Mucus production and tolerance to inhaled ovalbumin in a guinea pig model of chronic asthma

E John, A Jackson and K J Broadley, Welsh School of Pharmacy, Cardiff University, King Edward VII Avenue, Cardiff, CF10 3XF and Novartis, Horsham, RH12 4HE, UK.

In sensitised guinea pigs, a single inhalation of ovalbumin (OA) stimulates early (EAR) and late phase (LAR) bronchoconstriction, hyperreactivity to inhaled histamine (AHR), and inflammatory cell recruitment to the airways; all features of human asthma (Toward et al ,2004). However, excessive mucus production is also an important feature of asthma, which may contribute to airway obstruction ( Jackson, 2001). Few studies have investigated the relationship between airway mucus accumulation and lung function responses following allergen challenge. This study investigated the effect of chronic OA challenges on EAR, LAR, AHR, cellular infiltration and mucus production.

Male Dunkin-Hartley guinea pigs (200-250g) were sensitised with 100μg OA and 100mg AlOH3 i.p. on days 1 and 5. Guinea pigs received one of 3 OA exposure (60mins) protocols, commencing on day 15: nebulised OA 0.01% on day 15; nebulised OA 0.01% on days 15, 17, 19, 21, 23, 25, 27 and 29; nebulised OA 0.01% on day 15, and 0.1% (60 mins) on days 17, 19, 21, 23, 25, 27 and 29 with mepyramine maleate (30 mg.kg-1i.p.) 60mins prior to OA challenge on days 17-27. Whole body plethysmography measured specific airway conductance (sGaw) before and at intervals up to 12 hrs and at 24 hrs after OA challenge on days 15 and 29. 24 hrs after the last OA challenge, guinea pigs received a lethal dose of sodium pentobarbitone (400mg/kg, i.p.), bronchoalveolar lavage carried out to determine cell numbers and lungs inflated with formaldehyde (1ml.100g-1). Fixed tissues were embedded in paraffin wax, sectioned using a Leica microtome and stained with alcian blue/periodic acid-Schiff (ABPAS) for histological analysis of mucus content. Total mucus content of the bronchiolar epithelium was expressed as % ABPAS-positive area.

Sensitised guinea pigs (n=6) exposed to single OA challenge revealed an immediate EAR with a reduction in sGaw (-74 ± 4.2%), and LAR (-22 ± 4.5%) 7-10 hrs later. AHR, and increased total cells, eosinophils and macrophages (3.5-fold, 11-fold and 2-fold respectively, p<0.5) was observed 24hrs after OA challenge, but no increase in ABPAS-positive bronchiole epithelial area, suggesting no mucus production. Chronic low-dose OA challenge revealed no EAR or LAR following OA challenge on day 29, but did demonstrate AHR, increased total cells, eosinophils and macrophages (4-fold, 12-fold and 2.5-fold respectively, p<0.5) and a 5-fold increase in the ABPAS-positive bronchiole epithelial area. After chronic high-dose OA challenge, the final OA exposure revealed an EAR (-62 ± 4.8%) and a LAR (-21 ± 3.2%) between 6-10 hrs, AHR, and increases in total cells, eosinophils and macrophages (6-fold, 15-fold and 2.5-fold respectively, p<0.5). A 6-fold increase in the ABPAS-positive bronchiole epithelial area was observed.

This study shows that both chronic low and high-dose OA challenges stimulate an increase in bronchiolar epithelial mucus of sensitised guinea pigs. However, chronic low-dose OA challenge failed to stimulate either EAR or LAR, suggesting tolerance of these responses. Guinea pigs need chronic challenge with high-dose OA to demonstrate EAR, LAR, AHR, cellular infiltration to the airways and increased mucus production.

Toward, TJ et al (2004) Amer J Resp Crit Care Med. 169,227-234.
Jackson AD (2001) Trends Pharmacol Sci. 22, 38-45.

Acknowledgement:MRC, Novartis, June Giddings, Gareth Jones and Francis Schindler.