Treatment with the TLR7 agonist R848 induces regulatory T‐cell‐mediated suppression of established asthma symptoms

Abstract

BelgiumThe evolution of allergic asthma is tightly controlled by effector and regulatory cells, as well as cytokines such as IL-10 and/or TGF-b, and it is widely acknowledged that environmental exposure to allergens and infectious agents can influence these processes. In this context, the recognition of pathogen-associated motifs, which trigger TLR activation pathways, plays a critical role with important consequences for disease progression and outcome. We addressed the question whether the TLR7 ligand resiquimod (R848), which has been shown to be protective in several experimental allergic asthma protocols, can also suppress typical asthma symptoms once the disease is established. To this end, we used an OVA-induced experimental model of murine allergic asthma in which R848 was injected after a series of challenges with aerosolized OVA. We found that the treatment attenuated allergic symptoms through a mechanism that required Tregs, as assessed by the expansion of this population in the lungs of mice having received R848, and the loss of R848-mediated suppression of allergic responses after in vivo Treg depletion. IL-10 provided only a minor contribution to this suppressive effect that was largely mediated through a TGF-b-dependent pathway, a finding that opens new therapeutic opportunities for the pharmacological targeting of Tregs.Key words: Asthma . TLR7 agonist . Tregs IntroductionEpidemiological studies have established that in recent decades the prevalence of allergic asthma has significantly increased in developed countries. Emerging evidence attests that early life events, including exposure to allergens and infections, are critical in programming effective regulatory pathways to maintain pulmonary homeostasis. Toll-like receptor (TLR) signaling contributes prominently to CD41 T-cell activation by connecting innate and acquired immunity. Mouse models of allergic asthma have long been used to dissect the immunological mechanisms leading to asthma, and despite the heterogeneity of experimental protocols in terms of strain differences, type and dose of antigen, time and route of administration, most 1992studies have provided evidence for protective effects of natural or synthetic TLR2, TLR4, or TLR9 ligands in allergen-induced lung inflammation [1][2][3][4][5]. Similarly, the synthetic ligand of TLR7, R848 (or resiquimod), has been reported to prevent typical respiratory syndromes (airway hyperresponsiveness) and allergic inflammation (recruitment of eosinophils to the lung, production of IgE Abs, and Th2-driven cytokine production) [5][6][7][8] in treated mice.R848 was initially developed as a potential antiviral agent and exerts its immunostimulatory function via mouse TLR7 as well as human TLR7 and TLR8 [9,10]. TLR7 stimulation activates APCs through the MyD88 pathway leading to the induction of proinflammatory cytokines, chemokines, and the release of large amounts of type I IFNs together with upregulation of costimulatory molecules [11]. In the rat, R848 has been shown to inhibit the inflammatory re...

Development of a Novel Severe Triple Allergen Asthma Model in Mice Which Is Resistant to Dexamethasone and Partially Resistant to TLR7 and TLR9 Agonist Treatment

“…The phenotype induced reflects the atopic phenotype with a strong Th2 plus weaker Th1 response. This chronic model is steroid insensitive and less sensitive to therapeutic intervention with the TLR7 and TLR9 agonist resiquimod or CpG-ODN than other published severe asthma models [12], [40], [46], [47]. Nevertheless, the strongest suppressive effects were found when the TLR9 agonist was used suggesting that TLR9 agonists may also be effective in treating steroid insensitive severe asthma patients.…”

The Therapeutic Potential of Toll-Like Receptor 7 Stimulation in Asthma

“…Attenuation of airway inflammation occurs whether TLR7 agonists are given at the time of ovalbumin sensitization or challenge, suggesting TLR7 stimulation may be effective against both disease development and exacerbation. TLR7 stimulation reduces levels of eotaxin, a potent eosinophil chemoattractant in the lung [49], which may contribute to the reduction in eosinophilic infiltration in this model. TLR7 stimulation also reduces IgE [38].…”

“…Similarly, Foxp3-expressing regulatory T (Treg) cells have been implicated in the prevention of airway inflammation. Depleting Treg cells or blocking the Treg cell mediator TGF-β reverses TLR7-mediated suppression of lung inflammation [49]. In contrast, depletion of cytotoxic CD8 + T cells has no effect on acute TLR7-mediated prevention of airway inflammation [43].…”

“…TLR7 agonists, given either intranasally [39, 45], intraperitoneally [40, 48, 49, 52] or via inhalation [35, 36, 37] attenuate ovalbumin-induced AHR. TLR7 stimulation ameliorates AHR when animals are treated either 24-hours before or 30-minutes after antigen challenge, suggesting a therapeutic role of TLR agonists for both chronic disease progression and acute exacerbations.…”

Toll-Like Receptor 7-Targeted Therapy in Respiratory Disease

“…For example, resiquimod given prophylactically or therapeutically reduced inflammatory cell recruitment as well as IL-4, IL-5, IL-13 and IgE levels following allergen challenge in sensitized animals [47,48,49,50]. Resiquimod reversed airway reactivity [47,48,50] and prevented airway smooth muscle proliferation and goblet cell hyperplasia following chronic allergen challenge [49]. Thus, TLR7-targeted therapy may prevent both pathologic remodeling of the airway and airflow obstruction.…”