Quantifying HIV-1 exposure to illuminate resistance to infection
Richard Jefferys, TAG
In the early 1990s, the research group of Frank Plummer at the University of Manitoba drew considerable attention – and some controversy – when they reported that, among a large cohort of female sex workers in Nairobi, a subset showed evidence of resistance to HIV infection. 
The evidence emerged over the course of a long-term study that found that women starting sex work faced a very high risk of seroconverting in the first two years. However, for a subset of women who remained HIV negative, the risk of becoming infected subsequently declined significantly over time, which Plummer and colleagues interpreted as evidence of resistance to acquisition. Despite initial skepticism, the findings prompted efforts to identify individuals with possible resistance to HIV in other settings, such as among serodiscordant couples (in which one partner is HIV positive and the other negative). There is now a substantial amount of literature on the topic and ongoing workshops and research collaborations, but one of the challenges in the field is that there are no widely accepted criteria for defining high exposure to HIV (an important part of assessing whether an individual may be resistant as opposed to simply unexposed).
In a new paper in the Journal of Infectious Diseases, Romel Mackelprang and colleagues from the Partners in Prevention HSV/HIV Transmission Study describe a method for quantifying HIV exposure among serodiscordant couples, with the goal of enhancing efforts to identify individuals with potential resistance to infection. 
The study draws on data from a trial involving 3,408 serodiscordant heterosexual African couples. Risk factors that were associated with HIV transmission—such as unprotected sex, viral load in the HIV positive partner, and genital ulcer disease—were used to create “exposure scores” for the study participants. Unsurprisingly, high exposure was associated with a 6.9-fold increased risk of infection compared to low exposure. But these scores also identified a subset of 475 individuals with persistently high exposure who remained seronegative. Echoing Frank Plummer’s findings, the risk of HIV infection appeared to decline over the time in the high exposure group, while remaining constant among participants with lower exposure.
To further validate these findings, the researchers applied a simplified version of the exposure score model to an independent cohort of 485 serodiscordant couples from Kampala, Uganda, and Soweto, South Africa. Again, the analysis identified a group (comprising 48 individuals) with high exposure but a decreasing risk of acquiring HIV infection over time. The study authors conclude: “our approach to estimating HIV-1 exposure using longitudinal data from both partners in HIV-1–serodiscordant couples provides an objective tool to identify subsets of HESN [HIV-1–exposed seronegative] individuals to target for identification of host factors protecting against HIV-1.” Additional details on the exposure scoring system are provided as supplementary data on the journal website. 
Source: TAG basic science web log. (12 September 2012).
- Plummer FA et al. Evidence of resistance to HIV among continuously exposed prostitutes in Nairobi, Kenya. Int Conf AIDS 1993 Jun 6-11; 9:23 (abstract no. WS-A07-3).
- Mackelprang R et al. Quantifying Ongoing HIV-1 Exposure in HIV-1–Serodiscordant Couples to Identify Individuals With Potential Host Resistance to HIV-1. J Infect Dis. (2012)doi: 10.1093/infdis/jis480. First published online: August 27, 2012
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