Chemical- and Drug-Induced Allergic, Inflammatory, and Autoimmune Diseases Via Haptenation

Abstract

Haptens are small molecules that only elicit an immune response when bound to proteins. Haptens initially bind to self-proteins and activate innate immune responses by complex mechanisms via inflammatory cytokines and damage-associated molecular patterns and the subsequent upregulation of costimulatory signals such as cluster of differentiation 86 (CD86) on dendritic cells. Subsequent interactions between CD86 and CD28 on T cells are critically important for properly activating naive T cells and inducing interleukin 2 production, leading to the establishment of adaptive immunity via effector and memory T cells. Accumulating evidence revealed the involvement of haptens in the development of various autoimmune-like diseases such as allergic, inflammatory, and autoimmune diseases including allergic contact dermatitis, atopy, asthma, food allergy, inflammatory bowel diseases, hemolytic anemia, liver injury, leukoderma, and even antitumor immunity. Therefore, the development of in vitro testing alternatives to evaluate in advance whether a substance might lead to the development of these diseases is highly desirable. This review summarizes and discusses recent advances in chemical- and drug-induced allergic, inflammatory, and autoimmune diseases via haptenation and the possible molecular underlying mechanisms, as well as in vitro testing alternatives to evaluate in advance whether a substance might cause the development of these diseases.

Keywords: allergic disease; autoimmune disease; hapten; in vitro coculture; inflammatory disease; pro-haptens; sensitization.

Figure 1

Chemicals and drugs induce autoimmune-like responses via haptenation, resulting in skin allergy, respiratory allergy, inflammatory bowel disease, hemolytic anemia, hepatotoxicity, leukoderma, and antitumor immunity. The upregulation of costimulatory molecule CD86 via DAMPs and inflammatory cytokines is critically important for the proper activation of naive T cells, leading to the establishment of adaptive immunity, including Th1, Th2, and Th17 differentiation; CTL generation; and antibody production. T cells activated without co-stimulation with CD86 become anergic, a state in which cells are not responsive to the stimulation thereafter and are tolerant to self-antigens. Recently, in addition to the hapten theory, non-covalent binding models such as the pharmacological interaction with immune receptors (p-i) model and the altered peptide model have also been proposed.

Figure 2

In vitro cell culture systems created for predicting the sensitizing potential and allergenicity of chemicals and drugs. Due to the current limited utilization of animal experimentation for risk evaluation, several in vitro cell culture systems as alternative methods have been developed to predict the sensitizing potential and allergenicity of chemicals and drugs. (A) The h-CLAT is an in vitro evaluation method widely used as a guideline test for predicting the skin sensitizing potential of chemicals by measuring CD86 and CD54 on THP-1 cells, DC surrogate cells. However, h-CLAT is disadvantaged in not being able to evaluate pro-haptens, because THP-1 cells only have limited metabolic capacity. (B) The h-CLAT-w/M system consists of THP-1 cells and melanoma SK-MEL-37 cells, as surrogate melanocytes are established. This system is useful for evaluating whether whitening agents that target melanocytes, especially tyrosinase, cause leukoderma as an adverse effect. Using the h-CLAT-w/M, RD was shown to act on melanocytes, generate ROS, release ATP, and consequently upregulate CD86 and IL-12 in THP-1 cells, potentially leading to the generation of melanocyte-specific CTLs and eventually leukoderma. (C) The DC coculture system consists of the human upper airway epithelial cell line BEAS-2B cells, and human peripheral monocyte-derived proliferating cell line CD14-ML cells. This system successfully discriminates typical chemical respiratory sensitizers from typical skin sensitizers by measuring the mRNA expression of one of the critical molecules for Th2 differentiation in DCs, OX40L. (D) To more precisely reproduce the in vivo activation and migration of naive CD4⁺ T cells after stimulation with DCs treated by chemical sensitizers, a new two-step DC/T cell coculture system was established by further adding peripheral allogeneic naive CD4⁺ T cells to the DCs stimulated in the DC coculture system. In this two-step DC/T cell coculture system, the mRNA upregulation of IL-4 in T cells representing KE4 was successfully used to discriminate typical respiratory sensitizers from skin sensitizers.

Figure 3

Comparison between general hapten-induced sensitization and RD-induced sensitization. Generally, a skin sensitizer is considered to be hapten-specific for proteins present abundantly and broadly in serum and cells, inducing an allergic reaction across an expansive layer of skin when exposed again to the sensitizer (A). By contrast, RD is unique in being a pro-hapten specific for melanocytes that metabolizes into its hapten sensitizer, RD-quinone, by oxidation in melanocytes, resulting in ROS production and ATP release, and the resultant upregulation of CD86 and IL-12, which lead to the induction of leukoderma (B). The former can be detected by using h-CLAT and the latter can be detected by using h-CLATw/M.