A single inhalation of antigen in patients with atopic asthma results in an early phase bronchoconstrictor response (EAR), immediately after exposure, and approximately 60% of patients develop a late bronchoconstrictor response (LAR) 6 to 12 hours after the challenge (Pepys et al, 1975). The LAR is accompanied by airway hyperreactivity (AHR) to stimuli such as histamine, and cellular infiltration to the lungs (Booj-Noord et al, 1971). This study was designed to optimize sensitisation protocols to achieve airway function, airway hyperreactivity (AHR) and cellular infiltration responses following an ovalbumin challenge, in a guinea pig model of asthma.

Male Dunkin-Hartley guinea-pigs (300-350g) were sensitised with an i.p. injection of either 10µg or 100µg OA and 100mg Al₂OH₃ in 1ml of normal saline, as a single injection, or an additional injection of the same solution, administered on day 5. All procedures commenced 12 – 16 days later. Animals were exposed to low dose OA (10µg/ml) for 1h, in a sealed exposure chamber. Whole body plethysmography was used to monitor airway function before, and at intervals up to 24h post-challenge, recorded as specific airways conductance (sG_aw) (Griffiths-Johnson et al, 1988). Airway reactivity to 1mM histamine (20s, nose-only) was measured 24h before and after allergen challenge. Bronchoalveolar lavage was carried out 24h after challenge to measure total and differential cell counts.

Animals sensitised with 10 m g OA on day 1 did not reveal a bronchoconstrictor response to inhaled OA. The incorporation of an additional booster injection on day 5 resulted in an EAR immediately after exposure to OA, with a significant mean (n=6) reduction in sG_aw of –44.2 ± 3.8%. A LAR was not evident 7-10h after challenge. Sensitisation with a single injection of 100 µg OA revealed an EAR immediately after challenge, followed by a LAR 7h post-challenge, with reductions in sG_aw of –71.9 ± 3.5% and –22.3 ± 3.7% respectively. The LAR resolved within 1h of onset . Administration of the additional injection on day 5 resulted in an EAR immediately after exposure, and a LAR 7h post-challenge, with reductions in sG_aw  of –63.4 ± 2.8% and -22 ± 3.2% respectively. The LAR resolved within 2h . Inhaled histamine produced no bronchoconstriction 24h before any of the challenges. Sensitisation with 10 µg OA, administered as both single and booster injections revealed no response to histamine 24h post-OA challenge. Administration of 100 µg OA as single and booster injections revea led significant bronchoconstrictor responses to inhaled histamine, 24h after exposure to OA, with reductions in sG_aw of –14.4 ± 2.6% and –30.5 ± 3.4% respectively, indicating AHR. All sensitisation protocols used were able to reveal a significant increase in both total (minimum 3 fold) and differential (eosinophils- min 10 fold and macrophages- min 3 fold) cell counts 24h after exposure to OA, when compared to non-sensitised or saline-challenged sensitised animals. Sensitisation with 100 µg OA and the booster injection produced significantly greater total (16 fold) and differential cell counts than all other sensitisation protocols.

It can be concluded that an OA challenge can reveal an EAR, LAR, cellular infiltration, and AHR to histamine, following sensitisation with 100 µg OA on days 1 and 5. Lower levels of sensitization produced cell influx without other responses, indicating that it is not sufficient to induce cell infiltration for the other important parameters of the asthmatic response. Thus, cell influx may have to exceed a threshold level to become an index of the asthmatic response in animal models. We also showed that AHR to histamine is associated with the LAR.

Booj-Noord, H. et al(1971) J. Allergy Clin. Immunol.48, 344-354
Griffiths-Johnson, D.A. et al(1988) J. Pharmacol. Meth. 19, 233-242
Pepys, J & Hutchcroft, B.J. (1975) Am. Rev. Resp. Dis. 112, 829-8591

Supported by a BBSRC studentship to NS in collaboration with GlaxoSmithKline