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Even though the Epstein-Barr virus is a member of the Herpesviridae family, its life cycle has some unique characteristics that are not found in other members of the family. After transmission in the saliva to an uninfected person, it is believed that the virus infects naive B-lymphocytic cells in the oropharynx (1). Some researchers hypothesize that this B-cell infection occurs secondary to a primary lytic infection of the mucosal epithelium, but as yet this process has not been proven (2). As part of the immune system, B-cells are an ingenious host for a virus that sets up a life-long latent infection. The mechanisms by which the virus preferentially infects and transforms resting memory B-cells are under investigation as are the mechanisms for transition to the lytic life cycle.
When the virus comes across a target B-cell, viral protein gp350/220 attaches to the cellular receptor CD21, bringing the enveloped virus in close proximity to the plasma membrane (3). Additionally, gp42 binds to human leukocyte antigen (HLA) Class II molecules (2). This viral attachment induces endocytosis via non-clathrin coated vesicles. The viral envelope and the cell membrane fuse, utilizing viral proteins gB, gH, gL, and gp42. Structurally analogous to proteins found in other herpes viruses, EBV gB has been shown to take part in viral assembly as well as fusion. The other three proteins are closely associated with each other and form a complex that is required for fusion (4). Once across the plasma membrane the virus needs to get to the nucleus which is likely accomplished by travel along the cytoplasmic microtubules in a manner similar to the herpes simplex virus 1. This hypothesis is based on similarities between EBV capsid protein BFRF3 and its HSV homologue UL35 which is known to affiliate with dynein light chains (5).
The process of uncoating for EBV can only be speculated on, but based again on structural comparisons, it is believed that BVRF1, an EBV protein, may interact with the nuclear pore complex (NPC) as another HSV1 protein UL25 does. If this is true, the capsid likely stays localized to the microtubules after releasing the viral DNA to the nucleus (5).
Once in the nucleus, Epstein-Barr viral DNA could conceivably result in two separate outcomes. A lytic infection would produce huge numbers of virus in a relatively short period of time, but for EBV infections what occurs in almost every case is latent infection of the B-cells. Immediately upon entry into the nucleus, EBV linear DNA circularizes. This episome, or plasmid, will remain in the nucleus of infected cells and be replicated with the host cell components until a lytic infection is stimulated (6). Transformation of the B-cell to an immortalized state is dependent on the Epstein-Barr virus nuclear antigen 2 (EBNA-2) acting as a transcriptional activator for both host and viral genes (4).
Latent EBV infection of B-lymphocytes involves maintenance of the episome in the nucleus of infected cells, inducement of proliferation, and avoidance of host cell apoptosis. Host cell machinery is responsible for the production of EBV proteins and replication of the genome during latency. Of the many possible gene products, only a limited number are latently expressed, including six nuclear proteins and three membrane proteins (6). The nuclear proteins (EBNA-1, EBNA-2, EBNA-3A, EBNA-3B, EBNA-3C, and EBNA-LP) have critical roles in maintaining the episomal DNA and in latent viral gene transcription (4). Strictly speaking, EBNA-1 is the only one of these proteins that is absolutely required to maintain and replicate the viral episome (6; 7). It binds directly to the two functional domains of the origin of plasmid replication (OriP), the region of the EBV genome believed to be responsible for latent replication. In latency, the EBV genome is subject to the host cell cycle, regulated by host cell proteins, and is replicated only once during S phase. The origin recognition complex (ORC) is just one of the host protein complexes that is required for OriP replicative activity (4). The circular composition of the EBV genome in latency allows for the viral genome to be replicated right alongside the host chromosomal DNA and then segregated into the daughter cells (6).
While EBV can maintain a lifelong latent infection, it has also been observed that occasionally the virus will transition to a lytic state, resulting in prolific viral shedding and cell death. While the stimulus to make this transition in vivo has not been proven, lytic EBV infection relies much more heavily on viral proteins, two of which are immediate-early (IE) gene products BZLF1 and BRLF1 (6). BZLF1, also called ZEBRA or Zta, is a required transcription factor that binds to the EBV lytic origin of DNA replication (OriLyt) and has been speculated to be part of the initiation of the lytic phase. BRLF1 is believed to complex with BZLF1 as a transcriptional activator (8). A regulated cascade of viral gene expression ensues. BALF5 encodes for the catalytic portion of the viral DNA polymerase while BMRF1 codes for the accessory subunit. The BALF2 gene product is a single-stranded DNA-binding protein that acts to destabilize the double stranded helix. In addition, a complex of three proteins is coded for by BBLF4, BSLF1, and BBLF2/3. These proteins are believed to act at the replication forks to transcribe the viral genome (6; 8). The exact mechanisms for DNA replication are still under investigation, but one proposed model holds that the BALF2 single-stranded DNA binding protein works with a complex of BZLF1-BBLF4/BSLF1/BBLF2/3 to open up the double helix at OriLyt and synthesize RNA primers. If this model proves to be true, lytic replication is almost completely accomplished by viral proteins (6).
Lytic replication takes place in the nucleus, in specialized replication compartments (6). It is likely that replication is accomplished via a rolling-circle mechanism and once replicated, the now linear genome is prepared for nuclear exit by packaging it into virion particles (4). Late gene expression of the nucleocapsid proteins that make these virion particles is done in the cytoplasm and the proteins are transported into the nucleus. There are six structural proteins coded for by EBV. BdRF1 is a scaffolding protein that has been shown to transport the major capsid protein BcLF1 into the nucleus. BORF1 and BDLF1 are both triplex proteins that travel together into the nucleus. A small capsid protein, BFRF3, and BVRF2, a protease, also localize to the nucleus. The scaffold protein forms a spherical procapsid which is cleaved by the protease after the major capsid proteins, triplex proteins, and small capsid proteins self-assemble into a viral capsid containing DNA (9). It is believed that the capsid then buds through the nuclear membrane, acquiring a temporary envelope which is lost in the cytoplasm and then another envelope is gained, possibly from the trans-golgi network. The newly enveloped virus is then transported to the plasma membrane for exocytosis. This process has not been confirmed, but has been hypothesized based on studies of the alphaherpesviruses (10).