Toll-like Receptor Agonist Conjugation: A Chemical Perspective

Abstract

Toll-like receptors (TLRs) are vital elements of the mammalian immune system that function by recognizing pathogen-associated molecular patterns (PAMPs), bridging innate and adaptive immunity. They have become a prominent therapeutic target for the treatment of infectious diseases, cancer, and allergies, with many TLR agonists currently in clinical trials or approved as immunostimulants. Numerous studies have shown that conjugation of TLR agonists to other molecules can beneficially influence their potency, toxicity, pharmacokinetics, or function. The functional properties of TLR agonist conjugates, however, are highly dependent on the ligation strategy employed. Here, we review the chemical structural requirements for effective functional TLR agonist conjugation. In addition, we provide similar analysis for those that have yet to be conjugated. Moreover, we discuss applications of covalent TLR agonist conjugation and their implications for clinical use.

Peptide–TLR-7/8a conjugate vaccines chemically programmed for nanoparticle self-assembly enhance CD8 T-cell immunity to tumor antigens

“…Each of the neoantigens were formulated either as native LPs admixed with adjuvant (i.e. polyICLC + anti-CD40), which is representative of therapeutic cancer vaccines that have been widely used in clinical studies 10 ; as conjugate vaccines based on LPs linked to a hydrophobic molecule that form microparticles/aggregates (MP-7/8a), which is similar to current conjugate vaccine approaches that do not include a charge modifying group [22][23][24] ; or, as CM conjugates of the LPs, which self-assemble into nanoparticles (SNP-7/8a).…”

“…Indeed, formulating peptide antigens and adjuvants with particle technologies, including poly(lactic-co-glycolic acid) (PLGA) 16 , liposomes 17 , lipid nanodiscs 18 , polymersomes 19 and emulsions 20 , is an empirical process whereby peptide loading and other formulation characteristics may be different for each antigen. An alternative approach is to use conjugate vaccines based on peptide antigens linked to hydrophobic carriers (e.g., lipids 21 , fatty acids 22 and TLRa [23][24][25] ) that can induce particle assembly or bind albumin for more efficient delivery to lymph nodes 21,26 . Conjugate vaccines offer the potential advantages that antigen loading is chemically defined and that adjuvants can be covalently attached to ensure co-delivery of both components to antigen presenting cells (APCs), which may be needed for optimal T cell priming 27,28 .…”

“…An alternative approach is to use conjugate vaccines based on peptide antigens linked to hydrophobic carriers ( e.g. , lipids 21 , fatty acids 22 and TLRa 23 – 25 ) that can induce particle assembly or bind albumin for more efficient delivery to lymph nodes 21 , 26 . Conjugate vaccines offer the potential advantages that antigen loading is chemically defined and that adjuvants can be covalently attached to ensure co-delivery of both components to antigen presenting cells (APCs), which may be needed for optimal T cell priming 27 , 28 .…”

Recent Advances in Toll Like Receptor-Targeting Glycoconjugate Vaccines

“…Some of them form complexes when binding to ligands to be activated. TLR2 can form complexes with either TLR1 or TLR6, and TLR4 associates with myeloid differentiation factor 2 (MD2) [ 25 ] after being activated by lipopolysaccharide (LPS) ligand [ 26 ]. Each of these TLRs or their complexes recognize and are activated by multiple molecular patterns characteristic of bacteria or viruses.…”

“…Aside from these two major classes of ligands, other TLR ligands such as Flagellin (an agonist of TLR5), small heterocycle molecules (agonists of TLR7/8) and unmethylated cytidine-phosphate-guanosine (CpG) oligodeoxynucleotides (agonists of TLR9) have also been studied as built-in vaccine adjuvants. Most of these compounds are, however, linked to protein or peptide antigens and have been reviewed elsewhere [ 26 , 28 ], and conjugation of these ligands with carbohydrate antigen has not been reported.…”