The Major Histocompatibility Complex Class II Transactivator CIITA Inhibits the Persistent Activation of NF-kappa B by the Human T Cell Lymphotropic Virus Type 1 Tax-1 Oncoprotein

Human T cell lymphotropic virus type 1 (HTLV-1) Tax-1, a key protein in HTLV-1-induced T cell transformation, deregulates diverse cell signaling pathways. Among them, the NF-kappa B pathway is constitutively activated by Tax-1, which binds to NF-kappa B proteins and activates the I kappa B kinase (IKK). Upon phosphorylation-dependent I kappa B degradation, NF-kappa B migrates into the nucleus, mediating Tax-1-stimulated gene expression. We show that the transcriptional regulator of major histocompatibility complex class II genes CIITA (class II transactivator), endogenously or ectopically expressed in different cells, inhibits the activation of the canonical NF-kappa B pathway by Tax-1 and map the region that mediates this effect. CIITA affects the subcellular localization of Tax-1, which is mostly retained in the cytoplasm, and this correlates with impaired migration of RelA into the nucleus. Cytoplasmic and nuclear mutant forms of CIITA reveal that CIITA exploits different strategies to suppress Tax-1-mediated NF-kappa B activation in both subcellular compartments. CIITA interacts with Tax-1 without preventing Tax-1 binding to both IKK gamma and RelA. Nevertheless, CIITA affects Tax-1-induced IKK activity, causing retention of the inactive p50/RelA/I kappa B complex in the cytoplasm. Nuclear CIITA associates with Tax-1/RelA in nuclear bodies, blocking Tax-1-dependent activation of NF-kappa B-responsive genes. Thus, CIITA inhibits cytoplasmic and nuclear steps of Tax-1-mediated NF-kappa B activation. These results, together with our previous finding that CIITA acts as a restriction factor inhibiting Tax-1-promoted HTLV-1 gene expression and replication, indicate that CIITA is a versatile molecule that might also counteract Tax-1 transforming activity. Unveiling the molecular basis of CIITA-mediated inhibition of Tax-1 functions may be important in defining new strategies to control HTLV-1 spreading and oncogenic potential.