Background:  Although virus-specific CD8⁺ T cells likely mediate control over HIV replication in patients variably referred to as long-term nonprogressors (LTNP) or elite controllers, the underlying mechanisms remain incompletely understood. We examined the cytotoxic function of HIV-specific CD8⁺ T cells in LTNP, viremic progressors and treated patients with HIV RNA levels <50 copies/mL (Rx <50) using highly quantitative assays that examined effector and target cell frequencies, delivery of functional granzyme B (GrB), and elimination of primary autologous HIV-infected CD4⁺ T cell targets.

Methods:  A single copy assay (SCA) quantitated plasma HIV-1 RNA levels to 1 copy/mL. By flow cytometry, cytotoxicity of autologous HIV_SF162-infected CD4⁺ T cell targets mediated by day 0 or day 6 HIV-specific CD8⁺ T cells was measured by functional GrB activity in live targets and infected CD4 elimination (ICE). The true effector:target (E:T) ratios were measured by flow cytometric detection of interferon-g (IFN-g) -producing CD8⁺ T cells and the fraction of p24-expressing targets.

Results:  For equally low antigen levels (p = 0.3), HIV-specific CD8⁺ T cells of LTNP persisted at higher frequencies than those of Rx<50 (medians 2.8% vs 0.1%, respectively; p <0.001). Although cytotoxicity measured by target GrB activity or ICE was significantly greater using day 6 vs day 0 CD8⁺ T cells for each group, day 6 cells of LTNP had markedly greater cytotoxic capacity than either group of progressors (p <0.001). GrB activity strongly correlated with ICE when day 6 cells were used (r = 0.8, p <0.001). Perforin content was tightly correlated with GrB target activity and ICE (r = 0.9, p <0.001 and r = 0.8, p <0.001, respectively). Cytotoxic capacity of day 6 CD8⁺ T cells from LTNP remained significantly greater than results obtained with progressors’ cells with little overlap between patient groups when the data was analyzed in the context of the measured E:T ratios (p <0.001).

Conclusions:  In contrast to progressors, the HIV-specific CD8⁺ T cells of LTNP have extraordinary cytotoxic capacity on a per-cell basis. These results suggest that delivery of cytotoxic proteins to, and elimination of, HIV-infected CD4⁺ T cells by HIV-specific CD8⁺ T cells is an effector mechanism that clearly segregates with immune control of HIV. They also demonstrate that lytic granule loading of memory cells is a critical determinant of cytotoxicity that must be induced for maximal per-cell killing capacity.