Answer by Tirumalai Kamala:

To appreciate its novelty, let’s first summarize this study in layman terms.

Rhesus macaques vaccinated with what? Inactivated Simian Immunodeficiency Virus (iSIV) with three different live bacteria. Which ones?
1. BCG, the live attenuated TB vaccine.
2. Lactobacillus plantarum (LP) and Lactobacillus rhamnosus  (LR), common natural inhabitants of the human GI tract.
The controls got only iSIV.
Vaccinated how? Stomach or Vagina. Boosted two months later.
Infected with SIV how? Rectum or intravenous (iv).
When? 4 to 48 months later. Some two or three times, not just once.
What SIV? Either the same as in the vaccine (homologous) or different (heterologous).

What happened?
This unorthodox vaccination approach worked.
1. Vaccinated macaques had undetectable circulating virus after rectal or iv infection. Controls that only got iSIV had high circulating virus.
2. All three vaccines (iSIV+BCG or iSIV+LP or iSIV+LR) gave similar results. Each vaccine component alone was ineffective.
3. 23/24 vaccinated macaques were protected even when challenged 48 months after vaccination.
4. Protection against both homologous and heterologous SIV.

Circulating immune responses in the blood?
1. Undetectable or very low anti-SIV antibodies, unlike control macaques.
2. CD8 T cells.
a) strong antiviral response.
b) strongly inhibit CD4 T cell activation.
c) necessary because protection is lost when they are depleted.

The authors characterize this as immune tolerance. Rather than tolerance, this is a novel type of protective immunity, one that escapes easy characterization. In fact it has no place in current immune response classification. How so? The circulating CD8s prevent viral infection of circulating CD4s but without killing them (noncytolytic). This is novel CD8 function.

In a previous paper two years earlier (Page on cell.com), these authors had already shown that
1. The CD8 effect on inhibition of CD4 activation required cell-to-cell contact.
2. The CD8 effect required non-classical rather than classical MHC restriction.

How were these macaques protected?
Normally CD4s are activated in response to the virus. Their activation is the Trojan horse the virus then uses to gain a foothold in the body. Why is CD4 activation a Trojan horse? Because when CD4s are activated, they proliferate. The more they proliferate, more target cells available for the virus. By preventing CD4 activation, these non-classical SIV-specific CD8s are stopping viral spread. How are these non-classical CD8s activated in the first place? Obvious candidates?
1. Co-vaccination with live bacteria: BCG, Lactobacillus plantarum, Lactobacillus rhamnosus.
2. Route: vagina or stomach.

A combination of these two factors appears to initiate a protective SIV-specific immune response that consists of non-classical CD8, little or no anti-SIV antibody and little or no classical CD4 activation, at least when they examined some circulating immune responses. What are the other characteristics of this protective immune response? In circulation? Locally? Open questions as yet.

In science, novel and truly worthwhile answers typically open the door to more questions. Here as well.
1. Would transfer of SIV-specific CD8s alone from vaccinated macaques transfer protection to naive macaques?
2. Would human studies yield similar data?
a) By this group.
b) By other researchers.
3. How similar or different are the immune responses (local and circulating) when vaccine is given in the stomach or vagina?
a) Strength
b) Kinetics
c) Local site characteristics (stomach and vaginal washes): CD4, CD8, B, APCs (Antigen Presenting Cells), others?
4. How similar or different are the immune responses (local and circulating) when vaccine has BCG versus LP or LR?
5. Is a vaccine boost necessary?
6.CD8s typically need CD4 help for full function.
a) How are these CD8s activated?
b) Do these non-classical CD8s need CD4 help or not?
7. Are CD4s activated at all, albeit in a non-classical way, which prevents their subsequent infection?
8. What’s happening at the site of infection? Can only be assessed with rectal infection.
a) Local site characteristics (rectal washes): CD4, CD8, B, APCs, others?
b) Antibodies? Secretory IgA?
c) CD4: similar to those in circulation or not?
d) CD8: similar to those in circulation, i.e. non-classical, non-cytolytic and antiviral or different?

The most surprising fact about these studies? Their manner of funding. As Marc HV Van Regenmortel, wrote about their earlier 2012 study (An Oral Tolerogenic Vaccine Protects Macaques from SIV Infection without Eliciting SIV-Specific Antibodies nor CTLs) “This remarkable and totally unexpected breakthrough was obtained by an investigator-driven research that was not funded by the usual governmental and large scale organizations that support most of the ongoing HIV vaccine research world-wide. It was sponsored by a private benefactor who funded the project to the tune of 13 million Euros. This illustrates once again that success in basic vaccine research is unpredictable and that “risky” projects based on unorthodox thinking may deserve as much funding as the “safe” projects that are often preferred because they abide by current fashionable paradigms”.

Jose Esparza and  Marc HV Van Regenmortel write about this 2014 study (More Surprises in the Development of an HIV Vaccine), “The 2012 publication from this group had very little impact in the field, perhaps because it was received with a degree of skepticism. After all, 30 years of intense vaccine research had not resulted in a practical effective vaccine, although an HIV vaccine is sorely needed to bring the HIV epidemic under control. No stone should remain unturned in its search,and the approach reported in this journal should not be dismissed a priori. Instead, it should be carefully considered by other scientists and appropriately confirmed or refuted by additional research”. “Out-of-the- paradigm approaches, such as the one proposed by Andrieu et al., should be further explored”.

My personal opinion? Could we get a better scathing indictment of our currently gargantuan yet ineffectual biomedical research enterprise? That risk-averse and same-old, same-old approach yielded no effective HIV vaccine in 30 years. Private enterprise stepped in to fund real risk. It yielded a promising vaccine approach that could work not just for HIV but for other diseases as well, and true immunological surprise that could pave the way for toppling paradigms that are way past their expiration date, paradigms such as what should constitute a protective anti-HIV immune response, and what types of vaccines and routes are most likely to induce it.

How does the new vaccine work that completely blocks SIV infection in monkeys?