To further assess the role of immune modulation in the observed antitumor effects mediated by CpG-Stat3 siRNA conjugate treatment, we analyzed changes in Th1 cytokine/ chemokines and co-stimulatory molecule expression by DCs in the TDLNs. Lack of Stat3 in DCs has been shown to upregulate expression of Th1 cytokines/chemokines 10 , 20 , 42 , 43 , which can be greatly amplified by CpG treatment 25 . As shown in Fig. 5a , injection of the CpG-Stat3 siRNA at tumor site resulted in upregulation of several Th1 cytokines and chemokines, which were shown to be upregulated by Stat3 ablation 10 , 20 , 42 , 43 . It has also been documented that DCs with low expression levels of co-stimulatory molecules mediate immune tolerance 44 , which is one of the proposed mechanisms for tumor immune evasion induced by Stat3 activation in tumor-associated DCs 10 . We therefore analyzed expression of co-stimulatory molecules by DCs enriched from TDLNs. Results from these analyses indicated that CpG-Stat3 siRNA reduced the number of the DCs with low expression of costimulatory molecules, including MHC class II, CD80 and CD40, which was accompanied by a modest increase in expression of these co-stimulatory molecules ( Supplementary Fig.  12 ). Stat3 ablation in myeloid cells followed by local treatment has been shown to induce potent antitumor innate immune responses that involve neutrophils 10 . We therefore assessed whether CpG-Stat3 siRNA conjugate treatment could lead to neutrophil-associated tumor cell apoptosis. Co-staining B16 tumor tissue sections with antibodies specific to activated caspase-3 and neutrophils revealed that CpG-Stat3 siRNA treatment-induced massive tumor cell apoptosis (activated caspase 3-positive), which was associated with an increase in tumor-infiltrating neutrophils (Fig. 5b ). An increase in neutrophils in tumor after CpG-Stat3 siRNA treatment was further confirmed by flow cytometry (Fig. 5c) .