Characteristics of EBV and its antigens

EBV is a member of the gamma-herpesvirus family. It is an enveloped virus of about 150-180 nm diameter with an icosahedral nucleocapsid containing a double-stranded DNA viral genome of about 172 000 base pairs. The nucleocapsid is composed of a major 160 kDa nonglycosylated polypeptide and a minor 125 kDa glycoprotein, both of which form part of the viral capsid antigen (VCA) complex. The virus lipoprotein envelope contains at least three virally-encoded glycoproteins, designated the membrane antigen (MA) complex. Two antigenically-related MA glycoproteins, gp340 and gp220, potentiate binding of the virus to a specific cell surface receptor molecule during the infection of target cells. The third MA glycoprotein, gp85, is involved in the fusion of bound virus with the host cell membrane. Following fusion, viral DNA is released into the cell and is transcribed and replicated in the nucleus, where it persists as multiple episomal copies.

The 140 kDa C3d complement receptor molecule (CD21) is the virus receptor in the human host. CD21 expression is largely restricted to mature B lymphocytes, hence the predominant B cell tropism of EBV. However, the B lymphocyte is not the only cell type that is infectable with EBV, as has long been apparent from the association of EBV with undifferentiated nasopharyngeal carcinoma and other non-B cell malignancies (see below). Infection of primary epithelial cells and of immature thymocytes has been achieved in vitro, but these infections were demonstrated with a difficulty that contrasts with the ease and reproducibility with which B cells can be infected. EBV infection of resting B cells results in growth-transformation and the establishment of immortalized lymphoblastoid cell lines (LCLs) in which infection is largely nonproductive and only a limited number of the viral genes are transcribed. The transformation-associated or so-called 'latent' viral proteins expressed in LCLs comprise six nuclear antigens (EBNAs, for EBV-induccd nuclear antigens) and three membrane-associated proteins (LMPs, for Latent membrane proteins), whose functions are summarized in Table 1.

Some LCLs are partially permissive for virus production in vitro. In such lines, a minor subpopulation of cells express the BZLF-1 protein (encoded by a gene in the Bam HI Z restriction fragment of EBV, beginning with the first leftward reading frame), which is a transcriptional activator that can trigger a cascade of viral gene expression from more than 80 lytic cycle genes. The lytic cycle antigens were firsr identified using human sera from EBV-positive individuals which contained antibodies reactive in immunofluorescence with virus-producer cells. Historically, these antigens were classified into groups according to whether their expression was independent of DNA synthesis, i.e. early antigens (EAs), or blocked by inhibitors of DNA synthesis, i.e. late antigens. The EAs were further subdivided into diffuse (EA-D) or restricted (EA-R) components according to their cellular localization and their sensitivity to methanol fixation in immunofluorescence tests. The EA genes encode nonstructural proteins, which include transcriptional activators and enzymes. The late antigens are structural proteins of the EBV nucleocapsid (VCA) and lipoprotein envelope (MA).

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