Epidemiology and HIV’s Physical Properties

OCT.19 '95 7:35AM 310-446-1664 LAYNE @ UCLA. +1 310 446 1664.10
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FIGURE CAPTIONS
FIG. 1 a, The diagram illustrates the release of newly manufactured HIV particles from CD4+
cells. Experiments have demonstrated that a very small fraction (< 0.01%) of these particles arc
infectious. More transmissible HIV isolates may have larger infectious fractions compared to less
transmissible ones. b, The diagram illustrates the spontaneous loss of HIV infectivity. With
time, gpl20 complexes fall off the virus and reverse transcriptases lose their enzymatic activity.
Both molecules are necessary for maintaining infectivity - gpl20 initiates viral entry by binding to
CD4 receptors on ccli1 surfaces and reverse transcriptase initiates replication by converting RNA to
DNA. More transmissible HIV isolates may have slower rates of decay compared to less
transmissible ones.
FIG. 2 a, The diagram illustrates how the reproductive number influences the growth of viral
infection. Each infected CD4+ cell manufactures a certain number of virions - the reproductive
number - that diffuse and infect neighboring susceptible cells. This viral "chain reaction" is
shown for three time cycles and three different reproductive numbers. At time I, all boxes contain
one infected cell. A reproductive number of one leads to a constant burden of infected cells (top).
A reproductive number of two leads to infections that double per cycle (middle) and a reproductive
number of three leads to infections that triple per cycle (bottom). Reproductive numbers greater
than one thus lead to infections that grow exponentially with time. More transmissible HIV
isolates may have greater reproductive numbers compared to less transmissible ones. b, The
diagram illustrates how steady-state kinetics govern viral loads in vivo. Viral production by
CD4+ cells is represented by liquid pouring into the cistern and viral clearance by the immune
system is represented by liquid pouring out. Over time, the viral load (represented by the liquid's
height) builds up to a level where the total input matches the total output. Both cisterns have
similar rates of clearance but the cistern on the right has a greater rate of production.
Consequently, the viral load is much greater in this cistern. More transmissible HIV isolates may
have greater rates of viral reproduction in viva which generate larger viral loads.