Abstract

Δ⁹-Tetrahydrocannabinol (Δ⁹-THC) has been widely established as a modulator of host immune responses. Accordingly, the objective of the present study was to examine the effects of Δ⁹-THC on the immune response within the lungs and associated changes in the morphology of the bronchiolar epithelium after one challenge with a nonlethal dose of the influenza virus A/PR/8 (PR8). C57BL/6 mice were treated by oral gavage with Δ⁹-THC and/or vehicle (corn oil) for 5 consecutive days. On day 3, mice were instilled intranasally with 50 plaque-forming units of PR8 and/or vehicle (saline) 4 h before Δ⁹-THC exposure. Mice were subsequently killed 7 and 10 days postinfection (dpi). Viral hemagglutinin 1 (H1) mRNA levels in the lungs were increased in a dose-dependent manner with Δ⁹-THC treatment. Enumeration of inflammatory cell types in bronchoalveolar lavage fluid showed an attenuation of macrophages and CD4⁺ and CD8⁺ T cells in Δ⁹-THC-treated mice compared with controls. Likewise, the magnitude of inflammation and virus-induced mucous cell metaplasia, as assessed by histopathology, was reduced in Δ⁹-THC-treated mice by 10 dpi. Collectively, these results suggest that Δ⁹-THC treatment increased viral load, as assessed by H1 mRNA levels, through a decrease in recruitment of macrophages and lymphocytes, particularly CD4⁺ and CD8⁺ T cells, to the lung.