CD8+ lymphocyte p21cip1 production in FIV+ and FIV- cats following CD4+CD25+ co-culture. Levels of the CDKI p21cip1 begin to increase during G0 phase and reach maximal production in late G1/early S phase. However, p21cip1 is also increased in anergic T cells; thereby preventing the G to S phase transition. 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. p21cip1 production was increased by approximately 1.7 fold in FIV+ cats following a twelve hour CD4+CD25+ co-culture.