Biomaterial Immune Niches for Generating Strong and Persistent Humoral Immune Responses Luo Gu1,2, Alex Najibi2, Maxence Dellacherie2, Ting-Yu Shih2, Aileen Li2, and David Mooney2. 1Department of Materials Science and Engineering, Institute for NanoBioTechnology, Johns Hopkins University, Baltimore, MD 21218, USA. 2School of Engineering and Applied Sciences, Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA 02138, USA.

Introduction: Vaccination is one of the most important medical interventions, protecting the host from a variety of diseases. A key function of vaccines is to elicit humoral immune responses against disease- associated antigens. However, effective vaccination often requires several booster shots. In addition, it is challenging to generate potent response against small peptide antigens using conventional vaccination methods. One of the most promising strategies to overcome these challenges is to incorporate highly immunostimulatory factors or adjuvants. In recent years, new types of biomaterials and nanoparticle- based adjuvants are increasingly being developed, and they have demonstrated their advantages in generating effective immune responses due to their favorable physical and chemical features. Here, we created biomaterial scaffolds that can modulate the immune niches to generate strong and persistent humoral immune responses against peptide and protein antigens by recruiting immune cells and programing them in situ over a prolonged period of time.

Results and Discussion: Three types of biomaterial scaffolds were fabricated as vaccine platforms. They are: 1) porous poly(lactide-co-glycolide) (PLG) scaffolds, 2) alginate cryo-hydrogels (Cryogel), and 3) mesoporous silica microrods (MPS). Granulocyte-macrophage colony-stimulating factor (GM-CSF) was incorporated in the scaffolds as a recruitment factor for antigen-presenting cells (APCs). CpG Oligodeoxynucleotide was also incorporated in the scaffolds as a “danger signal” to activate the recruited APCs. A small peptide, gonadotropin-releasing hormone (GnRH), was coupled to a carrier protein and used as a model antigen in the three biomaterial vaccines (Fig 1a). Our previous works have shown that these biomaterial scaffolds can recruit and program APCs such as dendritic cells (DCs) in situ by creating a controlled immune niche. In this study, we investigated whether this strategy can be used to elicit long- term humoral immune responses. Using a mouse model, we found that all three biomaterial vaccines (PLG, Cryogel, and MPS) generated significantly stronger antigen-specific IgG antibody response than the bolus vaccine (solution containing the same components as the biomaterial vaccines but without the material scaffolds). More importantly, mice inoculated with the biomaterial vaccines maintained high serum antibody titers even over 1 year after a single vaccination, which demonstrated the advantage of using biomaterial scaffolds as vaccine platforms (Fig 1b). Among the three biomaterial vaccines, PLG and MPS scaffolds showed slightly stronger and more persistent antibody response than the Cryogel system. In order to understand the mechanism of how biomaterial scaffolds elicited strong and persistent antibody response, we investigated the germinal center (GC) activity in the draining lymph nodes in mice implanted with PLG vaccines. Interestingly, mice vaccinated with PLG scaffolds showed enhanced GC response even at 6 weeks after vaccination. In contrast, GC response in mice injected with bolus vaccine quickly dissipated after 2 weeks. Therefore, the potent humoral immune response to the biomaterial vaccines is likely due to the persistent GC activity induced by the long-term in situ immune modulation of the biomaterial scaffolds. abFigure 1. (a) Schematic depiction of biomaterial scaffold recruiting and programing immune cells to elicit humoral immune response. (b) Long-term serum antibody titer in mice vaccinated with different biomaterial scaffold vaccines containing a model antigen.