Evidence for CD8(+) antiviral activity in cats infected with feline immunodeficiency virus

Human immunodeficiency virus (HIV) causes a long, asymptomatic infection characterized by normal to elevated numbers of circulating CD8(+) cells and a progressive decline in CD4(+) cells. It has been speculated that HIV-specific antiviral activity driven by CD8(+) T cells may control viral replication during this period and maintain the clinically asymptomatic stage of disease. The disease induced in cats by feline immunodeficiency virus (FIV) is similar to HIV in that it is characterized by a long asymptomatic stage with a progressive decline in CD4(+) cells, culminating in AIDS. In the present study, we demonstrate that FIV is more readily isolated from CD8(+) T-cell-depleted peripheral blood mononuclear cells (PBMC) of FIV-infected cats than from unfractionated PBMC cultures. In addition, CD8(+) cells isolated from FIV-positive cats demonstrating anti-FIV activity in PBMC cultures inhibit FIV infection of FCD4E cells in vitro. Anti-FIV activity is not found in FIV-negative cats and is not characteristic of cats acutely infected with FIV but is present in the majority of chronically infected, clinically asymptomatic and symptomatic cats. Decreases in plasma and cell-associated viremia during the acute-stage FIV infection appears to precede the appearance of CD8(+) anti-FIV cells in the circulation. In summary, this study demonstrates a population(s) of CD8(+) T cells in chronically FIV-infected cats capable of suppressing FIV replication in cultured PBMC. The significance of anti-FIV CD8(+) cells in the immunopathogenesis of the infection and disease progression has yet to be determined.