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OK.  I think we'll get started.

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So we're going to continue our

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discussion today about viruses,
finish talking about influenza,

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influenza A virus and the flu,
and then talk a bit about HIV and

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AIDS.  Do you guys know more about
the tutorial session and so on and

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so forth for next week?
Has that been decided yet?  OK.

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So you'll get more information
Monday and Wednesday.

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And I'll also have an office hour,
probably a couple of office hours at

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the end of next week.
Most likely, if it doesn't conflict

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with this tutorial session that's
being planned,

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it will be during this hour from
11:00 probably until 1:00 in this

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room because it's free since we're
usually here.  But that may conflict

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with you guys so we'll have to see.
I'll let you know on Monday.

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So, again, the situation with
viruses is an ongoing one.

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And, actually, one of you brought
to my attention an outbreak of polio

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that's happening now both in Africa
and Indonesia.

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This is actually from today's CNN.
om where there's a fear that polio

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is starting to spread.
In this case in Jakarta,

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in Indonesia.  And there's a very
extensive reaction to this.

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There's a great fear that polio
will spread.

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This is an area that is not well
protected currently by polio

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vaccination.  And so if you look at
this you'll see that they are

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raising lots of money in order to
try to vaccinate with standard polio

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vaccines, which work extremely well,
5.2 million children, to try to stop

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the spread of this potential
epidemic of polio.

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It hasn't reached anything near
those levels to be that concerned,

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but in order to protect against that
possibility they're going

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to vaccinate.
And, obviously,

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if you vaccinate,
individuals cannot be successfully

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infected.  If they cannot be
successfully infected,

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the virus cannot propagate so they
cannot pass it on.

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So you just close off the
infectious cycle in a very small

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circle of infected individuals.
Also, in today's Globe, there was a

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discussion of a local company that
makes vaccines against smallpox.

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And I've mentioned to you that
smallpox is largely eliminated,

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if not fully eliminated in the world,
but there are stocks of smallpox in

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freezers around the world.
And there is a concern that an

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individual could access smallpox
virus or make one because the

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relevant sequences of the genome of
the smallpox virus is known.

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And so, in theory, you could
synthesize your own smallpox virus

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genome, and thereby create your own
smallpox virus and expose now

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unprotected individuals because we
don't get vaccinated currently

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against smallpox.
And so there are companies like

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this one that are making currently
and distributing smallpox vaccines.

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They distributed 182 million doses
in this country alone just in case

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somebody tried to deliberately
release some smallpox.

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So that's what we can do when we
know what we're dealing with.

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We can create effective vaccines.
In this case they're effective, but

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sometimes we get exposed to stuff
that we cannot effectively deal with.

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And I introduced this
to you last time.

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This was the major flu pandemic from
1918.  Some people worried that

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there might be another pandemic this
year because in the off season

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between 1918 and 1919 when this
happened, and 20 to 40 million

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people died, that's when the Red Sox
last won the World Series.

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[LAUGHTER]  So the Armageddon folks
thought maybe this was the year,

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but so far so good.  So, as I
mentioned last time,

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this was a major outbreak and a
major problem worldwide.

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But you also should know that
influenza is an annual problem.

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And I, for one, didn't appreciate
this.

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Where there are about 25 thousand

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deaths per year in the United States.
And, of course,

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to deal with that we make available,
especially to the elderly and

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infirmed immunodeficient flu shots.
And flu shots are simply flu

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vaccines.  And I'll tell you a
little bit about how they're made

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towards the end of this section,
but they're generally effective in

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combating against the flu that is
going to hit the population that

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year.  And they change every year
because, as you'll see,

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flu changes all the time as well.
So it's an annual problem.  There

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are occasional epidemics which you
could think of as --

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-- population-based infection where

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many individuals within a particular
population are infected.

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And the virus, in this case,
is spreading throughout that

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population.  And there are even more
occasional pandemics.

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And that's a worldwide infection.

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Where, because of the virulence of
the virus and its ability to access

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different populations and the
absence of immunity throughout the

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worldwide population,
lots and lots of people get infected.

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And that's what happened in 1918.
So how do we explain this?  How do

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we explain the occasional epidemics
of a virus that we're so familiar

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with?  Flu, as I said,
we get every year, lots of people

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get it every year.
And, yet, we seem to be susceptible

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year after year.
So why is that?  What causes the

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resistance to this immunity in small
measure and in large measure?

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And that's what I want to review
for you today.

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So this is the responsible agent.
It's a virus, of course.  More

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specifically, it's an enveloped
virus, which means it has its own

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lipid bilayer that it picked up from
the host cell.

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And you'll notice on the outside are
things sticking out of the bilayer.

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These are proteins encoded by the
virus which are going to be

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responsible for binding the virus to
the host cell.

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And also allowing the viral
membrane to fuse with the host cell

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membrane.  And you'll see that
that's done in the case of flu virus

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in a slightly different way than for
some other viruses.

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And then inside you have the capsid.
And wrapped up in this protein,

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this helical protein structure are
the nucleic acids of the virus.

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And the nucleic acids of this virus
are multiple.  That is it's not just

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one.  Oops.  It's not just one.
It's actually several.

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So, again, it's called influenza A

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virus.  It's an example
of an enveloped virus.

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And I said you should note the
envelope proteins,

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which are largely encoded by the
virus itself.  And we call these

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envelope glycoproteins.
Glycoproteins because when they

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emerge through the sorting pathway
of the cell they pick up sugars.

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And so when they're on the outside
of the cell, it's not just protein.

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There is some sugar there, so
glycoproteins.

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And sometimes the antibodies that
we make are directed against the

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sugar moieties alone or in
combination with peptides.

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So the envelope glycoproteins are
important.  As I said,

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it's a segmented RNA genome,
which means that there are multiple

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distinct nucleic acids in the viral
particle.  And in the case of flu

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there are eight.

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The RNA genome is single-stranded.
And all of the genome segments are

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in the minus configuration.
So their sequence is the complement

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to the mRNA.  OK?
And we talked about the

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significance of RNA viruses that
have a minus strand polarity in the

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sense that they need to have their
own RNA polymerase coming in with

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the virus in order to help,
in order to initiate the replication

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cycle.
So, hopefully,

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that's familiar from last time.
This is a diagram of the virus,

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and it just makes the same points I
just made.  So,

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again, it has a lipid bilayer.
It has envelope glycoproteins

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sticking out from the surface.
You'll see that they're important

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for different things in a moment.
Inside that is a nucleocapsid which

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surrounds the nucleic acids.
And you can see that there's a

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series of nucleic acids that are
enclosed within this.

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And, again, there are eight of them.
And they're all minus strand.

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And, importantly, contained within
here is a protein,

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an RNA dependent RNA polymerase,
the yellow dot.  And it has to get

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incorporated with the virus so that
when the virus infects it goes in

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there, too.  And it can then act on
the RNA genome and convert it into a

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plus strand, which will then serve
both as the RNA for translation,

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as well as the template for the
production of more of the

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minus strand RNA.
OK?  So this comes from your book,

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and it is a summary of the
infectious cycle of this virus.

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I've modified it a little bit
because some of the details which I

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think are important were missing.
So you might want to pay attention

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to this figure as you're reading the
section, the relevant

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section in the book.
But this is a typical viral

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lifecycle.  The virus attaches.
Remember, the terms that I used

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last time?  The virus attaches.
Here one of the viral glycoproteins

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binds to a protein on the surface of
the target cell.

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That then initiates an
internalization process,

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a penetration process in which the
virus gets brought in through an

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endosome.  So it gets brought in
through a separate vesicle,

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an endosome.  That vesicle, the
endosome then fuses with a lysosome,

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not shown here.
And you may recall that the

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lysosomes have very low pH.
And it's in the context of the low

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pH that one of the viral
glycoproteins changes its shape.

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And when it changes its shape it
allows the viral membrane to fuse

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with the membrane of the lysosome.
So this is a pH-dependant protein

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conformational change which turns an
inner protein into a protein that

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facilitates fusion.
And when that happens,

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the viral capsid can then slip out
into the cytoplasm.

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It gets unpackaged.
The RNA, which is the minus strand

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RNA gets released.
And then it gets acted on by that

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RNA dependent RNA polymerase that
came in and gets converted from

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minus strand to plus strand.
And the plus strand can then do two

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things.  It can get translated into
viral proteins,

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both the glycoproteins that go
through the sorting pathway and make

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it to the membrane,
as well as the structural proteins

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that form the capsid.
The structural proteins then join up

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with the viral RNA segments,
now the minus strand RNA segments.

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They meet at the membrane and then
bud off to form a new virus.

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OK?  So this is a standard
infectious cycle for this class of

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viruses.  And because it's going to
be relevant later,

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I just want to emphasize a couple of
points.

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Through interactions between the
cytoplasmic tail of the envelope

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glycoprotiens,
the capsid proteins coalesce

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underneath the plasma membrane.
And there's an interaction between

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these proteins and the tails of
these proteins which causes the

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virus to start to bud off the
surface of the cell.

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Now, there are separate interactions
with the proteins on the inside of

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the capsid which bring with them the
different viral RNA.

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So remember there are eight.
And there's a rather amazing

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process by which the eight distinct
RNA segments get brought together

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into the capsid,
and then the capsid goes to the

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membrane and gets budded.
So there is a sorting process,

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which we don't fully understand,
that ensures that viruses get at

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least one copy of each of the
genomic segments.

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And that's necessary because if a
virus doesn't have all eight,

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when it gets into the host cell that
it might infect,

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it's not going to be able to
replicate.  These RNA segments

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encode one, or at most two of the
viral proteins that are necessary

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for the infectious cycle that's
drawn up there.

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And you'll see why that's important
in a moment.

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OK.  So we get infected,
we develop antibodies against the

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proteins of the virus,
and we typically develop them

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against the glycoproteins that are
sitting on the outside.

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We develop T cells that can
recognize virus infected cells and

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kill them.  And yet we keep getting
flu.  So why is that?

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How is it that we can,
how is it that the virus can

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overcome this immune resistance?
And the answer is that the virus

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changes.  And this is a major
problem with all viruses,

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but it's a particular problem with
RNA viruses and a particular problem

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with flu.  So here's a depiction of
the virus again.

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It's got its genomic segments in
here.  And on the surface it has

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these glycoproteins.
And I'm going to draw a little bit

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more detail in these
glycoproteins now.

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There are actually two distinct
glycoproteins on the surface of the

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cell.  There's one that's called HA
and there's another that's called NA.

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And they carry out different
functions.  When you get infected,

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your immune system sees these
epitopes sitting on the surface of

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the virus and you make antibodies
against them.  And there are three

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particular places on these proteins
where good antibodies can be made

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inside your body.
One of them is here,

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00:16:08,000 --> 00:16:15,500
another one is here, and a third one
is here.  And different people make

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different of these antibodies.

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And these antibodies that are made
have a name which is neutralizing or

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blocking antibodies.
They're called that because if you

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have one of these antibodies,
and it will bind to the surface of

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the viral particle,
this virus can now not infect the

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cell.  There's an interference
between the, what the hell?

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00:17:04,000 --> 00:17:08,000
Between the glycoproteins on the
surface and the receptors to which

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00:17:08,000 --> 00:17:13,000
they bind by the antibody.
So they block binding.  And if you

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00:17:13,000 --> 00:17:18,000
cannot bind you cannot infect.
And this is very efficient.  This

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00:17:18,000 --> 00:17:23,000
works.  So that's how you can
overcome the infection to that

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00:17:23,000 --> 00:17:28,000
particular virus.
The problem is that viruses change.

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During their replication there are
mutations.

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00:17:33,000 --> 00:17:38,000
And sometimes these mutations affect
the structure of the proteins that

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are sitting on the surface.
So you might imagine the strain

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here gives rise to variant here
which has an HA protein which looks

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more or less the same,
but it has an NA protein which has

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00:17:53,000 --> 00:17:59,000
changed.  So now this epitope,
which was recognized by antibody

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three, cannot be recognized by
antibody three because it's got a

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00:18:04,000 --> 00:18:09,000
difference sequence.
OK?  If you were a person who made

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00:18:09,000 --> 00:18:13,000
predominantly antibody three in
response to this infection,

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00:18:13,000 --> 00:18:17,000
you would not be susceptible to this
virus.  So if this one came along

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next year you'd get the flu.
If you were a person who made

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00:18:21,000 --> 00:18:25,000
antibody one or antibody two as your
major protective antibody,

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00:18:25,000 --> 00:18:30,000
you'd still be resistant to this.
You'd be one of the lucky ones.

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00:18:30,000 --> 00:18:34,000
And because there's heterogeneity
within our population,

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even if these new variants arise,
they don't spread all that much

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because there are enough people in
the population who are pretty well

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protected.  So those are low-level
spread or low-level epidemics.

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OK?  Now, this process, one day I'm
going to figure out how this works.

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This process of generating variant

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00:18:57,000 --> 00:19:02,500
viruses through this mutational
mechanism is called

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00:19:02,000 --> 00:19:12,000
antigenic drift.

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00:19:12,000 --> 00:19:16,000
It's a slow drift of the antigens on
the virus.  And it happens because

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the virus is highly mutagenic.
And it's true of RNA viruses in

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general.

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00:19:33,000 --> 00:19:38,000
And there are two reasons why RNA
viruses create variants at such high

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00:19:38,000 --> 00:19:44,000
levels.  One is that RNA is less
stable than DNA.

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00:19:44,000 --> 00:19:49,000
Remember, there's a difference in
the structure of RNA versus DNA.

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00:19:49,000 --> 00:19:55,000
There's an extra hydroxyl in the
structure of RNA which can cause

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00:19:55,000 --> 00:20:01,000
increased mutation,
increased breaks and base

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00:20:01,000 --> 00:20:07,000
substitutions in our RNA molecules
compared to DNA molecules.

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00:20:07,000 --> 00:20:15,000
So RNA is inherently less stable
than DNA.  And also RNA polymerases

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00:20:15,000 --> 00:20:25,000
lack proofreading functions.

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00:20:25,000 --> 00:20:30,000
Which means they have an inherently
higher mutation rate.

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00:20:30,000 --> 00:20:34,000
And hopefully,
again, this is familiar from our

252
00:20:34,000 --> 00:20:38,000
discussions about DNA replication.
All polymerases make mistakes but

253
00:20:38,000 --> 00:20:42,000
your DNA polymerases have what's
called a proofreading function which

254
00:20:42,000 --> 00:20:46,000
can recognize the mistakes and
correct them.  RNA polymerases lack

255
00:20:46,000 --> 00:20:50,000
that, and so they make the mistakes
and they stay as mistakes.

256
00:20:50,000 --> 00:20:54,000
So the mutation rates for RNA
polymerases are at least ten fold

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00:20:54,000 --> 00:20:58,000
higher than for DNA polymerases,
and in some places even higher still.

258
00:20:58,000 --> 00:21:02,250
OK.
So that's what's called antigenic

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00:21:02,000 --> 00:21:06,750
shift, sorry, antigenic drift.
And it happens all the time and is

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00:21:06,000 --> 00:21:11,250
responsible for why we keep getting
mild cases of the flu.

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00:21:11,000 --> 00:21:15,750
But that's not what's responsible
for the Spanish flu of 1918.

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Instead of antigenic drift --

263
00:21:23,000 --> 00:21:27,000
-- these are due to a phenomenon
called antigenic shift.

264
00:21:27,000 --> 00:21:31,000
Rather than generating subtly
different variants,

265
00:21:31,000 --> 00:21:35,000
like I've shown up here,
this is the process of generating

266
00:21:35,000 --> 00:21:47,000
essentially entirely new strains.

267
00:21:47,000 --> 00:21:50,000
And it happens in the case of flu
all the time.  And one of the

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00:21:50,000 --> 00:21:54,000
reasons that it happens is that
there are lots and lots of types of

269
00:21:54,000 --> 00:21:58,000
flu viruses which are kind of
similar that infect other species.

270
00:21:58,000 --> 00:22:02,000
There are duck flus,
swine flus, horse flus,

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00:22:02,000 --> 00:22:07,000
seal flus that are all caused by
versions of this influenza A virus.

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00:22:07,000 --> 00:22:12,000
Similar.  Not identical but similar.
And sometimes those viruses cross

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00:22:12,000 --> 00:22:17,000
over into the human population.
And this is an example of a

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00:22:17,000 --> 00:22:35,000
zoonotic infection --

275
00:22:35,000 --> 00:22:38,000
-- which is an infection from
another species into the human

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00:22:38,000 --> 00:22:41,000
population.  And actually happens
all the time.  Many of the new

277
00:22:41,000 --> 00:22:44,000
pathogens that arise in the human
population arise through zoonotic

278
00:22:44,000 --> 00:22:47,000
infection.  Now,
most viruses that are evolved to

279
00:22:47,000 --> 00:22:50,000
grow in the cells of one given
species, at the temperature of a

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00:22:50,000 --> 00:22:54,000
given species,
will not replicate very well inside

281
00:22:54,000 --> 00:22:57,000
human cells.  So even if they did
infect, they wouldn't reproduce

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00:22:57,000 --> 00:23:01,000
themselves very well.
And that would be true of these

283
00:23:01,000 --> 00:23:06,000
viruses, too, swine flu virus,
bird flu viruses.  The problem is,

284
00:23:06,000 --> 00:23:10,000
in the case of flu, the human virus
is so common and the mechanism of

285
00:23:10,000 --> 00:23:15,000
replication of this virus is so
complex and amenable to generation

286
00:23:15,000 --> 00:23:20,460
of recombinant viruses that we can
generate new viral forms.

287
00:23:20,000 --> 00:23:25,230
And that's what I want to talk to
you about now.

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00:23:25,000 --> 00:23:30,000
So here's an example from not too
long ago.

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00:23:30,000 --> 00:23:34,000
This is the collection and culling
of a lot of birds somewhere in Asia.

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00:23:34,000 --> 00:23:39,000
I'm not really sure where this was.
But there was a concern that a new

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00:23:39,000 --> 00:23:44,000
virus was developing in a bird
population and was crossing over

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00:23:44,000 --> 00:23:49,000
into humans.  And it caused some
deaths.  And so to avoid that

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00:23:49,000 --> 00:23:54,000
further they just wiped out some
millions of these birds in order to

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00:23:54,000 --> 00:23:59,000
prevent further cross infection.
But, again, it's not the presence of

295
00:23:59,000 --> 00:24:04,000
the virus, the bird virus or swine
virus itself that's the problem.

296
00:24:04,000 --> 00:24:08,000
It's the fact that the two viruses,
the human virus and the bird virus

297
00:24:08,000 --> 00:24:13,000
can recombine to form a dangerous
new virus which is rather similar to

298
00:24:13,000 --> 00:24:18,000
the human virus but carries some
segments of the bird virus.

299
00:24:18,000 --> 00:24:22,000
And that's illustrated here,
and I'll show you on the board in a

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00:24:22,000 --> 00:24:27,000
second.  But basically the idea is
that a human virus carrying its

301
00:24:27,000 --> 00:24:32,000
eight segments,
including the segment for HA and NA

302
00:24:32,000 --> 00:24:37,000
infects an animal which also gets
infected by a bird virus.

303
00:24:37,000 --> 00:24:41,000
So that in the same cell there are
the segments for the human virus,

304
00:24:41,000 --> 00:24:45,000
as well as the segments for the bird
virus.  And during this process of

305
00:24:45,000 --> 00:24:49,000
the generation of a new viral
particle, you can get mixing of the

306
00:24:49,000 --> 00:24:53,000
segments so that most of the
segments come from the human virus

307
00:24:53,000 --> 00:24:57,000
and, therefore,
will replicate well in the context

308
00:24:57,000 --> 00:25:02,000
of the human cell.
But two new segments come in from

309
00:25:02,000 --> 00:25:08,000
the bird that encode its HA and NA,
a version that's never been seen in

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00:25:08,000 --> 00:25:13,000
the human population.
Nobody has antibodies against it so

311
00:25:13,000 --> 00:25:18,000
the virus can spread like wildfire.
And that's what we believe happened

312
00:25:18,000 --> 00:25:24,000
then and we fear continues to happen
over time.  So,

313
00:25:24,000 --> 00:25:29,000
again, imagine a cell which has been
infected by two different viruses,

314
00:25:29,000 --> 00:25:35,000
the avian virus, in this case it was
a duck, as well as the human virus

315
00:25:35,000 --> 00:25:41,000
in that given year.
In that cell there are going to be

316
00:25:41,000 --> 00:25:47,000
lots of different segments of virus
RNA, the human ones H1,

317
00:25:47,000 --> 00:25:53,000
2, 3, 4, 5, 6, 7, 8 and so on,
as well as the duck ones, duck 1,

318
00:25:53,000 --> 00:25:59,000
duck 2 and so on and so forth, duck
7 and duck 8.

319
00:25:59,000 --> 00:26:05,000
When the viral capsids coalesce
underneath the viral glycoproteins

320
00:26:05,000 --> 00:26:12,000
to produce a new virus,
they cannot necessarily distinguish

321
00:26:12,000 --> 00:26:19,000
between the human and the duck
subunits.  So there might be human 1,

322
00:26:19,000 --> 00:26:26,000
human 2, human 3,
duck 4, human 5, duck 6,

323
00:26:26,000 --> 00:26:32,000
human 7 and human 8.
OK?  This then makes a virus which

324
00:26:32,000 --> 00:26:36,000
is perfectly able to replicate in
human cells because it's got mostly

325
00:26:36,000 --> 00:26:40,000
the genes that are optimized for
human cells, but it's got new HA and

326
00:26:40,000 --> 00:26:44,000
NA forms that have never been seen
by the human population.

327
00:26:44,000 --> 00:26:49,000
So there is nobody who has
antibodies against it.

328
00:26:49,000 --> 00:26:53,000
So the virus will spread like crazy.
OK?  So this is an example of

329
00:26:53,000 --> 00:26:57,000
antigenic shift,
and it's responsible for these

330
00:26:57,000 --> 00:27:01,000
worldwide pandemics.
OK.  Any questions about flu?

331
00:27:01,000 --> 00:27:05,000
One thing I failed to mention,
sort of trivial but you probably

332
00:27:05,000 --> 00:27:09,000
know, is it spread very well because
it can spread in water droplets.

333
00:27:09,000 --> 00:27:13,000
If you sneeze on your friends
they'll also get the flu.

334
00:27:13,000 --> 00:27:17,000
It's a very efficient virus so you
don't need very many viral particles

335
00:27:17,000 --> 00:27:21,000
to go inside of your respiratory
system in order to initiate the

336
00:27:21,000 --> 00:27:25,000
infection.  And this is another good
example.  When you have a very

337
00:27:25,000 --> 00:27:29,000
active infection inside of you,
you start producing lots of the

338
00:27:29,000 --> 00:27:33,000
cytokines, that I mentioned last
time, like interferons.

339
00:27:33,000 --> 00:27:37,000
And, again, it's largely those that
are causing the symptoms of the flu,

340
00:27:37,000 --> 00:27:41,000
and in large enough quantities can
cause death.  You can also get

341
00:27:41,000 --> 00:27:45,000
secondary infections with bacteria
when you're that sick,

342
00:27:45,000 --> 00:27:49,000
which can be another reason why
folks die.  OK.

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00:27:49,000 --> 00:27:53,000
So if there are no other questions
about flu, we'll now turn our

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00:27:53,000 --> 00:27:57,000
attention to HIV.
And I think HIV is probably fairly

345
00:27:57,000 --> 00:28:02,000
familiar as a topic.
It's, of course,

346
00:28:02,000 --> 00:28:08,000
a major, major worldwide health
problem.  The initiation of this

347
00:28:08,000 --> 00:28:18,000
epidemic goes back a very long way.

348
00:28:18,000 --> 00:28:24,000
The first cases were reported in
1975, so about 30 years ago.

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00:28:24,000 --> 00:28:30,000
And so that's considered to be the
initiation of this epidemic

350
00:28:30,000 --> 00:28:35,000
or pandemic.
The first case in the United States

351
00:28:35,000 --> 00:28:39,000
is considered to have occurred in
1980.  And here's another example.

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00:28:39,000 --> 00:28:43,000
We think that this probably
occurred by one of the

353
00:28:43,000 --> 00:28:51,000
zoonotic infections.

354
00:28:51,000 --> 00:28:55,000
But it happened a long time ago.
It happened at least before 1950

355
00:28:55,000 --> 00:28:59,000
because there are samples of
individuals who died of unknown

356
00:28:59,000 --> 00:29:03,000
causes that have now been tested and
can be shown to have HIV

357
00:29:03,000 --> 00:29:08,000
sequences by PCR.
So whenever this happened,

358
00:29:08,000 --> 00:29:12,000
and from whatever species it
happened, it happened quite a while

359
00:29:12,000 --> 00:29:17,000
ago.  Why it then initiated in this
epidemic form much later is not so

360
00:29:17,000 --> 00:29:26,000
clear.  The etiological agent --

361
00:29:26,000 --> 00:29:33,000
-- is a virus called HIV,
human immunodeficiency virus.

362
00:29:33,000 --> 00:29:37,000
It is a retro virus.
And we'll briefly review how it

363
00:29:37,000 --> 00:29:41,000
replicated, but I told you more or
less how that happens last time.

364
00:29:41,000 --> 00:29:46,000
And this was another good example.
The syndrome was first discovered

365
00:29:46,000 --> 00:29:50,000
in 1975, gained a lot of attention
in this country starting in the

366
00:29:50,000 --> 00:29:55,000
early 1980s, and the virus was first
purified and characterized in 1983.

367
00:29:55,000 --> 00:29:59,000
So it didn't take too long to
identify the agent and then

368
00:29:59,000 --> 00:30:04,000
ultimately sequence its genome.
And also to develop tests for the

369
00:30:04,000 --> 00:30:10,000
presence of actually not the virus
itself.  The AIDS test is a test for

370
00:30:10,000 --> 00:30:15,000
the presence of antibodies in you
that recognize the viral

371
00:30:15,000 --> 00:30:21,000
glycoproteins.
And this was very important,

372
00:30:21,000 --> 00:30:27,000
obviously, in screening populations,
screening blood banks and so on for

373
00:30:27,000 --> 00:30:32,000
contaminated samples.
Now, since this time,

374
00:30:32,000 --> 00:30:36,000
in the early 1980s, the situation
has, of course,

375
00:30:36,000 --> 00:30:40,000
gotten much, much worse.
In this country, there are

376
00:30:40,000 --> 00:30:52,000
currently --

377
00:30:52,000 --> 00:30:56,000
-- a million people infected with
HIV.  That's between,

378
00:30:56,000 --> 00:31:00,000
you know, one and 300 individuals
total.

379
00:31:00,000 --> 00:31:08,000
And, amazingly,
about 20% of those people don't know

380
00:31:08,000 --> 00:31:16,000
it.  Worldwide,
anybody have any idea how many

381
00:31:16,000 --> 00:31:24,000
people are infected with HIV in the
world?  40 million.

382
00:31:24,000 --> 00:31:32,000
And total about 60 million people
have been infected because since the

383
00:31:32,000 --> 00:31:40,000
early 1980s, 20 million people have
died from this disease.

384
00:31:40,000 --> 00:31:45,750
More than 20 million.
And the prevalence is remarkable in

385
00:31:45,000 --> 00:31:51,500
certain places in the world.
In Southern Africa, for example,

386
00:31:51,000 --> 00:31:57,250
not the country of South Africa but
in the Southern Region of Africa,

387
00:31:57,000 --> 00:32:03,000
Saharan Africa, there are 25 million
people infected.

388
00:32:03,000 --> 00:32:08,500
And in certain countries that's one
in four sexually active individuals.

389
00:32:08,000 --> 00:32:14,000
Remarkable infection rates.  And
the effect of this disease and the

390
00:32:14,000 --> 00:32:19,500
death that it causes has also had
dramatic effects on the population.

391
00:32:19,000 --> 00:32:25,000
So that in Zimbabwe, for example,
where the life expectancy in 1990

392
00:32:25,000 --> 00:32:30,500
was 52 years old,
not a huge number but 52 years old,

393
00:32:30,000 --> 00:32:36,000
in 2003, because of AIDS, it's less
then 35 years.

394
00:32:36,000 --> 00:32:42,000
So it has dramatically affected the
lives of people throughout the world.

395
00:32:42,000 --> 00:32:48,000
OK.  And, as you know,
I suspect you know that transmission

396
00:32:48,000 --> 00:32:54,000
for this virus occurs through direct
contact of contaminated fluids.

397
00:32:54,000 --> 00:33:00,000
Sexual contact, which is both
homosexual and heterosexual.

398
00:33:00,000 --> 00:33:04,000
As well as blood transfusion,
direct blood contact, through

399
00:33:04,000 --> 00:33:09,000
transfusion, and also needle sharing.
And sometimes occasionally organ

400
00:33:09,000 --> 00:33:14,000
donation, but that's very,
very rare.  OK.

401
00:33:14,000 --> 00:33:21,750
So people die from AIDS because they

402
00:33:21,000 --> 00:33:25,250
develop infections.
And we'll talk a bit more about

403
00:33:25,000 --> 00:33:28,750
that in a moment.
But here's one patient who has

404
00:33:28,000 --> 00:33:32,250
developed an infection with a virus,
a particular type of herpes virus

405
00:33:32,000 --> 00:33:36,000
now called Kaposi's sarcoma virus.
That virus then is able to replicate

406
00:33:36,000 --> 00:33:40,000
in certain cells of the blood
vasculature and cause tumors.

407
00:33:40,000 --> 00:33:44,000
These are actually tumors.  And
this individual will die from a

408
00:33:44,000 --> 00:33:48,000
disease called Kaposi's sarcoma,
a type of cancer, another example of

409
00:33:48,000 --> 00:33:52,000
a virus-associated cancer of humans.
And in this country AIDS, as you

410
00:33:52,000 --> 00:33:56,000
probably know,
has taken over as the leading killer

411
00:33:56,000 --> 00:34:01,000
of young male adults.
It was not known in the early 1980s,

412
00:34:01,000 --> 00:34:06,000
and by 1992, just ten years later
had become the major killer of young

413
00:34:06,000 --> 00:34:10,000
male adults in this country.
And, of course, likewise around the

414
00:34:10,000 --> 00:34:15,000
world.  So here's the virus.
As I said, it's a retrovirus.

415
00:34:15,000 --> 00:34:20,000
Retroviruses also are enveloped.
They have glycoproteins on the

416
00:34:20,000 --> 00:34:25,000
outside, envelope glycoproteins.
They have a capsid like all viruses

417
00:34:25,000 --> 00:34:30,000
do.  They have a genome.
The genome is made of RNA.

418
00:34:30,000 --> 00:34:34,000
And, as you know,
through the retroviral lifecycle,

419
00:34:34,000 --> 00:34:38,000
the RNA is converted into a DNA form.
We'll go over that next time.

420
00:34:38,000 --> 00:34:42,000
And that's accomplished by an
enzyme called reverse transcriptase

421
00:34:42,000 --> 00:34:46,000
which gets prepackaged with the
virus.  Again,

422
00:34:46,000 --> 00:34:50,000
it has to be there because your
cells don't have an RNA dependent

423
00:34:50,000 --> 00:34:54,000
RNA polymerase.
So this gets prepackaged.

424
00:34:54,000 --> 00:34:58,000
The virus then infects cells.
And this, again, is a somewhat

425
00:34:58,000 --> 00:35:01,000
oversimplification.
I'll show you more details in a

426
00:35:01,000 --> 00:35:05,000
second.  But the virus attaches via
glycoproteins on its surface,

427
00:35:05,000 --> 00:35:08,000
membrane proteins on the surface of
the host cell.

428
00:35:08,000 --> 00:35:12,000
In this case it's a familiar
protein to you called CD4.

429
00:35:12,000 --> 00:35:15,000
That leads to a fusion event.
So now at the plasma membrane,

430
00:35:15,000 --> 00:35:19,000
which is different from what we saw
with flu virus which happened in an

431
00:35:19,000 --> 00:35:22,000
internal membrane,
at the plasma membrane there's a

432
00:35:22,000 --> 00:35:26,000
fusion event.  So now the viral
membrane fuses with the host cell

433
00:35:26,000 --> 00:35:30,000
membrane and the viral capsid can
lead into the cytoplasm.

434
00:35:30,000 --> 00:35:33,000
There's a little bit of unpackaging
that goes on.  And then reverse

435
00:35:33,000 --> 00:35:36,000
transcriptase,
that enzyme that got prepackaged

436
00:35:36,000 --> 00:35:39,000
there acts on the viral genome,
which is made of RNA and converts it

437
00:35:39,000 --> 00:35:43,000
to a DNA form,
a double-stranded DNA form which is

438
00:35:43,000 --> 00:35:46,000
called a provirus.
And that provirus then gets

439
00:35:46,000 --> 00:35:49,000
integrated into the host cell genome
in a random process,

440
00:35:49,000 --> 00:35:53,000
random integration, the host cell
genome.  This is also accomplished

441
00:35:53,000 --> 00:35:56,000
by a pre-packed enzyme called
integrase.  Once in the genome it

442
00:35:56,000 --> 00:36:00,000
gets treated like any
old piece of DNA.

443
00:36:00,000 --> 00:36:03,000
It gets transcribed into actually
multiple different RNAs.

444
00:36:03,000 --> 00:36:07,000
They get translated into viral
proteins which come together at the

445
00:36:07,000 --> 00:36:10,000
plasma membrane and the
glycoproteins,

446
00:36:10,000 --> 00:36:14,000
the capsid proteins which
incorporate the genome,

447
00:36:14,000 --> 00:36:18,000
plus the reverse transcriptase,
and you make a new particle.  OK?

448
00:36:18,000 --> 00:36:21,000
So this is more or less the
lifecycle.  There is some detail

449
00:36:21,000 --> 00:36:25,000
that I want to share with you in a
moment because it's interesting.

450
00:36:25,000 --> 00:36:29,000
This is what it looks like in real
life.

451
00:36:29,000 --> 00:36:33,000
These are HIV particles budding off
the surface of an infected cell.

452
00:36:33,000 --> 00:36:37,000
They have a very characteristic
electron dense structure.

453
00:36:37,000 --> 00:36:41,000
You can actually tell it is HIV
from just the way it looks in the

454
00:36:41,000 --> 00:36:45,000
electron microscope.
And this is an HIV infected T cell.

455
00:36:45,000 --> 00:36:49,000
Now, CD4 should have rung a bell
that it's one of the proteins on the

456
00:36:49,000 --> 00:36:53,000
surface of helper T cells,
CD4 positive T cells.  That is a

457
00:36:53,000 --> 00:36:58,000
major cell of infection for HIV.
And here is one such cell infected.

458
00:36:58,000 --> 00:37:02,000
And all these little blebs budding
off the surface of this cell are

459
00:37:02,000 --> 00:37:06,000
viruses getting out.
This process actually doesn't

460
00:37:06,000 --> 00:37:11,000
itself, the budding process doesn't
itself kill the T cell but,

461
00:37:11,000 --> 00:37:15,000
nevertheless, T cells, this class of
T cells dies.  And,

462
00:37:15,000 --> 00:37:20,000
actually, that's a critical event in
the development of HIV-AIDS,

463
00:37:20,000 --> 00:37:24,000
the disease.  We'll come to that in
a second, but let me first tell you

464
00:37:24,000 --> 00:37:44,000
one of the interesting details.

465
00:37:44,000 --> 00:37:48,000
The situation is a little more
complicated than the figure that I

466
00:37:48,000 --> 00:37:52,000
gave you on that slide.
There are two glycoproteins on the

467
00:37:52,000 --> 00:37:56,000
surface of HIV.
One of them is called GP120 for

468
00:37:56,000 --> 00:38:02,000
glycoprotein 120.
It's linked by a disulfide linkage

469
00:38:02,000 --> 00:38:08,000
to another protein called GP41.
And we now know that both of these

470
00:38:08,000 --> 00:38:14,000
proteins participate in binding and
fusion.  And on the surface of the

471
00:38:14,000 --> 00:38:20,000
infected cell is CD4,
as was indicated on that slide.

472
00:38:20,000 --> 00:38:26,000
And that is the major viral
receptor that is present on helper T

473
00:38:26,000 --> 00:38:33,000
cells, CD4 positive T cells.
It's also present on macrophages.

474
00:38:33,000 --> 00:38:37,500
As well as certain cells in the
brain called glial cells.

475
00:38:37,000 --> 00:38:42,000
So there are CD4 positive
macrophages and glial cells.

476
00:38:42,000 --> 00:38:46,500
AIDS-associated dementia, which you
may know about,

477
00:38:46,000 --> 00:38:51,000
is caused by the destruction of the
glial cells in the brain.

478
00:38:51,000 --> 00:38:55,500
Now, there's another protein which
participates, not so much in the

479
00:38:55,000 --> 00:39:00,000
binding but in the fusion event that
allows the virus to get in.

480
00:39:00,000 --> 00:39:02,000
And this is actually a class of
proteins called chemokine

481
00:39:02,000 --> 00:39:10,000
receptors.

482
00:39:10,000 --> 00:39:13,000
There are actually multiple
chemokine receptors on your cells.

483
00:39:13,000 --> 00:39:17,000
And, in fact, different HIV forms
bind to different of these.

484
00:39:17,000 --> 00:39:20,000
But just to simplify, let's say
that there is one of these which

485
00:39:20,000 --> 00:39:24,000
binds to the GP41 portion.
And it's that binding that allows

486
00:39:24,000 --> 00:39:28,000
the virus to fuse.
If you don't have this you cannot

487
00:39:28,000 --> 00:39:32,000
fuse.
And, therefore,

488
00:39:32,000 --> 00:39:36,000
the cells are not infected.
What's interesting about that is

489
00:39:36,000 --> 00:39:40,000
two-fold.  One,
it could represent a therapeutic

490
00:39:40,000 --> 00:39:44,000
target.  That interaction could be a
therapeutic target.

491
00:39:44,000 --> 00:39:49,000
But it came to light in an
interesting way which was there are

492
00:39:49,000 --> 00:39:53,000
people who have been followed a long
time who lived in high-risk

493
00:39:53,000 --> 00:39:57,000
populations, drug-users or
homosexual men with multiple

494
00:39:57,000 --> 00:40:01,000
contacts with known HIV infected
people who have not themselves

495
00:40:01,000 --> 00:40:06,000
gotten HIV.
They seem to be naturally resistant

496
00:40:06,000 --> 00:40:11,000
to the development of HIV,
despite documented exposure to the

497
00:40:11,000 --> 00:40:17,000
virus.  And they were studied.
And when that observation was made

498
00:40:17,000 --> 00:40:22,000
it was found that these individuals
lack the chemokine receptor.

499
00:40:22,000 --> 00:40:27,000
And if you lack the chemokine
receptor, you may have CD4 on the

500
00:40:27,000 --> 00:40:33,000
surface of your cells but you don't
have that critical co-receptor.

501
00:40:33,000 --> 00:40:39,000
And so there can be binding but no
fusion.  So your cells are resistant.

502
00:40:39,000 --> 00:40:45,000
So these individuals are naturally
resistant to HIV.  OK.

503
00:40:45,000 --> 00:40:58,000
So HIV infects.

504
00:40:58,000 --> 00:41:02,000
It infects a lot of T cells,
helper T cells and other cells in

505
00:41:02,000 --> 00:41:05,000
the body.  Why do you get sick?
Well, as I mentioned, people get

506
00:41:05,000 --> 00:41:09,000
sick because of the loss,
the absence --

507
00:41:09,000 --> 00:41:19,000
During infection,

508
00:41:19,000 --> 00:41:23,000
the CD4 positive T cells get
eliminated.  How exactly they get

509
00:41:23,000 --> 00:41:27,000
eliminated, why exactly they get
eliminated we don't yet know,

510
00:41:27,000 --> 00:41:31,000
but we know that the levels of these
cells goes down inside the

511
00:41:31,000 --> 00:41:35,500
infected individual.
And, as you probably remember,

512
00:41:35,000 --> 00:41:45,000
CD4 positive T cells --

513
00:41:45,000 --> 00:41:48,000
-- are required for the development
of these cells.

514
00:41:48,000 --> 00:41:55,000
So if you don't have CD4 positive T

515
00:41:55,000 --> 00:41:57,000
cells you cannot make antibody
producing cells.

516
00:41:57,000 --> 00:42:00,000
You cannot make functional
antibodies.

517
00:42:00,000 --> 00:42:05,000
And, to some extent,
they're also required for the

518
00:42:05,000 --> 00:42:10,000
development of cytotoxic T cells.
So that part of the immune system

519
00:42:10,000 --> 00:42:15,000
is also compromised.
So the virus wipes out these cells

520
00:42:15,000 --> 00:42:20,000
and, therefore,
the immune system crashes.

521
00:42:20,000 --> 00:42:26,000
And that's why it's called AIDS for
acquired immunodeficiency.

522
00:42:26,000 --> 00:42:32,000
Acquired because it comes through an

523
00:42:32,000 --> 00:42:36,000
infectious agent.
Immunodeficiency because your

524
00:42:36,000 --> 00:42:40,000
immune system crashes.
And when your immune system crashes

525
00:42:40,000 --> 00:42:44,000
you cannot fight infections.
And AIDS patients largely die

526
00:42:44,000 --> 00:42:48,000
because they get one or another type
of infection.

527
00:42:48,000 --> 00:42:54,750
So this is a graph which shows the

528
00:42:54,000 --> 00:42:58,250
time course of HIV infection.
And it kind of makes the point that

529
00:42:58,000 --> 00:43:01,000
T cells go away.
Shortly after you are infected,

530
00:43:01,000 --> 00:43:04,000
the level of HIV in your blood goes
way, way up, as you can see here.

531
00:43:04,000 --> 00:43:08,000
It spikes in the first several
months after your infection.

532
00:43:08,000 --> 00:43:11,000
Some people actually manifest that
aspect of the disease.

533
00:43:11,000 --> 00:43:14,000
Many don't so they don't know that
they have such a high concentration

534
00:43:14,000 --> 00:43:17,000
of virus.  And,
importantly, your body deals with it

535
00:43:17,000 --> 00:43:20,000
so you make antibodies,
in black.  You make anti-HIV

536
00:43:20,000 --> 00:43:24,000
antibodies.  And,
remember, it's these that are

537
00:43:24,000 --> 00:43:27,000
detected by the AIDS test.
If you've been exposed then you

538
00:43:27,000 --> 00:43:31,000
make antibodies.
And they stick around so that you

539
00:43:31,000 --> 00:43:35,000
know that you've been infected.
Likewise, T cells, cytotoxic T

540
00:43:35,000 --> 00:43:40,000
cells, CD8 positive T cells go up.
And that controls the level of the

541
00:43:40,000 --> 00:43:45,000
virus.  And it stays low for a while,
but after a while,

542
00:43:45,000 --> 00:43:49,000
because of the affects on the CD4
positive T cells,

543
00:43:49,000 --> 00:43:54,000
the other cells in the immune system
are no longer sustained so the B

544
00:43:54,000 --> 00:43:59,000
cells start to drop off and the T
cells start to drop off.

545
00:43:59,000 --> 00:44:02,000
And now you're starting to lose the
battle against the HIV,

546
00:44:02,000 --> 00:44:06,000
so the levels of HIV go up.
And as they go up, you lose more

547
00:44:06,000 --> 00:44:09,000
CD4 positive T cells and the
situation gets worse and worse.

548
00:44:09,000 --> 00:44:13,000
So after a while, usually in the
several years out time point,

549
00:44:13,000 --> 00:44:17,000
your immune system is now no longer
functional and you're very

550
00:44:17,000 --> 00:44:20,000
susceptible to opportunistic
infections.  So AIDS patients will

551
00:44:20,000 --> 00:44:24,000
develop infections,
and the infections will progress in

552
00:44:24,000 --> 00:44:28,000
a way that won't happen
in a healthy person.

553
00:44:28,000 --> 00:44:32,000
Certain bacterial infections that
you would clearly very easily take

554
00:44:32,000 --> 00:44:37,000
hold and can cause major problems.
Kaposi's sarcoma is caused by

555
00:44:37,000 --> 00:44:42,000
infection of this particular virus.
AIDS patients are very susceptible

556
00:44:42,000 --> 00:44:47,000
to tuberculosis,
this particular fungal infection,

557
00:44:47,000 --> 00:44:51,000
certain other viral infections.  And
in aggregate, the exposure to these

558
00:44:51,000 --> 00:44:56,000
various infectious agents and the
damage that they're causing leads to

559
00:44:56,000 --> 00:45:01,000
the death of the patient.  OK.
So that's bad news.

560
00:45:01,000 --> 00:45:06,000
And obviously it's have major and
devastating consequences around the

561
00:45:06,000 --> 00:45:11,000
world.  So what can we do about it?
Well, there is some good news here.

562
00:45:11,000 --> 00:45:33,000
There are antiviral therapies.

563
00:45:33,000 --> 00:45:44,000
Remember that viruses largely use

564
00:45:44,000 --> 00:45:48,000
your enzymes.  And you cannot make
good drugs against your own enzymes

565
00:45:48,000 --> 00:45:52,000
for treatment of a viral disease,
but I've told you about one viral

566
00:45:52,000 --> 00:45:56,000
protein to which you might be able
to make drugs.  And

567
00:45:56,000 --> 00:46:04,000
indeed we have.

568
00:46:04,000 --> 00:46:10,000
HIV has an RNA genome that gets
converted to a DNA form by an enzyme

569
00:46:10,000 --> 00:46:17,000
reverse transcriptase.
There have been drugs produced that

570
00:46:17,000 --> 00:46:23,000
block that enzyme.
And you've probably heard of the

571
00:46:23,000 --> 00:46:29,000
drug AZT.
There are other drugs like

572
00:46:29,000 --> 00:46:33,000
dideoxyinosine and dideoxycytosine
that likewise bind to and block the

573
00:46:33,000 --> 00:46:37,000
activity of this enzyme.
They actually cause chain

574
00:46:37,000 --> 00:46:42,000
termination in the same way that DNA
sequencing works.

575
00:46:42,000 --> 00:46:46,000
And they work because this enzyme
is sensitive to these drugs at

576
00:46:46,000 --> 00:46:51,000
concentrations where your
polymerases are not,

577
00:46:51,000 --> 00:46:55,000
so that gives you some selected
effect in the treatment of the virus.

578
00:46:55,000 --> 00:47:00,000
And these agents,
when used singly, work for a while.

579
00:47:00,000 --> 00:47:04,000
And then the virus becomes resistant
and comes roaring back.

580
00:47:04,000 --> 00:47:08,000
How does it become resistant?
It becomes resistant through

581
00:47:08,000 --> 00:47:13,000
mutation, exactly the way cancer
cells become resistant to targeted

582
00:47:13,000 --> 00:47:17,000
therapies.  Variant forms of HIV
arise which have slightly altered

583
00:47:17,000 --> 00:47:22,000
reverse transcriptases that now
won't bind to AZT or DDI or DDC.

584
00:47:22,000 --> 00:47:26,000
And the problem is very, very
serious because viruses,

585
00:47:26,000 --> 00:47:31,000
RNA viruses in particular are highly
mutagenic.

586
00:47:31,000 --> 00:47:36,000
They create variant forms at high
frequency.  An HIV infected person

587
00:47:36,000 --> 00:47:41,000
makes ten to the tenth viral
particles a day.

588
00:47:41,000 --> 00:47:46,000
At a mutation rate of ten to the
minus fifth, that would mean ten to

589
00:47:46,000 --> 00:47:52,000
the fifth variants of every
nucleotide.  OK?

590
00:47:52,000 --> 00:47:57,000
So variants come up frequently that
are resistant to this

591
00:47:57,000 --> 00:48:14,000
therapy.  Oops.

592
00:48:14,000 --> 00:48:24,000
Fortunately, there are alternatives.
The viral RNA gets translated into

593
00:48:24,000 --> 00:48:40,000
a large polyprotein --

594
00:48:40,000 --> 00:48:44,000
-- which is processed by a viral
protease encoded by the virus into

595
00:48:44,000 --> 00:48:48,000
different mature proteins.
Reverse transcriptase is one of

596
00:48:48,000 --> 00:48:53,000
them.  The integrase that I
mentioned is another.

597
00:48:53,000 --> 00:48:57,000
And there are others.
This is a viral protein,

598
00:48:57,000 --> 00:49:02,000
a viral enzyme.
And so, in theory,

599
00:49:02,000 --> 00:49:06,000
it's possible to make inhibitors
against the protease.

600
00:49:06,000 --> 00:49:11,000
And that's been successful.
Several companies have made

601
00:49:11,000 --> 00:49:15,000
inhibitors that will specifically
inhibit the viral protease that work

602
00:49:15,000 --> 00:49:20,000
well.  They work for a while and
then resistant forms arise for

603
00:49:20,000 --> 00:49:24,000
exactly the same reason I said.
You can create versions of protease

604
00:49:24,000 --> 00:49:29,000
which still work but won't bind the
drug.  OK?  So what do you do?

605
00:49:29,000 --> 00:49:34,000
Does anybody know what you do to
overcome this problem?

606
00:49:34,000 --> 00:49:46,000
You put the drugs together.

607
00:49:46,000 --> 00:49:54,000
So currently HIV patients,

608
00:49:54,000 --> 00:49:58,000
in this country anyway, and
hopefully soon around the world will

609
00:49:58,000 --> 00:50:02,000
get triple therapy when
they're diagnosed.

610
00:50:02,000 --> 00:50:08,000
And triple therapy includes two
reverse transcriptase inhibitors and

611
00:50:08,000 --> 00:50:18,000
one protease inhibitor.

612
00:50:18,000 --> 00:50:22,000
Since the development of resistance
against each of these agents is

613
00:50:22,000 --> 00:50:26,000
independent of the other,
to get resistance to all three is

614
00:50:26,000 --> 00:50:30,000
the product of the individual
frequency.

615
00:50:30,000 --> 00:50:35,000
So I said that the frequency of
developing resistance to any one is

616
00:50:35,000 --> 00:50:40,000
ten to the minus fifth.
To be resistant to all three would

617
00:50:40,000 --> 00:50:45,000
occur at ten to the minus fifteenth,
which is highly, highly unlikely.

618
00:50:45,000 --> 00:50:50,000
And so people tend not to develop
resistance to all three and,

619
00:50:50,000 --> 00:50:55,000
therefore, triple therapy tends to
keep the virus in-check for

620
00:50:55,000 --> 00:51:00,000
sustained periods of time.
As you probably know, this works.

621
00:51:00,000 --> 00:51:03,750
The fear is that it is starting not
to work.  And there are starting to

622
00:51:03,000 --> 00:51:07,500
be some resistant forms developing
here and they are starting to make

623
00:51:07,000 --> 00:51:11,250
their way into the population,
so we're going to have to develop

624
00:51:11,000 --> 00:51:15,000
even better and more different
inhibitors to now combat these

625
00:51:15,000 --> 00:51:18,750
variant forms.
Before I let you go,

626
00:51:18,000 --> 00:51:22,500
the last thing I wanted to mention,
and I'll just throw it out there and

627
00:51:22,000 --> 00:51:26,250
you can think about it,
is that what we'd really like to

628
00:51:26,000 --> 00:51:30,000
return to for HIV is what
we can do for polio.

629
00:51:30,000 --> 00:51:34,500
We'd like to be able to treat people
routinely in this country or out in

630
00:51:34,000 --> 00:51:39,000
the field in more distant regions
with an effective vaccine,

631
00:51:39,000 --> 00:51:43,500
which isn't possible today.
We don't have one for HIV.  You

632
00:51:43,000 --> 00:51:48,000
might think about what you would do
to make one and why it has been so

633
00:51:48,000 --> 00:51:51,000
hard.  And I'll touch
on that next time.