In the 1950s, Denis Burkitt described the existence of B-cell lymphomas in 2–14 year-old African children from malaria endemic areas. In 1964, continuous B-lymphocyte cell lines derived from these tumors were found by Epstein and Barr to spontaneously release a herpesvirus. It was Gertrud and Werner Henle who demonstrated that the Epstein-Barr virus (EBV) is ubiquitous in the human population where it is usually the cause of infections that are not apparent though it may cause infectious mononucleosis. The more severe, albeit rare, result of EBV infection is malignant transformation and cancer development in various forms, including Burkitt’s lymphoma and nasopharyngeal carcinoma, one of the most common cancers in China.
The primary site of Epstein-Barr virus (EBV) infection is the oropharyngeal cavity. Children and teenagers are commonly afflicted usually after oral contact, hence the name “kissing disease”. Based on serology, about 95% of the world adult population has been infected with EBV and, following primary infection, remains lifelong carriers of the virus. In developed countries, exposure to EBV occurs relatively late: only 50–70% of adolescents and young adults are EBV seropositive. About 30% of the seronegative group will develop infectious mononucleosis as a result of primary EBV infection. The disease is characterized by fever, sore throat, generalized lymphadenopathy, splenomegaly, intense asthenia, hyper-lymphocytosis (>50%) with atypical lymphocytes and elevated transaminase levels. In developing countries, EBV antibodies are acquired early in life and the disease is mostly asymptomatic.
EBV is associated with Burkitt’s B-cell lymphoma and nasopharyngeal carcinoma. Burkitt's lymphoma (BL) is a malignant form of tumor associated with EBV that is endemic to central parts of Africa and New Guinea with an annual incidence of 6–7 cases per 100 000 and a peak incidence at 6 or 7 years of age. The epidemiological involvement of EBV in Burkitt's lymphoma is based on the recognition of the EBV viral genome in tumor cells, associated with an elevated antibody titre against EBV viral capsid antigen (VCA). The highest prevalence of BL is found in the "lymphoma belt," a region that extends from West to East Africa between the 10th degree north and 10th degree south of the equator and continues south down the Eastern coast of Africa. This area is characterized by high temperature and humidity, which is probably the reason why an association of malaria with BL was suspected at one time. In African countries such as Uganda, in the lymphoma belt, the association of BL with EBV is very strong (97%), whereas it is weaker elsewhere (85% in Algeria; only 10–15% in France and the USA).
Nasopharyngeal cancer (NPC) incidence rates are less than 1 per 100 000 in most populations, except in populations in southern China, where an annual incidence of more than 20 cases per 100 000 is reported. Isolated northern populations such as Eskimos and Greenlanders also show high incidence. There is a moderate incidence in North Africa, Israel, Kuwait, the Sudan and parts of Kenya and Uganda. Men are twice as likely to develop NPC as women. The rate of incidence generally increases from ages 20 to around 50. In the USA, Chinese-Americans comprise the majority of NPC patients, together with workers exposed to fumes, smoke and chemicals, implying a role for chemical carcinogenesis. Studies related to nutrition and diet have shown an association between eating highly salted foods and NPC. Vitamin C deficiency at a young age also may be a contributing factor. Finally, a study of HLA haplotypes revealed a genetically distinct subpopulation in southern China, with an increased frequency of haplotype A-2/B-Sin-2 which may account for the higher disease incidence in the area.
Recent studies have shown that EBV also is associated with B-cell malignancies such as Hodgkin’s lymphoma (HL) and lymphoproliferative disease in immunosuppressed patients, as well as with some T-cell lymphomas and other epithelial tumors such as gastric cancers. These tumors are characterized by the presence of multiple extrachromosomal copies of the viral genome in tumor cells and the expression of part of the EBV genome.
EBV, together with HHV-8 (Kaposi sarcoma-associated virus), belongs to the genus Lymphocryptovirus, in the subfamily Gammaherpesvirinae, family Herpesviridae. These are complex enveloped DNA viruses, which multiply in the nucleus of the host cell (see 5.3). EBV infects resting human B-lymphocytes and epithelial cells, multiplies in the latter and establishes latent infection in memory B-lymphocytes. Thus, infected individuals may produce virions, carry virus-specific CTLs, produce EBV-specific antibody, and yet harbor latently infected memory B-cells. These maintain the latent EBV genome as an episome that expresses only part of its genetic information, including EBV nuclear antigens EBNA-1 (a latent DNA replication factor), EBNA-2 (a transcriptional activator) and EBNA-3A and -3C (involved in the establishment of latency), together with integral membrane proteins LMP-1 and LMP-2 which play major roles in maintenance of latency and escape from the immune response of the host. Latently infected cells do not produce the B7 coactivator receptor and, therefore, are not killed by CTLs. When peripheral blood from an infected individual is cultured, latently infected B-cells begin to replicate and yield immortalized progeny lymphoblasts that can be indefinitely propagated in the laboratory.
The major EBV external surface glycoprotein is a 350 kD antigen, gp350/220, which binds the CD21 receptor on B-cells. Another envelope glycoprotein, gp42, is responsible for the fusion between the virus envelope and the host cell membrane. The EBV genome, a 172 kbp linear double-stranded DNA molecule, becomes circular for replication and latency. Viral capsid antigens (VCA) are late gene products.
The development of an EBV vaccine could protect individuals against primary infection and hence presumably reduce the burden of EBV-associated cancers.
The principal target of EBV neutralizing antibodies is the major virus surface glycoprotein gp350/220. Several vaccine candidates based on gp350/220 have been developed. Live recombinant vaccinia virus vectors have been used to express the gp350/220 antigen and were found to confer protection in primates and elicit antibodies in EBV-negative Chinese infants.
Soluble recombinant gp350/220 produced in CHO cells was found to be safe in humans but needed strong adjuvants to elicit acceptable immunogenicity (co-development by MedImmune, GSK and Henogen). Phase II clinical trials of this candidate vaccine are under way.
Clinical trials of an EBNA-3A peptide are being conducted in Australia.