Some viruses, like the flu, are a flash in the pan. They infect our cells, our immune system fights back - which is what makes us sick with flu-like symptoms - and then the virus is eliminated, until the next infection.
Other viruses never leave our cells.
Epstein-Barr virus (EBV) -known primary as the cause of mononucleosis, “the kissing disease” a disease that can also elicit relatively mild flu-like symptoms - is also linked to cancers including Burkitt and Hodgkin lymphomas, and autoimmune diseases such as multiple sclerosis.
Although EBV was the first virus shown to cause cancer in humans in 1964, there is still not a licensed vaccine against it.
Recently, in the journal of Cell Reports Medicine, Dr. Andrew McGuire and his team evaluated the efficacy of a vaccine made of nanoparticles displaying 60 copies of the viral proteins required for cell entry, known as gH/gL, using two adjuvants to enhance the immune response.
The idea behind the vaccine is to train our immune system to recognize the gH/gL proteins on the nanoparticles and generate antibodies against them. So, if EBV -the real virus- infects our cells, the antibodies will prevent EBV cell entry and therefore, EBV infection.
The problem with testing this vaccine is that EBV can only infect humans. To overcome this problem, the researchers tested the EBV vaccine in rhesus macaques that were infected with the rhesus orthologue of EBV called rhesus lymphocryptovirus (rhLCV). “rhLCV infection of rhesus macaques recapitulates several aspects of EBV infection in humans, including a natural route of oral transmission and a similar course of infection in the host,” the authors commented.
Previously, McGuire and his team isolated an antibody -called AMMO1- from the blood of EBV-infected humans, which can block EBV infection by blocking the EBV gH/gL proteins. Excitingly, the team also found that AMMO1 can cross-neutralize rhLCV in their model system with this virus. These findings suggested that a vaccine using the EBV gH/gL proteins can produce antibodies that can cross-protect against rhLCV indicating that the vaccine could be efficacious at preventing EBV infection in humans.
The group immunized their model “with proteins involved in the fusion machinery of Epstein-Barr virus -gH/gL- to look at the protection mediated by our vaccine,” said Kristina Edwards, a graduate student in the McGuire lab and leading author of the study. “When we did this, we saw one animal was protected from challenge after vaccination," Edwards added.
First, McGuire and his team tested the vaccine using two different adjuvants. The animals received 3 doses of the vaccine with plasma collected each time. The results showed that only the vaccine with one of the adjuvants – SMNP, a saponin-based immune-stimulating nanoparticle- elicited a better immune response. Then, the authors assessed if this vaccine formulation can prevent rhLCV infection. After a couple of weeks of immunization, the animals were infected with rhLCV. Surprisingly, only one animal was protected!
“When we investigated what mediated the protection in that animal, we discovered that there was more of a difference between the two viruses than we initially thought,” commented Edwards. “While the gH/gL proteins have about 90% similarity at the amino acid level, the differences seem to substantially impact the known antigenic sites on EBV gH/gL.”
Electron microscope analysis revealed differences between vaccine-elucidated plasma antibodies' binding sites to EBV and rhLCV gH/gL complexes. Based on these findings, it seems that the epitopes recognized by vaccine-elicited antibodies against EBV gH/gL are different from those identified by antibodies against rhLCV gH/gL. Additional assays indicated that EBV gH/gL neutralizing epitopes are poorly conserved on rhLCV gH/gL. Based on these findings, EBV vaccine evaluation in these animals is hindered by the differences between EBV and rhLCV. Nonetheless, this study supports clinical development of gH/gL based nanoparticles vaccine against EBV, but also highlights the limitations of testing using rhLCV in this model system.
“The findings in this study will influence how we think about the best way to evaluate EBV vaccines in the future. It's a virus that is specific for humans, and both of the animal models we commonly use to study it (humanized mice infected with EBV or rhesus macaques infected with the orthologue rhLCV) have their own pros and cons to consider when we want to look at the protection mediated by candidate vaccines,” Edward concluded.
The spotlighted work was funded by the National Institute of Health, Diseases of Public Health Importance training grant, and Vaccine and Infectious Disease Division Initiative grant.
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium members Drs. Andrew T. McGuire, Marie Pancera, Stephen C. De Rosa, Evan W. Newell and Martin Prlic, contributed to this work.
Edwards KR, Schmidt K, Homad LJ, Kher GM, Xu G, Rodrigues KA, Ben-Akiva E, Abbott J, Prlic M, Newell EW, De Rosa SC, Irvine DJ, Pancera M, McGuire AT. Vaccination with nanoparticles displaying gH/gL from Epstein-Barr virus elicits limited cross-protection against rhesus lymphocryptovirus. Cell Rep Med. 2024.