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The progress toward developing an HIV vaccine may be hindered by the results of a new study by UCLA researchers, which found differences in the way identical twins’ immune systems responded to the virus.

The study, led by Dr. Otto Yang, an associate professor in the departments of medicine and microbiology, immunology and molecular genetics, tracked a pair of male twins first infected with HIV via blood transfusions from the same donor just after they were born in 1983.

Researchers have only been studying the twins since 2000, but did research on the twins’ prior histories, including a study of blood samples from 1995. Both of the twins have very similar medical histories, from taking most of the same medications to treat HIV and having similar illnesses and HIV complications at about the same times throughout their lives.

However, when it comes to their bodies’ reactions to HIV, the similarities end.

To protect the body from viruses, cells in the immune system have receptors on their surfaces which bind to proteins on the viruses. This binding keeps the viruses from damaging the body. HIV harms the body by avoiding the immune system’s attacks through mutations that change the shape of its proteins so that they cannot fit into the receptors of the cellular immune system.

“If the virus mutates the protein being bound, then that receptor becomes useless in clearing the virus ... The reason that’s important is there’s a degree of randomness in the way the immune system reacts which we can’t control,” Yang said.

The immune system responds through the generation of randomly shaped receptors, which can bind to various proteins of different viruses. Therein lies the difference between the twins, whose immune systems generated different receptors in response to HIV. Researchers said this difference between identical twins is an indication of how difficult it will be to create a vaccine to accommodate the masses of infected people across the world.

“The implication of our study for vaccine research (is that it) helps show our inability to predict the immune responses that people will have in attempting to control HIV growth within the body,” said Paul Krogstad, professor of pediatrics and pharmacology and another researcher for the study.

“Identical twins (are) as identical as two people can be. These data help demonstrate even identical twins are, in more than one way, obviously distinct human beings,” he said.

Both Krogstad and Yang said this study is just one part of a larger study on how HIV affects patients and does not yet indicate the end of the search for a vaccine.

In the study, researchers said, “Sterilizing immunity against HIV is considered an unlikely, and possibly unattainable, goal for vaccine development, but protection against disease may be possible if persistently effective immune responses can be generated.”

Yang said there have not been any significant steps taken toward developing a vaccine, so this should not be viewed as a step backwards.

“We need to be careful about making conclusions abroad about one pair of twins ... The twins are a good example of something that theoretically would be predicted by theories about how the immune system works,” he said.

Krogstad said the fact that the twins are even alive today is a sign of hope for the future of HIV research.

“(That they are alive) by itself is remarkable and something that wouldn’t be expected 15 years ago. Their survival into adulthood is a testament in advances in our ability to monitor and treat HIV infections both in adults and children,” he said.

Krogstad also said there is no reason to be pessimistic about the future.

“I see progress. I see hope in seeing so many kids who were infected making it into adulthood, many of them in good health, many with the opportunity to look forward to long life.”