Important properties of parvoviruses are listed in Table 1. It is noteworthy that there are both autonomously replicating and defective parvoviruses that require a helper virus for replication. Structure and Composition The icosahedral, nonenveloped particles are 18–26 nm in diameter (Figure 1). The particles have a molecular weight of 5.5–6.2 × 10⁶ and a heavy buoyant density of 1.39–1.42 g/cm3. Virions are extremely resistant to inactivation. They are stable between a pH of 3 and 9 and withstand heating at 56°C for 60 minutes, but they can be inactivated by formalin, β-propiolactone, and oxidizing agents.

Table1. Important Properties of Parvoviruses

Fig2. Electron micrograph of parvovirus particles. (Courtesy of FA Murphy and EL Palmer.)

Virions contain two coat proteins that are encoded by an overlapping, in-frame DNA sequence, so that VP2 is identical in sequence to the carboxy portion of VP1. The major capsid protein, VP2, represents about 90% of virion protein. The genome is about 5 kb, linear, single-stranded DNA. Autonomous parvoviruses usually encapsidate primarily DNA strands complementary to viral mRNA; defective viruses tend to encapsidate DNA strands of both polarities with equal frequency into separate virions.

Classification

There are two subfamilies of Parvoviridae: the Parvovirinae, which infect vertebrates, and the Densovirinae, which infect insects. The Parvovirinae comprise five genera. Human parvo virus B19 is the most common member of the Erythroparvovirus genus. The three human bocaviruses are in the Bocaparvovirus genus. Feline panleukopenia virus and canine parvovirus, both serious causes of veterinary diseases, are classified as members of the Protoparvovirus genus, as are isolates from many other animals. The genus Dependovirus contains members that are defective and depend on a helper virus (an adenovirus or herpesvirus) for replication. Human adeno-associated viruses (AAVs) have not been linked with any disease but are candidate vectors for gene therapy treatments.

Parvovirus Replication

It is difficult to culture human B19 parvovirus. Only primary erythroid progenitors are known to be permissive for B19 infection. The cellular receptor for B19 is blood group P antigen (globoside). P antigen is expressed on mature erythrocytes, erythroid progenitors, megakaryocytes, endothelial cells, placenta, and fetal liver and heart, which helps explain the narrow tissue tropism of B19 virus. The α5β1 integrin is believed to be a coreceptor for B19 entry.

The parvoviruses are highly dependent on cellular functions for replication. Viral DNA replication occurs in the nucleus. The parvoviruses do not have the ability to stimulate resting cells to initiate DNA synthesis, so they must infect dividing cells. One or more cellular DNA polymerases are involved. The nonstructural protein, NS1, is required for virus replication. There are two capsid proteins. Viral replication results in cell death.

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مواضيع ذات صلة

HIV Infections in Humans: Laboratory Diagnosis

HIV Infections in Humans :Clinical Findings

HIV Infections in Humans : Pathogenesis and Pathology

Animal Lentivirus Systems

Classification of Lentiviruses

Viral DNA Is Integrated into the Host-Cell Genome in Some Nonlytic Viral Growth Cycles

Lytic Viral Growth Cycles Lead to Death of Host Cells

Viral Capsids Are Regular Arrays of One or a Few Types of Protein

Structure and Composition of Lentiviruses

HERPESVIRUSES

POLYOMAVIRUSES

Human T-Lymphotropic Viruses