MECHANISMS OF HUMAN EOSINOPHIL ACTIVATION BY COMPLEMENT PROTEIN C5A AND PLATELET-ACTIVATING-FACTOR - SIMILAR FUNCTIONAL-RESPONSES ARE ACCOMPANIED BY DIFFERENT MORPHOLOGIC ALTERATIONS

The complement system is an important amplification system for the pro pagation of allergic as well as pseudoallergic inflammatory reactions. In the present study, the effect of the major anaphylatoxin C5a was c ompared with that of platelet-activating factor (PAF) on highly purifi ed eosinophils (greater than or equal to 95%) by functional as well as morphologic criteria. Upon stimulation with C5a, eosinophils maintain ed their spheric structure, developing short, pseudopodia-like protrus ions, whereas PAF induced the generation of a number of digitating pro trusions. As shown by functional and ultrastructural assay systems, bo th stimuli provoked significant extracellular and intracellular H2O2 p roduction in eosinophils, which was inhibited by cytochalasin B. With C5a, a pronounced H2O2 production was detected within the small cytopl asmic vesicles, whereas PAF-induced H2O2 production was observed on th e outer surface of the plasma membrane in the contact zones between ad jacent cells. Morphologic signs of degranulation induced by C5a and PA F were accompanied by the significantly increased release of eosinophi l cationic protein and eosinophil peroxidase in the presence of cytoch alasin B. Like PAF, C5a induced a significant production of reactive o xygen species in eosinophils, as measured by lucigenin-dependent chemi luminescence (CL) responses in eosinophils. Maximal responses, compara ble with those of interleukin-5 (100 U/ml), were observed at concentra tions of 10(-5)-10(-6) and 10(-7)-10(-8) M for PAF and C5a, respective ly. Separation of eosinophils by discontinuous density gradients revea led the existence of two hypodense eosinophil populations, one of them showing significantly reduced CL responses upon stimulation with C5a and PAF. In addition, CL responses upon stimulation with C5a and PAF w ere abrogated by cytochalasin B, staurosporine, and wortmannin, and we re almost completely blocked by pertussis toxin. In conclusion, these data indicate that C5a induces events in human eosinophils comparable to those induced by PAF in the assay systems tested. Thus, C5a, genera ted after activation of the complement system, may be of major importa nce for the eosinophil activation observed in eosinophil-related disea se.