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The Body Covers: The 15th Conference on Retroviruses and Opportunistic Infections
Peptide in Human Semen May Enhance Infectiousness of HIV

February 5, 2008

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Frank Kirchhoff: It's long been known that compounds in the human body, in tissues or in fluids, sometimes have the ability to inhibit the virus. But it's obvious that, normally, tissues or body fluids do not come in large quantities. Our collaborators, particularly Professor [Wolf-Georg] Forssmann from Hannover [Medical School], had the great idea to use hemofiltrate, which contains all the peptides that are present in the blood but becomes available in quantities up to 20,000 liters. This allowed us to establish methods to identify and isolate, out of a mixture of an estimated 1 million natural compounds, those that really affect HIV.2 It took us quite a while to get the method going.

Frank Kirchhoff, M.D., Ph.D.
Frank Kirchhoff, M.D., Ph.D.
As Beatrice [Hahn] mentioned [in her introduction], we have actually found one inhibitor. I think the interesting thing about this inhibitor is that it has a new target on HIV, and that the target is also available on other viruses. I think, although it's quite unclear at the moment whether what we found will work in the clinic, it will create opportunities for additional antivirals.

Now in the second study, the methods were improved so much that we could basically scale it down and analyze the semen-derived peptide library.1 We used semen because this is the most relevant fluid for sexual transmission of HIV: About 60% of all [HIV] transmission occurs by contaminated semen.3,4 To our surprise, we did not find a single inhibitor. But we found a peptide that forms so-called "fibrils" -- basically, little charged rods that can bind the virus's variants and facilitate their attachment to the cells to greatly enhance [HIV] infection.

We need to do further studies to really obtain proof that this is relevant for sexual transmission [of HIV] in vivo. But all the data from cell cultures, tissue cultures and these studies do strongly suggest that this may well be highly relevant for transmission, and may also provide a new target to reduce the rate of transmission.

Reporter #1: If I'm [understanding you correctly], the target would be in the host.

Frank Kirchhoff: Yes.

Reporter #1: And what is it called?

Frank Kirchhoff: It's a protein called prostatic acid phosphatase [PAP] that is secreted from the prostate gland. It's present in relatively large quantities in the semen, and for that reason it has been used as a semen marker.

Reporter #2: In your presentation this morning you showed that you could modify alpha-1 antitrypsin to get a small peptide, and modify it to enhance its possibility to block the virus. Did you, or are you going to, do the same manipulation with the PAP? Is it possible to imagine having something which could counteract the PAP's actions?

Frank Kirchhoff: This is very actively ongoing research. This is actually done in collaboration with an American group, with Warner Greene from the Gladstone Institute. It seems that there are ways to block both: the formation of the fibrils and their enhancing activity. But this is just in the test tube. It's very difficult to do this with semen. We are trying. Whether it's going to be possible in real life, we don't know. It certainly works in the test tube.

Reporter #3: I think you were speculating that perhaps it plays a role in fertility, [which would explain] why it's present naturally? Is that speculative?

Frank Kirchhoff: Yes. During fertilization, you have membrane interaction and fusion. This is what this molecule seems to be doing, so I think it's straightforward, but it's pure speculation at this point.

Reporter #3: If you developed inhibitors, might they be used in, say, a microbicide?

Frank Kirchhoff: No, what I said was that we found that the presence in semen of this inhibitor [PAP] affects the efficacy of microbicides and antiretrovirals. The problem is that, normally, when you test the efficiency of microbicides you do it in a normal medium. In our hands, when we had this enhancer present, it's basically the same effect as if you have, like, 10 or 24 [times] more virus. So you need much higher concentrations of microbicides or inhibitors to get the same effect. And I think this has to be taken into account when you evaluate these things.

Beatrice Hahn: But I think the question was, could that be a target for new microbicides?

Frank Kirchhoff: I'm sorry, yes. I definitely think that it is. And we are working on this, but we don't really know if it's going to be feasible at this point.

Reporter #5: When ejaculation occurs, semenogelin sort of clots up everything in there. Do you have a sense as to how much of this molecule is free at this point -- in a real-world, practical situation, as opposed to in a test tube? How do those dynamics change? Do you have any sense of that?

Frank Kirchhoff: Actually, we're also very new in this field, and I'm a virologist. The shortest time that my people waited to test these things was half an hour because, before [that time], it was simply not possible to pipette the semen, because it needs to liquefy to some extent. What we do know, though, is that -- at least from the earliest point -- you already have a substantial enhancing activity. When you compare it with [unintelligible] -- because the fibrils are larger than most compounds -- then we see that there is a constant formation, reformation, probably because the precursor is cleaved again, and you have, once again, these things. So I think that even agents that do not work in the first milieu -- in the first and second -- may still have a beneficial effect. On the other hand, I think it may be easier to block the enhancing activity of the fibrils or aggregates than to block their formation. But we are pursuing both possibilities.

Reporter #6: How many individuals have you tested -- human individuals? Could there be possible variations between people?

Frank Kirchhoff: You mean tested with regard to the effect of semen on HIV infection?

Reporter #6: I mean on the presence of fibrils, for example.

Frank Kirchhoff: The thing is that you can test, you control the semen enhancers -- and we have done some experiments to detect the peptides and the enhancement of infection -- [but] you cannot really detect large fibrils in semen. But it's the same: Even when we add the synthetic peptide to semen, we get an enhancement of the enhancing effect. But it seems that the fibrils that form in the solution -- basically take it the other way around -- that we don't see these large fibrils in the semen. So we think smaller aggregates form, which is actually well known in the [unintelligible] field. When you have a complex mixture of things, it's similar to crystallization; you don't really get these nice big aggregates, but smaller ones.

You also asked if there is variation. We don't really know yet. We started to [study] this, but it's not very easy to quantify the peptides in the semen, particularly since there are different length variants that they all enhanced. So it's easier for us just to test the overall effect on infection. So far, it always enhanced -- in our hands -- and we tested, I think, maybe 10 or 15 people so far.

Reporter #7: I had a question about the entry inhibitor. I was wondering whether you've had any discussions, or there has been any expressed interest on the part of the pharmaceutical industry, for developing that compound, or a small-molecule analog of that compound?

Frank Kirchhoff: Yes, I think it would be extremely interesting for us. I presented the data to a couple of biotech companies and they are interested, but it's not really something that is going to be done [soon]. Our strategy at the moment is to try to get some proof-of-concept evidence with the peptide in Phase 1, Phase 2 study, and then hope to motivate a company to develop this further, because I would agree that small-molecule inhibitors targeting the fusion peptide would have major advantages compared to peptides.

Reporter #2: Regarding this same type of question, did you test the small inhibitor on other viruses which are responsible for chronic infections in other diseases -- like hepatitis, for instance? Broadening the spectrum of the market, if there is a market for these drugs one day. I think that "big pharma" and even biotechs [biotechnology companies] are maybe a little bit shy this year, restricted back, especially in poor countries where there is no money. They may be more interested if you can show the same mechanism in a hepatitis virus or herpes virus.

Frank Kirchhoff: I agree. The inhibitor that we pulled out is highly specific for HIV-1. We started experiments on hepatitis C virus [HCV] and herpes simplex virus type 2. For HCV we have hits, but it's not always easy to purify the active compounds and we don't really know what they are targeting yet.

We tried to get money to screen for small-molecule inhibitors of other fusion peptides, because we have a good assay. Unfortunately, we did not get funding from the European Union. It was very competitive. There were only about 200 groups funded -- this was for the [European] Young Investigator Award. I didn't like their comments: We [initially] proposed to improve the inhibitor that we now have and to make a small-molecule inhibitor. They said that this is not really innovative. So we said, we are going to try to use this approach to inhibit the activity of other fusion peptides. They said that this is very risky. So in the same project, proof of concept doesn't help us, but we go immediately from not so interesting to too risky. But we will try to do this.

Reporter #8: As a means of expanding research in this area, could you promote it as a male contraceptive?

Beatrice Hahn: She's asking whether targeting the SEVI [semen-derived enhancer of virus infection] factor could potentially lead to contraception if, indeed, the fibrils increase fertility. The logical conclusion would be if you do something about it, fertility would go down.

Frank Kirchhoff: I'm a person who believes very much in experiments. I think it should be evaluated [to see] if it plays a role, and then one can discuss these issues. I think it's far too early to really say anything about this, because we don't even know if it plays a role. But I think in many cases, it's also possible to dissect different activities. Maybe it's possible in the long run -- I'm just speculating here -- to somehow inhibit the enhancing activity on HIV, but not other functions. This is commonly done with other approaches. It just takes a lot of research and time.

This transcript has been lightly edited for clarity.


  1. Münch J, Standker L, Forssmann WG, Kirchhoff F. Natural enhancers and inhibitors of HIV infection. In: Program and abstracts of the 15th Conference on Retroviruses and Opportunistic Infections; February 3-6, 2008; Boston, Mass. Abstract 66.
  2. Münch J, Ständker L, Adermann K, et al. Discovery and optimization of a natural HIV-1 entry inhibitor targeting the gp41 fusion peptide. Cell. April 20, 2007;129(2):263-275.
  3. Haase AT. Perils at mucosal front lines for HIV and SIV and their hosts. Nat Rev Immunol. October 2005;5(10):783-792.
  4. Royce RA, Sena A, Cates Jr. W Cohen MS. Sexual transmission of HIV. N Engl J Med. April 10, 1997;336(15):1072-1078.

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