Epstein-Barr virus (EBV) is a common virus that has infected ~95% adults worldwide. While primary infection typically occurs during childhood or teenage years and is usually asymptomatic, it can cause infectious mononucleosis. After the initial infection, EBV remains in the body for life, persisting in a latent form. Persistent, latent EBV infection can lead to the development of several types of malignancies including lymphoma, nasopharyngeal and gastric cancer. Recently, EBV infection has been linked to autoimmune diseases such as multiple sclerosis, myalgic encephalomyelitis (chronic fatigue syndrome), and rheumatoid arthritis. “Despite the significant level of EBV-associated morbidity and mortality, an EBV vaccine has so far remained elusive,” said Dr. Andrew McGuire, senior author on a recent paper in npj vaccines studying the efficacy of a new class of EBV vaccines.
Several EBV vaccine candidates are in various stages of development and are derived from the viral glycoproteins gp350, gH, gL, and gp42 involved in viral attachment and entry. gp350 binds to human CD21 and CD35 on the surface of B cells to initiate viral infection. Then, the gH/gL heterodimer mediates fusion of the EBV virion with the cellular membrane, triggered by gp42 binding to human leukocytes antigen class II molecules. In naturally infected individuals, the immune system produces antibodies that recognize gp350, gH/gL, and gp42, suggesting that vaccines derived from these viral proteins could produce neutralizing antibodies that prevent EBV infection.
Early EBV vaccine attempts focused on gp350. The most advanced gp350 vaccine protected against infectious mononucleosis but did not prevent infection, suggesting that the inclusion of other viral proteins might improve vaccine efficacy. “We previously isolated monoclonal anti-EBV antibodies and identified one, AMMO1, that binds to gH/gL and potently neutralized EBV infection,” said Dr. McGuire. In follow up work, the McGuire lab developed a protein-based vaccine derived from gH/gL complex that elicited high titers of neutralizing antibodies and protected humanized mice against lethal viral challenge.
In their most recent work, the McGuire lab worked in collaboration with HDT bio to develop and test EBV replicon RNA (repRNA) vaccine candidates, a novel class of nucleic acid-based vaccine. repRNA vaccines are derived from an attenuated variant of the Venezuelan equine encephalitis virus and, in some cases, show enhanced humoral and cellular immune responses compared to conventional mRNA vaccine technology used in the Pfizer and Moderna COVID-19 vaccines.
The team first tested repRNA constructs encoding gp350, gH/gL, or gH/gL/gp42. The gH/gL repRNA vaccine candidate elicited the highest titers of neutralizing antibodies against EBV infection in an in vitro assay and became the focus of the rest of the study. After optimization of the gH/gL construct and dosing regimen, vaccine-induced antibodies were evaluated for their ability to block EBV infection and tumorigenesis in a humanized mouse model. Since EBV only infects human and not mouse cells, immune-compromised “NSG” mice are first irradiated to deplete their bone marrow and then engrafted with human hematopoietic stem cells that can develop into human B cells which can then be infected with EBV. Prior to challenge with a lethal dose of EBV virus, purified antibodies from gH/gL repRNA immunized wild-type mice were infused into the humanized mice. Mice that received control antibodies survived for 8 weeks or less, developed splenomegaly and splenic tumors, and had high levels of viral DNA in the blood. In contrast, mice receiving gH/gL repRNA antibodies all survived for 12 weeks following challenge, had normal spleen weight, and were free of tumors and viral DNA. Furthermore, the gH/gL repRNA antibodies provided greater protection than antibodies produced by a conventional gH/gL protein-based vaccine, where only 60% of mice survived for 12 weeks and there was evidence of elevated levels of viral DNA in the spleen, splenomegaly and splenic tumors. While the focus of the study was on the contribution of antibodies to protection against EBV infection, the McGuire lab also noted that gH/gL repRNA vaccine candidate also elicited a vaccine-specific CD8+ T cell response, which may offer further protection against EBV infection in humans.
Looking to the future Dr McGuire says, “the robust immunogeneticity of repRNA-encoded gH/gL, relative ease of manufacturing, and favorable safety and reactogenicity profile of the delivery platform warrant the development of repRNA EBV vaccines for human clinical trials.”
The spotlighted research was funded by the National Institutes of Health.
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium members Drs. Andrew McGuire, James Olson, and M. Juliana McElrath contributed to this work.
Edwards KR, Malhi H, Schmidt K, Davis AR, Homad LJ, Warner NL, Chhan CB, Scharffenberger SC, Gaffney K, Hinkley T, Potchen NB, Wang JY, Price J, McElrath MJ, Olson J, King NP, Lund JM, Moodie Z, Erasmus JH, McGuire AT. 2024. A gH/gL-encoding replicon vaccine elicits neutralizing antibodies that protect humanized mice against EBV challenge. npj vaccines. 9:120.