PD-1^highCM9⁺CD8⁺ T cells are characterized by low proliferative capacity. (A) Flow cytometry plots showing the CFSE profile of annexin V⁻CM9⁺CD8⁺ T cells. Cells were cultured for 6 days in the absence or presence of CM9 peptide. A representative sample from 5 macaques tested is shown. (B) PD-1⁺CD3⁺ and PD-1⁻CD3⁺ cells were sorted, labeled with CFSE (PD-1⁺ CD3⁺) or FarRed (PD-1⁻CD3⁺), and cultured in the absence or presence of CM9 peptide. The PD-1 control (FMO) and specific staining in a presort sample is shown on the left. Cells were harvested on day 6 and stained for PD-1 expression. Histograms depict the CFSE^low and FarRed^low populations in the annexin V⁻CM9⁺CD8⁺ T cells. A representative example from 3 macaques analyzed is shown.