Unit: Pathogenesis of Lentiviral Infections
Director: Jérôme ESTAQUIER
AIDS raises questions about the nature of the mechanisms by which infectious agents the human immunodeficiency virus (HIV) in this case cause the complete collapse of the immune system via disappearance of the CD4+ T-lymphocyte population. The development of a vaccine and improvement of HIV treatments will only be possible if we improve our understanding of the physiopathology of this infection. Using the model of rhesus macaques infected with the simian homologue of HIV, SIVmac, the aim of our team was to contribute to this field, in two main areas: T-cell dynamics and the mechanisms leading to the depletion of CD4+ T lymphocytes.
1) Analysis of the mechanisms controlling viral replication during primary SIV infection: relationship to disease progression
The three or four weeks following initial infection the primary infection phase are characterised by the initiation of massive viral replication, the spread of the virus in the organism, its implantation in lymphoid organs and the development of an immune response. The characteristics of the primary infection are now considered to be a key element in the future development of AIDS. However, only in primate models of infection can we follow precisely the virological and immunological events in both the blood and lymphoid organs the major sites of viral replication and the immune response. We have demonstrated that the intense apoptosis occurring during this period conditions the prognosis of infection by preventing the antiviral immune response. Understanding the relationships between the processes (viral replication-cell activation and immune response-apoptosis) occurring during primary infection is essential for the design of an effective vaccine for HIV infection.
2) Analysis of the molecular mechanisms involved in CD4+ depletion during AIDS
There is some debate about the precise nature of the mechanism or mechanisms involved in CD4+ T lymphocyte depletion during HIV infection: cytopathogenic effect of the virus; destruction of infected CD4+ T lymphocytes by cytotoxic CD8+ T lymphocytes; exhaustion of the renewal capacity of CD4+ T lymphocytes; abnormal induction of apoptosis of adult peripheral T lymphocytes and/or their immature precursors, a phenomenon affecting both infected and uninfected cells.
Our studies, carried out during the asymptomatic phase in the rhesus macaque, have indicated that the level of CD8+ T lymphocyte activation and proliferation in the blood and lymphoid organs is proportional to viral load and predictive of progression to AIDS. Although CD4+ activation is weaker than CD8+ activation, it increases in proportion to viral replication. However, despite the increase in CD4+ T lymphocyte proliferation, we showed that this population does not expand in lymph nodes, suggesting the death of these cells.
Many studies have highlighted the important role of apoptosis in humans during the asymptomatic phase, but there are few data concerning rhesus macaques. We thus decided to re-examine this question and found a relationship between the degree of CD4+ T cell apoptosis, viral load and progression to AIDS. However, the apoptosis of these cells did not depend on an interaction between Fas and its ligand or on caspase activation. Instead, it involved mitochondrial depolarisation associated with an increase in the pro-apoptotic proteins of the Bcl-2 family. These results provide the first evidence that these pro-apoptotic proteins are involved in the physiopathological response to an infectious agent.
3) Analysis of the molecular mechanisms involved in CD4+ T lymphocyte depletion during attenuated infection
The introduction of highly active antiretroviral therapy (HAART) has made it possible to lower plasma viral load considerably, sometimes even below the detection threshold. In parallel, HAART increases the number of circulating CD4+ T lymphocytes. However, about 20% of treated patients present satisfactory virological status but with no notable increase in the number of circulating CD4+ T lymphocytes. These patients have a high risk of developing AIDS.
Our studies in macaques infected with the attenuated SIVmac251Δ nef, in which the gene encoding nef has been deleted, have indicated that, more than eight years after inoculation, this infection leads to an immune defect and AIDS in half of all infected subjects, despite very low levels of viral replication. Studies on this model showed that animals that develop AIDS have a slightly higher level of apoptosis of CD4+ T lymphocytes than healthy macaques and a lack of thymic CD4+ cells. These observations may help to explain why some patients receiving tri-therapy develop AIDS.
4) Analysis of the molecular mechanisms involved in the sensitization of CD4+ T cells for apoptosis
One of our aims was to identify the factors involved in the sensitisation of CD4+ T lymphocytes to apoptotic death. Our previous in vitro work showed that the virus can induce the apoptosis of CD4+ T lymphocytes both after lymphocyte activation (productive infection) and in the absence of viral replication and lymphocyte activation (bystander infection).
Our work has shown that the level of expression and nature of chemokine receptors are key determinants in the sensitisation of CD4+ T lymphocytes to apoptotic death. Thus, although the CD4 molecule was initially thought to be the major element involved in the apoptosis of CD4+ T lymphocytes, our observations suggest that chemokine receptors (HIV/SIV co-receptors) are the determinant elements.