Eosinophil granule proteins inhibit substance P-induced histamine release from human skin mast cells

We have investigated the activity of the four principal cationic proteins of the eosinophil granules, major basic protein (MBP), eosinophil peroxidase (EPO), eosinophil-derived neurotoxin, and eosinophil cationic protein on histamine release from human skin mast cells. These four cationic proteins, over the concentration range of 10 to 200 micrograms/ml, did not induce significant histamine release, nor did they prime anti-IgE-induced histamine release from human skin mast cells significantly. However, a brief incubation (15 minutes) of two of the four principal eosinophil granule proteins, MBP and EPO, at concentrations of 50 to 200 micrograms/ml, caused a significant concentration-related inhibition of histamine release induced by 30 mumol/L substance P. The concentrations producing 50% inhibition for MBP and EPO on substance P-induced histamine release were 30 micrograms/ml and 100 micrograms/ml, respectively. This inhibitory effect appears to be a direct effect of these proteins on skin mast cells because purified (78% to 85%) skin mast cells displayed a similar response to MBP and EPO (n = 4). Also, when skin mast cells were incubated with 100 micrograms/ml MBP and EPO for 15 minutes and washed twice before activation by substance P, the inhibitory effect was not altered. These two proteins also inhibited histamine release induced by 10 micrograms/ml compound 48/80. These results suggest that MBP and EPO affect the same binding site(s) on skin mast cells as those of substance.