Johan Verhagen

Immune Responses in Healthy and Allergic Individuals Are Characterized by a Fine Balance between Allergen-specific T Regulatory 1 and T Helper 2 Cells

The mechanisms by which immune responses to nonpathogenic environmental antigens lead to either allergy or nonharmful immunity are unknown. Single allergen-specific T cells constitute a very small fraction of the whole CD4+ T cell repertoire and can be isolated from the peripheral blood of humans according to their cytokine profile. Freshly purified interferon-γ–, interleukin (IL)-4–, and IL-10–producing allergen-specific CD4+ T cells display characteristics of T helper cell (Th)1-, Th2-, and T regulatory (Tr)1–like cells, respectively. Tr1 cells consistently represent the dominant subset specific for common environmental allergens in healthy individuals; in contrast, there is a high frequency of allergen-specific IL-4–secreting T cells in allergic individuals. Tr1 cells use multiple suppressive mechanisms, IL-10 and TGF-β as secreted cytokines, and cytotoxic T lymphocyte antigen 4 and programmed death 1 as surface molecules. Healthy and allergic individuals exhibit all three allergen-specific subsets in different proportions, indicating that a change in the dominant subset may lead to allergy development or recovery. Accordingly, blocking the suppressor activity of Tr1 cells or increasing Th2 cell frequency enhances allergen-specific Th2 cell activation ex vivo. These results indicate that the balance between allergen-specific Tr1 cells and Th2 cells may be decisive in the development of allergy.

Mechanisms of immune suppression by interleukin-10 and transforming growth factor-β: the role of T regulatory cells

Specific immune suppression and induction of tolerance are essential processes in the regulation and circumvention of immune defence. The balance between allergen‐specific type 1 regulatory (Tr1) cells and T helper (Th) 2 cells appears to be decisive in the development of allergy. Tr1 cells consistently represent the dominant subset specific for common environmental allergens in healthy individuals. In contrast, there is a high frequency of allergen‐specific interleukin‐4 (IL‐4)‐secreting T cells in allergic individuals. Allergen‐specific immunotherapy can induce specific Tr1 cells that abolish allergen‐induced proliferation of Th1 and Th2 cells, as well as their cytokine production. Tr1 cells utilize multiple suppressor mechanisms, such as IL‐10 and transforming growth factor‐β (TGF‐β) as secreted cytokines and various surface molecules, such as cytotoxic T‐lymphocyte antigen 4 and programmed death‐1. IL‐10 only inhibits T cells stimulated by low numbers of triggered T‐cell receptors, which depend on CD28 costimulation. IL‐10 inhibits CD28 tyrosine phosphorylation, preventing the binding of phosphatidylinositol 3‐kinase p85 and consequently inhibiting the CD28 signalling pathway. In addition, IL‐10 and TGF‐β secreted by Tr1 cells skew the antibody production from immunoglobulin E (IgE) towards the non‐inflammatory isotypes IgG4 and IgA, respectively. Induction of antigen‐specific Tr1 cells can thus re‐direct an inappropriate immune response against allergens or auto‐antigens using a broad range of suppressor mechanisms.

Negative feedback control of the autoimmune response through antigen-induced differentiation of IL-10–secreting Th1 cells

Regulation of the immune response to self- and foreign antigens is vitally important for limiting immune pathology associated with both infections and hypersensitivity conditions. Control of autoimmune conditions can be reinforced by tolerance induction with peptide epitopes, but the mechanism is not currently understood. Repetitive intranasal administration of soluble peptide induces peripheral tolerance in myelin basic protein (MBP)–specific TCR transgenic mice. This is characterized by the presence of anergic, interleukin (IL)-10–secreting CD4+ T cells with regulatory function (IL-10 T reg cells). The differentiation pathway of peptide-induced IL-10 T reg cells was investigated. CD4+ T cells became anergic after their second encounter with a high-affinity MBP peptide analogue. Loss of proliferative capacity correlated with a switch from the Th1-associated cytokines IL-2 and interferon (IFN)-γ to the regulatory cytokine IL-10. Nevertheless, IL-10 T reg cells retained the capacity to produce IFN-γ and concomitantly expressed T-bet, demonstrating their Th1 origin. IL-10 T reg cells suppressed dendritic cell maturation, prevented Th1 cell differentiation, and thereby created a negative feedback loop for Th1-driven immune pathology. These findings demonstrate that Th1 responses can be self-limiting in the context of peripheral tolerance to a self-antigen.

Regulation of adaptive immunity; the role of interleukin-10.

Since the discovery of interleukin-10 (IL-10) in the 1980s, a large body of work has led to its recognition as a pleiotropic immunomodulatory cytokine that affects both the innate and adaptive immune systems. IL-10 is produced by a wide range of cell types, but for the purposes of this review we shall focus on IL-10 secreted by CD4+ T cells. Here we describe the importance of IL-10 as a mediator of suppression used by both FoxP3+ and FoxP3− T regulatory cells. Moreover, we discuss the molecular events leading to the induction of IL-10 secretion in T helper cell subsets, where it acts as a pivotal negative feedback mechanism. Finally we discuss how a greater understanding of this principle has allowed for the design of more efficient, antigen-specific immunotherapy strategies to exploit this natural phenomenon clinically.

A Second Step of Chemotaxis After Transendothelial Migration: Keratinocytes Undergoing Apoptosis Release IFN-γ-Inducible Protein 10, Monokine Induced by IFN-γ, and IFN-γ-Inducible α-Chemoattractant for T Cell Chemotaxis Toward Epidermis in Atopic Dermatitis

Activation and skin-selective homing of T cells and their effector functions in the skin represent sequential immunological events in the pathogenesis of atopic dermatitis (AD). Apoptosis of keratinocytes, induced mainly by T cells and mediated by IFN-γ and Fas, is the essential pathogenetic event in eczema formation. Keratinocyte apoptosis appears as activation-induced cell death in AD. By IFN-γ stimulation, chemokines such as IFN-γ-inducible protein 10, monokine induced by IFN-γ, and IFN-γ-inducible α-chemoattractant are strongly up-regulated in keratinocytes. These chemokines attract T cells bearing the specific receptor CXCR3, which is highly expressed on T cells isolated from skin biopsies of AD patients. Accordingly, an increased T cell chemotaxis was observed toward IFN-γ-treated keratinocytes. Supporting these findings, enhanced IFN-γ-inducible protein 10, monokine induced by IFN-γ, and IFN-γ-inducible α-chemoattractant expression was observed in lesional AD skin by immunohistochemical staining. These results indicate a second step of chemotaxis inside the skin after transendothelial migration of the inflammatory cells. Keratinocytes undergoing apoptosis in acute eczematous lesions release chemokines that attract more T cells toward the epidermis, which may further augment the inflammation and keratinocyte apoptosis.

Antigen-specific immunotherapy of autoimmune and allergic diseases

Nearly a century has passed since the first report describing antigen-specific immunotherapy (antigen-SIT) was published. Research into the use of antigen-SIT in the treatment of both allergic and autoimmune disease has increased dramatically since, although its mechanism of action is only slowly being unravelled. It is clear though, from recent studies, that success of antigen-SIT depends on the induction of regulatory T (T reg) cell subsets that recognise potentially disease-inducing epitopes. The major challenge remaining for the widespread use of antigen-SIT is to safely administer high doses of immunodominant and potentially pathogenic epitopes in a manner that induces T cell tolerance rather than activation. This review illustrates that intelligent design of treatment agents and strategies can lead to the development of safe and effective antigen-SIT.

T Regulatory Cells in Allergy and Health: A Question of Allergen Specificity and Balance

Anergy, tolerance and active suppression may not be independent events, but rather involve similar mechanisms and cell types in immune regulation. Induction of allergen-specific regulatory/suppressor T cells (TReg) seems essential for the maintenance of a healthy immune response to allergens. Allergen-specific immunotherapy can induce specific TReg cells that abolish allergen-induced proliferation of T helper 1 (Th1) and Th2 cells, as well as their cytokine production. TReg cells utilize multiple suppressive mechanisms, interleukin-10 (IL-10) and transforming growth factor-β (TGF-β) as secreted cytokines and CTLA-4, PD-1, mTGF-β, mIL-10, TGF-βR and IL-10R as surface molecules. An important aspect of TReg cells is the regulation of antibody isotypes and suppression of proinflammatory cells. IL-10 and TGF-β secreted by TReg cells skew production of IgE towards the noninflammatory isotypes, IgG4 and IgA, respectively. Furthermore, TReg cells may directly or indirectly suppress effector cells of allergic inflammation such as basophils and eosinophils. In conclusion, induction of antigen-specific TReg cells may redirect an inappropriate immune response against allergen or autoantigens with the help of a broad range of suppressor mechanisms.

Sequential transcriptional changes dictate safe and effective antigen-specific immunotherapy

Antigen-specific immunotherapy combats autoimmunity or allergy by reinstating immunological tolerance to target antigens without compromising immune function. Optimization of dosing strategy is critical for effective modulation of pathogenic CD4+ T-cell activity. Here we report that dose escalation is imperative for safe, subcutaneous delivery of the high self-antigen doses required for effective tolerance induction and elicits anergic, interleukin (IL)-10-secreting regulatory CD4+ T cells. Analysis of the CD4+ T-cell transcriptome, at consecutive stages of escalating dose immunotherapy, reveals progressive suppression of transcripts positively regulating inflammatory effector function and repression of cell cycle pathways. We identify transcription factors, c-Maf and NFIL3, and negative co-stimulatory molecules, LAG-3, TIGIT, PD-1 and TIM-3, which characterize this regulatory CD4+ T-cell population and whose expression correlates with the immunoregulatory cytokine IL-10. These results provide a rationale for dose escalation in T-cell-directed immunotherapy and reveal novel immunological and transcriptional signatures as surrogate markers of successful immunotherapy.

Comment on “Expression of Helios, an Ikaros Transcription Factor Family Member, Differentiates Thymic-Derived from Peripherally Induced Foxp3+ T Regulatory Cells”

Thornton et al. ([1][1]) reported recently that in both mouse and man, ∼70% of peripheral Foxp3+ regulatory T cells (Tregs) express the transcription factor Helios. Interestingly, they found that Foxp3+ cells generated in the thymus all coexpressed Helios, whereas Tregs generated in vitro from

Enhanced selection of FoxP3+ T-regulatory cells protects CTLA-4-deficient mice from CNS autoimmune disease

It is generally acknowledged that cytotoxic T-lymphocyte–associated antigen-4 (CTLA-4/CD152) plays a pivotal role in the regulation of T-cell activation and the establishment of self-tolerance in the periphery. CTLA-4–deficient (CTLA-4KO) mice develop a lymphoproliferative disorder and die within 4 weeks of birth, suggesting a role for CTLA-4 in T-cell homeostasis or the development and activity of T-regulatory (Treg) cells. To study the role of CTLA-4 in the control of experimental autoimmune encephalomyelitis (EAE), we have generated a CTLA-4KO mouse in which >90% of all CD4+ T cells bear a Vβ8.2 transgenic T-cell receptor that is specific for myelin basic protein peptide Ac1–9 (ASQKRPSQR). These mice do not develop spontaneous lymphoproliferative disease or EAE and are resistant to disease induction. This correlates with a higher frequency of functional FoxP3+ Treg cells in the spleen and thymus of CTLA-4KO mice. The absence of CTLA-4–mediated suppression of CD28 signaling resulted in the early expression of FoxP3 on double-positive cells in the thymic cortex. We conclude that CTLA-4 is not essential for the peripheral function of FoxP3+ Treg cells but plays a pivotal role in their thymic selection.