The protein product of one of these accessory genes -- vpr -- is found in significant quantities in the extracellular free virus, suggesting that it may be crucial in the early stages of infection by the virus.
Now, researchers at the University of Pennsylvania Medical Center report that Vpr, the protein product of vpr, blocks the production of cytokines in the cells first infected by HIV, called macrophages, thus blunting immune activation of these cells. Cytokines are chemicals used by the immune system to initiate and control much of the its response to infection.
Additionally, the investigators found that Vpr is able to prevent apoptosis -- or programmed cell death -- in the infected cells, thus preserving them for use as viral production factories. And, in a further blow to the body's ability to defend against the infection, Vpr actively induces apoptosis in neighboring immune system cells that have not yet been infected, known as CD4 T cells. It also interferes with the ability of those cells to proliferate. The new findings appear in the October issue of Nature Medicine.
"In the test tube, Vpr suppresses production of the cytokines needed to fight any infection, bacterial or viral," says David B. Weiner, PhD, senior author and an associate professor of pathology and laboratory medicine. "It also prevents apoptosis in the cells it infects, so those cells can stay alive to make more virus. And then it kills the uninfected cells that would otherwise be involved in providing immune protection."
Noting that Vpr mimics the actions of a class of steroidal drugs called glucocorticoids, the researchers hypothesized that Vpr might act through the same molecular pathway -- binding to the same receptors -- as those drugs. Glucocorticoids are used as chemotherapeutic agents against some cancers and autoimmune disorders, but are also known to inhibit cytokine production and to induce cell death in lymphocytes, the category of immune cells to which macrophages and T cells are belong.
To test this possibility, the researchers conducted experiments with the antisteroidal compound RU-486, perhaps better known for its use in Europe as an abortion-inducing agent, to see if that drug could counter the effects of Vpr. They added Vpr to populations of cultured lymphocytes in the laboratory, documented its impact, and then added RU-486 to see if the results were altered in any way.
"When we added RU-486 to our cell cultures, it markedly reversed many of the effects of Vpr," says Velpandi Ayyavoo, PhD, lead author on the study and a research associate in Weiner's laboratory. "These results suggest that antisteroids such as RU-486 should be considered for development as potential HIV therapies."
The cytokines suppressed by Vpr include tumor necrosis factor (TNF) alpha and the interleukins IL-2, IL-4, IL-10, and IL-12. Because glucocorticoids are known to suppress nuclear factor kappa-B, a gene transcription factor that regulates cytokine production, the scientists examined the effects of Vpr on nuclear factor kappa-B. They found that Vpr activated another factor called inhibitor of kappa-B to neutralize nuclear factor kappa-B, largely shutting down production of the cytokines.
In addition to Weiner and Ayyavoo, coauthors based at Penn include S. Mahalingam, PhD; Sagar Kudchodkar, PhD; and William V. Williams, MD. The remaining coauthors are Artin Mahboubi, MS, and Douglas R. Green, PhD, at the La Jolla Institute of Allergy and Immunology, and R. Ramalingam, PhD, at the Roche Institute of Molecular Biology.
This work was supported in part by the National Institutes of Health and the Pediatrics AIDS Foundation.
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