Blocking a protein called PD1 on immune system cells helped reduced viral loads and increased survival in monkeys infected with a virus similar to HIV, according to the authors of a study published online December 10 in Nature and reported by ScienceDaily.

PD1 receptors are a critical component of the immune system. Their presence on CD8 cells keeps the immune system from over-reacting to infections. Two years ago, researchers reported that there is an overabundance of PD1 proteins on CD8 cells in people with HIV. The scientists proved that the excessive PD1 signaled other immune system cells to stop fighting HIV even though the virus was still present and reproducing. Additional research showed that blocking PD1 in mice improved the ability of CD8 cells to respond to a viral infection. No one knew, however, whether it would be safe to block PD1 in larger animals, such as monkeys and humans.

To explore the safety of blocking PD1, a team of researchers led by Rama Amara, PhD, from the Vaccine Research Center at Emory University in Atlanta, gave PD1-blocking antibodies to nine monkeys infected with SIV (simian immunodeficiency virus). They compared these monkeys, treated four times over a period of 10 days, with five other monkeys who were given a neutral inactive antibody.

All nine of the monkeys who received the anti-PD1 antibodies were still alive seven months after the injections, whereas four of the five monkeys who received the control antibodies died within four months. The immune systems of the monkeys who received the anti-PD1 antibodies also improved significantly, including an increased number of CD8 cells targeting SIV and production of SIV-fighting antibodies.

“It…is important to note that this therapy was effective without antiretroviral drugs and in monkeys with severe AIDS,” Amara said. The researchers plan to continue developing and refining the process for blocking PD1. They also plan to try it against other chronic diseases, such as hepatitis C virus (HCV) infection.