Consensus on the Key Characteristics of Immunotoxic Agents as a Basis for Hazard Identification

Abstract

Background: Key characteristics (KCs), properties of agents or exposures that confer potential hazard, have been developed for carcinogens and other toxicant classes. KCs have been used in the systematic assessment of hazards and to identify assay and data gaps that limit screening and risk assessment. Many of the mechanisms through which pharmaceuticals and occupational or environmental agents modulate immune function are well recognized. Thus KCs could be identified for immunoactive substances and applied to improve hazard assessment of immunodulatory agents.

Objectives: The goal was to generate a consensus-based synthesis of scientific evidence describing the KCs of agents known to cause immunotoxicity and potential applications, such as assays to measure the KCs.

Methods: A committee of 18 experts with diverse specialties identified 10 KCs of immunotoxic agents, namely, 1) covalently binds to proteins to form novel antigens, 2) affects antigen processing and presentation, 3) alters immune cell signaling, 4) alters immune cell proliferation, 5) modifies cellular differentiation, 6) alters immune cell-cell communication, 7) alters effector function of specific cell types, 8) alters immune cell trafficking, 9) alters cell death processes, and 10) breaks down immune tolerance. The group considered how these KCs could influence immune processes and contribute to hypersensitivity, inappropriate enhancement, immunosuppression, or autoimmunity.

Discussion: KCs can be used to improve efforts to identify agents that cause immunotoxicity via one or more mechanisms, to develop better testing and biomarker approaches to evaluate immunotoxicity, and to enable a more comprehensive and mechanistic understanding of adverse effects of exposures on the immune system. https://doi.org/10.1289/EHP10800.

Figure 1.

The key characteristics (KCs) of immunotoxicants. Various classes of exposures (outside) may exhibit any one or more of the 10 identified KCs (middle) leading to hypersensitivity, inappropriate enhancement, immunosuppression, or autoimmunity (inside). The figure was partly generated using Servier Medical Art, provided by Servier, licensed under a Creative Commons Attribution 3.0 unported license.

Figure 2.

Cyclosporine A (CsA) exhibits six key characteristics (KCs) of immunotoxicants. CsA is a widely used immunosuppressive drug whose mode of action has been well characterized in humans and experimental animals. Its main therapeutic indication is the treatment and prevention of organ rejection in kidney, liver, and heart allogeneic transplants. As a consequence of its immunosuppressive activity, infections and cancer are observed in long-term treated patients.^,, CsA acts on key mechanisms needed for many aspects of the immune response and exhibits six KCs of immunotoxicants as detailed in the respective KC descriptions. KC3: In T lymphocytes CsA binds to cyclophilin A, forming a complex inhibiting the phosphatase activity of calcineurin A and, consequently, the translocation of the nuclear factor of activated T cells (NFAT) transcription factor into the nucleus. The absence of NFAT translocation alters the transcription of key genes implicated in T cell proliferation and function (IL-2, IL-4, CD40 ligand). CsA also affects the activities of the AP-1 and $\text{NF}-\mathrm{\kappa }\mathrm{B}$ transcription factors. KC4: Via its effect on NFAT, CsA inhibits IL-2 synthesis and, consequently, T-cell proliferation. KC5: The effects of CsA on transcription factors and key molecular mechanisms lead to altered cytokine production, T-cell polarization, B-cell differentiation in plasmocytes, and cytotoxic T lymphocyte activation. KC6: Inhibition of cytokine production alters CsA-mediated $\text{CD}{4}^{+}$ T lymphocytes help to B lymphocytes. KC7: Alteration of cell differentiation and cell–cell communication lead to altered antibody production by plasmocytes and cell killing by cytotoxic T lymphocytes. KC9: The mitochondrial permeability transition pore (mPTP) involved in stress and calcium cell death is sensitive to CsA. Note: AP-1, activator protein 1; IL, interleukin; $\text{NF}-\mathrm{\kappa }\mathrm{B}$, nuclear factor kappa-light-chain-enhancer of activated B cells.

Figure 3.

AhR ligands exhibit nine key characteristics (KCs) of immunotoxicity. The aryl hydrocarbon receptor (AhR) is a transcription factor that is broadly expressed, including in immune cells. AhR ligands, including 2,3,7,8-tetrachlorodibenzo-$p$-dioxin (TCDD) and FICZ, are considered immunomodulators because they have the potential to produce immune suppression or immune enhancement through several of the KCs. KC2: Attenuation of dendritic cell (DC) ability to activate naïve T cells and changes to the expression of cell surface receptors may contribute to KC8.^,– KC3: Effects are mediated via AhR, leading to changes in gene expression and cell signaling.^,,– KC4: AhR ligands reduce T-cell clonal expansion^,, and impair proliferation of B cells^– and hematopoietic stem and progenitor cells (HSPCs).^– Effects on proliferation can contribute to KC5, KC6, and KC7. KC5: AhR ligands skew T-cell differentiation, reduce B cell differentiation, and affect context-dependent alteration of monocyte differentiation.^,,,,,– Effects on differentiation can contribute to KC6 and KC7. KC6: AhR ligands induce modulation of cytokines, chemokines, and adhesion molecules.^– Perturbation of cell–cell communication can contribute to all other KCs. KC7: AhR ligands were shown to inhibit B-cell activation and antibody production,^,, T-cell activation, and induce cytotoxicity of $\text{CD}{8}^{+}$ T cells.^,,, Alterations in effector cell functions can contribute to KC4 and KC5. KC8: Neutrophil accumulation in inflamed tissues, and reduced DC trafficking,^,– can contribute to KC2–5. KC9: Thymocyte apoptosis and B cell death may contribute to KC4, KC7, and KC10. KC10: Enhanced Treg cell frequency and tolerogenic DCs^,,– can contribute to KC2 and KC7. Note: DC, dendritic cell; FICZ, 6-formylindolo[3,2-b]carbazole; Treg, regulatory T cell.

Figure 4.

Implications of the Key Characteristics (KCs) of immunotoxicants for understanding disease. Each of the KCs may contribute to a health hazard or clinical disease, with KC1 and KC2 being the predominant mechanism for increased hypersensitivity (orange shading and arrows with a horizontal stripe pattern), and KC10 contributing mainly to increased risk of autoimmunity, inflammation, and recurrent miscarriage (green shading and arrows with a dotted pattern). The remaining KCs, KC2–7, jointly contribute to multiple outcomes including cytopenias and increased infection. (indicated by blue shading and solid arrows). In the authors’ opinion, the KCs can be used to protect human health by enhancing understanding of the pathogenesis of related disease processes and by informing the development of less immunotoxic medicines and consumer products. The KCs can also be used as an organizational framework that provides mechanistic insight for identifying and evaluating risks to the human immune system from environmental chemicals.