Invited Perspective: Environmental Chemical-Sensing AHR Remains an Enigmatic Key Player in Toxicology.

Abstract

Vol. 131, No. 3 Invited PerspectiveOpen AccessInvited Perspective: Environmental Chemical-Sensing AHR Remains an Enigmatic Key Player in Toxicologyis accompanied byMultidimensional Analysis of Lung Lymph Nodes in a Mouse Model of Allergic Lung Inflammation following PM2.5 and Indeno[1,2,3-cd]pyrene Exposure Barbara L.F. Kaplan and B. Paige Lawrence Barbara L.F. Kaplan Address correspondence to Barbara L.F. Kaplan, Center for Environmental Health Sciences, Department of Comparative Biomedical Sciences, 240 Wise Center Dr., Mississippi State University, Mississippi State, MS 39762 USA. Email: E-mail Address: [email protected] https://orcid.org/0000-0002-1992-4145 Center for Environmental Health Sciences, Department of Comparative Biomedical Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi, USA Search for more papers by this author and B. Paige Lawrence Department of Environmental Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York, USA Search for more papers by this author Published:28 March 2023CID: 031307https://doi.org/10.1289/EHP12535AboutSectionsPDF ToolsDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InReddit Over the last several years, the aryl hydrocarbon receptor (AHR) has gone from being characterized as not only a receptor that modulates cellular responses to myriad external environmental changes but also as one that regulates intricate aspects of immune homeostasis in response to endogenously produced ligands. Indeed, several endogenous AHR ligands have now been identified, including chemicals derived from tryptophan, phytochemicals, or commensal microbiota.1,2 Although AHR might play many roles, no doubt it remains a critical receptor in modulating immune responses within the context of exposure to environmental chemicals.In this issue of Environmental Health Perspectives, Liu et al. provide a detailed characterization of immune effects of fine particulate matter [PM with an aerodynamic diameter of less than or equal to 2.5 micrometers≤2.5μm (particulate matter begin subscript 2.5 end subscriptPM2.5)] obtained from atmospheric monitoring stations in Taiwan.3 This particulate matter begin subscript 2.5 end subscriptPM2.5 contained various polycyclic aromatic hydrocarbons (PAHs), including indeno[1,2,3-cd]pyrene (IP).4 They observed that intratracheal administration of particulate matter begin subscript 2.5 end subscriptPM2.5 or IP alone exacerbated pathology and modulated immune responses by using a mouse model for house dust mite (HDM)-mediated asthma.3 Importantly, they used mass cytometry to determine that both particulate matter begin subscript 2.5 end subscriptPM2.5 and IP exposure increased the percentage and number of uppercase t c r lowercase beta positive uppercase c d 4 positive uppercase c x c r 5 positive bcl-6 positive uppercase p d-1 begin superscript high end superscriptTCRβ+CD4+CXCR5+Bcl-6+PD-1hi T follicular helper (Tfh) cells in the lung-draining lymph nodes. In addition, the enhancement in Tfh cells was dependent on AHR expression in T cells, at least for particulate matter begin subscript 2.5 end subscriptPM2.5. Tfh cells are uppercase c d 4 positiveCD4+ T cells that play a critical role in inducing isotype-switched antibody responses in germinal centers (GCs).5 Therefore, it was important that they also demonstrated that IP enhanced levels of HDM-specific immunoglobulin E (IgE) and IgG in serum.3 Previously, they had demonstrated that IP-mediated exacerbation in a mouse ovalbumin-induced model of allergic lung inflammation was attenuated with the AHR antagonist CH223191.4Interestingly, the observation that IP, as an AHR ligand, increased the percentage and number of Tfh cells contradicts another recent paper, in which other AHR ligands suppressed Tfh cell percentage and number in mice infected with influenza A virus.6 In that paper, the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the tryptophan derivative 2-(1′H-indole-3′-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE), and the endogenous ligand kynurenic acid (KYNA) all suppressed uppercase c d 4 positive uppercase c d 44 positive uppercase c x c r 5 positive uppercase p d-1 begin superscript high end superscriptCD4+CD44+CXCR5+PD-1hi Tfh in infected mice.6 Using TCDD, Houser and Lawrence also showed in lung-draining lymph nodes that AHR activation significantly suppressed Tfh cell number as early as day 3 postinfection, that the GC B cell number directly correlated with Tfh cell number and, further, that AHR activation suppressed GC B cells. Consistent with these findings, AHR activation with ligands, including TCDD and ITE, also reduced virus-specific IgG levels in plasma.6The juxtaposition of the results from these papers displays a common theme in the AHR field: Different AHR ligands produce different, and sometimes opposite, effects in the immune system (Figure 1). The reasons for this remain uncertain, but they could be due to ligand affinity, ligand bioavailability or metabolism, strength of response, or differences in the immune stimulus. Indeed, the two juxtaposed studies used different disease models, with Houser and Lawrence6 using a replicating pathogen (influenza virus), and Liu et al.3 using a representative allergen (HDM).Figure 1. Relationship between AHR ligand affinity, metabolism, and biological effect. The bubble plot depicts the relative biological or toxicological consequences of agonists that bind AHR with either greater or lesser affinity and that are metabolized either more or less rapidly. The size of the bubble denotes relative effect. Note: AHR, aryl hydrocarbon receptor; FICZ, 6-formylindolo(3,2-b)carbazole; IP, indeno[1,2,3-cd]pyrene; ITE, (1′H-indole-3′-carbonyl)-thiazole-4-carboxylic acid methyl ester; KynA, kynurenic acid; PAHs, polyaromatic hydrocarbons; PCBs, polychlorinated biphenyls; TCDD, 2,3,7,8-tetrachlorodibenzo-p-dioxin.However, even in the same model system, two different AHR ligands can produce opposite effects. For instance, during influenza A virus infection in which TCDD suppressed Tfh cell percentage and number, the tryptophan derivative 6-formylindolo(3,2-b)carbazole (FICZ)—which, like TCDD, binds to AHR with high affinity—elevated Tfh cell percentage and number in the lung-draining lymph nodes.7 In addition to this contrasting effect on Tfh cells in influenza virus–infected mice,7 opposite effects of AHR ligands were observed in the experimental autoimmune encephalomyelitis model of multiple sclerosis, in which TCDD suppressed but FICZ exacerbated disease.8,9 Thus, ligand source might not necessarily be an ideal predictor of whether the outcome will be beneficial or detrimental.The demonstration that two environmentally relevant ligands exhibit opposite effects on the Tfh cell population (i.e., IP enhanced and TCDD suppressed)3,6 could also be explained by differences in AHR affinity, ligand metabolism, or bioavailability. IP was determined to have a half maximal effective concentration (effective concentration, 50 percentEC50) for induction of ethoxyresorufin-O-deethylase, an enzymatic measure of AHR-responsive cytochrome P4501A1, of 5.2 times 10 begin superscript negative 1 end superscript micromolar5.2×10−1 μM, which is 10,000 times less effective than TCDD.10 Unlike TCDD, which is relatively resistant to metabolism,11–13 there were several hydroxylated metabolites of IP identified in vitro using rat liver enzyme homogenates.14 Further support that metabolism of AHR ligands can influence immune effects, such as uppercase c d 4 positive uppercase c d 44 begin superscript high end superscript uppercase c x c r 5 positive uppercase p d-1 positiveCD4+CD44hiCXCR5+PD-1+ Tfh populations in influenza virus infection was shown by comparing TCDD and FICZ in mice.7 Specifically, oral TCDD (administered once) suppressed the percentage and number of Tfh cells in the lung-draining lymph nodes, whereas oral FICZ (administered once per day for 8 d) had no effect. However, in cytochrome P 450, family 1, subfamily A, polypeptide 1 (cytochrome P 450, family 1, subfamily A, polypeptide 1 begin superscript negative per negativeCyp1a1−/−) mice infected with influenza virus, both TCDD and FICZ suppressed the number of Tfh cells.7 This indicates that attenuating the clearance of FICZ in vivo produced an effect that was not observed in the presence of normal metabolism. Thus, the metabolism of ligands, regardless of whether the ligands are endogenous or exogenous, likely influences their effects in immune cells.Another issue that cannot be ignored, regardless of the source of AHR ligand being studied, is the fact that exposure to a single chemical is unlikely. In their paper, Liu et al. acknowledged this critical fact by showing that exposure to IP, both on its own and in a particulate matter begin subscript 2.5 end subscriptPM2.5 mixture, exacerbated the HDM-induced Tfh populations.3 In addition to drilling down from particulate matter begin subscript 2.5 end subscriptPM2.5 to PAH to a specific moiety, the leveraging of in vitro results strengthened the in vivo findings. Specifically, Liu et al. differentiated Tfh cells in vitro and connected altered Tfh cells to changes in gene expression.3 In particular, the associated increase in Il4 and Il21 gene expression was found to be due, in part, to AHR binding to xenobiotic response elements in the promoters of both Il4 and Il21. Importantly, Tfh cells producing interleukin-4 (IL-4) is necessary for IgE class switch15–17 and, as noted, IP increased HDM-mediated IgE.3Overall, the work by Liu et al.3 advances the field of immunotoxicology through the use of mass cytometry to identify a specific immune cell population that was sensitive to regulation by an environmental contaminant via AHR. Further, although this is likely not the sole consequence of AHR activation by IP, or by PAHs in general, the authors identified a specific role that AHR played in regulating Il4 and Il21 gene expression.3 This work provides a compelling and complete story about how the environmentally relevant AHR ligand IP-induced respiratory disease in vivo identified that exacerbated disease directly correlated with an increased Tfh cell population and IgE production and then showed that AHR was involved in IP-induced Il4 production in vitro, something that is necessary for IgE production.It will be exciting to see where future work leads, including further investigation into the mechanisms by which IP alone, and in combination with other contaminants, alters immune responses. For instance, it was recently suggested by using in silico modeling that IP would bind to toll-like receptor 4 with high affinity.18 This is an intriguing finding, considering that Liu et al.3 found that IP exacerbated airway disease. Hopefully, future research will also identify the mechanistic explanation as to why different AHR ligands sometimes cause similar effects but at other times can cause different, and even completely divergent, consequences. Understanding how differences in AHR affinity, ligand metabolism, and other characteristics influence immune function will be critical to parsing and predicting the immunological consequences of exposure to the myriad ligands of this fascinating environment-sensing transcription factor.