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HIV-1 vs HIV-2

Patrick O'Neil patrick at corona
Wed Mar 15 23:25:34 EST 1995


I didn't have my references available with my first reply so I decided to 
do a quick follow-up.

On 15 Mar 1995, RYBICKI, ED wrote:

> > 
> > HIV in general appears to have jumped into humans from Gabonese 
> mandrills> from 1 to 2 centuries ago.  From this jump, the two HIV 
> strains diverged,
> ...
> > The most parsimonious scenario of all has an HIV ancestor 
> incubating > within human populations for about a thousand years or 
> so, with jumps > into various simian lines within the last few 
> centuries.
> 
> Whaaaaaaaat?  Says who?!  All of a sudden, HIV has been in HUMANS 
> for a thousand odd years, and gets BACK into simians??

Though it is somewhat up in the air whether HIV-1 jumped from humans to 
chimps or vice versa, the HIV-1 strains diverged from each other (HIV-1 
and HIVant70) before 1960 and diverged from a chimpanzee virus 50 to 100 
years before.  The HIVant70 variant diverged BEFORE the human - chimp 
divergence, which would require two jumps from chimps to account for this 
whereas if it derives from humans, then only a single jump to chimps in 
needed.  My reference here is Ewald's book _Evolution of Infectious 
Disease_ and a number of sources he cites, which I subsequently acquired:
 
(Peeters, M., Fransen, K., Delaporte, E., Van den Haesevelde, 
M., Gershy-Damet, G. M., Kerstens, L., van der Groen, G., and Piot, P. 
1992.  Isolation and characterization of a new chimpanzee lentivirus 
(simian immunodificiency virus isolate cpz-ant) from a wild-captured 
chimpanzee.  AIDS 158, 866-868.

Li, W. H., Tanimura, M., and Sharp, P. M. 1988.  Rates and dates of 
divergence between AIDS virus nucleotide sequences.  Mol. Bio. Evol. 5, 
313-330.

Myers, G., MacInnes, K., and Korber, B.  1992.  The emergence of 
simian/human immunodificiency viruses.  AIDS Res. Hum. Retroviruses 8, 
373-386.

> 
> Surely a most parsimonious explanation is that HIVs separated in 
> simians, and went merrily along until humans were infected in recent 
> times?  Far mor eparsimonious than postualting all of the jumps into 
> monkeys that would have been necessary to acccount for all of the 
> SIVs, 

Depending on the method used, the main HIVs diverged anywhere from 1000 
to 100 years ago (the most recent date I come across is no less than 50 
years).  A divergence 900 years ago comes from:

Eigen, M., and Nieselt-Struwe, K. 1990.  How old is the immunodificiency 
virus?  AIDS 4 (supp 1), S85-93.

> several of which are quite divergent.  And some of which, I 
> might point out, don't cause immunodeficiency disease in their 
> natural hosts - indicating a better adaptation to these than to 
> other hosts, such as lab monkeys.

As for the argument arising from the idea that a benign infection must 
mean long association, I wish to quote from Ewald's book on HIV in 
particular as concerns virulence:

"...the idea that old associations are benign.  HIV causes AIDS in 
humans.  SIVs do not seem to cause AIDS in mangabeys, green monkeys, or 
chimpanzees, but can cause an AIDS-like disease in rhesus monkeys and 
macaques.  If old associations are benign, green monkeys, mangabeys, and 
chimps must be the older hosts... Because immunodificiency viruses are 
venereally transmitted, they must survive in the host for a period 
comparable to the time between partner changes.  Generally this 
requirement means that they must infect long-lived cells without 
triggering immunological destruction of the cells.  Specialization on 
cells of the immune system allows them to do so, but also requires 
suppressed reproduction if partner changes are infrequent.  If, however,  
rates of partner change increase, natural selection should favor 
increased rates of viral reproduction, such as those exhibited by HIV-1...
If HIV were to replicate explosively, the infected person would probably 
die during the early symptomatic stage rather than after many years of 
infection.  Indeed, a variant of SIV that was transmitted experimentally 
in a way that favored such variants, soon replicated explosively, 
causing death of infected macaques within two weeks of innoculation... 
Severe immunodificiency could develop in an old association, as a result 
of increases in sexual partner rates causing evolution of increased 
virulence."

It is the last two sentences that are particularly telling in that they
illustrate how the mode of transmission itself, the selection placed upon
the virus for slow or rapid replication, is more important than time in
host. 

Patrick



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