Brown Norway Rat Asthma Model of Diphenylmethane 4,4′-Diisocyanate

Abstract

Allergic asthma is a complex chronic inflammatory disease of the airways and its etiology is multifactorial. It involves the recruitment and activation of many inflammatory and structural cells, all of which release inflammatory mediators that result in typical pathological changes of asthma. The features of asthma addressed in this Brown Norway (BN) rat animal model include an analysis of cellular infiltrations in the lung, inflammatory factors in bronchoalveolar lavage (BAL), total immunoglobulin E (IgE) production in serum, and changes in delayed-onset respiratory reactions upon four inhalation challenges (every 2 wk) with polymeric diphenylmethane diisocyanate (MDI) aerosol in two groups of topically sensitized rats. The dependence on the induction-related variables was analyzed by using almost identical surface area doses but different total doses per animal. This regimen caused acute exacerbations of delayed-onset respiratory reactions, for which intensity increased with each challenge. After the fourth challenge BAL neutrophils, lymphocytes, eosinophils, cell counts, protein, and lactate dehydrogenase (LDH) as well as lung weights were significantly increased in sensitized rats relative to naive but challenged controls. Histopathology revealed activated bronchial lymphatic tissue, increased recruitment of inflammatory cells, the beginning of peribronchial/peribronchiolar fibrosis, thickening of alveolar septae, and vascular hypertrophy. Total IgE in serum was significantly increased in sensitized rats. Thus, high-dose topical induction to, and repeated inhalation challenges with, MDI was associated with a marked neutrophilic and a less consistent eosinophilic inflammatory response. With regard to the relative sensitivity of endpoints, those that integrate independently a series of complex physiological events appeared to be most practical to probe positive responses in this animal model. These include postchallenge changes in Penh to identify respiratory responses delayed in onset as well as inflammatory changes in BAL. In summary, this extension of a previous study that used 16 mg MDI/m³ instead of 39 mg MDI/m³ that was used in the current study for challenge exposures demonstrates that protocol variables are most critical for the outcome of test. Moreover, the sensitivity of this bioassay to define the typical asthma phenotype can be markedly improved by measurements of respiratory responses delayed in onset rather than immediate in onset. Accordingly, to increase the efficacy of this asthma model moderately irritant concentrations of the hapten have to be used for challenge and at least three to four adequately spaced challenge exposures are required to elicit a typical asthma phenotype.