Towards Targeted Delivery Systems: Ligand Conjugation Strategies for mRNA Nanoparticle Tumor Vaccines

Abstract

The use of nanoparticles encapsulating messenger RNA (mRNA) as a vaccine has recently attracted much attention because of encouraging results achieved in many nonviral genetic antitumor vaccination studies. Notably, in all of these studies, mRNA nanoparticles are passively targeted to dendritic cells (DCs) through careful selection of vaccination sites. Hence, DC-targeted mRNA nanoparticle vaccines may be an imminent next step forward. In this brief report, we will discuss established conjugation strategies that have been successfully applied to both polymeric and liposomal gene delivery systems. We will also briefly describe promising DC surface receptors amenable for targeting mRNA nanoparticles. Practicable conjugation strategies and receptors reviewed in this paper will provide a convenient reference to facilitate future development of targeted mRNA nanoparticle vaccine.

Figure 1

Established strategies for the conjugation of ligands onto polymeric and liposomal nanoparticles. (a) (Top) PLGA (poly(lactic-co-glycolic acid)) nanoparticles formed by copolymer PLGA-PEG-COOH are stabilized with normal surfactant and subsequently reacted with ligands bearing compatible linking groups. (Bottom) PLGA nanoparticles are stabilized with amphiphilic surfactants containing functionalizable molecules. PLGA nanoparticles, susceptible to hydrolysis, are purified by centrifugation to reduce water exposure time. (b) DC-targeting antibodies bearing compatible cross-linkers (e.g., -SH) are reacted with preformed liposomes to form immunoliposomes, which are purified by dialysis. (c) Electrostatically neutral ligands (mannose) are covalently conjugated to cationic polymers and directly used to formulate targeted nanoparticles. (d) Postinsertion functionalization of liposomes/lipopolyplexes. Formulated liposomes/lipopolyplexes are heated with micelles bearing targeting ligands at 55°C for at least 15 mins. The resultant ligand conjugated liposomes/lipopolyplexes can be used without further purification.

Figure 2

The genetic vaccination model. Antigen presentation occurs directly by transfected DCs through gene expression of the antigen. DCs also cross present antigens secreted by transfected bystander cells, or derived from phagocytosis of apoptotic cells. Cross presentation mechanisms in DCs may facilitate delayed lysosomal delivery leading to higher delivery efficiencies.