A new oral vaccine has been found to completely stop rhesus macaque monkeys from being infected with SIV, the monkey equivalent of HIV. The vaccine also reduced the number of SIV particles that were present in monkeys who were already infected with SIV.
The international research team, involving scientists from the Paris-Descartes University in France and the University of Chinese Medicine in Guangzhou, China, described the success of the vaccine as “surprising” and “unexpected”, mainly because it's so simple.
The new vaccine works in the opposite way to most vaccines - by suppressing, rather than triggering, an immune response. This is because HIV and SIV actually require immune cells known as CD4 T-cells in order to proliferate and establish an infection in the body.
So one of the goals in HIV prevention has been to develop a vaccine that introduces the body to the virus without causing an immune response. This requires the vaccine to somehow trick the body into thinking HIV is harmless, causing an ‘immune tolerance’ to the virus.
The scientists have now managed to do just that in monkeys, and their results are published in Frontiers in Immunology.
The new vaccine works by stimulating the production of a previously unknown group of immune cells known as CD8 T-cells, which then stop the monkeys’ CD4 cells from recognising SIV. This prevents any immune response and stops SIV from hijacking the CD4 cells and spreading around the body.
The vaccine works because it's made up of inactivated SIV, but, importantly, it’s given alongside doses of bacteria that are familiar to the body and are recognised as ‘friendly’ - in this case, strains used in probiotics.
The researchers tested variations of the vaccine, involving different bacteria strains, in 29 rhesus macaques - they were either given it as a vaginal gel, a rectal douche, an oral vaccine or an oral probiotic-type drink.
So far all 15 monkeys given the vaccine orally have been completely protected against SIV infection, with the vaccinated monkeys still able to suppress viral reproduction four years after vaccination.
The results are extremely positive, but the researchers admit they’re still mystified as to why something as simple as giving inactivated SIV alongside what is essentially a probiotic preparation produces such a strong immune-suppressing result. When inactive SIV is given alone, it stimulates the immune system as normal vaccines would.
Either way, the researchers are now looking into whether the same approach would work in humans. Of course, SIV isn’t identical to HIV, and previously vaccines that have looked extremely promising in animals studies have failed to work in humans, so it’s important to keep these results in perspective.
Two initial safety trials are now planned in humans both in HIV-negative and HIV-positive volunteers.
José Esparza, a HIV vaccine researcher and professor at the Institute of Human Virology in Baltimore in the US, who wasn’t involved in the study, commented in an accompanying article in Frontiers in Immunology: “Although the number of monkeys is relatively small, the levels of protection are impressive”. He admits there was some scepticism surrounding the team’s work, because their approach was so unorthodox and appeared so simple, but he believes more funding is needed for out of the box approaches such as this one.
“The solution to the HIV vaccine challenge will require genius, which…is characterised not only by originality and usefulness, but also by surprising results,” said Esparza.