Txb21-deficient NK cells fail to reject MHC class I-negative tumors. (A) CD11b and CD27 expression in peritoneal NKp46⁺CD3⁻ NK cells in Txb21^+/+ (left panels) and Txb21^−/− (right panels) mice injected with RMA (top panels) or RMA-S (bottom panels) 48 hours after tumor inoculation. (B) Absolute CD11b^hiCD27^low NK-cell number (mean ± SD) in the peritoneum of tumor injected mice from 2 independent experiments with 5 mice per group. (C) RMA and RMA-S tumor cells were labeled with CMTMR and CFSE, respectively, and coinjected into Txb21^+/+ and Txb21^−/− C57BL/6 mice. Forty-eight hours after tumor challenge, peritoneal cells were recovered and analyzed by flow cytometry for CFSE and CMTMR. Data are mean ± SD residual tumor cells of 3 independent experiments, including 3 mice per group. (D) A total of 2 × 10⁵ RMA-S-CFSE tumor cells were injected into Txb21^−/− mice as in panel C, and mice were transferred intravenously with spleen CD27^hi or CD11b^hiCD27^low NK cells enriched from Txb21^+/+ mice. Forty-eight hours after tumor injection, peritoneal cells were recovered and analyzed by flow cytometry for CFSE. Data are mean ± SD of 2 independent experiments, including 4 mice per group.