SUMMARY Virus-specific CD8+ T cells probably mediate control over HIV replication in rare individuals, termed long-term nonprogressors (LTNPs) or elite controllers. Despite extensive investigation, the mechanisms responsible for this control remain incompletely understood. We observed that HIV-specific CD8+ T cells of LTNPs persisted at higher frequencies than those of treated progressors with equally low amounts of HIV. Measured on a per-cell basis, HIV-specific CD8+ T cells of LTNPs efficiently eliminated primary autologous HIV-infected CD4+ T cells. This function required lytic granule loading of effectors and delivery of granzyme B to target cells. Defective cytotoxicity of progressor effectors could be restored after treatment with phorbol ester and calcium ionophore. These results establish an effector function and mechanism that clearly segregate with immunologic control of HIV. They also demonstrate that lytic granule contents of memory cells are a critical determinant of cytotoxicity that must be induced for maximal per-cell killing capacity.

IntroductionThe hematopoietic system is in a constant state of self-renewal as stem cells continuously replenish short-lived blood cells. 1 All blood cells are derived from the long-term hematopoietic stem cell (LT-HSC) subset that maintains peripheral homeostasis by undergoing continual self-renewal for the life of the organism. LT-HSCs differentiate into multipotent progenitor cells (MPPs), a more mature subset that lacks the long-term ability to self-renew but retains the capacity to reconstitute all blood lineages. 1,2 Both LT-HSCs and MPPs are found in the lineage-negative, c-Kitpositive, and Sca-1-positive (LSK) populations. 3 Studying the LSK subset itself has contributed to the understanding of HSC biology because it is a population highly enriched for HSCs. Within the LSK subset, those cells positive for CD150 expression and negative for CD48 identify LT-HSCs that are capable of long-term self-renewal (CD150 ϩ CD48 Ϫ LSK), whereas those cells that are negative for both (CD150 Ϫ CD48 Ϫ LSK) identify a multipotential population that has a comparably limited contribution to long-term hematopoiesis and is enriched for MPPs. 1,4,5 Although definitive HSC experiments require a test of the functional capacity of the populations, the surface phenotype of these populations allows for an estimation of the presence of stem cells with long-term self-renewal capacity or those with more limited hematopoietic potential.Among the signal transduction pathways that have attracted considerable attention as possibly being involved in HSC selfrenewal is the phosphoinositide 3-kinase (PI3K)-AKT pathway. PI3K is a lipid kinase 6 critical for the activation of AKT, a family of serine threonine kinases essential for the control of cellular metabolism and survival in multiple tissues. 7,8 Haneline et al 9 demonstrated that HSCs with decreased PI3K activity exhibit defective hematopoietic reconstitution and a reduced proliferative capacity. Concordantly, conditional deletion of phosphatase and tensin homolog (PTEN), a phosphatase that negatively regulates PI3K, 10 promotes differentiation and proliferation at the expense of self-renewal, leading to depletion of the HSC pool. 11,12 Researchers have also examined the importance of molecules downstream of the PI3K/AKT pathway. For instance, FOXO, a family of transcription factors negatively regulated by AKT, 13 controls HSC quiescence by maintaining a low threshold of intracellular reactive oxygen species (ROS). 14 HSCs that lack multiple FOXO family members are hyperproliferative and fail to self-renew but are normalized by treatment with antioxidants. 14 A recent report by Kharas et al 15 showed that constitutive activation of AKT in hematopoietic HSCs results in a hyperproliferative state and subsequent HSC depletion, akin to the phenotype of PTEN-deleted HSCs. However, to fully appreciate the biologic role of AKT in HSC development, complementary studies in the absence of AKT are necessary. A main challenge to depleting AKT from HSCs is the expression of 3 isoforms in mammalian ...