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Involvement of proteases in the binding of EAC14OXY23b to complement receptor cells.

Involvement of proteases in complement-dependent rosette formation was tested by using binding of EAC14 oxy 23b to complement receptor cells (CR + C) as a model. The evidence for such a mechanism was 4-fold: a) Interaction between EAC14 oxy 23b and CR + C (Raji cells, PBL, PMNL) could be inhibited by DFP, PMSF, or TLCK; pretreatment of EAC14 oxy 23b by the inhibitors had the same effect. b) EAC43b and EAC143b showed less rosettes than EAC14 oxy 23b, an effect for which oxy C2 could be made directly responsible; the oxy C2a-dependent increase in reactivity could be avoided by DFP. c) Inhibitor-induced reduction and oxy C2a-induced promotion of cell interaction could be correlated with reduction or promotion of the hemolytic activity of EAC14 oxy 23b toward C5. d) Treatment of Raji cells with a monospecific anti-C3 serum resulted in inhibition of the oxy C2-dependent part of the interaction. The enzyme-dependent mechanism started to function only after the protease-carrying cell (EAC14 oxy 23b) and the substrate-carrying cell (e.g., Raji cell) were brought into close enough contact via interaction between C3b and C3-receptors. This suggests that after the initial nonenzymatic contact through interaction of C3 and C3-receptor or possibly other ligand-receptor cooperation, membrane-bound proteases might be enabled to cleave suitable substrates on the adjacent cell and to liberate new binding sites on the substrates. These new binding sites might then intensify or stabilize an otherwise labile cell-cell contact.