What an HIV vaccine would have to do

A vaccine is essentially a ‘fake infection’. It is a way of priming the body by getting it to mount an immune response to essentially harmless microbes – or to parts of microbes called antigens – so that these immune responses also work against a similar but disease-causing microbe later on. The principle is essentially unchanged since Robert Jenner observed that dairymaids exposed to the relatively harmless cowpox virus (though he did not know it was a virus then) were later immune to the ravages of the smallpox virus.

Vaccines set in motion an immune response the body would mount against the dangerous pathogen (disease-causing organism) anyway.

The reason most diseases kill is not that the body mounts no fight against them but because there is always a timelag between an invasion by a previously-unknown infection and the immune system learning how to fight it.

In a few cases the invader will win and kill or cripple – either by directly causing damage before the immune system can stop it, or by generating an immune response so extreme that it starts to damage the body’s own cells (this is what is thought to happen in illnesses like SARS and bird flu, and it is also the cause of the liver damage in chronic hepatitis B infection).

In most cases the immune system will eventually win and the invader will be driven out. What the vaccines to most diseases do is prime the immune system to an invader so that when it eventually arrives, it is already ‘known’ to the immune system and there is a much shorter timelag between infection and the generation of an effective immune response.

Vaccines do this in the same way that infections do. An antigen is any foreign protein or protein component that causes an immune reaction in the body. Once an antigen of any sort – a bacterium, a virus, a parasite, even certain chemicals, drugs and dust – enters the body for the first time, the immune system sets about devising an immune response that will, in future, defeat this invader. A vaccine is an antigen designed to create a very specific response. Antigens set off three different types of immune response:

1. Innate immunity. This, evolutionarily the most primitive part of the immune system, is a set of chemicals that recognise and neutralise common foreign protein sequences non-specifically. However it does not ‘remember’ infections and so cannot be used as the basis for a conventional vaccine.

2. Adaptive immunity, which is subdivided into

2a. Humoral immunity. This comprises a set of free-floating proteins called antibodies that link to proteins from specific invaders, and when they reappear either chemically neutralise them or tag them for destruction. Antibodies are generated by B-cells and are immensely variable molecules that have the capacity to ‘remember’ infections – see below.

2b. Cellular immunity. This is a set of roving cells that destroy infected cells by recognising bits of foreign antigen, called epitopes, that cells display on their surfaces. Much of the time these cells exist in embryo form but during infection conditions differentiate – first into T-cells, then into CD4 cells (which orchestrate the immune response) and CD8 cells (which do the actual cell-killing) and finally into memory cells.

In the case of both humoral and cellular responses, the initial attack leaves behind a few memory cells. These are cells that have ‘learned’ the signature of the invader so that when the same one (or apparently the same one) turns up again, the immune system can spring into action far faster and contain an infection before it has any time to do damage.

It is this memory effect that vaccines exploit, and the goal of an HIV vaccine would be to produce enough broadly-effective memory B-cells (which make antibodies) and T-cells (which direct and operate the cell-killing mechanism) to recognise any strain of HIV when it arrives and quickly neutralise it.

Vaccination happens all the time naturally, in the spirit of Nietzsche’s saying “That which does not kill us, makes us stronger.”

Malaria, for instance, is a particularly tricky infection because, like HIV, it constantly changes its shape in order to fool the immune system. However children in Africa who do not die of repeated malaria infections within their first three years will eventually develop a broad-enough immune response to malaria to either repel further infections or develop only mild symptoms.

One theory as to why allergies like asthma are so much more common in the modern world is the so-called hygiene hypothesis. This states that children these days are not exposed to enough allergens and germs when they are young. As a result, their immune system does not ‘learn’ to respond appropriately to certain foreign substances and mounts a disproportionate response when it finally encounters them.

This provides a clue as to why it has proven so difficult to develop a vaccine against HIV. The body does mount an immune response to HIV – indeed, without one, the virus would destroy the average person’s immune system within weeks rather than years.

However in the case of HIV infection the immune response is sufficient neither to prevent infection in the first place nor to prevent the virus circumventing the body’s immune defences in the long run.

An HIV vaccine, therefore, would have to do ‘better than nature’ – and that is why it has proven so difficult to develop.

An HIV vaccine would have to do one of three things, which we will explore in more detail.

Humoral immunity

An HIV vaccine could prevent infection in the first place by generating so-called sterilising immunity.

Sterilising immunity, broadly speaking, happens when the body mounts an antibody or humoral response to the infection. Antibodies are extremely variable Y-shaped protein molecules that are produced in huge quantities by the B-cells of the immune system. They either destroy invading microbes themselves or tag them for destruction by other components of the immune system. If the invader is one the body already recognises, an antibody response can be generated so fast that an infection never becomes established. If it is not recognised, it may take some time for enough antibodies that ‘fit’ the invader to be generated.

Some vaccinations, so-called passive ones, actually consist of antibodies rather than of antigens that generate an antibody response. Passive inoculation with anti-hepatitis B antibodies, for instance, is used to strengthen the immune response and augment the regular vaccine, especially in cases where exposure may have already happened as in a needlestick injury. However passive inoculation is similar to using a drug – the antibodies quickly disappear from the body and no permanent immunity is generated.

Antibodies generally only recognise the surface molecules of bacteria, viruses, parasites etc. The first generation of candidate HIV vaccines, therefore, used this principle. They consisted of parts of HIV’s envelope – the outer viral covering. In particular, they used the gp120 protein that forms the ‘knobs’ on the surface of HIV that are the virus’s mechanism for entering cells.

Hope that an envelope vaccine might work died when the AIDSVAX vaccine trial (see below) proved ineffective in February 2003(rpg120 HIV Vaccine Study Group), and were finally buried when the second AIDSVAX trial in Thailand proved equally ineffective two years later (Pitisutithum 2004).

Why did they not work? The answer lies in the hyper-variability of the HIV envelope.

The gp120 protein, and in particular the part of the molecule called the V3 loop that actually makes contact with cellular receptors, is the most variable part of HIV. Not only is the amino acid sequence that makes up the core chain of the protein more variable than any other part of HIV, but it is also heavily glycosylated. This means that HIV, as it evolves, coats its envelope protein with an immensely variable ‘fuzz’ of sugar molecules that frustrate the attempts of antibodies to latch on to it.

What this means, essentially, is that an HIV envelope vaccine would produce an antibody response – but only one that worked against the exact strain of virus that the vaccine was developed from, or imitated. The first generation of vaccines did not work because they, and the antibodies they elicited, were uselessly specific.

HIV does have highly ‘conserved’ regions. These are areas of the viral genome and of viral proteins that are forced to stay evolutionarily stable because they are essential to its core tasks. These regions vary little from one virus to the next. However in the case of HIV they are also areas that evolution has taken great care to guard. An example is the fusion peptide of the gp41 viral protein, the part of the HIV ‘spike’ that changes shape so that it can inject HIV’s genetic material into the cell. This part of the virus is only exposed for a fraction of a second in the sequence of events that comprise infection so antibodies have to be extremely specific and potent to neutralise these conserved regions.

Cellular immunity

The other thing a vaccine could do is delay or halt the damage that an established infection can do.

It would do this by stimulating the other branch of the immune system – the cellular immunity.

The prime movers in the cellular immune system are the cytotoxicT-lymphocytes (CTLs), otherwise known as the CD8 cells. This branch of the immune system developed to deal with the problem that once a virus is inside a cell, it essentially becomes invisible to the humoral immune system.

However cells have a mechanism whereby they ‘advertise’ their contents by displaying tiny fragments of their internal constituents, called epitopes, on their surface. This is the way the body distinguishes between self and not-self – and between healthy cells and ones subverted into virus-making factories.

When the immune system senses the presence of foreign epitopes, a cascade of immune activation is generated which ends with the CTL cells destroying the infected cell.

The advantage of this kind of immunity is that the cell displays protein fragments from all parts of the invading virus and not just its envelope. In the case of HIV, this means that an immune response can be generated against deeper, more conserved parts of HIV.

The disadvantage of the cellular immune response is that it does not prevent an infection, but acts against already-infected cells.

In most illnesses, this does not matter; the cellular immune response wipes the body clean of sick cells and the disease is gone. Serious damage only occurs if so many cells are infected that the immune response itself becomes harmful.

However in the case of retroviruses like HIV and the HTLV viruses, the virus becomes incorporated into the cell’s genetic code itself – as proviral DNA.

By the time this has happened, the virus has essentially lost its identity as an independent entity and become so much part of the cell that it is not recognised as foreign. It is only when the cell is activated and starts producing new viruses that the immune system can recognise it as infected.

For this reason, a vaccine that generated cellular immunity could have immensely variable effects depending on whether it acted in time to prevent the incorporation of HIV’s genes into the human cells’ DNA.

At best, it might be able to turn people into exposed seronegatives. In the natural history of HIV, these are people who remain HIV antibody-negative but where extremely sensitive tests detect signs of a historical infection by HIV – one that remains so well-contained that not enough virus is ever present to trip the humoral immune response and induce antibodies to HIV. There is evidence that exposed seronegativity is quite common, but the majority of exposed seronegatives remain little-studied and we do not know how many there are and why they do not develop HIV infections.

Even though exposed seronegatives do not have antibodies to HIV, immune experiments showed that their T-cells ‘recognise’ HIV in the test tube – so they must have seen it before(Shearer 1996). The types of cellular responses detected included both CD4 and CD8 cell responses to HIV and the production of immune-activating cytokines in response to HIV. These CD4 and CD8 responses have been reported in sexual partners of HIV-infected individuals, as well as in seronegative health care workers who were accidentally exposed to HIV-infected blood via a needle stick.

At the time of the above study, in 1996, no HIV was detectable within these people by PCR viral load testing. Subsequent extremely sensitive PCR testing, however, has found that many exposed seronegatives may have extremely small viral loads – in the order of 0.05 copies This means that they do have some cells that have been infected by HIV and contain proviral DNA.

What appears to be the case with most of them, however, is that by good luck, good genes or good timing, their immune system developed a CD8 response against actively-infected cells so efficient that it nipped any productive viral infection in the bud – exactly what we hope a truly efficient CD8 vaccine could do.

One example of this phenomenon was a study by Tuofu Zhu(Zhu 2004) presented at the Bangkok International AIDS Conference. Zhu was studying long-term exposed seronegative partners of HIV-positive gay men. The group consisted of the HIV-negative partners of HIV-positive men who had been diagnosed between 1994 and 1998.

Out of 94 HIV-negative regular partners of positive men, he found 14 who had in fact become HIV-positive.

Two of the partners appeared to have caught HIV from their partners early on in their relationship, but to have mounted a successful immune response to it. They had no antibodies to HIV and therefore did not test HIV-positive. The fact that they had HIV at all could only be detected by hypersensitive viral-load testing, which picked up HIV in their blood at a count of 0.05 copies - one thousandth of the amount usually called "undetectable" by standard "ultrasensitive" tests.

The ultimate goal of a CD8 vaccine, therefore, would be to turn people into ‘fake’ exposed seronegatives.

However no CD8 vaccine has come anywhere close to producing this effective an immune response, and the exposed seronegatives – or at least the ones that have been studied, who are mainly multiply-exposed people that somehow do not become HIV-positive – remain with their immunity secrets tantalisingly elusive (it appears now that they may also generate broadly neutralising antibodies – see below).

What CD8 vaccines have done until now, at least in animal studies, is to blunt HIV infection. Though the vaccine-generated immune response may not be able to stop people becoming HIV positive, it may be enough to slow down viral production by interfering with the chain-reaction of viral infection and reproduction. A vaccine of this kind might not be able to prevent people becoming HIV positive (and in many cases would actually generate a ‘false positive’ result itself). But it might be able to contain HIV reproduction and enable people to develop a much lower viral load; so low, possibly, that progression to AIDS might never happen.

This kind of vaccine essentially blurs the distinction between a preventive vaccine and a therapeutic one. The latter are not the subject of this chapter as they are a treatment. But the objective of scientists who are trying to develop therapeutic vaccines is essentially identical: by manipulating parts of the immune system of people with HIV in such a way that their anti-HIV CD8 responses are amplified, they are aiming to contain HIV infection to the point where it becomes non-pathogenic.

The other very important thing therapeutic or cellular vaccines could do would be to do what HIV treatment can also do – they would act to prevent onward HIV transmission by lowering the average viral load in the infected population.

Mucosal immunity

There is a third kind of immunity a vaccine might be able to generate, but it is not one that previous vaccines have attempted to stimulate. This refers to humoral or cellular immune responses that are concentrated at the mucosal surfaces where most HIV transmission takes place, such as the vagina and rectum. Vaccines may be able to induce immune responses acting only at these surfaces, to prevent HIV transmission through sex. They would not work against infection by injection, but since the majority of HIV in the world is spread through sex, they would potentially contain the epidemic.

What would a mucosal vaccine look like? It might look a lot more like a microbicide than a vaccine, though it would be one that generated an immune response. It does not take a big leap of science to move from the idea of a microbicide that would work by getting genetically-altered versions of natural gut and genital bacteria to develop microbicidal substances like cyanovirin-N to getting genetically-altered bacteria to develop bits of HIV proteins that would then generate an immune response.

Such an approach has indeed been developed by Dean Hamer of the US National Institutes of Health. Because its method of delivery is more like a microbicide than a vaccine, it is described under microbicide-expressing bacteria in the Microbicides section.

In December 2005 one of the leading exponents of both microbicide and vaccine technology, Dr Robin Shattock of St George’s Hospital, London, told a vaccines meeting organised by the National AIDS Trust that the first effective HIV ‘vaccine’ to be developed might indeed look more like a microbicide or a long-acting contraceptive device than a standard injection. He said that the first vaccines might also only work for months at a time, and his talk was a useful summary of the challenges HIV throws at vaccine developers and why this might be so.

After talking about the difficulties of improving on the insufficiently effective natural immune response to HIV and of coping with HIV’s immense genetic variability, Shattock added that most diseases for which a successful vaccine had been developed got into the body via the lungs or the digestive system. Apart from hepatitis B we had little experience of a vaccine against something that usually entered through the genital tract. He said that the direction his own research was taking might not be a truly preventive vaccine, but one that blunted the huge surge in viral load people get when they are first infected with HIV (within the first six weeks). It is estimated that because people are so much more infectious at this time, anything from 25-60% of all HIV is transmitted by people who have just got it themselves.

He said: “If we could do this it might give infected individuals a better prognosis – and it would have a major impact on transmission within the community.”

However he warned that it might require “regular and repeated vaginal [or rectal] exposure” to have an effect.

So he was looking at technologies like intravaginal rings and caps that could deliver a sustained-release dose of a substance that would stimulate HIV-specific immunity. Such devises might reinforce or potentiate the effect of a more conventional injected vaccine.

Shattock said he was not pessimistic about the eventual discovery of a vaccine against HIV. “We have found out that conventional approaches don’t work against HIV, but we only know that because of 20 years of intensive research,” he said.

Broadly neutralising antibodies

As we said above, exposed seronegative people have also been found that have antibodies that are broadly effective against a wide range of different strains of HIV infection rather than just very specific ones.

Researchers such as Robert Gallo have argued that CD8 vaccines will not prove to be enough to prevent HIV and that a completely new ‘third generation’ approach to an HIV vaccine should be developed using a combination of CD8 stimulation and broadly neutralising antibodies.

Broadly neutralising antibodies are extremely rare; the first was not isolated from a person’s blood and described till 2001 (Stiegler 2001). They are also not typical of most antibodies. Molecular analysis and crystallography has found that they tend to have unusual structures featuring molecular ‘spikes’ that can penetrate into the normally well-concealed conserved areas of HIV’s envelope. So far only a few have been isolated from the blood of exposed seronegative individuals. A study in 2004(Binley 2004) found that just one antibody, 4E10, neutralised every one of a panel of 90 HIV viruses with moderate potency. One called 2F5 neutralized 67% of isolates, but none from clade C of HIV, the most common type in Africa. An antibody called b12 neutralised 50% of viruses, including some from almost every clade, while one called 2G12 neutralised 41% of the viruses, but none from clades C or E.

Experiments with these antibodies have so far mainly involved using them as passive inoculations and studying how they are eliminated in the body. Here they act more like potential long-lasting anti-HIV drugs, as they are eliminated from the body over a timescale of one to three weeks. Some artificially-created antibodies such as the experimental drug TNX-355 use the same principle.

Developing a vaccine which induces the body to generate them will be much more difficult. Because these antibodies act against parts of the viral infection mechanism that are only exposed for a fraction of a second during the intricate unfolding and insertion process that happens during the infection of a cell, it is challenging to establish what epitopes could elicit such antibodies in the average person, and so far we are nowhere near achieving this feat.

An alternative approach, since finding epitopes that elicit broadly neutralising antibodies for everyone may be impossible, is to follow the Shattock approach and to construct genetically-engineered bacteria that express these antibody molecules.

Vaccines against viral proteins

Vaccines can be made against toxins that bacteria and viruses produce as well as against parts of the mature virus. An example is the tetanus toxoid vaccine. It is an inactivated version of the bacterial toxin the tetanus bacterium produces, and it induces antibodies against the toxin itself.

HIV produces several harmful proteins that could be vaccine targets. The most promising so far is the Tat protein, which is produced early on in the viral lifecycle, before HIV integrates into the host genome. It stimulates the host cell’s genes to become active and, released outside the cell, influences immune cells to develop in directions that make them more receptive to HIV.

The Tat protein is so important to HIV that it is highly conserved, and studies have shown that people with high levels of anti-Tat antibodies progress more slowly to AIDS, so a Tat vaccine might also act as a therapeutic vaccine too. A small trial involving 47 volunteers in Italy ending in 2006 produced a strong immune response in 80% of subjects given it. The Italian team studying it is currently preparing for phase IIa studies in humans and is trying to get funding for a large African trial scheduled to end in 2011(Ensoli 2006).

The Tat vaccine has been controversial, however. Lead researcher in the development programme, Barbara Ensoli of Istituto Superiore di Sanità in Italy sued another researcher who criticised protocol changes and violations in the 2006 trial and some scientists are extremely concerned about Tat’s possible toxicity; studies have found that it has a multiplicity of other systemic effects. In particular, it was first detected because it caused the angiogenesis (proliferation of blood vessels) seen in Kaposi’s sarcoma, and there are fears it could have generally carcinogenic properties.