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David Ho: [00:00:00] Good morning. Let me thank the organizers for this opportunity to speak. This is to John [Mellors], I indeed do dynamics, and I speak English. [laughter] I'm going to focus my presentation solely on the story of unraveling of HIV dynamics. I'm sure George [Shaw] [00:00:30] will tell a good part of that story as well. Before I get to that, let me just tell you some events that led up to those studies. I was a physician in training when I encountered two of the first five patients described by Mike Gottlieb from Los Angeles. Then in the subsequent year, I was chief medical resident, and I saw approximately 20 such cases [00:01:00] in the west side of Los Angeles. That certainly piqued my interest in this new disease. When I went to Mass General to do my infectious disease fellowship, I was assigned by my mentor Marty Hirsch to look at this new mysterious illness. I'm also very grateful that in the spring of 1984, Bob Gallo allowed me to train in his lab for a week. This may not be known to someone like George, [00:01:30] but I learned a great deal. I worked on HIV throughout most of the 1980s.

As you just heard from John [Mellors], the techniques weren't great. Therefore, even though HIV as the causative agent of AIDS was well established, the virus was difficult to culture, difficult to find in places. That led to all sorts of theories [00:02:00] back in those days. That HIV was actually an autoimmune destruction process or autoimmune disease. Having worked with this virus in the laboratory seeing it reap through CD4 cell cultures, it just seemed counterintuitive.

David Ho (何大一, b. 1952) is Director of the Aaron Diamond AIDS Research Center at Columbia University.


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While it remain possible that it was an autoimmune disease even though I thought it was more likely that it was a viral disease, and our techniques [00:02:30] simply were not good enough. By the late 1980s, when I had my own lab, we did a systematic evaluation of trying to quantify HIV in blood. This we did by the old-fashioned limiting dilution culture technique. (1)

To our surprise, we found that in a lot of cases, a few hundred, a few thousand PBMCs [00:03:00] would turn a culture positive. In several dozen patients that we studied in the late 1980s, we were able to isolate virus from everyone from the plasma, including some when the plasma is diluted 100 fold or 1,000 fold. It was clear that there was a lot more virus than being reported previously. Then, a year later, I moved to the newly established [00:03:30] Aaron Diamond AIDS Research Center. And then because of this interest, ChironRoche, the various makers of viral assays came to see us about correlating the new results with the old-fashioned techniques. This allowed us to then get a clear picture of how much viremia there actually was in infected individuals.

And so knowing this information and going back [00:04:00] to read some of the old papers from old time biologists working in other fields, including Neal Nathanson, we realized that people in the 1960s had done viral dynamics. In fact, looked at particle clearance from the bloodstream of infected animals, and found that the halflife was in the order of five minutes to 30 minutes. Neil, for example, reported the langat flavivirus had a half-life of 30 minutes in the [00:04:30] circulation.

Knowing those facts, we realized that there was, in fact—if you had so much free virus, there had to be lots of de novo infection and productive infection. And so finally, 1992 came along and that was the election year when Bill Clinton (b. 1946) had a famous phrase, "It's the economy stupid" and we coined the [00:05:00] phrase it's the virus stupid, pointing out that this, we believe is firmly an infectious disease, not an autoimmune process.

I should point out two fortuitous events. About the same time in 1991, I went to a conference in Florida. This was in a small place. We had to drive from the airport to the conference, I was at the [00:05:30] Avis Hertz rental line. There was a chap standing behind me. We started talking, it turns out he was going to the same meeting. We go to the meeting together. That chap was Dale Kempf who is a chemist for Abbott. He had told me that he was working on these small molecules that will intercalate into the protease enzyme. In their enzymatic assay, inhibited protease function. Long story short, that started a collaboration that will [00:06:00] continue for about 10 years.

Throughout this period, we were working on various things on pathogenesis, protease inhibitors. Then at a conference in 1993, and for the clinical investigators in the audience, you would know that that was the year of AZT & 3TC combination. Charles Boucher from the Netherlands [00:06:30] showed a piece of preliminary data with one patient on AZT & 3TC. It showed the viral load at time zero and day 28. It's about a log and half and he drew a line that connected those two dots. I asked Charles, "How do you know to draw the line that way?" He says, "Why not?" That also piqued my [00:07:00] interest in wanting to know what the shape of that decay was. By early 1994, we had a protease inhibitor that was ready to go into the clinic, we had assays to measure viral load, and we had a persistent question that we wanted to answer.

In 1994, we were ready to launch this. After working on an injectable protease inhibitor, we were ready to work on a orally bioavailable [00:07:30] protease inhibitor. It turns out now to be only used as a booster by ritonavir but we're ready to do so. A guy named Charles Farthing across the street from us at NYU School of Medicine was ready to do the trial with us. But a couple of weeks before the trial started, he took a job in Los Angeles. And so there was this poor postdoc in my group named Marty Markowitz [00:08:00] who was doing resistance to protease inhibitor. He was thrusted into the role as the PI for the trial. Marty ever since has been doing our clinical studies.

Very quickly, we enrolled 20 patients into this initial protease inhibitor trial. Here are just three examples shown. (2) Here's the steady [00:08:30] state or quasi-steady-state that you could see prior to the intervention. Then after the drug is administered, you see the virus load fall precipitously about two orders of magnitude. Then after that, the decay became slower. We noticed then that there was the bi-phasic decay, but the effect was not durable. Remember, this is monotherapy. In many of the patients, there was a rebound due [00:09:00] to drug resistance. So We focused our attention on the first phase decay. It turned out to be remarkably consistent from case to case, with no more than two-fold variation in the slope of the decay. All 20 cases that are summarized here, by plotting the slope of the initial decay. The mean half-life was two and a half days.

Now, I have to [00:09:30] tell you that I spent weeks thinking about this, thinking about the ramifications while the Abbott team and the clinical team was celebrating the remarkable antiviral activity of this new agent, even though many of the patients had severe nausea and vomiting and other side effects. Now, going back to my undergraduate days, as a physics major at Caltech, I believe I could have solved this if they were on the testing [00:10:00]. 30 minutes. However, it took me weeks to realize what this meant. and to be confident about it.

To put it in very simple terms, we believe these are the HIV particles that come from productively infected CD4 T cells, and they go on to infect a new crop of susceptible [00:10:30] T cells. (3) This is the cyclic process that maintains the steady-state. When we administer a protease inhibitor, we render these particles non-infectious and thereby blocking de novo infection. When the protease inhibitors is on board, we would expect the viral particles to be clear at their natural rate, and these cells to die at a certain rate. So one mathematically would expect [00:11:00] a decay for this population and a decay for that population. But we saw a single phaze early on, and that was puzzling. I also knew that particles should be clear, like other viruses, in the order of minutes. There was a hint that the slower component was probably the death rate of the productively infected CD4 T cells, but I wasn't sufficiently confident. [00:11:30] 

As I was struggling with this, my colleague, Rick Koup said, "Well, I just got this call from Alan Perelson wanting to model some immune stuff. It looks like Alan could help you model this." In fact, that's how Alan and I began our collaboration, which also continued for about a decade.

There's another story here. By [00:12:00] the summer of 1994, a lot of this dynamic information was pretty clear in our heads, but I recall, and George [Shaw] may not, I recall a dinner that was being hosted by Dani Bolognesi with George and I sitting across the table from one another. In the middle of the dinner conversation, Danny asks, "I wonder how long does an infected cell live in the body?" [00:12:30] I recall George and I blurted out approximately the same answer at the same time. That's when I, at least, knew that George was doing pretty much the same thing. 

By that point, we recognized that the half-life of two days or so is a composite decay of these two parameters, but the math could not work out each parameter separately at the time. [00:13:00] I went on to present these results in the fall that year at the Orlando ICAAC (Interscience Conference on Antimicrobial Agents and Chemotherapy) meeting, the infectious disease meeting. I was actually asked to give a keynote presentation on long-term survivors, before that name was changed. Instead, I talked about this and in a subsequent talk at the same conference, George spoke about their results, which were [00:13:30] nearly identical. And of course, a few months later, the two papers were published in January 1995. (2, 3)

But we wanted to do more, and I figured George and his collaborators were busy trying to do the follow-up study. Alan had also been busy trying to formalize the modeling of what might be going on. [00:14:00] These are taken from Alan's equations. Two differential equations, one describing the kinetics of infected T cell population, the other one, the viremia. You could see there's a term for the death of productively infected T cells, the rate constant δ, and then the susceptible T cells could be infected by the viral population at a rate constant k[00:14:30] And then same for the viremia. There's a viral low clearance constant, and then the productively infected T cells die at rate δ producing birth size N. Of course, at steady-state, this parameter (kVT) equals that (δT*), and this parameter (NδT*) equals that (cV). Alan could explain to you how this is then derived.

Over [00:15:00] in late 1994, before the first papers were published, we got to work to say, "Let's get it done with just a few cases." In fact, we enrolled five patients into the hospital over Christmas and New Year. These five patients stayed in the hospital for approximately a week. And you can see, we let bled frequently in the first two days, and then daily thereafter. All five cases yield nearly [00:15:30] the identical profile and all five cases were again, treated with ritonavir. You could see that there is a delay of approximately a day or on average, 1.2 days and then after that, there's an inflection and we see the exponential decay. (4) What is this telling us? Well, the inflection and the more careful modeling and data fitting allow us to derive now a more accurate [00:16:00] half-life for virions of 0.22 days and for infected T cells about 1.4 days.

Why do we see this delay? Yes, you would expect a pharmacokinetic delay of an hour or a few hours, but this was about a day. After thinking about it, it became very clear: that is telling us the [00:16:30] eclipse phase of the viral life cycle, that is the time the virus penetrates into a susceptible T cell until the T cell becomes productively infected and making progeny virus. And so, think of it this way: if a virion escapes just before the pharmacologic effect of the drug, that virion is going to be infectious, it's going to go on and produce progeny virus one cycle [00:17:00] later, and you would still see that. And it's only the virion that's about to come out, who feels the effect of the drug that will have missing progenies. That's basically the duration of the eclipse phase for the viral life cycle. That second study gave us a more accurate assessment is about 1.4, 1.5 day [00:17:30], but certainly less than two days the half-life of the cells. That means there has to be equal amount of the de novo infection to match the cell death rate during the steady-state. The virion half-life, we thought, had to be less than six hours.

These are all minimum estimates, because we assume the antiviral agent completely blocks de novo infection, which may or may not be the case. [00:18:00] From the infected half-life, we could calculate the lifespan to the eclipse phase of one day, and at that time calculated 140 generations of HIV replication per year. If you now think about the huge number of viruses being produced each day, undergoing new rounds of infection, going through reverse transcription [00:18:30] with a rate that is high as [Louis] Mansky and [Howard] Temin (1934–1994) had reported (5) there is no question that the consequence is generation of lots and lots of mutants.

That number, based on our calculation and based on John Coffin's calculation, suggests that it would be in the tens of millions of variants per day in the body of an infected individual. That means at [00:19:00] every mutation at every position in the genome is possible. In fact, you could calculate the likelihood of certain unique double and triple combinations. It was the triple combination of mutations that we show, mathematically, became less and less probable.

And so this picture painted a dynamic evolution that was being fast forwarded with generations [00:19:30] of mutants at a fast clip, and we could only deal with multiple drugs and not one at a time. Monotherapy was certainly going to be doomed.

This picture of relentless replication and destruction of CD4 T cells suggested that we ought to intervene and hope this destructive process as early as possible.

In fact, one didn't need the second [00:20:00] study to draw these conclusions. This is why I wrote this particular editorial in 1995 in the New England Journal. (6) And I believe John Coffin wrote pretty much the same thing in his review in Science in the same year. (7) It's amazing, John wrote a review when there were only two papers published, but it was very insightful, as more [00:20:30] editorial commentary.

I would say that taking this to heart by the spring of 1995, Marty Markowitz and I, and Alan, we thought we had to test the concept. We launched three studies simultaneously. Each of the study used a different protease inhibitor. There was indinavir [00:21:00] done with Emilio Emini, who's in the audience. There was nelfinavir done with the company Agouron, which I'm sure it has been absorbed by one entity or another. And then Abbott with ritonavir, which John Mellors and colleagues. It was quite remarkable about a month or two into the study. In all three studies, it became clear that we were controlling the virus well, in a durable fashion, [00:21:30] that many of the sick patients actually became functional, and some even went back to work. We knew that a dramatic turning point had been reached, but we wanted to see if this effect was durable, not just for a few weeks or a few months, we wanted to see if that could last a year.

We waited till Spring of 1996, and it became very clear, that for [00:22:00] almost all the subjects who could stay on the medicines, this is exactly what we saw. (8) The viral load becoming undetectable and staying so for approximately one year. I was invited to present at the international conference in Vancouver in 1996 when these results were first reported, and these results were then wrapped into a viral dynamics paper in the subsequent year. [00:22:30] Since this is more the session focused on history, it's certainly for those of us who started looking at AIDS as a automatic death sentence, this was a remarkable turning point. It's rather unbelievable that 70 million or more individuals are now on ARV.

Other than the [00:23:00] clinical success, we focused on the viral dynamics, which was our fundamental interest. Of course, in a subset of these patients, particularly those who were treated with nelfinavir combination, we were able to build in a number of studies on dynamics. We, in particular, tried to characterize in more detail, more [00:23:30] precisely the second phase decay. The second phase half-life range from one to four weeks, with an average of about two weeks. We were able also to measure better the first-phase decay done not with a single drug, but with multiple drugs, assuming that we would approach 100% efficacy much more that way. [00:24:00] 

You can see from this study that we're really looking at two compartments decaying simultaneously. There's this compartment that's just decaying with a half-life of one to two days, and then once it dips below this level, the decay is dominated by the other compartment which is decaying more slowly. Then that compartment if you project this [00:24:30] back to here, constituted about 5% of the total viral load, or about 1% of the viral load. 

So we were able to say that this process that we have understood accounted for most of the virus, but there is another population or populations that accounted for a minor fraction of the viral load that we measure in our patients. In that paper, we had hints [00:25:00] that monocyte macrophages may fit that role, but the precise identity of those cell populations remain unknown as far as I'm concerned.

As we were reporting these results, we recognized that if you shut down virus production, particles would be essentially all gone in a day. Infected CD4 T cells would be all gone in [00:25:30] a matter of weeks. And then this population, assuming a large reservoir size, would be all gone in a matter of a year or two, few years. It was conceivable that a cure could be achieved, but the last paragraph of our paper, we pointed out that residual virus replication could still be an issue, [00:26:00] sanctuary sites be an issue, and additional compartments that have yet to be studied could remain an issue. That read like an introduction for Bob Siliciano paper that unbeknownst to us, Nature had published back-to-back, and he described this latent reservoir infected CD4 resting memory T cells. (9) I'm sure Bob [00:26:30] will amply cover that subject.

In the subsequent period, we went on to say, "Let's see if we could define this a bit more precisely, and this even more precisely." Marty Markowitz undertook another study in patients, treating them with, in this case, four different drugs shutting down the virus load, and deriving a half-life that was in the order [00:27:00] of 0.7 day or lifespan of a year. (10) We then said, "Let's do the viral clearance more precisely," and we carried out an interventional plasma of apheresis study. (11) This is before and after the intervention production equals clearance. And during the intervention, when we take blood out, return the cells and the fluid, but take away [00:27:30] the large proteins and particular matter, you see that production will not change in the short-term, clearance were not changed in the short-term, but the added clearance reflected by ε epsilon is something we control, so this is we can control, this we can measure, and there's only one unknown to solve. In fact, that's what happens, and we could solve any. For these patients, we show that the virion [00:28:00] half-life is in the order of 30 minutes. At the same time, two of the patients who were coinfected with HCV, and the virion half-life is two to three times slower for that virus.

This is the conclusion, that all the viral dynamics studies that we've done now, with the subsequent refinements, allow us to show that the lifespan of a productive infected T cell is about a day, with the eclipse phase being a day. That [00:28:30] comes to 180 generations per year. The half-life is 30 minutes. And up to trillions of particles are produced in the blood of the untreated infected patient. (2, 3, 4, 8, 10, 11)

I would need another half an hour to talk about all the lymphocyte dynamics, but I will tell you that we carried out two studies that consistently show that there's a 6 to 10 fold increase in [00:29:00] not only CD4 T cell turnover, but CD8 T cell turnover in SIV infection or HIV infection, and that this process had to be halted as soon as possible. (12, 13) It took 20 years for the treat-early concept to be adopted, only following the report of the results of the SMART study (2002–2006).

[00:29:30] I'd like to make sure I thank my colleague, Marty Markowitz who carried out most of the clinical studies that I described, and of course, our chair, Alan Perelson for not only his modeling and analysis but his insights and his engaging discussions. I like to recognize Marty as an exemplary mentor, [00:30:00] Bob Gallo for allowing me to learn some lab secrets early on, David Baltimore for years of inspiration and for serving as chair of SAB, George Shaw for lots of friendly competition and free exchange, John Coffin for engaging conversations on dynamics and to our founder, Irene Diamond, for entrusting the Aaron Diamond Age Research Center to me [00:30:30] when I still had lots to learn. Thank you very much.

Alan Perelson (Moderator): Well, thank you, David. [applause] Well, we're running a little bit late, maybe just one burning question if anyone has it? David was so clear. Sharon.

Sharon LewinThank you, David. Brilliant, very clear and amazing insights at that time. How do you think we can use [00:31:00] those viral dynamics to now understand more about the reservoir as we measure? We've got more things to measure, [unintelligible 00:31:09] viremia, cell-associated RNA and DNA. How can we use viral dynamics in that setting now?

David: Something I've been thinking a lot about, but I don't have clear answers. Obviously, that compartment is behaving quite differently from the compartments we've been studying, with much, much slower decay [00:31:30] and also, we need more assays that are much more precise and accurate in order to do the math that have been done.

John CoffinWe're coming back to this later on probably, but the slow decay of the viral reservoir is not like the decay in patients. It's really a statistical average and in some patients, in fact, it's going up. It's [00:32:00]important to keep that in mind.

David: Yes, and when you show those decays, you need to show the error bars, and the error bars suggests that there's a lot of variation.

Alan: There's perforation going on in that compartment, unlike what's going on with drugs where we're stopping everything, it's a balance between proliferation and loss from various mechanisms.

[00:32:19] [END OF AUDIO]


  1. Ho, David D., Tarsem Moudgil, and Masud Alam. “Quantitation of Human Immunodeficiency Virus Type 1 in the Blood of Infected Persons.” New England Journal of Medicine 321, no. 24 (December 14, 1989): 1621–25. doi:10.1056/NEJM198912143212401.
  2. Ho, David D., Avidan U. Neumann, Alan S. Perelson, Wen Chen, John M. Leonard, and Martin Markowitz. “Rapid Turnover of Plasma Virions and CD4 Lymphocytes in HIV-1 Infection.” Nature 373, no. 6510 (January 12, 1995): 123–26. doi:10.1038/373123a0.
  3. Wei, Xiping, Sajal K. Ghosh, Maria E. Taylor, Victoria A. Johnson, Emilio A. Emini, Paul Deutsch, Jeffrey D. Lifson, et al. “Viral Dynamics in Human Immunodeficiency Virus Type 1 Infection.” Nature 373, no. 6510 (January 12, 1995): 117–22. doi:10.1038/373117a0.
  4. Perelson, Alan S., Avidan U. Neumann, Martin Markowitz, John M. Leonard, and David D. Ho. “HIV-1 Dynamics in Vivo: Virion Clearance Rate, Infected Cell Life-Span, and Viral Generation Time.” Science 271, no. 5255 (March 15, 1996): 1582–86. doi:10.1126/science.271.5255.1582.
  5. Mansky, Louis M., and Howard M. Temin. “Lower in Vivo Mutation Rate of Human Immunodeficiency Virus Type 1 than That Predicted from the Fidelity of Purified Reverse Transcriptase.” Journal of Virology 69, no. 8 (August 1, 1995): 5087–94.
  6. Ho, David D. “Time to Hit HIV, Early and Hard.” New England Journal of Medicine 333, no. 7 (August 17, 1995): 450–51. doi:10.1056/NEJM199508173330710.
  7. Coffin, John M. “HIV Population Dynamics In Vivo: Implications for Genetic Variation, Pathogenesis, and Therapy.” Science 267, no. 5197 (January 27, 1995): 483–89. doi:10.1126/science.7824947.
  8. Perelson, Alan S., Paulina Essunger, Yunzhen Cao, Mika Vesanen, Arlene Hurley, Kalle Saksela, Martin Markowitz, and David D. Ho. “Decay Characteristics of HIV-1-Infected Compartments during Combination Therapy.” Nature 387, no. 6629 (May 8, 1997): 188–91. doi:10.1038/387188a0.
  9. Chun, Tae-Wook, Lucy Carruth, Diana Finzi, Xuefei Shen, Joseph A. DiGiuseppe, Harry Taylor, Monika Hermankova, et al. “Quantification of Latent Tissue Reservoirs and Total Body Viral Load in HIV-1 Infection.” Nature 387, no. 6629 (May 8, 1997): 183–88. doi:10.1038/387183a0.
  10. Markowitz, Martin, Michael Louie, Arlene Hurley, Eugene Sun, Michele Di Mascio, Alan S. Perelson, and David D. Ho. “A Novel Antiviral Intervention Results in More Accurate Assessment of Human Immunodeficiency Virus Type 1 Replication Dynamics and T-Cell Decay In Vivo.” Journal of Virology 77, no. 8 (April 15, 2003): 5037–38. doi:10.1128/JVI.77.8.5037-5038.2003.
  11. Ramratnam, Bharat, Sebastian Bonhoeffer, James Binley, Arlene Hurley, Linqi Zhang, John E Mittler, M Markowitz, John P Moore, Alan S Perelson, and David D Ho. “Rapid Production and Clearance of HIV-1 and Hepatitis C Virus Assessed by Large Volume Plasma Apheresis.” The Lancet 354, no. 9192 (November 20, 1999): 1782–85. doi:10.1016/S0140-6736(99)02035-8.
  12. Mohri, Hiroshi, Sebastian Bonhoeffer, Simon Monard, Alan S. Perelson, and David D. Ho. “Rapid Turnover of T Lymphocytes in SIV-Infected Rhesus Macaques.” Science 279, no. 5354 (February 20, 1998): 1223–27. doi:10.1126/science.279.5354.1223.
  13. Mohri, Hiroshi, Alan S. Perelson, Keith Tung, Ruy M. Ribeiro, Bharat Ramratnam, Martin Markowitz, Rhonda Kost, et al. “Increased Turnover of T Lymphocytes in HIV-1 Infection and Its Reduction by Antiretroviral Therapy.” Journal of Experimental Medicine 194, no. 9 (October 29, 2001): 1277–88. doi:10.1084/jem.194.9.1277.




Found 16 search result(s) for "David Ho".

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Page: 8.1 John Mellors — MACS and Beyond: Epidemiology, Viremia and Pathogenesis (HIV/AIDS Research: Its History & Future Meeting)
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...  4.3 Beatrice Hahn — Apes to Humans: The Origin of HIV  8.2 David Ho — Unraveling of HIV Dynamics In Vivo  Auerbach, David  Belgium  blood — banks, donors, plasma, screening, transfusions, clotting ...
Apr 27, 2021
Page: 6.2 Dennis Burton — How Does HIV Evade the Antibody Response? (HIV/AIDS Research: Its History & Future Meeting)
... 8.1 John Mellors — MACS and Beyond: Epidemiology, Viremia and Pathogenesis 8.2 David Ho — Unraveling of HIV Dynamics In Vivo Alt, Frederick W. antibody ... ...
Apr 27, 2021

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