Different thresholds of T cell activation regulate FIV infection of CD4+CD25+ and CD4+CD25− cells

Cellular activation plays an important role in retroviral replication. Previously, we have shown that CD4+CD25+ T cells by the virtue of their partially activated phenotype represent ideal candidates for a productive feline immunodeficiency virus (FIV) infection. In the present study, we extended our previous observations with regard to FIV replication in CD4+CD25+ and CD4+CD25− cells under different stimulation conditions. Both CD4+CD25+ and CD4+CD25− cells remain latently infected in the absence of IL-2 or concanvalinA (ConA), respectively; harboring a replication competent provirus capable of reactivation several days post-infection. While CD4+CD25+ cells require low levels of exogenous IL-2 and virus inputs for an efficient FIV replication, CD4+CD25− T cells can only be productively infected in the presence of either high concentrations of IL-2 or high virus titers, even in the absence of mitogenic stimulation. Interestingly, while high virus input activates CD4+CD25− cells to replicate FIV, it induces apoptosis in a high percentage of CD4+CD25+ T cells. High IL-2 concentrations but not high virus inputs lead to surface upregulation of CD25 and significant cellular proliferation in CD4+CD25− cells. These results suggest that CD4+CD25+ and CD4+CD25− T cells have different activation requirements which can be modulated by both viral and cytokine stimuli to reach threshold activation levels in order to harbor a productive FIV infection. This holds implications in vivo for CD4+CD25+ and CD4+CD25− cells to serve as potential reservoirs of a productive and latent FIV infection.