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.

HTLV-1 is more pathogenic than HTLV-2 despite having a similar genome and closely related transactivating oncoproteins. Both Tax-1 protein from HTLV-1 and Tax-2 from HTLV-2 activate the NF-κB pathway. The mechanisms involved in Tax-1 deregulation of this signalling pathway have been thoroughly investigated, but little is known about regulation by Tax-2. We have compared the interaction of Tax-1 and Tax-2 with two key NF-κB signalling factors: TAK1-binding protein 2 (TAB2), an adaptor involved in the activation of TAK1 kinase, and RelA, the active subunit of the canonical RelA/p50 NF-κB transcription factor. Tax-2 formed stable complexes with both RelA and TAB2. These two NF-κB factors colocalized with Tax proteins in dotted cytoplasmic structures targeted by calreticulin, a multi-process calcium-buffering chaperone. Co-expression of RelA and/or TAB2 markedly increased Tax-mediated NF-κB activation. These findings provide new insights into the role of RelA, TAB2 and Tax in the deregulation of the NF-κB pathway.

Human T-lymphotropic viruses type 1 (HTLV-1) and type 2 (HTLV-2) present very similar genomic structures but HTLV-1 is more pathogenic than HTLV-2. Is this difference due to their transactivating Tax proteins, Tax-1 and Tax-2, which are responsible for viral and cellular gene activation? Do Tax-1 and Tax-2 differ in their cellular localization and in their interaction pattern with cellular factors? In this review, we summarize Tax-1 and Tax-2 structural and phenotypic properties, their interaction with factors involved in signal transduction and their localization-related behavior within the cell. Special attention will be given to the distinctions between Tax-1 and Tax-2 that likely play an important role in their transactivation activity.

HTLV-1 is more pathogenic than HTLV-2B. The difference is generally attributed to the properties of their individual transactivating Tax proteins. By using internal Flag-6His tagged Tax-1 and Tax-2B, which display transcriptional activities comparable to the untagged proteins and can be recognized by a single anti-Flag antibody, we demonstrate that Tax-2B is modified by ubiquitination and sumoylation. In addition, Tax2B is distributed in punctuate nuclear structures that include the RelA subunit of NF-kappaB, as has been previously demonstrated for Tax-1.

Human T-cell lymphotropic virus (HTLV) type II has spread among intravenous drug users (IDUs), many of whom are coinfected with HIV-1. We have investigated the rate of HTLV-II infection in 3574 Italian IDUs screened for HIV-1, HTLV-I, and HTLV-II from 1986 to the present. HTLV-II proviral load was determined by a real-time polymerase chain reaction specifically designed for tax amplification. The frequency of HTLV-II infection was 6.7% among HIV-1-positive subjects and 1.1% among HIV-1-negative subjects (P < 0.0001). For examination of AIDS progression, a group of 437 HIV-1-monoinfected subjects and another group of 96 HIV-1/HTLV-II-coinfected subjects were monitored. Enrollees were matched at entry by CD4 cell counts and followed for an average of 13 years. HIV-1/HTLV-II coinfection was associated with older age (P < 0.0001) and higher CD4 (P < 0.0001) and CD8 (P < 0.001) cell counts compared with monoinfected IDUs. The number of long-term nonprogressors for AIDS was significantly higher (P < 0.0001) among coinfected patients (13 [13.5%] of 96 patients) than HIV monoinfected patients (5 [1.1%] of 437 patients), showing that HTLV-II exerts a protective role. An increased incidence of liver disease and hepatitis C virus positivity among coinfected IDUs was observed. Five coinfected subjects undergoing antiretroviral therapy showed a significant (P < 0.05) increase in HTLV-II proviral load concomitant to a decrease in HIV-1 viremia, suggesting that the treatment is ineffective against HTLV-II infection.