The COVID-19 pandemic highlighted the importance of vaccines and fueled researchers’ interests in advancing vaccine technologies. Along with efforts to combat pathogens, researchers also strive to create vaccines against diseases such as cancer. In a Drug Discovery News seminar, scientists shared their latest research on innovative vaccine strategies for preventing infectious diseases and cancer. 

Developing hybrid mRNA vaccines

Currently approved SARS-CoV-2 mRNA vaccines include viral spike protein-encoding mRNA encapsulated in lipid nanoparticles (LNPs). These vaccines work by expressing spike peptides on cell surfaces that recruit various immune cells. On the other hand, recombinant virus-like particle (VLP)-based vaccines, which consist of assembled viral proteins that lack viral genetic material, can stimulate potent antibody responses. “We are trying to develop a hybrid mRNA technology that combines these two vaccine strategies into one,” said Magnus Hoffmann, a biochemist at the California Institute of Technology, who kicked off the presentations. 

To accomplish this, Hoffmann and his team took advantage of the endosomal sorting complex required for transport (ESCRT) pathway, a universal cellular process that drives the outward budding of membranes from the cytosol. By inserting a small amino acid sequence termed ESCRT- and apoptosis-linked gene 2-interacting protein X-binding region (EABR) at the cytoplasmic tail of the SARS-CoV-2 spike protein, the team created a spike-EABR construct. EABR recruits ESCRT-associated proteins to induce the self-assembly of enveloped VLPs budding from cells.

The team injected spike-EABR encoding mRNA packaged in LNPs into mice. This spike-EABR mRNA simultaneously generated spike peptides on cell surfaces and released VLPs displaying spike antigens from cells. When they compared the efficacy of their vaccine to conventional mRNA vaccines that lacked EABR or recombinant VLP vaccines, they found that the spike-EABR mRNA vaccine elicited more than five-fold higher neutralizing antibody titers and stronger T cell responses than other vaccine types three months after a booster shot. “This hybrid mRNA vaccine approach could potentially induce more durable responses, leading to longer periods of protection,” Hoffmann concluded. Currently, his team is optimizing their EABR platform and expanding its application to other viruses, including influenza and HIV.

Improving RNA delivery 

The effectiveness of mRNA vaccines heavily depends on the delivery method. Anna Blakney, a bioengineer at the University of British Columbia, presented her team’s work designing efficient delivery systems for self-amplifying RNA (saRNA), a type of RNA that encodes a virus-derived replicase and amplifies itself once it enters the cell. Due to its high protein expression, saRNA offers an attractive strategy for minimizing vaccine doses and reducing side effects. 

Blakney and her team tested saRNA formulated with either a biocompatible polymer or LNPs, and characterized the protein expression and vaccine immunogenicity of both approaches. They observed that while the biocompatible polymer resulted in greater protein expression in cells, LNPs elicited higher humoral and cellular immunity. 

The team then compared the cytokine responses of mice that received intramuscular or intravenous injections of the biocompatible polymer or LNP saRNA vaccines. They noticed a significant increase in interleukin 6 (IL-6) secretion with intramuscular injections of the LNP saRNA vaccine. “This tells us that the intracellular sensing of delivery vehicles does influence downstream immunogenicity,” said Blakney. “We really need to think about these delivery vehicles when we’re using them for vaccine or therapeutic applications and what the cellular activation is in response to the delivery vehicle itself.”

These insights helped Blakney’s team to optimize LNP compositions and find the ideal parameters for achieving specific cellular activation for different vaccine applications. They also built an analytical model that accurately predicted quality attributes, protein expression, and cellular activation for different LNP formulations. Using this model, Blakney and her team are designing nanoparticles for more effective saRNA vaccines and RNA therapies.

Boosting antitumor immunity

A major roadblock in developing effective cancer vaccines is the inadequate immune response to antigens. Aliasger Salem, a pharmaceutical scientist at the University of Iowa, discussed his team’s innovative strategies to boost antitumor therapeutic efficacy by codelivering immunostimulatory adjuvants in biodegradable microparticle vaccines. 

The main adjuvants used by Salem’s team are cytosine–phosphorothioate–guanine (CpG) oligonucleotides, short single-stranded DNA molecules that are common components of bacterial DNA. CpG oligonucleotides activate natural killer cells to secrete interferons and stimulate cell-mediated immune responses. Salem described how his team incorporated synthetic CpG oligonucleotides with a variety of tumor antigens in biodegradable microparticles. Administering these vaccines in mice with melanoma and breast cancer remarkably reduced tumor growth and metastasis. 

Like Blakney, Salem also investigates formulations of biodegradable microparticles as vaccine delivery systems. During his presentation, he illustrated how different microparticle sizes and polymer chemistries affect antitumor vaccine activity. “We’ve seen that the more hydrophobic the polymer is, the more effectively it leads to tumor survival,” said Salem. Based on this observation, Salem developed a polyanhydride-based vaccine that elicited sustained tumor suppression in mice with a single dose. 

Lastly, Salem discussed his team’s work combining CpG-loaded microparticle vaccines with checkpoint blockade therapies in various tumor models. These in situ vaccines transformed the tumor microenvironment (TME) from immunosuppressive to immunostimulatory, leading to enhanced therapeutic efficacy. “In situ immunization of cancer can overcome resistance to checkpoint blockade agents, giving really great responses in mice and humans,” Salem said. 

Restoring tumor suppression

While Salem employs synthetic CpG oligonucleotides to overcome tumor immunosuppression, some scientists are exploring ways to restore the function of tumor suppressors. In the last presentation of the seminar, Jinjun Shi, a cancer biologist at Brigham and Women’s Hospital, discussed how his team used nanoparticle-mediated mRNA to reactivate cellular tumor antigen p53 and phosphatase and tensin homolog (PTEN), two tumor suppressors frequently mutated in human cancers. 

“RNA delivery is challenging because there are multiple steps involved in delivering RNA all the way to the cytosol of tumor cells,” said Shi. “Nanotechnology can help us solve this problem.” Shi and his team developed a self-assembly method for generating lipid-polymer hybrid nanoparticles that encapsulate p53 or PTEN mRNA. These particles can efficiently permeate the TME, inducing p53 or PTEN expression in deeper tumor tissues.

By delivering their p53 or PTEN mRNA nanoparticles to cancer cells and animal models, Shi’s team observed a series of boosted antitumor immune responses, including autophagy, immunogenic cell death, and increased production of proinflammatory cytokines and cytotoxic T cells. The results demonstrated that these mRNA nanoparticles can reverse immunosuppression in the TME and enhance therapeutic efficacy. 

Shi’s team also applied their mRNA nanoparticle strategy to developing a cancer vaccine. This vaccine consists of a tumor antigen encoding mRNA and an adjuvant agent, both packaged in a lipid-polyethylene glycol particle. In a proof-of-concept study in mice models of lymphoma and prostate cancer, the vaccine significantly prevented tumor growth. The team is now engineering their mRNA and delivery systems to further improve delivery efficiency and cell targeting. 

From designing novel approaches to advancing formulations and improving delivery systems, researchers continue to expand the frontiers of vaccine development. Hoffman, Blakney, Salem, and Shi each provided a unique glimpse of the promising future of disease prevention. “It’s an exciting time to be in this field, explore all these questions, and move these technologies forward,” said Salem.

To learn more about Hoffman’s, Blakney’s, Salem’s, and Shi’s work on developing novel vaccine technologies, view the live-on-demand seminar on the Drug Discovery News website.