B7-H6 is not detected on blood cells but is expressed on tumor cells. (A and B) Assessment of B7-H6 cell surface staining on PBMCs, P815 cell transfectants, and tumor cells was performed by flow cytometry using anti–B7-H6 mouse antiserum. (C) NKp30-Fc cell surface staining on tumor cells is also represented. HIg, control human Ig; mIg, control mouse Ig. (D) Freshly isolated primary blood NK cells in PBMCs were tested for their activation induced by the indicated cell lines in the presence or absence of anti-NKp30 F(ab′)₂ mAbs, B7-H6 mouse antiserum, or control mouse Ig (mIg). 500,000 PBMCs were added to 100,000 tumor cells. The fraction of reactive NK cells (percentage of responding NK cells) was assessed by adding the percentage of CD107⁺IFN-γ⁻, CD107⁺IFN-γ⁺, and CD107⁻IFN-γ⁺ NK cells (mean + SD; n = 3 independent experiments). Statistical analysis was performed using a Mann-Whitney U test. *, P < 0.05. Open histograms, B7-H6 staining; filled histograms, control mIg staining.