Cholesterol is instrumental in HIV's ability to infiltrate cells, and removing this fatty material from a cell's membrane blocks infection, according to a Johns Hopkins study. The discovery, reported in the July 20 issue of AIDS Research and Human Retroviruses, may provide new opportunities to stop HIV transmission.

"With a vaccine not immediately on the horizon, microbicides that can remove cholesterol from cell membranes, rendering HIV noninfectious, may play an important part in controlling the AIDS pandemic," says James Hildreth, associate professor of pharmacology and molecular sciences at Johns Hopkins and principal investigator of the study.

Researchers found that a starchy substance that drains cholesterol from a cell's membrane can completely block HIV transmission. Using microbicides that contain this cholesterol depletor during sex should be able to reduce or stop HIV transmission, according to the study.

Researchers have long known that HIV steals many proteins from a cell membrane when it exits the cell. The theft of adhesion proteins, for example, allows the virus to bind to many cell types, increasing its infectious nature. Scientists wondered, however, why a particular protein called CD45 was plentiful in cell membranes but ignored in HIV's robbery efforts. Looking closely at the issue, Hildreth observed that lipid rafts, subregions of a cell's membrane enriched with certain lipids and cholesterol, do not contain this protein.

"We thought that because HIV leaves behind CD45, and because CD45 doesn't exist in the lipid rafts, then maybe HIV emerges from the lipid rafts when it exits a cell," Hildreth says. Studies showed that indeed this was true, and in the current report, researchers focused on solving the next question.

"We wondered why lipid rafts might be important for HIV biology, and our attention focused on cholesterol in the rafts because it's important in a number of biological functions, including fusion," Hildreth says. "We also knew that certain other viruses require cholesterol for infection."

To test their hypothesis, the scientists manipulated cholesterol levels by using cyclodextrins, chains of sugar polymers that form circles or rings. The insides of these rings provide a hydrophobic environment, one that is appealing to molecules that hate water. Since molecules such as cholesterol are hydrophobic, they like to sit inside the rings. The researchers removed cholesterol from several primary cell lines by treating them with cyclodextrins, allowing the rings to absorb the fatlike material. They then exposed these cells to virus particles and cells infected with HIV.

To their surprise, the scientists discovered that cholesterol was not only crucial for the virus's exit but also for its entrance into a cell. When exposed to the virus, treated cells resisted HIV infection.

Upon further inspection, the researchers discovered that removing cholesterol reduces the number of so-called chemokine receptors, HIV coreceptors that the virus must latch onto to gain entry into a cell. Hildreth believes that in the absence of cholesterol, a rigid substance, these receptors lose their shape, become unstable and are destroyed. Cholesterol's stiffening quality is actually responsible for the name "lipid raft." This fatty material stiffens the lipid soup, and subregions then appear as little boats or rafts floating in the more liquid components of the membrane.

The researchers have tested the cyclodextrins with both HIV1 and SIV, a close cousin of HIV that affects monkeys, and thus believe cyclodextrins will similarly affect all viral strains. "Cholesterol plays a crucial role in the biology of the virus. The finding that cyclodextrins can stop infection is allowing us to develop an entirely new strategy for blocking sexual transmission of HIV using microbicides," Hildreth says. "Cyclodextrins have been used in humans for many years as a carrier for hydrophobic drugs, so the molecule already has a proven safety profile. And remarkably, it's nontoxic to cells."

In the future, researchers hope that creams that contain the cholesterol-depleting sugar rings will be used to coat the inside of the vagina or rectum to reduce infection. Topical creams could be applied immediately before intercourse, or cervical rings containing the microbicides could provide a slow release of the chemical over many weeks.

Other authors of the study include Zhaohao Liao, Lisa Cimakasky, Richard Hampton and Dzung Nguyen, all from Hopkins. The study was funded by grants from the Public Health Service, an institutional grant from the Johns Hopkins University School of Medicine and the Johns Hopkins Fund for Medical Discovery.

--Kate O'Rourke