The TLR7 Agonist R848 Alleviates Allergic Inflammation by Targeting Invariant NKT Cells To Produce IFN-γ

Abstract

It has been documented that TLR7 stimulation triggers not only antiviral responses, but also alleviates experimental asthma. Considering the implication of invariant NKT (iNKT) cells in both situations, we postulated that they might contribute to the anti-inflammatory effect of TLR7 ligands. We show in this study that spleen cells activated by the TLR7 agonist resiquimod (R848) attenuate allergic inflammation upon adoptive transfer when they are recovered from wild-type, but not from iNKT cell-deficient Jα18−/− mice, which proves the specific involvement of this regulatory population. Furthermore, we provide evidence that IFN-γ is critical for the protective effect, which is lost when transferred iNKT cells are sorted from IFN-γ–deficient mice. In support of a direct activation of iNKT cells through TLR7 signaling in vivo, we observed a prompt increase of serum IFN-γ levels, associated with upregulation of CD69 expression on iNKT cells. Moreover, we demonstrate that iNKT cells effectively express TLR7 and respond to R848 in vitro by producing high levels of IFN-γ in the presence of IL-12, consistent with the conclusion that their contribution to the alleviation of allergic inflammation upon treatment with TLR7 ligands is mediated through IFN-γ.

IL-27 Is Essential for Suppression of Experimental Allergic Asthma by the TLR7/8 Agonist R848 (Resiquimod)

“…In a murine asthma model, intranasal application of R848 efficiently inhibited the asthma phenotype, including airway hyperreactivity (AHR), airway inflammation, mucus hypersecretion, and Th2 cytokine production, confirming findings of previous studies in asthma models (18,(23)(24)(25)(26). Furthermore, R848 strongly induced IL-27 secretion, which was crucial for its therapeutic effect as demonstrated by in vivo neutralization of IL-27 via an mAb.…”

“…It activates APCs via the MyD88 pathway, leading to induction of proinflammatory cytokines and chemokines, especially large amounts of type 1 IFNs (22). In different rodent models of experimental asthma, R848 effectively inhibited the development of asthma and even reversed already established asthma symptoms after systemic and intranasal application (18,(23)(24)(25).…”

“…However, IFN-g cannot solely account for the protective effect because the R848-mediated protection is neither abolished in IFN-g-deficient mice nor through depletion of NKT cells (23,26). In addition to inducing IFN-g (18), application of R848 enhanced secretion of IL-27 in human and murine cells (27,28). Moreover, in allergen-induced airway inflammation models, secretion of IL-27 together with IFN-g was essential for attenuation of the Th2 response (29).…”

“…TLR7 and TLR8 are receptors with high affinity for viral ssRNA (14). TLR7 is expressed most abundantly on plasmacytoid dendritic cells (DCs), myeloid DCs, B cells, and epithelial cells (15), whereas TLR8 is mainly expressed on monocytes, myeloid DCs, and neutrophils (14,(16)(17)(18). R848 (resiquimod) is an imidazoquinoline compound initially developed as a potential antiviral agent that exerts its immunostimulatory function via mouse TLR7 as well as human TLR7 and TLR8 (19)(20)(21).…”

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

“…In agreement with this notion, we have recently shown that R848-induced NKT cell activation alleviates asthma symptoms through IFN-g production in an IL-12-dependent manner [27]. The present study shows that CD25 depletion reverses the effect of R848 on all asthma hallmarks tested, namely AHR, cellular infiltration in BALF and production of Th2 cytokines (IL-4, IL-5, IL-10, and IL-13) as well as chemokines (eotaxin) and the Th17 cytokine IL-17.…”