HIV Vaccine Primer: Basic Science and Concepts

A DNA vaccine is a vaccine that uses the genetic code of a pathogen to invoke an immune response. With a DNA vaccine, trial participants cannot become infected because only some of the genes are used, but never the whole virus. DNA vaccines are similar to subunit vaccines, except instead of using purified antigens, they use circles of DNA called plasmids, which produce the antigens that train the immune system to recognize the targeted pathogen. This approach has shown promising results, and trials are ongoing.

Perhaps the most common vaccine approach used in HIV research is using a recombinant vector vaccine. This approach uses genetic material from a pathogen and delivers that material into the body by placing it onto a "vector." A vector is the scientific name for any weakened bacterium or virus that does not normally cause disease in humans. A common vector being used in HIV vaccine candidates is weakened adenovirus -- a virus that, when not weakened, can cause colds and sore throats. Many ongoing trials use recombinant vector vaccine candidates to "trick" the body into thinking that it has been exposed to live HIV even though it has not, and thus to trigger a protective immune response.

Vaccine trials often use a prime-boost approach to induce immunity. This means administering a first vaccine dose (the "prime") to trigger an initial set of immune responses. The prime may be coupled with an adjuvant, which is a substance used to improve the body's ability to fight disease or infection. Then a second type of vaccine (the "booster") is administered to further amplify the immune response. Finding the right combinations and order of vaccine doses can lead to better immune responses.

Types of Immune Responses

A successful HIV vaccine will elicit two main immune responses that, in conjunction, can ward off the virus.

The first type of response is humoral immunity, meaning protection provided by antibodies. An antibody is an infection-fighting protein molecule made naturally by our immune system; it can tag, destroy or neutralize specific bacteria, viruses or other harmful toxins. Researchers are trying to improve the process by which antibodies identify an HIV-infected cell as something abnormal that needs to be killed. This process activates our body's natural killer (NK) cells, which then kill the infected cell.

Because of HIV's ability to mutate, researchers are looking for ways to induce "broadly neutralizing" antibodies, which are antibodies that can identify many different strains and mutations of HIV. This would be the ideal form of humoral immunity to HIV.

The second type of immune response is cellular immunity, meaning protection provided by the cells of the body's immune system. These cells include killer T cells (CD8) and helper T cells (CD4), among other important types of cells.

CD8 cells kill foreign cells marked for destruction. They can destroy cancer cells and cells infected with viruses or bacteria. CD4 cells are a group of T cells that help produce antibodies, activate CD8 cells and make sure the immune system is running smoothly.

In addition to using CD4 and CD8 cells to respond to pathogens, the immune system also gathers an army of memory B cells and memory T cells that can quickly detect and neutralize the invader should it ever try to re-enter the body.

With an effective vaccine, the immune system would prepare all these components to work together in preventing HIV infection.

RV144: First Proof That an HIV Vaccine Can Work

In 2009, RV144, sometimes referred to as the "Thai prime-boost AIDS vaccine trial," was the first HIV vaccine study conducted in humans that showed some level of protection against HIV. More than 16,000 Thai men and women volunteered for the study, in which they took a prime-boost vaccine regimen consisting of an ALVAC vector followed by an AIDSVAX boost. The vaccine candidate was shown to reduce HIV infection by 31 percent overall. It wasn't enough to truly call the vaccine a success, but after many years of research in which candidates showed no protection at all, it was a watershed moment in HIV vaccine research.

Soon after the results were made public, researchers began setting up tests to identify what specific aspects of the RV144 vaccine made people more or less likely to be protected against HIV. In the analysis, researchers were able to identify a particular antibody that may be associated with vaccine-induced immunity. Further research is going on to better understand the response so that the vaccine can be improved. Some of the volunteers from RV144 have begun to be "re-boosted" to see if their immune responses to HIV can be improved. Meanwhile, three follow-up trials are planned.

HVTN 505 and Other Vaccine Trials

On April 25, HVTN 505, a large phase-2b HIV vaccine trial, was halted by the National Institute of Allergy and Infectious Diseases (NIAID) after it determined there was a lack of efficacy for the vaccine regimen being studied. At the time, HVTN 505 was the furthest along in terms of clinical progress and had 2,504 participants across 19 U.S. cities. The trial had used a prime-boost approach similar to RV144, administering a DNA-based vaccine as the prime and a recombinant vaccine as the boost.

The trial halting was certainly disappointing news, however, as Mitchell Warren, executive director of AVAC (AIDS Vaccine Advocacy Coalition) pointed out, "This trial has provided a clear, swift answer about a specific vaccine strategy. It's not the answer we hoped for, but the search doesn't end here. There are other approaches that must be pursued without delay, and this result will help to focus and guide research efforts."

While no new vaccinations will be given in HVTN 505, the researchers will continue to follow and collect data on the study participants. Moreover, there are still more than 30 ongoing HIV vaccine trials around the world, according to AVAC. They are still in early phases, but research continues.

Overall, we may still be years from developing an HIV vaccine that is considered successful enough to be used in the general public, but after a long period of time in which many started to believe HIV vaccine research was a dead end, hope now burns as brightly as ever that the "holy grail" of HIV prevention can still be found.

Warren Tong is the research editor for TheBody.com and TheBodyPRO.com.

Follow Warren on Twitter: @WarrenAtTheBody.