Antigen-specific intracellular IFN-γ production. Cells isolated from various tissues at necropsy of macaque 53 were used for determining intracellular IFN-γ producing T cells by immunofluorescence staining after stimulation with the HIV-1 envelope peptide mixture (pep-mix), cell fixation and permeabilization as described in the methods section. To define T cell subsets, lymphocytes were stained for cell surface CD4, CD8, and CD95, the later to distinguish memory-phenotype. Among the CD95+ T cells, IFN-γ + CD8+ and CD4+ T cells were detected in the intestinal lymphocytes, but not in peripheral blood, spleen, and lymph nodes.