By binding to the surface of the virus the HIV-1 Tat protein increases virus infectivity, identifying a target for new therapies.
The HIV Tat protein binds the Env viral protein, generating a virus entry complex that appears to play a key role in HIV vulnerability to neutralizing antibodies and might contribute to the maintenance of latent virus reservoirs resistant to therapy. Upon the identification of this complex (the Tat/Env complex) CNAIDS has combined Tat and Env in an experimental vaccine that was tested first in monkeys and then in a phase I clinical trial. CNAIDS is now committed to continue the research, aimed to clarify in more detail the role of the Tat/Env complex in virus infectivity and how it could be used to induce antibodies capable of neutralizing the infection.
HIV enters the target cells by binding the cellular receptors CD4 and CCR5 expressed on the surface of CD4+ T lymphocytes, macrophages and dendritic cells. Env also recognizes and binds other receptors, called membrane lectins. Some of these lectins promote the binding of Env to the CD4 receptor. Furthermore, lectins expressed by dendritic cells "capture" HIV transmitting the virus directly to CD4+ T lymphocytes. This process, called "trans-infection", ensures a very efficient pathway for virus spreading within the host. The Env molecule is, therefore, the natural target of antibodies able to neutralize the virus and many of the vaccines tested so far have used, or are using, Env as an immunogen to induce a neutralizing response against HIV.
The Env molecule is very immunogenic and during the course of the infection induces a variety of poorly or non-neutralizing antibodies. This is due to the conformational dynamics of Env, which “spends” most of the time in a "closed" conformation, in which both the Env CD4 and CCR5 binding sites remain inaccessible to antibodies. However, to infect the target cells, Env must temporarily shift to an "open" conformation, exposing the CD4 and CCR5 binding sites for binding to the target cell receptors. The open form of Env, therefore, is very vulnerable to neutralizing antibodies. On the contrary, the closed form of Env is particularly resistant to either neutralizing antibodies produced during infection or those induced by the vaccines tested in humans so far. However, a limited number of HIV-infected people develop antibodies that "block" Env in the closed form, preventing the virus from attacking cells; these antibodies are currently the subject of intensive research to develop new generation immunotherapies and protective vaccines.
Studies performed at CNAIDS have shown that the Tat protein is able to modify Env conformational dynamics by easing the transition between the closed and open conformations. The data suggest that Tat increases virus infectivity for CD4+ lymphocytes and dendritic cells by rendering Env more open but more vulnerable. Therefore, CNAIDS is conducting studies to clarify the role of the Tat protein in the conformational transitions of Env, in virus infectivity and neutralization by Env antibodies. These studies have further shown that, by binding Env, the Tat protein "deflects" the virus from dendritic cells Env canonical receptors "redirecting" it to other surface molecules (integrins) that act as receptors for Tat. Therefore, thanks to the Tat/Env complex, the virus penetrates dendritic cells even in the presence of effective anti-Env antibodies. In turn, however, anti-Tat antibodies block the integrin-dependent entry route. These antibodies appear only rarely during natural infection but can be induced with high efficiency by the Tat vaccine developed at CNAIDS, as demonstrated by the results of phase II vaccine trials. The integrins acting as Tat receptors are also expressed at high levels by activated endothelial cells. These cells mark inflamed tissue that actively recruits activated CD4+ T lymphocytes, the most efficient targets for HIV infection. Thus, CNAIDS conducts studies aimed at clarifying the role of Tat and the Tat/Env complex in the trans-infection of HIV, in the recruitment of HIV at sites of inflammation and in the use of the Tat/Env complex to induce neutralizing antibodies. All these studies could provide useful information to clarify the protective action of the Tat vaccine against HIV and new scenarios for a preventive, sterilizing vaccine based on the Tat/Env complex, which was already successfully tested in monkeys and in a phase I clinical trial in humans (ClinicalTrials.gov identifier: NCT01441193).