No abstract

Recent studies indicate that the time required for virus‐infected cells to become vulnerable for the activity of CTL is of significance for the capacity of CTL to control ongoing viral reproduction. To investigate whether this applies to the effectiveness of HIV‐1‐specific CTL, we measured virus production in cultures containing CD4+ T cells inoculated with HIV at low multiplicity of infection, and CTL directed against an early protein, Rev, or a late protein, RT. The Rev‐specific CTL prevented at least 2 log10 more HIV‐1 production, in 10 days, than similar numbers of RT‐specific CTL. To study how CTL effectiveness depends on variations in the potency of effector functions and kinetics of HIV protein expression, we developed a mathematical model describing CTL‐target cell interactions during successive infection cycles. The results show that substantially higher CTL‐mediated target cell elimination rates are required to achieve control as there is less time for CTL to act before infected cells release progeny virions. Furthermore, in vitro experiments with HIV recombinant viruses showed that the RT‐specific CTL were at least as effective as the Rev‐specific CTL, but only if the RT epitope was expressed as part of the early protein Nef. Together these results indicate that CTL control ongoing HIV reproduction more effectively if they are able to recognize infected cells earlier during individual viral replication cycles. This provides rationale for immunization strategies that aim at inducing, boosting or skewing CTL responses to early regulatory proteins in AIDS vaccine development.

Ex vivo detection of virus-specific cytotoxic T lymphocyte (CTL) responses is limited to the use of methods assessing cytokine production, degranulation, or perforin contents of antigen-specific CD8+ T cells. Generally, their cytotoxic activity is detectable only after cultivation. We describe the fluorescent antigentransfected target cellCTL (FATT-CTL) assay, which measures antigen-specific cytotoxicity ex vivo. Target cells were generated by nucleofection with DNA vectors encoding antigengreen fluorescent protein (GFP) fusion proteins. After coculture at various effector : target (E : T) cell ratios, viable and dead GFP-positive cells were quantified by flow cytometry, and antigen-specific target-cell elimination was calculated. The assay was validated with human immunodeficiency virus (HIV) and influenza virusspecific CTL clones and revealed cytotoxicity at lower E : T cell ratios than standard 51Cr-release assays. Moreover, antigen-specific cytotoxicity was detected ex vivo within 1 day in peripheral blood mononuclear cells from HIV-infected individuals. The FATT-CTL assay provides a versatile tool that will advance our understanding of cell-mediated immunity.

SummaryIn the present study, we examined the effect of the loss of the human leucocyte antigen (HLA)-B*3501-restricted nucleoprotein (NP)418-426 epitope on interferon (IFN)-g-production and lytic activity of the human cytotoxic T lymphocyte (CTL) response in vitro. Extensive amino acid variation at T cell receptor contact residues of the NP418-426 epitope has led to repeated evasion from specific CTL. We generated recombinant influenza viruses with variants of the NP418-426 epitope, which were used to stimulate peripheral blood mononuclear cells obtained from six HLA-B*3501-positive study subjects in order to expand virus-specific CTL. Loss of the NP418-426 epitope resulted in a significant reduction of IFN-g-expressing CD8+ T cells, similar to that observed previously after the loss of the HLA-B*2705-restricted NP383-391 epitope. In addition, the effect of the loss of the NP418-426 epitope on the lytic activity of the virus-specific CTL response was assessed. Also this functional property of the virus-specific CTL response was affected significantly by the loss of this and the NP383-391 epitope, as determined using the newly developed fluorescent antigen-transfected target cell (FATT)-CTL assay. These findings indicate that the loss of single immunodominant epitopes affects the functionality of the virus-specific CTL response significantly.