Human T-cell lymphotropic virus type 1 (HTLV-1) is the causative agent of an aggressive malignancy of CD4 + T lymphocytes. Since the viral transactivator Tax-1 is a major player in T-cell transformation, targeting Tax-1 protein is regarded as a possible strategy to arrest viral replication and to counteract neoplastic transformation. We demonstrate that CIITA, the master regulator of major histocompatibility complex class II gene transcription, inhibits HTLV-1 replication by blocking the transactivating function of Tax-1 both when exogenously transfected in 293T cells and when endogenously expressed by a subset of U937 promonocytic cells. Tax-1 and CIITA physically interact in vivo via the first 108 amino acids of Tax-1 and two CIITA adjacent regions (amino acids 1 to 252 and 253 to 410). Interestingly, only CIITA 1-252 mediated Tax-1 inhibition, in agreement with the fact that CIITA residues from positions 64 to 124 were required to block Tax-1 transactivation. CIITA inhibitory action on Tax-1 correlated with the nuclear localization of CIITA and was independent of the transcription factor NF-YB, previously involved in CIITA-mediated inhibition of Tax-2 of HTLV-2. Instead, CIITA severely impaired the physical and functional interaction of Tax-1 with the cellular coactivators p300/CBP-associated factor (PCAF), cyclic AMP-responsive element binding protein (CREB), and activating transcription factor 1 (ATF1), which are required for the optimal activation of HTLV-1 promoter. Accordingly, the overexpression of PCAF, CREB, and ATF1 restored Tax-1-dependent transactivation of the viral longterminal-repeat promoter inhibited by CIITA. These findings strongly support our original observation that CIITA, beside increasing the antigen-presenting function for pathogen antigens, acts as an endogenous restriction factor against human retroviruses by blocking virus replication and spreading.
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