Previous studies have called attention to heterogeneity in eosinophil phenotypes among people with asthma (7
), but estimates for the prevalence of noneosinophilic asthma in these prior studies were generated from single sputum samples. The importance and novelty of our study is that we show that sputum eosinophilia is persistently absent in a large subgroup of mild-to-moderate asthma when sputum is analyzed repeatedly over time. Subjects whose asthma was persistently noneosinophilic had a very poor response to a period of intense combined treatment with asthma controller treatments, despite showing good responses to bronchodilator therapy. These data suggest that a sizeable subgroup of mild-to-moderate asthma has a disease phenotype that is not the usual eosinophilic, steroid-responsive subtype, but a different subtype whose mechanisms are poorly understood and for which new controller treatments are needed.
Among subjects with asthma with eosinophilia, some had it persistently and others had it intermittently. The clinical characteristics of eosinophilic asthma are most evident in the persistently eosinophilic subtype. Here we found a relatively homogeneous asthma phenotype characterized by normal body weight and greater bronchial hyperresponsiveness. Our data reporting the association of normal body mass index with persistent eosinophilic asthma are novel, and a relative paucity of eosinophils in the airway of overweight or obese people with asthma may contribute to the relative glucocorticoid insensitivity observed in obese people with asthma (34
). Our findings also support the emerging concept that eosinophils play a role in metabolic homeostasis and prevention of obesity (37
). Specifically, studies in mice show that eosinophils in white adipose tissue secrete Th2 cytokines to induce alternative activation of macrophages, promote glucose homeostasis, and prevent obesity.
The intermittently eosinophilic subgroup was larger than the persistently eosinophilic subgroup and was characterized by lower average eosinophil percentages that often fell below the 2% cutoff for eosinophilia in repeated sputum analyses. Importantly, the treatment responses of intermittently eosinophilc asthma mirrored those of persistently eosinophilic asthma. Thus, intermittently eosinophilic asthma can be considered a less severe form of eosinophilic asthma and not necessarily a distinct pathologic phenotype. The advantage of categorizing intermittent and persistent eosinophil subtypes separately is that this categorization emphasizes that repeated measures of sputum eosinophils are necessary to properly identify eosinophilic asthma. In this regard, it is notable that peripheral blood eosinophils and FENO cannot be relied on to identify eosinophilic asthma phenotypes, because both of these tests were not sufficiently sensitive as biomarkers of sputum eosinophilia.
Persistently noneosinophilic asthma comprised approximately half of subjects with asthma with mild-to-moderate disease in our study. The median values for sputum eosinophils in this subgroup were consistently closer to 0% than to the 2% cutoff on each of the repeated measure time-points. This provides confidence that these subjects indeed were noneosinophilic and were not a subgroup falling just below the 2% cutoff and thereby possibly explained by measurement variability. We considered the possibility that the low sputum eosinophil percentage could reflect good asthma control in the subjects studied and that estimates could be much higher in patients with less optimal disease control. In analyses to address this possibility, we surprisingly found that only 15% of subjects who did not achieve good control with ICS treatment had sputum eosinophilia; this compared with 26% of subjects with sputum eosinophilia among subjects with asthma who did achieve good asthma control with ICS treatment. We interpret this apparent discrepancy as revealing how the poor asthma control subgroup is enriched in noneosinophilic people with asthma (because steroids do not control asthma in patients with this disease subtype).
The subjects with asthma who were persistently noneosinophilic did not have an FEV1
response to a 2-week period of intense treatment with oral and ICSs and oral zafirlukast. They had the potential to respond as evidenced by a 10% improvement in FEV1
with albuterol treatment. Although other studies have shown sputum eosinophils to be important in treatment responses to corticosteroids in asthma (7
), and we have shown previously that the Th2-low molecular phenotype of asthma (which has low airway eosinophils) does not respond well to corticosteroids (10
), the data we report here provide stronger evidence that noneosinophilic asthma represents a disease phenotype that does not benefit from corticosteroid treatment. The high doses of corticosteroids given in the period of intense combined therapy and the documentation of a bronchodilator response to albuterol show that inadequate steroid dosing and fixed airflow obstruction are not explanations for poor steroid responsiveness in this subgroup. Our careful classification of eosinophil phenotype is also important because we show that it is the persistently noneosinophilic subgroup that does not respond; the intermittently eosinophilic subgroup has treatment responses that mirror those of the persistently eosinophilic subgroup. Additional studies of steroid responsiveness in persistently noneosinophilic asthma are needed and should include evaluation of asthma exacerbation outcomes. However, previous clinical trials of corticosteroids in asthma have also identified sizeable subgroups with poor treatment responses (9
), and we argue that these poor steroid treatment responses are because of asthma phenotypes, such as persistent noneosinophilia, that are not steroid sensitive.
The clinical data from the persistently noneosinophilic subjects with asthma showed that this is not a homogeneous group of patients with obvious clinical identifiers. In preliminary unsupervised analyses to identify possible clusters of clinical characteristics within the persistently noneosinophilic asthma subgroup, we identified sex, body weight, presence or absence of neutrophilia, and sensitivity to fungal aeroallergens as important variables likely to underlie mechanisms of noneosinophilic asthma. Some of these variables have also been associated with noneosinophilic asthma phenotypes in other studies (31
). Notably, however, our analysis identified a novel cluster characterized by asthma occurring in nonobese males who develop asthma in childhood and who have sensitivity to Alternaria
. Male children frequently have sensitization to Alternaria
, and epidemiologic studies suggest that, unlike other forms of male childhood asthma, it does not remit (42
). Thus, adult males with asthma and Alternaria
sensitivity may represent a cohort of males whose asthma persists from childhood. The mechanisms by which Alternaria
causes sensitization and asthma in males are unknown, but Th17 cells may be involved because they are already implicated in asthma, are known to be involved in fungal immune responses at mucosal surfaces, and do not always use eosinophils as effector cells.
The identification of persistently noneosinophilic asthma as a common phenotype of mild to moderate asthma that responds poorly to currently available asthma controller medications has important implications for future research in asthma. One implication is that clinical studies in subjects with asthma should characterize the eosinophil phenotype of enrolled patients to enable better understanding of disease mechanisms and treatment responses. Another is that in vitro and mouse models of asthma, currently heavily weighted toward study of mechanisms of eosinophilic airway disease, need to better emphasize the study of mechanisms of noneosinophilic airway disease.