Systemic Cancer Therapy with a Small Molecule Agonist of Toll-like Receptor 7 Can Be Improved by Circumventing TLR Tolerance

Abstract

Topical application of small molecule Toll-like receptor 7 (TLR7) agonists is highly effective for the treatment of skin tumors, whereas their systemic application has been largely unsuccessful for cancer therapy. One reason may be that repeated systemic application of TLR ligands can induce a state of immune unresponsiveness, termed TLR tolerance. We show here that a single injection of the TLR7 agonist R848 in mice induces a short period of increased response to TLR stimulation followed by a state of hyporesponsiveness lasting several days. This state is characterized by inhibited secretion of the key cytokines interleukin (IL)-12p70 and IL-6 as well as by a block in IFN-a production. We show for the first time that at the cellular level, TLR7 tolerance occurs in both plasmacytoid and myeloid dendritic cells, two cell populations that play a critical role in the initiation and amplification of antitumor immune responses. We further show that TLR7 tolerance in plasmacytoid dendritic cells is accompanied by downregulation of the adaptor protein IL-1 receptor-associated kinase 1. On the basis of these findings, we have designed a novel strategy for the treatment of tumors by using cycles of repeated R848 injections separated by treatment-free intervals. We show in CT26 tumor-bearing mice that this protocol circumvents TLR7 tolerance and improves the efficacy of cancer immunotherapy. Cancer Res; 71(15); 5123-33. Ó2011 AACR.

Reprogramming of TLR7 signaling enhances antitumor NK and cytotoxic T cell responses

“…We have previously demonstrated that this block in cytokine secretion is due to a state of tolerance that lasts for several days after TLR7 stimulation. 18 As expected, the simultaneous application of poly(I:C) and R848 at a single time point led to higher cytokine production in bone marrow cells than stimulation with R848 alone. However, stimulation of bone marrow cells with first poly(I:C) followed by R848 24 h later consistently resulted in even higher production of the cytokines IL-12p40, IL-12p70, and TNF-a (Fig.…”

“…Indeed, we found earlier that repetitive stimulation of the TLR7 pathway led to unresponsiveness to TLR7 ligands and other MyD88-dependent agonists. 18 Im-portantly, it was possible to circumvent this tolerance by well-timed applications of TLR7 agonists and thus improve the efficacy of antitumor therapy. 18 Whether the reprogramming of PRR signaling pathways seen following viral exposure can affect NK and effector T-cell responses, and whether this phenomenon and the associated enhancement of TLR7 signaling can be harnessed for tumor therapy is currently unknown.…”

“…18 Im-portantly, it was possible to circumvent this tolerance by well-timed applications of TLR7 agonists and thus improve the efficacy of antitumor therapy. 18 Whether the reprogramming of PRR signaling pathways seen following viral exposure can affect NK and effector T-cell responses, and whether this phenomenon and the associated enhancement of TLR7 signaling can be harnessed for tumor therapy is currently unknown. Here, we investigated whether TLR7-targeted cancer immunotherapy can be further improved by timing-dependent combination strategies employing poly(I:C) to take advantage of TLR reprogramming and facilitate crosstalk of MyD88-dependent and independent agonists.…”

Polymer-based nanoparticles loaded with a TLR7 ligand to target the lymph node for immunostimulation

“…However, these small molecules also induce inflammatory responses with high serum levels of proinflammatory cytokines when applied systemically. Indeed, even subcutaneous application of R848 leads to a rapid increase of the cytokines IFNα and IL-6 in the serum (Bourquin et al, 2011). Since these unspecific and generalized inflammatory responses can result in doselimiting toxicity (Kobold et al, 2014), it is of great therapeutic interest to develop particulate delivery systems that may focus the immuneactivating effect of these molecules to the target tissues.…”

“…Unfortunately, systemic administration has met with limited success in clinical trials so far (Iribarren et al, 2016). Part of the lack of efficacy may lie in a phenomenon of tolerance that leads to immune unresponsiveness upon repetitive applications of these TLR7 ligands (Bourquin et al, 2011). In addition, upon systemic application in clinical studies, dose-limiting toxicity restricted the therapeutic potential of these drugs (Kobold et al, 2014).…”