Peter J Lachmann, University of Cambridge, SmithKline Beecham Microbial Immunology Laboratory, Centre for Veterinary Science, Cambridge, UK
The term 'alternative pathway of complement activation' is used to describe the activation pathway that is alternative to the so-called 'classical' pathway of activation which was the model of complement activity used to characterize the system. The classical pathway is the reaction sequence by which antibody-sensitized sheep erythrocytes are lysed by human or guinea pig complement. It was already inherent in the original description of the third component of complement, by Coca in 1914, that there must be a separate pathway of complement activation used by yeast cell walls which depleted complement activity without depleting either CI or C2. It was not until the 1950s, however, that Pillemer and his colleagues recognized that there was a novel pathway of complement activation which they described as the 'properdin pathway'. This pathway did not require antibody for its initiation and they postulated it to be a major mediating pathway for nonspecific immunity. They described a new protein - 'properdin' - which they ascribed the central role in this pathway and major importance in resistance to infection and tumors. Towards the end of the 1950s this group also described the existence of other new factors that were needed for the inactivation of C3 by zymosan. Although much of this work later turned out to be essentially correct, it was much contested at the time, largely by RA Nelson (in 1958), and the existence of the alternative pathway was not fully accepted until the discovery of sera genetically deficient in C2 and C4 in the 1960s. It was also at this time that the complement components were first isolated as proteins and as antigens so that the biochemistry of their interactions could be worked out. It was not until the 1970s, however, that the biochemistry and reaction pathway of the alternative pathway was clarified after a period of considerable confusion.
At the heart of the alternative pathway is the C3 feedback cycle (Figure 1). The primary large cleavage product of C3 is C3b, which combines in the presence of magnesium ions with factor B - the C2-like protein of the alternative pathway - to form a bimol-ecular complex, C3b,B. This acts as a substrate for factor D - the CI analog of the alternative pathway -which cleaves a bond in factor B splitting off a fragment, Ba, and leaving the complex C3b,Bb. This is the 'C3'convertase' of the alternative pathway and is the homologous enzyme to C4b,2b; the C3-con-vertase of the classical pathway. Generating a C3 cleaving enzyme from C3b provides a positive feedback amplification loop, generating more C3b and consuming more factor B until one or other of these components is exhausted. Factor D is a single domain serum protease occurring in plasma in active form. It was a great surprise when it was recently discovered, following the cloning of the molecule,
that factor D is identical to a serine protease known as 'adipsin' which occurs in fat tissue and in nerves. The function of adipsin is unknown, but its levels are strikingly reduced in certain genetic forms of obesity and of diabetes.
The C3b feedback cycle is subject to homeostatic control by an analogous reaction cycle where C3b combines with another normal serum protein -factor H - to give a complex C3b,H which is cleaved by another serine protease present in active form in plasma - factor I. Factor I cleaves two bonds in C3b, cleaving out a small fragment - C3f - and leaving the large fragment iC3b which still has important functions as an inflammatory mediator but is no longer capable of combining with factor B or giving rise to a C3 convertase. The rate of C3 activation by the alternative pathway is controlled by the relative velocities of the C3b feedback cycle on the one hand and the C3b breakdown cycle on the other.
The alternative pathway is therefore fired by any mechanism which either increases the rate of C3b production or reduces the rate of C3b breakdown. Mechanisms that increase C3b production include:
1. The generation of extrinsic enzymes that cleave C3. The principal of these is the C3-convertase of the classical complement pathway, C4b,2b. Kallikrein and plasmin also split C3 as does elas-tase and a number of proteases from phagocytic cells or from bacteria. At inflammatory sites a number of these enzymatic systems may be active.
2. Mechanisms that stabilize the alternative pathway C3 convertase. Physiologically this enzyme is stabilized by properdin which binds to it. This is the part that properdin plays in the alternative pathway - not at all what the Pillemer group had envisaged. Although properdin levels have not been found of great diagnostic value, the total absence of properdin leads to (usually severe) immunodeficiency. A more complete stabilization of C3b,Bb is produced by autoantibodies that react with it. Such autoantibodies are called 'nephritic factors' since they were first described in mesangiocapillary glomerulonephritis. However, the renal disease follows the formation of the nephritic factors rather than causing it and the mechanisms underlying the formation of the nephritic factor remain unknown.
An artefactual way of producing a stable C3-con-vertase is the use of cobra venom factor. This is a reptilian analog of a C3 breakdown product that functionally resembles C3b and has the curious property that it combines with mammalian factor B but is not dissociated by mammalian factor H and therefore produces a stable C3-convertase not susceptible to homeostatic control which causes the C3b feedback cycle to run to exhaustion.
The principal mechanism that decreases the rate of C3b breakdown is the 'protected surface' phenomenon; C3b bound on certain surfaces is relatively poorly reactive with factor H and therefore resistant to breakdown whilst supporting the C3b feedback cycle by reaction with factor B. Such protected surfaces include particulate polysaccharides (for example zymosan and particulate inulin); the surfaces of most parasites and of many virus-infected and some tumor cells; and certain immunoglobulin aggregates particularly those of IgA. More dramatic failure of C3b breakdown occurs in the genetic deficiencies of factor I or of factor H. A further mechanism of reducing C3b breakdown is provided by local removal of factor H. This was first described for 'sulfated sephadex' and has been seen also with an intestinal glycoprotein and a myeloma light chain dimer. The physiological or pathophysiological relevance of this mechanism is still unclear.
The view that the alternative pathway is essentially governed by the velocity of competing reactions requires that there is a minimal level of C3b (or of a C3b-like protein) always available. This is the essential feature of the 'tickover' model of the alternative pathway. It was originally proposed that the tickover was maintained by minimal levels of proteolysis of C3 by any of the enzymes that can cleave it and indeed this is likely to be the case. However, another mechanism has been suggested: namely that the hydrolysis by water of the internal thioester bond generates a C3b-like molecule and that this spontaneous hydrolysis of C3 may be sufficient to keep the tickover working. The levels of C3b or C3b-like proteins that are required for the maintenance of the tickover are so low that it has not so far been possible to define critically the relative roles of proteolysis and thioester hydrolysis in maintaining the tickover in vivo.
See also: Immunoconglutinins; Complement, alternative pathway; Clotting system; Cobra venom factor; Complement, classical pathway; Complement deficiencies; Complement, genetics; Complement receptors.
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