergism between T1 (IFN-γ) and T2 (IL-13) cytokines augments expression of the enzymes inducible nitric oxide synthase (iNOS) and dual oxidase 2 (DUOX2) in the airway epithelial cells driving production of nitric oxide (NO) and H2O2, respectively, with increases in nitrative and oxidative stress. APC, antigen presenting cell; FeNO, fraction exhaled nitric oxide.

FIGURE 3.

“Inflammasome” linked interleukin (IL)-1β, IL-18, and IL-33 pathways in relation to downstream pathway activation and cellular responses. Both genetic and transcriptomic studies have strongly supported involvement of these cytokines in asthma phenotypes. These pathways are all activated by danger signals/receptors, with responses likely dependent on initiating signal, cell type, and perhaps genetic factors. IL-1β activation, through a classical inflammasome process, leads to broad activation of many cell types, generally linked to neutrophilic inflammation, perhaps through interferon (IFN)-γ or IL-17 pathways. Genetically associated IL-33 activation, arising from epithelial damage and augmented by inflammatory cell protease activity, stimulates ILC2 cells and mast cells/basophils to generate type 2 cytokines increasing type 2 immune responses. IL-18, strongly linked genetically with asthma through IL-18R1, can augment either type 1 or type 2 immune responses depending on the presence of IL-12. Interestingly, all 3 receptor pathways signal through MyD88, NF-κB, and Jun kinase (JNK) pathways. IL18RAP, IL-18 receptor activating protein; IL-18BP, IL-18 binding protein; sST2, soluble ST2/IL1RL1.

FIGURE 4.

Timeline in establishment of the Th1/Th2 paradigm, identification of master regulators of T helper cells, and association with asthma. Naive CD4+ T cells differentiate in response to antigens and costimulation to different subsets, each characterized by its expression of specific cytokines, a paradigm established in the mid 1980s. Increased numbers of Th2 cells were identified in the airways of asthmatics. A surge of interest in molecules that are essential for lineage development led to the association of GATA-3 with interleukin (IL)-5 gene expression and as a master regulator of Th2 cells and subsequently of ILC2 cells as well. Likewise, T-bet and RORγt are essential for the development of Th1 and Th17 cells, respectively. Increase in GATA-3 expression was observed in human asthma, and expression of a dominant-negative (DN) mutant of GATA-3 in mice blocked allergic eosinophilic airway inflammation in a mouse model. IFN, interferon; BAL, bronchoalveolar lavage; ConA, concanavalin A; TCR, T-cell receptor; HSC, hematopoietic stem cell; CLP, common lymphoid progenitor.

FIGURE 5.

Schematic of the current understanding of T2-Hi molecular and clinical phenotypes. Type 2 inflammation is characterized by enhanced activity of interleukin (IL)-4, -5, and -13 which commonly leads to eosinophilic inflammation, mast cells, mucus, and elevated fraction exhaled nitric oxide (FeNO), all of which are likely to overlap to varying degrees. However, depending on the mix of cytokines and these accompanying immune processes, at least 4 distinct type 2 molecular phenotypes are recognizable, with varying response to therapy, ranging from simple mild allergic early-onset asthma to very severe, T2-Hi+ asthma. IFN, interferon; CS, corticosteroid.

FIGURE 6.

Simplified schematic of potential microbiome-immune interactions in relation to molecular and inflammatory asthma phenotypes. Emerging data support the importance of gut microbiota/dysbiosis in the development of atopic-allergic disease, with certain bacteria including staphylococcus and ruminococcus associated with atopy, while Lactobacillus johnsonii has been considered protective. These alterations could promote the development of mild-moderate T2-Hi allergic asthma. In all cases, loss of diversity of the microbiome is believed to be a disadvantage and drives development of ILC2 and/or ILC3 cells which, under certain circumstances, can migrate to the airways. Abnormalities in the lung microbiome are more controversial and difficult to confirm. An abnormal airway epithelium could predispose to bacterial contamination, colonization, and infection with pathogenic bacteria being found more commonly in T2-Lo/neutrophilic asthma. Interestingly, in mild asthma, the presence of T2-Hi immune processes is associated with significantly less evidence for bacterial 16S RNA, suggesting the T2 environment/phenotype may inhibit bacterial presence. Similarly fungal RNA has also been observed in lower abundance.

FIGURE 7.

Schematic of the current understanding of T2-Lo molecular and clinical phenotypes. T2-Lo phenotypes are characterized by lack of clear biomarkers, obesity, and poor corticosteroid (CS) responsiveness, often despite high doses of CSs. Unlike the overlapping cellular and molecular characteristics which defined T2-Hi phenotypes, the characteristics of T2-Lo asthma are more variably overlapping. The subphenotypes include a broad range of severity from the most mild to severe patients, including elements of long disease duration (suppression of T2 immunity), a bronchitic, perhaps neutrophil associated phenotype, and a late-onset obese phenotype. Precision approaches to treating these patients remain poorly understood. IL, interleukin.

FIGURE 8.

Overview of the intersection of asthma and obesity. Obesity and asthma enjoy a complex relationship. Top left: persistence of severe, type 2 (T2) associated asthma, poorly responsive to standard treatment can lead to increasing obesity, worsening symptoms, and more exacerbations, which respond to improved asthma treatment (often with concurrent weight loss). Bottom right: however, an obesity-associated T2-Lo asthma arising in adulthood also exists in association with both metabolic and mechanical dysfunction. While balancing certain metabolic pathways may improve outcomes, weight loss appears to be the most effective therapy. FeNO, fraction exhaled nitric oxide; SOB, shortness of breath.

FIGURE 9.

General paradigm for development and progression of childhood-onset asthma. It is likely over 60% of all asthma phenotypes have their origins in childhood. Genetic predisposition to childhood asthma combined with environmental exposures/changes in gut microbiota lead to atopy, asthma persistence, and potential progression. While an IgE-dependent, likely T2-Hi asthma phenotype is recognized, the association with cellular and molecular profiles is less clear. T2-Lo asthma likely also exists in childhood, although its trajectory is even less clear. Finally, the immune or hormonal factors that drive the resolution, remission, or persistence of disease into adulthood remain largely unknown.

FIGURE 10.

In situ hematopoiesis (ISH) as a potential mechanism for expansion of T2 cytokine-producing cells in the airways of nonallergic asthmatics. The airway epithelium at the interface of the environment and the respiratory system is vulnerable to breaching by pathogens and allergens. Factors such as thymic stromal lymphopoietin (TSLP), interleukin (IL)-33, and TSLP released by disrupted and dysfunctional epithelial cells can be expected to induce IL-5 and IL-13 production by tissue-resident ILC2 cells. TSLP and IL-33 may also promote expansion of hematopoietic stem and progenitor cells (HSPCs) that are resident in the submucosa or are recruited from the periphery. Upon activation stimulation by these cytokines and potentially other mediators, the HSPCs would undergo differentiation into various myeloid lineages that have the ability to produce T2 cytokines. Eos, eosinophil; Baso, basophil; MC, mast cell.