Cannabinoid Receptor 2 (CB2) Signals via G-alpha-s and Induces IL-6 and IL-10 Cytokine Secretion in Primary Human Leukocytes

Cannabinoid receptor 2 (CB2) is a promising therapeutic target for immunological modulation. There is, however, a deficit of knowledge regarding CB2 signaling and function in primary human immunocompetent cells. We applied an experimental paradigm which closely models the in situ state of primary human leukocytes (PBMC; peripheral blood mononuclear cells) to characterize activation of a number of signaling pathways in response to a CB2-selective ligand (HU308). Application of G protein inhibitors revealed that an apparent "lag" phase of unchanged cAMP concentration prior to development of classically-expected Gαi-mediated inhibition of cAMP synthesis was a result of counteraction by concurrent Gαs activation, a phenotype not previously observed for CB2. Monitoring downstream signaling events, p38 was mediated by Gαi whereas ERK1/2 and Akt phosphorylation were mediated by Gαi-coupled βγ. Activation of CREB integrated multiple components; Gαs and βγ mediated ~85% of the response, while ~15% was attributed to Gαi. Responses to HU308 had an important functional outcome - secretion of interleukins 6 (IL-6) and 10 (IL-10). IL-2, IL-4, IL-12, IL-13, IL-17A, MIP-1α, and TNF-α were unaffected. IL-6/IL-10 induction had a similar G protein coupling profile to CREB activation. All response potencies were consistent with that expected for HU308 acting via CB2. Additionally, signaling and functional effects were completely blocked by a CB2-selective inverse agonist, giving additional evidence for CB2 involvement. This work expands the current paradigm regarding cannabinoid immunomodulation and reinforces the potential utility of CB2 ligands as immunomodulatory therapeutics.