ASSESSMENT OF RESPIRATORY HYPERSENSITIVITY IN GUINEA PIGS SENSITIZED TO TOLUENE DIISOCYANATE: A COMPARISON OF SENSITIZATION PROTOCOLS

Evaluation of animal toxicity studies with diisocyanates regarding presence of thresholds for induction and elicitation of respiratory allergy by quantitative weight of evidence

Animal toxicity studies on diisocyanates were evaluated using quantitative weight of evidence (QWoE) to test the hypothesis that the dose-response curve shows a threshold for the induction and/or elicitation of respiratory sensitization. A literature search identified 59 references that included at least two concentration groups of the diisocyanate and a vehicle-exposed concurrent control in the study design. These studies were subjected to a QWoE-assessment applying scoring criteria for quality and relevance/strength of effects relevant to the selected endpoint of respiratory sensitization. Overall, the studies assessing dose/concentration-response for diisocyanates with the endpoint, respiratory sensitization, were heterogenous regarding study design, animal models used, endpoints assessed, and quality. Only a limited number of the studies subjected to the QWoE-assessment allowed drawing conclusions about possible thresholds for respiratory sensitization. Highest quality and relevance/strength of effects scores were obtained by a series of studies specifically designed to investigate a potential threshold for elicitation of respiratory sensitization in the Brown Norway (BN) rat. These studies applied an elaborate study design to optimize induction of respiratory sensitization and reduce interference by respiratory tract irritation. In summary, the available studies provided moderate to good support for the existence of a threshold for elicitation and limited to moderate support for a threshold regarding induction of respiratory allergy by diisocyanates in experimental animals. However, a quantitative extrapolation of threshold values established in rodents to humans remains complex.

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.

Chemical allergy: translating biology into hazard characterization

The induction by chemicals of allergic sensitization and allergic disease is an important and challenging branch of toxicology. Skin sensitization resulting in allergic contact dermatitis represents the most common manifestation of immunotoxicity in humans, and many hundreds of chemicals have been implicated as skin sensitizers. There are far fewer chemicals that have been shown to cause sensitization of the respiratory tract and asthma, but the issue is no less important because hazard identification remains a significant challenge, and occupational asthma can be fatal. In all areas of chemical allergy, there have been, and remain still, intriguing challenges where progress has required a close and productive alignment between immunology, toxicology, and clinical medicine. What the authors have sought to do here is to exemplify, within the framework of chemical allergy, how an investment in fundamental research and an improved understanding of relevant biological and biochemical mechanisms can pay important dividends in driving new innovations in hazard identification, hazard characterization, and risk assessment. Here we will consider in turn three specific areas of research in chemical allergy: (1) the role of epidermal Langerhans cells in the development of skin sensitization, (2) T lymphocytes and skin sensitization, and (3) sensitization of the respiratory tract. In each area, the aim is to identify what has been achieved and how that progress has impacted on the development of new approaches to toxicological evaluation. Success has been patchy, and there is still much to be achieved, but the journey has been fascinating and there have been some very important developments. The conclusion drawn is that continued investment in research, if coupled with an appetite for translating the fruits of that research into imaginative new tools for toxicology, should continue to better equip us for tackling the important challenges that remain to be addressed.

Comparative Analysis of Pulmonary Irritation by Measurements of Penh and Protein in Bronchoalveolar Lavage Fluid in Brown Norway Rats and Wistar Rats Exposed to Irritant Aerosols

The object of this study was to compare in two strain of rats (Brown Norway and Wistar) that were acutely exposed (1 x 6 hrs) to aerosols of polymeric methylenediphenyl-4,4'-diisocyanate (MDI), the relative sensitivity of the functional endpoint "enhanced pause (Penh)" with endpoints in bronchoalveolar lavage fluid (BALF) indicative of lower respiratory tract irritation. This approach included an analysis of both the concentration-response and the time-course of effects. Penh was measured repeatedly prior to (baseline) and during a stepped methacholine (MCh) bronchoprovocation challenge in barometric plethysmographic chambers on postexposure days 1, 3, and 7. Results show that total protein in BALF was a sensitive endpoint to probe early effects caused by exposure to irritant polyisocyanate aerosol. Baseline Penh increased in a concentration-dependent manner and paralleled closely the magnitude of increased BALF-protein on postexposure day 1. The time-course of changes in Penh complemented those of BALF-protein observed in previous studies. The incremental increase of Penh during a stepped MCh-challenge was attenuated with increasing baseline Penh values. In summary, these data show that the exposure to irritant concentrations of this aerosolized polyisocyanate caused a transient perturbation of the blood/air-barrier, which can suitably be probed by measurements of protein in BALF. Although Penh appears to parallel these changes, it displays substantially greater variability. The increase in Penh is apparently more related to changes in breathing patterns either caused by stimulation of receptors or changes in the mechanical properties of lung parenchyma. In regard to Penh and BALF-protein, both strains of rats were equally susceptible, although some endpoints displayed a higher variability in Brown Norway rats when compared to Wistar rats. This supports the conclusion that Wistar rats afford a higher diagnostic resolution than BN rats.

Experimental approaches to evaluate respiratory allergy in animal models

Asthma is defined as a chronic disease of the entire lung and asthma attacks may either be immediate, delayed or dual in onset. 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 mediators that result in typical pathological changes of asthma. A wealth of clinical and experimental data suggests that allergic asthma is due to an aberrant lung immune response mediated through T-helper type 2 (Th2) cells and associated cytokine-signaling pathways. The pathology of asthma is associated with reversible narrowing of airways, associated with prominent features that involve structural changes in the airway walls and extracellular matrix remodeling including abnormalities of bronchial smooth muscle, eosinophilic inflammation of the bronchial wall, hyperplasia and hypertrophy of mucous glands. The primary objective of respiratory allergy tests is to determine whether a low-molecular-weight chemical (hapten) or high-molecular-weight compound (antigen) exhibits sensitizing properties to the respiratory tract. This may range from reactions occurring in the nose (allergic rhinitis), in the bronchial airways (i.e., allergic bronchitis, asthma) or alveoli (e.g., hypersensitivity pneumonitis). Current assays utilize several phases, viz. an induction phase, which includes multiple exposures to the test compound (sensitization) via the respiratory tract (e.g., by intranasal or intratracheal instillations), by inhalation exposures or by dermal contact, and a single or multiple challenge or elicitation phase. The challenge can either be with the chemical (hapten), the homologous protein conjugate of the hapten or the antigen. The choice depends both on the irritant potency and the physical form (vapor, aerosol) of the hapten. The appropriate selection of concentrations (dosages) both for the induction and elicitation of respiratory allergy appears to be paramount for the outcome of test. Endpoints to characterize positive response range from the induction of immunoglobulins, cytokine or lymphokine patterns in serum (or the lung) to (patho-)physiological reactions typifying asthma. None of the currently applied animal models duplicate all features of human asthma. Accordingly, the specific pros and cons of the selected animal model, including protocol variables, animal species and strain selected, must be interpreted cautiously in order to arrive at a meaningful extrapolation for humans.

Respiratory hypersensitivity in guinea pigs sensitized to 1,6-hexamethylene diisocyanate (HDI): comparison of results obtained with the monomer and homopolymers of HDI

This study used guinea pigs that were sensitized to the biuret or isocyanurate type homopolymers of 1,6 hexamethylene diisocyanate (HDI). Induction was either by intradermal injection or repeated inhalation exposures. For comparison, groups of guinea pigs were sensitized to monomeric HDI. Naive animals served as negative controls. Two and three weeks following induction, animals were challenged by inhalation with the hapten and homologous protein conjugate of the hapten, respectively. Assessments were based on changes in respiratory rate, serum IgG(1)-antibody titer, and influx of eosinophilic granulocytes in airways. Guinea pigs induced and challenged with the HDI-monomer did not display appreciable changes in respiratory rate, whilst the re-challenge with the HDI-protein conjugate caused unequivocal changes in respiratory patterns, including a marked bronchial influx of eosinophilic granulocytes. In contrast, animals induced and challenged with either the free or conjugated aerosols of HDI-homopolymers failed to elicit specific physiological or morphological pulmonary responses. IgG(1) antibodies were observed in all groups receiving monomeric HDI or HDI-homopolymers. Based on the comparative assessment of antibody titers following intradermal injections, it appeared that monomeric HDI was more potent to induce specific IgG(1) antibodies than the homopolymers of HDI. In summary, with respect to induction of allergy and asthma, the data presented here suggest that the homopolymeric forms of HDI appear to be less potent asthmagens, if any, than monomeric HDI.