Protein nanocages: A new frontier in mucosal vaccine delivery and immune activation

Abstract

Mucosal infectious diseases represent a significant global health burden, impacting millions of people worldwide through pathogens that invade the respiratory, gastrointestinal, and urogenital tracts. Mucosal vaccines provide a promising strategy to combat these diseases by preventing pathogens from entering through the portals as well as within the systemic response compartment. However, challenges such as antigen instability, inefficient delivery, suboptimal immune activation, and the complex biology of mucosal barriers hinder their development. These limitations require integrating specialized adjuvants and delivery systems. Protein nanocages, self-assembling nanoscale structures that can be engineered, may provide an innovative solution for co-delivering antigens and adjuvants. With their remarkable stability, biocompatibility, and design versatility, protein nanocages can potentially overcome existing challenges in mucosal vaccine delivery and enhance protective immune responses. This review highlights the potential of protein nanocages to revolutionize mucosal vaccine development by addressing these challenges.

Keywords: Protein nanocage; adjuvant display; mucosal vaccine; self-assembling nanocarrier; vaccine delivery.

Graphical abstract

Figure 1.

Different types of self-assembling protein nanocages (PNCs). Structures of representative PNCs. VLPs: (1) surface structure of T4 GALA PNC (PDB: 7MH2), (2) surface structure of bacteriophage AP205 VLP (PDB: 5LQP), (3) surface structure of bacteriophage MS2 VLP (PDB: 2MS2). Non-VLP: (1) surface structures of ferritin (PDB: 3BVE), (2) surface structures of lumazine synthase (PDB: 1HQK), (3) surface structures of encapsulin (PDB: 3DKT)), (4) surface structures of vault (PDB: 7PKZ). Computationally designed particles: (1) surface structures of T3 tetrahedral nanocage (PDB: 8TL7), (2) surface structures of VelcroVax tandem HBcAg with SUMO-Affimer inserted at MIR (PDB: 7ZQA), (3) surface structures of O43_129_+4 (PDB: 8V3B), (4) surface structures of mTIP60-ba (metal-ion induced TIP60 (K67E) complex with barium ions (PDB: 7XM1).

Figure 2.

Advantages of PNC-Based vaccines. PNCs offer several unique advantages for vaccine delivery. They can present antigens in a highly organized, multivalent manner, allow ease in modification with functional elements like adjuvants and targeting moieties, co-deliver both antigens and other immunostimulatory molecules, and potentially enhance immune responses.

Figure 3.

Enhanced humoral immune response in antigen displayed with PNCs compared to antigen alone. The humoral immune responses triggered by soluble antigens interacting with B-cell receptors (BCRs) are less effective and shorter in duration compared to the responses elicited by protein nanocages (PNCs) that display an organized arrangement of antigens. In contrast to soluble antigens, PNCs present multiple antigen copies, enabling concurrent engagement with numerous BCRs, a process known as BCR clustering. This results in robust and long-lasting antigen recognition by B cells, which initiates intracellular signaling cascades, antigen internalization, and processing of MHC class II presentation to T follicular helper (tfh) cells. This sequence of events stimulates tfh cells to release regulatory cytokines, facilitating the differentiation of B cells into plasma cells that produce antigen-specific neutralizing antibodies. The intensity of these responses is indicated as follows: high - ***, low - *.

Figure 4.

Modification of PNCs at different interfaces. Self-assembling PNCs can be modified at different interfaces, namely, the exterior surface, the interior surface, and the interface region between the exterior and interior surfaces.

Figure 5.

Strategies to conjugate antigens on PNPs. Three major strategies exist for presenting antigens: (1) genetic fusion: involves the direct fusion of antigen with the corresponding PNC subunit and expressed genetically (2) chemical conjugation: involves chemical crosslinking agents to form irreversible bonds between the chemically active amino acid side chains of both PNCs and antigens or drug moieties (3) coupling with tags: involve genetically fusing the catcher to one entity and tag to another, thereby it results in strong affinity interaction between catcher and tag system forming PNCs displaying antigens on to their surface in an orderly manner.