Eosinophil granule major basic protein regulates generation of classical and alternative-amplification pathway C3 convertases in vitro.

Eosinophil major basic protein (MBP), a highly charged polycation, forms the core of the eosinophil granule and mediates tissue damage in allergic disease. Purified MBP was studied for capacity to regulate the generation of classical and alternative-amplification pathway C3 convertases because previous studies have shown that other polycations (protamine, poly-L-lysine) and polyanions (heparin) may play important roles in regulating C activation. MBP inhibited the generation of EAC1,4b,2a and EAC4b,3b,Bb,P but appeared to inhibit the generation of classical pathway convertase more than the alternative amplification pathway convertase at a given dose. Dose-response curves with MBP were steeper than curves seen with polyanion (heparin). MBP did not lyse cellular intermediates at concentrations that caused almost total inhibition of convertase generation. One mechanism of inhibition of convertase generation may have been through an action on C3b, because preincubation of MBP with an EAC4b,3b cellular intermediate interfered with the ability of this cellular intermediate to be lysed. Furthermore, MBP prevented consumption of B in a reaction mixture that contained factors B, D, and C3b, also suggesting an action on C3b. Reduced and alkylated MBP (A-MBP) was compared with native MBP, which possesses two reactive sulfhydryl groups, to determine whether charge alone is responsible for blocking convertase generation; native MBP rapidly associates and is relatively insoluble at neutral and alkaline pH whereas A-MBP remains soluble. A-MBP impaired convertase generation, did not appear to remain bound to cellular intermediates and did not suppress B consumption in the fluid phase assay. This suggests that the ability of MBP to regulate C activation is complex and not entirely through its net charge. Finally, although heparin or MBP alone may prevent C activation, when these substances were present at the same time there was no effect on C activation suggesting that charge neutralization may abrogate the effects of these charged substances on C activation. Taken together, these studies suggest that MBP at physiologic concentrations may regulate in vivo C activation at the tissue level.