"The bottom line is you cannot routinely smoke marijuana without it affecting your immune system," said Thomas Klein, PhD, professor of medical microbiology and immunology at USF. "However, because of the complexity of the immune system, we can't say yet whether the effect we've observed in humans is good or bad."
A study by USF and UCLA is the first to show that healthy humans who smoke marijuana appear to alter the expression of marijuana receptors, or molecules, on immune cells in their blood. The findings were reported in the June issue of the Journal of Neuroimmunology.
Pot's influence on the immune system continues to be hotly debated. While more human studies are needed, overwhelming evidence from animal studies indicates that marijuana and its psychoactive compounds, known as cannabinoids, suppress immune function and inflammation.
"This suggests marijuana or cannabinoids might benefit someone with chronic inflammatory disease, but not someone who has a chronic infectious disease such as HIV infection," said Dr. Klein, lead investigator of the study.
The USF/UCLA group is one of few in the world conducting studies to define the role of cannabinoid receptors in regulating immunity in both drug abusers and nonusers.
If the results in animals hold true in humans, their work might lead to the development of safe and effective cannabinoid drugs for certain diseases, Dr. Klein said. "If the cannabinoids in marijuana are effective immune suppressors, this property might be harnessed to treat patients with overly aggressive immune responses or inflammatory diseases like multiple sclerosis and rheumatoid arthritis."
Receptors that react to delta-9 tetrahydrocannabinol or THC, the compound in marijuana that produces a high, have been found in tissues throughout the body and in the brain. A naturally circulating THC-like substance called anandamide also binds to and activates these marijuana, or cannabinoid, receptors, indicating that the body's own cannabinoid system plays a physiological role in normal immunity as well as defining moods, Dr. Klein said.
In the USF/UCLA study, researchers analyzed blood samples from 56 healthy volunteers -- including 10 chronic marijuana smokers, ages 22 to 46, participating in lung and immune function studies at UCLA. The marijuana smokers denied use of any other drugs, and the nonsmokers denied all illegal drug use. Because no accurate way yet exists to directly study the expression of cannabinoid receptors on immune cells, the researchers looked at the genetic material (messenger RNA) that is the direct predecessor, or precursor, of the receptor.
They found that the baseline genetic expression of precursor RNA was consistent across all age, gender and ethnic groups. But, the peripheral blood cells from the marijuana users expressed significantly higher levels of cannabinoid receptor messenger RNA than blood cells from non-users. The levels increased regardless of the amount of marijuana use, although all users in the study had a history of smoking pot several times or more a week.