Intracellular IFN-γ expression in response to peptide stimulation was performed as previously described (19, 25). A total of 106 brain-derived monocytes or splenocytes per well were cultured for 5 h at 37°C in 200 μl of RPMI 1640 medium, supplemented with 5% fetal calf serum, 10 U of human recombinant interleukin-2, and brefeldin A (Golgiplug [1 μl/ml]; PharMingen) either with or without peptides. The peptides were used at a concentration of 1 μg/ml. Cells were then stained as described above for surface expression of CD8 and incubated overnight at 4°C. For intracellular IFN-γ staining, cells were then fixed and permeabilized using the Cytofix/Cytosperm kit (PharMingen) and stained with a fluorescein isothiocyanate-conjugated monoclonal rat anti-mouse IFN-γ antibody (clone XMG 1.2; PharMingen). Cells were washed and then suspended in PBS containing 2% paraformaldehyde and analyzed by FACSan flow cytometry (Becton-Dickinson).