Omalizumab reverses the phenotypic and functional effects of IgE-enhanced Fc epsilonRI on human skin mast cells.

The dramatic effects of the anti-IgE mAb omalizumab to lower free IgE levels and Fc epsilonRI levels on basophils contrast with more modest clinical effects. Accordingly, whether IgE modulates Fc epsilonRI levels and Fc epsilonRI-dependent mediator release in vitro on human skin mast cells (MC(TC) type) that had matured in vivo is of interest. IgE reversibly enhanced Fc epsilonRI levels on MC(TC) cells in a dose- and time-dependent manner (up-regulation t(1/2) of 4-5 days with 1-3 microg/ml IgE), without affecting cell proliferation. A molar ratio of omalizumab to IgE of 0.9 at baseline prevented receptor up-regulation by 50%, whereas adding omalizumab to MC(TC) cells already with IgE-enhanced Fc epsilonRI levels at molar ratios of 5, 12.5, and 31 reduced Fc epsilonRI levels to baseline with respective t(1/2) values of 8.7, 6.3, and 4.8 days. MC(TC) cells with IgE-enhanced Fc epsilonRI levels were more sensitive to stimulation with a low dose of anti-Fc epsilonRI mAb in terms of degranulation and production of PGD(2), GM-CSF, IL-6, IL-13, and TNF-alpha. Reducing up-regulated Fc epsilonRI levels with omalizumab also reduced mediator release to a low dose of anti-Fc epsilonRI mAb to baseline by 3-4 wk. Thus, reducing free IgE should decrease the hypersensitivity of allergic individuals to low naturally occurring concentrations of allergens.

Role of cannabinoid receptors in inhibiting macrophage costimulatory activity.

Delta(9)-tetrahydrocannabinol (THC) inhibits several immunologic functions of macrophages. THC's impact on peritoneal macrophages to deliver costimulatory signals to a helper T cell hybridoma was investigated by T cell interleukin-2 production stimulated with immobilized anti-CD3 antibody. The drug's inhibition of costimulatory activity depended on the macrophages. THC decreased costimulation provided by peritoneal cells elicited with polystyrene beads and thioglycollate, but the drug had no influence with macrophages elicited with thioglycollate alone. Bead administration induced CB2 mRNA expression in macrophages, while CB1 mRNA was not detected. Although inhibition was associated with functional heat-stable antigen, a costimulatory molecule, on macrophages, THC exposure did not alter cell surface heat-stable antigen expression. Inhibition by THC and anti-heat-stable antigen antibody was not additive suggesting the inhibitory mechanisms may overlap. Cannabinoid suppression was stereoselective; low affinity synthetic isomer CP56,667 did not diminish the T cell response. CB1-selective antagonist SR141716A completely reversed, and CB2-selective antagonist SR144528 partially blocked THC's inhibition. Both antagonists appeared to behave as inverse agonists in a receptor-selective manner. Although T cells expressed a low level of CB2 mRNA, neither THC nor SR141716A affected T cell activation in a system independent of macrophages, while SR144528 was inhibitory. High affinity synthetic agonist CP55,940, but not partial agonist THC, impaired costimulation by macrophages from mice lacking CB2 receptor. Although CB1 mRNA was not detected in CB2 null macrophages, CP55,940 reversed the inverse agonist activity of SR141716A. Hence, CB2 and possibly another receptor subtype may be involved in mediating cannabinoid suppression of macrophage costimulation.