Summaries
Track A: Basic Science

The innate immune response, which has been relatively less studied may provide insights into other factors important for viral control and therefore alternate routes for immunotherapy and vaccine design. The need for increased understanding of the roles of dendritic cells, interferons and soluble factors secreted by CD8 + T cells is becoming increasingly clear. A clearer more precise understanding of the interplay between these more primitive immune responses and adaptive immune responses may provide insights critical for viral control.

Sessions examining the evolution of virus sequence in different regions of the world highlighted the ongoing changes in the predominating forms. The process of viral recombination was documented in an individual infected with two virus types and this mechanism is probably common in countries where multiple viruses circulate. However, the factors that determine the success of particular recombinant forms is unclear. Most interesting was the first study to indicate that the viral sequence diversity is tied to the replicative properties of these viruses. A detailed analysis was presented comparing the replication capacity of different viral subtypes in vitro. The study showed that within the different virus groups considerable differences exist in the capacity to replicate. However overall replication of group M (representing most predominant subtypes, including subtype B and C) viruses was generally better than the other two groups seen less frequently representing HIV 2 and subtype O. Interestingly within group M subtype C was one of the poorest replicating viruses, future studies will address whether this may result in a virus being less pathogenic and or less transmissible.


TUESDAY
13 July

Today the presentations in the basic science track A examined aspects of HIV virology and the complex interplay between the virus and host. A unifying theme was the heterogeneity of the virus as it has successfully adapted to a range of different environments.

A session today examined dual infections and recombination in communities with several different circulating strains. The rapid rate of change documented in several studies highlighted the complexity of the task of vaccine makers in these communities. Evidence was presented showing that during early subtype B infections the rate of dual infection or superinfection with a subtype B virus could be as high as 5%. However, other studies have found the lack of superinfection in the chronic stage of disease. Superinfection in patients with stable CD4 cell counts and lower viral loads may actually result in more rapid progression (e.g. drops in CD4 cell counts and increased viral loads). Although intriguing, more superinfections need to be identified to confirm these studies.

Also examined today were several host genetic factors that have an impact on disease progression, focusing in particular on two groups of individuals: exposed-uninfected and long term non progressors. In studies of exposed seronegative individuals in China, viral HIV-1 DNA was detected at very low levels in patient PBMC. In some instances, the exposed seronegative individuals did get infected with HIV-1 (12/94) and became seropositive. Another study in China of these patients identified new mutations in the RANTES promoter that conferred a degree of resistance to virus.

Several studies reported today showed that cofactors modulating cytokines, such as co-infections with HIV and agents as diverse as scrub typhus, HTLV-2 and measles can dramatically shape the course of HIV infection.

Mechanisms of HIV infection and pathogenesis in non-lymphoid tissues, showed that nestin-expressing cells (ie progenitor cells) can be infected with HIV. These can differentiate into neurones or astrocytes these progenitor cells in brain may actually be a significant reservoir for HIV in the CNS. Apoptosis in neurones was found to result from the stimulation by viral components rather than the effects of products liberated from infected cells.

Cellular events associated with mother-to-child HIV transmission were brought into focus by showing that infection of placental trophoblast cells can be readily achieved independent of CD4, CXCR4,and CCR5 expression on target cells. The entry mechanism is instead mediated by endosomes. HIV infection is reduced in vitro by using agents that inhibit endosomes, probably because these agents reduce escape of HIV to cytoplasm.

Symposia examining the novel and experimental approaches to therapy highlighted that several interesting approaches warrant vigorous research. The quest to eradicate latent viral pools with IL-7 showed that it was effective at driving viral replication from quiescent sites, but the strains were different from that driven out by IL-2 or OKT3, suggesting that if such purging protocols are to be pursued then perhaps multiple T cell stimuli will be required in combination to drive out latent virus from as many T cell reservoirs as possible. A new therapeutic approach that uses short interfering RNA was discussed. This approach uses an endogenous cellular mechanism that uses short RNA sequences to cleave and degrade HIV RNA. This strategy can silence HIV replication, potentially targeting HIV RNA at both the pre- and post-integration stages of its life cycle. In terms of developing dsRNA as a therapeutic strategy the main hurdles will be: i) viral variability. The effect is highly sequence specific, mutational escape to dsRNA has already been described in vitro. Targeting conserved regions and using multiple dsRNAs simultaneously may overcome this problem.ii) effective delivery of the constructs. While it is possible to deliver these molecules preformed in certain animal systems to certain organs, the effective targeting of these constructs to lymphoid tissue as preformed molecules is likely to be problematical and require intravenous infusion.

Other cell determinants were proposed as target for drugs or a route to drug discovery. Stimulants of innate immunity, such as Toll-like receptor ligands may be useful and comparative genomics may identify further novel cellular targets akin to the recent descriptions of APOBEC3G and TRIM5alpha that protect from the harmful effects of HIV-infection. Finally pharmacogenomics was identified as a promising way to drive the personalisation of therapy with the aim of minimising side effects known to be related to genetic polymorphisms by tailoring drug regimens to a personalised pharmacogenetic profile.


WEDNESDAY
14 July

Targeting Cellular Functions as an Approach to Antiretroviral Therapy

While antiviral drug development efforts have classically focused on agents aimed at a distinct viral target, such as HIV reverse transcriptase or protease, there is another intriguing alternative: targeting cellular functions required for viral replication.

Targeting cellular functions may offer some significant advantages. Cellular functions represent a completely new set of targets. Agents developed against such targets would be unlikely to exhibit cross-resistance with other agents, and resistance to agents aimed at cellular targets may be expected to develop more slowly than conventional antiviral agents. Asking a virus to mutate so that it can replicate without the cellular functions it has evolved to use over long periods of time may be a much more stringent requirement than asking it to mutate so that a small molecule can no longer access an active site.

Several presentations described the development of agents targeting obvious cellular targets, for example HIV co-receptors. These have been the subject of much prior work, and some agents, for example Schering-D, are in advanced stages of clinical development. Several other approaches targeting the chemokine co-receptors and entry were discussed. Jim Demarest from GSK described a novel CCR5 antagonist, 873140. M. Franti discussed PRO140, a humanized mouse monoclonal antibody to CCR5, and presented new data showing activity in a SCID-hu mouse model. Shizuko Sei described the identification of a new class of entry inhibitors that target the interaction of gp120 and CD4. Thomas Klimkait showed data about new peptide epitope mimetics (e.g. POL2438) that target CXCR4. And new cellular targets continue to be identified. For example, Roger Pomerantz described in the molecular virology session on Tuesday a newly appreciated cellular DEAD box protein, DDX-1, that, together with Rev, may play an important role in exporting HIV unspliced RNA from the nucleus, above and beyond the well-known CRM-1 export pathway.

Some cellular systems, as yet unidentified, may in fact be the mechanisms through which the long elusive CD8 suppressive factor described by Jay Levy (MoSy145), or the activities responsible for the antiviral effects of co-infection with scrub typhus by S Philpot (TuOrA1184), or measles virus by J-C Grivel (TuOrA1185).

The response of host cell functions, however, may not be entirely favorable from the viewpoint of inhibiting HIV replication. Nathalie Dereuddre-Bosquet presented evidence that HIV infection may lead to increased expression of ABC transporters that can increase the efflux of AZT from the host cell, making at least this antiretroviral less effective than it otherwise might be (WeOrA1275).

HIV probably requires a large number of cellular functions for optimal replication, and some of these functions may need to be finely tuned to optimally support HIV replication. Disrupting these functions may inhibit HIV replication. For example, a few years ago, inhibitors of MAPK could suppress activation of lytic replication in latently infected cells. In addition to the obvious cellular targets, a few presentations at this meeting have found other, less obvious cellular targets, which may offer avenues for the development of new, effective antiviral agents. In data presented in a poster by K. Muthumani (WePe5652) a novel compound, RWJ67657, which inhibits the serine/threonine kinase p38 MAPK was found to suppress HIV replication. Reasoning that HIV replication may lead to changes in the expression of host cell genes that are important for HIV replication, S Zeichner used a broad-based expression profiling approach to identify cell genes that had altered expression during HIV replication in induced, latently infected cells (ThOrA1397). He found that about 15% of the genes examined had altered expression during HIV replication and that many genes in certain pathways exhibited coordinate changes in expression. Several of these genes had clear links to HIV replication. Ras was observed to be upregulated. Since the abl kinase regulates ras, and since ras and abl synergistically regulate NF-kB, this lead to the hypothesis that inhibiting Ras might inhibit HIV replication. This was a particularly interesting hypothesis since a Ras inhibitor, imatinib (Gleevec), has recently been clinically approved for the treatment of certain cancers. Zeichner studied the effect of imatinib on HIV replication in latently infected cell lines, and in cells exposed to virus, imatinib inhibited HIV replication, supporting the idea that altering the host cell environment in a way to make it less hospitable to viral replication may offer new therapeutic opportunities. This study demonstrates the usefulness of comparative genomics to identify novel targets, as was stressed by Amalio Telenti in his symposium presentation on Tuesday (TuSy174), and suggests that other useful targets still await discovery.


THURSDAY
15 July

HIV vaccine development: two steps forward, one step back Today basic scientists compared their efforts towards progress with a safe and effective HIV vaccine. While significant progress has been made, the clear and consistent message is that this task will be even more difficult than first appreciated.

Vaccines are clearly of great interest to all participants at this meeting. An overview of the current state of this field was delivered to a capacity audience in the main auditorium by Dr. Jose Esparza, now with the Bill and Melinda Gates Foundation. His message to the greater audience was echoed throughout the basic science sessions in smaller venues throughout the day: the scientific difficulty of this quest is immense.

As Wayne Koff from IAVI said today although the theme of this conference is “Access for all” unfortunately we were not discussing issues of access to prophylactic vaccines, simply because none of the candidates have any proven efficacy. The vaccine related presentations today highlighted a number of issues that complicate vaccine development. The immune correlates of protection are still not understood and the ideal animal model for viral challenge experiments is a matter of ongoing discussion. Therefore, it could be cogently argued that the phrase “ rationale vaccine design” is an oxy-moron where HIV vaccine development is concerned.

However, many of the presentations added to the breadth of knowledge accumulating in terms of understanding the fine specificity of the natural and vaccine induced immune responses and how they compare. Furthermore, it is becoming clear that subtle changes to vaccine constructs can impact upon their immunogenicity and the effectiveness of viral control post viral challenge. Several novel approaches were described including encouraging data on cytokine and chemokine adjuvants. Importantly, many of the constructs that have undergone exhaustive pre-clinical evaluation are now in or will come to clinical trial evaluation over the next 2-5 years. This should result in identification of lead candidates and provide insight into what is immunogenic in humans as compared to what looks promising in mice or primates. The ongoing pathogenesis studies and the ongoing studies related to refining vaccine constructs and adjuvants will hopefully ensure that at some time in the not too distant future we will be discussing access for prophylactic vaccines.

The success of HIV-1 vaccines obviously hinges on understanding HIV-1 pathogenesis. Our current knowledge of this area was summarized in a thorough and thought provoking manner by Dr. Fauci in the special plenary session. He clearly outlined the dysfunction and dysregulation of the basic immune cells, T, B, and NK cells in HIV disease by contrasting gene expression profiles and critical surface protein expression in both viremic and aviremic patients. He then outlined an intriguing theory on HIV pathogenesis and immune control or lack there of. HIV is the key and it activates the cells, which it will infect. This stimulation can in turn lead to a general immune activation that results in a positive feedback to enhance virus replication. Continuous immune activation coupled with infection will then cause a slow depletion of the CD4+ T cell subset as well as dysfunctional responses and aberrant turnover of lymphocytes and other immune cells. Surprisingly, this whole disease process could be reversed or even shut off if HIV replication is blocked. Thus the key to resumption of normal immune function may simply be dependent on treatment with antiretroviral drugs, further emphasizing the need for increased drug access for all HIV infected patients. Alternatively an effective and safe vaccine could block this infection from even starting.

Another key to a successful vaccine trial is an understanding the molecular epidemiology of this disease. Today continued on a theme that was a focus of this entire conference, i.e. how, what, and where are different HIV-1 subtypes spreading the worldwide epidemic. In addition, it is clear that the rapid emergence of new intersubtype recombinants in Africa, Southeast Asia, China, South America, and even Europe will complicate any design of the proposed monoclade vaccines. We had initially thought that HIV-1 recombinants such as CRF or unique recombinants forms were a recent phenomenon in the epidemic. However, two presentations today clearly indicate that a high prevalence of HIV-1 intersubtype recombinants (at least 40% in the Congo basin) was evident as early as 1984. This insight like many others from today constantly remind us of the complexity of the challenge that we are attempting to overcome.