Differential Expression of Intracellular and Extracellular CB2 Cannabinoid Receptor Protein by Human Peripheral Blood Leukocytes

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Abstract
MRNA encoding for the CB2 cannabinoid receptor is expressed by many subsets of human peripheral blood leukocytes (PBL), but little is known about the resulting protein expression and function. Employing clones from the A549 and 293T cell lines that were constructed to express both full-length human CB2 and GFP, we developed a flow cytometry assay for characterizing CB2 protein expression. A monoclonal antibody directed against human CB2 selectively stained the surface of transduced but not parental cell lines. When cells were fixed and permeabilized, imaging flow cytometry identified large stores of intracellular protein. Total cellular staining for CB2 corresponded closely with the level of GFP expression. When exposed to Δ9-tetrahydrocannabinol, CB2-expressing cells internalized cell surface CB2 receptors in a time- and dose-dependent manner. Applying these approaches to human PBL, CB2 protein was identified on the surface of human B cells but not on T cells or monocytes. In contrast, when PBL were fixed and permeabilized, intracellular CB2 expression was readily detected in all three subsets by both conventional and imaging flow cytometry. Similar to the protein expression pattern observed in fixed and permeabilized PBL, purified B cells, T cells, and monocytes expressed relatively equal levels of CB2 mRNA by quantitative real-time RT-PCR. Our findings confirm that human PBL express CB2 protein but that its distribution is predominantly intracellular with only B cells expressing CB2 protein at the extracellular membrane. The differential role of intracellular and extracellular CB2 receptors in mediating ligand signaling and immune function remains to be determined.

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