Toll-like receptor agonists in cancer therapy

Abstract

Toll-like receptors (TLRs) are pattern-recognition receptors related to the Drosophila Toll protein. TLR activation alerts the immune system to microbial products and initiates innate and adaptive immune responses. The naturally powerful immunostimulatory property of TLR agonists can be exploited for active immunotherapy against cancer. Antitumor activity has been demonstrated in several cancers, and TLR agonists are now undergoing extensive clinical investigation. This review discusses recent advances in the field and highlights potential opportunities for the clinical development of TLR agonists as single agent immunomodulators, vaccine adjuvants and in combination with conventional cancer therapies.

Keywords: CpG; Toll-like receptor; Toll-like receptor agonist; bacillus Calmette–Guerin; imiquimod; immunotherapy; monophosphoryl lipid A; polyriboinosinic-polyribocytidylic acid; resiquimod; vaccine adjuvant.

Figure 1. Toll-like receptor signaling

Toll-like receptors (TLRs) recognize microbial products, endogenous ligands (such as HGMB1, not shown) and synthetic agonists (not shown) either directly or aided by accessory molecules, such as CD14 and MD2. Dimerization of the receptor (heterodimerization for TLR2/6 and TLR1/6) is followed by downstream signaling. All TLRs engage the MyD88 adaptor molecule with the exception of TLR3, which signals through TRIF. TLR4 signaling can follow MyD88/TIRAP-dependent and/or TRIF/TRAM-dependent pathways. Activation of NF-κB, activator protein (AP)-1 and interferon regulatory factors (IRFs) induces expression of genes encoding inflammatory cytokines, type 1 IFN and IFN-inducible genes. MyD88: Myeloid differentiation factor-88; TIRAP: TIR-associated protein; TRAM: Toll-receptor-associated molecule; TRIF: Toll receptor-associated-activator of interferon.

Figure 2. Toll-like receptor agonists and their immune effects on the tumor microenvironment

Left panel: several immunosuppressive mechanisms inhibit Teff function in the tumor microenvironment. Tumor-derived soluble factors such as IL-10, TGF-β and VEGF recruit MDSCs, prevent maturation of dendritic cells and inhibit Teff function. Furthermore, tumors deficient in B7–1 and B7–2 expression lead to inefficient T-cell stimulation; and expression of programmed death (PD)-ligand 1 on tumors inhibits T-cell activation (not shown). Tregs, often infiltrating tumors, can also directly suppress Teff function and trigger IL-10 production, which leads to upregulation of B7-H4 on antigen-presenting cells (APCs), and inhibition of DC maturation. B7-H4⁺ APCs such as TAM can induce cell cycle arrest in T cells. iDCs promote the accumulation of Tregs and result in T-cell anergy. Right panel: Toll-like receptor (TLR) activation enhances antitumor immunity via several mechanisms. TLR-mediated DC activation leads to enhanced phagocytosis, maturation with upregulation of MHC and costimulatory molecules (CD80, CD86 and CD40), secretion of Th1 cytokines (especially IL-12) and antigen presentation to lymphocytes resulting in the generation of Teff and antigen-specific B cells. Furthermore, local TLR activation can enhance natural killer cell-mediated cytotoxicity (not shown), upregulate MHC class I on tumor cells, directly induce tumor cell apoptosis and shift the tumor microenvironment toward an inflammatory milieu conducive to antigen cross-presentation and infiltration by functional Teff. DC: Dendritic cell; iDC: Immature dendritic cell; MDSC: Myeloid-derived suppressor cell; Mono/Macro: Monocyte/macrophage; TAM: Tumor-associated macrophage; Teff: Effector T cell; Treg: Regulatory T cell.

Figure 3. Dendritic cells recruited into skin treated with a Toll-like receptor-7 agonist

Imiquimod 5%, applied topically to patients' healthy skin for five days (in order to condition the site for vaccination) results in marked cellular infiltration in the dermis (H&E-stained sections, ×40 magnification). Upper left panel: untreated skin (control), Upper right panel: imiquimod-treated skin with perivascular and periadnexal mononuclear cell infiltrates (insert ×200 magnification). Infiltrates were mainly composed of lymphocytes as well as myeloid and plasmacytoid dendritic cells (as shown in representative immunohistochemistry sections). Lower left panel: mature DC (CD83), Lower center panel: activated myeloid dendritic cells (DC-LAMP) and Lower right panel: plasmacytoid dendritic cells (CD123).