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Why some individuals develop AIDS rapidly whereas others remain healthy without treatment for many years remains a central question of HIV research. Of the quantities that predict how quickly an untreated infection progresses, the most widely used is set-point viral load. This measure varies by orders of magnitude between infected individuals and is predictive of infectiousness and time to onset of AIDS. Host factors, predominantly linked to the immune system, are known to influence the set point, but much variation remains unexplained.
We review recent evidence showing that HIV genotype influences the set-point viral load far more than anticipated. Our summary of published estimates suggests that 33% (95% confidence interval, 20 to 46%) of the variation is attributable to the virus. Because set-point viral load is heritable (partially controlled by virus genotype) and is linked to transmissibility, it is likely to have evolved to maintain transmission fitness and may continue to evolve in response to diverse selection pressures. These findings are unexpected and paradoxical because rapid and error-prone viral replication should favor within-host adaptation and rapidly scramble signals of viral genotype as infection progresses, rather than leaving a lasting footprint that is preserved throughout an infection and from one infection to the next in transmission chains.
We propose that resolving the paradox of heritability of set-point viral load will provide new insights into the mechanisms of HIV pathogenesis. To this end, we provide three parsimonious, testable, and nonexclusive explanatory mechanisms. The first states that HIV evolution in virulence genes is more functionally constrained than previously thought. The second proposes that virulence of HIV is mediated through the virus’s capacity to systemically activate target cells in which it can efficiently replicate. The capacity to activate would not be expected to evolve rapidly because it does not provide a specific selective advantage to virus strains that activate more cells; rather, it is an advantage shared by all viruses. The third mechanism implicates the preferential transmission of viruses that are stored in nonreplicating cells or during early infection, and the disproportionate influence on long-term pathogenesis of these early viruses.
In addition to these insights into mechanisms of pathogenesis, we believe that this research highlights a major gap in our knowledge of HIV. The identification of the genetic determinants of HIV virulence, which appear to vary between closely related strains of the virus, should be a major priority. Thus, whole-genome association studies that are focused on the virus genome should be pursued and expanded, as well as more functional and mechanistic studies, which could be guided by hypotheses such as those presented here.
A major focus of research on HIV is on host responses to infection—understandably, because the virus targets the immune system and because of the interest in vaccine development. In reviewing what little research has been done on viral virulence determinants, Fraser et al. (10.1126/science.1243727) present evolutionary explanations for some of the poorly understood phenomena that mark HIV infection, including long-term survivorship, latency, rapid within-host evolution, and inheritability of between-host virulence.
Why some individuals develop AIDS rapidly whereas others remain healthy without treatment for many years remains a central question of HIV research. An evolutionary perspective reveals an apparent conflict between two levels of selection on the virus. On the one hand, there is rapid evolution of the virus in the host, and on the other, new observations indicate the existence of virus factors that affect the virulence of infection whose influence persists over years in infected individuals and across transmission events. Here, we review recent evidence that shows that viral genetic factors play a larger role in modulating disease severity than anticipated. We propose conceptual models that reconcile adaptive evolution at both levels of selection. Evolutionary analysis provides new insight into HIV pathogenesis.