Cannabis suppresses the immune system

A study just published in the European Journal of Immunology resolves how Cannabis sativa (marijuana) suppresses the immune system. The authors state:

"Cannabinoid receptor activation by agents such as Δ9-tetrahydrocannabinol (THC) is known to trigger immune suppression. Here, we show that administration of THC in mice leads to rapid and massive expansion of CD11b+Gr-1+ myeloid-derived suppressor cells (MDSC) expressing functional arginase and exhibiting potent immunosuppressive properties both in vitro and in vivo."

Myeloid-derived suppressor cells (MDSC) are a class of white blood cells that act to regulate the immune system by suppressing immune activity. While their expansion can help to reduce the inflammation and pain of autoimmunity, in cancer they suppress immune system activity in the environment of a tumor. The authors further state:

"Use of selective antagonists SR141716A and SR144528 against cannabinoid receptors 1 and 2, respectively, as well as receptor-deficient mice showed that induction of MDSC was mediated through activation of both cannabinoid receptors 1 and 2. These studies demonstrate that cannabinoid receptor signaling may play a crucial role in immune regulation via the induction of MDSC."

In other words, THC stimulates cannabinoid receptors 1 and 2 which triggered the proliferation of the immune suppressing MDSC. This is also likely to be at least part of the mechanism by which THC can cause an increase in susceptibility to opportunistic infections. Although this may be a rational therapy for organ transplantation where immune suppression is called for, it makes the use of marijuana worrisome for conditions where a deficient immune response needs support such as most cancers.