"In the future, we'll be able to control HIV without taking drugs every day"

Frank Kirchhoff, winner of Germany's Leibniz Prize, talked to Agência FAPESP about promising strategies to combat the virus responsible for the AIDS pandemic.

By Karina Toledo

Agência FAPESP – Several subtypes of the human immunodeficiency virus (HIV) can cause acquired immune deficiency syndrome (AIDS), but scientific evidence suggests that the AIDS pandemic that currently affects some 35 million people worldwide is caused mainly by one specific subtype, HIV-1 M.

Understanding the evolutionary mechanism that has made this lineage so much more successful than others is the goal of Professor Frank Kirchhoff, Director of the Institute of Molecular Virology at the Ulm University Medical Center in Germany. For more than two decades, he has been comparing HIV with similar viruses found in apes, from which HIV originated.

For this research, Kirchhoff won the 2009 Gottfried Wilhelm Leibniz Prize, awarded by the German Research Foundation (DFG). Considered equivalent to Nobel prizes, ten Leibniz prizes are awarded each year. Each winner receives up to €2.5 million to be spent on research within seven years.

Kirchhoff presented some of his main research findings at FAPESP’s headquarters in São Paulo, Brazil, on August 17. The presentation was entitled “Why was HIV-1 able to cause the AIDS pandemic?” FAPESP has a long-running cooperation agreement with the DFG.

According to Kirchhoff, the origins of HIV-1 date back some 10,000 years, when the virus developed in the chimpanzees of southern Cameroon, Central Africa, through the recombination of simian viruses transmitted by small monkeys such as those of the genus Cercopithecus.

From chimpanzees, the virus jumped to gorillas and later to humans. The differences in the geographic distribution of HIV-1 groups M, N, O and P on the African continent are due to the distinct independent processes of adaptation to the human host. The relatively uncommon HIV-2 strain is concentrated in West Africa and is rarely found elsewhere.

The first case of HIV-1 transmission to humans is believed to have occurred in 1920 in the Congo region, possibly to bushmeat hunters who came into contact with contaminated blood. Group M was probably transmitted for the first time around 1940.

HIV-1 P has been detected in only two individuals, and HIV-1 N has been detected in about ten. In the case of HIV-1 O, there have been thousands of infections detected, but infection by this group has rarely been detected outside Central Africa. According to Kirchhoff, HIV-1 M is the only subtype that has become pandemic because it is the only one to succeed in defeating all of the human organism’s natural antiviral defense mechanisms.

In an interview given to Agência FAPESP, Kirchhoff explained how human antiviral barriers work and described how HIV-1 M became resistant to them. He also spoke about potential and ongoing applications of knowledge from this and other research projects to new strategies to control HIV.
 

Agência FAPESP – Why has only HIV-1 M become pandemic?
Frank Kirchhoff – During millions of years of interacting with various retroviruses, humans developed antiviral defense mechanisms. The problem with HIV-1 is that it became resistant to these mechanisms, as well as to some drugs. Basically, the success of HIV-1 M is due to the fact that it’s the only strain to have developed factors capable of shutting down all human defenses against itself. It’s able to avoid or defeat whatever interferes with it and all the mechanisms that prevent intracellular infection.

Agência FAPESP – Could you give examples?
Kirchhoff – One of the human antiviral factors is the protein TRIM5α (tripartite motif-containing protein 5 alpha), which normally binds to the viral capsid and degrades it. But TRIM5α is no longer able to bind to HIV-1. It’s still effective against SIV [simian immunodeficiency virus], but no longer against the human virus. Another important viral restriction factor is the protein APOBEC3G, which attacks the virus’s genome and introduces so many mutations that it [the virus] ceases to be infectious. But HIV-1 produces a protein called VIF [virion infectivity factor], which is able to degrade APOBEC3G, thereby drastically increasing the virus’s pathogenicity. HIV-1 also produces VPU [viral protein U], which binds to the human protein tetherin and induces its degradation by the proteasome [the complex responsible for cleansing cells of damaged proteins]. Tetherin prevents the virus from leaving the cell to infect new cells once it has used the cellular machinery to replicate. Group M viruses are the only ones that are able to deactivate tetherin efficiently.

Agência FAPESP – The other HIV-1 groups continue to evolve. Do you think they may become as dangerous as group M some day?
Kirchhoff – They’re already relatively pathogenic. The main difference isn’t their pathogenicity but their ability to spread. The non-pandemic lineages are already able to deactivate APOBEC3G and TRIM5α, but they’re less effective against tetherin. And we believe it’s precisely tetherin that prevents release of the virus into body fluids such as semen or vaginal liquid. In the case of these non-pandemic viruses, tetherin may still be capable of preventing transmission in an efficient manner. However, this hypothesis is based only on in vitro experiments with cell cultures. It hasn’t yet been confirmed in apes or humans, but I think it’s a very plausible hypothesis.

Agência FAPESP – Could knowledge of the role of these viral and antiviral factors translate into new therapies or prevention strategies?
Kirchhoff – It’s hard to say. In theory, there are many ways of using this knowledge, but for ethical reasons, it’s difficult to implement these strategies in humans. For example, we could genetically modify antiviral factors so as to reactivate them against HIV-1, but that would mean manipulating human immune cells. We could manipulate human TRIM5α or APOBEC3G. That would be technically feasible, but it’s hard to assess the risks. It might affect not just the specific target but also secondary targets. This needs to be very well evaluated in animal models and other systems before we know whether it’s a safe approach. An experiment has been performed in which the TRIM5α protein from apes was placed in cats, which became completely resistant to feline immunodeficiency virus (FIV). But this is a very new technology, and it’s hard to foresee whether it could be used in humans.

Agência FAPESP – But could viral proteins like VIF or VPU be targets for new drugs?
Kirchhoff – They are targets, but from the pharmacological standpoint, there are better targets. I don’t think inhibiting the virus is the problem at the moment. Protease and integrase inhibitors do this job very well. If you block protease, you kill the virus completely. If you block VPU, you destroy 90% [of the virus], so it’s not such a good target.

Agência FAPESP – In an article published in 2010 in the journal Science Translational Medicine, you described a peptide found in human blood that was promising against HIV. Could you describe the research and the stage reached so far?
Kirchhoff – We began with the hypothesis that human blood might contain agents capable of affecting HIV. We then embarked on a screening program in which we tested all the small peptides or small blood proteins. We found a very small fragment of a protein called alpha 1-antitrypsin present in large quantities and observed that it blocks virus entry into the cell. For invasion to occur, a protein found in the viral envelope has to penetrate the cell membrane and function as an anchoring mechanism. The peptide we identified is able to bind to this viral protein and prevent it from penetrating the cell membrane. The original peptide wasn’t very active, but we made a few modifications and produced derivatives that are 400 times more potent. In collaboration with other groups, we performed phase I clinical trials and succeeded in reducing viral load by more than 90%. However, the problem is that because it’s a peptide, it has to be administered intravenously, and relatively large amounts are required for treatment. It was well tolerated and there was no cross-resistance with other compounds, but it’s a very expensive treatment. Not being able to administer it orally is a major drawback.

Agência FAPESP – Are you still working on enhancement of the compound?
Kirchhoff – Yes. We’re currently trying to create modified forms with greater stability. In collaboration with other groups, we’re endeavoring to package the drug in nanoparticles. In its present form, it’s not suitable as medication, but I believe it has advantages that justify investment. First, it has a different mechanism from all other drugs now used against HIV, and a problem of resistance is unlikely. Second, although there are effective anti-HIV drugs, there are still many other viruses that are pathogenic for humans and use similar anchoring mechanisms. If we can find peptides with a similar action to alpha 1-antitrypsin, they could be used to combat other diseases.

Agência FAPESP – If you succeed in creating this drug against HIV, would the idea be to use it in conjunction with today’s prescription cocktail?
Kirchhoff – Yes, I believe the best strategy is always combined use rather than monotherapy. The virus’s resistance to drugs remains a major challenge. There aren’t many different classes, and sometimes the patient is infected by a multi-resistant virus. In these cases, this drug would be an effective weapon.

Agência FAPESP – Your group has also described in the journal Cell a factor found in human semen that favors infection. Could you explain the mechanism involved?
Kirchhoff – Sexual transmission of HIV isn’t always efficient, so we imagined there might be inhibitory factors in semen. What we did was separate all the small proteins in semen to see how each one affects the infectious capacity of HIV. To our surprise, we failed to find any inhibitors. Instead, we found a peptide that increases infectious capacity. It’s a fragment of prostatic acid phosphatase (PAP), a protein that’s secreted by the prostate gland. These fragments form small amyloid fibers with a positive charge. The virus normally has a negative charge. Because the cell surface also has a negative charge, it usually repels the virus. So it’s really difficult for the virus to bind to cells. These fibers facilitate the process. They can increase infectious capacity by a factor of 100,000.

Agência FAPESP – Could this discovery pave the way to some method for preventing transmission?
Kirchhoff – Yes. An unexpected outcome is that we currently use similar molecules in gene therapy because in these cases, we want to facilitate cell infection by retroviruses. We’ve also developed agents capable of blocking this interaction between semen amyloid fibers and HIV. We thought this inhibition might increase the effectiveness of a microbicidal approach [using compounds that protect the individual against several STIs, including HIV, when administered vaginally or rectally] because so far microbicides haven’t been very successful. One of the reasons they don’t work well may be the presence of these fibers. To date, all the tests performed with these inhibitors have been carried out in vitro in cellular models. Studies with animals are very costly, and it’s hard to obtain funding in Germany.

Agência FAPESP – Do you believe that humans will win the battle against HIV some day?
Kirchhoff – A great deal of progress has already been made. The number of new AIDS cases and HIV infections is falling because more and more people are receiving treatment, even in Africa. I think that this delays the advance of the virus. I can’t say with certainty whether we’ll ever eradicate HIV infection, but I believe that one day, we’ll be able to control the virus without the patient needing to take drugs every day. We’re already seeing the use of long-lasting drugs that can be taken at intervals of months. This will be very important in Africa, where many people are unable to go to clinics frequently.

Agência FAPESP – During your presentation at FAPESP, you said developing more effective therapies is more important than developing a vaccine for control of the virus. Why?
Kirchhoff – With the right treatment, viral load remains very low, and these individuals are unlikely to transmit the virus to anyone. In my view, alongside the use of condoms, this is currently the most efficient way to avoid HIV transmission. If all carriers are treated, they won’t transmit the virus, and this will be more effective than any foreseeable vaccine.