Today, Highly Active Anti-Retroviral Therapy (HAART) makes it possible to interrupt, or at least significantly reduce replication of the HIV virus inside the blood cells of infected patients. Yet even after years of treatment, if patients quit the treatment, HIV replication starts anew. Somewhere inside the body, hidden HIV particles manage to escape drugs, and indefinitely provide new supplies of the virus. For nearly a decade, research has focused on localizing and understanding the mechanisms of what experts call the “HIV reservoir.” Recent studies by CNRS teams are shedding new light on two systems by which the virus remains undetectable and unaffected by treatment. One is at the anatomical level, the other at the cellular level. “These are two different notions of what the reservoir is, but they are not incompatible,” says researcher Jérôme Estaquier.
Working on macaque monkeys, his team1 identified a region of the body where HIV replication remains active, even when viral load in the plasma is undetectable after 10 years of infection (Long Term Non Progressor).2 Investigating these animals at various stages of illness, the researchers consistently found higher levels of virus replication in lymph nodes of the intestines' mesentery, compared to other regions. “These levels were 20 to 30 times what they were in other organs,” adds Estaquier.
The researchers also observed that the immune system's CD8+ T cells–those responsible for killing HIV-infected cells–die prematurely in the mesenteric region. “These organs are peculiar when it comes to immunology. They are flooded with bacteria and viruses on a daily basis–when we ingest foods–and some of these microorganisms need to be tolerated to avoid immune inflammation,” he says. “Therefore, the immune system sets up mechanisms of immunosuppression in those areas”–which might explain why killer cells are silenced in those regions. The authors suggest that an excess of TGF-ß, an immunosuppressive protein also involved in cancers, could be responsible for the killer cells' weakness in the mesentery. Inhibiting TGF-ß or finding ways of preventing death of CD8+ T cells could thus help drain the reservoir, or at least block the diffusion of new virus particles from the reservoir to the blood circulation.
In parallel, a series of recent findings in the field of molecular genetics suggests other mechanisms are also at play in this reservoir process. Some virus particles seem to escape medication by hiding inside cells of the immune system itself–in resting memory CD4+ T cells, also known as T-helper cells, that carry an integrated provirus that is transcriptionally silent.
T-helper cells are present in all compartments of the body. They play a central role in the immune response by signaling other cells of the system to perform their functions. But during HIV infection, their cellular machinery is hijacked by the virus which multiplies and releases viral particles in the blood circulation.
“However, in a very few CD4 cells, the virus 'decides' not to replicate and remains in a latent state,” explains CNRS researcher Monsef Benkirane, from IGH in Montpellier.3 “The quasi-absence of viral gene expression enables the virus to remain 'invisible' and escape antiretrovirals–which mainly target replication.” Some reactivation process may occur, and replication can start over, thus providing new supplies of the virus. “Our idea is to induce this reactivation process–so that dormant infected cells become visible and sensitive to antiretrovirals,” says Benkirane. “The idea would be to use a treatment which activates the virus along with one that suppresses it.”
In a series of studies published this year,4 Benkirane's team suggests that at least three molecular regulators inside CD4 helpers are involved in putting the virus to sleep. Micro RNAs, proteasomes (involved in protein degradation), and heterochromatin (involved in gene expression). “In vitro, the inhibition of one of these molecular regulators enables the reactivation of the virus,” says Benkirane, who is about to start in vivo tests in animals.
1. Unité de physiopathologie des infections lentivirales (CNRS / Institut Pasteur / INSERM).
2. MC. Cumont et al., “TGF-b in intestinal lymphoid organs contributes to the death of armed effector CD8 T cells and is associated with the absence of virus containment in rhesus macaques infected with the simian immunodeficiency virus,” Cell Death Differ. 14(10): 1747-58. 2007.
3. Institut de génétique humaine (CNRS).
4. R. Triboulet, et al., Science. 315(5818): 1579-82. 2007. I. du Chene et al., EMBO J. 26(2): 424-35. 2007. I. Lassot et al., Mol Cell. 25(3): 369-83. 2007.