(Inside Science) -- Since the HIV epidemic began in the 1980s, scientists have been exploring the idea that cocaine and other stimulants could increase the chance of infection and assist the progression of HIV into full-blown AIDS.
New research on unusual mice with implanted human immune systems adds weight to previous work, suggesting a link between cocaine and HIV is legitimate.
Earlier studies measured the rate of HIV infection in human cells exposed to cocaine compared to cells not under the influence of the stimulant. Those findings favored the idea that cocaine increases the risk of HIV infection. So have other experiments that gave infected laboratory mice cocaine.
Despite a fairly substantial body of research, not everyone is convinced. "Surprisingly, some people still seem to discount it," said Dimitrios Vatakis, an assistant researcher at UCLA's AIDS Institute.
Results from animal experiments can be easily questioned by pointing out the vast differences between humans and mice, while Petri dish research with cells -- also known as "in-vitro" experiments -- doesn't always translate to the real world, as it overlooks the many variables that exist beyond the lab.
Vatakis recently published a paper in the journal Scientific Reports that helps bridge the gaps between animal and Petri dish studies. "Our results support the little bits and pieces from all the epidemiological, in vitro and animal studies that have come before," he said.
He contaminated 38 mice with HIV and gave half of them cocaine and the other half salt water. Unlike in previous research, Vatakis used what is known as "humanized mice," rodents transplanted with human blood cells and liver and thymus tissues.
Such mice have, according to Vitakis, something close to a functioning human immune system. This experimental model has been deployed before to study treatments that use the body's immune system to fight cancer, but this is the first time it has been used to examine HIV infection and drug use.
Two weeks after injecting HIV into each group of 19 mice, the UCLA team took blood and tissue samples to gauge infection. Nine in the saline group had no detectable trace of HIV compared to just three in the cocaine group. Despite the low number of mice used in the study, the researchers found the data to be statistically significant. Additionally, the HIV-positive mice that were given cocaine had a higher concentration of the virus than the saline-administered HIV-positive mice.
Cocaine appears to activate a type of white blood cell that plays an important role in the body's immune response. But this activation makes the cells less resistant to HIV. Whether this effect is observed with other diseases hasn't been studied in depth, Vatakis says, but some research has been done with HPV and found a similar outcome.
Vatakis said most people attribute the higher rates of HIV among drug users to dirty needles and risky behavior associated with drug use such as unprotected sex. The skepticism isn't manifested in research papers or articles, said Vatakis, but it's something he’ll hear from time to time in feedback when he's applying for grant money or trying to get his research published in a journal.
"People have an attitude that drug users are engaging in risky behavior so that's what makes them get HIV -- end of story," explained Vatakis. "But that's not the whole story."
While behavioral factors undoubtedly have a significant impact on HIV risk, it doesn't mean that a potential biological link should be ignored, he argued. It's an issue of raising awareness in the wider scientific community.
"The data from the study shows that cocaine use means a higher chance of HIV independent of risky behavior," said Adam Carrico, an assistant professor in the department of community health systems at the University of California, San Francisco. He was not involved with the research. In 2010, Carrico published a review paper of previous epidemiology studies that compared the pace of HIV progression between stimulant users and non-users. He says the effect is profound.
"We find that frequent stimulant use is associated with a 50 percent higher risk of HIV progression into AIDS or dying," said Carrico. It is, however, hard to control for risky behavior in these population studies, he added, making it difficult to know how much of the risk should be attributed to any direct influence of the stimulant itself.
Vatakis said that replications of his experiment are needed to put a definitive percentage on the increased risk. But he added that it's important to study the link between stimulants and HIV because the effect is large enough to matter in public health.
Drug addiction is an extremely difficult disease to tackle. The most effective way to halt cocaine's impact on HIV would be to abstain; however that's not always possible. But one day there could be blockers for cocaine users to take alongside their HIV medication to prevent activation of white blood cell proteins. "That would be amazing," said Carrico, "it's about bridging the gap between clinical and basic science."
Editors’ note (July 9, 2015) : After publication, we have made multiple changes to clarify language used in the story.