CD8+ lymphocyte Rb phosphorylation in FIV+ and FIV- cats following CD4+CD25+ co-culture. Hyperphosphorylation of Rb by cyclin/CDK complexes and subsequent separation of E2F proteins from Rb signals the irreversible commitment of the cell to S phase; while lack of Rb phosphorylation suggest either quiescence (G0) or anergy (G1 cell cycle arrest). As depicted here, CD8+ LN cells from either FIV+ (left) or FIV- (right) cats were either untreated (first column), or co-cultured with autologous CD4+CD25+ Treg cells (second column). Shown above is a representative blot for experiments from FIV+ (n = 4) and FIV- (n = 2) cats. There was a lack of Rb phosphorylation in both FIV+ and FIV- cats following an eighteen hour CD4+CD25+ co-culture.