Bacillus subtilis-derived poly-gamma-glutamic acid (gamma PGA) stimulates dendritic cells (DCs) to produce IL12, leading to CD4(+) T cell differentiation toward the Th1 phenotype, but DCs consist of heterogeneous subpopulations with a variety of immune functions. Among these, natural killer dendritic cells (NKDCs) play an important role in anti-tumor immune responses. Herein, we demonstrate the role of NKDCs in gamma PGA-meditated anti-tumor immune responses. NK1.1(+) CD11c(+) NKDCs were stimulated upon gamma PGA stimulation in vitro and in vivo to up-regulate lymphocyte activation markers, MHC class I and II, and co-stimulatory molecules. In particular, NKDCs were activated by gamma PGA to produce IFN gamma and TNF alpha, like NM cells, as well as IL12, like DCs, implying that NKDCs have unique and multifunctional roles. Importantly, NKDCs stimulated by gamma PGA conferred stronger anti-tumor effects in mice and showed increased cytotoxicity against various tumor cell lines in vitro. In conclusion, NKDCs are one of the key players in anti-tumor immunity induced by gamma PGA. (C) 2013 Elsevier Inc. All rights reserved.