A novel and relatively simple vaccine that can be administered orally has managed to completely block rectal infection with SIV, the monkey equivalent of HIV, in rhesus macaques and produced rapid re-suppression of viral load in monkeys who were previously infected with SIV.

The vaccine, whose success at blocking infection was described by its own designers as "surprising" and "unexpected", appears to work by stimulating the production of a previously unknown group of CD8 T-cells that, while recognising HIV themselves, stopped the monkeys’ CD4 cells from recognising SIV as a foreign invader, thereby preventing an immune response to SIV. This suppressant effect – which works in the opposite way to a traditional vaccine – means that the SIV is deprived of the SIV-specific immune-activated CD4 cells it needs in order to proliferate and establish an infection in the body.

The vaccine consisted of inactivated SIV administered alongside doses of familiar bacteria – in the first case the TB-suppressant bacterium BCG, and subsequently with gut bacteria of the Lactobacillus genus, including one type commonly used in probiotic supplements. This suggests that if human studies replicate the success seen in monkeys (by no means assured in vaccine studies) the vaccine could be administered in a drink.

Glossary simian immunodeficiency virus (SIV) An HIV-like virus that can infect monkeys and apes and can cause a disease similar to AIDS. Because HIV and simian immunodeficiency virus (SIV) are closely related viruses, researchers study SIV as a way to learn more about HIV. However, SIV cannot infect humans, and HIV cannot infect monkeys. CD8 A molecule on the surface of some white blood cells. Some of these cells can kill other cells that are infected with foreign organisms. CD4 cells The primary white blood cells of the immune system, which signal to other immune system cells how and when to fight infections. HIV preferentially infects and destroys CD4 cells, which are also known as CD4+ T cells or T helper cells. oral Refers to the mouth, for example a medicine taken by mouth. immune response The immune response is how your body recognises and defends itself against bacteria, viruses and substances that appear foreign and harmful, and even dysfunctional cells.

Two initial safety trials are now planned in humans. In one, HIV-negative volunteers at low risk of HIV will be given the vaccine to see if it stimulates the same immune- and virus-suppressant responses. In the other, volunteers living with HIV who are on fully suppressive antiretroviral therapy (ART) will be given the vaccine and then taken off ART six months later if test tube results suggest the vaccine has produced such responses.

The vaccine

The vaccine programme is a collaboration between scientists at the French Institute for Research into Cancer and HIV Vaccines at Paris-Descartes University and scientists at the Chinese Tropical Medicine Institute at the University of Chinese Medicine in Guangzhou.

The researchers note that their approach was based on a long-held hypothesis that conventional vaccines – that rely on stimulating an immune response to a microbe in advance of exposure to it – could not work in HIV, as this virus uses the very cells that proliferate in an immune response (primarily CD4 T-cells) as the ones it chooses to reproduce in. The trick with an HIV vaccine would therefore be to induce the body to recognise HIV but not mount a proliferative response to it. This could work if the body is induced to respond to HIV as if it was harmless – to induce so-called immune tolerance to it.

The researchers had previously had limited success in using immune-suppressant drugs to induce brief increases in CD4 counts, and in attaching HIV to inactivated dendritic cells that would ‘show’ HIV to the immune cells without activating them. The dendritic-cell vaccine showed promise in suppressing viral loads in people living with HIV but would be too high-tech and expensive for general use.

The team therefore decided to see if combining inactivated SIV with bacteria that either strongly bind to dendritic cells (in the case of BCG) or are recognised as ‘friendly’ by the immune system (in the case of Lactobacillus) would also work as a vaccine.

They started off with six monkeys given an inactivated SIV/BCG vaccine as a vaginal gel; then gave it to seven monkeys as a rectal douche; then to eight monkeys as an oral vaccine (via gastric tube). When the oral inactivated SIV (iSIV)/BCG vaccine proved completely effective, they then gave eight monkeys an oral vaccine of iSIV plus the Lactobacillus plantarum bacterium, as a drink; then another eight iSIV plus Lactobacillus rhamnosus, which is a common species found in the human gut and in many probiotic supplements.

Results

So far 15 of the 29 monkeys have been completely protected from SIV infection. The effect appears to last; the last challenge was three years after infection and vaccinated monkeys’ immune systems show ability to suppress viral reproduction four years after vaccination.

Vaginal and rectal vaccine formulations were not 100% protective. In addition, when monkeys were injected with SIV they developed infections (see below). But not a single monkey given the vaccine as an oral formulation, and who also received SIV rectally, could be infected, despite repeated challenge (apart from one ‘breakthrough’ – see below).

In contrast, 26 'control' monkeys that were either given no vaccine, inactivated SIV without bacteria, or bacteria without vaccine, were all infected, with typical SIV viral loads.

Five monkeys given a vaccine containing BCG and dosed rectally or vaginally were not fully protected against SIV. They developed SIV infections after challenge with a typical peak viral load of about 100,000 copies/ml, but developed a ‘set point’ (viral load after the period of acute infection) that was lower than usual – below 1000 copies/ml.

Seven monkeys were challenged with injected SIV. In these cases, infection did develop, but signs of it then quickly disappeared. Three monkeys given a vaginal BCG-containing vaccine developed a viral load of about 1000 copies/ml (as opposed to a more typical 100,000-1 million/ml) ten days after challenge, but which fell to undetectable levels (below ten copies/ml) 30 days after challenge. A similar pattern was seen in four monkeys given an oral vaccine but in this case viral load never rose above 200 copies/ml.

There was one ‘vaccine breakthrough’. In one of the monkeys vaccinated with iSIV/Lactobacillus plantarum, its CD8 cells lost their ability to suppress HIV replication in CD4 cells in the test tube a year after vaccination. The researchers predicted it would become infected when challenged rectally 16 months after vaccination, and this duly happened, but this monkey is the only one – out of 24 that were initially completely protected against rectal infection – to lose its immune protection up to four years after vaccination.

The researchers wanted to check that the CD8 cells were the ones responsible for the suppressive immune response and to do this took four of the iSIV/Lactobacillus plantarum-vaccinated monkeys and temporarily deleted their CD8 cells using an anti-CD8 antibody before challenging them once again with live SIV. The monkeys were all infected.

Excitingly, however, once the anti-CD8 antibody was withdrawn and the monkeys’ CD8 cells started repopulating the immune system, which took four to seven weeks, the SIV viral load in these newly infected cases rapidly became undetectable and stayed so. This, and the response of the monkeys challenged with injected SIV, shows that this vaccine could work both as a sterilising vaccine that prevents infection altogether, and as a therapeutic vaccine that rapidly suppresses viral replication in those already infected.

How does it work?

How does this vaccine work? The researchers found that it generates a previously unknown class of CD8 T-cells called T-regulatory (Treg) MHC-IB/E-restricted cells. Treg cells generally are a newish discovery as a class and the only other Treg cells so far seen work in a different way.

HIV ‘boots’ itself into the body precisely by getting the immune system to react to it and thereby make lots of new activated CD4 cells to infect: if the CD4 cells don’t recognise it as foreign, no infection gets established. What these cells appear to do is to suppress the response in CD4 cells to HIV by inducing the CD4 cells to change the MHC molecules on their surface. The job of these molecules is to detect foreign proteins.

The researchers, however, are mystified as to why doing something as simple as giving inactivated SIV along with what is essentially a simple probiotic preparation produced such a strong immune-suppressant response, especially as iSIV given by itself produced a more typical immune-stimulant response.

It is important to emphasise that the MHC system of self-recognition proteins is the most variable in the body, and between species, and there is no guarantee that what works in rhesus monkeys will work in people: previous vaccines such as the one used in the STEP Study looked promising in animal studies but failed to work in humans. One thing that has tripped up HIV vaccine research is that HIV in the field is much more variable than the experimental viruses used in lab research, although eight of the vaccinated monkeys were challenged with a significantly different strain of SIV and were protected against that too.

This research is not only of use to HIV vaccine research. The strategy of vaccine-induced immune tolerance, rather than immune activation, is so new that, the researchers say, “it opens up a vast array of research on the suppressive arm of the immune system and its potential manipulation in human and veterinary medicine.”

In an accompanying commentary, José Esparza, a veteran HIV vaccine researcher who is currently a professor at the Institute of Human Virology in Baltimore in the US, comments that the levels of protection seen in this vaccine are “impressive”.

The team’s previous work, he says, was received “with a degree of scepticism” partly because their approach was so unorthodox and appeared so simple. He advocates for more funding for “out-of-paradigm approaches” like this one.

Esparza comments: “The solution to the HIV vaccine challenge will require genius, which…is characterised not only by originality and usefulness, but also by surprising results.”