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HIV Viral Dynamics: viral fitness, genetic diversity, progresion, co-receptor use
Written by Mike Youle, MD, Royal Free Hospital, London, UK
2nd IAS Conference
July 13-16, Paris, 2003
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The only criticism I had of the Paris meeting, apart from the perennial rudeness of French waiters was that some of the parallel sessions became so full that there was only standing room (sometimes 5 deep) and since my advanced age no longer allows me to stand for more than 15 minutes without calf pain I retreated into rooms that normally I would not visit. Thus I found myself in a fascinating series of presentations on viral dynamics, chaired by Mark Wainberg and Jean-Luc Darlix.
Firstly a group from the Institut Pasteur and The University of Saarland in Germany showed data on quantifying HIV DNA from the reverse transcription to integration so essentially trying to work out how many virions are needed to infect a new cell [abstract 146]. They showed in an elegant series of experiments that the intracellular enzyme proteasome degrades up to 75% of incoming virions. Combining this data with other studies the conclusion was that the fraction of HIV RNA that gets converted into provirus could be as low as 1%. Thus since the spleen cells they examined had on average 3-4 proviruses each it suggests that 300-400 virus particles are required for the infection of an individual cell. They also showed that there is rampant recombination of viruses with up to 29% difference between viruses within an individual cell. Doug Richmann questioned if this data could be validated in cells within infected individuals whilst Vincent Calvez from Paris suggested that cell to cell spread of virus may be more important explaining why the switch from non-syncytial inducing (R5) virus to syncytial inducing (X4) virus may herald a rapid decline in health.
This led nicely onto the second presentation from the team of Eric Arts at Case Western Reserve University, who working with the Institute of Tropical Medicine in Antwerp are following a cohort of patients over a protracted period of time to evaluate the relationship between HIV fitness and viral genetic diversity [147]. It is known that several factors have an impact on progression of disease; viral load, possession of nef, use of either CCR5 or CXCR4 co-receptor. In this study they used growth competition assays to compare the fitness of viruses from 12 individuals followed for 2-5 years as well as assessing the genetic diversity of the HIV quasispecies by sequencing a section of the envelope of at least 10 clones form the same patient sample used to estimate the fitness. What they showed was that as time passed the viruses within an individual become gradually fitter and their genetic diversity (variability) and divergence from the original strain increase. So for example: Patient K was followed over 6 years during which his T4 count dropped from 800 to 400 with a concomitant rise in viral load of 1 log. Four isolates were examined over this time period which showed a doubling of viral fitness with a rise in diversity and divergence. Across the 12 patients there was a marked correlation between ex vivo fitness and genetic diversity (p<0.003). When HAART was commenced there was a reduction in viral fitness that paralleled the drop in viral load and rise in T4 count. HIV viral fitness was also correlated with time from seroconversion (r=0.08; p<0.001). The presenter questioned whether treatment might reduce the fitness of the virus. Doug Richmann suggested that resistance might be a simpler explanation. However none was found in the samples examined. This study raises the question of why the epidemic is not accelerating if fitness is rising constantly form time of infection. Much as in the previous abstract this may be a reflection of a genetic bottleneck the cellular level during transmission which reduces fitness back to lower levels. A further question was asked about what co-receptor these viruses were using. The answer was all R5 except for the last sample of Patient K where the virus had become dual tropic, fitting well with idea that time is an important factor in determining co-receptor usage and disease progression. What this study suggests is that perhaps it may be better to treat early to reduce viral fitness, diversity and the speed of progression, a nice idea if we had completely non-toxic therapy.
A study from Albany Medical College and the Department of Health examined the dynamics of R5 and X4 viruses after the commencement of HAART in 8 individuals (6 female) [abstract 149]. In patients with X4 virus there was a marked suppression of these viruses within 2 weeks, form the blood, whilst the trajectory of clearance of R5 virus was much slower. A shift in co-receptor usage in the female genital tract also occurred but the dynamics differed from the blood due to R5 viruses persisting for a much longer period. These changes did not seem to be linked to the density of particular co-receptors, cellular activation or cytokine production. Work on characterising any resistance in these isolates is ongoing.
Jacques Reynes from Montpellier then showed data that argued however that the density of CCR5 receptors may be an important determining factor in HIV replication [abstract 151]. Using flow cytometric techniques he determined the density of the CCR5 co-receptor on peripheral blood T4 cells in 23 subjects (8 women and 15 men) who underwent a 30 day interruption in HAART. T4 varied between 300 and 1739 (median 724) and their viral load was <200. CCR5 density appears to vary for around 4-24,000 molecules/cell and remains reasonably stable over time. In this study there was a strong correlation between the CCR5 expression and viral load rebound. For R5 densities <8,000 molecules/cell viral load rose to <100,000 copies/ml whereas those who rebounded to >100,000 copies/ml had densities over 8,000 molecules/cell (r=0.71; p<0.001). These results emphasize the role of CCR5 density in in vivo HIV replication. Thus therapies such as the new CCR5 receptor blockers SCH D (Schering) and UK-427,857 (Pfizer) may offer a therapy which not only is effective but has a greater efficacy in those patients with the greatest potential for progression. Only clinical studies will inform as to whether this is true, however it is tantalising to hope that the arrival of these blockers may have an added advantage to those with high viral loads.
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