Researchers have known for years that HIV can infect specialized immune system cells called macrophages, but new research suggests these cells may play a larger role in HIV infection than previously believed. In the current online early edition of Proceedings of the National Academy of Sciences, scientists from the National Institute of Allergy and Infectious Diseases (NIAID) report that macrophages contain and continue to produce large amounts of an HIV-like virus in monkeys even after the virus depletes CD4+ T cells, the primary HIV target in infected individuals. This discovery provides new insight on how the virus might survive in the midst of antiretroviral drugs and suggests new strategies for eliminating the virus from the body.
"Our research suggests that macrophages are an underappreciated reservoir of virus in HIV infection," says study author Malcolm A. Martin, M.D., chief of NIAID's Laboratory of Molecular Microbiology. "These cells become infected immediately after exposure to HIV, are relatively resistant to virus killing, and are able to produce lots of new virus. Most currently available treatments target HIV during its infection of T cells, but if the virus also infects and accumulates in large amounts in macrophages, additional drugs may be required."
Highly active antiretroviral therapy, or HAART, can reduce HIV to undetectable levels in a person's blood, but the virus usually bounces back when the drugs are stopped. Because HIV enters and destroys CD4+ T cells early in infection, many researchers believe those cells are the most likely source of the rebounding virus. Several studies have questioned that idea, however, leaving scientists uncertain of the key reservoir for latent HIV. Dr. Martin and colleagues studied SHIV, a specially designed hybrid virus in monkeys, to see if macrophages might be a major virus source.
SHIV strains do not exist in nature but can be generated in the laboratory by combining the outer envelope proteins of HIV with a core of simian immunodeficiency virus (SIV), a closely related monkey retrovirus that also induces AIDS in inoculated animals. SHIV infection in monkeys is an extremely rapid and exaggerated model of HIV infection in humans that allows scientists to address certain clinical aspects of retrovirus biology that are difficult or impossible to study in people. "SHIV viruses rapidly deplete CD4+ T cells yet continue to grow to very high levels in the animal, allowing us to look at where else the virus might be hiding," explains Dr. Martin. "Our studies would be impossible to do in people because natural HIV infection does not deplete T-cell populations as quickly or as completely."
Dr. Martin and his colleagues infected macaques with a highly virulent SHIV strain and watched the virus as it infected cells. SHIV rapidly entered the animals' CD4+T cells and quickly eliminated most of those cells from the blood, lymph nodes and other tissues. The infected monkeys continued to produce high levels of virus, however, suggesting that SHIV also had infected another cell type. When the researchers examined lymphoid organs such as lymph nodes and spleen for the source of the remaining virus, they found that 95 percent of the virus-producing cells were macrophages and only 1 to 2 percent were T cells.
Anthony S. Fauci, M.D., director of NIAID, says this might partially explain the results of studies conducted recently by his laboratory and others. "When patients on HAART stop their therapy, the virus that bounces back usually differs from the latent virus in their CD4+ T cells, suggesting another reservoir for HIV. If the SHIV model is giving us a glimpse of what is happening with HIV in humans, it might point to macrophages as an important reservoir of HIV in humans receiving HAART."
Dr. Martin believes this information might change how physicians treat HIV infections. Currently available drugs designed to attack the virus in T cells do not eliminate all of the virus in the body, particularly those in macrophages. The researchers tested this idea by checking whether a potent reverse transcriptase (RT) inhibitor could control both the early T-cell and late macrophage stages of the SHIV infection. RT inhibitors are antiretroviral drugs and key components of HAART. The team first treated macaques early in the infection, when SHIV was predominant in CD4+T cells. In this group of animals, the drug readily and efficiently reduced virus levels and protected the monkeys from any T-cell loss. However, when the RT inhibitor was given only during the later macrophage phase of infection, the drug had no effect.
"If we want to completely eliminate the virus, we need to attack it everywhere it lives, not just in the T cells," says Dr. Martin. "Our studies suggest we need new classes of antiretroviral agents that can target HIV during infections of tissue macrophages. They potentially could eliminate this reservoir of virus and obviously complement currently available drugs."
The researchers plan to use the SHIV model to further investigate the role of macrophages in HIV infections. SHIV infection in monkeys provides an ideal opportunity to study infected macrophages in a living animal, a better alternative than the tissue culture experiments scientists have used to date. Studies are under way to see how macrophages respond to SHIV infection and how long these infected cells can survive. The researchers also will test additional drugs, which they hope eventually will include new candidate antiretroviral compounds designed to stop latent virus from rebounding.
A copy of this article is available online at www.pnas.org/papbyrecent.shtml.