Unit: Pathogenesis of Lentiviral Infections
Director: ESTAQUIER Jérôme
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 and the absence of . 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.
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 of CD4+ T-lymphocyte occurring during this period conditions the prognosis of infection by preventing the antiviral immune response (1, manuscript in revision). Moreover, we found, by comparing pathogenic and non-pathogenic primate models (in collaboration with Michaella Müller-Trutwin), a positive correlation between the extent of CD8+ T cell activation and the progression to AIDS2,3. Moreover, this greater T cell activation is associated with an abortive profile of CD8 T cell differentiation2.
2) Attenuated infection and pathogenesis.
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. In collaboration with Olivier Benveniste, we found that CD4 T cell apoptosis and thymic output are critical factors impacting T cell reconstitution in those patients4.
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+ output5.
3) Analysis of the molecular mechanisms involved in the death of lymphocytes.
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, programmed cell death (PCD) 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 (6, manuscript in revision). AIF, a pro-apoptogenic mitochondrial factor, has been proposed to be one of the main candidates to explain caspase-independent PCD. However, ours findings did not confirm a role of AIF. Since we found that a large variety of apoptogenic stimuli induced caspase-independent PCD7 (in collaboration with Nuala mooney), we therefore searched for other biochemical pathways that might account for this pathway of PCD. The observation that mitochondria were degraded and then eliminated by a caspase-independent pathway led us to describe the mechanisms involved in this form of death, which we call "mitoptosis", a new concept8,9. This work was done in collaboration with Craig Blackstone (NIH, USA).
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 could induce the apoptosis of CD4+ T lymphocytes in the absence of viral replication and lymphocyte activation ("bystander" effect).
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. We found that in vitro IL-7 favors apoptosis-induced by HIV-1, in modulating the expression of chemokine receptors10. This result might be a limiting factor for immune reconstitution in the trial performed in macaques using IL-711 (in collaboration with Nicole Israel).
Keywords: AIDS, CD4, Apoptosis, Primate