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1.The
human immunodeficiency virus (HIV), the causative agent of AIDS, is a retrovirus. In order
to reproduce, retroviruses enter living cells and use the host cell´s replication
machinery. The outer envelope of HIV consists of a lipid bilayer with protruding
spikes
(glycoprotein 120).
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Inside the envelope lies the nucleocapsid which surrounds the central
core protein. Packed within this core is the viral genome (two RNA strands), and several
copies of the enzyme reverse transcriptase. This enzyme converts the viral RNA into
proviral DNA during replication.
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2.During HIV
infection, millions of viral particles are present in the blood stream. They continuously
infect cells of the immune system. As the immune system becomes progressively more
damaged, HIV disease becomes more
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advanced and eventually the Acquired Immunodeficiency
Syndrome (AIDS) develops.
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3.HIV binds to cells
via a molecule, known as the CD4 antigen. Macrophages and a subset of T lymphocytes (T
cells), known as helper T cells, express CD4 and so are vulnerable to HIV infection.
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4.Each helper T cell
expresses the CD4 antigen. HIV enters the cell by binding to CD4 and other cell surface
molecules. A helper T cell is known colloquially as a CD4 cell.
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5.Helper T
cells express the CD4 antigen. HIV enters cells by binding to CD4. Helper T cells are
known colloquially as CD4 cells. A healthy individual normally has between 500 and 1 500
helper T cells per mm3 of plasma.
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6.The anchor protein
gp 120 on the surface of HIV enables the virus to bind to a target cell. CD4 and other
molecules on the cell surface form a complex with gp 120 to initiate the fusion of the
virus with the target cell.
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7.After the gp 120
protein has enabled the virus to bind to CD4 and other host cell surface molecules, HIV
starts to fuse with the target cell. The infectious cycle of HIV begins.
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8.Following the
establishment of a stable contact, the viral membrane fuses with the membrane of the CD4
cell. During fusion, the nucleocapsid is released into the cell.
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9.As HIV enters the
host cell, it loses its outer envelope. The genetic information and the enzymes for viral
replication are contained within the nucleocapsid. Two viral molecules, which are of
particular importance for viral replication,
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are viral RNA and the enzyme reverse
transcriptase.
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10.The presence of the
viral nucleocapsid within the host cell has been documented by electron microscopy. The
nucleocapsid contains the genetic information of HIV (viral RNA) and viral proteins,
including reverse transcriptase.
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