By Lisa Seachrist
WASHINGTON —A year ago, fusin made a news splash as a key for HIV's entry into the T cells that serve as its replicative machines. A few months later, several research groups identified its chemical cousin, CCR-5, as the key to entry for a separate strain of HIV.
These entry keys were members of a group of receptors known as chemokine receptors, which in turn belong to a family of receptors, like rhodopsin in the eye, known as the G-protein coupled receptors. But these proteins, as facilitators of HIV infection, have generated new ways of thinking about how the virus infects, evolves and ultimately causes disease. And those discoveries have resulted in new strategies for developing vaccines and therapies to prevent the virus from infecting healthy T cells.
"It is amazing how quickly we began to understand the chemokine receptor story," said Edward Berger, of the National Institute of Allergy and Infectious Diseases (NIAID). "These receptors, as members of the G-protein superfamily, are already targets for pharmaceutical and biotech companies."
For a dozen years or more, researchers have known that HIV would infect T cells and macrophages and that the main target for that interaction was CD4. They also knew from studies of animal cells producing human CD4 that CD4 alone was not enough for HIV to infect a cell. Berger and his colleagues at NIAID began the search for the cofactors that would allow HIV into the cell.
The researchers focused on a strain of HIV that infected and grew very well in T cells that had been adapted to growing in cell culture. They put the human gene for CD4 into an animal cell and added a human cDNA library. The cells that could fuse with the HIV envelope protein were probed further to find the cofactor.
Fusin Molecule Discovered
What the researchers found was a molecule littering the surface of T cells that they named fusin. Fusin turned out to be a chemokine receptor —A a molecule that receives chemokine messages and spurs macrophages and T cells into areas of inflammation.
"Fusin had already been cloned when we looked in the data base," said Berger. "But no one knew the co-factor would be a chemokine receptor."
HIV has two different infection persuasions. One is T-tropic, or HIV isolated from patients who have late-stage AIDS, which appears to show a preference for infecting T cells; the other is M-tropic, where early in infection and during the latent phase of the disease, the isolated virus strain has a preference for macrophages. The Berger lab had isolated a cofactor that facilitated T-tropic strains of HIV.
Identifying the M-tropic strain cofactor occurred in short order as the result of a fortuitous discovery by Robert Gallo, former National Institutes of Health scientist and co-discoverer of HIV, and his team. Some long-term survivors and people who had been exposed many times to AIDS produced high levels of b-chemokines called RANTS, MIP 1a and MIP 1b.
By searching for a chemokine receptor that bound those precise chemokines, five different laboratories discovered a receptor called CCR-5, that when blocked, prevented M-tropic strains from infecting cells.
In addition, Richard Kaup, of the Aaron Diamond AIDS Research Center, in New York, found a cohort of highly exposed people who never became infected. These people turned out to have a 32 nucleotide deletion of the CCR-5 gene called delta-32. People with the deletion mutation are healthy which means that CCR-5 is unnecessary for human development.
"It is unclear what CCR-5 does in the human body," said Nathaniel Landau, of Aaron Diamond and one of the co-discoverers of CCR-5. "It may at some point in evolution have protected against a pathogen that no longer threatens Europeans."
Landau pointed out that the mutations have shown up only in Caucasians so far. However, evidence is now growing that some patients who show a level of HIV resistance may have single amino acid changes in CCR-5 that offer some measure of protection. "But we can't be sure that either the delta-32 or the amino acid changes completely protect against infection," said Landau.
Each of these discoveries grabbed headlines. However, the researchers presented the updates on their research earlier this month at the Fourth Conference on Retroviruses and Opportunistic Infections, in Washington, DC.
The HIV envelope protein clearly binds to CCR-5, but Landau and his group wanted to determine whether the binding site for the receptor's natural chemokines was identical to that of the HIV envelope protein. He presented results showing that the chemokines and HIV bind to different areas. As a result, the researchers can more precisely direct antibodies and potential small molecules to block HIV. If there is as yet undetermined activity for CCR-5, it may be possible to design a small molecule drug that doesn't block the activity of the natural chemokines.
In addition, Landau and his colleagues presented data showing that while the deletion mutation can only be found in Europeans, CCR-5 is identical around the world. "What this means is that should a drug be developed," Landau told BioWorld Today, "it will be effective irrespective of ethnicity."
Stimulating T Cells May Help
Berger and his colleagues presented work on methods of stimulating T cells that have been taken from a patient. Presumably, you could help a patient fight the virus by removing some uninfected cells, multiplying them and returning them to the patient. Berger's lab has developed a method of stimulating these cells to grow that shuts off CCR-5 production, making them immune to infection by M-tropic viruses. "However, these cells are very susceptible to the T-tropic strains," said Berger.
Because T-tropic viruses are associated with a decrease in T cells and the illnesses associated with AIDS, it is possible that providing these T cells could harm the patient. But Berger noted that the advances in our knowledge about CCR-5 and fusin--now known as CXCR-4--should allow researchers to develop small animal models of HIV infection and AIDS that will answer these questions.
"We are all working to develop small molecule drugs to block these receptors," said Berger. "Perhaps we can prevent infection or prevent disease progression."
Progenics Pharmaceuticals, in Tarrytown, New York, and Leukosite in Cambridge, Mass., have initiated programs to identify potential small molecules that can block CCR-5 and CXCR-4. *