Pediatric Cytomegalovirus Infection 

Of all the human herpesviruses described to date, cytomegalovirus (CMV) arguably causes the most morbidity and mortality. Although primary infection with this agent generally does not produce symptoms in healthy adults, several high-risk groups, including immunocompromised organ transplant recipients and individuals infected with human immunodeficiency virus (HIV), are at risk of developing life-threatening and sight-threatening cytomegalovirus disease. In addition, cytomegalovirus has emerged in recent years as the most important cause of congenital infection in the developed world, commonly leading to mental retardation and developmental disability.

In 1904, Ribbert first identified histopathological evidence of cytomegalovirus, probably in tissues from a congenitally infected infant. Ribbert mistakenly assumed that the large inclusion-bearing cells he observed at autopsy were from protozoa (incorrectly named Entamoeba mortinatalium). In 1920, Goodpasture correctly postulated the viral etiology of these inclusions.^[1] Goodpasture used the term cytomegalia to refer to the enlarged, swollen nature of the infected cells. Human cytomegalovirus (HCMV) was first isolated in tissue culture in 1956, and the propensity of this organism to infect the salivary gland led to its initial designation as a salivary gland virus.

In 1960, Weller designated the virus cytomegalovirus;^[2] during the 1970s and 1980s, knowledge of the role of cytomegalovirus as an important pathogen with diverse clinical manifestations increased steadily.^[3] Although enormous progress has recently been made in defining and characterizing the molecular biology, immunology, and antiviral therapeutic targets for cytomegalovirus, considerable work remains in devising strategies for prevention of cytomegalovirus infection and in understanding the role of specific viral genes in pathogenesis.

Furthermore, development of a vaccine against this virus is a major public health priority (reviewed below).^[4]

Cytomegalovirus is a member of a family of 8 human herpesviruses, designated as human herpesvirus 5 (HHV-5). Taxonomically, cytomegalovirus is referred to as a Betaherpesvirinae, based on its propensity to infect mononuclear cells and lymphocytes and on its molecular phylogenetic relationship to other herpesviruses. Cytomegalovirus is the largest member of the herpesvirus family, with a double-stranded DNA genome of more than 240 kbp, capable of encoding more than 200 potential protein products. The function of most of these proteins remains unclear. As with the other herpesviruses, the structure of the viral particle is that of an icosahedral capsid, surrounded by a lipid bilayer outer envelope.

An understanding of the process of viral replication provides insights into molecular mechanisms of antiviral therapy and protective immunity. Cytomegalovirus replicates very slowly in cell culture, mirroring its very slow pattern of growth in vivo (in contrast to herpes simplex virus [HSV] infection, which progresses very rapidly). The replication cycle of cytomegalovirus is temporally divided into the following 3 regulated classes: immediate early, early, and late.

Immediate early gene transcription occurs in the first 4 hours following viral infection, when key regulatory proteins that allow the virus to take control of cellular machinery are made. The major immediate early promoter of this region of the cytomegalovirus genome is one of the most powerful eucaryotic promoters described in nature; this has been exploited in modern biotechnology as a useful promoter for driving gene expression in gene therapy and vaccination studies.

Following synthesis of immediate early genes, the early gene products are transcribed. Early gene products include DNA replication proteins and some structural proteins.

Finally, the late gene products are made approximately 24 hours after infection, and these proteins are chiefly structural proteins that are involved in virion assembly and egress. Synthesis of late genes is highly dependent on viral DNA replication and can be blocked by inhibitors of viral DNA polymerase, such as ganciclovir. The lipid bilayer outer envelope contains the virally encoded glycoproteins, which are the major targets of host neutralizing antibody responses. These glycoproteins are candidates for human vaccine design. The proteinaceous layer between the envelope and the inner capsid, the viral tegument, contains proteins that are major targets of host cell–mediated immune responses. The most important of these tegument proteins is the so-called major tegument protein, UL83 (phosphoprotein 65 [pp65]).

Another clinically important gene product, the UL97 gene product, is a phosphotransferase. Although the function of this protein in the viral life cycle is unknown, this gene is clinically important because a substrate of the kinase is the antiviral drug ganciclovir, which, once phosphorylated, becomes a highly effective cytomegalovirus therapy.^[5]

In clinical specimens, one of the classic hallmarks of cytomegalovirus infection is the cytomegalic inclusion cell. These strikingly enlarged cells (the property of "cytomegaly," from which cytomegalovirus acquires its name) contain intranuclear inclusions that have the histopathological appearance of owl's eyes. The presence of these cells indicates productive infection, although they may be absent even in actively infected tissues. In most cell lines, cytomegalovirus is difficult to culture in the laboratory; however, in vivo infection seems to chiefly involve epithelial cells. In severe disseminated cytomegalovirus disease, involvement can be observed in most organ systems.

Little is known about the molecular mechanisms responsible for the pathogenesis of tissue damage caused by cytomegalovirus, particularly for congenital cytomegalovirus infection. Although the CNS is the major target organ for tissue damage in the developing fetus, culturing cytomegalovirus from the cerebrospinal fluid of symptomatic infants with congenital infection is surprisingly difficult. Because cytomegalovirus can infect endothelial cells, some authors have postulated that a viral angitis may be responsible for perfusion failure in the developing brain with resultant maldevelopment. Others have postulated a direct teratogenic effect of cytomegalovirus on the developing fetus. Observation of cytomegalovirus–induced alternations in the cell cycle and cytomegalovirus–induced damage to chromosomes supports this speculation; however, this hypothesis has been difficult to experimentally verify.

Immunity to cytomegalovirus is complex and involves humoral and cell-mediated responses. Several cytomegalovirus gene products are of particular importance in cytomegalovirus immunity. The outer envelope of the virus, which is derived from the host cell nuclear membrane, contains multiple virally encoded glycoproteins. Glycoprotein B (gB) and glycoprotein H (gH) appear to be the major determinants of protective humoral immunity. Antibody to these proteins is capable of neutralizing virus, and gB and gH are targets of investigational cytomegalovirus subunit vaccines; however, although humoral responses are important in control of severe disease, they are clearly inadequate in preventing transplacental infection, which can occur even in women who are cytomegalovirus seropositive.

The generation of cytotoxic T-cell (CTL) responses against cytomegalovirus may be a more important host immune response in control of infection. In general, these CTLs involve major histocompatibility complex (MHC) class I restricted CD8+ responses. Although many viral gene products are important in generating these responses, most cytomegalovirus–specific CTLs target an abundant phosphoprotein in the viral tegument, pp65, the product of the cytomegalovirus UL83 gene. In passive transfer experiments involving high-risk bone marrow transplant recipients, the value of these responses was dramatically demonstrated using adoptive transfer of cytomegalovirus–specific CD8+ T cells that target the cytomegalovirus UL83 gene, which was able to control cytomegalovirus disease.

Recent investigations into the molecular biology of cytomegalovirus have revealed the presence of many viral gene products, which appear to modulate host inflammatory and immune responses. Several cytomegalovirus genes interfere with normal antigen processing and generation of cell-mediated immune responses. To date, 3 viral gene products have been identified that inhibit MHC class I antigen presentation. One is the US11 gene product, which exports the class I heavy chain from the endoplasmic reticulum (ER) to the cytosol (rendering it nonfunctional). Another is the US3 gene product, which retains MHC molecules in the ER, preventing them from traveling to the plasma membrane. Finally, the US6 protein inhibits peptide translocation by transporters associated with antigen processing (TAP).

Other viral gene products, the UL33, US27, and US28 genes, are functional homologs of cellular G-protein coupled receptors which may, via molecular mimicry, subvert normal inflammatory responses and, in the process, promote tissue dissemination of the virus and interfere with host immune response. The cytomegalovirus genome also encodes a homolog of the cellular major histocompatibility class I gene, which appears to contribute to the ability of cytomegalovirus to evade host defense. The UL144 open reading frame found in clinical isolates of cytomegalovirus encodes a structural homolog of the tumor necrosis factor receptor superfamily, which may contribute to the ability of HCMV to escape immune clearance.

Other cytomegalovirus genes interfere with natural killer (NK) cell responses, including the UL18 gene product. A better understanding of the impact of viral immune evasion genes on the development of protective immunity to cytomegalovirus infection should enable the design of improved vaccines.

Frequency

United States

Every mammal appears to be infected with its own species-specific cytomegalovirus, and no evidence suggests that infections cross species. Hence, humans are the only natural host for HCMV infection. Although most adults eventually become infected with cytomegalovirus, the epidemiology of this infection is complex, and the age at which an individual acquires cytomegalovirus greatly depends on geographic location, socioeconomic status, cultural factors, and child-rearing practices.

In developing countries, most children acquire cytomegalovirus infection early in life, with adult seroprevalence approaching 100% by early adulthood. In contrast, in developed countries, the seroprevalence of cytomegalovirus approximates 50% in young adults of middle-upper socioeconomic status. This observation has important implications for congenital cytomegalovirus epidemiology because women of childbearing age who are cytomegalovirus seronegative are at major risk of giving birth to infants with symptomatic congenital infection if primary infection is acquired during pregnancy.

Transmission of cytomegalovirus infection may occur throughout life, chiefly via contact with infected secretions.^[6] Acquisition of cytomegalovirus in the newborn period is common. Approximately 1% (range, 0.5-2.5%) of all newborns are congenitally infected with cytomegalovirus. Most of these infections occur in infants born to mothers with preexisting immunity and are clinically asymptomatic at birth; however, long-term sequelae, including deafness, can occur (see History).

The route of congenital infection is presumed to be transplacental. Cytomegalovirus may also be transmitted perinatally, both by aspiration of cervicovaginal secretions in the birth canal and by breastfeeding. More than 50% of infants fed with breast milk that contains infectious virus become infected with cytomegalovirus.^[7] In particular, a recent study reported 5 cases of severe morbidity and mortality in very low birth weight infants with cytomegalovirus infection acquired postnatally through breast milk.^[8] Infants who are not infected congenitally or perinatally with cytomegalovirus are at high risk to acquire infection in daycare centers. According to some studies, the prevalence of cytomegalovirus infection in children who attend daycare, particularly children younger than 2 years, approximates 80%.

The virus may be readily transmitted to susceptible children via saliva, urine, and fomites; these children, in turn, may transmit infection to their parents. Such horizontal transmission of infection in daycare centers appears to play a major role in the epidemiology of many cytomegalovirus infections in young parents.

In adulthood, sexual activity is probably the most important route of acquisition of cytomegalovirus,^[9] although the observation that virus is present in saliva, cervicovaginal secretions, and semen obscures which route or routes of transmission are primarily responsible for establishment of infection. Saliva alone appears to be sufficient for transmission of cytomegalovirus, and this route of transmission may be responsible for those cases of heterophile-negative mononucleosis, which are attributable to cytomegalovirus. Kissing appears to be a way in which cytomegalovirus is transmitted from toddlers to seronegative parents. Recent work by the Centers for Disease Control and Prevention (CDC) has emphasized the need for greater public awareness of these risks and for educational interventions for young women of childbearing age.^[10]

Other important routes of transmission include blood transfusion and solid organ transplantation. Before screening of blood products, transfusion-associated cytomegalovirus was an important cause of morbidity and mortality in premature infants; however, the routine use in many neonatal intensive care units of cytomegalovirus–negative blood products has largely eliminated this problem. Posttransfusion cytomegalovirus is still a risk in cytomegalovirus–seronegative trauma and in surgery patients, often manifesting as hepatitis.

International

The risk of congenital cytomegalovirus infection is not well defined in the developing world. Because seroepidemiologic studies indicate that, in many developing countries, seroprevalence for cytomegalovirus approaches 100% very early in childhood, little attention has been given to the question of potential morbidities in these populations.

Mortality/Morbidity

Cytomegalovirus is a substantial cause of morbidity in newborns. As the most common so-called toxoplasmosis, rubella, cytomegalovirus, and herpes simplex (TORCH) infection in the developed world, cytomegalovirus accounts for extensive neurodevelopmental morbidity, including sensorineural deafness in infants. Cytomegalovirus also accounts for substantial mortality in immunocompromised patients.

Race

The effects of race and genetics on clinical manifestations of cytomegalovirus infection are not well understood. In some studies in the United States, prevalence of congenital cytomegalovirus appears to be higher in infants born to black women.^[11] More work is required to understand the basis for the differences in the epidemiology of cytomegalovirus infection in various ethnic groups in the United States.

Sex

Both sexes are equally susceptible to infection and morbidity from cytomegalovirus, although only women are at risk for transplacental transmission of infection.

Age

The annual seroconversion rate for acquisition of cytomegalovirus infection is approximately 1%. However, two age groups have higher rates of acquisition of infection: toddlers who attend group daycare and adolescents. Accordingly, these represent two potential groups in which to implement vaccination.