TAG HIV Basic Science, Vaccines, and Cure Project Blog

HIV and the Politics of Molecular Epidemiology - Haiti Study Makes Waves

Today has seen a flood of press stories about a new study which strongly suggests that HIV-1 subtype B migrated into the US from Central Africa by way of Haiti. The stories were instigated by a press release from the University of Arizona, publicizing a paper by Michael Worobey and colleagues which is apparently due for online publication in PNAS later this week. Rather awkwardly, the paper does not seem to be online as yet (UPDATE 10/31: the PNAS article is now online, although to the journal's discredit they have not made it open access. UPDATE 11/1: I guess I wasn't the only person who thought the paper should be open access, to their credit PNAS has now made the PDF available free of charge). Anyone interested in finding out more details about the study can watch Michael Worobey's presentation at CROI earlier this year; just go to the webcast page and then scroll down to the session entitled "Oral Abstracts: Epidemiology: Transmission Dynamics and Risk." Click on "Play Video" and Worobey's presentation is the first in the session.

Sadly, the study is re-igniting controversies regarding Haitians somehow being blamed for HIV (as happened, with horrible consequences, in the early 1980s). You can hear an angry response to Worobey's presentation by a Haitian researcher at the end of the CROI webcast, and Jean Pape is quoted in an article in Science by Jon Cohen saying that the study authors "restate prejudices advanced 2 decades ago." However, it seems clear that the researcher's interest is in using molecular techniques to track the history of HIV's spread, not in judging any person, nation or ethnicity. Some press stories have highlighted the suggestion that one transmission event from Haiti around 1969 triggered the US epidemic, and made the leap that - if true - the individual in question must have been Haitian.  But the study makes no such implication; only that the genetics of early HIV sequences provides strong evidence that the virus circulated in Haiti prior to the US and that a single transfer event occurred; if a single individual did transfer the virus between the countries, they could have been of any ethnicity and the transfer was almost certainly an inevitability. The study authors do suggest that it might have been an infected Haitian worker returning from the Congo that transferred HIV from Africa to Haiti around 1966 but this is speculation; the molecular data supports the hypothesis that the virus moved between those two places around that time but says nothing about how the transfer occurred. It is dismal to realize that the stigma associated with HIV because of how it is transmitted remains strong enough to severely complicate the discussion of this type of study in 2007.

The Emergence of HIV/AIDS in the Americas and Beyond

Gilbert MTP, Rambaut A, Wlasiuk G, Spira TJ, Pitchenik AE & Worobey M

(2007) PNAS In press

HIV-1 group M subtype B was the first human immunodeficiency virus discovered and is the predominant variant of AIDS virus in most countries outside of sub-Saharan Africa. However, the circumstances of its origin and emergence remain unresolved. Here we propose a geographic sequence and timeline for the origin of subtype B and the emergence of pandemic HIV/AIDS out of Africa. Using HIV-1 gene sequences recovered from archival samples from some of the earliest known Haitian AIDS patients, we find that subtype B likely moved from Africa to Haiti in or around 1966 [1962-1970] then spread there for some years before successfully dispersing elsewhere. A ?pandemic? clade, encompassing the vast majority of non-Haitian subtype B infections in the US and elsewhere around the world, subsequently emerged after a single migration of the virus out of Haiti in or around 1969 [1966-1972]. Haiti appears to have the oldest HIV/AIDS epidemic outside sub-Saharan Africa and the most genetically diverse subtype B epidemic, which might present challenges for HIV-1 vaccine design and testing. The emergence of the pandemic variant of subtype B was an important turning point in the history of AIDS but its spread was likely driven by ecological rather than evolutionary factors. Our results suggest that HIV-1 circulated cryptically in the US for approximately twelve years before the recognition of AIDS in 1981.

Do Most Lymphocytes in Humans Reside in the Gut?

Anyone who has seen a presentation or read a review regarding the putative importance of gut CD4 T cell depletion in HIV infection will have come across statements like this: “the majority of CD4 T cells are mucosal” or “gut-associated lymphoid tissue (GALT) harbors the majority of T lymphocytes in the body.” However, closer inspection reveals that such claims are not very well supported. As Reinhardt Pabst and Hermann Rothkötter wrote in a review published last year in Advances in Experimental Medicine and Biology: "It is often stated that lamina propria lymphocytes represent the largest lymphocyte pool in humans. This is not the case. Only when the number of plasma cells and in particular IgA-producing plasma cells are counted, does the lamina propria achieve such an outstanding position for plasma cells."

Now, Vitaly Ganusov and Rob De Boer from Utrecht University in the Netherlands have published an opinion piece that offers a detailed review of the topic in Trends in Immunology. They note that two observations seem to have led to the dogma that most lymphocytes reside in the gut: first is the large surface area involved (estimated to be 400m² in one paper) and second, as referenced by Pabst and Rothkötter, studies have estimated that 80% of IgA-producing cells in mice and humans are in the gut. However, it turns out that surface area estimates include the microvilli, which do not harbor lymphocytes, and the surface area of the gut excluding microvilli is on the order of 2-10m². Furthermore, IgA-secreting lymphocytes comprise only a small fraction of all the lymphocytes in the body. Ganusov and De Boer point out that “surprisingly, most of the widely cited literature claiming that the gut is the largest immune compartment in humans neither has information on the total number of lymphocytes nor on the distribution of B- and T-cell numbers in humans.”

Ganusov and De Boer scour the literature for studies that use data to estimate the total number of lymphocytes in the body and in the gut. They report that, based on studies in several mammalian species (including humans, macaques, pigs and mice), it appears approximately 5% of the entire body lymphocyte population resides in the lamina propria of the gut and approximately 12% of lymphocytes are found in the gut overall. The largest populations of lymphocytes are found in the lymph nodes (~41%) and spleen (~15%).

Ganusov and De Boer conclude by highlighting the fact that most immunology studies present observations in terms of cell frequencies rather than cell numbers, because calculating total numbers is far more challenging. They argue that, despite the difficulties involved, “studies estimating the number and phenotype of T and B lymphocytes in different organs of healthy humans or monkeys are desperately needed…measuring population sizes should allow for a more quantitative understanding of immunology, and it might ultimately allow for new insights into mechanisms of HIV pathogenesis.”

Trends in Immunology
Article in Press, Corrected Proof -
doi:10.1016/j.it.2007.08.009   

Opinion

Do most lymphocytes in humans really reside in the gut?

Vitaly V. Ganusov and Rob J. De Boer

Available online 26 October 2007.

It is widely believed that the gut, and particularly the lamina propria (LP) of the gut, contains most of the lymphocytes in humans. The strong depletion of CD4+ T cells from the gut LP of HIV-infected patients was, therefore, suggested to be such a large, irreversible insult that it could explain HIV disease progression. However, reviewing data from different mammalian species, we found that only 5%–20% of all lymphocytes reside in the gut, and that only 1%–9% of the total lymphocyte number is located in the gut LP. Our findings suggest that spleen and lymph nodes, rather than the gut, are the largest immune compartments in mammals.

Anatomy of a Communications Disaster

AVAC's excellent statement regarding the Phambili trial announcements is now online.

What is quite stunning about the way events have unfolded this week is the lack of information, and the complete failure of just about all of the large, well-funded HIV vaccine research entities to provide any guidance about what has happened. Here are some examples:

The HIV Vaccine Trials Network, who ran both the STEP and Phambili trials, has a link to the Washington Post article "Warning Sent to AIDS Vaccine Volunteers" on their front page. As far as I can see, there is no other information regarding what has happened on the site; the last press release is dated February 7, 2007, the last story in their "In the News" section is dated September 28, 2007.

The Global HIV/AIDS Vaccine Enterprise has a large article about the appointment of their new director on their front page, dated October 11, 2007. Their "latest news" section has a press release about the STEP study from back on September 21, 2007.

The Center for HIV/AIDS Vaccine Immunology also makes no mention of this week's events. The last article in their "In the News" section is also about the appointment of the new director for the Global HIV/AIDS Vaccine Enterprise.

The International AIDS Vaccine Initiative, even though it is the only organization that is independent and not directly linked to NIAID, HVTN, the Global HIV/AIDS Vaccine Enterprise or CHAVI, at least has a link to the NIAID Phambili statement on its front page.

Which brings us to NIAID itself. It is clear that NIAID's response to the media stories is the same response that I obtained on Wednesday from Kathy Stover at NIAID:

"The data and safety monitoring board (DSMB) for the Phambili /HVTN 503 HIV vaccine study examined the available findings from the STEP/HVTN 502 study, which involved the same investigational HIV vaccine. That data showed 24 cases of HIV infection among those volunteers who received at least one dose of the investigational vaccine compared to 21 cases of HIV infection among those who received the placebo. In those volunteers who received at least two vaccinations, there were 19 cases of HIV infection among those who received at least two doses of the investigational vaccine versus 11 cases of HIV infection among those who received two doses of the placebo.

Based on this information, the Phambili DSMB felt it necessary to inform study volunteers of the possibility that the vaccine potentially could increase susceptibility to acquiring HIV infection and to counsel them accordingly on avoiding exposure to HIV. NIAID noted this in its statement and accompanying Q&A document in the spirit of transparency and sharing information with the study volunteers and the research community as it becomes available.

Although the STEP data continues to be analyzed, available findings will be presented at the HIV Vaccine Trials Network meeting in Seattle on November 7. The study volunteers are being informed of the developments and encouraged to return to their study sites for counseling and follow-up."

Like the original NIAID statement and Q&A that were released this past Tuesday, no information is provided as to why the Phambilii DSMB responded differently to what is - supposedly - the same data as reviewed by the STEP DSMB.

So we are left guessing. Did the Phambili DSMB simply insist that they saw evidence of enhancement where the STEP DSMB didn't? This is the implication of the phrase "felt it necessary." If it's just a case of differing interpretations of the same data, is their some rule that precludes anyone from clearly stating that is what has happened here? Sarah Alexander, the Associate Director of Communications at the HVTN, is quoted extensively in yesterday's Washington Blade  article, and the way the quotes are presented makes it sound like data from the Phambili trial has also been reviewed. But it's possible that this is just a misunderstanding on the part of the writer, as Alexander makes no direct reference to Phambili data herself (UPDATE II: in an email on 10/27, Sarah Alexander confirmed that both DSMBs reviewed the same data).

It still seems possible at this point that everything that has occurred is related to an understandable desire on the part of the Phambili DSMB to act with an excess of caution. If this is the case, however, it is bizarre that no one is willing to say as much.

UPDATE: Representatives from Merck have confirmed that additional data from the Phambili trial was not involved in the DSMB's decision to caution vaccine recipients about the possibility of enhanced susceptibility to HIV infection. Because distressing evidence of enhancement emerged from a microbicide efficacy trial in South Africa earlier this year, it is not surprising that the Phambili DSMB decided to take an ultra-cautious approach to the issue. What is surprising and very disappointing is that the basis for the DSMB's recommendations could not be articulated more clearly by the trial sponsors.

Vaccine Enhancement Stories Hit the Press

As many people feared when the Phambili trial statements were released on Tuesday, the mainstream media has picked up on the reported possibility that vaccine recipients experienced an increased risk of acquiring HIV infection.

AIDS Vaccine May Increase Infection Risk - Researchers (Reuters)
HIV vaccine trial participants warned of possible increased risk of HIV (Washington Blade)
Warning is Sent to AIDS Vaccine Volunteers (Washington Post)

Despite a large investment on the part of NIAID in HIV vaccine communications, the totality of what has occurred over this past week can best be described as a horrific mess of uncertainty and confusion. The Washington Blade article, the most comprehensive of the three, suggests that the Phambili DSMB did not just review data from the STEP trial, but also preliminary data from the ~800 people enrolled in the Phambili trial. If a similar disconnect was seen between the number of infections among people that received one vs. two immunizations, that might offer another explanation for the recommendations regarding the possibility of vaccine-induced enhancement.

The AIDS Vaccine Advocacy Coalition is releasing a statement today emphasizing that it is the safety of the STEP and Phambili trial participants that must be prioritized, and that caution should be exercised about over-interpreting statements regarding enhancement until details of the actual data are available.

Merck Results Among MSM

Despite the confusing nature of the releases regarding the Phambili trial, it is very hard to imagine a DSMB recommending counseling people about the possibility of vaccine-induced enhancement without good reason. With that in mind, Glenda Gray's quote in the SA Business Day article merited a closer reading than I gave it in the prior post. Gray specifically refers to the STEP trial results in men who have sex with men, and because approximately one third of STEP participants were women, it is possible that the statistical evidence of enhancement derives from an analysis of the results restricted to gay men who received at least two immunizations. An article by Jon Cohen in Science reported that only one woman in the STEP trial has become infected, which might add to the likelihood that the results among gay men came worryingly close to showing statistically significant enhancement of the risk of acquiring HIV infection among vaccine recipients.

Merck HIV Vaccine Trials: Updating the Updates

After receiving helpful emails from a number of NIAID communications staff, it appears (although it does not seem to be certain as yet) that the Phambili trial DSMB were not privy to any more information about the STEP trial results than has already been reported. Specifically, among STEP participants that received one immunization there were 5 infections in the vaccine group and 10 in the placebo group, but among participants that received at least two immunizations (and were uninfected for the first 12 weeks of the trial), there were 19 infections in the vaccine group and 11 in the placebo group. Overall totals came to 24 infections in the vaccine group and 21 in the placebo group.

What remains very unclear at this time is why the Phambili DSMB considered there to be a possible risk of enhancement of HIV infection based on these data, when the STEP DSMB (as reported in the announcements regarding their findings that were released on September 21) did not. None of the differences between the vaccine and placebo groups are statistically significant or even reach what might legitimately be called a trend. Nevertheless, Business Day in South Africa picked up on yesterday’s releases and quoted the Principal Investigator of the Phambili trial, Glenda Gray, as saying: “in the US, among men who had sex with men, there seems to be an increased risk of susceptibility (of HIV infection) among people who got the vaccine.”

The only explanation (or maybe not - see next post) at this juncture is that it is the difference between people that received one immunization (where the non-significant results appear to favor the vaccine group) vs. people that received at least two immunizations (where the non-significant results appear to favor the placebo group) that caused the Phambili DSMB concern. If you ignore the statistics – never a wise thing to do when looking at clinical trial results – then it might appear that there is a 180 degree shift in the balance of infections when recipients that received one vs. two immunizations are compared. Perhaps this suggested to the DSMB that the second immunization had some kind of enhancing effect. But then the converse of this interpretation is that one immunization may have had a protective effect. One of the potential problems with phase IIb trials like STEP is that the relatively small overall sample size (just 1,500 people in the initial analysis) can lead to this type of confusion; one possibility is that a larger trial might have found statistically significant evidence of enhancement but it is equally – if not more - likely that the 19 vs. 11 difference is the result of chance.

Both TAG and the AIDS Vaccine Advocacy Coalition are calling for NIAID to further clarify yesterday’s statements regarding the possibility of enhancement in the STEP trial.

Merck HIV Vaccine Trial Updates: The Enhancement Bell Rings Louder

One of the outstanding issues after the recent cessation of immunizations in the Merck/HVTN HIV vaccine trial (STEP/HVTN 502) was the fate of a sister trial, HVTN 503 (also known as the Phambili trial). Today, the outcome of the Data Safety Monitoring Board (DSMB) review of whether to continue the Phambili trial was officially announced. This is the pertinent quote from the press release issued by the South African AIDS Vaccine Initiative (SAAVI):

“Based on that additional review of the STEP data, an independent DSMB has now concluded that there is no basis for anticipating more favourable results in the South African clinical trial known as HVTN 503, or the ‘Phambili’ study. Therefore, the HVTN 503 oversight committee has permanently suspended immunisations and enrolment in the study in South Africa.”

NIAID also issued a statement and updated their Q&A about the two trials. What is particularly worrying about all three documents is the repeated suggestions that the vaccine may have enhanced susceptibility to HIV infection.

From the SAAVI statement:

"Based on data from the STEP Study, there is a possibility that people in the vaccine arm may have increased susceptibility to acquiring HIV infection."

From the NIAID statement:

"The Phambili DSMB also recommended that HVTN 503 volunteers be told whether they received the vaccine or placebo, be strongly encouraged to return to their study sites for protocol-related tests, and be counseled about the possibility that those who received the vaccine may have an increased susceptibility to acquiring HIV infections. Volunteers in the STEP study also will be counseled about this possibility, and discussions are under way to define the details of continued follow-up for the STEP volunteers, including when those participants will be told whether they received the vaccine or placebo.

Detailed analyses of the available data are being conducted, including analyses to better understand if there may be an increased susceptibility to acquiring HIV infection among those volunteers who received the vaccine."

From the NIAID Q&A:

"Volunteers in the HVTN 502 study are being encouraged to return to their study sites for HIV risk-reduction counseling and protocol-related tests, so that investigators can fully evaluate the effects of the vaccine on HIV acquisition, including whether there is any increased susceptibility to acquisition of HIV infection among those who received the vaccine. Volunteers are being counseled about the possibility that the investigational vaccine may have caused an increased susceptibility to acquiring HIV infection among those who received it."

"The DSMB for the HVTN 503 study also recommended that all volunteers be told whether they received vaccine or placebo and be strongly encouraged to return to their study sites for HIV risk-reduction counseling, protocol-related tests and counseling about the possibility that those who received the vaccine might have an increased susceptibility to acquiring HIV infection."

"Researchers, however, are analyzing available data to better understand if there may be an increased susceptibility to acquiring HIV infection among those volunteers who received the vaccine."

At the current time, it is completely unclear what these hints about enhancement are based upon. It may be that the South African DSMB decided that the difference between the vaccine and placebo arms for people that received at least two immunizations looked worrying enough to mention the possibility (there were 19 infections in the vaccine group and 11 in the placebo group), but it is important to stress that the results do not even amount to a statistical trend: the p value for this difference between the vaccine and placebo arms is p=0.1042.

Alternatively, perhaps the DSMB reviewed additional data that has not yet been made public. The official presentation of the data from the STEP/HVTN 502 trial is slated for the HVTN conference that is taking place in Seattle from November 7-9. In a very welcome move, the meeting has been opened to the public.

However, it is extremely frustrating that it appears that interested scientists, community members and trial participants will have to wrestle with the implications of these hints about enhancement without additional details until the HVTN conference. TAG will seek clarification about the content of today’s statements from NIAID and provide updates as new information becomes available. Future blog posts will also look at the literature on the potential associations between immune activation and susceptibility to HIV infection and discuss whether the data may have implications for vaccine research.

CD4 T cells & Dendritic Cells on Film

A new paper in the "online ahead of issue" section of the journal Immunity captures interactions between CD4 T cells and dendritic cells (DCs) on film using intravital imaging. Although access to the full text of the article requires a subscription to the journal, the Quicktime videos can be viewed free of charge online. DCs are stained green, CD4 T cells are stained red. Some similar footage has been published previously, but this study goes a step further by using a system that allows the first encounter between DCs and antigen-specific CD4 T cells to be captured in real time.

Immunity
10.1016/j.immuni.2007.08.018

Real-Time Manipulation of T Cell-Dendritic Cell Interactions In Vivo Reveals the Importance of Prolonged Contacts for CD4+ T Cell Activation

Susanna Celli,1,2 Fabrice Lemaître,1,2 and Philippe Bousso1,2,

1 Institut Pasteur, G5 Dynamiques des Réponses Immunes, Paris, F-75015, France
2 Inserm, Equipe Avenir, U668, Paris, F-75015, France

T cells interact with dendritic cells (DCs) for periods lasting from minutes to hours. However, a causal link between the duration of this interaction and the efficiency of T cell activation has not been established in vivo. Employing intravital two-photon imaging, we manipulated T cell-DC interactions in real time and found that the first T cell-DC encounter often resulted in a long-lived interaction. Moreover, the cessation of T cell receptor-major histocompatibility complex signals promoted cellular dissociation, suggesting that antigen availability on DCs regulates contact duration. Finally, at least 6 hr of in vivo T cell-DC interaction were required for naive CD4+ T cells to undergo clonal expansion. These results establish the importance of prolonged T cell-DC interactions for efficient CD4+ T cell activation in vivo.

Hijacking Dendritic Cells: HIV-1 vs. HIV-2

A new paper in the Journal of Virology reports that dendritic cells (DCs) are more susceptible to infection by HIV-1 than its far less pathogenic retroviral sibling, HIV-2. The work is the product of collaboration between researchers at the NIH’s Vaccine Research Center and Oxford University in the UK. DCs act as immune system sentinels that capture pathogens as they first enter the body and transport them to lymph nodes in order to initiate immune responses by T cells and B cells. DCs are referred to as antigen-presenting cells for this reason.

The study authors point out that previous studies have shown that DCs can be susceptible to HIV-1 infection, but that infection of DC’s also leads to presentation of HIV antigens to HIV-specific CD4 T cells. They write: “this suggests that the infection of DCs by HIV-1 and subsequent processing and presentation of HIV-1 antigens on the surface of DCs is a double-edged sword. In order to induce an HIV-specific immune response, it is necessary for DCs to deliver appropriate signals to memory CD4+ T cells. However, in doing so, the intimate contact of DCs and T cells leaves HIV-1-specific CD4+ T cells susceptible to becoming infected by these HIV-1-carrying DCs. This could help to explain mechanistically how HIV-1-specific CD4+ T cells become preferentially infected in vivo.” However, although HIV-2 was extremely inefficient at infecting DCs, the researchers found that HIV-2-specific CD4 T cells were still more likely to be infected by HIV-2 compared to CD4 T cells targeting other pathogens, suggesting that transfer from DCs may not be the only means by which the virus preferentially infects HIV-2-specific CD4 T cells.

The findings may also suggest that HIV-1’s superior ability to target DCs contributes to the different courses of disease associated with HIV-1 and HIV-2 infection. Recent studies by the same researchers have shown that HIV-2-specific CD4 T cell responses are relatively preserved compared to the loss and dysfunction of HIV-1-specific CD4 T cells that occurs in progressive HIV-1 infection. Further investigations of the role of DCs in HIV-1 vs. HIV-2 infection may help shed light on the reasons for these differences. This area of research may also have implications for HIV-1 vaccines; although T cell-based candidates like the Merck Ad5 vaccine reliably induce HIV-specific memory CD4 T cells, it could be the case that HIV-1 infection of DCs (or other antigen-presenting cells such as macrophages) somehow prevents these CD4 T cell responses from being activated efficiently.

JVI Accepts, published online ahead of print on 3 October 2007
J. Virol. doi:10.1128/JVI.00976-07

Dendritic Cells Are Less Susceptible to HIV-2 Than HIV-1 Infection

Melody G. Duvall, Karin Loré, Hetty Blaak, David A. Ambrozak, William C. Adams, Kathlyn Santos, Christof Geldmacher, John R. Mascola, Andrew J. McMichael, Hilton C. Whittle, Sarah L. Rowland-Jones, and Richard A. Koup

HIV-1 infection of dendritic cells (DCs) has been documented in vivo, and may be an important contributor to HIV-1 transmission and pathogenesis. HIV-1-specific CD4+ T cells respond to HIV antigens presented by HIV-1-infected DCs and in this process become infected, thereby providing a mechanism through which HIV-1-specific CD4+ T cells could become preferentially infected in vivo. HIV-2 disease is attenuated with respect to HIV-1 and host immune responses are thought to be contributory. Here, we investigated the susceptibility of primary myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) to infection by HIV-2. We found that neither CCR5-tropic primary HIV-2 isolates nor a lab-adapted CXCR4-tropic HIV-2 could efficiently infect mDCs or pDCs, though these viruses could infect primary CD4+ T cells in vitro. HIV-2-exposed mDCs were also incapable of transferring virus to autologous CD4+ T cells. Despite this, we found that HIV-2-specific CD4+ T cells contained more viral DNA than memory CD4+ T cells of other specificities in vivo. These data suggest that either infection of DCs is not an important contributor to infection of HIV-2-specific CD4+ T cells in vivo, or that infection of DCs by HIV-2 occurs at a level undetectable in vitro. The frequent carriage of HIV-2 DNA within HIV-2-specific CD4+ T cells, however, does not appear to be incompatible with preserved numbers and functionality of HIV-2-specific CD4+ T cells in vivo, suggesting that additional mechanisms contribute to maintenance of HIV-2-specific CD4+ T cell help in vivo.

CD4 T cell Responses to HIV: Walking a Tightrope between Infection and Protection

Continuing the recent theme of covering issues raised by the Merck HIV vaccine trial results, one longstanding concern with T cell-based vaccines is that they might create more CD4 T cell targets for HIV infection. HIV has a well-described proclivity for activated, dividing CD4 T cells, as is evinced by studies showing that CD4 T cell activation by licensed vaccines and co-infections can often lead to a transient increase in viral load in HIV-infected individuals.

Immunization of an HIV-negative person with a T cell-based HIV vaccine creates a pool of “memory” CD4 and CD8 T cells that specifically recognize the HIV components included in the vaccine. If that individual is subsequently exposed to HIV, the HIV-specific CD4 T cells will respond by becoming activated. On one hand, this is a good thing in that these CD4 T cells help initiate an immune response by assisting HIV-specific CD8 T cells in their efforts to kill virus-infected cells and by signaling to B cells to make antibodies. On the other hand, these activated HIV-specific CD4 T cells also represent ideal targets for the virus and some proportion will inevitably become infected. One grim possibility is that infection of even a small proportion of the HIV-specific memory CD4 T cell response could be enough to compromise the overall immune response to HIV, rendering vaccination useless.

However, in the absence of memory CD4 T cells targeting HIV, the first-responders in a person exposed to HIV are naïve (inexperienced) CD4 T cells that recognize the virus. Naïve CD4 T cells have to become activated and undergo multiple divisions in order to develop into memory CD4 T cells, and the few studies that have looked at this issue have found that activated naïve CD4 T cells are even more susceptible to HIV infection than activated memory CD4 T cells. Furthermore, a recent study found that developing memory CD4 T cells are preferentially infected with HIV during acute infection. These findings, although very limited, suggest that having vaccine-induced memory CD4 T cells on board prior to exposure might confer an advantage. Additionally, particular types of memory CD4 T cell may be less susceptible to infection than others; recently presented data has shown that CMV-specific memory CD4 T cells that make the chemokine MIP-1alpha resist HIV infection significantly better than memory CD4 T cells that do not.

The only study to look closely at this issue in the SIV model was published last year in the Journal of Experimental Medicine. Joseph Mattapallil and colleagues studied macaques that were immunized with a DNA prime/adenovirus vector boost regimen encoding SIVmac239 Env and a Gag-Pol fusion protein. Animals were challenged with SIVmac251(an uncloned virus with similar virulence to SIVmac239) a month after their last booster immunization. Post-infection viral loads were significantly lower in vaccinated animals (by approximately 1 log) compared to controls and vaccination was also associated with a relative preservation of gut CD4 T cells (controls lost 50-80% of this CD4 T cell population, vaccine recipients 5-20%). Mattapallil and colleagues specifically evaluated the extent to which SIV infected SIV-specific CD4 T cells; in both vaccine recipients and controls, SIV-specific CD4 T cells were preferentially infected to a similar extent (containing ~2-fold more SIV DNA than CD4 T cells of other specificities). The only difference was that the preferential infection of SIV-specific CD4 T cells was detectable a few days earlier in vaccinated macaques. The researchers conclude that the infection of SIV-specific CD4 T cells did not abrogate the virological and immunological benefits of vaccination in this study.

At worst, the Merck results could indicate that these findings in the macaque/SIV model do not translate to HIV-infected people, and that the susceptibility of CD4 T cells to HIV infection is an Achilles heel for all vaccination approaches. Among the less pessimistic possibilities, it could be the case that the vaccine-induced memory CD4 T cells were overly susceptible to HIV infection either because of their activation state at the time of exposure (see prior post on resting vs, activated memory T cells) or because they lacked the ability to make potentially protective substances like MIP-1alpha. The breadth of the memory CD4 T cell response could also come into play; Merck's vaccine induces responses only to Gag, Pol and Nef, meaning naive CD4 T cells would be responsible for responding to the HIV proteins not included in the vaccine (Env, Tat, Rev, Vif, Vpu and Vpr). This issue of HIV infection of virus-specific CD4 T cell responses is among the many that will need to be explored as the Merck results are investigated in detail.