Beyond toxicity: aryl hydrocarbon receptor-mediated functions in the immune system

The aryl hydrocarbon receptor is a ligand-activated transcriptional regulator that binds dioxin and other exogenous contaminants and is responsible for their toxic effects, including immunosuppression. New evidence suggests, however, that the aryl hydrocarbon receptor has a physiological role in the immune system, and the immunosuppressive effects of dioxin may reflect a more subtle disruption of the regulatory interactions between immune cells

An intracellular self protein synthesized in macrophages is presented but fails to induce tolerance

Mice deficient for the fifth component of murine complement (C5), unlike normal mice, do not possess the secreted form of C5 in their body fluids and can be readily immunized to serum-derived normal C5. Although macrophages from C5-deficient mice do not secrete C5, they synthesize the precursor form (pro-C5). Therefore contact of T cells with autologous pro-C5 presented by macrophages is theoretically possible. We show that macrophages from C5-deficient mice can Indeed stimulate a class II restricted CS-specific T cell clone without addttion of exogenous C5. lmmunization of C5-deficient mice with autologous pro-C5 induces vigorous C5-specific T cell proliferation and pro-C5 is recognized by C5-specific T cells in vitro, demonstrating that this protein fails to induce tolerance under physiological condltions. Thus, intraceliuiar pro-C5 is processed and presented by C5-deficient macrophages and can activate T cell clones in vitro, yet is neither immunogenic nor tolerogenic for T cells in viv

Novel pathways of antigen presentation for the maintenance of memory

Follicular dendritic cells (FDC) store native antigen for long periods In tymphoid follicles and so provide a source ofcontinued stimulation for specific B cells. The expression of MHC class II by FDC suggested they might act as antigen-presenting cells for MHC class ll-restricted T cells. We show here, however, that the MHC class II molecules found on their surface are not synthesized by the FDC but are picked up from surrounding B cells In germinal centres. Although FDC by themselves cannot present native antigen to T cells, acquired MHC class ll-peptlde complexes can be recognized by T cells. The true physiological role of FDC seems to be as long-term antigen depots. We demonstrate that antigen localized onto FDC In vivo can be retrieved by antigen-specific B cells, which In turn process and present It to T cells. Thesepresentation pathways are likely to be crucial In both the maintenance of long-term Immune responses and the continued survival of memory cell

Involvement of Avidity for Major Histocompatibility Complex in Homeostasis of Naive and Memory T Cells

The requirements for survival and self-renewal of peripheral T cells and the nature of mechanisms controlling the size of the naive and memory pool are not completely understood. Here, we examine the involvement of the major histocompatibility complex (MHC) in survival and homeostatic expansion of naive and memory T cells. We show that the homeostatic behavior of naive T cell receptor (TCR)-transgenic T cells can be deduced by the expression levels of TCR and CD5, a negative regulator of TCR signaling. Both these factors determine the strength of TCR stimulation by MHC-derived signals. We further show that, similarly to naive T cells, MHC-derived signals influence the homeostatic expansion capacity of memory T cells under lymphopenic conditions. In contrast to naive T cells, however, memory T cells can reach a homeostatic equilibrium, in which survival/self-renewal of each clone is dissociated from their avidity for MHC-derived signals

Natural agonists for aryl hydrocarbon receptor in culture medium are essential for optimal differentiation of Th17 T cells

Th17 cell differentiation is dependent on interleukin (IL)-6 and transforming growth factor (TGF)-β, and it is modulated by activation of the aryl hydrocarbon receptor (AhR). In this study, we show that differentiation of Th17 cells, but not Th1 or induced regulatory T (iT reg) cells, is increased by endogenous AhR agonists present in culture medium. Th17 development from wild-type mice is suboptimal in the presence of the AhR antagonist CH-223191, similar to the situation in AhR-deficient mice, which show attenuated IL-17 production and no IL-22 production. The presence of natural AhR agonists in culture medium is also revealed by the induction of CYP1A1, a downstream target of AhR activation. However, the most commonly used medium, RPMI, supports very low levels of Th17 polarization, whereas Iscove's modified Dulbecco's medium, a medium richer in aromatic amino acids, which give rise to AhR agonists, consistently results in higher Th17 expansion in both mouse and human cells. The relative paucity of AhR agonists in RPMI medium, coupled with the presence of factors conducive to IL-2 activation and enhanced Stat5 phosphorylation, conspire against optimal Th17 differentiation. Our data emphasize that AhR activation plays an essential part in the development of Th17 cells and provide a rational explanation for the poor in vitro polarization of Th17 cells that is reported in the majority of publications for both mouse and human cells

CD4 memory T cells survive and proliferate but fail to differentiate in the absence of CD40

Secondary T cell responses are enhanced because of an expansion in numbers of antigen-specific (memory) cells. Using major histocompatibility complex class II tetramers we have tracked peptide-specific endogenous (non–T cell receptor transgenic) CD4 memory T cells in normal and in costimulation-deficient mice. CD4 memory T cells were detectable after immunization for more than 200 days, although decay was apparent. Memory cells generated in CD40 knockout mice by immunization with peptide-pulsed wild-type dendritic cells survived in the absence of CD40 and proliferated when boosted with peptide (plus adjuvant) in a CD40-independent fashion. However, differentiation of the memory cells into cytokine-producing effector cells did not occur in the absence of CD40. The data indicate that memory cells can be generated without passing through the effector cell stage

Characterising the original anti-C5 function-blocking antibody, BB5.1, for species specificity, mode of action and interactions with C5

The implication of complement in multiple diseases over the last twenty years has fuelled interest in developing anti‐complement drugs. To date, the focus has been on C5; blocking cleavage of C5 prevents formation of two pro‐inflammatory activities, C5a anaphylatoxin and membrane attack complex. The concept of C5 blockade to inhibit inflammation dates back thirty years to the description of BB5.1, an anti‐C5 blocking monoclonal antibody raised in C5‐deficient mice. This antibody proved an invaluable tool to demonstrate complement involvement in mouse disease models and catalysed enthusiasm for anti‐complement drug development, culminating in the anti‐human C5 monoclonal antibody ecuizumab, the most successful anti‐complement drug to date, already in the clinic for several rare diseases. Despite its key role in providing proof‐of‐concept for C5 blockade, the mechanism of BB5.1 inhibition remains poorly understood. Here we characterised BB5.1 cross‐species inhibition, C5 binding affinity and chain specificity. BB5.1 efficiently inhibited C5 in mouse serum but not in human or other rodent sera; it prevented C5 cleavage and C5a generation. BB5.1 bound the C5 α‐chain with high affinity and slow off‐rate. BB5.1 complementarity determining regions were obtained and docking algorithms used to predict the likely binding interface on mouse C5

Natural agonists for aryl hydrocarbon receptor in culture medium are essential for optimal differentiation of Th17 T cells

Th17 cell differentiation is dependent on interleukin (IL)-6 and transforming growth factor (TGF)-β, and it is modulated by activation of the aryl hydrocarbon receptor (AhR). In this study, we show that differentiation of Th17 cells, but not Th1 or induced regulatory T (iT reg) cells, is increased by endogenous AhR agonists present in culture medium. Th17 development from wild-type mice is suboptimal in the presence of the AhR antagonist CH-223191, similar to the situation in AhR-deficient mice, which show attenuated IL-17 production and no IL-22 production. The presence of natural AhR agonists in culture medium is also revealed by the induction of CYP1A1, a downstream target of AhR activation. However, the most commonly used medium, RPMI, supports very low levels of Th17 polarization, whereas Iscove's modified Dulbecco's medium, a medium richer in aromatic amino acids, which give rise to AhR agonists, consistently results in higher Th17 expansion in both mouse and human cells. The relative paucity of AhR agonists in RPMI medium, coupled with the presence of factors conducive to IL-2 activation and enhanced Stat5 phosphorylation, conspire against optimal Th17 differentiation. Our data emphasize that AhR activation plays an essential part in the development of Th17 cells and provide a rational explanation for the poor in vitro polarization of Th17 cells that is reported in the majority of publications for both mouse and human cells

IL-22 Protects Against Liver Pathology and Lethality of an Experimental Blood-Stage Malaria Infection

The host response following malaria infection depends on a fine balance between levels of pro-inflammatory and anti-inflammatory mediators resulting in the resolution of the infection or immune-mediated pathology. Whilst other components of the innate immune system contribute to the pro-inflammatory milieu, T cells play a major role. For blood-stage malaria, CD4+ and γδ T cells are major producers of the IFN-γ that controls parasitemia, however, a role for TH17 cells secreting IL-17A and other cytokines, including IL-17F and IL-22 has not yet been investigated in malaria. TH17 cells have been shown to play a role in some protozoan infections, but they also are a source of pro-inflammatory cytokines known to be involved in protection or pathogenicity of infections. In the present study, we have investigated whether IL-17A and IL-22 are induced during a Plasmodium chabaudi infection in mice, and whether these cytokines contribute to either protection or to pathology induced during the infection. Although small numbers of IL-17- and IL-22-producing CD4 T cells are induced in the spleens of infected mice, a more pronounced induction is observed in the liver, where increases in mRNA for IL-17A and, to a lesser extent, IL-22 were observed and CD8+ T cells, rather than CD4 T cells, are a major source of these cytokines in this organ. Although the lack of IL-17 did not affect the outcome of infection or pathology, lack of IL-22 resulted in 50% mortality within 12 days after infection with significantly greater weight loss at the peak of infection and significant increase in alanine transaminase in the plasma in the acute infection. As parasitemias and temperature were similar in IL-22 KO and wild-type control mice, our observations support the idea that IL-22 but not IL-17 provides protection from the potentially lethal effects of liver damage during a primary P. chabaudi infection

Feedback control of AHR signalling regulates intestinal immunity

The aryl hydrocarbon receptor (AHR) recognizes xenobiotics as well as natural compounds such as tryptophan metabolites, dietary components and microbiota-derived factors, and it is important for maintenance of homeostasis at mucosal surfaces. AHR activation induces cytochrome P4501 (CYP1) enzymes, which oxygenate AHR ligands, leading to their metabolic clearance and detoxification. Thus, CYP1 enzymes have an important feedback role that curtails the duration of AHR signalling, but it remains unclear whether they also regulate AHR ligand availability in vivo. Here we show that dysregulated expression of Cyp1a1 in mice depletes the reservoir of natural AHR ligands, generating a quasi AHR-deficient state. Constitutive expression of Cyp1a1 throughout the body or restricted specifically to intestinal epithelial cells resulted in loss of AHR-dependent type 3 innate lymphoid cells and T helper 17 cells and increased susceptibility to enteric infection. The deleterious effects of excessive AHR ligand degradation on intestinal immune functions could be counter-balanced by increasing the intake of AHR ligands in the diet. Thus, our data indicate that intestinal epithelial cells serve as gatekeepers for the supply of AHR ligands to the host and emphasize the importance of feedback control in modulating AHR pathway activation