Partial C-fiber ablation modulates diphenylmethane-4,4′-diisocyanate (MDI)-induced respiratory allergy in Brown Norway rats

Development of a respiratory sensitization/elicitation protocol of toluene diisocyanate (TDI) in Brown Norway rats to derive an elicitation-based occupational exposure level

Toluene diisocyanate (TDI), a known human asthmagen, was investigated in skin-sensitized Brown Norway rats for its concentration×time (C×t)-response relationship on elicitation-based endpoints. The major goal of study was to determine the elicitation inhalation threshold dose in sensitized, re-challenged Brown Norway rats, including the associated variables affecting the dosimetry of inhaled TDI-vapor in rats and as to how these differences can be translated to humans. Attempts were made to duplicate at least some traits of human asthma by using skin-sensitized rats which were subjected to single or multiple inhalation-escalation challenge exposures. Two types of dose-escalation protocols were used to determine the elicitation-threshold C×t; one used a variable C (Cvar) and constant t (tconst), the other a constant C (Cconst) and variable t (tvar). The selection of the "minimal irritant" C was based an ancillary pre-studies. Neutrophilic granulocytes (PMNs) in bronchoalveolar lavage fluid (BAL) were considered as the endpoint of choice to integrate the allergic pulmonary inflammation. These were supplemented by physiological measurements characterizing nocturnal asthma-like responses and increased nitric oxide in exhaled breath (eNO). The Cconst×tvar regimen yielded the most conclusive dose-response relationship as long C was high enough to overcome the scrubbing capacity of the upper airways. Based on ancillary pre-studies in naïve rats, the related human-equivalent respiratory tract irritant threshold concentration was estimated to be 0.09ppm. The respective 8-h time-adjusted asthma-related human-equivalent threshold C×t-product (dose), in 'asthmatic' rats, was estimated to be 0.003ppm. Both thresholds are in agreement of the current ACGIH TLV(®) of TDI and published human evidence. In summary, the findings from this animal model suggest that TDI-induced respiratory allergy is likely to be contingent on two interlinked, sequentially occurring mechanisms: first, dermal sensitizing encounters high enough to cause systemic sensitization. Second, when followed by inhalation exposure(s) high enough to initiate and amplify an allergic airway inflammation, then a progression into asthma may occur. This bioassay requires an in-depth knowledge on respiratory tract dosimetry and irritation of the involved test substance to clearly understand the dosimetry causing C- and/or C×t-dependent respiratory tract irritation and eventually asthma.

Ovalbumin sensitizes vagal pulmonary C-fiber afferents in Brown Norway rats

Sensitization of vagal lung C fibers has been postulated to contribute to the development of asthma, but support for this notion is still lacking. We investigated the characteristics and function of pulmonary C fibers (PCFs) in ovalbumin (OVA)-sensitized Brown Norway rats, an established animal model of asthma. Rats were sensitized with intraperitoneal injection of OVA or were treated with saline (control). In study 1, with the use of open-chest and artificially ventilated rats, inhalation of 5% OVA aerosol evoked an augmented increase in total lung resistance in the OVA-sensitized rats, compared with the control rats. Bilateral vagotomy or subcutaneous pretreatment with a high-dose of capsaicin for blocking of C-fiber function equally attenuated this augmented total lung resistance response, suggesting the involvement of PCFs. In study 2, with the use of anesthetized, spontaneously breathing rats, right atrial injection of capsaicin (1 microg/kg; a PCF stimulant) evoked an augmented apneic response in the OVA-sensitized rats, compared with the control rats. In study 3, with the use of open-chest, paralyzed, and artificially ventilated rats, the afferent PCF responses to right atrial injection of capsaicin (0.5 and 1.0 microg/kg), phenylbiguanide (8 microg/kg; a PCF stimulant), or adenosine (0.2 mg/kg; a PCF stimulant) were enhanced in the OVA-sensitized rats, compared with the control rats. However, the baseline activities of PCFs and their afferent responses to mechanical stimulation by lung hyperinflation in the OVA-sensitized and control rats were comparable. Our results suggested that OVA-sensitized Brown Norway rats possess sensitized vagal PCFs, which may participate in the development of the airway hyperreactivity observed in these animals.

Brown Norway rat asthma model of diphenylmethane-4,4'-diisocyanate (MDI): impact of vehicle for topical induction

The development of this Brown Norway (BN) rat asthma model was focused on the duplication of at least some hallmarks of human diisocyanate asthma using the skin as the initial priming route of exposure. Equal total doses of polymeric diphenylmethane-diisocyanate (MDI) were applied to similar surface areas either dissolved in di-n-octyl sebacic acid ester (20%) (SEBA), in acetone:olive oil (20%) (AOO) or undiluted. The elicitation of respiratory allergy utilized four repeated nose-only inhalation challenges of 30 min with 39 mg/m(3) MDI-aerosol approximately every 2 weeks. Emphasis was directed towards the analysis of respiratory responses delayed in onset. Endpoints suggestive of an allergic inflammatory response were examined by bronchoalveolar lavage (BAL) 1 day after the last inhalation challenge and comprised protein, LDH, cytodifferentiation of BAL cells, MCP-1, and some Th1 and Th2 cytokines. MCP-1 and cytokines were comparatively determined in three compartments: BAL fluid, BAL cells, and lung-associated lymph nodes (LALN). In all groups sensitized topically to MDI typical delayed-onset respiratory responses occurred. The lung and LALN weights, BAL-protein and -LDH were significantly increased as compared to the naïve control group challenged identically. There was compelling evidence of a neutrophilic rather than an eosinophilic inflammatory response. The patterns of interleukin (IL) IL-1alpha, TNF-alpha, IFN-gamma, GM-CSF, and IL-4 differed appreciably from one compartment to another and were essentially maximal in BAL cells. In contrast, MCP-1 was increased to the same extent in all compartments measured. Collectively, changes were slightly, although consistently more pronounced when using SEBA as vehicle when compared with the vehicle AOO or undiluted MDI. Notable was a discordance of cytokine profiles and respiratory responses. In conclusion, the priming potency of topically administered MDI and subsequent asthma-like responses following repeated inhalation exposures appear to be dependent on multiple factors, one of them appears to be associated with the type of matrix used to dissolve MDI. This animal model provides a versatile and robust experimental tool to evaluate and assess at least some features of MDI-related asthma.