Discovering Biomarkers of Neutrophilic Asthma: A Clinician’s Perspective

Although asthma is a disease characterized by chronic airway inflammation, for decades, its diagnosis has been through physiological assessment, such as detection of airway hyperresponsiveness or reversible obstruction. Unfortunately, the physiological diagnosis is often not precise and does not directly inform the choice of treatment. It has not been long that a pathological assessment, such as detection of inflammatory markers, was incorporated into asthma management. It is now widely accepted that asthma is a clinical syndrome with various phenotypes and endotypes, and the treatment ideally needs to be determined by inflammatory biomarkers and phenotype stratification.1 The pathological diagnosis would be a key to successful management, especially for patients with treatment-refractory or severe asthma, as shown in the early clinical trials with mepolizumab.2, 3

The introduction of T2-biomarkers and novel biologicals is the second breakthrough in the history of asthma management, after anti-inflammatory inhalers (inhaled corticosteroids). T2-targeted biologics are helpful in T2-high severe asthma, not only in controlling asthma symptoms and attacks in the affected individuals but also in reducing future risks of the disease- and oral steroid-related complications.4 However, there is a sizable proportion of patients without T2 inflammation or even with elevated neutrophils in the airways (sputum neutrophilia of ≥ 61%–65%). Although there is a controversy over the clinical relevance of “being neutrophilic”,5 patients with neutrophilic asthma, comprise approximately 20%–30% of all asthma cases in adults. They are older, comprising more males, and frequently present with more late-onset, severe, or steroid-resistant asthma.6, 7 Thus, the identification and treatment of severe non-T2 asthma remains an area with major unmet needs.8

In this issue of Allergy, Asthma & Immunology Research, Bich et al.9 investigated human samples and experimental models to test if serum amyloid A1 (SAA1) is a potential biomarker for neutrophilic asthma. They recruited asthmatic patients and healthy controls and found that the serum levels of SAA1 were higher in neutrophilic asthmatic patients (n = 78; 2.7 ± 0.3 ng/mL) than in non-asthmatic controls (n = 60; 2.4 ± 0.2 ng/mL) or non-neutrophilic asthmatic patients (n = 44; 2.3 ± 0.5 ng/mL). Asthmatic patients with high serum SAA1 levels (≥ 2.6 ng/mL) had significantly lower forced expiratory volume in 1 second (% of predicted) (81.4 ± 20.5% vs. 89.4 ± 14.3%; P = 0.018), more severe asthma (45% vs. 27%; P = 0.008), and higher sputum neutrophil counts (77.3 ± 28.0% vs. 61.8 ± 34.7%; P = 0.031), than those with low-SAA1 levels (< 2.6 ng/mL), suggesting the potential relevance of SAA1 in severe neutrophilic asthma. Furthermore, the authors demonstrated the mechanistic plausibility of SAA1 present in neutrophilic airway inflammation particularly induced by viral infections, using different in vitro and in vivo asthma models with polyinosinic:polycytidylic acid. SAA1 treatment led to the activation of neutrophils and macrophages isolated from peripheral blood, with the formation of neutrophil extracellular traps and secretion of proinflammatory cytokines presenting M1 phenotype of macrophages.

This work not only suggests a potential biomarker for severe neutrophilic asthma but also explores the mechanistic plausibility of SAA1 in detail.9 SAA1 is an acute phase protein, blood levels of which are elevated during infection, trauma, surgery, burns, tissue infarction, inflammation, neoplasia, and stress.10 A significant relationship between serum SAA1 levels and asthma prevalence has been suggested previously.11, 12 Serum SAA1 levels are also elevated in patients with acute exacerbation of chronic obstructive pulmonary disease with neutrophilic inflammation.13 More recently, it was suggested that SAA1 could bind to house dust mite allergens, driving type 2 responses via inducing the production of interleukin (IL)-33 by airway epithelial cells in vitro.14 The current study by Bich et al.9 broadened the knowledge on SAA1 in asthma, and added the details of SAA1 that are specific to the pathogenesis of neutrophilic asthma, particularly induced by viral infection.

In this editorial, however, we would like to appraise the findings and raise questions from a clinician’s perspective. First, at least presently, neutrophilic inflammation is not being assessed in usual clinical practice because it does not inform treatment decision. The absence of T2 (or eosinophilic) inflammation is a useful guide in clinical decision-making, but it does not indicate that the presence of neutrophilic inflammation is clinically relevant. Moreover, clinical trials with brodalumab (anti-IL-17 receptor antibody), golimumab (anti-tumor necrosis factor-α antibody), and AZD5069 (CXCR2 receptor antagonist) failed to show significant improvements over placebos, in patients with moderate or severe asthma.15, 16, 17 Treatment with risankizumab, blocking the p19 subunit of IL-23, was rather associated with worse clinical outcomes in severe asthmatic patients compared with those treated with placebo, although it led to the downregulated expression of sputum genes involved in the activation of natural killer and cytotoxic T cells and the activation of type 1 helper T and type 17 helper T transcription factors.18

Secondly, owing to the nature of neutrophils as acute phase reactant to microbial or pollutant exposure,19, 20 it is uncertain if neutrophilic asthma is a stable phenotype in individuals. Neutrophilic inflammation is also confounded by oral corticosteroid treatments in severe asthmatic patients, as steroids can prolong neutrophil survival and prevent apoptosis.21 In this regard, a question is raised: a simple counting of neutrophil numbers in sputum is not precise enough to define a “clinically relevant” phenotype of neutrophilic asthma? Neutrophil activation markers may be more useful than cell number quantitation, although the former is technically more challenging.22 These indicate the limitations of current phenotyping approaches and emphasize the need for endotype research to precisely define neutrophilic or non-T2 asthma as well as discover its biomarkers and treatment targets. The integration of clinical trials with omics analyses may help reveal novel pathways and biomarkers.23 As there are multiple pathways leading to neutrophilic inflammation,20, 24 the key mechanisms are likely heterogeneous across different disease context (or depending on triggers); thus, the experimental works by Bich et al.9 are relevant in that they evaluated the roles of SAA1 in virus-induced neutrophilic asthma.

Thirdly, the clinical utility of SAA1 as a biomarker needs to be validated. Given the multi-faceted nature of neutrophilic inflammation, participants should be further characterized based on exacerbation status, viral infection, and oral steroid use. Diagnostic utility, usually assessed with area under the curve analyses, was not presented in this study.9 Also, the degree of diagnostic precision is not clear from the data in Table 2,9 as sputum neutrophilia was not infrequent even in the low-SAA1 group. Although serum SAA1 levels were significantly higher in neutrophilic asthmatic patients than in the compared groups, the levels were not different between non-asthmatic controls and non-neutrophilic asthmatic patients (Table 1),9 indicating that SAA1 is a broad marker of systemic neutrophilic inflammation rather than asthma-specific. Comorbidities that may confound serum levels of SAA1 should also be explored and controlled in further validation studies. The diagnostic utility should be confirmed in external cohorts and compared with other biomarkers previously reported for neutrophilic asthma.

Despite the prevailing questions, the work by Bich et al.9 is a welcoming addition, contributing to high hopes in the search of neutrophilic asthma biomarkers. Neutrophilic asthma has major unmet needs, and further understanding as to why neutrophil numbers are increased in asthmatic airways will facilitate the development of biomarkers and help develop precision therapies for severe asthmatic patients who are not responsive to T2-targeted biologics. These advances would be the next breakthrough in asthma management.