Age-related NK cell infiltration into the brain. A, CD161+ NKp30+ cells was increased in the hippocampus of aged compared with young rats (*p < 0.05; student's t-test for independent means; n = 6). The panel to the right in Figure 2b shows a sample flow cytometry plot, demonstrating increased expression of NKp30 in the aged brain, after gating on the CD161+ cells. B, IL-2 increased IFNγ release from CD161+ cells obtained from rat hippocampus and C, cortex, and D, from a human NK cell line (*p < 0.05; ***p < 0.001; student's t-test for independent means; n = 6). E, IL-2 concentration was not significantly altered in hippocampus of aged and young rats.