epstein–barr virus: opportunities for prophylactic and therapeutic vaccines

james e. turner & graham s. taylor

the burden of ebv-associated diseases

given that it infects over 90% of
people worldwide, epstein–barr virus (ebv) is a highly successful pathogen. we have co-evolved with this virus over millions of years, achieving a state of détente. ebv infection during childhood, which until recently was the normal state of affairs, usually passes unnoticed. improvements in living standards mean that, increasingly, infection is delayed until adolescence or later, whereupon ebv infection can cause infectious mononucleosis (glandular fever) with symptoms of fever, sore throat, swollen lymph nodes and a debilitating fatigue that can persist for months. while most people mount a strong immune response able to control their ebv infection, the virus nevertheless establishes a lifelong infection of b cells hidden from the immune system. periodically, the virus reactivates in some of these cells to complete its life cycle, producing new viruses that are spread to other people via saliva.

however, since its discovery in 1964 in burkitt’s lymphoma cells, several other cancers have been linked to ebv. in terms of morbidity and mortality two cancers stand out. nasopharyngeal carcinoma (npc) is rare in most western countries but prevalent throughout south east asia. some 75,000 cases occur each year, of which almost all are positive for ebv. more recently, ebv has been found in almost 10% of gastric carcinoma tumours. although the proportion of virus-positive gastric carcinomas is much lower than npc, there are far more cases of this disease worldwide (an estimated 1 million cases per year). this means that the number of virus-positive cases of these two cancers is actually very similar. ebv is also associated with several different lymphomas, the most common being hodgkin lymphoma. in total, almost 200,000 cases of ebv-associated cancer occur each year worldwide.

prophylactic vaccines to prevent ebv infection

prophylactic or preventative vaccines have been successfully developed for several viruses including two human tumour viruses (hepatitis b virus and human papillomavirus). given the burden of ebv-related disease in terms of cancer, but also the debilitating effects of infectious mononucleosis, a similar vaccine to prevent ebv infection would have a substantial health and economic impact. most vaccine efforts to date have focused on a single ebv protein, called gp350, because antibodies to this protein brought about by natural virus infection are also able to inhibit virus infection of cells in the laboratory. a clinical trial performed in ebv-uninfected young adults showed that a vaccine consisting of purified gp350 protein reduced the incidence of infectious mononucleosis, but was unable to protect against virus infection. while such a vaccine could offer some benefit, for example by reducing the incidence or severity of infectious mononucleosis in uninfected adolescents, the need for an effective prophylactic vaccine remains.

since the gp350 vaccine alone could not protect against ebv infection, a truly preventative vaccine is likely to need additional antibodies targeting other virus proteins, as well as perhaps also enlisting the t cell arm of the immune system. ideally, this would be achieved using a live but attenuated vaccine strain of ebv. however, the fact that ebv is associated with cancer development raises enormous, probably insurmountable, regulatory issues for such a vaccine. one possible alternative therefore could be to use a vaccine comprised of ebv virus-like particles. since these particles lack virus genetic material, the risk of the vaccine causing cancer is removed while the presence of multiple viral proteins, displayed in a particulate form, broadens and enhances the antiviral immune response. however, the key question of whether a strong immune response against ebv can protect against infection remains unanswered. our research group is addressing this fundamental question in collaboration with professor paul kellam’s research group at the wellcome trust sanger institute in cambridge. since different ebv strains circulate in the population, and each strain can be identified by its own genetic signature, we are using deep sequencing to see if people are protected against acquiring additional ebv strains following their initial infection. evidence of such natural protection following ebv infection would be a strong stimulus for developing second-generation prophylactic ebv vaccines.

therapeutic vaccines to treat ebv-associated cancers

even if the perfect prophylactic ebv vaccine became available tomorrow, millions of people already infected with the virus will go on to develop virus-associated cancers. irrespective of progress towards a prophylactic vaccine, the desperate need for better treatments for ebv-positive npc, gastric carcinoma and lymphoma remains. one way this could be achieved is by making use of the fact that all ebv-associated cancers express one or more ebv proteins. it should therefore be possible to eliminate these cancers by harnessing relevant antiviral immune responses. this has clearly proved to be the case for post transplant lymphoma, an ebv-associated cancer that can occur in patients who are immunosuppressed following organ or stem cell transplantation. these tumours result from the loss of the normal anti-ebv immune responses that usually prevent virus-infected cells from growing out of control. consequently, restoring these immune responses by infusing ebv-specific immune cells grown in the laboratory often results in the elimination of the cancer.

post-transplant lymphoma does, however, represent a somewhat idealised situation for such immunotherapy because the cancer cells contain at least eight ebv proteins and some of these are good immune targets. in contrast, all other ebv-associated cancers contain fewer viral proteins that are poorer targets for the immune system. this is presumably because these other cancers have developed in individuals with relatively ‘normal’ immunity. nevertheless, several clinical trials have shown that infusing ebv-specific immune cells into patients with ebv-positive hodgkin lymphoma or npc can result in clinical benefit. these trials serve as very important proofs of principle, but producing the immune cells requires highly specialised facilities and staff; consequently, the cost per patient is high. the challenge now is therefore to develop ways to focus the immune response against the smaller number of viral proteins present in these cancers in a way that can be applied to large numbers of patients worldwide. to achieve this goal, our research group has developed a genetically engineered vaccine based on modified vaccinia ankara (mva), a safe attenuated strain of poxvirus. into mva we introduced the genes for the two ebv proteins that are present in almost all ebv-positive cancers. this vaccine has recently completed testing in two early-phase clinical trials performed in collaboration with cancer research uk and colleagues in hong kong, london, manchester and birmingham. the results of these trials show that the vaccine can boost immune responses to one or both of the ebv proteins when administered to npc patients and, equally importantly, is well-tolerated with relatively minor side effects. the vaccine is now being tested in two new much larger clinical trials that will measure the ability of the vaccine to shrink, or even eliminate, ebv-associated tumours.

conclusion

ebv is a fascinating virus and its study has revealed novel aspects of the interaction between viruses, the immune system and cancer. the results of many years of basic research have already been translated into the clinic and efforts by several research groups worldwide continue to develop and refine exciting new therapies that will benefit human health.

james e. turner & graham s. taylor

school of cancer sciences, university of birmingham, vincent drive, birmingham
b15 2tt; tel. +44 (0)121 414 7983; email [email protected]