July 26, 2012
The extent to which virus-specific CD8 T cell responses contribute to control of viral load and prevention of disease progression in HIV infection has long been a subject of conflicting data and controversy. Early tests of the ability of CD8 T cells to kill target cells expressing HIV antigens were misleading because they relied on extended culture with IL-2, a process that turned out to have restorative properties on otherwise dysfunctional cells. The advent of class I MHC tetramers, which facilitate the counting of CD8 T cells specific for individual HIV epitopes, was an advance but also initially caused confusion: an early paper using tetramers reported an inverse correlation between CD8 T cells specific for epitopes from HIV Gag and Pol and viral load, but a subsequent more comprehensive analysis of CD8 T cell responses to all viral proteins found the opposite: a positive correlation with viral load.
To some extent, these disparate findings have been reconciled by data indicating that CD8 T cell targeting of certain epitopes, especially those in Gag, may be particularly important in controlling HIV. But it has also become clear that cell function is critical, and a number of research groups have developed assays designed to directly measure the ability of CD8 T cells to kill HIV-infected CD4 T cells in vitro. For example, the laboratory of Mark Connors at NIAID has pioneered this approach and demonstrated that elite controllers display functionally superior HIV-specific CD8 T cells compared to individuals with progressive infection.
In new a study published in the current issue of the Journal of Infectious Diseases, Lucy Dorrell and colleagues use an assay developed in their lab to investigate the relationship between the killing function of HIV-specific CD8 T cells and the rate of decline in peripheral blood CD4 T cell counts. The researchers first established that the CD8 T cell antiviral activity they were measuring was specific: at a CD8:CD4 ratio of 2:1 in the in vitro assay, the median inhibition was 78% in HIV-positive individuals compared to 0% in HIV-negative controls. The activity was only seen when the CD4 T cell targets shared a class I HLA allele with the study participant from whom the CD8 T cells were obtained.
The researchers also studied a cohort of 20 individuals recruited during acute HIV infection in China (in the Beijing PRIMO study) followed prospectively. CD4 T cell slopes were calculated from a median of 13.5 measurements over an average of around 2.5 years of follow-up. The results showed that the antiviral activity of CD8 T cells in early infection was predictive of the rate of CD4 T cell loss over time. The relationship was strongest when reduced ratios of CD8:CD4 T cells were used (either 1:10 or 1:100), leading the authors to state that: "titration of CD8 T cells to sub-optimal levels improved the capacity of the assay to distinguish fast and slow progressors." The percentage of CD8 T cell inhibition was calculated to explain 73% of the variance in CD4 slopes at a CD8:CD4 ratio of 1:10. The researchers found that adding HIV viral load set point into a model together with percentage of CD8 T cell inhibition improved their ability to predict CD4 T decline: at a CD8:CD4 ratio of 1:10, the combined measures explained a remarkable 91% of the variance in the slope of CD4 T cell loss.
These findings make an important contribution to the understanding of differences in individual rates of CD4 T cell loss and disease progression among HIV-positive individuals. The researchers note, however, that their study has some limitations: their model assumes constant CD8 T cell antiviral activity, when in reality it is likely that this function becomes impaired over the course of infection. The retrospective nature of the analysis of the chronic infection cohort also makes discerning cause and effect difficult, and the prospective cohort only had limited samples from early infection available. To address these limitations, they recommend the conduct of a longitudinal study assessing CD8 T cell antiviral activity repeatedly from the earliest stages of infection onwards.
Richard Jefferys is the coordinator of the Michael Palm HIV Basic Science, Vaccines & Prevention Project Weblog at the Treatment Action Group (TAG). The original blog post may be viewed here.
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