Purified components of guinea-pig complement; reaction of erythrocytes with antibody and the first four components

The amplification loop of the complement pathways

The C3 amplification loop lies at the core of all the complement pathways, rather than the alternative pathway alone. It is, in evolutionary terms, the oldest part of the complement system and its antecedents can be seen in insects and in echinoderms. The amplification loop is the balance between two competing cycles both acting on C3b: the C3 feedback cycle which enhances amplification and the C3 breakdown cycle which downregulates it. It is solely the balance between their rates of reaction on which amplification depends. The C3 breakdown cycle generates iC3b as its primary reaction product. iC3b, through its reaction with the leukocyte integrins (and complement receptors) CR3 (CD11b/CD18) and CR4 (CD11c/CD18), is the most important mechanism by which complement mediates inflammation. A variety of genetic polymorphisms in components of the amplification loop have been shown to predispose to two kidney diseases-dense deposit disease and atypical haemolytic uraemic syndrome-and to age-related macular degeneration. All predisposing alleles enhance amplification, whereas protective alleles downregulate amplification. This leads to the conclusion that there is a "hyperinflammatory complement phenotype" determined by these polymorphisms. This hyperinflammatory phenotype protects against bacterial infections in early life but in later life is associated with immunopathology. Besides the diseases already mentioned, there is evidence that this hyperinflammatory complement phenotype may predispose to accelerated atherosclerosis and also shows an association with Alzheimer's disease. Downregulation of the amplification loop therefore constitutes an important therapeutic target.

Interactions of the complement system with native and chemically modified endotoxins

Endotoxic lipopolysaccharides (LPS) isolated from Serratia marcescens, Veillonella alcalescens, and Salmonella typhosa were potent in their ability to induce fixation of complement (C') in normal guinea pig, rabbit, mouse, and human serum. The C'-fixing ability of LPS was pronounced even when assays were performed in undiluted serum, and was lost after each of four chemical modifications which resulted in loss of biological toxicities. The detoxification procedures had in common the cleavage of ester-bound, long-chain carboxylic acids. The ability of biologically active LPS to fix C' in normal guinea pig serum was reflected chiefly in dramatic uptake of classical C'3 (C'3t); fixation of C'1, C'4, and C'2 was virtually undetectable. Hence, it was the capacity for fixation of C'3t which was lost most overtly during detoxification. Addition of immune serum to the assay mixtures resulted in detectable fixation of C'1 and C'4. Biologically active LPS also fixed more of these components than did detoxified LPS. Immune serum restored the ability of detoxified LPS to fix C'3t, but whether this is by the original pathway is not yet clear. We concluded that the loss of certain biological activities and the loss of ability to fix C'3t in normal serum after LPS detoxification involved loss or rearrangement of substrates on LPS which either initiated or supported, or both, its interaction with the complement system. It was apparent that the ability to fix C' can serve as a valuable in vitro indicator of the integrity of the toxic conformation of biologically active LPS membrane fragments. These experiments supported the hypothesis that certain of the biological activities induced by endotoxins are mediated via the complement system.