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Increased detection of interleukin-;5 in sputum by addition of protease inhibitors

Asthma Research Group, Depts of ¹ Medicine and ² Pathology, St. Joseph's Hospital-McMaster University, Hamilton, Canada. ³ Dept of Immunology, Mayo Clinic Medical School, Rochester, USA

CORRESPONDENCE: G. Cox, Firestone Institute for Respiratory Health, Joseph's Hospital, 50 Charlton Avenue East, Hamilton, Ontario, L8N 4A6, Canada. Fax: 1 9055216132

Keywords: asthma, inflammation, interleukin-;5, proteases, protease inhibitors, sputum chemistry

Received: November 21, 2000
Accepted June 12, 2001

This study was supported by the Asthma Society of Canada. R. Leigh is a South African Pulmonology Society Boehringer-Ingelheim Fellow. G. Cox and F.E. Hargreave were supported in part by the Father Sean O'Sullivan Research Centre at St. Josephs's Hospital, Hamilton. F.E. Hargreave and G.J. Gleich were supported in part by a grant from NIH AI 34577.

The measurement of interleukin (IL)-;5 in sputum is problematic, with interfering factors affecting immunoassay. The authors investigated whether sputum proteases could be acting as interfering factors by studying the effect of protease inhibitors (PI) on sputum IL-;5 measurement.

Induced sputa from 20 subjects with asthma were divided into aliquots, processed with and without protease inhibitors (in low and high concentrations) and the levels of IL-;5 (spiked and endogenous) measured by enzyme immunoassay were compared.

The concentration of sputum IL-;5 was significantly increased by PI, with median (interquartile range) levels processed with no, low and high PI concentrations being 0 (0), 41.8 (75.6) and 66.1 (124.4) pg·mL^–1, respectively. There was also a significant increase in percentage recovery of spiked IL-;5. Although high concentrations of PI reduced cell viability, there was no effect on total or differential cell counts and low concentrations of PI had no effect on cell counts or viability.

Levels of endogenous interleukin-;5 in sputum of asthmatic subjects can be significantly increased by the addition of protease inhibitors, and samples which would be regarded as negative for interleukin-;5 without protease inhibitors may instead have considerable amounts of interleukin-;5 detected.

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