Posted on November 6th, 2010 No comments
HIV infection in HIV antibody negative individuals
There is another post on this topic: HIV exposed individuals who are seronegative.
The possibility that there are individuals who are infected with HIV but who are negative on the test for HIV antibodies has always been theoretically possible. Considerable evidence has accumulated for many years that there are indeed such individuals. Despite the importance of this phenomenon, it receives relatively little comment.
It sometimes seemed to me ever since I first tried to discuss this possibility in the mid 1980s that there was a wilful discouragement of any discussion of this topic.
In 1989 David Imagawa reported that that 31 of 133 HIV antibody negative men showed the presence of HIV.
In 27 of them this persisted for 36 months despite remaining seronegative (1). This resulted in a vigorous response culminating in what almost looked like a retraction by the authors. At that time many unsuccessful attempts to replicate these results were reported, and the findings of David Imagawa were generally presented as due to technical errors, such as incorrect specimen labelling. In view of many subsequent findings, the likelihood is that David Imagawa and his colleagues were correct. The furious response to Imagawa’s paper is an indication of how non rational considerations can influence the progress of science. This is of course nothing new.
Curiously in a recent book, Imagawa’s findings are included in a list of what are stated to be errors and controversies in the HIV/AIDS epidemic that impeded scientific progress (2).
What in fact impeded progress was a rigid adherence to what was only a hypothetical, not an empirical model of the course of HIV infection.
David Imagawa died shortly after this controversy, and sadly did not live to see that his initial conclusions were absolutely consistent with what has been learned of the complexity and diversity of individual responses to HIV infection.
I certainly experienced considerable resistance and disbelief when I raised the possibility of silent HIV infections. In the late 1980s I took part in a NPR program, and was quite abruptly dismissed by another scientist (I have forgotten who) when I raised the absolutely reasonable theoretical possibility of persistent latent infections in antibody negative individuals.
Apart from very few exceptions there was an almost complete lack of interest in HIV seronegative, but infected individuals, by science writers; there was no shortage of community commentators who also seemed to be oblivious or uncaring of this phenomenon.
To be sure there were occasional reports of seronegative but infected individuals. Gus Cairns, a UK journalist wrote about this in the UK magazine, Positive nation. I wrote something about this as a result of an interview with him in 2000, which he published. I have scanned the article. I was unable to make a perfect copy, but a legible version can be seen by clicking HERE.
In the US reports confirming the existence of seronegative infected people continued to receive very little comment; what little there being generally quite hostile..
Today this issue was again brought to my attention by an article I saw reporting the presence of HIV proteins and HIV RNA in cervical biopsies from women who were persistently HIV seronegative negative, at least for the duration of the study which was one year. They did not have antibodies to HIV despite being infected ; of course it is possible that they are in an unusually long “window” period and will eventually seroconvert.
I expect that, as is usual this report will provoke little or absolutely no interest.
But it is enormously interesting; (just one of many questions: can these women infect their male partners?) Seeing this article is the reason why I decided to make this issue the subject of this post.
It was no great surprise when evidence appeared that there were some individuals who were HIV infected but remained negative on the HIV antibody test. It must be said that there were probably more papers in the early years in which silent HIV infections in HIV antibody negative individuals was not observed. In another approach, reports started to appear that HIV antibody negative individuals had T lymphocyte responses to HIV which means that they were exposed to the virus, not necessarily that they were infected – although that is quite a real possibility. Some early papers, before 2000, including those showing T cell responses can be seen by clicking HERE. There was quite an extensive literature at that time, but most, as mentioned reported that there was no such thing as a silent antibody negative infection, apart from the short window period following infection.
Why has the possibility of prolonged latency always been theoretically possible?
As part of its life cycle HIV is turned into DNA and is then incorporated into the host genome. In infected cells it effectively becomes part of our genetic material. Once inserted into human DNA, it must be activated to start the process of making new virus particles. Cellular signals that start the process of activating HIV DNA include cytokines, which are messenger molecules produced and released by cells, which can then act on other cells to evoke a variety of responses. Amongst these HIV activating cytokines are those that are produced during inflammation. Once HIV DNA is activated, and at least some of its proteins made, these then mediate further activation.
There are some other factors that can activate HIV DNA.
Alloantigens are antigens expressed on foreign cells. When these antigens are in contact with a cell containing integrated HIV DNA, activation occurs; HIV DNA is transcribed and new viral particles made. In earlier days HIV was isolated from infected lymphocytes in this way. Latently Infected lymphocytes were induced to produce HIV by culturing them together with lymphocytes from an uninfected donor.
It is the nature of HIV infection that it is frequently acquired in situations which involve exposure to foreign cells (to alloantigens). This may be exposure to semen in sexual transmission, or blood cells in the case of infection by shared needles, or by blood transfusion.
Herpes viruses have the ability to activate HIV if a cell is infected with both viruses. I suppose this must happen but I imagine doubly infected cells may not be too frequent. Of course active herpetic infections in non HIV infected cells may be associated with the production of pro inflammatory cytokines, which circulate and can activate HIV DNA at a distance.
There is absolutely no reason not to expect that in some circumstances incorporation of HIV DNA into human DNA will result in a state of stable integration. This means that HIV DNA remains in the genome, it is not activated, and no virus is produced. Since antibodies are made as a response to viral proteins, and as none are made, the HIV antibody test will be negative.
So it was no surprise when such individuals were again reported in 1999 . These individuals remained in good health and were reported to be antibody negative as long as they were observed.
We cannot know if these individuals may seroconvert (or maybe already have), but what is established is that stable integration of HIV DNA without seroconversion can occur. In such individuals limited expression of HIV can occur, at least sufficient to induce, if not antibodies, a cellular immune response.
The presence of such cellular immune responses in HIV antibody negative individuals is further evidence consistent with HIV DNA persistence, but in itself does not indicate this.
Demonstration of cell mediated immunity to HIV:
Apart from the identification of antibodies, specific immunity to HIV can also be detected by a much more elaborate test that measures cellular immunity rather than immunity determined by detecting specific anti HIV antibodies. In this case what is measured is the ability of lymphocytes to recognize HIV. They will do so only if they have been exposed to the virus, which would obviously be the case if they were taken from an infected individual.
The detection of such lymphocyte responses in the mid 1990s was one of the first indications that there may be infected people who don’t make antibodies. Other interpretations are that the infection was overcome, or that that the individual was infected with defective virus.
Gene Shearer was i believe the first to report this phenomenon. HIV antibody negative sexual partners of HIV positive people, as well as individuals who had occupational contact with HIV were among those showing these responses.
It is unknown how widespread this phenomenon of silent HIV infection is. It may be exceedingly rare. It is also unknown if this condition of stable integration is really just a prolonged “window” period that always follows all HIV infections.
But It is entirely possible that there are individuals in whom the ability to control HIV is such that they will remain healthy and HIV negative.
A number of different possible outcomes of HIV infection are possible:
Some of the factors that influence this:
Host genetic factors.
Size of the inoculum – the amount of infecting virus.
Route of infection.
The particular virus strain.
The presence of associated systemic infections.
these provide signals activating HIV proviral DNA. In the case of some tropical infections there may be cytokines (IL 10) that blunt immune responses.
Sexually transmitted infections with genital ulcers.
Double infection of a cell with HIV and herpes viruses – probably an unusual occurrence.
Exposure to alloantigens; a theoretical possibility.
These are some of the known influences.
Maybe the most common outcome is a productive infection where viral DNA is activated within a few weeks.
But this scenario is also possible:
Infection is followed by insertion of HIV DNA into cellular nuclear DNA. Possibly with small inoculums, and in the absence of strong or sustained activation signals, the proviral DNA remains silent. This has been observed.
Or this one:
There is a limited burst of viral production, not sufficient to elicit an antibody response but enough to induce a cell mediated response with the generation of lymphocytes that recognize HIV antigens and can kill HIV infected cells. HIV seronegative individuals with such specific lymphocyte responses have certainly been observed. In this case if there is an incipient burst of HIV production, the producing cells are promptly killed. Each time this happens the cellular immune response is primed and strengthened. Such a mechanism has been well studied in EBV infections. This common virus is totally unlike HIV, but it does similar things. It remains present in B lymphocytes rather than T lymphocytes for life. The mechanism of persistence is quite different – EBV is not a retrovirus. But the majority of individuals carry this virus – which in rare situations can have lethal effects, in their B lymphocytes for life. We have evolved many mechanisms to keep this virus in check. The ability of some types of lymphocytes to kill EBV infected cells which start to make virus is well understood. Similar mechanisms must exist for HIV – but obviously for most are insufficiently effective. But in those with very limited HIV production these killer lymphocytes may actually be what allows such rare fortunate individuals to remain HIV seronegative.
With this outcome, one can view the infection as actually having an immunizing effect.
If there were not yet enough reason to study the phenomenon of persistently seronegative HIV infection, here is an important one. What are the circumstances that produce this outcome? Individuals who are seronegative but have lymphocyte responses to HIV are of great interest.
Yet another scenario is one of stable integration, but where some HIV proteins, but not complete virus, are produced. Maybe the women referred to whose cervical biopsies contained HIV antigens might be in this category. This is a strange situation as antigens were detected but these women apparently did not develop antibodies.
Shortly after HIV was discovered a description of the course of HIV infection was produced. Everyone interested in this disease will have seen this picture: Here it is again:
The life span after untreated infection was said to be 10 -11 years. But this period had not even passed since the start of the epidemic. This picture is a total construct, but presented as something that had empirical foundations.
Within 5-6 years of the start of the epidemic scientists confidently presented this conjectural picture as fact. It does not allow for the immense variation in the course of HIV disease and has I believe adversely affected research progress as well as had implications for treatment, where a one size fits all approach seems to be standard.
The uncritical acceptance of the above depiction of the course of HIV infection is particularly odd as not only was the disease new – we have no precedents of human retroviral diseases (apart from HTLV-1 associated disease, but the techniques used to study the disease were themselves new. The ability to identify T lymphocyte subsets with monoclonal antibodies is about as old as the HIV epidemic. So we had no idea then of the variation in T subset numbers in health and disease. Other immunological and virological techniques were being introduced as the epidemic was proceeding.
The validity of the very concept of a “standard” course of HIV infection is in question. HIV infection can progress in different ways. If we were more open to this there may greater interest and funding into research that investigates the various factors that influence how the disease progresses. HIV disease is in this sense like almost every other infectious disease. I could probably leave out the almost.
I have often wondered why there has been such resistance to not only the reasonable idea, but also to actual evidence that HIV disease may take a different course to that shown above.
It may be that most untreated infections follow such a course; we may never know now that we have treatments.
But we do know that there are other responses to HIV infection.
In conclusion, the study of prolonged HIV seronegativity in infected people is important. Some reasons are:
1. There are obvious implications for vaccine development.
2. Seroprevalence may significantly underestimate the prevalence of HIV infection.
3. Understanding the phenomenon will advance our understanding of the pathogenesis of this disease, which in turn will open new therapeutic approaches.
1. Imagawa, D.T., M.H. Lee. S.M Wolinsky. et al.. Human immunod-eficiency virus type 1 infection in homosexual men who remain seronegative for prolonged periods. New EnglandJournal ofMedicine 1989 320:1458-1462.
2. Scientific Errors and Controversies in the U.S. HIV/AIDS Epidemic: How They Slowed Advances and Were Resolved
By Scott D. Holmberg
Published by Greenwood Publishing Group, 2008