Nanoparticle orientationally displayed antigen epitopes improve neutralizing antibody level in a model of porcine circovirus type 2

Ding, Peiyang; Zhang, Teng; Li, Yafei; Teng, Man; Sun, Yaning; Liu, Xiao; Chai, Shujun; Zhou, Enmin; Jin, Qianyue; Zhang, Gaiping

Vaccines are a cost-effective and powerful weapon against human and animal infectious diseases. 1 Although many diseases have been neutralized, potential vaccine-preventable diseases continue to threaten human and animal health. 2, 3 Considering the safety of vaccines, subunit vaccines have become increasingly popular. Improvements and innovation in heterologous protein expression technology have led to a sufficient source for candidate subunit antigens. 4, 5 Regrettably, most subunit vaccines induce lower levels of NA, which is the key factor in providing protective immunity that is accomplished via reducing or abolishing the biological activity of a living microorganism. 6 Therefore, the major challenge in the development of subunit vaccines is identification of strategies that can enhance the levels of NA. Over the past few decades, advances in immunological and high-throughput genome sequencing technology have enabled the identification of pathogen-associated protective antigens. 7 However, some viruses, such as human immunodeficiency virus (HIV), influenza viruses, and porcine reproductive and respiratory syndrome virus, rapidly vary their surface antigens such that NA cannot effectively recognize new epitopes, leading to escape from the host's anti-infective immune process. [8] [9] [10] In addition, some pathogens reduce the level of NA level by generating antibodies against immunodominant epitopes, thus exacerbating diseases, as seen in Dengue virus, respiratory syncytial virus, porcine circovirus type 2 (PCV2), and Middle East respiratory syndrome coronavirus. [11] [12] [13] [14] Development of structural biology has increased the throughput of protein structure determination enabling the determination of the threedimensional structure and structured electrostatic landscapes of most vaccine antigens and their epitopes. 15, 16 Based on the surface electrostatic landscapes of the antigen, stabilized and fully displayed neutralizing epitopes on the antigenic surface or the flexible scaffold allow for easy recognition by B cells, promoting proliferation and antibody affinity maturation, while removing or hiding the immunodominant epitopes. 17 Subunit vaccine monomers are variedly and chaotically distributed within the vaccine system and thus cannot efficiently activate B cells. 18 The orientation of antigen epitopes is critical for activation of B cells. Reliable affinity maturation of NA requires stable and full display of neutralizing epitopes rather than transient orientations through a state. 17 Crosslinking of B-cell receptors (BCRs) is also critical for activation of B cells. An ideal immunogen should have multiple copies of epitopes, and bind to multiple BCRs, allowing for multimerization of signal transduction molecules involved in antigen recognition on the surface of B cells. 19, 20 Antigen processing would be better facilitated if antigens were nanoparticles with granular structure and repetitive surface organization, which would increase phagocytosis and the ability to activate the complement system and recruit other immune molecules. 21 One reasonable strategy to improve NA level is based on antigenic epitope structure and to orientationally and repeatedly display neutralizing epitopes on the outer surface of nanoparticles by orientation coupling between antigen and nanoparticles. To verify this strategy, gold nanoparticles (AuNPs) as delivery vehicles have been developed and have shown several features that make them w