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Host Immunosurveillance Controls Tumor Growth via IFN Regulatory Factor-8–Dependent Mechanisms

¹ Laboratory of Tumor Immunology and Biology, ² Biostatistics and Data Management Section, National Cancer Institute, NIH, Bethesda, Maryland; and ³ Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta, Georgia

Requests for reprints: Scott I. Abrams, Laboratory of Tumor Immunology and Biology, Center for Cancer Research, National Cancer Institute, NIH, Building 10, Room 5B46, 10 Center Drive, Bethesda, MD 20892-1402. Phone: 301-496-4343; Fax: 301-496-2756; E-mail: sa47z{at}nih.gov.

IFN regulatory factor (IRF)-8 plays an important role in normal myelopoiesis. The loss of IRF-8 in myeloid cells results in a chronic myelogenous leukemia–like syndrome, suggesting that IRF-8 behaves as a tumor suppressor gene in certain hematopoietic malignancies. We have been investigating the molecular determinants of solid tumor progression, with an emphasis on apoptotic resistance. Recently, we showed that IRF-8 expression was directly correlated with Fas-mediated apoptosis, and inversely related to malignant phenotype. However, the functional role of IRF-8 in solid tumors is unresolved. We stably silenced IRF-8 expression via RNA interference in IRF-8–expressing mouse tumor cells, and evaluated them for changes in apoptotic phenotype and malignant behavior. Apoptosis induced by Fas engagement or irradiation was markedly reduced in IRF-8–deficient tumor cells, despite unaltered proliferation, cell surface Fas, or MHC class I expression. Moreover, in syngeneic immunocompetent mice, IRF-8–deficient tumor cells grew more aggressively than their control counterparts. However, in IFN-– or Fas ligand–deficient mice, but not T cell–deficient mice, both control and IRF-8–deficient tumor populations grew similarly. Furthermore, both tumor populations grew similarly in mice with defects in innate immunity. Although subsequent studies precluded a role for natural killer cells, immunohistochemical analysis supported the involvement of macrophages. Overall, our findings show that IRF-8 expression in solid tumor cells is important for efficient host immunosurveillance and response to apoptotic stimuli. Therefore, IRF-8 down-regulation may represent a previously unrecognized tumor escape mechanism that facilitates tumor progression. Conversely, strategies aimed at up-regulating or restoring IRF-8 expression in neoplastic cells may improve therapeutic efficacy. [Cancer Res 2007;67(21):10406–16]