EOSINOPHIL GRANULE CATIONIC PROTEINS REGULATE THE CLASSICAL PATHWAY OF COMPLEMENT

Major basic protein, the primary constituent of eosinophil granules, r egulates the alternative and classical pathways of complement. Major b asic protein and other eosinophil granule cationic proteins, which are important in mediating tissue damage in allergic disease, regulate th e alternative pathway by interfering with C3b interaction with factor B to assemble an alternative pathway C3 convertase. In the present stu dy, eosinophil peroxidase, eosinophil cationic protein and eosinophil- derived neurotoxin, as well as major basic protein, were examined for capacity to regulate the classical pathway. Eosinophil peroxidase, eos inophil cationic protein and major basic protein inhibited formation o f cell-bound classical pathway C3 convertase (EACl, 4b, 2a), causing 5 0% inhibition of complement-mediated lysis at about 0.19, 0.75 and 0.5 mu g/10(7) cellular intermediates, respectively. Eosinophil-derived n eurotoxin had no activity on this pathway of complement. The eosinophi l granule proteins were examined for activity on the formation of the membrane attack complex. Major basic protein and eosinophil cationic p rotein had no activity on terminal lysis. In contrast, eosinophil pero xidase inhibited lysis of EACl,4b, 2a, 3b, 5b, but had only minimal ac tivity on later events in complement lysis. These polycations were the n examined to determine the site(s) at which they regulated the early classical pathway. Eosinophil granule polycationic proteins: (1) reduc ed the Z(max) at all time points but had only minimal effect on the T- max during the formation of the classical pathway C3 convertase (EACl, 4b, 2a); (2) inhibited formation of EPCl, 4b, 2a proportional to C4 b ut independent of C2 concentration; (3) inhibited fluid phase formatio n of C1, 4b, 2a, as reflected by a decrease in C1-induced consumption of C2 over time; and (4) inhibited C1 activity over time without a dir ect effect on either C4 or C2. These observations suggest that polycat ions regulate the early classical pathway by interfering with C1 and m ay exert this activity in vivo.