Antigen-adjuvant conjugates (AACs) are antigenic proteins or polypeptides coupled to synthetic PRR ligands that potentiate immune responses by triggering the production of pro-inflammatory cytokines and chemokines. These types of bioconjugates represent an opportunity for novel vaccination approaches [1,2]. Indeed, AACs ensure the co-delivery of antigen and adjuvant to the same antigen-presenting cells (APCs). Moreover, the AAC strategy helps overcome the dissociation of the antigen and PRR agonist upon mixture administration and allows a prolonged antigen presentation for the induction of adaptive immune responses, probably through an intracellular storage/depot effect [1,3,4]. Numerous pre-clinical vaccination studies have demonstrated the superior efficacy of AACs over unconjugated mixtures [5-8].
Key factors
Different covalent attachment strategies may be employed to build AACs, including the generation of recombinant fusion proteins, enzyme-mediated conjugation, or chemical conjugation [1]. InvivoGen offers a series of conjugatable PRR ligands for conventional chemical ligation to antigenic proteins/polypeptides. This strategy involves the use of thiol- or amine-reactive reagents to generate conjugates through the side-chain of cysteine or lysine residues [1,5,6]. The selection of target antigen and the conjugation method are key determinants of AAC potency. Indeed, if the ligation reaction requires the denaturation of the protein antigen in order to expose the reaction sites, the absence of correct protein folding may preclude appropriate immune responses [1].
Mechanisms of action
AACs potentiate immune responses by:
- Co-delivering antigens and adjuvant to the same APC
- Extending the duration of antigen presentation after AAC internalization
Biological tools for building AACs
- Conjugatable PRR ligands: a series of TLR7 and STING agonists, either pre-linked or not
- OVA antigens: ovalbumin protein, MHC I-restricted ovalbumin peptide (OVA257-264), and MHC II-restricted ovalbumin peptide (OVA323-339)
References:
1. Xu Z. & Moyle P.M, 2018. Bioconjugation approaches to producing subunit vaccines composed of protein or peptide antigens and covalently attached Toll-Like receptor ligands. Bioconjug. Chem. 29(3):572.
2. Moyle P.M., 2017. Biotechnology approaches to produce potent, self-adjuvanting antigen-adjuvant fusion protein subunit vaccines. Biotechnol. Adv. 35:375.
3. Van Montfoort N. et al., 2009. Antigen storage compartments in mature dendritic cells facilitate prolonged cytotoxic T lymphocyte cross-priming capacity. PNAS. 106(16):6730.
4. Ho N.I. et al., 2021. Distinct antigen uptake receptors route to the same storage compartments for cross-presentation in dendritic cells. Immunology. 164(3):494.
5. Tighe H. et al., 2000. Conjugation of protein to immunostimulatory DNA results in a rapid, long-lasting and potent induction of cell-mediated and humoral immunity. Eur Journal Immunol 30(7):1939.
6. Kastenmüller K. et al., 2011. Protective T cell immunity in mice following protein-TLR7/8 agonist-conjugate immunization requires aggregation, type I IFN, and multiple DC subsets. J Clin Invest. 121(5):1782.
7. Zom G.G. et al., 2018. Novel TLR2-binding adjuvant induces enhanced T cell responses and tumor eradication. J Immunother Cancer. 6(1):146.
8. Bhagchandani S. et al., 2021. Evolution of Toll-like receptor 7/8 agonist therapeutics and their delivery approaches: from antiviral formulations to vaccine adjuvants. Adv Drug Deliv Rev. 175:113803.
