B220 is not up-regulated on mNK cells even on in vivo activation. CD27⁺ and CD27⁻ mNK-cell subsets sorted from the spleen of B6.SJL (CD45.1) mice were adoptively transferred into C57BL/6 (CD45.2; A-C) or B6.Rag1^−/− (CD45.2; D-F) mice through tail vein injections. The next day, mice were injected intraperitoneally with PBS (A,D), poly I:C (B), or anti-CD40 Ab (E). The expression of CD49b, CD11c, and B220 was analyzed on CD45.1⁺ CD45.2⁻ cells 18 hours after poly I:C injection (A-B) or 40 hours after anti-CD40 Ab injection (D-E). (C,F) CD69 expression is depicted on CD45.1⁺ CD45.2⁻ cells from PBS injected (gray filled) or poly I:C or anti-CD40 treated (bold) mice. Results are representative of ≥ 3 independent experiments.