SUPPRESSION OF THE HUMORAL IMMUNE-RESPONSE BY CANNABINOIDS IS PARTIALLY MEDIATED THROUGH INHIBITION OF ADENYLATE-CYCLASE BY A PERTUSSIS-TOXIN-SENSITIVE G-PROTEIN COUPLED MECHANISM

Cannabinoid compounds, including the major psychoactive component of m arihuana, Delta(9)-tetrahydrocannabinol (Delta(9)-THC), have been wide ly established as being inhibitory on a broad array of humoral and cel l-mediated immune responses. The presence of cannabinoid receptors has been identified recently on mouse spleen cells, which possess structu ral and functional characteristics similar to those of the G-protein c oupled cannabinoid receptor originally identified in rat brain. These findings, together with those demonstrating that Delta(9)-THC inhibits adenylate cyclase in splenocytes, strongly suggest that certain aspec ts of immune inhibition by cannabinoids may be mediated through a cann abinoid receptor-associated mechanism. The objective of the present st udies was to determine whether inhibition of adenylate cyclase is rele vant to mouse spleen cell immune function and, if so, whether this inh ibition is mediated through a G(i)-protein coupled mechanism as previo usly described in neuronal tissue. Spleen cell activation by the phorb ol ester phorbol-12-myristate-13-acetate (PMA), plus the calcium ionop hore ionomycin, produced a rapid but transient increase in cytosolic c AMP, which was inhibited completely by immunosuppressive concentration s of Delta(9)-THC (22 mu M) and the synthetic bicyclic cannabinoid CP- 55940 (5.2 mu M), which produced no effect an cell viability. Inhibiti on by cannabinoids of lymphocyte proliferative responses to PMA plus i onomycin and the sheep erythrocyte (sRBC) IgM antibody-forming cell (A FC) response, was abrogated completely by low concentrations of dibuty ryl-cAMP (10-100 mu M). Inhibition of the sRBC AFC response by both De lta(9)-THC (22 mu M) and CP-55940 (5.2 mu M) was also abrogated by pre incubation of splenocytes for 24 hr with pertussis toxin (0.1-100 ng/m L). Pertussis toxin pretreatment of spleen cells was also found to dir ectly abrogate cannabinoid inhibition of adenylate cyclase, as measure d by forskolin-stimulated accumulation of intracellular cAMP. These re sults indicate that inhibition of the sRBC AFC response by cannabinoid s is mediated, at least in part, by inhibition of adenylate cyclase th rough a pertussis toxin-sensitive G(i)-protein coupled cannabinoid rec eptor. Additionally, these studies further support the premise that cA MP is an important mediator of lymphocyte activation.