1 00:00:15 --> 00:00:28 OK. I think we'll get started. 2 00:00:28 --> 00:00:36 So we're going to continue our 3 00:00:36 --> 00:00:41 discussion today about viruses, finish talking about influenza, 4 00:00:41 --> 00:00:47 influenza A virus and the flu, and then talk a bit about HIV and 5 00:00:47 --> 00:00:52 AIDS. Do you guys know more about the tutorial session and so on and 6 00:00:52 --> 00:00:58 so forth for next week? Has that been decided yet? OK. 7 00:00:58 --> 00:01:02 So you'll get more information Monday and Wednesday. 8 00:01:02 --> 00:01:06 And I'll also have an office hour, probably a couple of office hours at 9 00:01:06 --> 00:01:10 the end of next week. Most likely, if it doesn't conflict 10 00:01:10 --> 00:01:14 with this tutorial session that's being planned, 11 00:01:14 --> 00:01:18 it will be during this hour from 11:00 probably until 1:00 in this 12 00:01:18 --> 00:01:22 room because it's free since we're usually here. But that may conflict 13 00:01:22 --> 00:01:27 with you guys so we'll have to see. I'll let you know on Monday. 14 00:01:27 --> 00:01:31 So, again, the situation with viruses is an ongoing one. 15 00:01:31 --> 00:01:35 And, actually, one of you brought to my attention an outbreak of polio 16 00:01:35 --> 00:01:39 that's happening now both in Africa and Indonesia. 17 00:01:39 --> 00:01:43 This is actually from today's CNN. om where there's a fear that polio 18 00:01:43 --> 00:01:47 is starting to spread. In this case in Jakarta, 19 00:01:47 --> 00:01:51 in Indonesia. And there's a very extensive reaction to this. 20 00:01:51 --> 00:01:56 There's a great fear that polio will spread. 21 00:01:56 --> 00:02:00 This is an area that is not well protected currently by polio 22 00:02:00 --> 00:02:04 vaccination. And so if you look at this you'll see that they are 23 00:02:04 --> 00:02:08 raising lots of money in order to try to vaccinate with standard polio 24 00:02:08 --> 00:02:12 vaccines, which work extremely well, 5.2 million children, to try to stop 25 00:02:12 --> 00:02:16 the spread of this potential epidemic of polio. 26 00:02:16 --> 00:02:20 It hasn't reached anything near those levels to be that concerned, 27 00:02:20 --> 00:02:24 but in order to protect against that possibility they're going 28 00:02:24 --> 00:02:27 to vaccinate. And, obviously, 29 00:02:27 --> 00:02:31 if you vaccinate, individuals cannot be successfully 30 00:02:31 --> 00:02:34 infected. If they cannot be successfully infected, 31 00:02:34 --> 00:02:38 the virus cannot propagate so they cannot pass it on. 32 00:02:38 --> 00:02:41 So you just close off the infectious cycle in a very small 33 00:02:41 --> 00:02:44 circle of infected individuals. Also, in today's Globe, there was a 34 00:02:44 --> 00:02:48 discussion of a local company that makes vaccines against smallpox. 35 00:02:48 --> 00:02:51 And I've mentioned to you that smallpox is largely eliminated, 36 00:02:51 --> 00:02:55 if not fully eliminated in the world, but there are stocks of smallpox in 37 00:02:55 --> 00:02:59 freezers around the world. And there is a concern that an 38 00:02:59 --> 00:03:03 individual could access smallpox virus or make one because the 39 00:03:03 --> 00:03:08 relevant sequences of the genome of the smallpox virus is known. 40 00:03:08 --> 00:03:13 And so, in theory, you could synthesize your own smallpox virus 41 00:03:13 --> 00:03:17 genome, and thereby create your own smallpox virus and expose now 42 00:03:17 --> 00:03:22 unprotected individuals because we don't get vaccinated currently 43 00:03:22 --> 00:03:27 against smallpox. And so there are companies like 44 00:03:27 --> 00:03:32 this one that are making currently and distributing smallpox vaccines. 45 00:03:32 --> 00:03:36 They distributed 182 million doses in this country alone just in case 46 00:03:36 --> 00:03:40 somebody tried to deliberately release some smallpox. 47 00:03:40 --> 00:03:44 So that's what we can do when we know what we're dealing with. 48 00:03:44 --> 00:03:49 We can create effective vaccines. In this case they're effective, but 49 00:03:49 --> 00:03:53 sometimes we get exposed to stuff that we cannot effectively deal with. 50 00:03:53 --> 00:03:58 And I introduced this to you last time. 51 00:03:58 --> 00:04:02 This was the major flu pandemic from 1918. Some people worried that 52 00:04:02 --> 00:04:07 there might be another pandemic this year because in the off season 53 00:04:07 --> 00:04:11 between 1918 and 1919 when this happened, and 20 to 40 million 54 00:04:11 --> 00:04:16 people died, that's when the Red Sox last won the World Series. 55 00:04:16 --> 00:04:20 [LAUGHTER] So the Armageddon folks thought maybe this was the year, 56 00:04:20 --> 00:04:25 but so far so good. So, as I mentioned last time, 57 00:04:25 --> 00:04:30 this was a major outbreak and a major problem worldwide. 58 00:04:30 --> 00:04:35 But you also should know that influenza is an annual problem. 59 00:04:35 --> 00:04:40 And I, for one, didn't appreciate this. 60 00:04:40 --> 00:04:47.5 Where there are about 25 thousand 61 00:04:47 --> 00:04:52.5 deaths per year in the United States. And, of course, 62 00:04:52 --> 00:04:57.5 to deal with that we make available, especially to the elderly and 63 00:04:57 --> 00:05:02 infirmed immunodeficient flu shots. And flu shots are simply flu 64 00:05:02 --> 00:05:06 vaccines. And I'll tell you a little bit about how they're made 65 00:05:06 --> 00:05:11 towards the end of this section, but they're generally effective in 66 00:05:11 --> 00:05:16 combating against the flu that is going to hit the population that 67 00:05:16 --> 00:05:20 year. And they change every year because, as you'll see, 68 00:05:20 --> 00:05:25 flu changes all the time as well. So it's an annual problem. There 69 00:05:25 --> 00:05:30 are occasional epidemics which you could think of as -- 70 00:05:30 --> 00:05:37 -- population-based infection where 71 00:05:37 --> 00:05:43 many individuals within a particular population are infected. 72 00:05:43 --> 00:05:49 And the virus, in this case, is spreading throughout that 73 00:05:49 --> 00:05:55 population. And there are even more occasional pandemics. 74 00:05:55 --> 00:06:04 And that's a worldwide infection. 75 00:06:04 --> 00:06:07 Where, because of the virulence of the virus and its ability to access 76 00:06:07 --> 00:06:11 different populations and the absence of immunity throughout the 77 00:06:11 --> 00:06:15 worldwide population, lots and lots of people get infected. 78 00:06:15 --> 00:06:19 And that's what happened in 1918. So how do we explain this? How do 79 00:06:19 --> 00:06:23 we explain the occasional epidemics of a virus that we're so familiar 80 00:06:23 --> 00:06:27 with? Flu, as I said, we get every year, lots of people 81 00:06:27 --> 00:06:31 get it every year. And, yet, we seem to be susceptible 82 00:06:31 --> 00:06:35 year after year. So why is that? What causes the 83 00:06:35 --> 00:06:40 resistance to this immunity in small measure and in large measure? 84 00:06:40 --> 00:06:45 And that's what I want to review for you today. 85 00:06:45 --> 00:06:49 So this is the responsible agent. It's a virus, of course. More 86 00:06:49 --> 00:06:54 specifically, it's an enveloped virus, which means it has its own 87 00:06:54 --> 00:06:59 lipid bilayer that it picked up from the host cell. 88 00:06:59 --> 00:07:02 And you'll notice on the outside are things sticking out of the bilayer. 89 00:07:02 --> 00:07:06 These are proteins encoded by the virus which are going to be 90 00:07:06 --> 00:07:09 responsible for binding the virus to the host cell. 91 00:07:09 --> 00:07:13 And also allowing the viral membrane to fuse with the host cell 92 00:07:13 --> 00:07:17 membrane. And you'll see that that's done in the case of flu virus 93 00:07:17 --> 00:07:20 in a slightly different way than for some other viruses. 94 00:07:20 --> 00:07:24 And then inside you have the capsid. And wrapped up in this protein, 95 00:07:24 --> 00:07:28 this helical protein structure are the nucleic acids of the virus. 96 00:07:28 --> 00:07:34.5 And the nucleic acids of this virus are multiple. That is it's not just 97 00:07:34 --> 00:07:41 one. Oops. It's not just one. It's actually several. 98 00:07:41 --> 00:07:54 So, again, it's called influenza A 99 00:07:54 --> 00:08:00 virus. It's an example of an enveloped virus. 100 00:08:00 --> 00:08:04 And I said you should note the envelope proteins, 101 00:08:04 --> 00:08:08 which are largely encoded by the virus itself. And we call these 102 00:08:08 --> 00:08:12 envelope glycoproteins. Glycoproteins because when they 103 00:08:12 --> 00:08:16 emerge through the sorting pathway of the cell they pick up sugars. 104 00:08:16 --> 00:08:20 And so when they're on the outside of the cell, it's not just protein. 105 00:08:20 --> 00:08:24 There is some sugar there, so glycoproteins. 106 00:08:24 --> 00:08:28 And sometimes the antibodies that we make are directed against the 107 00:08:28 --> 00:08:33 sugar moieties alone or in combination with peptides. 108 00:08:33 --> 00:08:39 So the envelope glycoproteins are important. As I said, 109 00:08:39 --> 00:08:45 it's a segmented RNA genome, which means that there are multiple 110 00:08:45 --> 00:08:51 distinct nucleic acids in the viral particle. And in the case of flu 111 00:08:51 --> 00:09:06 there are eight. 112 00:09:06 --> 00:09:11 The RNA genome is single-stranded. And all of the genome segments are 113 00:09:11 --> 00:09:16 in the minus configuration. So their sequence is the complement 114 00:09:16 --> 00:09:21 to the mRNA. OK? And we talked about the 115 00:09:21 --> 00:09:26 significance of RNA viruses that have a minus strand polarity in the 116 00:09:26 --> 00:09:32 sense that they need to have their own RNA polymerase coming in with 117 00:09:32 --> 00:09:37 the virus in order to help, in order to initiate the replication 118 00:09:37 --> 00:09:41.75 cycle. So, hopefully, 119 00:09:41 --> 00:09:45.25 that's familiar from last time. This is a diagram of the virus, 120 00:09:45 --> 00:09:48.75 and it just makes the same points I just made. So, 121 00:09:48 --> 00:09:52.25 again, it has a lipid bilayer. It has envelope glycoproteins 122 00:09:52 --> 00:09:55.75 sticking out from the surface. You'll see that they're important 123 00:09:55 --> 00:09:59.25 for different things in a moment. Inside that is a nucleocapsid which 124 00:09:59 --> 00:10:03 surrounds the nucleic acids. And you can see that there's a 125 00:10:03 --> 00:10:07 series of nucleic acids that are enclosed within this. 126 00:10:07 --> 00:10:11 And, again, there are eight of them. And they're all minus strand. 127 00:10:11 --> 00:10:15 And, importantly, contained within here is a protein, 128 00:10:15 --> 00:10:19 an RNA dependent RNA polymerase, the yellow dot. And it has to get 129 00:10:19 --> 00:10:23 incorporated with the virus so that when the virus infects it goes in 130 00:10:23 --> 00:10:27.72 there, too. And it can then act on the RNA genome and convert it into a 131 00:10:27 --> 00:10:31 plus strand, which will then serve both as the RNA for translation, 132 00:10:31 --> 00:10:35 as well as the template for the production of more of the 133 00:10:35 --> 00:10:40.1 minus strand RNA. OK? So this comes from your book, 134 00:10:40 --> 00:10:44 and it is a summary of the infectious cycle of this virus. 135 00:10:44 --> 00:10:48.5 I've modified it a little bit because some of the details which I 136 00:10:48 --> 00:10:52.7 think are important were missing. So you might want to pay attention 137 00:10:52 --> 00:10:56.9 to this figure as you're reading the section, the relevant 138 00:10:56 --> 00:11:01 section in the book. But this is a typical viral 139 00:11:01 --> 00:11:05 lifecycle. The virus attaches. Remember, the terms that I used 140 00:11:05 --> 00:11:09 last time? The virus attaches. Here one of the viral glycoproteins 141 00:11:09 --> 00:11:13 binds to a protein on the surface of the target cell. 142 00:11:13 --> 00:11:18 That then initiates an internalization process, 143 00:11:18 --> 00:11:22 a penetration process in which the virus gets brought in through an 144 00:11:22 --> 00:11:26 endosome. So it gets brought in through a separate vesicle, 145 00:11:26 --> 00:11:30 an endosome. That vesicle, the endosome then fuses with a lysosome, 146 00:11:30 --> 00:11:35.25 not shown here. And you may recall that the 147 00:11:35 --> 00:11:39.75 lysosomes have very low pH. And it's in the context of the low 148 00:11:39 --> 00:11:44.25 pH that one of the viral glycoproteins changes its shape. 149 00:11:44 --> 00:11:48.75 And when it changes its shape it allows the viral membrane to fuse 150 00:11:48 --> 00:11:53.25 with the membrane of the lysosome. So this is a pH-dependant protein 151 00:11:53 --> 00:11:57.75 conformational change which turns an inner protein into a protein that 152 00:11:57 --> 00:12:02 facilitates fusion. And when that happens, 153 00:12:02 --> 00:12:06 the viral capsid can then slip out into the cytoplasm. 154 00:12:06 --> 00:12:10 It gets unpackaged. The RNA, which is the minus strand 155 00:12:10 --> 00:12:14 RNA gets released. And then it gets acted on by that 156 00:12:14 --> 00:12:18 RNA dependent RNA polymerase that came in and gets converted from 157 00:12:18 --> 00:12:22 minus strand to plus strand. And the plus strand can then do two 158 00:12:22 --> 00:12:26 things. It can get translated into viral proteins, 159 00:12:26 --> 00:12:30 both the glycoproteins that go through the sorting pathway and make 160 00:12:30 --> 00:12:34 it to the membrane, as well as the structural proteins 161 00:12:34 --> 00:12:38 that form the capsid. The structural proteins then join up 162 00:12:38 --> 00:12:44 with the viral RNA segments, now the minus strand RNA segments. 163 00:12:44 --> 00:12:50 They meet at the membrane and then bud off to form a new virus. 164 00:12:50 --> 00:12:55 OK? So this is a standard infectious cycle for this class of 165 00:12:55 --> 00:13:01 viruses. And because it's going to be relevant later, 166 00:13:01 --> 00:13:07 I just want to emphasize a couple of points. 167 00:13:07 --> 00:13:11 Through interactions between the cytoplasmic tail of the envelope 168 00:13:11 --> 00:13:16 glycoprotiens, the capsid proteins coalesce 169 00:13:16 --> 00:13:20 underneath the plasma membrane. And there's an interaction between 170 00:13:20 --> 00:13:25 these proteins and the tails of these proteins which causes the 171 00:13:25 --> 00:13:30 virus to start to bud off the surface of the cell. 172 00:13:30 --> 00:13:35 Now, there are separate interactions with the proteins on the inside of 173 00:13:35 --> 00:13:40 the capsid which bring with them the different viral RNA. 174 00:13:40 --> 00:13:46 So remember there are eight. And there's a rather amazing 175 00:13:46 --> 00:13:51 process by which the eight distinct RNA segments get brought together 176 00:13:51 --> 00:13:57 into the capsid, and then the capsid goes to the 177 00:13:57 --> 00:14:01 membrane and gets budded. So there is a sorting process, 178 00:14:01 --> 00:14:05 which we don't fully understand, that ensures that viruses get at 179 00:14:05 --> 00:14:08 least one copy of each of the genomic segments. 180 00:14:08 --> 00:14:12.35 And that's necessary because if a virus doesn't have all eight, 181 00:14:12 --> 00:14:15.88 when it gets into the host cell that it might infect, 182 00:14:15 --> 00:14:19.41 it's not going to be able to replicate. These RNA segments 183 00:14:19 --> 00:14:22 encode one, or at most two of the viral proteins that are necessary 184 00:14:22 --> 00:14:26 for the infectious cycle that's drawn up there. 185 00:14:26 --> 00:14:30 And you'll see why that's important in a moment. 186 00:14:30 --> 00:14:35 OK. So we get infected, we develop antibodies against the 187 00:14:35 --> 00:14:40 proteins of the virus, and we typically develop them 188 00:14:40 --> 00:14:45 against the glycoproteins that are sitting on the outside. 189 00:14:45 --> 00:14:50 We develop T cells that can recognize virus infected cells and 190 00:14:50 --> 00:14:55 kill them. And yet we keep getting flu. So why is that? 191 00:14:55 --> 00:15:00 How is it that we can, how is it that the virus can 192 00:15:00 --> 00:15:05 overcome this immune resistance? And the answer is that the virus 193 00:15:05 --> 00:15:09 changes. And this is a major problem with all viruses, 194 00:15:09 --> 00:15:13 but it's a particular problem with RNA viruses and a particular problem 195 00:15:13 --> 00:15:17 with flu. So here's a depiction of the virus again. 196 00:15:17 --> 00:15:21 It's got its genomic segments in here. And on the surface it has 197 00:15:21 --> 00:15:25 these glycoproteins. And I'm going to draw a little bit 198 00:15:25 --> 00:15:30 more detail in these glycoproteins now. 199 00:15:30 --> 00:15:35 There are actually two distinct glycoproteins on the surface of the 200 00:15:35 --> 00:15:40 cell. There's one that's called HA and there's another that's called NA. 201 00:15:40 --> 00:15:46 And they carry out different functions. When you get infected, 202 00:15:46 --> 00:15:51 your immune system sees these epitopes sitting on the surface of 203 00:15:51 --> 00:15:56 the virus and you make antibodies against them. And there are three 204 00:15:56 --> 00:16:02 particular places on these proteins where good antibodies can be made 205 00:16:02 --> 00:16:08.5 inside your body. One of them is here, 206 00:16:08 --> 00:16:15.5 another one is here, and a third one is here. And different people make 207 00:16:15 --> 00:16:31 different of these antibodies. 208 00:16:31 --> 00:16:37 And these antibodies that are made have a name which is neutralizing or 209 00:16:37 --> 00:16:44 blocking antibodies. They're called that because if you 210 00:16:44 --> 00:16:50 have one of these antibodies, and it will bind to the surface of 211 00:16:50 --> 00:16:57 the viral particle, this virus can now not infect the 212 00:16:57 --> 00:17:04 cell. There's an interference between the, what the hell? 213 00:17:04 --> 00:17:08 Between the glycoproteins on the surface and the receptors to which 214 00:17:08 --> 00:17:13 they bind by the antibody. So they block binding. And if you 215 00:17:13 --> 00:17:18 cannot bind you cannot infect. And this is very efficient. This 216 00:17:18 --> 00:17:23 works. So that's how you can overcome the infection to that 217 00:17:23 --> 00:17:28 particular virus. The problem is that viruses change. 218 00:17:28 --> 00:17:33 During their replication there are mutations. 219 00:17:33 --> 00:17:38 And sometimes these mutations affect the structure of the proteins that 220 00:17:38 --> 00:17:43 are sitting on the surface. So you might imagine the strain 221 00:17:43 --> 00:17:48 here gives rise to variant here which has an HA protein which looks 222 00:17:48 --> 00:17:53 more or less the same, but it has an NA protein which has 223 00:17:53 --> 00:17:59 changed. So now this epitope, which was recognized by antibody 224 00:17:59 --> 00:18:04 three, cannot be recognized by antibody three because it's got a 225 00:18:04 --> 00:18:09 difference sequence. OK? If you were a person who made 226 00:18:09 --> 00:18:13 predominantly antibody three in response to this infection, 227 00:18:13 --> 00:18:17 you would not be susceptible to this virus. So if this one came along 228 00:18:17 --> 00:18:21 next year you'd get the flu. If you were a person who made 229 00:18:21 --> 00:18:25 antibody one or antibody two as your major protective antibody, 230 00:18:25 --> 00:18:30 you'd still be resistant to this. You'd be one of the lucky ones. 231 00:18:30 --> 00:18:34 And because there's heterogeneity within our population, 232 00:18:34 --> 00:18:38 even if these new variants arise, they don't spread all that much 233 00:18:38 --> 00:18:42 because there are enough people in the population who are pretty well 234 00:18:42 --> 00:18:46 protected. So those are low-level spread or low-level epidemics. 235 00:18:46 --> 00:18:50 OK? Now, this process, one day I'm going to figure out how this works. 236 00:18:50 --> 00:18:57.5 This process of generating variant 237 00:18:57 --> 00:19:02.5 viruses through this mutational mechanism is called 238 00:19:02 --> 00:19:12 antigenic drift. 239 00:19:12 --> 00:19:16 It's a slow drift of the antigens on the virus. And it happens because 240 00:19:16 --> 00:19:20.8 the virus is highly mutagenic. And it's true of RNA viruses in 241 00:19:20 --> 00:19:33 general. 242 00:19:33 --> 00:19:38 And there are two reasons why RNA viruses create variants at such high 243 00:19:38 --> 00:19:44 levels. One is that RNA is less stable than DNA. 244 00:19:44 --> 00:19:49 Remember, there's a difference in the structure of RNA versus DNA. 245 00:19:49 --> 00:19:55 There's an extra hydroxyl in the structure of RNA which can cause 246 00:19:55 --> 00:20:01 increased mutation, increased breaks and base 247 00:20:01 --> 00:20:07 substitutions in our RNA molecules compared to DNA molecules. 248 00:20:07 --> 00:20:15 So RNA is inherently less stable than DNA. And also RNA polymerases 249 00:20:15 --> 00:20:25 lack proofreading functions. 250 00:20:25 --> 00:20:30 Which means they have an inherently higher mutation rate. 251 00:20:30 --> 00:20:34 And hopefully, again, this is familiar from our 252 00:20:34 --> 00:20:38 discussions about DNA replication. All polymerases make mistakes but 253 00:20:38 --> 00:20:42 your DNA polymerases have what's called a proofreading function which 254 00:20:42 --> 00:20:46 can recognize the mistakes and correct them. RNA polymerases lack 255 00:20:46 --> 00:20:50 that, and so they make the mistakes and they stay as mistakes. 256 00:20:50 --> 00:20:54 So the mutation rates for RNA polymerases are at least ten fold 257 00:20:54 --> 00:20:58 higher than for DNA polymerases, and in some places even higher still. 258 00:20:58 --> 00:21:02.25 OK. So that's what's called antigenic 259 00:21:02 --> 00:21:06.75 shift, sorry, antigenic drift. And it happens all the time and is 260 00:21:06 --> 00:21:11.25 responsible for why we keep getting mild cases of the flu. 261 00:21:11 --> 00:21:15.75 But that's not what's responsible for the Spanish flu of 1918. 262 00:21:15 --> 00:21:23 Instead of antigenic drift -- 263 00:21:23 --> 00:21:27 -- these are due to a phenomenon called antigenic shift. 264 00:21:27 --> 00:21:31 Rather than generating subtly different variants, 265 00:21:31 --> 00:21:35 like I've shown up here, this is the process of generating 266 00:21:35 --> 00:21:47 essentially entirely new strains. 267 00:21:47 --> 00:21:50 And it happens in the case of flu all the time. And one of the 268 00:21:50 --> 00:21:54 reasons that it happens is that there are lots and lots of types of 269 00:21:54 --> 00:21:58 flu viruses which are kind of similar that infect other species. 270 00:21:58 --> 00:22:02 There are duck flus, swine flus, horse flus, 271 00:22:02 --> 00:22:07 seal flus that are all caused by versions of this influenza A virus. 272 00:22:07 --> 00:22:12 Similar. Not identical but similar. And sometimes those viruses cross 273 00:22:12 --> 00:22:17 over into the human population. And this is an example of a 274 00:22:17 --> 00:22:35 zoonotic infection -- 275 00:22:35 --> 00:22:38 -- which is an infection from another species into the human 276 00:22:38 --> 00:22:41 population. And actually happens all the time. Many of the new 277 00:22:41 --> 00:22:44 pathogens that arise in the human population arise through zoonotic 278 00:22:44 --> 00:22:47 infection. Now, most viruses that are evolved to 279 00:22:47 --> 00:22:50 grow in the cells of one given species, at the temperature of a 280 00:22:50 --> 00:22:54 given species, will not replicate very well inside 281 00:22:54 --> 00:22:57 human cells. So even if they did infect, they wouldn't reproduce 282 00:22:57 --> 00:23:01 themselves very well. And that would be true of these 283 00:23:01 --> 00:23:06 viruses, too, swine flu virus, bird flu viruses. The problem is, 284 00:23:06 --> 00:23:10 in the case of flu, the human virus is so common and the mechanism of 285 00:23:10 --> 00:23:15 replication of this virus is so complex and amenable to generation 286 00:23:15 --> 00:23:20.46 of recombinant viruses that we can generate new viral forms. 287 00:23:20 --> 00:23:25.23 And that's what I want to talk to you about now. 288 00:23:25 --> 00:23:30 So here's an example from not too long ago. 289 00:23:30 --> 00:23:34 This is the collection and culling of a lot of birds somewhere in Asia. 290 00:23:34 --> 00:23:39 I'm not really sure where this was. But there was a concern that a new 291 00:23:39 --> 00:23:44 virus was developing in a bird population and was crossing over 292 00:23:44 --> 00:23:49 into humans. And it caused some deaths. And so to avoid that 293 00:23:49 --> 00:23:54 further they just wiped out some millions of these birds in order to 294 00:23:54 --> 00:23:59 prevent further cross infection. But, again, it's not the presence of 295 00:23:59 --> 00:24:04 the virus, the bird virus or swine virus itself that's the problem. 296 00:24:04 --> 00:24:08 It's the fact that the two viruses, the human virus and the bird virus 297 00:24:08 --> 00:24:13 can recombine to form a dangerous new virus which is rather similar to 298 00:24:13 --> 00:24:18 the human virus but carries some segments of the bird virus. 299 00:24:18 --> 00:24:22 And that's illustrated here, and I'll show you on the board in a 300 00:24:22 --> 00:24:27 second. But basically the idea is that a human virus carrying its 301 00:24:27 --> 00:24:32 eight segments, including the segment for HA and NA 302 00:24:32 --> 00:24:37 infects an animal which also gets infected by a bird virus. 303 00:24:37 --> 00:24:41 So that in the same cell there are the segments for the human virus, 304 00:24:41 --> 00:24:45 as well as the segments for the bird virus. And during this process of 305 00:24:45 --> 00:24:49 the generation of a new viral particle, you can get mixing of the 306 00:24:49 --> 00:24:53 segments so that most of the segments come from the human virus 307 00:24:53 --> 00:24:57 and, therefore, will replicate well in the context 308 00:24:57 --> 00:25:02 of the human cell. But two new segments come in from 309 00:25:02 --> 00:25:08 the bird that encode its HA and NA, a version that's never been seen in 310 00:25:08 --> 00:25:13 the human population. Nobody has antibodies against it so 311 00:25:13 --> 00:25:18 the virus can spread like wildfire. And that's what we believe happened 312 00:25:18 --> 00:25:24 then and we fear continues to happen over time. So, 313 00:25:24 --> 00:25:29 again, imagine a cell which has been infected by two different viruses, 314 00:25:29 --> 00:25:35 the avian virus, in this case it was a duck, as well as the human virus 315 00:25:35 --> 00:25:41 in that given year. In that cell there are going to be 316 00:25:41 --> 00:25:47 lots of different segments of virus RNA, the human ones H1, 317 00:25:47 --> 00:25:53 2, 3, 4, 5, 6, 7, 8 and so on, as well as the duck ones, duck 1, 318 00:25:53 --> 00:25:59 duck 2 and so on and so forth, duck 7 and duck 8. 319 00:25:59 --> 00:26:05 When the viral capsids coalesce underneath the viral glycoproteins 320 00:26:05 --> 00:26:12 to produce a new virus, they cannot necessarily distinguish 321 00:26:12 --> 00:26:19 between the human and the duck subunits. So there might be human 1, 322 00:26:19 --> 00:26:26 human 2, human 3, duck 4, human 5, duck 6, 323 00:26:26 --> 00:26:32 human 7 and human 8. OK? This then makes a virus which 324 00:26:32 --> 00:26:36 is perfectly able to replicate in human cells because it's got mostly 325 00:26:36 --> 00:26:40 the genes that are optimized for human cells, but it's got new HA and 326 00:26:40 --> 00:26:44 NA forms that have never been seen by the human population. 327 00:26:44 --> 00:26:49 So there is nobody who has antibodies against it. 328 00:26:49 --> 00:26:53 So the virus will spread like crazy. OK? So this is an example of 329 00:26:53 --> 00:26:57 antigenic shift, and it's responsible for these 330 00:26:57 --> 00:27:01 worldwide pandemics. OK. Any questions about flu? 331 00:27:01 --> 00:27:05 One thing I failed to mention, sort of trivial but you probably 332 00:27:05 --> 00:27:09 know, is it spread very well because it can spread in water droplets. 333 00:27:09 --> 00:27:13 If you sneeze on your friends they'll also get the flu. 334 00:27:13 --> 00:27:17 It's a very efficient virus so you don't need very many viral particles 335 00:27:17 --> 00:27:21 to go inside of your respiratory system in order to initiate the 336 00:27:21 --> 00:27:25 infection. And this is another good example. When you have a very 337 00:27:25 --> 00:27:29 active infection inside of you, you start producing lots of the 338 00:27:29 --> 00:27:33 cytokines, that I mentioned last time, like interferons. 339 00:27:33 --> 00:27:37 And, again, it's largely those that are causing the symptoms of the flu, 340 00:27:37 --> 00:27:41 and in large enough quantities can cause death. You can also get 341 00:27:41 --> 00:27:45 secondary infections with bacteria when you're that sick, 342 00:27:45 --> 00:27:49 which can be another reason why folks die. OK. 343 00:27:49 --> 00:27:53 So if there are no other questions about flu, we'll now turn our 344 00:27:53 --> 00:27:57 attention to HIV. And I think HIV is probably fairly 345 00:27:57 --> 00:28:02 familiar as a topic. It's, of course, 346 00:28:02 --> 00:28:08 a major, major worldwide health problem. The initiation of this 347 00:28:08 --> 00:28:18 epidemic goes back a very long way. 348 00:28:18 --> 00:28:24 The first cases were reported in 1975, so about 30 years ago. 349 00:28:24 --> 00:28:30 And so that's considered to be the initiation of this epidemic 350 00:28:30 --> 00:28:35 or pandemic. The first case in the United States 351 00:28:35 --> 00:28:39 is considered to have occurred in 1980. And here's another example. 352 00:28:39 --> 00:28:43 We think that this probably occurred by one of the 353 00:28:43 --> 00:28:51 zoonotic infections. 354 00:28:51 --> 00:28:55 But it happened a long time ago. It happened at least before 1950 355 00:28:55 --> 00:28:59 because there are samples of individuals who died of unknown 356 00:28:59 --> 00:29:03 causes that have now been tested and can be shown to have HIV 357 00:29:03 --> 00:29:08 sequences by PCR. So whenever this happened, 358 00:29:08 --> 00:29:12 and from whatever species it happened, it happened quite a while 359 00:29:12 --> 00:29:17 ago. Why it then initiated in this epidemic form much later is not so 360 00:29:17 --> 00:29:26 clear. The etiological agent -- 361 00:29:26 --> 00:29:33 -- is a virus called HIV, human immunodeficiency virus. 362 00:29:33 --> 00:29:37 It is a retro virus. And we'll briefly review how it 363 00:29:37 --> 00:29:41 replicated, but I told you more or less how that happens last time. 364 00:29:41 --> 00:29:46 And this was another good example. The syndrome was first discovered 365 00:29:46 --> 00:29:50 in 1975, gained a lot of attention in this country starting in the 366 00:29:50 --> 00:29:55 early 1980s, and the virus was first purified and characterized in 1983. 367 00:29:55 --> 00:29:59 So it didn't take too long to identify the agent and then 368 00:29:59 --> 00:30:04 ultimately sequence its genome. And also to develop tests for the 369 00:30:04 --> 00:30:10 presence of actually not the virus itself. The AIDS test is a test for 370 00:30:10 --> 00:30:15 the presence of antibodies in you that recognize the viral 371 00:30:15 --> 00:30:21 glycoproteins. And this was very important, 372 00:30:21 --> 00:30:27 obviously, in screening populations, screening blood banks and so on for 373 00:30:27 --> 00:30:32 contaminated samples. Now, since this time, 374 00:30:32 --> 00:30:36 in the early 1980s, the situation has, of course, 375 00:30:36 --> 00:30:40 gotten much, much worse. In this country, there are 376 00:30:40 --> 00:30:52 currently -- 377 00:30:52 --> 00:30:56 -- a million people infected with HIV. That's between, 378 00:30:56 --> 00:31:00 you know, one and 300 individuals total. 379 00:31:00 --> 00:31:08 And, amazingly, about 20% of those people don't know 380 00:31:08 --> 00:31:16 it. Worldwide, anybody have any idea how many 381 00:31:16 --> 00:31:24 people are infected with HIV in the world? 40 million. 382 00:31:24 --> 00:31:32 And total about 60 million people have been infected because since the 383 00:31:32 --> 00:31:40 early 1980s, 20 million people have died from this disease. 384 00:31:40 --> 00:31:45.75 More than 20 million. And the prevalence is remarkable in 385 00:31:45 --> 00:31:51.5 certain places in the world. In Southern Africa, for example, 386 00:31:51 --> 00:31:57.25 not the country of South Africa but in the Southern Region of Africa, 387 00:31:57 --> 00:32:03 Saharan Africa, there are 25 million people infected. 388 00:32:03 --> 00:32:08.5 And in certain countries that's one in four sexually active individuals. 389 00:32:08 --> 00:32:14 Remarkable infection rates. And the effect of this disease and the 390 00:32:14 --> 00:32:19.5 death that it causes has also had dramatic effects on the population. 391 00:32:19 --> 00:32:25 So that in Zimbabwe, for example, where the life expectancy in 1990 392 00:32:25 --> 00:32:30.5 was 52 years old, not a huge number but 52 years old, 393 00:32:30 --> 00:32:36 in 2003, because of AIDS, it's less then 35 years. 394 00:32:36 --> 00:32:42 So it has dramatically affected the lives of people throughout the world. 395 00:32:42 --> 00:32:48 OK. And, as you know, I suspect you know that transmission 396 00:32:48 --> 00:32:54 for this virus occurs through direct contact of contaminated fluids. 397 00:32:54 --> 00:33:00 Sexual contact, which is both homosexual and heterosexual. 398 00:33:00 --> 00:33:04 As well as blood transfusion, direct blood contact, through 399 00:33:04 --> 00:33:09 transfusion, and also needle sharing. And sometimes occasionally organ 400 00:33:09 --> 00:33:14 donation, but that's very, very rare. OK. 401 00:33:14 --> 00:33:21.75 So people die from AIDS because they 402 00:33:21 --> 00:33:25.25 develop infections. And we'll talk a bit more about 403 00:33:25 --> 00:33:28.75 that in a moment. But here's one patient who has 404 00:33:28 --> 00:33:32.25 developed an infection with a virus, a particular type of herpes virus 405 00:33:32 --> 00:33:36 now called Kaposi's sarcoma virus. That virus then is able to replicate 406 00:33:36 --> 00:33:40 in certain cells of the blood vasculature and cause tumors. 407 00:33:40 --> 00:33:44 These are actually tumors. And this individual will die from a 408 00:33:44 --> 00:33:48 disease called Kaposi's sarcoma, a type of cancer, another example of 409 00:33:48 --> 00:33:52 a virus-associated cancer of humans. And in this country AIDS, as you 410 00:33:52 --> 00:33:56 probably know, has taken over as the leading killer 411 00:33:56 --> 00:34:01 of young male adults. It was not known in the early 1980s, 412 00:34:01 --> 00:34:06 and by 1992, just ten years later had become the major killer of young 413 00:34:06 --> 00:34:10 male adults in this country. And, of course, likewise around the 414 00:34:10 --> 00:34:15 world. So here's the virus. As I said, it's a retrovirus. 415 00:34:15 --> 00:34:20 Retroviruses also are enveloped. They have glycoproteins on the 416 00:34:20 --> 00:34:25 outside, envelope glycoproteins. They have a capsid like all viruses 417 00:34:25 --> 00:34:30 do. They have a genome. The genome is made of RNA. 418 00:34:30 --> 00:34:34 And, as you know, through the retroviral lifecycle, 419 00:34:34 --> 00:34:38 the RNA is converted into a DNA form. We'll go over that next time. 420 00:34:38 --> 00:34:42 And that's accomplished by an enzyme called reverse transcriptase 421 00:34:42 --> 00:34:46 which gets prepackaged with the virus. Again, 422 00:34:46 --> 00:34:50 it has to be there because your cells don't have an RNA dependent 423 00:34:50 --> 00:34:54 RNA polymerase. So this gets prepackaged. 424 00:34:54 --> 00:34:58 The virus then infects cells. And this, again, is a somewhat 425 00:34:58 --> 00:35:01 oversimplification. I'll show you more details in a 426 00:35:01 --> 00:35:05 second. But the virus attaches via glycoproteins on its surface, 427 00:35:05 --> 00:35:08 membrane proteins on the surface of the host cell. 428 00:35:08 --> 00:35:12 In this case it's a familiar protein to you called CD4. 429 00:35:12 --> 00:35:15 That leads to a fusion event. So now at the plasma membrane, 430 00:35:15 --> 00:35:19 which is different from what we saw with flu virus which happened in an 431 00:35:19 --> 00:35:22 internal membrane, at the plasma membrane there's a 432 00:35:22 --> 00:35:26 fusion event. So now the viral membrane fuses with the host cell 433 00:35:26 --> 00:35:30 membrane and the viral capsid can lead into the cytoplasm. 434 00:35:30 --> 00:35:33 There's a little bit of unpackaging that goes on. And then reverse 435 00:35:33 --> 00:35:36 transcriptase, that enzyme that got prepackaged 436 00:35:36 --> 00:35:39 there acts on the viral genome, which is made of RNA and converts it 437 00:35:39 --> 00:35:43 to a DNA form, a double-stranded DNA form which is 438 00:35:43 --> 00:35:46 called a provirus. And that provirus then gets 439 00:35:46 --> 00:35:49 integrated into the host cell genome in a random process, 440 00:35:49 --> 00:35:53 random integration, the host cell genome. This is also accomplished 441 00:35:53 --> 00:35:56 by a pre-packed enzyme called integrase. Once in the genome it 442 00:35:56 --> 00:36:00 gets treated like any old piece of DNA. 443 00:36:00 --> 00:36:03 It gets transcribed into actually multiple different RNAs. 444 00:36:03 --> 00:36:07 They get translated into viral proteins which come together at the 445 00:36:07 --> 00:36:10 plasma membrane and the glycoproteins, 446 00:36:10 --> 00:36:14 the capsid proteins which incorporate the genome, 447 00:36:14 --> 00:36:18 plus the reverse transcriptase, and you make a new particle. OK? 448 00:36:18 --> 00:36:21 So this is more or less the lifecycle. There is some detail 449 00:36:21 --> 00:36:25 that I want to share with you in a moment because it's interesting. 450 00:36:25 --> 00:36:29 This is what it looks like in real life. 451 00:36:29 --> 00:36:33 These are HIV particles budding off the surface of an infected cell. 452 00:36:33 --> 00:36:37 They have a very characteristic electron dense structure. 453 00:36:37 --> 00:36:41 You can actually tell it is HIV from just the way it looks in the 454 00:36:41 --> 00:36:45 electron microscope. And this is an HIV infected T cell. 455 00:36:45 --> 00:36:49 Now, CD4 should have rung a bell that it's one of the proteins on the 456 00:36:49 --> 00:36:53 surface of helper T cells, CD4 positive T cells. That is a 457 00:36:53 --> 00:36:58 major cell of infection for HIV. And here is one such cell infected. 458 00:36:58 --> 00:37:02 And all these little blebs budding off the surface of this cell are 459 00:37:02 --> 00:37:06 viruses getting out. This process actually doesn't 460 00:37:06 --> 00:37:11 itself, the budding process doesn't itself kill the T cell but, 461 00:37:11 --> 00:37:15 nevertheless, T cells, this class of T cells dies. And, 462 00:37:15 --> 00:37:20 actually, that's a critical event in the development of HIV-AIDS, 463 00:37:20 --> 00:37:24 the disease. We'll come to that in a second, but let me first tell you 464 00:37:24 --> 00:37:44 one of the interesting details. 465 00:37:44 --> 00:37:48 The situation is a little more complicated than the figure that I 466 00:37:48 --> 00:37:52 gave you on that slide. There are two glycoproteins on the 467 00:37:52 --> 00:37:56 surface of HIV. One of them is called GP120 for 468 00:37:56 --> 00:38:02 glycoprotein 120. It's linked by a disulfide linkage 469 00:38:02 --> 00:38:08 to another protein called GP41. And we now know that both of these 470 00:38:08 --> 00:38:14 proteins participate in binding and fusion. And on the surface of the 471 00:38:14 --> 00:38:20 infected cell is CD4, as was indicated on that slide. 472 00:38:20 --> 00:38:26 And that is the major viral receptor that is present on helper T 473 00:38:26 --> 00:38:33 cells, CD4 positive T cells. It's also present on macrophages. 474 00:38:33 --> 00:38:37.5 As well as certain cells in the brain called glial cells. 475 00:38:37 --> 00:38:42 So there are CD4 positive macrophages and glial cells. 476 00:38:42 --> 00:38:46.5 AIDS-associated dementia, which you may know about, 477 00:38:46 --> 00:38:51 is caused by the destruction of the glial cells in the brain. 478 00:38:51 --> 00:38:55.5 Now, there's another protein which participates, not so much in the 479 00:38:55 --> 00:39:00 binding but in the fusion event that allows the virus to get in. 480 00:39:00 --> 00:39:02 And this is actually a class of proteins called chemokine 481 00:39:02 --> 00:39:10 receptors. 482 00:39:10 --> 00:39:13 There are actually multiple chemokine receptors on your cells. 483 00:39:13 --> 00:39:17 And, in fact, different HIV forms bind to different of these. 484 00:39:17 --> 00:39:20 But just to simplify, let's say that there is one of these which 485 00:39:20 --> 00:39:24 binds to the GP41 portion. And it's that binding that allows 486 00:39:24 --> 00:39:28 the virus to fuse. If you don't have this you cannot 487 00:39:28 --> 00:39:32 fuse. And, therefore, 488 00:39:32 --> 00:39:36 the cells are not infected. What's interesting about that is 489 00:39:36 --> 00:39:40 two-fold. One, it could represent a therapeutic 490 00:39:40 --> 00:39:44 target. That interaction could be a therapeutic target. 491 00:39:44 --> 00:39:49 But it came to light in an interesting way which was there are 492 00:39:49 --> 00:39:53 people who have been followed a long time who lived in high-risk 493 00:39:53 --> 00:39:57 populations, drug-users or homosexual men with multiple 494 00:39:57 --> 00:40:01 contacts with known HIV infected people who have not themselves 495 00:40:01 --> 00:40:06 gotten HIV. They seem to be naturally resistant 496 00:40:06 --> 00:40:11 to the development of HIV, despite documented exposure to the 497 00:40:11 --> 00:40:17 virus. And they were studied. And when that observation was made 498 00:40:17 --> 00:40:22 it was found that these individuals lack the chemokine receptor. 499 00:40:22 --> 00:40:27 And if you lack the chemokine receptor, you may have CD4 on the 500 00:40:27 --> 00:40:33 surface of your cells but you don't have that critical co-receptor. 501 00:40:33 --> 00:40:39 And so there can be binding but no fusion. So your cells are resistant. 502 00:40:39 --> 00:40:45 So these individuals are naturally resistant to HIV. OK. 503 00:40:45 --> 00:40:58 So HIV infects. 504 00:40:58 --> 00:41:02 It infects a lot of T cells, helper T cells and other cells in 505 00:41:02 --> 00:41:05 the body. Why do you get sick? Well, as I mentioned, people get 506 00:41:05 --> 00:41:09 sick because of the loss, the absence -- 507 00:41:09 --> 00:41:19 During infection, 508 00:41:19 --> 00:41:23 the CD4 positive T cells get eliminated. How exactly they get 509 00:41:23 --> 00:41:27 eliminated, why exactly they get eliminated we don't yet know, 510 00:41:27 --> 00:41:31 but we know that the levels of these cells goes down inside the 511 00:41:31 --> 00:41:35.5 infected individual. And, as you probably remember, 512 00:41:35 --> 00:41:45 CD4 positive T cells -- 513 00:41:45 --> 00:41:48 -- are required for the development of these cells. 514 00:41:48 --> 00:41:55 So if you don't have CD4 positive T 515 00:41:55 --> 00:41:57 cells you cannot make antibody producing cells. 516 00:41:57 --> 00:42:00 You cannot make functional antibodies. 517 00:42:00 --> 00:42:05 And, to some extent, they're also required for the 518 00:42:05 --> 00:42:10 development of cytotoxic T cells. So that part of the immune system 519 00:42:10 --> 00:42:15 is also compromised. So the virus wipes out these cells 520 00:42:15 --> 00:42:20 and, therefore, the immune system crashes. 521 00:42:20 --> 00:42:26 And that's why it's called AIDS for acquired immunodeficiency. 522 00:42:26 --> 00:42:32 Acquired because it comes through an 523 00:42:32 --> 00:42:36 infectious agent. Immunodeficiency because your 524 00:42:36 --> 00:42:40 immune system crashes. And when your immune system crashes 525 00:42:40 --> 00:42:44 you cannot fight infections. And AIDS patients largely die 526 00:42:44 --> 00:42:48 because they get one or another type of infection. 527 00:42:48 --> 00:42:54.75 So this is a graph which shows the 528 00:42:54 --> 00:42:58.25 time course of HIV infection. And it kind of makes the point that 529 00:42:58 --> 00:43:01 T cells go away. Shortly after you are infected, 530 00:43:01 --> 00:43:04 the level of HIV in your blood goes way, way up, as you can see here. 531 00:43:04 --> 00:43:08 It spikes in the first several months after your infection. 532 00:43:08 --> 00:43:11 Some people actually manifest that aspect of the disease. 533 00:43:11 --> 00:43:14 Many don't so they don't know that they have such a high concentration 534 00:43:14 --> 00:43:17 of virus. And, importantly, your body deals with it 535 00:43:17 --> 00:43:20 so you make antibodies, in black. You make anti-HIV 536 00:43:20 --> 00:43:24 antibodies. And, remember, it's these that are 537 00:43:24 --> 00:43:27 detected by the AIDS test. If you've been exposed then you 538 00:43:27 --> 00:43:31 make antibodies. And they stick around so that you 539 00:43:31 --> 00:43:35 know that you've been infected. Likewise, T cells, cytotoxic T 540 00:43:35 --> 00:43:40 cells, CD8 positive T cells go up. And that controls the level of the 541 00:43:40 --> 00:43:45 virus. And it stays low for a while, but after a while, 542 00:43:45 --> 00:43:49 because of the affects on the CD4 positive T cells, 543 00:43:49 --> 00:43:54 the other cells in the immune system are no longer sustained so the B 544 00:43:54 --> 00:43:59 cells start to drop off and the T cells start to drop off. 545 00:43:59 --> 00:44:02 And now you're starting to lose the battle against the HIV, 546 00:44:02 --> 00:44:06 so the levels of HIV go up. And as they go up, you lose more 547 00:44:06 --> 00:44:09 CD4 positive T cells and the situation gets worse and worse. 548 00:44:09 --> 00:44:13 So after a while, usually in the several years out time point, 549 00:44:13 --> 00:44:17 your immune system is now no longer functional and you're very 550 00:44:17 --> 00:44:20 susceptible to opportunistic infections. So AIDS patients will 551 00:44:20 --> 00:44:24 develop infections, and the infections will progress in 552 00:44:24 --> 00:44:28 a way that won't happen in a healthy person. 553 00:44:28 --> 00:44:32 Certain bacterial infections that you would clearly very easily take 554 00:44:32 --> 00:44:37 hold and can cause major problems. Kaposi's sarcoma is caused by 555 00:44:37 --> 00:44:42 infection of this particular virus. AIDS patients are very susceptible 556 00:44:42 --> 00:44:47 to tuberculosis, this particular fungal infection, 557 00:44:47 --> 00:44:51 certain other viral infections. And in aggregate, the exposure to these 558 00:44:51 --> 00:44:56 various infectious agents and the damage that they're causing leads to 559 00:44:56 --> 00:45:01 the death of the patient. OK. So that's bad news. 560 00:45:01 --> 00:45:06 And obviously it's have major and devastating consequences around the 561 00:45:06 --> 00:45:11 world. So what can we do about it? Well, there is some good news here. 562 00:45:11 --> 00:45:33 There are antiviral therapies. 563 00:45:33 --> 00:45:44 Remember that viruses largely use 564 00:45:44 --> 00:45:48 your enzymes. And you cannot make good drugs against your own enzymes 565 00:45:48 --> 00:45:52 for treatment of a viral disease, but I've told you about one viral 566 00:45:52 --> 00:45:56 protein to which you might be able to make drugs. And 567 00:45:56 --> 00:46:04 indeed we have. 568 00:46:04 --> 00:46:10 HIV has an RNA genome that gets converted to a DNA form by an enzyme 569 00:46:10 --> 00:46:17 reverse transcriptase. There have been drugs produced that 570 00:46:17 --> 00:46:23 block that enzyme. And you've probably heard of the 571 00:46:23 --> 00:46:29 drug AZT. There are other drugs like 572 00:46:29 --> 00:46:33 dideoxyinosine and dideoxycytosine that likewise bind to and block the 573 00:46:33 --> 00:46:37 activity of this enzyme. They actually cause chain 574 00:46:37 --> 00:46:42 termination in the same way that DNA sequencing works. 575 00:46:42 --> 00:46:46 And they work because this enzyme is sensitive to these drugs at 576 00:46:46 --> 00:46:51 concentrations where your polymerases are not, 577 00:46:51 --> 00:46:55 so that gives you some selected effect in the treatment of the virus. 578 00:46:55 --> 00:47:00 And these agents, when used singly, work for a while. 579 00:47:00 --> 00:47:04 And then the virus becomes resistant and comes roaring back. 580 00:47:04 --> 00:47:08 How does it become resistant? It becomes resistant through 581 00:47:08 --> 00:47:13 mutation, exactly the way cancer cells become resistant to targeted 582 00:47:13 --> 00:47:17 therapies. Variant forms of HIV arise which have slightly altered 583 00:47:17 --> 00:47:22 reverse transcriptases that now won't bind to AZT or DDI or DDC. 584 00:47:22 --> 00:47:26 And the problem is very, very serious because viruses, 585 00:47:26 --> 00:47:31 RNA viruses in particular are highly mutagenic. 586 00:47:31 --> 00:47:36 They create variant forms at high frequency. An HIV infected person 587 00:47:36 --> 00:47:41 makes ten to the tenth viral particles a day. 588 00:47:41 --> 00:47:46 At a mutation rate of ten to the minus fifth, that would mean ten to 589 00:47:46 --> 00:47:52 the fifth variants of every nucleotide. OK? 590 00:47:52 --> 00:47:57 So variants come up frequently that are resistant to this 591 00:47:57 --> 00:48:14 therapy. Oops. 592 00:48:14 --> 00:48:24 Fortunately, there are alternatives. The viral RNA gets translated into 593 00:48:24 --> 00:48:40 a large polyprotein -- 594 00:48:40 --> 00:48:44 -- which is processed by a viral protease encoded by the virus into 595 00:48:44 --> 00:48:48 different mature proteins. Reverse transcriptase is one of 596 00:48:48 --> 00:48:53 them. The integrase that I mentioned is another. 597 00:48:53 --> 00:48:57 And there are others. This is a viral protein, 598 00:48:57 --> 00:49:02 a viral enzyme. And so, in theory, 599 00:49:02 --> 00:49:06 it's possible to make inhibitors against the protease. 600 00:49:06 --> 00:49:11 And that's been successful. Several companies have made 601 00:49:11 --> 00:49:15 inhibitors that will specifically inhibit the viral protease that work 602 00:49:15 --> 00:49:20 well. They work for a while and then resistant forms arise for 603 00:49:20 --> 00:49:24 exactly the same reason I said. You can create versions of protease 604 00:49:24 --> 00:49:29 which still work but won't bind the drug. OK? So what do you do? 605 00:49:29 --> 00:49:34 Does anybody know what you do to overcome this problem? 606 00:49:34 --> 00:49:46 You put the drugs together. 607 00:49:46 --> 00:49:54 So currently HIV patients, 608 00:49:54 --> 00:49:58 in this country anyway, and hopefully soon around the world will 609 00:49:58 --> 00:50:02 get triple therapy when they're diagnosed. 610 00:50:02 --> 00:50:08 And triple therapy includes two reverse transcriptase inhibitors and 611 00:50:08 --> 00:50:18 one protease inhibitor. 612 00:50:18 --> 00:50:22 Since the development of resistance against each of these agents is 613 00:50:22 --> 00:50:26 independent of the other, to get resistance to all three is 614 00:50:26 --> 00:50:30 the product of the individual frequency. 615 00:50:30 --> 00:50:35 So I said that the frequency of developing resistance to any one is 616 00:50:35 --> 00:50:40 ten to the minus fifth. To be resistant to all three would 617 00:50:40 --> 00:50:45 occur at ten to the minus fifteenth, which is highly, highly unlikely. 618 00:50:45 --> 00:50:50 And so people tend not to develop resistance to all three and, 619 00:50:50 --> 00:50:55 therefore, triple therapy tends to keep the virus in-check for 620 00:50:55 --> 00:51:00 sustained periods of time. As you probably know, this works. 621 00:51:00 --> 00:51:03.75 The fear is that it is starting not to work. And there are starting to 622 00:51:03 --> 00:51:07.5 be some resistant forms developing here and they are starting to make 623 00:51:07 --> 00:51:11.25 their way into the population, so we're going to have to develop 624 00:51:11 --> 00:51:15 even better and more different inhibitors to now combat these 625 00:51:15 --> 00:51:18.75 variant forms. Before I let you go, 626 00:51:18 --> 00:51:22.5 the last thing I wanted to mention, and I'll just throw it out there and 627 00:51:22 --> 00:51:26.25 you can think about it, is that what we'd really like to 628 00:51:26 --> 00:51:30 return to for HIV is what we can do for polio. 629 00:51:30 --> 00:51:34.5 We'd like to be able to treat people routinely in this country or out in 630 00:51:34 --> 00:51:39 the field in more distant regions with an effective vaccine, 631 00:51:39 --> 00:51:43.5 which isn't possible today. We don't have one for HIV. You 632 00:51:43 --> 00:51:48 might think about what you would do to make one and why it has been so 633 00:51:48 --> 00:51:51 hard. And I'll touch on that next time.