Using Sputum to Measure Type 2 Airway Inflammation During a Severe Acute Exacerbation in Paediatric Asthmatics

Background: Asthma is an immunological disease which often starts in early childhood. Patients present with a wheeze which regularly coincides with a viral respiratory tract infection. Wheeze in pre-school children is driven by Th1 immunity, which progresses to Th2 immunity; thought to be due to allergic sensitisation. Underlying airway immune mechanisms have yet to be understood in children, especially during an acute exacerbation. Methods to measure airway inflammation in acute asthmatic children are limited with sputum providing a safe, non-invasive option however, it is unknown whether this medium is fit for purpose.

Aim: The aim of this study was to assess the suitability of sputum for measuring airway inflammation in asthmatic children during an exacerbation. As part of this assessment airway inflammation was compared between acutely unwell and clinically stable asthmatics.

Method: Children aged 5-16 years, with a doctor diagnosed wheeze (n=16), produced a spontaneous sputum sample. Sputum was induced from an unpaired ‘stable’ cohort (n=5). Airway inflammation was assessed using flow cytometry to identify cell populations including neutrophils, eosinophils, helper (Th2, peTh2) and cytotoxic (Tc2) T-lymphocytes, type-2 innate lymphoid cells (ILC2s) and basophils.

Result: A range of cell populations were identified in sputum; however, sample sizes can be small which reduces cell yield. This limits which cells can be measured with many samples too small to identify rare lymphocytes. CD4+ (acute 0.275% vs stable 2.12%) and CD8+ T-lymphocytes (acute 0.38% vs stable 1.16%) and macrophages (acute 24.19% vs stable 80.52%) were higher in stable children.

Conclusion: Sputum provides a safe way to assess airway inflammation during an acute exacerbation with caveat that sample size and quality are limited. Helper and cytotoxic T-lymphocyte airway populations are significantly higher in stable compared with acute children. Functional studies are required to explore drivers of acute and chronic asthma.