Immunity lures
The broader vaccines in development display viral antigens, such as spike proteins, to the immune system on diverse frameworks. Here are a few approaches.
Ferritin nanoparticles: A candidate vaccine from the emerging infectious diseases branch of Water Reed National Military Medical Center began phase 1 human trials in April 2021. Called SpFN, the vaccine consists of arrays of ferritin nanoparticles linked to spike proteins from various variants and species. Ferritin is a protein that binds and stores iron in the body.
“The repetitive and ordered display of the coronavirus spike protein on a multifaced nanoparticle may stimulate immunity in such a way as to translate into significantly broader protection,” said Walter Reed’s branch director and vaccine coinventor Kayvon Modjarrad, MD, PhD.
A second vaccine targets only the “bullseye” part of the spike that the virus uses to attach and gain access to human cells, called the receptor-binding domain (RBD), of SARS-CoV-2 variants and of the virus behind the original SARS. The preclinical data appeared in Science Translational Medicine.
Barton Haynes, MD and colleagues at the Duke Human Vaccine Institute are also using ferritin to design and develop a “pan-betacoronavirus vaccine,” referring to the genus to which SARS-CoV-2 belongs. They say their results in macaques, published in Nature, “demonstrate that current mRNA-based vaccines may provide some protection from future outbreaks of zoonotic betacoronaviruses.”
Mosaic nanoparticles: Graduate student Alexander Cohen is leading an effort at CalTech, in the lab of Pamela Bjorkman, PhD, that uses nanoparticles consisting of proteins from a bacterium (Strep pyogenes) to which RBDs from spike proteins of four or eight different betacoronaviruses are attached. The strategy demonstrates that the whole is greater than the sum of the parts.
“Alex’s results show that it is possible to raise diverse neutralizing antibody responses, even against coronavirus strains that were not represented on the injected nanoparticle. We are hopeful that this technology could be used to protect against future animal coronaviruses that cross into humans,” said Dr. Björkman. The work appeared in Science.
Candidate vaccines from Inovio Pharmaceuticals also use a mosaic spike strategy, but with DNA rings (plasmids) rather than nanoparticles. One version works against pre-Omicron variants and is being tested against Omicron, and another with “pan–COVID-19” coverage has tested well in animal models. Inovio’s vaccines are delivered into the skin using a special device that applies an electric pulse that increases the cells’ permeability.
Chimeric spikes: Yet another approach is to fashion vaccines from various parts of the betacoronaviruses that are most closely related to SARS-CoV-2 – the pathogens behind Middle East respiratory syndrome and severe acute respiratory syndrome as well as several bat viruses and a few pangolin ones. The abundance and ubiquity of these viruses provide a toolbox of sorts, with instructions written in the language of RNA, from which to select, dissect, recombine, and customize vaccines.
“SARS-like viruses can recombine and exhibit great genetic diversity in several parts of the genome. We designed chimeric spikes to improve coverage of a multiplexed vaccine,” said David Martinez, PhD.
His team at the University of North Carolina at Chapel Hill has developed mRNA vaccines that deliver “scrambled coronavirus spikes” representing various parts, not just the RBD, as described in Science.
In mice, the chimeric vaccines elicit robust T- and B-cell immune responses, which stimulate antibody production and control other facets of building immunity.