Effect of IL-15 on circulating CMV-specific CD8⁺ memory T cells. (A) Samples of PBMCs were obtained from CMV-immune macaques 97067, K00043, 02279, and T02392 at the indicated time before, during, or after intermittent IL-15 administration. The samples were stimulated with the CMV peptide, and examined by CFC for the presence of IFN-γ–producing CMV-specific CD8⁺ T cells. Controls consisted of PBMCs cultured in medium alone to subtract background levels. The absolute number of CMV-specific CD8⁺ T cells per microliter of peripheral blood was calculated based on the frequency of T cells that produced IFN-γ after CMV-peptide stimulation at each time point and the absolute number of CD8⁺ T cells. The fold increase of the absolute number of CMV-specific CD8⁺ T cells per microliter of peripheral blood compared with the start of the treatment is shown. The vertical bar represents the mean. indicates the duration of the IL-15 administration. *K00043 and T02392: no sample was available from week 8. (B-C) Representative data are shown for macaque K00043. PBMCs obtained before and on day 6 of the IL-15 treatment were stimulated with media alone or with CMV peptide, and examined by CFC for expression of IFN-γ and for Ki-67-expression, respectively. (B) The samples are gated on lymphocytes or (C) on peptide-stimulated CMV⁺ CD8⁺ IFN-γ⁺ T cells.