Background: Low doses of the demethylating agent decitabine have been shown to enhance the sensitivity of tumors to immune eVector cells and molecules through upregulation of tumor antigen presentation and apoptotic pathways. EVects on host immune eVector and suppressor responses have not been well characterized. Methods: Mice bearing B16 melanoma were treated with low-dose decitabine, cytokine, interleukin-2 (IL-2), tolllike receptor 9 agonist ODN1826, and/or a viral vectored vaccine targeting the melanoma antigen Trp2. Lymphoid and myeloid eVector and suppressor cells were examined both systemically and intratumorally with functional, Xow cytometric, and polymerase chain reaction-based assays. Results: Enhancement of tumor growth delay was observed when decitabine was applied sequentially but not concurrently with IL-2. In contrast, complete responses and prolonged survival were observed when decitabine was applied with ODN1826 as therapy and with ODN1826 as a Trp2 vaccine adjuvant. Decitabine decreased natural killer and antigen-speciWc cellular immune responses when administered concurrently with IL-2 and with ODN1826; the Th1-associated transcription factor Tbet also decreased. T regulatory cells were not aVected. When applied concurrently with ODN1826, decitabine increased macrophage cytotoxicity, M1 polarization, and dendritic cell activation. Myeloid-derived suppressor cells were reduced. Conclusion: Low-dose decitabine promotes both anti-and pro-tumor host immune responses to immunotherapeutics in melanoma-bearing mice. Macrophage eVector and dendritic cell activation increase, and myeloid suppressor cells decrease. Lymphoid eVector responses, however, can be inhibited. © Springer-Verlag 2012.