Innate and Adaptive Responses of Intratumoral Immunotherapy with Endosomal Toll-Like Receptor Agonists

Abstract

Toll-like receptors (TLRs) are natural initial triggers of innate and adaptive immune responses. With the advent of cancer immunotherapy, nucleic acids engineered as ligands of endosomal TLRs have been investigated for the treatment of solid tumors. Despite promising results, their systemic administration, similarly to other immunotherapies, raises safety issues. To overcome these problems, recent studies have applied the direct injection of endosomal TLR agonists in the tumor and/or draining lymph nodes, achieving high local drug exposure and strong antitumor response. Importantly, intratumoral delivery of TLR agonists showed powerful effects not only against the injected tumors but also often against uninjected lesions (abscopal effects), resulting in some cases in cure and antitumoral immunological memory. Herein, we describe the structure and function of TLRs and their role in the tumor microenvironment. Then, we provide our vision on the potential of intratumor versus systemic delivery or vaccination approaches using TLR agonists, also considering the use of nanoparticles to improve their targeting properties. Finally, we collect the preclinical and clinical studies applying intratumoral injection of TLR agonists as monotherapies or in combination with: (a) other TLR or STING agonists; (b) other immunotherapies; (c) radiotherapy or chemotherapy; (d) targeted therapies.

Keywords: TLR agonists; antitumoral immunotherapy; intratumoral administration; toll-like receptors; tumor associated macrophages.

Figure 1

Schematic representation of endosomal TLR activation pathways. Upon ligation of endosomal TLRs, stimulation of myD88 or TRIF signaling results in the activation of co-stimulatory molecules, secretion of pro-inflammatory cytokines and Type I IFNs responses. Furthermore, combination treatment with TLR agonists triggers a synergistic induction of NF-kB, IRFs, or MAPKs pathways, leading to enhanced outcomes.

Figure 2

Schematic representation of responses triggered by TLRag in cancer cells (mainly with a protumoral effect) versus immune cells (mainly with antitumoral immune responses). Many experimental studies have shown the activation of protumoral functions by TLR agonists on cancer cells, such as increasing their metabolism and proliferation, epithelial–mesenchymal/leucocytic transition, metastasis, and immunosuppression. On the other hand, TLR agonists activate antitumoral functions on immune cells, such as secretion of pro-inflammatory cytokines, Type I IFNs, increase in levels of perforins or granzyme B, as well as proliferation and recruitment of immune cells to fight against cancer cells.

Figure 3

Pharmacological strategies for the activation of endosomal TLRs and combinations with other therapeutic approaches. TLRag alone or in combination have demonstrated the ability to reprogram TAMs towards M1-like antitumor macrophages. This pharmacological approach has been tested in preclinical and clinical studies with other immunotherapies, such as STING agonists, immune checkpoint inhibitors, or adoptive cell therapy. Other combinations include chemotherapy, oncolytic viruses, or radiotherapy, which can kill cancer cells and trigger immunostimulatory responses. Finally, some studies have used combinations with targeted therapy, epigenetic drugs, or metabolic reprogramming drugs.