Measuring HIV-1—Specific T Cell Immunity: How Valid Are Current Assays?

Because success in eliciting broadly neutralizing antibodies has been limited to date and because data on the importance of the cytotoxic T lymphocyte (CTL) response in controlling HIV exist, recent vaccine candidates advancing in clinical trials have been focused primarily on inducing cellular immunity. Virusspecific CD8+ T cell responses are thought to play an important role in the control of natural HIV-1 infection, on the basis of the following observations: (1) the first appearance of HIV-l-specific CD8+ T cell responses in primary infection coincides with the decline of peak viremia [2, 3]; (2) depletion of CD8+ T cells in simian immunodeficiency virus (SIV)infected rhesus macaques results in increased viremia [4]; (3) polymorphisms in HLA class I alleles restricting CD8+ T cell responses are associated with differential HIV-1 disease outcome [5, 6]; and (4) HIV-1 evades virus-specific CD8+ T cell-mediated immune pressure by selecting for variant epitope sequences [7, 8]. However, despite this strong evidence for the important role of CD8+ T cell immunity in the control of HIV-1 infection, current assays to quantify T cell immunity by measuring antigen-specific cytokine production or cytotoxic activity have failed to correlate HIV-1 specific CD8+ T cell responses with protection from infection or control of viral replication [9-10]. After HIV-1 infection of a cell and reverse transcription of the viral RNA, the viral genome is integrated into the host cell genomic DNA. Once active viral replication starts, resulting in translation of viral proteins, parts of the viral proteome are processed and presented on the surface of infected cells by HLA class I molecules, allowing target cell recognition by the T cell receptor of epitope-specific CD8+ T cells. The rapid recognition and subsequent elimination of HIV-l-infected cells before production of new viruses represent critical effector functions of virus-specific CD8+ T cells that are not reflected in the quantification of cytokine secretion or cytotoxic activity after stimulation with high concentrations of exogenous peptide. In 1996, Drs. Otto Yang, Paul Johnson, and Bruce Walker described an in vitro HIV-1 replication inhibition assay that measured the ability of CD8+ T cells to inhibit viral replication in autologous CD4+ T cells infected with laboratory strains of HIV1 [11]. Studies by these investigators, as well as by additional groups adapting this in vitro assay, have demonstrated that virus-specific CD8+ T cells can inhibit, and under some conditions fully suppress, HIV-1 replication in vitro in a dose-dependent manner through direct cytotoxic activity as well as through the secretion of antiviral cytokines [12-16]. In new data published in this issue of the Journal, Bennett et al. compare the cross-clade antiviral activity of HIV-1specific CD8+ T cells using the in vitro viral replication inhibition assay with a peptide-based cytolysis assay [17]. The Received 4 October 2007; accepted 5 October 2007; electronically published 9 January 2008. Potential conflicts of interest: none reported. Reprints or correspondence: Dr. Marcus Altfeld, Partners AIDS Research Center, 149 13th St., Charlestown, MA 02129 (maltfeld@partners.org). The Journal of Infectious Diseases 2008; 197:337-9 C 2008 by the Infectious Diseases Society of America. All rights reserved. 0022-1899/2008/19703-0003$15.00 DOI: 10.1086/525288