CD8⁺-Cell-Mediated Suppression of Virulent Simian Immunodeficiency Virus during Tenofovir Treatment

Abstract

The ability of tenofovir to suppress viremia in simian immunodeficiency virus (SIV)-infected macaques for years despite the presence of virulent viral mutants with reduced in vitro susceptibility is unprecedented in this animal model. In vivo cell depletion experiments demonstrate that tenofovir's ability to suppress viremia during acute and chronic infection is significantly dependent on the presence of CD8 ؉ lymphocytes. Continuous tenofovir treatment was required to maintain low viremia. Although it is unclear whether this immunemediated suppression of viremia is linked to tenofovir's direct antiviral efficacy or is due to independent immunomodulatory effects, these studies prove the concept that antiviral immune responses can play a crucial role in suppressing viremia during anti-human immunodeficiency virus drug therapy.Although highly active antiretroviral therapy (HAART) regimens provide significant progress in the clinical management of human immunodeficiency virus (HIV) infection, their longterm use is often limited by problems such as incomplete virus suppression and drug resistance. To alleviate this problem, it is believed we need to invoke the help of the immune system to help limit virus replication and/or the virus-induced immune dysfunction.Immune responses, especially cell-mediated immune responses (CD4 ϩ -T-helper cells and CD8 ϩ T lymphocytes) are thought to play an important role in controlling virus replication and determining disease-free survival. Evidence for this comes from studies in untreated long-term nonprogressors (for a review, see reference 2). In addition, macaque data have shown conclusively that CD8 ϩ lymphocytes play an important role in limiting lentivirus replication because in vivo CD8ϩ -cell depletion of untreated macaques resulted in an increase in viremia (23,34,67).A number of dilemmas exist, however, for patients receiving HAART. HAART initiated during acute HIV infection can often preserve or increase antiviral immune responses, but episodes of transient viremia may be required to maintain these responses (54, 63). In contrast, when HAART is initiated during chronic HIV infection, HIV-specific immune responses are more difficult to restore (for a review, see reference 2).The goal of immunotherapeutic strategies is to augment these antiviral immune responses to prevent the emergence and/or the replication of drug-resistant mutants during HAART or to allow simplified regimens or periods without drug treatment. Although different approaches are investigated to stimulate antiviral immune responses (e.g., structured treatment interruptions, active immunizations, and cytokines), the results have been poor or success has been limited mainly to patients with acute infection with often transient results (2,3,25,63). Progress in this area may depend on a better understanding of the contribution of antiviral immune responses to the reduction of viremia during HAART and of the complex in vivo interactions of viral replication, drug resistance, immune responses, and drug pharmacok...

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