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.

Liposomal Delivery of an Immunostimulatory CpG Induces Robust Antitumor Immunity and Long‐Term Immune Memory by Reprogramming Tumor‐Associated Macrophages

“…Systemic administration of immune‐stimulating molecules can cause autoimmune toxicity, [ 25 ] an effect that can be avoided by direct administration of immune stimulants. [ 26 ] Accordingly, we next assessed whether direct (intratumoral) administration of SLNP@CpG caused side effects in mice. Mice injected with SLNP@CpG showed no significant changes in body weight compared with control mice.…”

Anti-EGFR Bioengineered Bacterial Outer Membrane Vesicles as Targeted Immunotherapy Agents in Triple-Negative Breast Tumor Murine Model

“…TLRs, natural triggers of both innate and adaptive immunity, are promising targets in cancer therapy. At present, only three FDA-approved TLR agonists are available for use in patients [21]. In the tumor microenvironment (TME), TLRs can be expressed by multiple immune cells, including monocytes, macrophages, dendritic cells, B cells, T cells, mast cells, and natural killer (NK) cells, as well as by nonimmune cells, such as epithelial cells, broblasts, and cancer cells [22,23].…”

Immunotherapeutic Agents for Intratumoral Immunotherapy

“…Different types of TLRs react to particular PAMPs in order to recognize and respond to invading microbial pathogens. Lipoteichoic acid and peptidoglycan of bacterial components stimulate TLR2/TLR6 heterodimer, double-stranded RNAs from viruses activate TLR3, lipopolysaccharides (LPS) from Gram-negative bacteria stimulate TLR4, guanosine-or uridine-rich single-stranded RNAs from viruses activate TLR7/8 and viral DNAs rich in unmethylated CpG motifs stimulate TLR9 [68]. In addition to TLRs to trigger protective innate and adaptive immune responses, the cyclic GMP-AMP synthase (cGAS)-stimulator of the interferon (IFN) genes (STING) pathway detects doublestranded DNA (dsDNA), a product resulting from viral or bacterial infection or severe tissue damage through the allosteric binding of the dsDNA to the nucleotide cyclase enzyme cGAS, which subsequently generates cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) dinucleotides to activate STING for the induction of innate immune reactions by inducing transcription of IFNs and numerous IFN-stimulated genes as well as NF-κB-mediated expression of proinflammatory cytokines genes [69,70].…”