Naturally occurring C-terminally truncated STAT5 is a negative regulator of HIV-1 expression.

CD4(+) cells of most individuals infected with HIV-1 harbor a C-terminally truncated and constitutively activated form of signal transducer and activator of transcription-5 (STAT5 Delta). We report that the chronically HIV-infected U1 cell line expresses STAT5 Delta but not full-length STAT5. Granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulation of U1 cells promoted early activation of STAT5 Delta and of extracellular signal regulated kinases (ERKs), followed by later activation of activator protein 1 (AP-1) and HIV expression. Inhibition of ERK/AP-1 by PD98,059 abolished, whereas either tyrphostin AG490 or a STAT5 small interfering RNA (siRNA) enhanced, virion production in GM-CSF-stimulated U1 cells. Chromatin immunoprecipitation demonstrated the induction of STAT5 Delta binding to STAT consensus sequences in the HIV-1 promoter together with a decreased recruitment of RNA polymerase II after 1 hour of GM-CSF stimulation of U1 cells. Down-regulation of STAT5 Delta by siRNA resulted in the up-regulation of both HIV-1 gag-pol RNA and p24 Gag antigen expression in CD8-depleted leukocytes of several HIV-positive individuals cultivated ex vivo in the presence of interleukin-2 but not of interleukin-7. Thus, the constitutively activated STAT5 Delta present in the leukocytes of most HIV-positive individuals acts as a negative regulator of HIV expression.