Adjuvant Effect of Toll-Like Receptor 9 Activation on Cancer Immunotherapy Using Checkpoint Blockade

Abstract

Immunotherapy using checkpoint blockade has revolutionized cancer treatment, improving patient survival and quality of life. Nevertheless, the clinical outcomes of such immunotherapy are highly heterogeneous between patients. Depending on the cancer type, the patient response rates to this immunotherapy are limited to 20-30%. Based on the mechanism underlying the antitumor immune response, new therapeutic strategies have been designed with the aim of increasing the effectiveness and specificity of the antitumor immune response elicited by checkpoint blockade agents. The activation of toll-like receptor 9 (TLR9) by its synthetic agonists induces the antitumor response within the innate immunity arm, generating adjuvant effects and priming the adaptive immune response elicited by checkpoint blockade during the effector phase of tumor-cell killing. This review first describes the underlying mechanisms of action and current status of monotherapy using TLR9 agonists and immune checkpoint inhibitors for cancer immunotherapy. The rationale for combining these two agents is discussed, and evidence indicating the current status of such combination therapy as a novel cancer treatment strategy is presented.

Keywords: CpG-ODN; adjuvant; cancer immunotherapy; immune checkpoint blockade; innate immune; toll-like receptor.

Figure 1

TLR9 signaling to produce inflammatory cytokines and type I IFNs. (A) TLR9 synthesized in the endoplasmic reticulum traffics through the ER, Golgi to endosome with the aid of UNC93B and AP2, where TLR9 interact with CpG-ODN, recruits MyD88 and downstream signaling molecules to activate NF-κB and IRF-7, resulting in the production inflammatory cytokines and type I IFNs (B,C). Two proposed models for the spatiotemporal activation of NF-κB and IRF-7 at different type of endosomes. In the first model, TLR9 triggers NF-κB activation from the VAMP3⁺ endosomal compartments within minutes after activation and initiates IRF-7 activation in the LAMP1/2 endosomal compartment in 30 min to hours (B). In the second model in plasmacytoid dendritic cells, class A CpG-ODNs activate IRF-7 to produce type I IFNs from the EEA1⁺, TfR⁺ early endosomes. In contrast, class B CpG-ODNs activate NF-κB for inflammatory cytokine production in LAMP1⁺ and lysoTracker⁺ late endosomes/lysosomes (C). The capability of class C CpG-ODNs to activate production of type I IFNs and inflammatory cytokines is in between the capabilities of CpG-ODNs in class A and class C.

Figure 2

Complementary mechanisms of TLR9 activation and checkpoint blockade in combinational cancer immunotherapy. DCs and T cells play key roles in the antitumor immune response. These two types of cells are major target for TLR9 agonists and immune checkpoint inhibitors, respectively. (A) Activation of TLR9 by CpG-ODN triggers innate immune responses, including cytokine production and the uptake and presentation of tumor antigen in DCs. These adjuvant effects, particularly the production of IL-12 and type I IFNs, facilitate a Th1 response of T cells and expansion of tumor-specific T cells during the priming phase. Immune checkpoint blockade by anti-PD-1/anti-PD-L1 antibody release inhibition of CD8⁺ cytotoxic T-cell activation during the effector phase. In contrast, anti-CTLA-4 inhibition activates T cells during both of the priming and effector phases. These events lead to a more effective and more specific adaptive immune response for tumor-cell killing. (B) DCs and T cells involved in the antitumor immune response serve different immunological functions in different locations, as illustrated.