Description
The alternative complement pathway provides innate protection against microbial agents in the absence of specific antibody.1-5 The activation of this complement pathway can be triggered by a variety of substances including microbial polysaccharides or lipids, gram-negative bacterial lipopolysaccharides, and surface determinants present on some viruses, parasites, virally infected mammalian cells, and cancer cells. In autoimmune diseases, the alternative complement pathway may contribute directly to tissue damage.
A centrally important reaction that occurs during alternative pathway activation is the conversion of the 93 Kd molecular weight Factor B zymogen to an active proteolytic enzyme. This is accomplished in a two-step reaction. During the first reaction step the Factor B forms a magnesium-dependent complex with C3(H20) or C3b.4 The C3(H20),B complex is formed only in fluid-phase while the C3b,B complex can be formed either in fluid-phase or on a target surface.1-4 Factor B, which is present in the C3(H20),B or the C3b,B complex, is cleaved into the Ba (33 Kd) and Bb (60 Kd) fragments in the second reaction step by the alternative pathway
enzyme, Factor D.1-4 The resulting C3b,Bb bimolecular complex is the C3 convertase enzyme of the alternative pathway. The Bb subunit is the catalytically active site of the complex that is capable of cleaving C3 to C3a and C3b fragments.1-4,6 The additional C3b fragments produced in this manner may form the C3b,Bb,C3b rimolecular complex that is the C5 convertase enzyme of the alternative pathway. This C5 convertase is capable of cleaving C5 to C5a and C5b fragments.1-4,6
The C3 and C5 convertases of the alternative pathway can be stabilized by Factor P (also called Properdin), a component of the alternative pathway normally present in human plasma or serum,1-4 or by C3 nephritic factor, an autoantibody produced in some patients experiencing extensive alternative pathway activation.5 The C3 and C5 convertases of the alternative pathway can be dissociated, and thereby inactivated, by spontaneous decay dissociation,7 or by the binding of Factor H or Complement Receptor 1 (CR1 ).4,8 The Bb fragment that is dissociated from either convertase retains some biological activities, e.g., retention of functional hemolytic activity,4,9 the ability to induce macrophage- spreading,10 and plasminogen activation.11
Although alternative pathway activation is thought to occur primarily in the absence of specific antibody, many situations arise in which alternative pathway activation can occur as the result of classical pathway activation. For example, immune complexes that are present in autoimmune disease patients can trigger classical complement pathway activation with resultant production of C3b fragments. As described above, these C3b molecules are capable of binding Factor B and initiating its cleavage into the Ba and Bb fragments. Thus, alternative pathway activation can occur in antibody-mediated autoimmune disease states and may contribute
significantly to enhanced complement activation and concomitant tissue destruction.
By assessing Factor B cleavage products in test specimens, one can estimate the extent of alternative pathway utilization occurring at the time of sample collection in the disease state under investigation. The Bb Plus EIA provides a simple, rapid, non-radioactive, highly specific, and quantitative procedure for measuring Factor B activation. It is ideal for investigations involving the role or status of the alternative complement pathway in numerous research and clinical settings, and for monitoring the generation of Bb in vitro