The development of autochtonous mammary tumors in HER-2/neu transgenic mice is facilitated by immune tolerance to the neu-transgene. However, appropriate vaccination strategies can initiate immune system-mediated antitumor response by a process that requires IFN-gamma. We investigated the role of inducible nitric oxide synthase (iNOS) induction by IFN-gamma to promote tumor cell apoptosis. Tumors from FVBN202 mice expressing the normal neu gene under the control of the MMTV-LTR were treated in slice cultures with IFN-gamma for up to 24 hr. Apoptosis was induced, which depended on iNOS enzymatic activity. iNOS expression was predominantly found in infiltrating CD11b(+)CD11c(+) myeloid cells and at much lower levels in the tumor epithelium. By contrast, IFN-gamma treatment of explant cultures of tumor epithelial cells was not sufficient to efficiently induce iNOS, emphasizing an important role of the integrity of tumor tissue architecture, which was preserved in the slice cultures. This notion was further supported by the upregulation of iNOS costimulatory cytokines TNF-alpha and IL-1beta in slice cultures but not in explants and the capability of purified CD11b(+)CD11c(+) cells to enhance iNOS expression of tumor cells in cocultures. The findings suggest that tumor-infiltrating myeloid cells in immuno-tolerant HER-2/neu transgenic mice possess tumor killing ability via induction of iNOS and underline the capacity of antitumor strategies designed to stimulate infiltrating myeloid cells.