Background:  Epitope mutation allowing viral evasion of HIV-1-specific cytotoxic T-lymphocytes (CTL) is believed to be a key mechanism of immune failure. A widely used measurement to detect escape mutations is “functional avidity” comparison of index to mutated epitope sequences. High avidity has been proposed to allow better CTL antiviral activity, but the true relationship between functional avidity and the interaction of CTL with infected cells is unknown. Contradictory reports of HIV-1 epitope mutation in vivo have cited a 10- to 100-fold reduction in avidity as either significant or insignificant for escape. Here we report detailed studies of multiple CTL clones, comparing their avidity to antiviral activity.

Methods:  For several HIV-1-specific CTL clones, the functional avidity (SD₅₀) against multiple epitope variants was measured by classical exogenous peptide titration chromium release assays. Corresponding HIV-1 molecular clones containing the same epitope variants were constructed by point mutagenesis. The CTL clones were then challenged with HIV-1-infected target cells containing epitope variants, for measurement of antiviral activity (ability to kill the infected cells and to suppress HIV-1 replication).

Results:  The CTL clones recognized the epitope variants with avidity varying over several orders of magnitude. Comparisons of avidity to antiviral activity indicated a tight “avidity threshold” required for antiviral activity (infected cell killing and suppression of HIV-1 replication) of CTL, above which, activity plateaued and below which, activity was lost. This avidity threshold spanned approximately 1 log for all CTL clones, but its location, reflected by the avidity required for 50% maximal infected cell killing efficiency (KE₅₀), differed depending on the epitope targeted.

Conclusions:  These data indicate that the functional avidity required for the antiviral activity of HIV-1-specific CTL varies depending on the epitope. When assessing for viral escape due to epitope mutation the absolute change in avidity is not sufficient; the relationship of avidity to the KE₅₀ better reflects whether CTL recognize the variant. Furthermore, greater avidity above the avidity threshold does not yield greater antiviral activity, consistent with the function of T-cell receptor binding as a simple trigger for CTL activity.