Anaphylatoxin in its relation to the complement system

The anaphylatoxin C5a: Structure, function, signaling, physiology, disease, and therapeutics

The complement system is one of the oldest known tightly regulated host defense systems evolved for efficiently functioning cell-based immune systems and antibodies. Essentially, the complement system acts as a pivot between the innate and adaptive arms of the immune system. The complement system collectively represents a cocktail of ∼50 cell-bound/soluble glycoproteins directly involved in controlling infection and inflammation. Activation of the complement cascade generates complement fragments like C3a, C4a, and C5a as anaphylatoxins. C5a is the most potent proinflammatory anaphylatoxin, which is involved in inflammatory signaling in a myriad of tissues. This review provides a comprehensive overview of human C5a in the context of its structure and signaling under several pathophysiological conditions, including the current and future therapeutic applications targeting C5a.

C4a: An Anaphylatoxin in Name Only

Activation of complement leads to generation of the 3 anaphylatoxins C3a, C4a, and C5a. Although all 3 peptides are structurally similar, only C3a and C5a share a similar functional profile that includes the classic inflammatory activities and, more recently, developmental homing and regenerative properties among others. In contrast, the functional profile of C4a is questionable in most cases owing to contamination of C4a preparations with physiologically relevant levels of C3a and/or C5a. Combined with the absence of an identified C4a receptor and the inability of C4a to signal through the C3a and C5a receptors, it is clear that C4a should not be included in the family of complement anaphylatoxins.

Complement: A nostalgic journey

The scientific career and research contributions of Hans J. Müller-Eberhard to the field of complement research are presented in historical context, and interpreted with regard to the state of the field and the research technologies available when the contributions were made.

A Novel Protein Derived from the MUC1 Gene by Alternative Splicing and Frameshifting

Genes that have been designated the name "MUC" code for proteins comprising mucin domains. These proteins may be involved in barrier and protective functions. The first such gene to be characterized and sequenced is the MUC1 gene. Here we report a novel small protein derived from the MUC1 gene by alternative splicing that does not contain the hallmark of mucin proteins, the mucin domain. This protein termed MUC1/ZD retains the same N-terminal MUC1 sequences as all of the other known MUC1 protein isoforms. The common N-terminal sequences comprise the signal peptide and a subsequent stretch of 30 amino acids. In contrast, the MUC1/ZD C-terminal 43 amino acids are novel and result from a reading frameshift engendered by a splicing event that forms MUC1/ZD. The expression of MUC1/ZD at the protein level in human tissues is demonstrated by Western blotting, immunohistochemistry, immunoprecipitation, and an ELISA. Utilization was made of affinity-purified MUC1/ZD-specific polyclonal antibodies as well as two different monoclonal antibodies that are monospecific for the MUC1/ZD protein. The MUC1/ZD protein is expressed in tissues as an oligomeric complex composed of monomers linked by disulfide bonds contributed by MUC1/ZD cysteine residues. MUC1/ZD protein expression did not parallel that of the tandem-repeat array-containing MUC1 protein. Results presented here demonstrate for the first time the expression of a novel MUC1 protein isoform MUC1/ZD, which is generated by an alternative splicing event that both deletes the tandem-repeat array and leads to a C-terminal reading frameshift.

Effect of oleanolic acid on complement in adjuvant- and carrageenan-induced inflammation in rats

The present work was aimed at evaluating the influence of oleanolic acid on complement-related inflammation. In adjuvant-induced arthritis and carrageenan-induced paw oedema in rats, oleanolic acid was found to possess significant anti-inflammatory and complement-inhibitory activities. The intraperitoneal injection of oleanolic acid (60 mg kg-1, twice a day), before and after Freund's Complete Adjuvant challenge and thereafter repeated for several days, significantly reduced foot-pad thickness of experimental animal models and simultaneously reduced complement activity. Oleanolic acid also produced marked reduction in complement levels and inflammatory effects on carrageenan-induced paw oedema in rats when injected intraperitoneally (60 mg kg-1, twice a day).

Anticomplementary activity of boswellic acids--an inhibitor of C3-convertase of the classical complement pathway

Boswellic acids (BA), an anti-inflammatory and anti-arthritic principle/s of Boswellia serrata, were found to possess anticomplementary activity. It inhibits the in vitro immunohaemolysis of antibody-coated sheep erythrocytes by pooled guinea-pig serum. The reduced immunohaemolysis was found to be due to inhibition of C3-convertase of the classical complement pathway. The threshold concentration for inhibiting C3-convertase was found to be 100 micrograms. However, higher concentrations of BA showed constant inhibitory effects on immunohaemolysis. BA also exhibited weak inhibitory effects on individual components of the complement system. In vivo administration of BA also showed the inhibitory effect on guinea-pig serum.

Chemotactic factor generation and cell accumulation in acute lung injury induced by endotracheal acid instillation

We studied the time course of chemotactic factor generation and inflammatory cell accumulation in the rabbit aspiration pneumonia model. Two major potent chemotactic factors, leukotriene B4 (LTB4) and C5a, in bronchoalveolar lavage fluid (BALF) were measured by radioimmunoassay, and cell analysis was also done. The level of LTB4 increased only in the early phase (2-6 h) after endotracheal acid instillation. The level of C5a increased gradually almost in parallel with the total protein level in BALF, and reached a maximum at 24 h. Neutrophil accumulation occurred early and reached a maximum at 24 h. In contrast, the number of alveolar macrophages increased from days 1 to 7. These findings suggest that the increases in LTB4 and C5a are responsible for accumulation of neutrophils and that C5a may be an important chemotactic factor for alveolar macrophage.

Resistance to the Sterne strain of B. anthracis: phagocytic cell responses of resistant and susceptible mice

Inflammatory responses were compared in vivo, and host phagocytic cell functions compared in vitro, of mice resistant (CBA/J) and susceptible (A/J) to lethal infection with the Sterne strain of Bacillus anthracis. Polymorphonuclear leukocyte (PMN) and macrophage responses at the initial site of infection were slower in A/J mice than in CBA/J mice. Whereas in A/J mice, the number of PMN ultimately responding to infection was equal to, or greater than, that in CBA/J mice, fewer macrophages accumulated. A/J mice failed to clear relatively low doses of the organisms and died. In vitro, chemotactic responses to both serum- and bacteria-derived attractants were similar for macrophages from A/J and CBA/J mice but were reduced for PMN from A/J mice. PMN and macrophages from the two mouse strains phagocytosed and killed spores in vitro to a similar extent, although killing by A/J PMN could be blocked by prior uptake of large numbers of killed spores. Thus susceptibility to lethal infection with Sterne strain correlated with the delayed influx (PMN) and reduced accumulation (macrophages) of phagocytes at the initial site of infection, but not with defective in vitro uptake or killing of spores.

Tumor necrosis factor/cachectin. Induction of hemorrhagic necrosis in normal tissue requires the fifth component of complement (C5)

TNF induces hemorrhagic necrosis (HN) when injected into skin exposed to bacterial agents but not when injected into normal skin. In this paper, we present several lines of evidence suggesting that TNF requires the fifth component of complement (C5) to induce HN in skin exposed to bacteria. First, mouse strains that do not have C5 did not develop HN after injection of TNF and bacteria into skin. Second, plasma from C5-sufficient mice could correct the defect in these C5-deficient mice. Third, heating at 56 degrees C for 30 min inactivated the capacity of plasma to reconstitute C5-deficient mice. Fourth, CVF, which is known to inactivate complement, abrogated the capability of C5-sufficient mice to respond. Fifth, depleting plasma of hemolytic activity while generating C5a did not affect the capacity of the activated plasma to reconstitute C5-deficient mice. Finally, only the plasma fraction containing molecules of the size range of C5a reconstituted C5-deficient mice. These findings indicate that C5a and not the membrane attack complex is required for HN. Although we do not know through which mechanism C5a participates in the development of HN, we propose that the described HN response is related to a local defense mechanism in which TNF and C5a lead to the disruption of capillaries in the direct vicinity of bacteria. By this mechanism the rapid spread of bacteria or their products into the circulation is prevented. Such a tissue response is consistent with the known higher susceptibility of C5-deficient mice to bacterial infections and provides a model with which to search for the multiple steps involved in this important local defense mechanism.

Angioedema induced by a peptide derived from complement component C2

Synthetic peptides that correspond to the COOH-terminal portion of C2b enhance vascular permeability in human and guinea pig skin. In human studies, 1 nmol of the most active peptide of 25-amino acid residues produced substantial local edema. A pentapeptide and a heptapeptide corresponding to the COOH-terminal sequence of C2b each induced contraction of estrous rat uterus in the micromole range; a peptide of 25 amino acids from this region induced a like contraction of rat uterus at a concentration 20-fold lower than the smaller peptides. The vascular permeability of guinea pig skin was enhanced by doses of these synthetic peptides in a similar fashion as that observed for the concentration of rat uterus. The induction of localized edema by intradermal injection in both the guinea pig and the human proceeds in the presence of antihistaminic drugs, suggesting that there is a histamine-independent component to the observed increase in vascular permeability. Cleavage of C2 with the enzymic subcomponent of C1, C1s, yields only C2a and C2b, and no small peptides, whereas cleavage of C2 with C1s and plasmin yields a set of small peptides. These plasmin-cleaved peptides are derived from the COOH terminus of C2b, and they induce the contraction of estrous rat uterus.

Pathogenesis and genetic control of resistance to the Sterne strain of Bacillus anthracis

The pathogenesis of lethal infection by the nonecapsulated, toxigenic Sterne strain of Bacillus anthracis and the genetic basis of resistance were characterized in mice. Lethal doses of Sterne spores produced disease in susceptible mice similar to that caused by toxigenic and encapsulated B. anthracis. At the inoculation site, the mice developed an edematous exudate with large concentrations of bacilli and toxin. In the susceptible A/J strain, lethal infection was accompanied by systemic invasion and serum anthrax toxin levels increased in parallel with systemic bacterial concentrations and with the mortality rate. Host resistance to Sterne infection was associated with the ability to synthesize the complement component 5 (C5). All Sterne-resistant mouse strains had a functional gene (Hc) encoding C5, whereas susceptible mice were deficient in C5. A/J mice could be passively protected from lethal challenge by C5-positive serum but not by serum from C5-negative congenic mice. Also resistance was linked to production of C5 in individual backcross (97%) and F2 (98%) mice. The distribution pattern for recombinant inbred mice was consistent with a major role in host resistance of Hc or a closely linked locus, although other genes probably contribute. This mouse model will be useful in characterizing the pathogenesis of anthrax and testing the safety and efficacy of new anthrax vaccines.

Characterization of vascular permeability-increasing component isolated from solid tumors and the effect of highly polymerized dextran sulfate on its activity

Increase in vascular permeability is usually seen at the growth site of a tumor implant in murine dermal tissue. Increased vascular permeability was inducible by the subcutaneous injection of a solid tumor extract rich in protein precipitable at 20-50% saturation of ammonium sulfate. The vascular permeability-increasing activity of the tumor extract was reducible in the presence of highly polymerized dextran sulfate (DS-500) which showed a strong anticomplementary activity, but not by other substances such as dextran sulfate with a low molecular weight, non-sulfated dextran, chondroitin sulfate or heparin. As the tumor extract includes gamma-globulins in aggregated or bound form and adsorbs complements, it is probable that the aggregated gamma-globulins increase vascular permeability by triggering the complement activation system in the skin. DS-500 might antagonize the process.

Demonstration of a specific receptor for human C5a anaphylatoxin on murine macrophages

Human C5a anaphylatoxin is known to be a potent mediator of the acute inflammatory response. It serves to trigger a wide variety of neutrophil responses after binding to a specific cellular receptor. We have now demonstrated that this bioactive glycopolypeptide is also bound to a specific receptor found on murine resident peritoneal macrophages, thioglycollate-induced exudate macrophages, and the murine cell line P388D1. The apparent Kd of the C5a receptors. Resident macrophages appear to have an average of 2 x 10(5) binding sites per cell, whereas thioglycollate- induced cells have only 4-5 x 10(4) binding sites. The continuous cell line P388D1 is intermediate between these two cell types, exhibiting 8-10 x 10(4) C5a receptors per cell. Neither murine lymphocytes nor the parent cell line P388 displays a measurable number of C5a receptors. Macrophage receptor-C5a binding interactions are followed by cellular uptake and degradation of 125I-C5a, much as is observed with neutrophils. As demonstrated in another paper, binding of C5a to macrophages results in augmentation of the primary humoral immune response as well as enhancement of mixed lymphocyte reactions. These observations suggest that C5a should not only be considered as an acute inflammatory mediator but as an immunopotentiating modulator as well, thus serving as a critical link between complement activation and subsequent immune responses.

Identification of classical anaphylatoxin as the des-Arg form of the C5a molecule: evidence of a modulator role for the oligosaccharide unit in human des-Arg74-C5a

A functionally active and potentially lethal fragment of the fifth component of complement (C5) is generated during complement activation in serum from animals of various species. This factor, termed the "classical" anaphylatoxin, was isolated from porcine serum and was identified chemically as the des-Arg derivative of the well-characterized C5a molecule. Unlike the C3a and C4a anaphylatoxins, porcine C5a does not require the COOH-terminal arginyl residue for spasmogenic activity. Further degradation of porcine des-Arg(74)-C5a by carboxypeptidase Y removed glycine-73 and leucine-72 and decreased the intrinsic spasmogenic activity by >90%. Hence, we conclude that, although the arginyl residue is not essential, the COOH-terminal sequence Leu-Gly-Arg contributes structural information that accounts for >90% of C5a activity. Human des-Arg(74)-C5a, like its porcine counterpart, has instrinsic anaphylatoxin activity; however, higher concentrations were needed to contract the guinea pig ileal tissue (i.e., 1 muM for human des-Arg(74)-C5a versus 1 nM for porcine des-Arg(74)-C5a). Furthermore, the des-Arg form of human C5a was only 0.1% as active as porcine des-Arg(74)-C5a for enhancing vascular permeability in guinea pig skin. In addition to these biological differences, numerous chemical differences exist between the human and porcine des-Arg(74)-C5a molecules, the most prominent feature being an oligosaccharide entity associated uniquely with the human C5a. When the oligosaccharide unit of human des-Arg(74)-C5a was removed by glycosidases, leaving a single glucosamine residue attached to the side chain of asparagine-64, activity was enhanced. The human des-Arg(74)-C5a molecule devoid of the complex oligosaccharide unit exhibited 10-fold stronger spasmogenic activity and 20- to 50-fold greater permeability-enhancing activity than did human des-Arg(74)-C5a containing the oligosaccharide. Consequently, the oligosaccharide associated with human C5a modulates or suppresses potentially harmful activities of this anaphylatoxin. The relatively high levels of spasmogenic activity associated with porcine des-Arg(74)-C5a indicate that this factor is poorly controlled by endogenous serum carboxypeptidase, whereas human C5a is virtually inactivated by the enzyme. Hence, the influence of this oligosaccharide in suppressing human des-Arg(74)-C5a activity is of major physiologic importance in protecting man from potentially toxic effects of this complement factor.

Deficiency of the fifth component of complement in human subjects. Clinical, genetic and immunologic studies in a large kindred

The discovery of a large kindred with a heritable deficiency of the fifth component of complement (C5) has permitted the accumulation of new clinical, genetic and immunologic data concerning the role of C5 in human subjects. The proband, who has had nine episodes of disseminated gonococcal infection, has a hemolytic C5 level of approximately 0.5 per cent of normal. No C5 protein was detectable, but low levels of functional C5 activity could be found using a sensitive bactericidal assay. The proband's twin as well as another sister also had extremely low levels of hemolytic C5(approximately 0.5 per cent normal), but both these subjects have been healthy. Hemolytic complement and bacteriolytic activity could be restored by the addition of purified C5. No chemotactic activity for polymorphonuclear leukocytes could be generated in the C5-deficient serums upon activation of either the classic or alternative pathways, again demonstrating the importance of C5 in human subjects for the production of chemotactic factors. The chemotactic responsiveness of the patients' polymorphonuclear leukocytes and monocytes to preformed chemotactic factors was not depressed. Twenty-two of 32 other family members from three generations had depressed whole hemolytic complement levels. In 19 of 30 family members, levels of hemolytic C5 ranged from 13 to 64 per cent of normal. No linkage for C5 deficiency and the A or B loci of the major histocompatibility complex could be found. These data suggest an autosomal codominant mode of inheritance of C5 deficiency. Deficiency of C5 is compatible with good health, but it can be associated with repeated disseminated gonococcal infection.

Shock and noncardiogenic pulmonary edema following meglumine diatrizoate for intravenous pyelography

In a 25 year old man shock and pulmonary edema developed following the intravenous administration of meglumine diatrizoate for an intravenous pyelogram. A pulmonary-capillary wedge pressure of 3 torr and a high protein content in the pulmonary edema effluent confirmed the diagnosis of noncardiogenic pulmonary edema. We suggest that the basis of the increased capillary permeability in this patient may be related to an immunologic reaction to intravenous pyelogram dye.

Homozygous human C3 deficiency. The role of C3 in antibody production, C-1s-induced vasopermeability, and cobra venom-induced passive hemolysis

Studies of the family of a patient with marked deficiency of the third component of complement (C3) demonstrated that the patient was homozygous for a blank allele at the C3 locus, C3-. Metabolic studies with purified radiolabeled C3 in the patient revealed a mildly elevated fractional catabolic rate and a markedly reduced synthesis rate, consistent with a lack of C3 synthesis as the patient's primary defect. There was also a mild increase in the rate of conversion of purified C3 added to her serum and incubated at 37 degrees C in vitro. Major blood group-compatible erythrocytes from a patient with paroxysmal nocturnal hemoglobinuria had the same shortened survival in the C3-deficient patient as in a normal control. Although no leukocytosis developed in the patient in spontaneous infection by pyogenic organisms, there was a normal leukocytosis in response to the injection of thyphoid vaccine. The intradermal injection of C-1s, which produces a marked increase in vasopermeability in the skin of normal subjects, produced no definite change in the patient, possibly implicating C3 or a protein in the alternative pathway as the normal mediator of this response. The patient's serum exhibited near-normal immune adherence activity, confirming the lack of requirement of C3 for this function. C5 inactivation and passive hemolysis of unsensitized guinea pig erythrocytes occurred normally in C3-deficient serum on incubation with cobra venom factor, indicating that C3 is not required for these reactions. The patient's humoral antibody response to both protein and carbohydrate antigens was entirely normal, making it unlikely that C3 is required for antigen processing.

Formation of C3a and C5a anaphylatoxins in whole human serum after inhibition of the anaphylatoxin inactivator

Two biologically and chemically distinct anaphylatoxins (ATs) could be generated in whole human serum after removal of the AT inactivator (AI) by immune-absorption or after inhibition of AI with 1 M epsilon-aminocaproic acid (EACA). Both human ATs could be generated by treatment of serum with antigen-antibody complexes, which activate the classical complement pathway, and with inulin or yeast, both of which trigger the alternate pathway. The ATs were isolated from serum in active form and characterized as C3a and C5a. Although human C3a had been characterized previously, C5a had not. The molecular weight of human C5a AT was 17,500; its electrophoretic mobility at pH 8.5 was -1.7 x 10(-5) cm(2) V(-1) s(-1). The minimal effective concentration in vitro was 7.5 x 10(-10) M. The minimal effective doses of human C5a in producing a wheal and erythema in the human skin was 1 x 10(-15) mol. The results strongly suggest a biological function for both ATs and indicate that the expression of their activity is controlled by the AI of normal blood plasma.