Inhibition of complement-derived enzymes

Complement activation in pneumonia

Complement analyses performed on serially collected plasma samples from 10 patients during acute infectious pneumonia or bronchopneumonia showed normal or increased values for hemolytic activity and Clq, C4, C3, and factor B levels. However, the levels of C3 breakdown products (C3d) were significantly (greater than 2 SD) increased in six patients during the first three days of observation, suggesting that hypercatabolism of complement components may occur during the acute phase of infectious pneumonia concomitant with a hypersynthesis of some complement components. Evidence of circulating immune complexes was obtained only in two patients with the 125I-Clq Binding Test.

The natural modulation of the amplification phase of complement activation

As C3 cleavage represents the most critical step in the elaboration of the biologic effects of the complement system, the modulation of this reaction by formation and function of the C3b-dependent C3 convertase may well determine whether the initial activation of the complement sequence eventuates in beneficial or detrimental effects to the host. Stabilization of the amplification C3b-dependent convertase, C3bBb, is achieved with P and C3NeF, respectively, after their binding which exhibits different molecular and temperature requirements. Control of this amplifying step occurs at three levels: intrinsic decay of the inherently labile C3bBb convertase; extrinsic decay by displacement of Bb from the convertase with beta1H; and inactivation by C3bINA of C3b after its generation from native C3 or removal of protective Bb by intrinsic or extrinsic decay. In the presence of the stabilizing factors the control proteins must function in sequence with beta1H-mediated decay preceding C3b inactivation.

Alternative pathway of complement: recruitment of precursor properdin by the labile C3/C5 convertase and the potentiation of the pathway

In this study the physiological role of properdin and the differential subunit composition of the solid phase enzymes of the pathway have been explored. Cell-bound C3 and C5 convertase differ in their C3b requirement. Apparently one molecule of C3b is sufficient to allow formation of C3 convertase (C3b,B), whereas two or more are required for generation of C5 convertase (C3bn,B). This conclusion was drawn from results indicating the critical role of the spacial distribution of C3b molecules on the cell surface in enzyme formation. While the C3/C5 convertase is fully capable of acting on C5 and thereby initiating the assembly of the cytolytic membrane attack complex, it is exceedingly labile and vulnerable to destruction by the C3b inactivator. It is the apparent role of properdin to confer a degree of stability upon the labile enzyme and to protect its C3 convertase function against enzymatic destruction. To achieve these effects, precursor properdin (pre-P) is recruited in a binding-activation reaction by the labile C3/C5 convertase. Multiple C3b molecules appear to be needed for the formation of properdin-activating principle. Three modes of regulation have been described, which involve spontaneous dissociation enzymatic degradation by C3b inactivator and disassembly by beta1H. The functional differences of pre-P and activated properdin (P) were delineated, pre-P displaying a weak affinity for C3b and P the capacity of strong interaction, P generating a soluble C3 convertase in serum and pre-P being unable to do so. Because of the profound differences between native pre-P and the laboratory product P, the question was raised as to whether soluble P represents an unphysiological form of the protein. On the basis of this and other studies, the conclusion was reached that in vitro properdin recruitment constitutes the terminal event of the properdin pathway, and that properdin augments the function of C3/C5 convertase without changing its substrate specificity.