Complement resistance in microbes

Increased severity of respiratory infections associated with elevated anti-LPS IgG2 which inhibits serum bactericidal killing

An antibody directed against the O-antigen of Pseudomonas aeruginosa LPS can block complement-mediated bacterial killing and contributes to the severity of respiratory infection.

Although specific antibody induced by pathogens or vaccines is a key component of protection against infectious threats, some viruses, such as dengue, induce antibody that enhances the development of infection. In contrast, antibody-dependent enhancement of bacterial infection is largely unrecognized. Here, we demonstrate that in a significant portion of patients with bronchiectasis and Pseudomonas aeruginosa lung infection, antibody can protect the bacterium from complement-mediated killing. Strains that resist antibody-induced, complement-mediated killing produce lipopolysaccharide containing O-antigen. The inhibition of antibody-mediated killing is caused by excess production of O-antigen–specific IgG2 antibodies. Depletion of IgG2 to O-antigen restores the ability of sera to kill strains with long-chain O-antigen. Patients with impaired serum-mediated killing of P. aeruginosa by IgG2 have poorer respiratory function than infected patients who do not produce inhibitory antibody. We suggest that excessive binding of IgG2 to O-antigen shields the bacterium from other antibodies that can induce complement-mediated killing of bacteria. As there is significant sharing of O-antigen structure between different Gram-negative bacteria, this IgG2-mediated impairment of killing may operate in other Gram-negative infections. These findings have marked implications for our understanding of protection generated by natural infection and for the design of vaccines, which should avoid inducing such blocking antibodies.

Functional characterization of exopolyphosphatase/guanosine pentaphosphate phosphohydrolase (PPX/GPPA) of Campylobacter jejuni

The inorganic polyphosphate (poly-P) is a key regulator of stress responses and virulence in many bacterial pathogens including Campylobacter jejuni. The role of exopolyphosphatases/guanosine pentaphosphate (pppGpp) phosphohydrolases (PPX/GPPA) in poly-P homeostasis and C. jejuni pathobiology remains unexplored. Here, we analyzed deletion mutants (∆ppx1, ∆ppx2) and the double knockout mutant (dkppx), all ∆ppx mutants exhibited increased capacity to accumulate poly-P; however only ∆ppx1 and dkppx mutants showed decreased accumulation of ppGpp, an alarmone molecule that regulates stringent response in bacteria, suggesting potential dual role for PPX1/GPPA. Nutrient survival defect of ∆ppx mutants was rescued by the supplementation of specific amino acids implying that survival defect may be associated with decreased ppGpp and/ or increased poly-P in ∆ppx mutants. The ppk1 and spoT were upregulated in both ∆ppx1 and ∆ppx2 suggesting a compensatory role for SpoT and Ppk1 in poly-P and ppGpp homeostasis. The lack of ppx genes resulted in defects in motility, biofilm formation, nutrient stress survival, invasion and intracellular survival indicating that maintaining a certain level of poly-P is critical for ppx genes in C. jejuni pathophysiology. Both ppx1 and ppx2 mutants were resistant to human complement-mediated killing; however, the dkppx mutant was sensitive. The serum susceptibility did not occur in the presence of MgCl 2 and EGTA suggesting an involvement of the classical or lectin pathway of complement mediated killing. Interestingly, the chicken serum did not have any effect on the ∆ppx mutants' survival. The observed serum susceptibility was not related to C. jejuni surface capsule and lipooligosaccharide structures. Our study underscores the importance of PPX/GPPA proteins in poly-P and ppGpp homeostasis, two critical molecules that modulate environmental stress responses and virulence in C. jejuni.

Aptamer–biotin–streptavidin–C1q complexes can trigger the classical complement pathway to kill cancer cells

Nucleic acid aptamers are regarded as rivals for antibodies and as such are being investigated for their therapeutic potential. In the present work, it is shown that two different high-affinity DNA aptamers developed previously by Ferreira et al. against MUC1 antigen (designated MUC1-5TR-1 and MUC1-S1.3/S2.2) on MCF7 breast cancer cells can be linked to the first component of complement (C1q) via a biotin–streptavidin system and induce significant killing of MCF7 cells in vitro. Cell viability was assessed by Trypan blue uptake and absorbance at 590 nm of stained cells following buffer washes and lysis in 1% SDS. While the killing effect is demonstrable versus various controls, dependent on aptamer dose, and reproducible, it appears to kill maximally about half of treated MCF7 cells. Possible reasons for the marginal killing effect include antigenic shedding in vitro and membrane-bound complement regulatory proteins (mCRPs) on the cell surface such as CD46, CD55, and CD59 which act to inhibit complement-mediated lysis of cells. Future in vitro research could benefit from application of mCRP-specific aptamers in combination with anti-MUC1 aptamers to overcome surface protective mechanisms while attacking the plasma membrane of MCF7 cells or other MUC1-expressing cancer cells. However, in vivo such a combination could have deleterious effects on normal MUC1-expressing cells as well.

Campylobacter capsule and lipooligosaccharide confer resistance to serum and cationic antimicrobials

The innate immune system plays a critical role in host defense against mucosal bacteria. Campylobacter jejuni is a major cause of human gastroenteritis that usually resolves spontaneously within several days, suggesting that innate mechanisms are important to control the infection. However, the specific means by which this occurs is not well understood. While diarrheal isolates of C. jejuni usually are susceptible to human serum, we found that a systemic strain of C. jejuni, isolated from the cerebrospinal fluid of an infant with meningitis, is relatively more resistant to human serum, the Bactericidal/Permeability-Increasing Protein (BPI), an endogenous cationic antimicrobial protein, and the cationic peptide antibiotic polymyxin B. To test the hypothesis that the surface properties of this strain contributed to its ability to withstand these innate host defenses, we constructed isogenic mutants in capsule (kpsM) and lipooligosaccharide (waaF) and complemented these mutants by insertion of the complementation construct in trans into hipO, a chromosomal locus. We found that capsule expression was essential for serum resistance, whereas lipooligosaccharide played no substantial role. In contrast, the lipooligosaccharide mutant showed increased sensitivity to polymyxin B, α-defensins, cathelicidins, and BPI. These findings suggest that the polysaccharides of C. jejuni strains contribute differently to resistance against host innate immunity; whereby capsule is more important for resisting human complement and lipooligosaccharide is more important for protection against killing mediated by cationic antimicrobial peptides and proteins.

Polysaccharide capsule and sialic acid-mediated regulation promote biofilm-like intracellular bacterial communities during cystitis

Uropathogenic Escherichia coli (UPEC) is the leading cause of urinary tract infections (UTIs). A murine UTI model has revealed an infection cascade whereby UPEC undergoes cycles of invasion of the bladder epithelium, intracellular proliferation in polysaccharide-containing biofilm-like masses called intracellular bacterial communities (IBC), and then dispersal into the bladder lumen to initiate further rounds of epithelial colonization and invasion. We predicted that the UPEC K1 polysaccharide capsule is a key constituent of the IBC matrix. Compared to prototypic E. coli K1 strain UTI89, a capsule assembly mutant had a fitness defect in functionally TLR4(+) and TLR4(-) mice, suggesting a protective role of capsule in inflamed and noninflamed hosts. K1 capsule assembly and synthesis mutants had dramatically reduced IBC formation, demonstrating the common requirement for K1 polysaccharide in IBC development. The capsule assembly mutant appeared dispersed in the cytoplasm of the bladder epithelial cells and failed to undergo high-density intracellular replication during later stages of infection, when the wild-type strain continued to form serial generations of IBC. Deletion of the sialic acid regulator gene nanR partially restored IBC formation in the capsule assembly mutant. These data suggest that capsule is necessary for efficient IBC formation and that aberrant sialic acid accumulation, resulting from disruption of K1 capsule assembly, produces a NanR-mediated defect in intracellular proliferation and IBC development. Together, these data demonstrate the complex but important roles of UPEC polysaccharide encapsulation and sialic acid signaling in multiple stages of UTI pathogenesis.

The complement system in systemic autoimmune disease

Complement is part of the innate immune system. Its major function is recognition and elimination of pathogens via direct killing and/or stimulation of phagocytosis. Activation of the complement system is, however, also involved in the pathogenesis of the systemic autoimmune diseases. Activation via the classical pathway has long been recognized in immune complex-mediated diseases such as cryoglobulinemic vasculitis and systemic lupus erythematosus (SLE). In SLE, the role of complement is somewhat paradoxical. It is involved in autoantibody-initiated tissue damage on the one hand, but, on the other hand, it appears to have protective features as hereditary deficiencies of classical pathway components are associated with an increased risk for SLE. There is increasing evidence that the alternative pathway of complement, even more than the classical pathway, is involved in many systemic autoimmune diseases. This is true for IgA-dominant Henoch Schönlein Purpura, in which additional activation of the lectin pathway contributes to more severe disease. In anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis the complement system was considered not to be involved since immunoglobulin deposition is generally absent in the lesions. However, recent studies, both in human and animal models, demonstrated complement activation via the alternative pathway as a major pathogenic mechanism. Insight into the role of the various pathways of complement in the systemic autoimmune diseases including the vasculitides opens up new ways of treatment by blocking effector pathways of complement. This has been demonstrated for monoclonal antibodies to C5 or C5a in experimental anti-phospholipid antibody syndrome and ANCA-associated vasculitis.

Pseudomonas aeruginosa lipopolysaccharide: a major virulence factor, initiator of inflammation and target for effective immunity

Pseudomonas aeruginosa is one of the most important bacterial pathogens encountered by immunocompromised hosts and patients with cystic fibrosis (CF), and the lipopolysaccharide (LPS) elaborated by this organism is a key factor in virulence as well as both innate and acquired host responses to infection. The molecule has a fair degree of heterogeneity in its lipid A and O-antigen structure, and elaborates two different outer-core glycoforms, of which only one is ligated to the O-antigen. A close relatedness between the chemical structures and genes encoding biosynthetic enzymes has been established, with 11 major O-antigen groups identified. The lipid A can be variably penta-, hexa- or hepta-acylated, and these isoforms have differing potencies when activating host innate immunity via binding to Toll-like receptor 4 (TLR4). The O-antigen is a major target for protective immunity as evidenced by numerous animal studies, but attempts, to date, to produce a human vaccine targeting these epitopes have not been successful. Newer strategies employing live attenuated P. aeruginosa, or heterologous attenuated bacteria expressing P. aeruginosa O-antigens are potential means to solve some of the existing problems related to making a P. aeruginosa LPS-specific vaccine. Overall, there is now a large amount of information available about the genes and enzymes needed to produce the P. aeruginosa LPS, detailed chemical structures have been determined for the major O-antigens, and significant biologic and immunologic studies have been conducted to define the role of this molecule in virulence and immunity to P. aeruginosa infection.

HIV and complement: hijacking an immune defense

The complement system is a central player of the innate immune system. Activation of the complement system protects the host against pathogens. However, uncontrolled synthesis can be detrimental to host. This concise review summarizes the current understanding of the mechanism(s) of complement activation, the mechanism of C3 regulation, and the role of complement in human immunodeficiency virus (HIV) pathogenesis with emphasis on the cross-talk between HIV and complement system in NeuroAIDS and HIV-associated nephropathy (HIVAN).

Complement receptor 2, natural antibodies and innate immunity: Inter-relationships in B cell selection and activation

Complement receptor type 2 (CR2) is a receptor that serves as an important interface between the complement system and adaptive immunity. Recent studies have shown that CR2 is also centrally involved in innate immunity, and one key area is the development of potentially pathogenic natural antibodies that target neo-epitopes revealed in ischemic tissue undergoing reperfusion. Mice lacking either total immunoglobulins or CR2 alone are protected from the development of ischemia-reperfusion injury, and this effect can be reversed by introducing CR2-sufficient B-1 cells or by transferring polyclonal natural IgM antibody from wild type mice as well as monoclonal antibodies that recognize phospholipids, DNA or non-muscle myosin. We will report at the XXI ICW an additional membrane-associated protein to which pathogenic IgM antibodies are directed. Whether B cells producing these natural antibodies are differentially selected in CR2-deficient mice is as yet not well understood, and the complement-related mechanism(s) whereby this differential repertoire selection process could occur have yet to be explored in any detail. In addition to this important role in innate immunity, CR2 can also act as a receptor for other components or activators of innate immunity. One such component is interferon-alpha, an anti-viral cytokine that binds CR2 and induces a component of its mRNA signature in B cells through this receptor. Other potential CR2 ligands are DNA and DNA-containing complexes such as chromatin. The biologic role of these CR2 interactions with interferon-alpha and DNA-containing complexes is not well understood, but may be important in the development of the autoimmune disease systemic lupus erythematosus that is characterized by enhanced interferon-alpha levels and loss of self tolerance to DNA-containing self antigens.

The Role of Noradrenergic Nerves in the Development of the Lymphoproliferative Disease in Fas-Deficient, lpr/lpr Mice

Lpr/lpr mice develop a lymphoproliferative, autoimmune, lupus-like disease. These mice lack functional Fas (CD95) expression and are resistant to Fas ligand (CD178)-mediated apoptosis, a critical mechanism for the maintenance of peripheral tolerance. In this study, we show that noradrenaline (NA), the main sympathetic neurotransmitter, can induce apoptosis of lymphoid cells independently of functional Fas. Based on this finding, we used lpr/lpr mice as model to study the role of noradrenergic nerves in the expression of a lymphoproliferative disease. Early in ontogeny, the concentration of NA was significantly increased in the spleen of lpr/lpr mice, compared with normal littermates. However, splenic sympathetic innervation gradually declined as the disease progressed, and IgM blood levels and splenic NA concentration inversely correlated when the disease was overtly manifested. When the loss of noradrenergic fibers that occurred naturally during adult life in lpr/lpr mice was experimentally advanced by neonatal sympathectomy, the concentration of IgM and IgG2a in blood was markedly higher than that of control lpr/lpr mice, and the appearance of lymphadenopathy was accelerated. Furthermore, although neonatal denervation did not affect the life span of normal animals, it shortened significantly the survival time of lpr/lpr mice. These data show that, in addition to defects in the Fas pathway, an altered sympathetic innervation in lpr/lpr mice also contributes to the pathogenesis of the autoimmune disease, and strongly support the hypothesis that the sympathetic nervous system can modulate the expression of lymphoproliferative diseases.

The central role of the alternative complement pathway in human disease

The complement system is increasingly recognized as important in the pathogenesis of tissue injury in vivo following immune, ischemic, or infectious insults. Within the complement system, three pathways are capable of initiating the processes that result in C3 activation: classical, alternative, and lectin. Although the roles that proinflammatory peptides and complexes generated during complement activation play in mediating disease processes have been studied extensively, the relative contributions of the three activating pathways is less well understood. Herein we examine recent evidence that the alternative complement pathway plays a key and, in most instances, obligate role in generating proinflammatory complement activation products in vivo. In addition, we discuss new concepts regarding the mechanisms by which the alternative pathway is activated in vivo, as recent clinical findings and experimental results have provided evidence that continuous active control of this pathway is necessary to prevent unintended targeting and injury to self tissues.

Selection of GM2, fucosyl GM1, globo H and polysialic acid as targets on small cell lung cancers for antibody mediated immunotherapy

Glycolipids GM2, GD2, GD3, fucosyl GM1, sialyl Lewis a (sLe(a)) and globo H, and polysialic acid on embryonal NCAM, are cell-surface antigens expressed on small cell lung cancer (SCLC) biopsy specimens. They are all candidates for inclusion in a polyvalent, antibody-inducing vaccine or for adoptive therapy with monoclonal antibodies (mAbs) against SCLC. To identify the minimum optimal combination of target antigens on SCLC and to confirm that antibodies against this combination might be able to mediate complement activation and lysis in the majority of cases, we tested ten SCLC cell lines with fluorescence activated cell sorter (FACS) and complement dependent cytotoxicity (CDC) assays using mAbs against these seven target antigens individually or pooled in different combinations. We find that (1) none of these mAbs demonstrated strong FACS reactivity with more than 6 of the 10 cell lines, (2) no mAb had strong CDC reactivity with more than 4 of the cell lines, (3) when the mAbs were pooled, nine cell lines were strongly positive by FACS and nine cell lines were strongly positive by CDC, and (4) mAbs against GM2, FucGM1, globo H and polysialic acid was the minimum optimal combination for inducing FACS reactivity. The addition of mAbs against sLe(a), GD2 and GD3 had no additional impact by FACS and only minimal additional impact in CDC assays. H345, the only cell line that had less than 30% CDC with the four mAb pool was strongly positive by FACS. To understand the lack of correlation between FACS and CDC in the case of H345, the ten cell lines were screened for expression of complement resistance factors CD55 and CD59. Three cell lines were strongly positive for CD55 and eight were strongly positive for CD59. Overall, no correlation was seen between expression of either of these factors on the ten cell lines and sensitivity to CDC. In the case of H345 however, complement resistance of H345 is demonstrated to be mediated primarily by CD59, and in the presence of mAb against CD59, the four mAb MEM-43 pool induced strong (94%) CDC. CD59 inhibits membrane attack complex formation but not activation of earlier complement components. Consequently, all ten cell lines are good targets for complement activation by the four antibody pool and for elimination by effector mechanisms including complement mediated inflammation and opsonization. These findings support our plan to develop a tetravalent vaccine against SCLC targeting GM2, fucosyl GM1, globo H and polysialic acid.

Antibodies against tumor cell glycolipids and proteins, but not mucins, mediate complement-dependent cytotoxicity

One of several effector mechanisms thought to contribute to Ab efficacy against cancer is complement-dependent cytotoxicity (CDC). Serological analysis of a series of clinical trials conducted over a 10-year period suggested that six vaccines containing different glycolipids induced Abs mediating CDC whereas four vaccines containing carbohydrate or peptide epitopes carried almost exclusively by mucin molecules induced Abs that did not mediate CDC. To explore this further, we have now compared cell surface reactivity using flow cytometry assays (FACS), complement-fixing ability, and CDC activity of a panel of mAbs and immune sera from these trials on the same two tumor cell lines. Abs against glycolipids GM2, globo H and Lewis Y, protein KSA (epithelial cell adhesion molecule, also known as EpCAM) and mucin Ags Tn, sialylated Tn, Thomsen Friedenreich (TF), and MUC1 all reacted comparably by FACS with tumor cells expressing these Ags. Compared with the strong complement binding and CDC with Abs against glycolipids and KSA, complement binding was diminished with Abs against mucin Ags and no CDC was detected. A major difference between these two groups of Ags is proximity to the cell membrane. Glycolipids and globular glycoproteins extend less than 100 A from the cell membrane while mucins extend up to 5000 A. Although complement activation at sites remote from the cell membrane has long been known as a mechanism for resistance from complement lysis in bacteria, it is identified here for the first time as a factor which may contribute to resistance from CDC against cancer cells.

Killing of dsrA mutants of Haemophilus ducreyi by normal human serum occurs via the classical complement pathway and is initiated by immunoglobulin M binding

Previously, we showed that serum resistance in Haemophilus ducreyi type strain 35000HP required expression of the outer membrane protein DsrA because the isogenic dsrA mutant FX517 is highly serum susceptible. In this study, we confirmed this finding by construction of additional serum-susceptible dsrA mutants in more recently isolated serum-resistant strains. We also demonstrated that killing of dsrA mutants required an intact classical complement cascade but not the alternative or mannan-binding lectin pathways. Between 5- and 10-fold more purified human immunoglobulin M (IgM) but not IgG was deposited onto dsrA mutant FX517 than onto parent strain 35000HP, consistent with IgM initiation of the classical cascade. Depletion of IgM, but not IgG, from complement-intact serum inhibited killing of FX517. As predicted from the amounts of IgM bound, more of the individual complement components were bound by FX517 than by parent strain 35000HP. Examination of the binding of negative regulators of complement as an explanation for serum resistance indicated that parent strain 35000HP bound more C4 binding protein and vitronectin than FX517 but not factor H. However, the degree and pattern of complement component binding observed suggested that IgM binding to the serum-susceptible mutant FX517 was responsible for the activation of the classical pathway and the observed killing of FX517 as opposed to binding of negative regulators of complement by the serum-resistant parent. We speculate that an undefined neo-epitope, possibly carbohydrate, is exposed in the dsrA mutant that is recognized by naturally occurring bactericidal IgM antibodies present in human sera.

In vivo neutralization of TNF-alpha promotes humoral autoimmunity by preventing the induction of CTL

Neutralization of TNF-alpha in humans with rheumatoid arthritis or Crohn's disease has been associated with the development of humoral autoimmunity. To determine the effect of TNF-alpha neutralization on cell-mediated and humoral-mediated responses, we administered anti-TNF-alpha mAb to mice undergoing acute graft-vs-host disease (GVHD) using the parent-into-F(1) model. In vivo neutralization of TNF-alpha blocked the lymphocytopenic features characteristic of acute GVHD and induced a lupus-like chronic GVHD phenotype (lymphoproliferation and autoantibody production). These effects resulted from complete inhibition of detectable antihost CTL activity and required the presence of anti-TNF-alpha mAb for the first 4 days after parental cell transfer, indicating that TNF-alpha plays a critical role in the induction of CTL. Moreover, an in vivo blockade of TNF-alpha preferentially inhibited the production of IFN-gamma and blocked IFN-gamma-dependent up-regulation of Fas; however, cytokines such as IL-10, IL-6, or IL-4 were not inhibited. These results suggest that a therapeutic TNF-alpha blockade may promote humoral autoimmunity by selectively inhibiting the induction of a CTL response that would normally suppress autoreactive B cells.

Induction of complement sensitivity in Escherichia coli by citric acid and low pH

AIMS

The lytic functions of the complement system play an important role in the control of Gram-negative infections. Complement-resistant Escherichia coli LP1395 (O18) grown under normal conditions can survive the bactericidal action of complement present in human serum. Towards elucidating the mechanisms of complement resistance, the resistance of E. coli LP1395 grown under conditions of low pH and in the presence of citric acid was tested.

METHODS AND RESULTS

E. coli LP1395 becomes sensitive to complement after growth in the presence of citric acid at pH 5. Complement resistance could be restored when the cells were transferred to pH 7 media. However, this recovery was greatly impaired when the cells were transferred to pH 7 media with chloramphenicol. This implies that protein synthesis may be involved in complement resistance. The cells exposed to citric acid at pH 5 showed no indication of a generalized outer membrane (OM) permeability when compared with those grown under normal conditions in terms of sensitivity to lysozyme, uptake of lipophilic dye, or sensitivity to a number of antibiotics.

CONCLUSION

Complement-resistant LP1395 may acquire a sensitivity to complement due not to a generalized disruption of the OM barrier, but possibly to the alteration of the activity of one or more normal complement resistance factors.

SIGNIFICANCE AND IMPACT OF THE STUDY

The elucidation of the mechanisms of complement resistance of Gram-negative pathogens would bring important information about bacterial infections. Complement resistance factors could also be potential targets in antimicrobial therapies.

BrkA protein of Bordetella pertussis inhibits the classical pathway of complement after C1 deposition

Bordetella pertussis produces a 73-kDa protein, BrkA (Bordetella resistance to killing), which inhibits the bactericidal activity of complement. In this study we characterized the step in the complement cascade where BrkA acts, using three strains: a wild-type strain, a strain containing an insertional disruption of brkA, and a strain containing two copies of the brkA locus. Following incubation with 10% human serum, killing was greatest for the BrkA mutant, followed by that for the wild-type strain, while the strain with two copies of brkA was the most resistant. Complement activation was monitored by enzyme-linked immunosorbent assay (ELISA) or Western blotting. ELISAs for SC5b-9, the soluble membrane attack complex, showed that production of SC5b-9 was greatest with the brkA mutant, less with the wild type, and least with the strain containing two copies of brkA. Deposition of complement proteins on the bacteria was monitored by Western blotting. A decrease in deposition on the bacteria of C4, C3, and C9 corresponded with decreased complement sensitivity. Deposition of C1, however, was not affected by the presence of BrkA. These studies show that BrkA inhibits the classical pathway of complement activation and prevents accumulation of deposited C4.

Complement deficiencies

The complement proteins play an important role in innate immunity, promoting inflammation and microbial killing. They play a role in the adaptive immune response, as well. Inherited total deficiencies of complement proteins are extremely rare. Table 1 lists more than 40 proteins that comprise the elements of the complement system. Deficiency of the proteins that promote lysis and opsonization is so rare that two papers are able to list all the observed cases to 1991. The exception is mannan-binding lectin, in which deficiency may be commoner. Diseases of regulatory proteins, such as occurs in hereditary angioedema or paroxysmal nocturnal hemoglobinuria, are commoner but still are quite rare. As we learn more about complement proteins and their mechanism of action, we will understand more clearly how these proteins function. Polymorphisms of the proteins exists. Learning how these polymorphisms contribute to the development of disease will be the focus of complement studies in the next decade.

Characterization of human bactericidal antibodies to Bordetella pertussis

The Bordetella pertussis BrkA protein protects against the bactericidal activity of complement and antibody; however, some individuals mount an immune response that overcomes this bacterial defense. To further characterize this process, the bactericidal activities of sera from 13 adults with different modes of exposure to B. pertussis (infected as adults, occupational exposure, immunized with an acellular vaccine, or no identified exposure) against a wild-type strain and a BrkA complement-sensitive mutant were evaluated. All of the sera killed the BrkA mutant, suggesting past exposure to B. pertussis or cross-reactive organisms. Several samples had no or minimal activity against the wild type. All of the sera collected from the infected and occupationally exposed individuals but not all of the sera from vaccinated individuals had bactericidal activity against the wild-type strain, suggesting that some types of exposure can induce an immune response that can overcome the BrkA resistance mechanism. Adsorbing serum with the wild-type strain removed the bactericidal antibodies; however, adsorbing the serum with a lipopolysaccharide (LPS) mutant or an avirulent (bvg mutant) strain did not always result in loss of bactericidal activity, suggesting that antibodies to either LPS or bvg-regulated proteins could be bactericidal. All the samples, including those that lacked bactericidal activity, contained antibodies that recognized the LPS of B. pertussis. Bactericidal activity correlated best with the presence of the immunoglobulin G3 (IgG3) antibodies to LPS, the IgG subtype that is most effective at fixing complement.

Characterization of BrkA expression in Bordetella bronchiseptica

BrkA confers resistance to killing by complement in Bordetella pertussis. Complement resistance in Bordetella bronchiseptica was examined. Four B. bronchiseptica strains possessed the brkA gene; however, only three expressed the protein. Only the strain lacking BrkA was susceptible to complement. Introduction of the B. pertussis brkA gene restored BrkA expression to this strain but did not confer resistance. brkA was mutated in the strains that naturally expressed BrkA, and loss of BrkA did not confer sensitivity to complement. As a species, B. bronchiseptica is more resistant to complement than B. pertussis, and BrkA does not mediate resistance.

High Dose Intravenous Immunoglobulin Does Not Affect Complement-Bacteria Interactions

Pooled IgG preparations for i.v. use (IVIg) have been shown to possess anticomplementary activity in autoimmune and systemic inflammatory diseases. Both in vitro and in vivo, IVIg is a preferential acceptor of activated C4 and C3, thus diverting complement activation from the target surface. We explored the effect of IVIg on complement-bacteria interactions in an attempt both to determine the safety of IVIg preparations in relation to natural immunity to bacteria and to extend our knowledge of the physiologic mechanism of action of IVIg. Using both complement-sensitive and complement-resistant bacterial strains, we investigated the effect of IVIg on C3 binding to bacterial surfaces. In all cases, whether complement could be directly activated by bacteria through the classical or the alternative pathway, IVIg had no effect on the amount of C3 bound to bacteria. In addition, IVIg did not inhibit complement-dependent bacterial lysis. Interestingly, increasing concentrations of IVIg induced an increase in C1q binding, suggesting the presence of low affinity complement-fixing antibacterial Abs in certain preparations. Using serum samples from patients treated with IVIg, complement binding to and lysis of complement-sensitive bacterial strains were not modified as compared with normal controls and pretreatment samples, although a decrease in C3 binding to sensitized human erythrocytes was observed. Our data suggest that IVIg does not affect direct complement-bacteria interactions, although it is a potent agent to use for diversion of complement activation on sensitized target surfaces.

Complement factors and their receptors

In summary, recent advances in molecular cloning of anaphylatoxins and the anaphylatoxin receptors add new dimensions to our investigations and understanding of the molecular mechanisms involved in anaphylatoxin action. Combining knowledge accumulated from peptide modeling of the ligands with mutagenesis studies of these ligands and their receptors makes it possible to more accurately model interactive sites and understand the sequence of molecular interactions required for cellular activation. In addition, these new developments provide valuable tools for investigating, yet unknown, activities and cellular targets of the anaphylatoxin molecules.

Animal models for complement deficiencies

The complement system plays a key role in host defense and in the development of autoimmunity. Three types of animal models of complement-mediated disease have traditionally been used: they involve normal animals, animals with spontaneously arising genetic deficiency, and animals treated with complement-inactivating agents. All of these approaches have had partial success in our attempts to understand complement mechanisms. Most animal models of genetic deficiency have been studied relatively little, as the availability of such animals is limited. C4, C2, and partial C3 deficiency in the guinea pig are well characterized, although only C4 deficiency in the guinea pig has been exclusively studied. C3 deficiency in the dog and C6 deficiency in the rabbit are well described, although studies are limited in number. C6 deficiency in the rat has been described recently and C5 deficiency in inbred mice strains has been studied fairly extensively. Factor H deficiency in the Yorkshire pig has also been described. Relatively few agents that inhibit complement are in use. Most widely used in animal studies is cobra venom factor. This inactivates the alternative complement pathway in the fluid phase and thereby depletes complement protein levels. The antigenicity of this protein, purified from the venom of cobras, limits its duration of use in most animal models. Complement-inhibiting agents are rare and, as yet, not widely used. We recently described the use of intravenous immune globulin for inhibiting complement in animal studies and present data on its use in animals, including discordant xenograft rejection, and its potential use in human disease. New developments in molecular biology provide the potential for a vast new array of deficiency models. A limited number of laboratories are actively engaged in the production of animals with inactivated genes. For example, gene knockout mice with no C3, and with no factor B, have been generated. Several complement control proteins have been prepared by genetic molecular biological techniques. Most promising among these is CR1, which limits complement damage in several animal models. Transgenic animals, which complement regulatory proteins expressed on their cells, have been prepared. As complement control proteins tend to be more efficient at regulating complement of the same species type as the regulatory protein, these animals may be useful in such areas as xenograft transplantation. The various animal models are reviewed and their potential application to understanding of human disease is emphasized.