CD11c^loB220⁺ interferon-producing killer dendritic cells are activated natural killer cells

Interferon-producing killer dendritic cells (IKDCs) are a recently described subset of CD11c^loB220⁺ cells that share phenotypic and functional properties of DCs and natural killer (NK) cells (Chan, C.W., E. Crafton, H.N. Fan, J. Flook, K. Yoshimura, M. Skarica, D. Brockstedt, T.W. Dubensky, M.F. Stins, L.L. Lanier, et al. 2006. Nat. Med. 12:207–213; Taieb, J., N. Chaput, C. Menard, L. Apetoh, E. Ullrich, M. Bonmort, M. Pequignot, N. Casares, M. Terme, C. Flament, et al. 2006. Nat. Med. 12:214–219). IKDC development appears unusual in that cytokines using the interleukin (IL)-2 receptor β (IL-2Rβ) chain but not those using the common γ chain (γ_c) are necessary for their generation. By directly comparing Rag2^−/−γ_c^−/y, Rag2^−/−IL-2Rβ^−/−, Rag2^−/−IL-15^−/−, and Rag2^−/−IL-2^−/− mice, we demonstrate that IKDC development parallels NK cell development in its strict IL-15 dependence. Moreover, IKDCs uniformly express NK-specific Ncr-1 transcripts (encoding NKp46), whereas NKp46⁺ cells are absent in Ncr1^gfp/+γ_c^−/y mice. Distinguishing features of IKDCs (CD11c^loB220⁺MHC-II⁺) were carefully examined on developing NK cells in the bone marrow and on peripheral NK cells. As B220 expression was heterogeneous, defining B220^lo versus B220^hi NK1.1⁺ NK cells could be considered as arbitrary, and few phenotypic differences were noted between NK1.1⁺ NK cells bearing different levels of B220. CD11c expression did not correlate with B220 or major histocompatibility complex (MHC) class II (MHC-II) expression, and most MHC-II⁺ NK1.1⁺ cells did not express B220 and were thus not IKDCs. Finally, CD11c, MHC-II, and B220 levels were up-regulated on NK1.1⁺ cells upon activation in vitro or in vivo in a proliferation-dependent fashion. Our data suggest that the majority of CD11c^loB220⁺ “IKDC-like” cells represent activated NK cells.