After the approval of three new antiretrovirals'two in new medication classes'within the past year, the slowdown in the drug pipeline for the foreseeable future might seem disheartening. But researchers haven't taken a break from their efforts to make new, viable anti-HIV meds. In fact, in recent months their efforts have led to discoveries about the immune system that will very likely open new frontiers for attacking the virus.
One such discovery could take the number of viable antiretroviral classes far beyond the current half a dozen. While HIV has to hijack human proteins that are part of CD4 cells to do its damage, scientists have known only a few dozen such targets. But Harvard researchers have unveiled a surprisingly longer list'an important first step in the hunt for new meds. On its face HIV is a simple virus, consisting of just nine genes. Yet it makes up for that bare-bones structure in a sinister and complex way'by literally taking over the cellular machinery of its victims so that it can multiply and then destroy. The proteins it exploits have been dubbed HIV dependency factors, and 36 had been discovered. The new research, published online in the journal Science, found 273 of these potential targets.
Led by geneticist Stephen Elledge of Brigham and Women's Hospital, the team used a technique called RNA interference that can disrupt a gene's ability to do its job and make a protein. One by one, they disrupted thousands of human genes in test tubes, dropped in some HIV, and watched what happened. If HIV couldn't grow well, it signaled the protein that the gene that had failed to produce must be the reason. It will take more research to figure out the role each of these proteins plays in HIV's life cycle, Elledge says.
But most of today's anti-HIV meds work by targeting the virus itself. In August, the government approved sale of the first drug that works by blocking one of HIV's dependency factors, a cellular doorway called CCR5. The hope is that this longer list of those factors will point toward spots where similar drugs might work.
And other research has experts hoping that they've uncovered an all-new way to defeat the virus. By outfitting immune-system killer cells with a new pair of genes, scientists have transformed them into weapons that destroy cells infected with HIV. Their strategy of genetically engineering immune cells to redirect their infection-fighting ability toward killing HIV-infected cells could lead to an entirely new approach for combating the virus, according to their study, reported in the March issue of the Journal of Virology.
After someone is infected with HIV, a subgroup of their immune cells known as CD8 cytotoxic T lymphocytes recognize cells infected with HIV and kill them before they become HIV-producing factories. This CD8 activity initially keeps the infection in check. But eventually the virus typically evades and ultimately overpowers the immune system. However, a small percentage of HIVers, known as elite controllers, manage to suppress their infection for years.
'Certain of the CD8s of elite controllers may be genetically equipped to bind tightly to HIV-infected cells and destroy them and thereby suppress the infection indefinitely,' says Harris Goldstein, MD, senior author of the study and director of the Einstein/Montefiore Center for AIDS Research at Yeshiva University. 'Our idea,' he says, 'was first to identify the elite controllers' 'super' CD8s and to isolate the genes that enable these cells to bind tightly to HIV-infected cells and kill them efficiently; then we would transfer these genes into CD8s that do not recognize HIV-infected cells and convert them into potent killers of those cells.'
The researchers injected mice with both HIV-infected human cells and with the reprogrammed naive CD8s into which the HIV-recognizing T-cell receptor genes had been inserted using a lentivirus delivery system. One week later when the researchers looked for HIV-infected human cells in the animals, they found that the infected cells had virtually been eliminated.
Goldstein notes that this study was done using genes for just a single CD8 T-cell receptor. 'To make this strategy even more effective, we're now in the process of isolating a 'cocktail' of CD8 receptor genes that are specific for many different HIV peptides,' he says. 'Ultimately, we'd like to remove CD8s from patients, convert them into potent HIV-specific CD8s by inserting a variety of HIV-specific CD8 receptor genes, and then reinfuse them back into patients. By reinforcing the immune system in this way, we hope to turn the tide of battle against HIV in favor of people infected with the virus.'