Sputum neutrophil counts are associated with more severe asthma phenotypes using cluster analysis

Abstract

Background: Clinical cluster analysis from the Severe Asthma Research Program (SARP) identified 5 asthma subphenotypes that represent the severity spectrum of early-onset allergic asthma, late-onset severe asthma, and severe asthma with chronic obstructive pulmonary disease characteristics. Analysis of induced sputum from a subset of SARP subjects showed 4 sputum inflammatory cellular patterns. Subjects with concurrent increases in eosinophil (≥2%) and neutrophil (≥40%) percentages had characteristics of very severe asthma.

Objective: To better understand interactions between inflammation and clinical subphenotypes, we integrated inflammatory cellular measures and clinical variables in a new cluster analysis.

Methods: Participants in SARP who underwent sputum induction at 3 clinical sites were included in this analysis (n = 423). Fifteen variables, including clinical characteristics and blood and sputum inflammatory cell assessments, were selected using factor analysis for unsupervised cluster analysis.

Results: Four phenotypic clusters were identified. Cluster A (n = 132) and B (n = 127) subjects had mild-to-moderate early-onset allergic asthma with paucigranulocytic or eosinophilic sputum inflammatory cell patterns. In contrast, these inflammatory patterns were present in only 7% of cluster C (n = 117) and D (n = 47) subjects who had moderate-to-severe asthma with frequent health care use despite treatment with high doses of inhaled or oral corticosteroids and, in cluster D, reduced lung function. The majority of these subjects (>83%) had sputum neutrophilia either alone or with concurrent sputum eosinophilia. Baseline lung function and sputum neutrophil percentages were the most important variables determining cluster assignment.

Conclusion: This multivariate approach identified 4 asthma subphenotypes representing the severity spectrum from mild-to-moderate allergic asthma with minimal or eosinophil-predominant sputum inflammation to moderate-to-severe asthma with neutrophil-predominant or mixed granulocytic inflammation.

Keywords: Severe Asthma Research Program; cluster analysis; eosinophils; neutrophils; phenotype; severe asthma; sputum.

FIGURE 1

Sputum inflammatory cells. Shown are box plots of % eosinophils and % neutrophils from sputum specimens. There was a wide range of values for both cell types in all sputum clusters. There are statistically significant differences among the four groups, (p=0.001 for eosinophils, p<0.0001 for neutrophils) with differentiation between clusters in contrast to blood inflammatory cells (see ). P values shown on graph are paired comparisons between two clusters of similar disease severity.

FIGURE 2

Sputum granulocyte patterns in the four clusters. Subjects were assigned into one of four possible patterns based on % sputum eosinophils (<2% or >=2%) and % sputum neutrophils (<40% or >=40%). All four granulocyte patterns are present in every cluster, but in general, as asthma severity increases (Clusters C and D), sputum neutrophils increase in nearly all subjects and sputum eosinophils persist in a third to half of these subjects despite increasing doses of inhaled and/or oral corticosteroids.