An Adverse Outcome Pathway for Sensitization of the Respiratory Tract by Low-Molecular-Weight Chemicals: Building Evidence to Support the Utility ofIn VitroandIn SilicoMethods in a Regulatory Context

Abstract

Sensitization of the respiratory tract is an important occupational health challenge, and understanding the mechanistic basis of this effect is necessary to support the development of toxicological tools to detect chemicals that may cause it. Here we use the adverse outcome pathway (AOP) framework to organize information that may better inform our understanding of sensitization of the respiratory tract, building on a previously published skin sensitization AOP, relying on literature evidence linked to low-molecular-weight organic chemicals and excluding other known respiratory sensitizers acting via different molecular initiating events. The established key events (KEs) are as follows: (1) covalent binding of chemicals to proteins, (2) activation of cellular danger signals (inflammatory cytokines and chemokines and cytoprotective gene pathways), (3) dendritic cell activation and migration, (4) activation, proliferation, and polarization of T cells, and (5) sensitization of the respiratory tract. These events mirror the skin sensitization AOP but with specific differences. For example, there is some evidence that respiratory sensitizers bind preferentially to lysine moieties, whereas skin sensitizers bind to both cysteine and lysine. Furthermore, exposure to respiratory sensitizers seems to result in cell behavior for KEs 2 and 3, as well as the effector T cell response, in general skewing toward cytokine secretions predominantly associated with T helper 2 (Th2) response. Knowledge gaps include the lack of understanding of which KE(s) drive the Th2 polarization. The construction of this AOP may provide insight into predictive tests that would in combination support the discrimination of respiratory-sensitizing from non-and skin-sensitizing chemicals, a clear regulatory need.

Applying the adverse outcome pathway (AOP) for food sensitization to support in vitro testing strategies

“…The simplification of the biological process of food allergy sensitization by applying the AOP concept is an effective strategy to identify in vitro methods that lead to investigate the sensitizing potential of food proteins. Previously, such an approach focusing on MIE and KE described in reported AOPs for skin and respiratory sensitization have been used successfully to study the sensitizing potential chemicals in skin and respiratory allergy (Ezendam, Braakhuis, & Vandebriel, 2016;Sullivan et al, 2017).…”

Towards a testing strategy for the identification of respiratory sensitizing chemicals

“…However, the determination of chemical thresholds for respiratory sensitization is of yet very difficult [32]. These described steps have recently been put together in an adverse outcome pathway for respiratory sensitization to organize information that may better inform the understanding of sensitization of the respiratory tract [33].…”

“…In those cases, the exposures have already led to personal adverse effects and associated socioeconomic consequences. In view of the recently proposed adverse outcome pathway for respiratory sensitization [33], it is likely that the limitations of each individual method will be resolved in the future. On the other hand, respiratory allergy is a complex toxicological endpoint, of which the adverse outcome depends on several sequential key elements [33].…”

“…In view of the recently proposed adverse outcome pathway for respiratory sensitization [33], it is likely that the limitations of each individual method will be resolved in the future. On the other hand, respiratory allergy is a complex toxicological endpoint, of which the adverse outcome depends on several sequential key elements [33]. A workshop on the assessment of respiratory sensitization was held in 2016, during which it was concluded that a testing strategy comprised of several different methods is necessary to come to a definitive conclusion on the respiratory sensitization potential of chemicals [139].…”

“…The individual methods described earlier could well fit in such a testing strategy, as they relate to several key elements of the respiratory sensitization AOP [33]. Two of those key elements are covalent binding of a chemical to proteins and the induction of cellular danger signals.…”