Site-Specific and Stable Conjugation of the SARS-CoV-2 Receptor-Binding Domain to Liposomes in the Absence of Any Other Adjuvants Elicits Potent Neutralizing Antibodies in BALB/c Mice

Next-Generation Vaccine Development with Nanomaterials: Recent Advances, Possibilities, and Challenges

Nanomaterials are becoming important tools for vaccine development owing to their tunable and adaptable nature. Unique properties of nanomaterials afford opportunities to modulate trafficking through various tissues, complement or augment adjuvant activities, and specify antigen valency and display. This versatility has enabled recent work designing nanomaterial vaccines for a broad range of diseases, including cancer, inflammatory diseases, and various infectious diseases. Recent successes of nanoparticle vaccines during the coronavirus disease 2019 (COVID-19) pandemic have fueled enthusiasm further. In this review, the most recent developments in nanovaccines for infectious disease, cancer, inflammatory diseases, allergic diseases, and nanoadjuvants are summarized. Additionally, challenges and opportunities for clinical translation of this unique class of materials are discussed.

Minimal Determinants for Lifelong Antiviral Antibody Responses in Mice from a Single Exposure to Virus-like Immunogens at Low Doses

The durability of an antibody (Ab) response is highly important for antiviral vaccines. However, due to the complex compositions of natural virions, the molecular determinants of Ab durability from viral infection or inactivated viral vaccines have been incompletely understood. Here we used a reductionist system of liposome-based virus-like structures to examine the durability of Abs from primary immune responses in mice. This system allowed us to independently vary fundamental viral attributes and to do so without additional adjuvants to model natural viruses. We show that a single injection of protein antigens (Ags) orderly displayed on a virion-sized liposome is sufficient to induce a long-lived neutralizing Ab (nAb) response. The introduction of internal nucleic acids dramatically modulates the magnitude of Ab responses without an alteration of the long-term kinetic trends. These Abs are characterized by very slow off-rates of ~0.0005 s-1, which emerged as early as day 5 after injection and these off-rates are comparable to that of affinity-matured monoclonal Abs. A single injection of these structures at doses as low as 100 ng led to lifelong nAb production in mice. Thus, a minimal virus-like immunogen can give rise to potent and long-lasting antiviral Abs in a primary response in mice without live infection. This has important implications for understanding both live viral infection and for optimizing vaccine design.

Minimal determinants for lifelong antiviral antibody responses in BALB/c mice from a single exposure to virus-like immunogens at low doses

However, due to the complex compositions of natural virions, the molecular determinants of Ab durability from viral infection or inactivated viral vaccines have been incompletely understood. Here we used a reductionist system of liposome-based virus-like structures to examine the durability of Abs in primary immune responses in mice. This system allowed us to independently vary fundamental viral attributes and to do so without additional adjuvants to model natural viruses. We show that a single injection of antigens (Ags) orderly displayed on a virion-sized liposome is sufficient to induce a long-lived neutralizing Ab (nAb) response. Introduction of internal nucleic acids dramatically modulates the magnitude of long-term Ab responses without alteration of the long-term kinetic trends. These Abs are characterized by exceptionally slow off-rates of ~0.0005 s-1, which emerged as early as day 5 after injection and these off-rates are comparable to that of affinity-matured monoclonal Abs. A single injection of these structures at doses as low as 100 ng led to lifelong nAb production in BALB/c mice. Thus, a minimal virus-like immunogen can give rise to potent and long-lasting antiviral Abs in a primary response in mice without live infection. This has important implications for understanding both live viral infection and for optimized vaccine design.

An integrated signaling threshold initiates IgG response towards virus-like immunogens

Class-switched neutralizing antibody (nAb) production is rapidly induced upon many viral infections. However, due to the presence of multiple components in typical virions, the precise biochemical and biophysical signals from viral infections that initiate nAb responses remain inadequately defined. Using a reductionist system of synthetic virus-like structures (SVLS) containing minimal, highly purified biochemical components commonly found in enveloped viruses, here we show that a foreign protein on a virion-sized liposome can serve as a stand-alone danger signal to initiate class-switched nAb responses in the absence of cognate T cell help or Toll-like receptor signaling but requires CD19, the antigen (Ag) coreceptor on B cells. Introduction of internal nucleic acids (iNAs) obviates the need for CD19, lowers the epitope density (ED) required to elicit the Ab response and transforms these structures into highly potent immunogens that rival conventional virus-like particles in their ability to elicit strong Ag-specific IgG. As early as day 5 after immunization, structures harbouring iNAs and decorated with just a few molecules of surface Ag at doses as low as 100 ng induced all IgG subclasses of Ab known in mice and reproduced the IgG2a/2c restriction that has been long observed in live viral infections. These findings reveal a shared mechanism for nAb response upon viral infection. High ED is capable but not necessary for driving Ab secretion in vivo . Instead, even a few molecules of surface Ag, when combined with nucleic acids within these structures, can trigger strong antiviral IgG production. As a result, the signaling threshold for the induction of neutralizing IgG is set by dual signals originating from both ED on the surface and the presence of iNAs within viral particulate immunogens.

One-sentence summary

Reconstitution of minimal viral signals necessary to initiate antiviral IgG.

Immunogenicity of Recombinant Lipid-Based Nanoparticle Vaccines: Danger Signal vs. Helping Hand

Infectious diseases are a predominant problem in human health. While the incidence of many pathogenic infections is controlled by vaccines, some pathogens still pose a challenging task for vaccine researchers. In order to face these challenges, the field of vaccine development has changed tremendously over the last few years. For non-replicating recombinant antigens, novel vaccine delivery systems that attempt to increase the immunogenicity by mimicking structural properties of pathogens are already approved for clinical applications. Lipid-based nanoparticles (LbNPs) of different natures are vesicles made of lipid layers with aqueous cavities, which may carry antigens and other biomolecules either displayed on the surface or encapsulated in the cavity. However, the efficacy profile of recombinant LbNP vaccines is not as high as that of live-attenuated ones. This review gives a compendious picture of two approaches that affect the immunogenicity of recombinant LbNP vaccines: (i) the incorporation of immunostimulatory agents and (ii) the utilization of pre-existing or promiscuous cellular immunity, which might be beneficial for the development of tailored prophylactic and therapeutic LbNP vaccine candidates.

Molecular basis for potent B cell responses to antigen displayed on particles of viral size

Multivalent viral epitopes induce rapid, robust and T cell-independent humoral immune responses, but the biochemical basis for such potency remains incompletely understood. We take advantage of a set of liposomes of viral size engineered to display affinity mutants of the model antigen (Ag) hen egg lysozyme. Particulate Ag induces potent 'all-or-none' B cell responses that are density dependent but affinity independent. Unlike soluble Ag, particulate Ag induces signal amplification downstream of the B cell receptor by selectively evading LYN-dependent inhibitory pathways and maximally activates NF-κB in a manner that mimics T cell help. Such signaling induces MYC expression and enables even low doses of particulate Ag to trigger robust B cell proliferation in vivo in the absence of adjuvant. We uncover a molecular basis for highly sensitive B cell responses to viral Ag display that is independent of encapsulated nucleic acids and is not merely accounted for by avidity and B cell receptor cross-linking.

Molecular basis for potent B cell responses to antigen displayed on particles of viral size

Although it has long been appreciated that multivalent antigens - and particularly viral epitope display - produce extremely rapid, robust, and T-independent humoral immune responses, the biochemical basis for such potency has been incompletely understood. Here we take advantage of a set of neutral liposomes of viral size that are engineered to display affinity mutants of the model antigen (Ag) hen egg lysozyme at precisely varied density. We show that particulate Ag display by liposomes induces highly potent B cell responses that are dose-and density-dependent but affinity-independent. Titrating dose of particulate, but not soluble, Ag reveals bimodal Erk phosphorylation and cytosolic calcium increases. Particulate Ag induces signal amplification downstream of the B cell receptor (BCR) by selectively evading LYN-dependent inhibitory pathways, but in vitro potency is independent of CD19. Importantly, Ag display on viral-sized particles signals independently of MYD88 and IRAK1/4, but activates NF- κ B robustly in a manner that mimics T cell help. Together, such biased signaling by particulate Ag promotes MYC expression and reduces the threshold required for B cell proliferation relative to soluble Ag. These findings uncover a molecular basis for highly sensitive B cell response to viral Ag display and remarkable potency of virus-like particle vaccines that is not merely accounted for by avidity and BCR cross-linking, and is independent of the contribution of B cell nucleic acid-sensing machinery.