qRT-PCR was used to quantify the levels ofdFasL andegranzyme B mRNA in each sample relative to one random sample in the WT 0Gy group. to result in tumor control can be enhanced. Our preclinical approach to study the effects of local increase in IFN- levels can be used to further optimize the combination LUCT therapy strategy in terms of dosing and scheduling, which may lead to better clinical outcome. == Electronic supplementary material == The online version of this article (doi:10.1007/s00262-013-1506-7) contains supplementary material, which is available to authorized users. Keywords:Radiation therapy, Interferon, B16 melanoma, CXCR3, CD8+T cells == Introduction == Ionizing radiation has been used for the treatment for cancer for more than 100 years. Although radiation therapy (RT) is best known to directly kill tumor cells by the induction of lethal DNA damage, it is now appreciated that RT induces an anti-tumor immune response that is essential for its therapeutic effects including tumor growth delay, tumor eradication, and even long-term tumor-free survival. Since the 1940s, patients have been treated with low dose (typically 13 Gy) of daily fractions of radiation delivered over weeks or months [1]. However, with the introduction of improved tumor imaging technology and highly targeted radiation delivery systems, there has been increasing interest in the use of hypofractionated, high-dose RT, also known as stereotactic body radiation therapy (SBRT). Despite promising data from many clinical trials that exhibited the effectiveness of SBRT in a variety of cancers [25], the mechanisms by which the treatment reduces tumor growth are still unclear. This lack of understanding is currently a limiting factor in optimizing the treatment strategy in the clinic. We previously reported, using the murine melanoma B16-OVA model, that this frequency and number of CD8+T cells increased after RT [6]. In a similar model (B16-SIY), the depletion of CD8+T cells abrogated the ability of RT to delay tumor growth [7]. Data from our laboratory and other investigators suggested that RT NF 279 causes the release of tumor antigens and danger signals [8], which enhance the ability of dendritic cells (DCs) both in tumor-draining lymph nodes (tdLN) [6] as well as in tumors [9], to present antigens. These effects are in turn required for priming of antigen-specific T cells, and the resulting T cell proliferation, activation, cytokine release, and cytolytic activity cause effective killing of tumor cells. Soon after its discovery as an anti-viral factor, exogenous type I IFNs were shown to exhibit anti-tumor effects. IFN- has been FDA-approved for the treatment for various types of cancer since 1997 [10,11]. In the last NF 279 decade, studies revealed that endogenous type I IFNs play a critical role in tumor growth control by the host immune system [1214]. More recently, type I IFNs were NF 279 implicated in radiation-mediated delay of tumor growth. Although it is usually well established that type I IFNs have direct anti-proliferative effects, the main target of RT-induced type I IFNs is the hematopoietic compartment [9]. Strikingly, deficiency in type I IFN signaling abrogated the capacity of DCs to cross-prime antigens, which may result in less efficient priming of CD8+T cell-mediated anti-tumor responses. However, type I IFNs may affect CD8+T cells in other ways, for example, by providing signal 3 that T cells require to acquire full effector functions [15]. These possible direct effects of type I IFNs on T cells are still unclear. Our current study discloses that radiation-induced type I IFNs play a role in increasing CXCR3 chemokine NF 279 levels within the tumor, which affects the initial recruitment of immune cells into the tumor microenvironment. In addition, type I IFNs are also required for CD8+T cells to acquire an activated phenotype and tumor cell-killing activity, at least partly through direct effects of type I IFNs on T cells. Using an inducible expression system, we show that increasing intratumoral IFN- levels exogenously after RT can further improve the ability of the therapy to bring about tumor control. Therefore, SBRT is usually a moderately effective therapy that activates immune responses against tumor, and better clinical outcomes can be achieved by using SBRT in combination with immunotherapies like exogenous IFN-. == Materials and methods == == Mice == C57BL/6J wild-type (WT), B6.129S7-Ifngtm1ts(IFN-KO), B6.129P2-Cxcr3tm1Dgen/J (CXCR3 KO), and B6-SJL-PtpreaPepeb/BoyJ. NF 279