Tat competes with CIITA for the binding to P-TEFb and blocks the expression of MHC class II genes in HIV infection

Two distinct domains within CIITA mediate self-association: involvement of the GTP-binding and leucine-rich repeat domains

CIITA is the master regulator of class II major histocompatibility complex gene expression. We present evidence that CIITA can self-associate via two domains: the C terminus (amino acids 700 to 1130) and the GTP-binding domain (amino acids 336 to 702). Heterotypic and homotypic interactions are observed between these two regions. Deletions within the GTP-binding domain that reduce GTP-binding and transactivation function also reduce self-association. In addition, two leucine residues in the C-terminal leucine-rich repeat region are critical for self-association as well as function. This study reveals for the first time a complex pattern of CIITA self-association. These interactions are discussed with regard to the apoptosis signaling proteins, Apaf-1 and Nod1, which share domain arrangements similar to those of CIITA.

Repression of MHC determinants in HIV infection

HIV infects CD4(+) macrophages and lymphocytes. Before the development of AIDS, HIV weakens the immune system in part by blocking antigen processing and presentation via major histocompatibility complex (MHC) molecules. In this report, we discuss how HIV escapes the immune surveillance by MHC II molecules.

Cellular immune responses to HIV

The cellular immune response to the human immunodeficiency virus, mediated by T lymphocytes, seems strong but fails to control the infection completely. In most virus infections, T cells either eliminate the virus or suppress it indefinitely as a harmless, persisting infection. But the human immunodeficiency virus undermines this control by infecting key immune cells, thereby impairing the response of both the infected CD4+ T cells and the uninfected CD8+ T cells. The failure of the latter to function efficiently facilitates the escape of virus from immune control and the collapse of the whole immune system.

The Cdk9 and cyclin T subunits of TAK/P-TEFb localize to splicing factor-rich nuclear speckle regions

TAK/P-TEFb is an elongation factor for RNA polymerase II-directed transcription that is thought to function by phosphorylating the C-terminal domain of the largest subunit of RNA polymerase II. TAK/P-TEFb is composed of Cdk9 and cyclin T and serves as the cellular cofactor for the human immunodeficiency virus transactivator Tat protein. In this study, we examined the subcellular distribution of Cdk9 and cyclin T1 using high resolution immunofluorescence microscopy and found that Cdk9 and cyclin T1 localized throughout the non-nucleolar nucleoplasm, with increased signal present at numerous foci. Both Cdk9 and cyclin T1 showed only limited colocalization with different phosphorylated forms of RNA polymerase II. However, significant colocalization with antibodies to several splicing factors that identify nuclear ‘speckles’ was observed for Cdk9 and especially for cyclin T1. The pattern of Cdk9 and cyclin T1 distribution was altered in cells treated with transcription inhibitors. Transient expression of cyclin T1 deletion mutants indicated that a region in the central portion of cyclin T1 is important for accumulation at speckles. Furthermore, cyclin T1 proteins that accumulated at speckles were capable of recruiting Cdk9 and the HIV Tat protein to this compartment in overexpression experiments. These results suggest that cyclin T1 functions to recruit its binding partners to nuclear speckles and raises the possibility that nuclear speckles are a site of TAK/P-TEFb function.

Antiapoptotic function of Cdk9 (TAK/P-TEFb) in U937 promonocytic cells

Cdk9 is the catalytic subunit of TAK (cyclinT1/P-TEFb), a cellular protein kinase that mediates human immunodeficiency virus type 1 (HIV-1) Tat transcriptional activation function. To examine Cdk9 function in cells relevant to HIV-1 infection, we used a murine leukemia virus retrovirus vector to transduce and overexpress the cDNA of a dominant negative mutant Cdk9 protein (Cdk9-dn) in Jurkat T cells and U937 promonocytic cells. In Jurkat cells, overexpression of Cdk9-dn specifically inhibited Tat transactivation and HIV-1 replication but had no inhibitory effect on induction of CD69, CD25, and interleukin-2 following T-cell activation. In U937 cells, overexpression of Cdk9-dn sensitized cells to apoptosis, especially after phorbol myristate acetate (PMA) treatment to induce differentiation to macrophage-like cells. Because Cdk9 function is induced in PMA-treated U937 cells, Cdk9 may play an antiapoptotic role during monocyte differentiation.

Reciprocal modulation of transcriptional activities between HIV-1 Tat and MHC class II transactivator CIITA

HIV-1 is the etiologic agent of acquired immune deficiency syndrome (AIDS). Functional loss of antigen-presenting cells (APC) in HIV-1 infection is considered to be involved in AIDS pathogenesis. We found that actions of the viral transactivator Tat and the transactivator of MHC class II genes, CIITA, are mutually inhibitory. While Tat inhibited expression of MHC class II genes in APC, overexpression of CIITA inhibited Tat and subsequently HIV-1 replication. This action of Tat appears to be mediated by sequestering the common cofactor, cyclin T1, but not p300 and CBP. These reciprocal actions between Tat and CIITA not only explains the functional impairment of APC in HIV-1 infection but also rationalizes the suppression of HIV-1 virus load by induction of CIITA such as IFN-gamma.

Therapeutic targeting of human immunodeficiency virus type-1 latency: current clinical realities and future scientific possibilities

Factors affecting HIV-1 latency present formidable obstacles for therapeutic intervention. As these obstacles have become a clinical reality, even with the use of potent anti-retroviral regimens, the need for novel therapeutic strategies specifically targeting HIV-1 latency is evident. However, therapeutic targeting of HIV-1 latency requires an understanding of the mechanisms regulating viral quiescence and activation. These mechanisms have been partially delineated using chronically infected cell models and, clearly, HIV-1 activation from latency involves several key viral and cellular components. Among these distinctive therapeutic targets, cellular factors involved in HIV-1 transcription especially warrant further consideration for rational drug design. Exploring the scientific possibilities of new therapies targeting HIV-1 latency may hold new promise of eventual HIV-1 eradication.

Identification of class II transcriptional activator-induced genes by representational difference analysis: discoordinate regulation of the DN alpha/DO beta heterodimer

Class II transcriptional activator (CIITA) is a master regulator of MHC class II genes, including DR, DP, and DQ, and MHC class II-associated genes DM and invariant chain. To determine the repertoire of genes that is regulated by CIITA and to identify uncharacterized CIITA-inducible genes, we used representational difference analysis. Representational difference analysis screens for differentially expressed transcripts. All CIITA-induced genes were MHC class II related. We have identified the alpha subunit, DN alpha, of the class II processing factor DO as an additional CIITA-inducible gene. Northern analysis confirmed that DN alpha is induced by IFN-gamma in 2fTGH fibrosarcoma cells, and CIITA is necessary for high-level expression in B cells. The beta subunit, DO beta, is not inducible in fibrosarcoma cells by IFN-gamma or exogenous CIITA expression. Moreover, in contrast to other class II genes, DO beta expression remains high in the absence of CIITA in B cells. The promoters for DN alpha and DO beta contain the highly conserved WXY motifs, and, like other class II genes, expression of both DN alpha and DO beta requires RFX. These findings demonstrate that both DN alpha and DO beta are regulated by RFX. However, DN alpha is defined for the first time as a CIITA-inducible gene, and DO beta as a MHC class II gene whose expression is independent of CIITA.

Modulation of host immune responses by clinically relevant human DNA and RNA viruses

Numerous mechanisms allow viruses to evade host immune surveillance, and new evasion strategies continue to be identified. In addition to interference with antigen processing and presentation, direct viral modulation of host immune responses can also be achieved by altering the host cytokine milieu and the development of immunoregulatory cells. A better understanding of these viral evasion strategies will help to define critical host defense mechanisms and will lead to novel immune-based therapeutic strategies in the future.

Functions of tetramer-stained HIV-specific CD4(+) and CD8(+) T cells

Significant insight has been gained into constraints on the sensitivity and specificity of staining with class I tetramers. The function of the populations that are defined varies with the clinical situation. Insight has also been gained into the determinants of the CD8(+) T cell response during primary and chronic HIV infection. Human class II tetramers have been synthesised but their role in defining CD4(+) T cell function in HIV infection remains to be determined.

Expression of MHC class II in T cells is associated with increased HIV-1 expression

HIV-1 replicates in activated T cells at significantly higher levels than in resting cells. Thus, certain molecules up-regulated during T cell activation appear to be important for HIV-1 replication. In this study, we present evidence suggesting that expression of MHC class II (class II) molecules on CD4+ T cells facilitate HIV-1 replication. T cells that expressed class II supported greater virus replication than T cells lacking class II. The class II+ cells, when either infected with HIV-1 or transfected with an env-minus HIV-1 provirus plasmid, produced 10-20-fold greater virus expression than class II- cells. Anti-class II antibody markedly inhibited virus expression in class II+ cells (but not class II- cells) and also decreased the nuclear binding activity of AP-1, an inducible transcription factor important in T cell activation and HIV-1 expression. Most importantly, the induction of class II expression by transfection of the MHC class II transactivator (CIITA) stimulated HIV-1 replication in Jurkat T cells. Taken together, these data suggest that expression of MHC class II molecules and/or CIITA in T cells enhances HIV-1 transcription.

Novel Tat-encoding bicistronic human immunodeficiency virus type 1-based gene transfer vectors for high-level transgene expression

We describe bicistronic single-exon Tat (72-amino-acid Tat [Tat72])- and full-length Tat (Tat86)-encoding gene transfer vectors based on human immunodeficiency virus type 1 (HIV-1). We created versions of these vectors that were rendered Rev independent by using the constitutive transport element (CTE) from Mason-Pfizer monkey virus (MPMV). Tat72-encoding vectors performed better than Tat86-expressing vectors in gene transfer experiments. CTE-containing vectors, produced in a Rev-independent packaging system, had gene transfer efficiencies nearly equivalent to those produced using a combination RNA transport (CTE and Rev-Rev response element)-based packaging system. The Tat72-encoding vectors could be efficiently transduced into a variety of cell types, showed higher levels of transgene expression than vectors with the simian cytomegalovirus immediate-early or the simian virus 40 early promoter, and provide an alternative to HIV-1 vectors with internal promoters.

P-TEFb kinase recruitment and function at heat shock loci

P-TEFb, a heterodimer of the kinase Cdk9 and cyclin T, was isolated as a factor that stimulates formation of productive transcription elongation complexes in vitro. Here, we show that P-TEFb is located at >200 distinct sites on Drosophila polytene chromosomes. Upon heat shock, P-TEFb, like the regulatory factor HSF, is rapidly recruited to heat shock loci, and this recruitment is blocked in an HSF mutant. Yet, HSF binding to DNA is not sufficient to recruit P-TEFb in vivo, and HSF and P-TEFb immunostainings within a heat shock locus are not coincident. Insight to the function of P-TEFb is offered by experiments showing that the direct recruitment of a Gal4-binding domain P-TEFb hybrid to an hsp70 promoter in Drosophilacells is sufficient to activate transcription in the absence of heat shock. Analyses of point mutants show this P-TEFb stimulation is dependent on Cdk9 kinase activity and on Cdk9's interaction with cyclin T. These results, coupled with the frequent colocalization of P-TEFb and the hypophosphorylated form of RNA polymerase II (Pol II) found at promoter-pause sites, support a model in which P-TEFb acts to stimulate promoter-paused Pol II to enter into productive elongation.