Marijuana is one of the most abused drugs due to its psychotropic effects. Interestingly, it is also used for medicinal purposes. The main psychotropic component in marijuana, Δ9-tetrahydrocannabinol (THC), has also been shown to mediate potent anti-inflammatory properties. Whether the immunomodulatory activity of THC is mediated by epigenetic regulation has not been investigated previously. In this study, we employed ChIP-Seq technology to examine the in vivo effect of THC on global histone methylation in lymph node cells of mice immunized with a superantigen, staphylococcal enterotoxin B (SEB). We compared genome-wide histone H3K4, H3K27, H3K9, H3K36 trimethylation and H3K9 acetylation patterns in such cells exposed to THC or vehicle. Our results showed that THC treatment leads to the association of active histone modification signals to Th2 cytokine genes and suppressive modification signals to Th1 cytokine genes, indicating that such a mechanism may play a critical role in THC-mediated switch from Th1 to Th2. At the global level, a significant portion of histone methylation and acetylation regions were altered by THC.
However, the overall distribution of these histone methylation signals among the genomic features were not altered significantly by THC, suggesting that THC activates the expression of a subset of genes while suppressing the expression of another subset of genes through histone modification. Functional classification of these histone marker associated genes showed that these differentially associated genes were involved in various cellular functions, from cell cycle regulation to metabolism, suggesting that THC had a pleiotropic effect on gene expression in immune cells. Together, the current study demonstrates for the first time that THC may modulate immune response through epigenetic regulation involving histone modifications.