To ensure proper identification of the NK cell population in PECs, NK1.1-positive cells were costained with markers identifying TCR-␣/␤ and TCR-␥/␦ T cells. A lymphocyte gate was set to maximize inclusion of NK1.1 ϩ cells at different times after infection, and it was identified as an "extended lymphocyte gate." Using this scheme, Ͻ5% of the NK1.1 ϩ cells were TCR-␦ ϩ under naive conditions; after LCMV infection, the population was reduced to Ͻ2%, and the cells were not producing IFN-␥ (data not shown). NK1.1 ϩ cells that were TCR-␤ ϩ (NK T cells) represented approximately 30% of the total NK1.1 ϩ population at day 0. After LCMV infection, this population was again diminished to Ͻ5%, and the IFN-␥ produced by these cells represented only about 3% of the total IFN-␥ ϩ cells. Thus, all experiments defined NK cells to be NK1.1 ϩ TCR-␤ Ϫ .