Toll-Like Receptor Agonists as Adjuvants for Allergen Immunotherapy

Toll-like receptors (TLRs) are essential components of innate immunity and provide defensive inflammatory responses to invading pathogens. Located within the plasma membranes of cells and also intracellular endosomes, TLRs can detect a range of pathogen associated molecular patterns from bacteria, viruses and fungi. TLR activation on dendritic cells can propagate to an adaptive immune response, making them attractive targets for the development of both prophylactic and therapeutic vaccines. In contrast to conventional adjuvants such as aluminium salts, TLR agonists have a clear immunomodulatory profile that favours anti-allergic T lymphocyte responses. Consequently, the potential use of TLRs as adjuvants in Allergen Immunotherapy (AIT) for allergic rhinitis and asthma remains of great interest. Allergic Rhinitis is a Th2-driven, IgE-mediated disease that occurs in atopic individuals in response to exposure to otherwise harmless aeroallergens such as pollens, house dust mite and animal dander. AIT is indicated in subjects with allergic rhinitis whose symptoms are inadequately controlled by antihistamines and nasal corticosteroids. Unlike anti-allergic drugs, AIT is disease-modifying and may induce long-term disease remission through mechanisms involving upregulation of IgG and IgG4 antibodies, induction of regulatory T and B cells, and immune deviation in favour of Th1 responses that are maintained after treatment discontinuation. This process takes up to three years however, highlighting an unmet need for a more efficacious therapy with faster onset. Agonists targeting different TLRs to treat allergy are at different stages of development. Synthetic TLR4, and TLR9 agonists have progressed to clinical trials, while TLR2, TLR5 and TLR7 agonists been shown to have potent anti-allergic effects in human in vitro experiments and in vivo in animal studies. The anti-allergic properties of TLRs are broadly characterised by a combination of enhanced Th1 deviation, regulatory responses, and induction of blocking antibodies. While promising, a durable effect in larger clinical trials is yet to be observed and further long-term studies and comparative trials with conventional AIT are required before TLR adjuvants can be considered for inclusion in AIT. Here we critically evaluate experimental and clinical studies investigating TLRs and discuss their potential role in the future of AIT.

Safety and pharmacodynamics of intranasal GSK2245035, a TLR7 agonist for allergic rhinitis: A randomized trial

BACKGROUND

Toll-like receptor 7 (TLR7) stimulation in the airways may reduce responses to aeroallergens by induction of type 1 interferons (IFNs). GSK2245035 is a novel selective TLR7 agonist in pharmaceutical development.

OBJECTIVE

Assessment of safety, pharmacodynamics and nasal allergic reactivity following repeated weekly intranasal (i.n.) GSK2245035.

METHODS

This randomized, double-blind, placebo-controlled study (TL7116958) was conducted over two pollen seasons (2013-2014) and follow-up study (204509) conducted 1 year later. Participants with allergic rhinitis (n=42) were randomized to receive eight weekly doses of i.n. GSK2245035 (20 ng [2014 Cohort; n=14] or 80 ng [2013 Cohort; n=14]) or placebo (n=14). Adverse events (AEs) including cytokine release syndrome AEs (CytoRS-AEs) and nasal symptoms were assessed. Nasal and serum IFN-inducible protein 10 (IP-10) were measured after doses 1 and 8, then 1 (follow-up visit [FUV] 1) and 3 (FUV2) weeks after final dose. Nasal allergen challenges (NACs) and allergic biomarker assessment (nasal, serum) were conducted at baseline, FUV1, FUV2 and at a FUV 1 year after final dose (FUV3; 2014 Cohort only). A Bayesian framework enabled probability statements for mean effect sizes.

RESULTS

GSK2245035 induced CytoRS-AEs (most commonly headache, median duration <1 day) in 93% of participants at 80 ng, while AE incidence at 20 ng was similar to placebo. There was no evidence of nasal inflammation. Dose-related increases in nasal and serum IP-10 were observed 24 hours after doses 1 and 8 (>95% certainty). Both doses showed a trend in reducing total nasal symptom score 15 minutes post-NAC at FUV1 and FUV2, but there was no reduction evident at FUV3. Nasal levels of selected allergic biomarkers demonstrated trends for reductions at FUV1, FUV2 and FUV3.

CONCLUSIONS AND CLINICAL RELEVANCE

Weekly i.n. GSK2245035 20 ng was well tolerated and reduced allergic reactivity to nasal challenge for 3 weeks post-treatment.

Discovery of 6-Amino-2-{[(1S)-1-methylbutyl]oxy}-9-[5-(1-piperidinyl)pentyl]-7,9-dihydro-8H-purin-8-one (GSK2245035), a Highly Potent and Selective Intranasal Toll-Like Receptor 7 Agonist for the Treatment of Asthma

Induction of IFNα in the upper airways via activation of TLR7 represents a novel immunomodulatory approach to the treatment of allergic asthma. Exploration of 8-oxoadenine derivatives bearing saturated oxygen or nitrogen heterocycles in the N-9 substituent has revealed a remarkable selective enhancement in IFNα inducing potency in the nitrogen series. Further potency enhancement was achieved with the novel (S)-pentyloxy substitution at C-2 leading to the selection of GSK2245035 (32) as an intranasal development candidate. In human cell cultures, compound 32 resulted in suppression of Th2 cytokine responses to allergens, while in vivo intranasal administration at very low doses led to local upregulation of TLR7-mediated cytokines (IP-10). Target engagement was confirmed in humans following single intranasal doses of 32 of ≥20 ng, and reproducible pharmacological response was demonstrated following repeat intranasal dosing at weekly intervals.

Inhibition of capsaicin-driven nasal hyper-reactivity by SB-705498, a TRPV1 antagonist

AIMS

To assess the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of intranasal SB-705498, a selective TRPV1 antagonist.

METHODS

Two randomized, double-blind, placebo-controlled, clinical studies were performed: (i) an intranasal SB-705498 first time in human study to examine the safety and PK of five single escalating doses from 0.5 to 12 mg and of repeat dosing with 6 mg and 12 mg twice daily for 14 days and (ii) a PD efficacy study in subjects with non-allergic rhinitis (NAR) to evaluate the effect of 12 mg intranasal SB-705498 against nasal capsaicin challenge.

RESULTS

Single and repeat dosing with intranasal SB-705498 was safe and well tolerated. The overall frequency of adverse events was similar for SB-705498 and placebo and no dose-dependent increase was observed. Administration of SB-705498 resulted in less than dose proportional AUC(0,12 h) and Cmax , while repeat dosing from day 1 to day 14 led to its accumulation. SB-705498 receptor occupancy in nasal tissue was estimated to be high (>80%). Administration of 12 mg SB-705498 to patients with NAR induced a marked reduction in total symptom scores triggered by nasal capsaicin challenge. Inhibition of rhinorrhoea, nasal congestion and burning sensation was associated with 2- to 4-fold shift in capsaicin potency.

CONCLUSIONS

Intranasal SB-705498 has an appropriate safety and PK profile for development in humans and achieves clinically relevant attenuation of capsaicin-provoked rhinitis symptoms in patients with NAR. The potential impact intranasal SB-705498 may have in rhinitis treatment deserves further evaluation.

Immunomodulation: the future cure for allergic diseases

Allergies are the result of aberrant immune reactivity against common innocuous environmental proteins (allergens). A pivotal component of allergic pathogenesis is the generation of allergen-specific Th cells with an effector phenotype. These Th cells activate a complex immune cascade that triggers the release of potent mediators and enhances the mobilization of several inflammatory cells types, which in turn elicit the acute allergic reactions and promote the development of chronic inflammation. The current therapies for allergic diseases focus primarily on pharmacological control of symptoms and suppression of inflammation. This approach is beneficial, but not curative, since the underlying immune pathology is not inhibited. In an attempt to develop more effective therapeutic strategies, the scientific interest has been directed toward methods down-modulating the immune mechanisms that initiate and maintain the allergic cascade. Today, the only widely used disease-modifying form of allergy treatment is the specific immunotherapy with allergen extracts. More recently the use of anti-IgE has been approved for patients with allergic asthma. Other immunomodulatory methods being currently explored are the administration of microbial adjuvants that inhibit Th2 reactivity and the design of molecules that interrupt the activity of key allergic cytokines, chemokines, or other Th2 effector mediators.

Enhancement of MEK/ERK signaling promotes glucocorticoid resistance in CD4+ T cells

Glucocorticoids have potent immunosuppressive properties, but their effects are often modulated by the conditions prevailing in the local immune milieu. In this study we determined whether the action of glucocorticoids is influenced by the degree of signaling during T cell activation. We found that dexamethasone (Dex) effectively suppressed T cell receptor-induced (TCR-induced) proliferation of naive CD4+ T cells, through a mechanism involving downregulation of c-Fos expression and inhibition of activator protein-1 (AP-1), nuclear factor of activated T cells (NF-AT), and NF-kappaB transcriptional activity. However, enhancement of TCR signaling by CD28- or IL-2-mediated costimulation abrogated the suppressive effect of Dex on c-Fos expression and AP-1 function and restored cellular proliferation. The amount of signaling through the MAPK pathway was critical in determining the effect of Dex on T cell activation. In particular, costimulatory signaling via MAPK kinase (MEK) and extracellular signal-regulated kinase (ERK) was essential for the development of T cell resistance to Dex. Selective blockade of MEK/ERK signal transduction abolished the costimulation-induced resistance. In contrast, transmission of IL-2 signals via STAT5 and CD28 signals via NF-kappaB remained inhibited by Dex. These results imply that the immune system, by regulating the degree of local costimulation through MEK/ERK, can modify the effect of glucocorticoids on T cells. Moreover, these findings suggest that MAPK inhibitors may offer a therapeutic solution for glucocorticoid resistance.

Critical role of B cells in the development of T cell tolerance to aeroallergens

Respiratory exposure to allergen induces the development of allergen-specific CD4(+) T cell tolerance that effectively protects against the development of allergic-sensitization and T(h)2-biased immunity. The establishment of T cell unresponsiveness to aeroallergens is an active process preceded by a transient phase of T cell activation that requires T cell co-stimulation and is critically influenced by the antigen-presenting cell type. In this study we examined the role of B cells in the development of respiratory tolerance following intranasal (i.n.) exposure to a prototypic protein antigen. We found that respiratory exposure of BCR-transgenic (Tg) mice to minute quantities of cognate antigen effectively induced T cell unresponsiveness, indicating that antigen presentation by antigen-specific B cells greatly enhanced the development of respiratory tolerance. In contrast, respiratory T cell unresponsiveness could not be induced in B cell-deficient JHD mice exposed to i.n. antigen, although T cell tolerance developed in JHD mice reconstituted with B cells, suggesting that B cells are required for the induction of respiratory T cell tolerance. Respiratory exposure of BCR-Tg mice to cognate antigen induced activation of antigen-specific T cells and partial activation of antigen-specific B cells, as demonstrated by enhanced expression by B cells of class II MHC and B7 molecules but lack of antibody secretion. Our data indicate that B cells critically influence the immune response to inhaled allergens and are required for the development of allergen-specific T cell unresponsiveness induced by respiratory allergen.

Respiratory infection with influenza A virus interferes with the induction of tolerance to aeroallergens

Viral respiratory infections have been implicated in influencing allergen sensitization and the development of asthma, but their exact role remains controversial. Because respiratory exposure to Ag normally engenders T cell tolerance and prevents the development of airway hyperreactivity (AHR) and inflammation, we examined the effects of influenza A virus infection on tolerance induced by exposure to intranasal (i.n.) OVA and the subsequent development of AHR. We found that concurrent infection with influenza A abrogated tolerance induced by exposure to i.n. OVA, and instead led to the development of AHR accompanied by the production of OVA-specific IgE, IL-4, IL-5, IL-13, and IFN-gamma. When both IL-4 and IL-5 were neutralized in this system, AHR was still induced, suggesting that influenza-induced cytokines such as IL-13, or mechanisms unrelated to cytokines, might be responsible for the development of AHR. The length of time between influenza A infection and i.n. exposure to OVA was crucial, because mice exposed to i.n. OVA 15-30 days after viral inoculation developed neither AHR nor OVA-specific tolerance. These mice instead acquired Th1-biased OVA-specific immune responses associated with vigorous OVA-induced T cell proliferation, and reduced production of OVA-specific IgE. The protective effect of influenza A on AHR was dependent on IFN-gamma, because protection was abrogated with a neutralizing anti-IFN-gamma mAb. These results suggest that viral respiratory infection interferes with the development of respiratory allergen-induced tolerance, and that the time interval between viral infection and allergen exposure is critical in determining whether viral infection will enhance, or protect against, the development of respiratory allergen sensitization and AHR.

Mechanisms preventing allergen-induced airways hyperreactivity: role of tolerance and immune deviation

BACKGROUND

Aeroallergens continuously enter the respiratory tract of atopic individuals and provoke the development of asthma characterized by airway hyperreactivity (AHR) and inflammation. By contrast, nonatopic individuals are exposed to the same aeroallergens, but airway inflammation does not develop. However, the mechanisms that prevent allergen-induced respiratory diseases in nonatopic subjects are poorly characterized.

OBJECTIVE

In this study we compared the role of allergen-specific T-cell tolerance and immune deviation in conferring protection against the development of allergen-induced AHR.

METHODS

We exposed mice to intranasal ovalbumin (OVA) to induce T-cell tolerance and examined its effects on the subsequent development of AHR and inflammation.

RESULTS

We demonstrated that exposure of mice to intranasal OVA resulted in peripheral CD4(+) T-cell unresponsiveness that very efficiently prevented not only the development of AHR but also greatly inhibited airway inflammation and OVA-specific IgE production. The induction of peripheral T-cell tolerance and protection against AHR were not dependent on the presence of IFN-gamma or IL-4. The development of AHR was also prevented by an OVA-specific T(H)1-biased immune response induced by inhalation of OVA in the presence of IL-12. However, the OVA-specific T(H)1 response was associated with a significant degree of pulmonary inflammation.

CONCLUSION

These results indicate that both allergen-specific T-cell tolerance and T(H)1-biased immune deviation prevent the development of AHR, but T(H)1 responses are associated with significantly greater inflammation in the lung than is associated with T-cell unresponsiveness. Therefore CD4(+) T-cell unresponsiveness critically regulates immune responses to aeroallergens and protects against the development of allergic disease and asthma.

Intranasal exposure to protein antigen induces immunological tolerance mediated by functionally disabled CD4+ T cells

In this study we examined the immunological parameters underlying the natural immunity to inhaled nonpathogenic proteins. We addressed this question by examining the effect of intranasal exposure to OVA in both wild-type mice and mice reconstituted with OVA-TCR transgenic CD4+ T cells. Intranasal administration of OVA induced an initial phase of activation during which CD4+ T cells were capable of proliferating and producing cytokines. Although many of the OVA-specific CD4+ T cells were subsequently depleted from the lymphoid organs, a stable population of such T cells survived but remained refractory to antigenic rechallenge. The unresponsive state was not associated with immune deviation due to selective secretion of Th1- or Th2-type cytokines, and the presence of regulatory CD8+ T cells was not required. Moreover, neutralization of the immunosuppressive cytokines IL-10 and TGF-beta did not abrogate the induction of tolerance. Inhibition of the interaction of T cells with CD86, but not CD80, at the time of exposure to intranasal Ag prevented the development of unresponsiveness, while selective blockade of CTLA-4 had no effect. Our results suggest that intranasal exposure to Ags results in immunological tolerance mediated by functionally impaired CD4+ T cells via a costimulatory pathway that requires CD86.

Intranasal Exposure to Protein Antigen Induces Immunological Tolerance Mediated by Functionally Disabled CD4+ T Cells

In this study we examined the immunological parameters underlying the natural immunity to inhaled nonpathogenic proteins. We addressed this question by examining the effect of intranasal exposure to OVA in both wild-type mice and mice reconstituted with OVA-TCR transgenic CD4+ T cells. Intranasal administration of OVA induced an initial phase of activation during which CD4+ T cells were capable of proliferating and producing cytokines. Although many of the OVA-specific CD4+ T cells were subsequently depleted from the lymphoid organs, a stable population of such T cells survived but remained refractory to antigenic rechallenge. The unresponsive state was not associated with immune deviation due to selective secretion of Th1- or Th2-type cytokines, and the presence of regulatory CD8+ T cells was not required. Moreover, neutralization of the immunosuppressive cytokines IL-10 and TGF-β did not abrogate the induction of tolerance. Inhibition of the interaction of T cells with CD86, but not CD80, at the time of exposure to intranasal Ag prevented the development of unresponsiveness, while selective blockade of CTLA-4 had no effect. Our results suggest that intranasal exposure to Ags results in immunological tolerance mediated by functionally impaired CD4+ T cells via a costimulatory pathway that requires CD86.

Regulation of cytokine production by human Th0 cells following stimulation with peptide analogues: differential expression of TGF-beta in activation and anergy

The different biological activities of T-cell-derived cytokines and their level of production influences the qualitative nature of immune responses and, in certain forms of T-cell tolerance, the lack of antigen responsiveness is associated with the production of transforming growth factor-beta (TGF-beta) and interleukin-4 (IL-4). In this study we have investigated the effects of T-cell receptor (TCR) ligation with peptide analogues and the native peptide, in the presence and absence of costimulation, on cytokine production by human T-helper type 0 (Th0) cells reactive with influenza virus haemagglutinin (HA) peptide (HA306-318) and restricted by HLA-DRB1*0101. We observed that resting Th0 cells constitutively produced TGF-beta, but when stimulated with peptide and antigen-presenting cells (APC) under conditions that induce clonal expansion, TGF-beta secretion was abrogated. Furthermore, exposure of the T cells to the wild-type HA peptide under conditions that induce T-cell anergy resulted in the secretion of TGF-beta, and subsequent antigenic rechallenge was unable to override this signal and down-regulate TGF-beta production. Stimulation with altered TCR ligands that failed to induce proliferation also resulted in marked production of TGF-beta, although in many instances the levels were less than those observed in the total absence of antigen, suggesting that partial signalling has occurred. Although in general, there was a direct positive correlation between proliferation and the production of IL-2, IL-4 and interferon-gamma (IFN-gamma) following stimulation with certain analogues, the production of selected cytokines was dissociated.

Altered T cell ligands derived from a major house dust mite allergen enhance IFN-gamma but not IL-4 production by human CD4+ T cells

Changes in the affinity of the interaction between T cell Ag receptors and their ligands can modulate selected T cell effector functions. Since both allergen-specific Th2 and Th0 cells are present in the peripheral CD4+ T cell pool of atopic individuals, the potential to inhibit cytokine production by Th2 cells and promote Th1-type cytokines from Th0 cells may contribute to the down-regulation of allergic inflammation. The aim of this study was to investigate the effects of peptide analogues of a dominant T cell epitope of the group II allergen derived from house dust mite, residues 28 to 40, on proliferation and cytokine production by human Th2 and Th0 cells. From both functional competition and proliferation assays, using analogues substituted with alanine or charged amino acids, the influence of different positions in p28-40 on TCR recognition and/or MHC class II binding was determined. For the specific Th0 cells, generally those analogues that lead to a reduction in proliferation also decreased both IL-4 and IFN-gamma production. However, the p28-40 analogues with alanine residues at positions 34 and 36 altered the IFN-gamma:IL-4 ratio by selectively enhancing IFN-gamma secretion. In the case of Th2 cells, stimulation with the peptide analogues induced different patterns of effector function. Selected analogues were capable of inducing IL-4 production in the absence of proliferation, whereas in response to other peptide variants of p28-40, both IL-4 production and proliferation were inhibited.

Altered T cell ligands derived from a major house dust mite allergen enhance IFN-gamma but not IL-4 production by human CD4+ T cells

Changes in the affinity of the interaction between T cell Ag receptors and their ligands can modulate selected T cell effector functions. Since both allergen-specific Th2 and Th0 cells are present in the peripheral CD4+ T cell pool of atopic individuals, the potential to inhibit cytokine production by Th2 cells and promote Th1-type cytokines from Th0 cells may contribute to the down-regulation of allergic inflammation. The aim of this study was to investigate the effects of peptide analogues of a dominant T cell epitope of the group II allergen derived from house dust mite, residues 28 to 40, on proliferation and cytokine production by human Th2 and Th0 cells. From both functional competition and proliferation assays, using analogues substituted with alanine or charged amino acids, the influence of different positions in p28-40 on TCR recognition and/or MHC class II binding was determined. For the specific Th0 cells, generally those analogues that lead to a reduction in proliferation also decreased both IL-4 and IFN-gamma production. However, the p28-40 analogues with alanine residues at positions 34 and 36 altered the IFN-gamma:IL-4 ratio by selectively enhancing IFN-gamma secretion. In the case of Th2 cells, stimulation with the peptide analogues induced different patterns of effector function. Selected analogues were capable of inducing IL-4 production in the absence of proliferation, whereas in response to other peptide variants of p28-40, both IL-4 production and proliferation were inhibited.

Induction of anergy in human T helper 0 cells by stimulation with altered T cell antigen receptor ligands

CD4+ T cells may become profoundly unresponsive to antigenic restimulation following ligation of TCR by immunogenic peptides bound to MHC class II molecules in the absence of costimulation. Furthermore, it has been reported that anergy can be induced as a consequence of engagement of TCR by analogues of antigenic peptides presented by live APCs. In this study, based on resolution of the crystal structure of an influenza virus hemagglutinin (HA) peptide (HA 306-318) bound to HLA-DRB1*0101, we investigated the potential of analogues with amino acid substitutions at those positions predicted to form interactions with TCR to differentially activate and/or anergize HA-specific human Th0 cells restricted by DR1 class II molecules. For some analogues altering the affinity of peptide/TCR interactions revealed a direct positive correlation between antigenicity and their ability to induce anergy. Nevertheless, certain HA peptide analogues functioned as partial agonists, which although they failed to stimulate clonal expansion, were capable of rendering the Th0 cells unresponsive to immunogenic rechallenge. Furthermore, differences were noticed in the characteristics of the anergic phenotype induced by selected analogues. Restimulation with the native peptide of Th0 cells pre-exposed to the HA analogues in the absence of costimulatory signals failed to uncouple IL-4 and IFN-gamma secretion; however, in some instances, dissociation of proliferation from cytokine production was observed. The ability to differentially signal T cells through changing the affinity of peptide/TCR interactions may have implications in the potential use of altered TCR ligands in immunotherapy.

Induction of anergy in human T helper 0 cells by stimulation with altered T cell antigen receptor ligands

CD4+ T cells may become profoundly unresponsive to antigenic restimulation following ligation of TCR by immunogenic peptides bound to MHC class II molecules in the absence of costimulation. Furthermore, it has been reported that anergy can be induced as a consequence of engagement of TCR by analogues of antigenic peptides presented by live APCs. In this study, based on resolution of the crystal structure of an influenza virus hemagglutinin (HA) peptide (HA 306-318) bound to HLA-DRB1*0101, we investigated the potential of analogues with amino acid substitutions at those positions predicted to form interactions with TCR to differentially activate and/or anergize HA-specific human Th0 cells restricted by DR1 class II molecules. For some analogues altering the affinity of peptide/TCR interactions revealed a direct positive correlation between antigenicity and their ability to induce anergy. Nevertheless, certain HA peptide analogues functioned as partial agonists, which although they failed to stimulate clonal expansion, were capable of rendering the Th0 cells unresponsive to immunogenic rechallenge. Furthermore, differences were noticed in the characteristics of the anergic phenotype induced by selected analogues. Restimulation with the native peptide of Th0 cells pre-exposed to the HA analogues in the absence of costimulatory signals failed to uncouple IL-4 and IFN-gamma secretion; however, in some instances, dissociation of proliferation from cytokine production was observed. The ability to differentially signal T cells through changing the affinity of peptide/TCR interactions may have implications in the potential use of altered TCR ligands in immunotherapy.