Endotyping severe asthma with qPCR for five mast cell transcripts in induced sputum correlates with microscopy

Asthma is a heterogenous disease characterized by airway inflammation and variable expiratory airflow limitation resulting in variable respiratory symptoms. Characterization of airway inflammation is important to choose the optimal treatment for severe asthma patients eligible for biological treatment. However, counting cells in induced sputum samples is a time-consuming process, highly dependent on personal skills. Replacing cell counting with qPCR for transcripts of selected Mast cell and basophil genes may provide more reproducible results. <b>Aims:</b> The objective of this study was to compare qPCR with microscopy in asthma endotyping. <b>Methods:</b> A qPCR method measuring 5 mast cell/basophil genes was applied on induced sputum samples from 30 severe asthma patients and compared with microscopy. Target gene Ct-values (CPA3, GATA2, HDC, MS4A2, TPSAB1/TPSB2) were referenced to household β-actin Ct values as a measure of relative mRNA abundance of the target in each sample. Target/β-actin-ratios in eosinophilic and non-eosinophilic groups determined by microscopy with an eosinophil threshold of 3% in 400 cells were compared using Wilcox rank sum test. <b>Results:</b> The study demonstrated a statistical difference in relative mRNA abundance for 4 mast cell/basophil specific genes. CPA3, GATA2, HDC and MS4A2 were elevated in eosinophilic asthma versus non-eosinophilic asthma patients. The 5 genes along with a target/β-actin-ratio cut-off ≥2 generated sensitivity=79%, specificity=94%, NPV=83% and PPV=92% compared to microscopy. <b>Conclusion:</b> This pilot study illustrates the potential of qPCR to endotype asthma at high resolution.&nbsp;It may improve the clinical impact of asthma endotyping.