The intestinal immune system can be influenced by various external factors, e.g., the diet (108). Therefore, it is more than likely that the modeling of the intestinal plasma cell survival niche and the support of plasma cell function does not depend only on the intestinal milieu itself. One transcription factor, the Aryl hydrocarbon receptor (AhR), has been described to connect nutritional intake with the microbiota composition and the immune response (109). The AhR was first discovered as a sensor for xenobiotics like polycyclic aromatic hydrocarbons (PAHs) and dioxins (110). The treatment of Hepa-1 cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces the translocation of the AhR from the cytosol to the nucleus, followed by the induction of drug-metabolizing enzymes (111). Therefore, AhR was revealed to function as a transcription factor involved in metabolic processes and detoxification early after its discovery. Today, we know that the AhR is a member of a transcription factor superfamily mainly characterized by two motifs, a basic N-terminal helix-loop-helix (bHLH) and a Per-Arnt-Sim (PAS) domain with two subunits (PAS-A and PAS-B) (112). While the bHLH domain allows dimerization of the protein and its binding to DNA, the PAS domain located at the C-terminal end of the bHLH domain is necessary to bind other PAS-proteins like the aryl hydrocarbon receptor nuclear translocator (ARNT) and the chaperone family member heat-shock protein 90 (HSP90) (113, 114). Essential and unique for AhR among the bHLH/PAS superfamily members, its PAS-B subdomain encodes for a ligand binding side specific for “all classes of dioxin receptor ligands”, which partially overlaps with the HSP90 binding side (115). The nuclear localization and the nuclear export signal can be found at the N-terminal side of the AhR protein, respectively (116). In contrast, the C-terminal region contains a glutamine-rich transactivation domain (TAD), which is essential for binding the AhR to its transcriptional co-activators and, therefore, for the induction of its target genes transcription (117). In the absence of AhR ligands, HSP90 binds to the PAS domain, thereby restricting its entrance to the nucleus and retaining AhR in an inactive conformation in the cytosol. Ligand binding to the PAS-B domain of the AhR in the cytosol results in the dissociation of HSP90, where after the AhR ligand complex translocates into the nucleus, binds to various transcriptional co-activators and induces its target gene expression (118). Important AhR-targets are the enzyme Cyp1a1 that degrades xenobiotics serving as AhR ligands (119), and the AhR repressor (AhRR) that suppresses AhR-activity by competing for the binding to the AhR co-activator ARNT (120). Both mechanisms result in a feedback loop regulating AhR activity. AhR-ARNT complex binding to the xenobiotic response element (XRE; also dioxin response element, DRE) in the target gene promotor region and its mediated gene expression is defined as the canonical AhR signaling (121–123). However, more recent studies demonstrated that AhR can interact with a variety of co-activators in a non-canonical signaling pathway, depending on the co-activator’s availability, and presumably the nature of the activating ligand (124, 125). Therefore, all studies addressing the AhR function by activation or inhibition through specific ligands must be compared carefully to allow reliable conclusions.

Among the initially described toxic chemicals functioning as AhR ligands, a vast number of naturally occurring (126) and synthesized AhR agonists and antagonists have been described (125, 127). The number of the latter is steadily increasing with the rising interest in AhR as a drug target in various diseases, including cancer, rheumatoid arthritis and inflammatory bowel disease (125). The interplay between the dietary composition, the variety of diet-derived AhR ligands generated by individual members of the intestinal microbiota or liver enzymes and the transport of these compounds through the organism to the site of action is incredibly complex and differs between individuals. Therefore, the prospect of treating diseases by directly manipulating AhR-activity through specifically designed ligands administrable as drugs seems promising. However, the possibility of treating or preventing diseases or even supporting our immune defenses with a diet rich or low in AhR ligands is tempting, especially as a mixture of AhR ligands can result in an altered AhR-functionality compared to their individual activity (128).

A wide range of naturally occurring, exogenous AhR ligands are generated during the metabolism of tryptophan or glucobrassicin found in vegetables (129–131). Metabolism of tryptophan by individual microorganisms of the intestinal microbiota results in the production of the AhR agonists indole acetic acid (IAA), indole-3-acetaldehyde and indole-3-aldehyde (IAld) (132–134). In addition, a very potent AhR agonist is the tryptophan photoproduct 6-formylindolo [3,2-b] carbazole (FICZ), which is generated by UV-radiation (135). In contrast to tryptophan, glucobrassicin from cruciferous vegetables of the family Brassicaceae is already metabolized in the oral cavity by myrosinases, resulting in the production of the AhR agonist precursor indole-3-carbinol (I3C) and indole-3-acetonitrile (I3ACN) (136). In the stomach, non-enzymatic acid condensation of I3C and I3ACN produces chemical compounds, such as 3,3’ diindolylmethane (DIM), 2-(indol-3-ylmethyl)-3,3’ diindolylmethane and indolo[3,2-b]carbazole (ICZ), that can function as AhR ligands (131, 137–139).

Diet-derived short-chain fatty acids (SCFAs) like butyrate can also induce AhR and AhR target gene expression in human IECs (140). This is particularly interesting as SCFAs are discussed for preventing or curing different diseases (141). In addition, by consuming stimulants like coffee (caffeine) or tobacco (nicotine), the AhR can be activated in the corresponding organs, while flavonoids found in, e.g., tea function mainly as AhR antagonists (142–144).

Even though studies of human and murine blood plasma showed biologically active concentrations of diet-derived AhR ligands that can potentially control AhR activity in distinct organs (145–148), the digestive tract is the first responding organ that is in contact with the highest concentrations of AhR ligands due to the primarily oral uptake of AhR ligands or their precursors.

AhR protein abundance has been identified in various cell types, including a wide range of immune cells as well as cells functioning at barrier sites like the lung, skin or intestine (149). Upholding intestinal homeostasis requires a delicate interplay between the microbiota commensals, the IECs, and immune cells found in the LP or IELs, with the majority expressing AhR in varying abundance (150). The analysis of an AhR-reporter mouse model revealed widespread expression of AhR in the intestinal epithelium with a proximal-distal gradient in the small intestine (151). A weaker and mosaic-structured AhR expression was detected in the colon of the same mice, indicating a more cell-type selective AhR function. Of note, the AhR gradient in the intestinal tract is comparable to the observed IgA gradient (1, 2). Therefore, AhR signaling may play a critical role in the intestinal plasma cell niche.

Of note, depending on the sensitivity of the used method, a gene expression analysis in whole organs does not always allow reliable distinguishing between different cell types. For this reason, more specific and detailed studies are required to identify the cell types in the intestinal tract that express AhR and to reveal how these cells are influenced by AhR activity. Therefore, advanced technologies like spatial transcriptomics will help to solve the remaining questions about the intestinal plasma cell niche and the functional relevance of AhR signaling therein.

However, several studies addressed the role of AhR, or more precisely of individual AhR ligands, for the intestinal epithelial barrier. One study revealed that FICZ supplementation prevents the intestinal permeability caused by damage of the epithelial layer due to decreased tight junction stability between epithelial cells during dextran sodium sulfate (DSS)–induced murine colitis (152). The authors demonstrated that FICZ-induced AhR signaling in a Caco-2 cell monolayer suppressed NF-kB p65 signaling and thereby protected against the tumor necrosis factor (TNF)-α/IFN-γ-mediated reduction in tight junction protein. However, FICZ-mediated rescue of DSS-colitis in vivo may not exclusively be attributed to the intrinsic role of AhR in IECs, as the activation of AhR also affects immune cells in the intestinal tissue (153). Supporting the importance of AhR in IECs, AhR-deficient mice or mice lacking AhR specifically in IECs were more susceptible to infection with the gram-negative bacterium Citrobacter rodentium (154–156). This phenotype could be re-produced by constitutive expression of the AhR-target Cyp1a1 (R26^Cyp1a1-mice) (109). In this mouse model, constant degradation of diet-derived AhR ligands by Cyp1a1-overexpression efficiently inhibited AhR signaling in IECs, resulting in an impaired immunity to C. rodentium and, thereby, an increasing systemic burden. However, the dietary supply of the AhR agonist precursor I3C enabled sufficient clearance of C. rodentium and rescued R26^Cyp1a1-mice.

Bacterial invasion of the LP does depend on the permeability of the IECs. Still, it is also controlled by the thickness of the intestinal mucus layer and its abundance of AMPs ( Figure 1 ). Therefore, an increased bacterial burden in AhR-deficient mice during a C. rodentium infection may also be caused by a diminished intestinal mucus layer. This hypothesis is supported by the observation that AhR is required for the differentiation of secretory cells in the intestinal epithelium, thereby enhancing the resistance to enteropathogenic Escherichia coli (157). In detail, activation of AhR by L-Kynurenine represses Notch1 signaling in murine intestinal cells, thereby allowing indoleamine 2,3-dioxygenase 1 (IDO1)-mediated promotion of differentiation into goblet and paneth cells. Furthermore, IEC-specific AhR-deficiency restricted stem cell differentiation to epithelial cells, e.g., goblet cells, while stem cells proliferated uncontrollably, resulting in an increased tendency to malignant transformation (154).

Studies in human colorectal cancer showed that upregulation of Wnt signaling at the bottom of the intestinal crypts through mutations in the Wnt-pathway component genes adenomatous polyposis coli (APC), β-catenin and/or AXIN2 causes this pathogenicity (158). Therefore, it is of particular interest that, besides its functions as a transcription factor, AhR was shown to support ubiquitination and, thereby, degradation of β-catenin by participating in an E3 ubiquitin ligase complex (159, 160). In addition, analyzing mice lacking AhR specifically in IECs demonstrated that dietary-induced AhR signaling prevents intestinal tumorigenesis by inhibiting the Wnt/β-catenin signaling in intestinal stem cells through induction of the transcription of E3 ubiquitin ligases (154). Furthermore, AhR was shown to be indispensable for the termination of the regenerative response in IECs after an injury (161). Here, the authors analyzed colonic organoid cultures under specific media conditions that either simulate differentiating or regenerative conditions. They demonstrated by RNAseq analysis that AhR regulates critical factors involved in the regenerative response or the reacquisition of intestinal identity post-injury. Interestingly, AhR signaling was also shown to control the differentiation of hematopoietic stem cells into lymphocytes (162). In addition, AhR can directly function as a tumor suppressor of acute myeloid leukemia by suppression of self-renewing of leukemia stem cells (163).

PCB 126, formerly produced as a lubricant in electronic equipment (164), is a very toxic AhR ligand (165). Treatment of human breast epithelial cell cultures with a very low dose of PCB 126 resulted in a significant reduction of CCL28 mRNA levels (166). As the parallel treatment with an AhR inhibitor nullified the observed phenotype, the authors concluded that AhR activation by PCB 126 inhibits CCL28 expression in an AhR-dependent manner. CCL28 secretion by IECs is associated with the homing of IgA plasmablasts to the small intestine (64, 74) ( Figure 2 ). Therefore, it should be considered as an AhR-controlled mechanism associated with antibody-secreting cell (ASC) migration in the gut. Of note, PCB126 treatment also altered the microbial community structure in the murine intestines (167).

Additional studies in AhR- or IL-10-deficient mice demonstrated that treatment with the Braccicacea-originating AhR ligand indole-3-carboxaldehyde (ICA) promotes goblet cell differentiation and proliferation of IECs through IL-10 in an AhR-dependent manner (168). It is well known that aging results in changes in the intestinal epithelium composition that involves the reduction of goblet cells, thereby, a decline in the epithelial barrier integrity and increased risk of inflammation. However, early colonization with bacteria known for their capacity to produce indoles, dampens this age-related phenotype. Furthermore, another xenobiotic AhR ligand called 2,3,7,8-tetrachlorodibenzofuran (TCDF) shifted the ratio of Firmicutes to Bacteroidetes, a change in the microbial community which was associated with the development of intestinal diseases (169). As the described phenotypes did not occur in AhR-deficient mice, the authors concluded that they are AhR-dependent.

In another study, the analysis of liver cells from TCDD-treated mice showed that AhR can potentially induce the expression of E-Cadherin when interacting with KLF6 as a co-activator (170). As E-cadherin may be necessary for cell-cell interaction between IECs and lymphocytes in the intestine (83), this mechanism has to be considered when analyzing the role of AhR in the intestinal plasma cell niche. However, functional studies are still missing, and it must be determined whether AhR interacts with KLF6 to induce E-Cadherin in IECs.

Interestingly, mice suffering from 2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced colitis showed significantly less damage in colonic tissue and reduced infiltration of inflammatory cells when treated with the AhR ligand TCDD prior to disease onset (171). Surprisingly, the treatment with TCDD led to an increased mRNA abundance of the plasma cell survival factor APRIL in colonic tissue three days after TNBS-induction. As the colon harbors a vast variance of cell types, the localization of the exact APRIL source seems challenging and could be addressed by spatial transcriptomics. However, in vitro LPS-activated IECs showed a profound increase in APRIL and BAFF mRNA when treated with TCDD. Therefore, TCDD-induced AhR signaling in IECs may support the viability of ASCs in the intestinal survival niche by inducing the secretion of the survival factors APRIL and BAFF ( Figure 2 ).

Based on these data, it is out of the question that AhR signaling plays a vital role in IECs. However, if AhR-regulated processes in IECs are essential for the intestinal plasma cell niche and healthy IgA secretion remains unanswered.

AhR-deficient LPS-activated macrophages secreted elevated amounts of IL-6, IL-12, and TNF-α (174). In detail, AhR mediates inhibition of IL-6 promotor activity by forming complexes with NFκB and signal transducer and activator of transcription 1 (Stat1). Furthermore, LPS-induced inflammation of bone marrow stromal cells in vitro results in an increase of IL-6 (175). However, this was abolished when cells were pre-treated with the AhR ligand TCDD. As IL-6 supports human long-lived plasma cells, AhR-controlled IL-6 production in the intestinal environment could play an important role in the intestinal plasma cell survival niche (93).

Nutrition studies demonstrated that a dietary lack of the AhR ligand precursor tryptophan alters the gut microbiota and results in an impaired intestinal immune system (176). Especially tryptophan degradation to indole-3-aldehyde by Lactobacilli induces AhR signaling in ILC3s, thereby promoting the production of IL-22 (132), an essential mechanism described in the immune response to Citrobacter rodentium (177). Furthermore, IL-22 is indispensable for a functional intestinal epithelial barrier, as it mediates the differentiation of intestinal stem cells into IECs and supports their viability (178). Thereby, IL-22 guarantees mucosal protection from damage and the repair of defects in the epithelial barrier integrity. Interestingly, IL-22 is expressed in the small intestine but barely in the colon (179), forming a gradient along the intestinal tract comparable to the one for AhR expression and IgA density. Thus, AhR-mediated IL-22 production may be beneficial for IgA plasma cell longevity in the intestine.

In contrast, the numbers of IL-5 and IL-13-secreting ILC2 cells were significantly elevated in AhR-deficient mice (180). Authors claimed that the AhR is the regulating factor that balances the intestinal ILC2 and ILC3 populations and, thereby, the intestinal immune homeostasis with an appropriate response to acute infections. As the function of ILCs in the intestinal tract is out of the question, it seems obvious to assume that they are also an AhR-regulated part of the intestinal plasma cell niche.

A variety of T cell subpopulations express AhR (181). However, there are contradicting reports about the abundance and importance of AhR in the same T cell subpopulations. AhR was shown to induce IL-22 production in CD4⁺ T cells by interacting with Stat3 as a co-activator (182). IL-22 is secreted by Th17 and Th22 cells, both of which require the activity of the transcription factor RORγt for differentiation (183–185). Of note, more recent studies revealed that AhR and RORγt can directly interact as a transcription factor complex (186) and are a potential target for treating lupus erythematosus (187). However, in vitro-activated T cells, primed for differentiation into Th17 cells, showed even further elevation of IL-22 production when treated with the AhR agonists β-naphthoflavone and FICZ (188). In strict contrast, AhR^-/- mice showed increased numbers of Th17 cells and elevated secretion of the cytokines IL-22 and IL-17 compared to AhR^+/- mice after imiquimod-induced psoriasiform skin inflammation (189). With IL-22 being a critical cytokine in the intestinal environment, the AhR-T cell interaction must be kept in mind when analyzing the role of nutritional AhR ligands and their impact on the intestinal IgA plasma cells niche.

Analysis of a Foxp3^Yfp-Cre reporter mouse demonstrated that Tregs located in the spleen and mLNs showed less AhR expression than intestinal Treg cells (190). The authors of this manuscript also pointed out that AhR has a cell-intrinsic role in intestinal Tregs. They demonstrated that AhR-deficient intestinal CD4⁺TCRβ⁺Foxp3⁺ Tregs lack the expression of CD103 and the chemokine CCL20, which is involved in cell migration. CD103 expression in Tregs is important for their activation and retention at the site of inflammation and CD103 can also be found on IgA plasma cells (83). Furthermore, mice treated with the AhR agonist FICZ expressed more CD103 in intestinal Tregs than control-treated animals, implying CD103 as a potential AhR target gene. As AhR-deficient intestinal Tregs express more inflammatory cytokines like IL-17 and IFN-γ, this study demonstrated the importance of AhR signaling in Treg cells for intestinal homing, and its requirement for their immunosuppressive function in colitis. T cells as a part of the intestinal micro-milieu regulate inflammatory responses and also the differentiation of epithelial cells. Therefore, T cells may undoubtedly be an AhR-controlled part of the intestinal plasma cell niche.

Various studies addressed the effect of AhR ligand treatment on B cells (153). Few of them focused on the intrinsic function of AhR in B cell activation and differentiation, and none discussed the role of AhR in intestinal plasma cells. However, the results of these studies are controversial, claiming either a supportive function of AhR during B cell activation or the exact opposite, i.e., inhibition of the B cell response. A study comparing activated human and murine B cell cultures claimed that TCDD treatment diminished IgM secretion in vitro, independent of the species (191). However, intracellular IgM abundance was reduced in activated murine B cells, but increased in human B cells after TCDD treatment. This indicates that TCDD: AhR inhibits the formation of IgM-secreting cells, even though the data are partially contradicting. There may also be ligand- or species-depending differences in AhR function in B cells.

Investigations of the role of AhR signaling in the B cell response in vivo using an adoptive transfer model demonstrated that AhR^-/- mature B cells are outcompeted by AhR^+/+ cells in the bone marrow and mLNs of mixed bone marrow chimeras (192). Even more interesting, challenging a recipient mouse reconstituted with AhR^-/- and AhR^+/+ splenocytes revealed that AhR-deficient B cells expanded less. Of note, B cell-specific AhR-deficient mice showed altered steady state serum Ig titers compared to controls, with elevated IgM but diminished IgG1 concentrations. IgA serum titers were unaltered. ASC numbers were reduced in the spleen of the same mice, but remained unchanged in their bone marrow. These data indicate a function of AhR in ASC formation, but very likely not in their maintenance in the bone marrow. Unfortunately, this study failed to determine ASC formation and maintenance in other organs or niches, e.g., the intestinal tract or their migration behavior. Even though IgA serum titers were unvaried in these mice, changes in intestinal plasma cells or the amount of mucus secreted IgA was not addressed and remains unclarified. However, this study indicates that AhR signaling may also be necessary for the formation of intestinal plasma cells.

One study analyzing B cell-specific AhR-deficient mice demonstrated that AhR signaling in regulatory B cells (Bregs) induces IL-10 secretion, thereby, dampening inflammatory Th1 and Th17 responses in a murine rheumatoid arthritis model (193). A more recent study of the same group indicates that AhR signaling induced by dietary SCFA butyrate supplementation favors the formation of Bregs during GC response and, thereby, alleviating inflammation in the cause of rheumatoid arthritis (194). In sharp contrast to the previously mentioned study (192), the authors claimed that butyrate-induced AhR signaling in B cells suppresses GC formation and plasmablast differentiation. But even though butyrate treatment reduced the numbers of CD19⁺ plasmablasts in the spleen of control animals, there was a significant increase in this population detected in butyrate-fed B cell-specific AhR-deficient mice. In contrast, numbers were unaltered between standard-fed knock-out and control animals. As B cells lack AhR in this model, why did butyrate still affect plasmablast numbers in the knock-out mouse? In addition, the population defined as Bregs showed a severe upregulation of the plasma cell signature transcription factor Blimp-1 (prdm-1 gene) when lacking AhR and butyrate feeding markedly reduced Blimp-1 expression independent of the genotype. This gene expression profile questions the nature of the analyzed cell population, especially as plasma cells can also secrete IL-10 in autoimmune inflammation (195) and express high amounts of CD24 (196, 197). Of note, the analysis of the B cell response in the intestinal tract was missing in this study.

Due to contradicting data, the role of AhR signaling in B cell activation and plasma cell differentiation is still unresolved. Especially the function of AhR in the intestinal ASC pool, and their migration behavior remains completely unclear.

In addition to several studies that described a potential function of AhR signaling in B cells, there are also analyses addressing a connection between AhR activity and IgA production.

Analyzing the effect of TCDD treatment during viral airway infection with Influenza A demonstrated an overall immunosuppressive phenotype (198). Interestingly, virus-specific IgG and IgM levels were reduced in the blood plasma of TCDD-treated infected mice, while antigen-specific IgA was 4-fold increased. In contrast, analyzing broncho-alveolar lavage fluid revealed that IgA was unaltered at the side of infection, even so, IgG levels were again diminished after TCDD application. Of note, all mice in this study were female and observations may reflect a gender-specific phenotype. Even though this data may indicate a connection between AhR signaling and IgA production, the authors failed to determine whether this phenotype was associated with a B cell-intrinsic AhR signaling or a secondary effect due to the overall observed alterations in T cells and cytokine production. Furthermore, organ-specific alterations in IgA levels indicate a differential role of AhR activity for secreted, mucus-associated IgA and systemic IgA in the blood. This contradicts a potential function of AhR signaling in p-IgR-mediated IgA transport through lung epithelial cells into the mucus. However, whether the same is true for the intestinal tract remains unanswered.

Research focusing on the formation of IgA-secreting cells in the intestine revealed that feeding wildtype C57Bl/6 mice with the AhR ligand TCDD alters fecal IgA concentrations independent of the circadian rhythm (199). However, the authors did not clarify whether the observed phenotype is mediated by a B cell-intrinsic AhR function or associated with AhR activity in the surrounding cells e.g., IECs. Furthermore, TCDD treatment elevated fecal and serum IgA levels in male mice independent of the analyzed dose. However, female mice showed reduced IgA levels when treated with low doses of TCDD, while results were comparable to their male counterparts during high-dose TCDD treatment. Notably, hormonal changes can affect IgA secretion (200). Regarding the female menstrual cycle, the AhR can recruit the estrogen receptor α to the promotor region of AhR-regulated genes (201) or even to the estrogen response elements (202). Therefore, gender-specific variances in AhR ligand-induced IgA secretion are not surprising and must be addressed in future experiments.

In support, AhR ligand 2,3,7,8-tetrachlorodibenzofuran (TCDF)-fed male wildtype mice showed increased fecal IgA abundance and elevated concentrations of inflammatory cytokines in the ileum (169). Surprisingly, LPS abundance in the blood serum was significantly increased in TCDF-treated mice, indicating diminished intestinal barrier integrity and increased translocation of bacteria from the gut lumen into the LP and adjacent organs. These data oppose previously described studies which showed that FICZ-mediated AhR activity in IECs strengthened intestinal barrier function by increasing tight junction protein production (152). This conflicting data may be caused by the authors missing to address the question if the TCDF-mediated disruption of the intestinal barrier is AhR-dependent or caused by the very toxic nature of the ligand. In addition, it has to be considered that altered IgA levels in TCDF-fed mice could be triggered by increasing bacterial infiltration and, therefore, may not depend on TCDF-induced AhR activity in ASCs.

A study assessing TCDD-mediated AhR signaling and its influence on the course of Crohn`s disease demonstrated faster recovery of TCDD-fed mice from 2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced colitis, which was associated with increased numbers of Foxp3⁺ Tregs in the intestines (171). Interestingly, IgA concentrations significantly increased in feces and in homogenized colon tissue after TCDD treatment. This phenotype was independent of TNBS-induced colitis but mediated by AhR activity, as IgA levels were comparable between TCDD- and control-fed AhR^-/- mice. As mentioned before, TCDD treatment of LPS-activated IECs led to elevated APRIL and BAFF mRNA abundance, cytokines known to support B cell and ASC viability, respectively. In addition, TCDD-induced AhR signaling in IECs or intestinal immune cells could trigger the production of cytokines that induce or support ASC differentiation. Therefore, augmentation of IgA levels in TCDD-treated mice may rather be attributed to AhR signaling in other cell types and not B cell or ASC intrinsic.

A unique study among AhR-focused research projects determined the influence of AhR activity on IgA production in neonatal mice (203). Therefore, researchers compared AhR^-/- mice and wildtype mice whose parents were already fed with a standard diet or an AhR ligand-free diet, which was continued after birth. Two-week-old AhR^-/- mice showed no fecal IgA, while their serum IgA levels were comparable to wildtype mice fed with a standard diet. In contrast, an AhR ligand-free diet abolished serum IgA in wildtype mice but did not affect fecal IgA concentrations. Furthermore, the authors claimed that IgA abundance in the gastric content were comparable between animals during the nursing period, indicating no influence of maternal IgA on the observed phenotype. However, a direct diet-mediated effect on 2 week-old suckling mice is hard to imagine. Therefore, it has to be considered that other components in the maternal milk besides IgA itself may contribute to the observed differences in the IgA concentrations in the different groups of neonatal mice. Interestingly, depending on the available diet for nutrient intake, eight-week-old weaned mice did not show any differences in serum IgA concentrations. In contrast, only mice fed with an AhR ligand-free diet lacked fecal IgA, likely caused by reduced CD19⁺B220⁺ B cell numbers in the GALT. Even though the authors provided a unique set of data analyzing the influence of a diet or AhR signaling in neonatal mice, the results of this study are contradicting. As the observed alterations in IgA concentrations differed between AhR ligand-free diet-fed mice and AhR^-/- mice, these results implicate an AhR-independent effect of the diet on the intestinal immune system. As obviously all cells of an organism are affected by the deficiency in AhR^-/- mice or mice fed with a specific diet, their analysis does not allow conclusions about the B cell-intrinsic role of AhR in the formation of IgA-secreting cells or IgA-secretion itself. However, the differences between fecal and serum IgA concentrations in mice of the same genotype or diet-fed group imply a differential AhR function for secreted mucus-associated IgA and serum IgA, again.

Regarding the significant number of studies addressing the AhR function, it was often neglected that TCDD is a toxic substance causing oxidative stress, DNA damage and endocrine disruption amongst other symptoms (204–207). Until now, it is just partially known which of these mechanisms are caused by TCDD : AhR or an AhR-independent pathway. Therefore, studies analyzing the effect of AhR by simply adding a chemical like, e.g., TCDD to the system must be viewed with caution.