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Towards an HIV Vaccine

HIV vaccine

A safe effective vaccine against HIV remains the “holy grail” of vaccine researchers. Source: iStockphoto

By Ivan Stratov and Stephen Kent

Potent antibodies force the HIV virus to mutate, opening up a new strategy for producing an effective HIV vaccine.

Ever since human immunodeficiency virus (HIV) was revealed as the cause of acquired immune deficiency syndrome (AIDS) 25 years ago, scientists have been trying to make a vaccine to prevent it. It was only in 2009 that a glimmer of light appeared at the end of what had been a very dark tunnel, full of dead ends and disappointments.

Scientific attention has now focused on highly potent natural killer cells that destroy HIV-infected cells and prevent the virus from spreading. Our research has conclusively shown that the HIV virus mutates to avoid being attacked by these natural killer cells, opening an entirely new strategy for developing a vaccine against HIV.

HIV is the most studied virus in the history of mankind. It has been the most significant global health issue of our generation, and has been responsible for approximately 25 million deaths since its effects were first described in 1981. In comparison, some 15 million people died during World War 1. Approximately 33 million people worldwide are infected with HIV, and annually there are 2.5 million new infections and two million deaths attributable to HIV.

HIV causes disease by infecting and destroying key immune white blood cells called CD4 T cells. Once these cells are depleted, a person’s immune system is damaged and he/she becomes susceptible to infections and cancers that are ultimately fatal. Amazingly, while nearly 60 million people have been infected with HIV, not one person has ever spontaneously cleared it from their body.

Great advances in the treatment of HIV have been made, especially since 1996 when highly active anti-retroviral therapy (HAART) was introduced. Combinations of these anti-retroviral drugs suppress the virus but cannot eradicate it, so lifelong treatment is required.

Global philanthropic efforts are making these expensive medications available to many disadvantaged people, and now approximately five million in the developed world are on HAART. However, it is estimated that many more are in critical need of HAART. Unfortunately, the demand for medications is being outstripped by accessibility at a rate of 2:1.

A vaccine for HIV is widely regarded as the best way to effectively halt this pandemic.

Vaccines have been one of the greatest advances in the history of human scientific endeavour. They have led to the eradication of smallpox and protected humans from diseases such as measles, rubella, poliomyelitis, tetanus, mumps, influenza and hepatitis B.

To date, the global scientific effort to produce a safe effective vaccine against HIV remains the “holy grail” of vaccine researchers. Live attenuated vaccines based on a mutated, weakened form of the virus (like the polio vaccine) are too dangerous as the virus can mutate back to a lethal form. A vaccine that produced neutralising antibodies to HIV (similar to the hepatitis B vaccine) was also unsuccessful, as was a vaccine strategy that utilised killer T cells that attack cells infected with HIV.

Recently, a 6-year HIV vaccine trial in Thailand involving 16,400 people produced a 30% reduction in new HIV infections. Although public health officials would normally require an effective vaccine to produce more than 90% protection, this was the first glimmer of light in a very long dark tunnel that has confronted HIV vaccine researchers for 25 years.

Having made this critical first step, researchers are now focusing on what exactly protected these people from infection. Neutralising antibody and killer T-cell responses were similar in people who received the vaccine and those who did not. However, vaccine recipients did have significantly higher levels of other types of antibodies, and these may be the key to how this vaccine worked.

Our research is investigating antibodies that target cells infected with HIV rather than trying to attack the free virus directly. The phenomenon is called antibody-dependent cellular cytotoxicity (ADCC), where antibodies induce the killing of cells infected with HIV.

It is important to understand that HIV can only replicate inside cells, so the ability to kill HIV while it is inside another cell and trying to replicate is potentially a very important tool to use in a vaccine strategy.

It is also worth noting that HIV can spread directly from one cell to another cell (by cells fusing to each other) without necessarily releasing free virus. This is the downside of using neutralising antibodies, but one of the great benefits of the ADCC approach.

Our research into this area started 7 years ago when we developed a laboratory technique for detecting exactly what part of the virus these ADCC antibodies were targeting. HIV is made up of nine genes that code for nine proteins that allow the virus to exist and replicate. Our technique works by breaking down these nine HIV proteins into tiny segments called peptides, and then seeing which of these segments react with ADCC antibodies and activate natural killer cells to produce “killer molecules” called cytokines. These cytokines destroy the cells harbouring HIV and prevent them from replicating.

Once we established exactly which specific parts of the HIV proteins were being targeted by ADCC antibodies, we were able to extract the virus from people infected with HIV and see whether or not the virus was mutating to avoid these antibodies.

For example, we studied one individual who was diagnosed with HIV in the year 2000 but had remained very well over the next 10 years and had not required treatment with HIV drugs. We found that he had strong ADCC responses directed at a part of the virus called the envelope protein, which coats the virus and protects it from being destroyed. We then checked the strain of HIV that he had when he was first diagnosed and compared it to the strain of HIV he had 8 years later.

We found that the envelope protein had mutated at the exact point targeted by the ADCC antibodies, and that this mutation meant that the ADCC antibodies no longer recognised the virus strain and activated natural killer cells. By avoiding this “immune pressure”, the mutated HIV virus could replicate more freely and continue to damage the immune system. This is termed “immune escape”.

The ability of replicating HIV to mutate and avoid ADCC antibodies reinforces just how slippery a customer HIV is. HIV has an “error-prone” mechanism of replication, which leads to mutations at each cycle of replication. Most of these random mutations are deleterious but occasionally mutations will be advantageous (such as immune escape variants). Viruses carrying advantageous mutations grow to dominate the viral population over time.

Our results provide a crucial clue for vaccine studies. The fact that HIV is forced to mutate to avoid ADCC antibodies suggests that these antibodies are “hurting” the virus: they are a potent force! Indeed, the mutated HIV strain is likely to be at least marginally weaker than its original form.

More importantly, though, our results suggest that if these ADCC antibodies were induced by a vaccine and present in the body prior to infection, then they may be able to prevent infection altogether or at least substantially reduce virus levels.

Most current vaccines work by mopping up virus or virus-infected cells at the initial site of infection before there has been widespread infection throughout the body. Killing just a few infected cells at the initial exposure to HIV is likely to be easier than coping with massive levels of replication seen in people with established HIV infection.

Using ADCC antibodies to limit the initial infection is also less likely to lead to mutations that escape ADCC antibodies, since the levels of new mutations are typically proportional to the amount of replication.

Where is our work with ADCC antibodies to fight HIV going? We have now successfully purified a small amount of these ADCC antibodies and shown that we can significantly inhibit the replication of HIV in the laboratory. We are currently designing experiments to produce larger quantities of these antibodies. We will be focusing our efforts on ADCC antibodies that target highly sensitive parts of the HIV virus and are likely to cause the most damage to the virus and less likely to result in escape variants.

We will test these ADCC antibodies in animal models and ultimately, if safe and effective, we will test them in humans to see if they can prevent (or at least substantially reduce) HIV infection.

Ivan Stratov and Stephen Kent are conducting their research at the University of Melbourne’s Department of Microbiology and Immunology.