The Biology of TAT

Role of TAT in HIV Pathogenesis

TAT is a transactivator protein essential for the initial high-level production of HIV. This early viral output facilitates the generation of mutational variants that outpace immune control, ultimately leading to chronic infection, immune system degradation, and progression to AIDS. During acute infection, infected cells release extracellular TAT, which enhances HIV gene expression and stimulates multiple pathological processes.

Extracellular TAT contributes to the growth of Kaposi’s sarcoma (KS) cells, promotes angiogenesis and tumor metastasis, and facilitates lymphoid hyperplasia. It also induces secretion of TGF-β, TNF-α and -β, and IL-6, promotes malignant transformation of keratinocytes, and suppresses IL-2 and IL-2 receptor gene expression. Additionally, it inhibits the antiviral interferon α/β system. These effects are mediated through distinct pathways governing cell proliferation and viral transactivation. Further discussion of TAT’s immunomodulatory properties is available in the accompanying essay The Folly of Modeling HIV Vaccine Efficacy in Chimpanzees.

TAT Domains and Cellular Uptake

TAT comprises two biologically significant functional domains. The amino-terminal domain, enriched in cysteines and basic residues, facilitates nuclear localization. The carboxyl-terminal domain features an RGD sequence, similar to fibronectin, that binds integrin proteins on cell membranes.

When TAT is secreted and reabsorbed into cells, the amino-terminal domain mediates binding via two primary receptor sites. The first is a low-affinity, sulfated proteoglycan-dependent site responsible for endocytosis and enhanced viral replication. This proteoglycan is under TGF-β control and rapidly turns over, suggesting TGF-β may enhance TAT uptake in both infected and uninfected cells. The second binding site is 100 to 1000 times more sensitive to TAT and is linked to the stimulation of cell proliferation.

TAT Interaction with the VEGF Pathway

A high-affinity receptor for TAT has been identified as FLK-1/KDR, the VEGF receptor. This tyrosine kinase regulates endothelial cell proliferation and vascular permeability. TAT mimics VEGF by activating this receptor, thereby inducing angiogenesis and endothelial activation. In transgenic models, TAT expression leads to lesions characteristic of KS, along with various carcinomas and lymphomas.

Extremely low extracellular concentrations of TAT (~0.1 ng/ml) can mimic VEGF signaling. This mechanism likely contributes to AIDS-associated endothelial dysfunctions such as KS, arteriopathy, and coagulopathies. Elevated von Willebrand factor levels, released from activated endothelium, correlate with disease progression and declining CD4 counts.

Effects on Dendritic Cells and B Cell Maturation

VEGF functions as a chemoattractant for monocytes and an inhibitor of dendritic cell (DC) maturation. In HIV-associated follicular hyperplasia, immature DCs are commonly observed and linked to alterations in B cell populations, specifically an inverted centroblast-to-centrocyte ratio. Since centroblasts undergo somatic hypermutation to produce high-affinity antibodies, impaired centrocyte development may hinder effective B cell selection.

Despite TAT and NEF stimulating hypergammaglobulinemia, this does not equate to the production of high-affinity or neutralizing antibodies. Both TAT and VEGF also inhibit apoptosis in various cell types, and VEGF acts as an autocrine growth factor in KS cells. TAT and VEGF likely synergize to enhance vascular lesion formation and vascular leakage characteristics of KS.

Vascular Permeability and CD4 Cell Dynamics

Given their shared activation of VEGF receptors, TAT may parallel VEGF in promoting vascular permeability and inhibiting dendritic cell development. VEGF is known to be approximately 50,000 times more potent than histamine in this regard. This is relevant because clinical AIDS development is closely tied to the ability of HIV-infected cells to cross endothelial barriers and access interstitial spaces, including the CNS.

TGF-β and glucocorticoids suppress immune cell migration across endothelium—a key mechanism for controlling inflammation. Therefore, the extent of TAT or VEGF-induced vascular permeability could influence the duration of the asymptomatic phase of HIV infection.

A reevaluation of findings by Ho et al. suggests that increases in CD4 T cell counts following protease inhibitor therapy may not solely reflect de novo proliferation. Telomere studies show no significant difference in CD4 cell proliferation between HIV-infected and uninfected individuals. Instead, elevated CD4 counts could stem from blocked egress of CD4 cells and enhanced CD4 expression on both infected and uninfected cells.

Cells in interstitial spaces may return to circulation via the thoracic duct, rapidly increasing CD4 counts. Protease inhibitors, which also act as proteasome inhibitors, induce apoptosis in infected cells and suppress viral production. With reduced viral synthesis, TAT and VEGF levels decline, lowering vascular permeability. Notably, vascular permeability has not previously been proposed as a factor contributing to CD4 cell dynamics during protease inhibitor therapy.

Endothelial Cells as HIV Reservoirs

Endothelial cells vastly outnumber circulating mononuclear cells and, when activated, can be infected with HIV. While viral secretion typically requires additional stimulation by IL-1β or TNF-α, activated endothelial cells express MHC class II and serve as antigen-presenting cells, supporting T cell proliferation and IL-2 production. This makes them capable of transmitting HIV to activated T cells and stimulating transcription in latently infected monocytes.

TNF-α, which is generally low during the asymptomatic phase, downregulates VEGF receptor expression. In its absence, endothelial and dendritic cells may be more responsive to TAT and VEGF at low concentrations.

TAT Effects on Thymocyte Development

Although the RGD-containing carboxyl terminus of TAT does not mediate endocytosis, it influences thymic epithelial cells by modulating fibronectin expression. In vitro, TAT-expressing stromal cells impair the development of CD4+CD8+ thymocytes from CD4–CD8– precursors. These thymic epithelial cells are known to be infected in vivo. TAT’s inhibitory effects on thymocyte maturation may involve TGF-β signaling.

TAT and NF-κB Activation

TAT enhances HIV transcription via NF-κB but does not directly bind DNA. Instead, it binds the TAR stem loop in viral RNA, increasing transcriptional elongation. TAT also binds a similar structure in TNF-β RNA, inducing its transcription in lymphoid cells. TAT synergizes with TNF-α to activate NF-κB by binding to a receptor on infected cell membranes—a process that anti-TAT antibodies can inhibit.

TAT may indirectly activate NF-κB by depolymerizing microtubules, which regulate the degradation of IκB, the NF-κB inhibitor.

Interference with Interferon Pathways

TNF-α/β exhibits strong antiviral activity and synergizes with interferons to induce resistance to RNA and DNA viruses. TAT antagonizes this response by two mechanisms: it binds the TAR RNA structure, preventing activation of interferon-inducible enzymes, and it directly binds PKR, the interferon-induced RNA-dependent kinase. This interaction inhibits PKR activity, disrupting protein synthesis inhibition in infected and uninfected cells. Consequently, TAT uptake by uninfected cells may suppress interferon-mediated antiviral defenses.

Anti-TAT Antibodies and Immune Protection

Natural anti-TAT antibodies exist in uninfected individuals and recognize the basic amino acid region of the amino terminus. Upon HIV infection, these antibodies decline, with an inverse correlation observed between anti-TAT titers and circulating p24 antigen. Monoclonal anti-TAT antibodies inhibit viral replication in vitro, suggesting a protective role in early infection.

Loss of natural anti-TAT antibodies may reflect a broader failure in germinal center selection for high-affinity antibody production. Notably, passive transfer of sera from asymptomatic HIV-positive individuals has been shown to reduce viremia in AIDS and ARCS patients to undetectable levels. If these sera contain high titers of anti-TAT antibodies, it would support the conclusion that TAT is a critical target for inclusion in HIV vaccine strategies.