HIV Structure and the Pathogenesis of AIDS

HIV (Human Immunodeficiency Virus) is a retrovirus with a fairly simple but distinctive structure. Its outermost layer is a lipid envelope, borrowed from the membrane of the host cell it last budded from, studded with glycoprotein spikes — gp120, anchored in the membrane by a second glycoprotein, gp41. Beneath the envelope is a protein coat (capsid) enclosing two identical copies of single-stranded RNA (ssRNA), the virus's genetic material, together with the enzyme reverse transcriptase that the virus carries with it.

The gp120 spike specifically recognises and binds the CD4 receptor, a surface marker found mainly on helper T-lymphocytes (TH cells). This is why HIV shows a strong tropism (preference) for infecting TH cells in particular, rather than cells at random.

Once HIV has entered a TH cell, its reverse transcriptase enzyme converts the viral RNA genome into a DNA copy — the defining feature of a retrovirus, since this reverses the usual DNA-to-RNA flow of information. This viral DNA then integrates into the host cell's own chromosomal DNA, forming what is called a provirus. The provirus may remain dormant for a time, or it may direct the host cell's own machinery to manufacture large numbers of new virus particles, which eventually burst out of and destroy the host TH cell, going on to infect further TH cells in the same way. …