The sleeping giant of HIV infection—a reservoir of viral DNA that lies dormant in human immune cells—could be far larger than previously believed.

A study published today in the journal Cell shows that the reservoir, consisting of proviruses integrated into resting CD4 immune cells, may be 60 times larger than scientists had estimated. Antiretroviral therapy (ART) kills replicating HIV but not the latent proviruses, which pose a major barrier to eradicating the virus from the body and curing infection.

Recent research had indicated that fewer than 1% of proviruses become infectious when resting CD4 cells are activated in a test tube. Without activation, proviruses can’t replicate. The proviruses that don’t cause infection have been considered defective, but investigators hadn’t described specifics about the deficiencies.

In the new study, a team of investigators used a more detailed method to study proviruses that didn’t switch on and become infectious when the resting CD4 cells they inhabited were activated in the laboratory. They cloned the genomes of 213 inactive proviruses from 8 HIV-infected patients treated with ART for more than 6 months. Their genetic analyses showed that about 88% of proviruses that didn’t turn on had obvious defects that prevented them from replicating. But nearly 12%—a far greater percentage than previously estimated—were capable of replicating and causing infection.

The investigators said their study suggests that there are enough proviruses that don’t turn on but are capable of replicating to boost the size of the latent reservoir by 60-fold. “These results indicate an increased barrier to cure, as all intact noninduced proviruses need to be eradicated,” senior author Robert Siliciano, MD, PhD, of Johns Hopkins University School of Medicine in Baltimore, said in a statement.

“We would like to use these findings by developing better ways to measure the size of the latent reservoir in patients who are participating in future trials of potentially curative strategies,” Siliciano added. “In this way, we think our analysis will contribute to HIV eradication efforts.”