Human immunodeficiency virus (HIV) types, derived from primate lentiviruses, are the etiologic agents of Acquired Immune Deficiency Syndrome (AIDS). The illness was first described in 1981, and HIV-1 was isolated by the end of 1983. Since then, AIDS has become a worldwide epidemic, expanding in scope and magnitude as HIV infections have affected different populations and geographic regions. Millions are now infected worldwide; once infected, individuals remain infected for life. Within a decade, if left untreated, the vast majority of HIV-infected individuals develop fatal opportunistic infections as a result of HIV-induced deficiencies in the immune system. AIDS is one of the most important public health problems worldwide at the start of the 21st century. The development of highly active antiretroviral therapy (HAART) for chronic suppression of HIV replication and prevention of AIDS has been a major achievement in HIV medicine.
PROPERTIES OF LENTIVIRUSES
Important properties of lentiviruses, members of a genus in the Retroviridae family, are summarized in Table 44-1.
TABLE 44-1Important Properties of Lentiviruses (Nononcogenic Retroviruses) ||Download (.pdf) TABLE 44-1 Important Properties of Lentiviruses (Nononcogenic Retroviruses)
|Virion: Spherical, 80–100 nm in diameter, cylindric core |
|Genome: Single-stranded RNA, linear, positive-sense, 9–10 kb, diploid; genome more complex than that of oncogenic retroviruses, contains up to six additional replication genes |
|Proteins: Envelope glycoprotein undergoes antigenic variation; reverse transcriptase enzyme contained inside virions; protease required for production of infectious virus |
|Envelope: Present |
|Replication: Reverse transcriptase makes DNA copy from genomic RNA; provirus DNA is template for viral RNA. Genetic variability is common. |
|Maturation: Particles bud from plasma membrane |
Members are nononcogenic and may be cytocidal
Infect cells of the immune system
Proviruses remain permanently associated with cells
Viral expression is restricted in some cells in vivo
Cause slowly progressive, chronic diseases
Replication is usually species-specific
Group includes the causative agents of AIDS
Structure and Composition
HIV is a retrovirus, a member of the Lentivirus genus, and exhibits many of the physicochemical features typical of the family (see Chapter 43). The unique morphologic characteristic of HIV is a cylindrical nucleoid in the mature virion (Figure 44-1). The diagnostic bar-shaped nucleoid is visible in electron micrographs in those extracellular particles that happen to be sectioned at the appropriate angle.
Electron micrographs of HIV-infected lymphocytes, showing a large accumulation of freshly produced virus at the cell surface (top, 46,450×, bar = 100 nm); newly formed virus budding from cytoplasmic membrane (lower left, 49,000×, bar = 100 nm); and two virions about to be cast off from cell surface (lower right, 75,140×, bar = 100 nm).
The RNA genome of lentiviruses is more complex than that of transforming retroviruses (Figure ...