HCMV Infection

Herpesviruses are divided into three groups: alpha-herpesviruses, beta-herpesviruses, and gamma-herpesviruses. The beta herpesviruses have a restricted host range. Their reproductive life cycle is long (days), with infection progressing slowly in cell culture systems. These viruses cause their host cells to enlarge, as exemplified by a human cytomegalovirus (HCMV) infection. These viruses can establish latent infection in secretory glands, cells of the reticuloendothelial system, and the kidneys.

Human Cytomegalovirus, or HCMV, is a common virus that infects people of all ages. In the United States, nearly one in three children are already infected with HCMV by age 5 years. Over half of adults by age 40 have been infected with HCMV. Once HCMV is in a person’s body, it stays there for life and can reactivate.

Cytomegalovirus causes three clinical syndromes:

(1) Congenital cytomegalovirus infection (when symptomatic) causes hepatosplenomegaly, retinitis, rash, and central nervous system involvement.

(2) In about 10 per cent of older children and adults, primary cytomegalovirus infection causes a mononucleosis syndrome with fever, malaise, atypical lymphocytosis, and pharyngitis.

(3) Immunocompromised hosts (transplant recipients and human immunodeficiency virus [HIV]-infected individuals) may develop life-threatening disseminated disease involving the lungs, gastrointestinal tract, liver, retina, and central nervous system.

Experimentally HCMV can be propagated in multiple cell lines. When propagated in human fibroblasts, HCMV clinical isolates acquire mutations in a manner that suggests a process of adaptation. Two strains of HCMV AD169 (grown from cultures of adenoid tissue taken from a 7-year-old girl) and Towne (developed as an attenuated vaccine by passaging 125 times in vitro) were initially used as the primary clinical strains. As only 26 % of HCMV canonical genes (45/171) are essential for viral replication in vitro it became important that a model strain be developed.

The Merlin BAC was derived for this use. Produced using a bacterial artificial chromosome (BAC) cloning system (to avoid adaptation/degradation of the genome with each passage) the Merlin strain contains a complete HCMV genome that is thought to accurately to represent the original clinical agent from which it was derived. It is also a reproducible source of clonal virus (via transfection) and is capable of reconstituting phenotypically wild-type virus.

The lifecycle represented here uses the Merlin strain where possible.Infectious Human Cytomegalovirus (HCMV) particles enter the cell through interaction with cellular receptors. Once in the cytoplasm capsid and tegument proteins are delivered to the cytosol. The capsid travels to the nucleus, where the genome is delivered and circularized. Tegument proteins regulate host cell responses and initiate the expression of viral I immediate early genes. This is followed by delayed early genes, which initiate viral genome replication, then late genes. Late gene expression initiates capsid assembly in the nucleus, followed by nuclear egress to the cytosol. Capsids associate with tegument proteins in the cytosol and are trafficked to the viral assembly complex that contains components from the endoplasmic reticulum, Golgi apparatus, and endosomal machinery. The capsids acquire additional tegument proteins and a viral envelope by budding into intracellular vesicles. These vesicles fuse with the plasma membrane to release enveloped infectious particles along with non-infectious dense bodies.