Many lines of evidence indicate that CD8 T cells targeting HIV play a key role in suppressing viral replication in individuals who maintain persistently low viral loads in the absence of treatment (typically dubbed “elite controllers” if the viral load is <50 copies/mL and “viremic controllers” if the viral load is <2,000 copies/mL). The main mechanism by which CD8 T cells kill virus-infected cells involves release of specialized cell-destroying substances, one of which is called perforin (named after its ability to perforate the membrane of cells). Until quite recently it was thought that CD8 T cells carried a stash of perforin that had to be replenished by cell proliferation after it was unleashed, but it is now appreciated that CD8 T cells can rapidly manufacture new supplies of perforin which allows serial killing of multiple infected targets. This revelation resulted from the discovery that the antibody normally used to detect perforin within cells was unable to detect the rapidly synthesized form. The researchers responsible for this advance – a group led by Michael Betts at the University of Pennsylvania – have now applied their method for detecting perforin expression to HIV-specific CD8 T cells from elite controllers.
The study found that elite controllers consistently displayed a higher proportion of HIV-specific CD8 T cells expressing perforin compared to individuals with progressive infection. This finding held true for a broad array of HIV antigens. The HIV-specific CD8 T cells of elite controllers also made a greater amount of perforin compared to both viremic controllers and progressors. The researchers documented a significant inverse correlation between the average HIV-specific CD8 T cell perforin expression within each study participant and their viral load. The correlation was still seen if CD8 T cells to each HIV antigen were considered separately, and if the analysis was restricted to only those individuals with detectable viral loads. Suppression of viral load by antiretroviral therapy was not able to significantly enhance perforin expression by HIV-specific CD8 T cells in progressors, leading the researchers to conclude: “Further studies are necessary to identify the mechanism(s) underlying the relative absence of perforin upregulation in progressive HIV infection, and, if possible, to discover a means by which this critical function can be regained or elicited through therapeutic intervention.”
PLoS Pathog 6(5): e1000917. doi:10.1371/journal.ppat.1000917
Adam R. Hersperger1, Florencia Pereyra2, Martha Nason3, Korey Demers1, Prameet Sheth4, Lucy Y. Shin4, Colin M. Kovacs5, Benigno Rodriguez6, Scott F. Sieg6, Leia Teixeira-Johnson7, Debbie Gudonis8, Paul A. Goepfert9, Michael M. Lederman6, Ian Frank8, George Makedonas1, Rupert Kaul4, Bruce D. Walker2,10, Michael R. Betts1
1 Department of Microbiology, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America, 2 Ragon Institute of MGH, MIT, and Harvard, Boston, Massachusetts, United States of America, 3 Biostatistics Research Branch, National Institutes of Health, Bethesda, Maryland, United States of America, 4 Department of Medicine, University of Toronto, Toronto, Ontario, Canada, 5 Canadian Immunodeficiency Research Collaborative, Toronto, Ontario, Canada, 6 Case Western Reserve University and University Hospitals of Cleveland, Cleveland, Ohio, United States of America, 7 Department of Infectious Diseases, Cleveland Clinic, Cleveland, Ohio, United States of America, 8 Division of Infectious Diseases, Department of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, United States of America, 9 Department of Medicine, University of Alabama at Birmingham, Birmingham, Alabama, United States of America, 10 Howard Hughes Medical Institute, Chevy Chase, Maryland, United States of America
Many immune correlates of CD8+ T-cell-mediated control of HIV replication, including polyfunctionality, proliferative ability, and inhibitory receptor expression, have been discovered. However, no functional correlates using ex vivo cells have been identified with the known ability to cause the direct elimination of HIV-infected cells. We have recently discovered the ability of human CD8+ T-cells to rapidly upregulate perforin—an essential molecule for cell-mediated cytotoxicity—following antigen-specific stimulation. Here, we examined perforin expression capability in a large cross-sectional cohort of chronically HIV-infected individuals with varying levels of viral load: elite controllers (n = 35), viremic controllers (n = 29), chronic progressors (n = 27), and viremic nonprogressors (n = 6). Using polychromatic flow cytometry and standard intracellular cytokine staining assays, we measured perforin upregulation, cytokine production, and degranulation following stimulation with overlapping peptide pools encompassing all proteins of HIV. We observed that HIV-specific CD8+ T-cells from elite controllers consistently display an enhanced ability to express perforin directly ex vivo compared to all other groups. This ability is not restricted to protective HLA-B haplotypes, does not require proliferation or the addition of exogenous factors, is not restored by HAART, and primarily originates from effector CD8+ T-cells with otherwise limited functional capability. Notably, we found an inverse relationship between HIV-specific perforin expression and viral load. Thus, the capability of HIV-specific CD8+ T-cells to rapidly express perforin defines a novel correlate of control in HIV infection.
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