Gammaherpesvirinae

Structure

Viruses in Gammaherpesvirinae are enveloped, with icosahedral, spherical to pleomorphic, and round geometries, and T=16 symmetry. The diameter is around 150-200 nm. Genomes are linear and non-segmented, around 180kb in length.

Life cycle

The main stages in the lifecycle of Gamma herpes virus are namely
• Virus attachment and entry
• Viral DNA injection through nuclear pore complex (NPC) into nucleus
• Assembly of nucleocapsids and encapsidation of viral genome
• Primary envelopment, invaginations of nuclear membranes and nuclear egress
• Tegumentation and secondary envelopment in the cytoplasm
• Egress and extracellular virions release

Lytic cycle

The lytic cycle of the gammaherpesviruses is initiated only on rare occasions. Therefore, the least contribution to pathogenicity has to be expected from this stage. The ORFs expressed during that stage are further divided into immediate-early, early, and late. Promoter activation mediated by these proteins has also a strong effect on DNA synthesis from the origins of lytic DNA replication. As a result, virions are generated and released from the productively infected cells.

Immune evasion strategies

Viruses that establish lifelong latent infections must ensure that the viral genome is maintained within the latently infected cell throughout the life of the host, yet at the same time must also be capable of avoiding elimination by the immune surveillance system especially must avoid being detected by host CD8+ cytotoxic T lymphocytes (CTLs). The gamma-herpesviruses are characteristically latent in lymphocytes and drive the proliferation that requires the expression of latent viral antigens. The majority of gammaherpesviruses encode a specific protein that is critical for maintenance of the viral genome within latently infected cells termed the genome maintenance protein (GMP). During latency, the genome persists in the nucleus of the infected cells as a circular episomal element. GMPs are DNA-binding proteins that ensure that, as the host cell progresses through mitosis, the viral episomes are partitioned to daughter cells. This provides continuous existence of the viral genome within the host cells.

MHV68 in genetically modified mice is used as a model for studying infection and host responses. Stable lifelong latency is a hallmark of the chronic phase of MHV68 infection. The spleen is a major infection site. The episomal maintenance protein for MHV68 is a latency-associated nuclear antigen (mLANA; ORF73). M2 protein mediates signaling pathways of infected B cells by interacting with SH2- and SH3-containing proteins. Host factors have been identified that can promote or antagonize MHV68 latency and reactivation.

Human health

Gammaherpesviruses are of primary interest due to the two human viruses, EBV (Epstein–Barr virus) and KSHV (Kaposi's sarcoma-associated herpesvirus) and the diseases they cause. The gammaherpesviruses replicate and persist in lymphoid cells but some are capable of undergoing lytic replication in epithelial or fibroblast cells. Gammaherpesviruses may be a cause of chronic fibrotic lung diseases in humans and in animals.

Murid herpesvirus 68 is an important model system for the study of gammaherpesviruses with tractable genetics. The gammaherpesviruses, including HVS, EBV, KSHV, and RRV, are capable of establishing latent infection in lymphocytes.

Attenuated virus mutants represent a promising approach towards gamma-herpesvirus infection control. Surprisingly, latency-deficient and, therefore, apathogenic MHV-68 mutants are found to be highly effective vaccines against these viruses. Research in this area is almost exclusively performed using MHV68 as KSHV and EBV (the major human pathogens of this family) do not productively infect model organisms typically used for this type of experimentation.

Growth de-regulating genes

Herpesviruses have large genomes containing a wide array of genes. Although the first ORF in these gammaherpesviruses have oncogenic potential, other viral genes may also play a role in viral transformation. A striking feature of the four gammaherpesviruses is that they contain distinct ORFs involved in lymphocyte signaling events. At the left end of each viral genome are located ORFs encoding distinct transforming proteins. Gammaherpesvirus genes are capable of modulating cellular signals such that cell proliferation and viral replication occur at the appropriate times in the viral life cycle.