T-cell immunity measured by IFN-γ ELISPOT. (A) IFN-γ spot-forming cells responding to SIV Gag, Pol, and DMSO (control antigen) antigens in each well from naïve and SHIV-immunized macaques before and after infusion of Gag- and Pol-peptide-pulsed autologous PBMC. Numbers of positive spots in actual ELISPOT wells are shown. (B) T cells secreting IFN-γ in response to either SIV Gag peptides or whole inactivated SIV prior to (□) and 1 week following (▪) infusion of peptide-pulsed autologous cells in two unimmunized macaques (E20 and E22) and four macaques previously immunized with DNA and FPV vaccines (B00, H8, H20, and H21).