Expression and activation of STAT5 isoforms in PBMC of HIV-positive individuals and in the chronically infected U1 cell line. Expression of both STAT5FL and STAT5Δ (upper panel) and of constitutively activated STAT5Δ in WCE of PBMC from HIV-positive individuals (lower panel) as detected by Western blotting using specific anti-STAT5 and antiphosphorylated STAT5 Ab, respectively. PBMC from all HIV-positive individuals studied are characterized by the dominant (patient nos. 1, 2, 3, 4, and 7) or exclusive (patient nos. 5 and 6) presence of STAT5Δ (upper panel). Variable levels of constitutive activation of STAT5Δ were detected by using anti-phospho-STAT5 (lower panel). Exclusive expression of either STAT5Δ or STAT5FL was detected in U1 and ACH-2 cells, respectively (upper panel). GM-CSF stimulation (15 minutes) of U1 cells led to the phosphorylation of STAT5Δ (U1, lower panel), whereas no stimuli were identified as capable of activating STAT5FL in ACH-2 cells (see text).