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Journal of Virology, May 2005, p. 6299-6311, Vol. 79, No. 10
0022-538X/05/$08.00+0     doi:10.1128/JVI.79.10.6299-6311.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Distinct Mechanisms of CD4⁺ and CD8⁺ T-Cell Activation and Bystander Apoptosis Induced by Human Immunodeficiency Virus Type 1 Virions

Department of Cancer Immunology and AIDS, Dana-Farber Cancer Institute,¹ Department of Neurology, Harvard Medical School, Boston, Massachusetts 02115²

Received 19 August 2004/ Accepted 13 January 2005

Apoptosis of uninfected bystander T cells contributes to T-cell depletion during human immunodeficiency virus type 1 (HIV-1) infection. HIV-1 envelope/receptor interactions and immune activation have been implicated as contributors to bystander apoptosis. To better understand the relationship between T-cell activation and bystander apoptosis during HIV-1 pathogenesis, we investigated the effects of the highly cytopathic CXCR4-tropic HIV-1 variant ELI6 on primary CD4⁺ and CD8⁺ T cells. Infection of primary T-cell cultures with ELI6 induced CD4⁺ T-cell depletion by direct cell lysis and bystander apoptosis. Exposure of primary CD4⁺ and CD8⁺ T cells to nonreplicating ELI6 virions induced bystander apoptosis through a Fas-independent mechanism. Bystander apoptosis of CD4⁺ T cells required direct contact with virions and Env/CXCR4 binding. In contrast, the apoptosis of CD8⁺ T cells was triggered by a soluble factor(s) secreted by CD4⁺ T cells. HIV-1 virions activated CD4⁺ and CD8⁺ T cells to express CD25 and HLA-DR and preferentially induced apoptosis in CD25⁺HLA-DR⁺ T cells in a CXCR4-dependent manner. Maximal levels of binding, activation, and apoptosis were induced by virions that incorporated MHC class II and B7-2 into the viral membrane. These results suggest that nonreplicating HIV-1 virions contribute to chronic immune activation and T-cell depletion during HIV-1 pathogenesis by activating CD4⁺ and CD8⁺ T cells, which then proceed to die via apoptosis. This mechanism may represent a viral immune evasion strategy to increase viral replication by activating target cells while killing immune effector cells that are not productively infected.

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