Natural immunity to bacterial infections: the relation of complement to heat-labile opsonins

A versatile assay to determine bacterial and host factors contributing to opsonophagocytotic killing in hirudin-anticoagulated whole blood

Most bacteria entering the bloodstream will be eliminated through complement activation on the bacterial surface and opsonophagocytosis. However, when these protective innate immune systems do not work optimally, or when bacteria are equipped with immune evasion mechanisms that prevent killing, this can lead to serious infections such as bacteremia and meningitis, which is associated with high morbidity and mortality. In order to study the complement evasion mechanisms of bacteria and the capacity of human blood to opsonize and kill bacteria, we developed a versatile whole blood killing assay wherein both phagocyte function and complement activity can easily be monitored and modulated. In this assay we use a selective thrombin inhibitor hirudin to fully preserve complement activity of whole blood. This assay allows controlled analysis of the requirements for active complement by replacing or heat-inactivating plasma, phagocyte function and bacterial immune evasion mechanisms that contribute to survival in human blood.

Differential effects of serum heat treatment on chemotaxis and phagocytosis by human neutrophils

Neutrophils, in cooperation with serum, are vital gatekeepers of a host’s microbiome and frontline defenders against invading microbes. Yet because human neutrophils are not amenable to many biological techniques, the mechanisms governing their immunological functions remain poorly understood. We here combine state-of-the-art single-cell experiments with flow cytometry to examine how temperature-dependent heat treatment of serum affects human neutrophil interactions with “target” particles of the fungal model zymosan. Assessing separately both the chemotactic as well as the phagocytic neutrophil responses to zymosan, we find that serum heat treatment modulates these responses in a differential manner. Whereas serum treatment at 52°C impairs almost all chemotactic activity and reduces cell-target adhesion, neutrophils still readily engulf target particles that are maneuvered into contact with the cell surface under the same conditions. Higher serum-treatment temperatures gradually suppress phagocytosis even after enforced cell-target contact. Using fluorescent staining, we correlate the observed cell behavior with the amounts of C3b and IgG deposited on the zymosan surface in sera treated at the respective temperatures. This comparison not only affirms the critical role of complement in chemotactic and adhesive neutrophil interactions with fungal surfaces, but also unmasks an important participation of IgGs in the phagocytosis of yeast-like fungal particles. In summary, this study presents new insight into fundamental immune mechanisms, including the chemotactic recruitment of immune cells, the adhesive capacity of cell-surface receptors, the role of IgGs in fungal recognition, and the opsonin-dependent phagocytosis morphology of human neutrophils. Moreover, we show how, by fine-tuning the heat treatment of serum, one can selectively study chemotaxis or phagocytosis under otherwise identical conditions. These results not only refine our understanding of a widely used laboratory method, they also establish a basis for new applications of this method.

Bacteria, toxins, and the peritoneum

Intraperitoneal infections are caused by members of the gastrointestinal flora, mainly Escherichia coli, enterococci, Klebsiella, Enterobacter, Proteus, Bacteroides, anaerobic cocci, Clostridia, and Fusobacteria. The Gram-negative aerobic bacteria exert their pathogenic potential mainly through endotoxin which acts by way of mediators, causing systemic septic response and, initially, the local response of the peritoneal cavity. The main virulence factors of anaerobic bacteria are exoenzymes and capsular polysaccharides. Peritoneal infections are truly synergistic infections. The most important synergistic mechanisms are protection against host defense and creation of a suitable environment by one member of the flora for another. Aside from bacteria, certain adjuvant substances, i.e., bile, gastric juice, blood, and necrotic tissue, play a role in the pathogenesis of peritonitis. The peritoneum deals with an infection in 3 ways: first, the direct absorption of bacteria into the lymphatics via the stoma of the diaphragmatic peritoneum; second, the local destruction of bacteria through phagocytosis by either resident macrophages or polymorphonuclear granulocytes attracted to the peritoneal cavity; and third, the localization of the infection in the form of an abscess.

Quantitation of the third component of human complement attached to the surface of opsonized bacteria: opsonin-deficient sera and phagocytosis-resistant strains

The role of the third component of human complement (C3) in the opsonization of bacteria in nonimmune human sera was evaluated. The amount of C3 that becomes attached to the surface of bacteria upon incubation in serum was measured in a quantitative fluorescent immunoassay using fluorescein-conjugated monospecific antiserum to human C3. The intensity of the fluorescence from opsonized bacteria was found to be directly proportional to the absolute amount of C3 fixed, and this enabled the detection of as few as 300 molecules of bound C3 per bacterium. In normal serum the rate of C3 fixation was closely correlated with an increase in opsonization of the bacteria for human PMNs. Both C3 fixation and opsonization were maximal after 15 min of incubation. C3 fixation was also observed, albeit at a significantly slower rate, in human serum with a nonfunctional classical pathway but an intact alternative complement pathway and in serum deficient in immunoglobulins. Again, the kinetics of C3 fixation correlated with bacterial opsonization. Using a total of 21 strains of several bacterial species, including Staphylococcus aureus and Escherichia coli, encapsulation of bacteria was found to interfere with the process of C3 fixation in normal human serum, rendering these organisms resistant to subsequent phagocytosis by human polymorphonuclear leukocytes.

Efficacy of antimicrobial therapy in experimental rat pneumonia: effects of impaired phagocytosis

The importance of intact host defense mechanisms for successful antimicrobial therapy was investigated in an animal model. Recovery from lobar pneumococcal pneumonia as a result of penicillin therapy was studied in normal rats and in rats treated with cobra venom factor. This factor was used to selectively suppress the phagocytosis of pneumococci as a result of complement depletion. Although complete recovery from the infection occurred in normal rats after appropriate penicillin therapy, this was not the case in cobra venom factor-treated rats. Within the limitations of this study, evidence is presented for loss of antibiotic activity as a consequence of impaired phagocytosis.

Interactions of Pseudomonas aeruginosa with immunoglobulins and complement in sputum

The interactions of Pseudomonas aeruginosa with humoral factors in the sputum of patients with cystic fibrosis were investigated by using an indirect immunofluorescent technique. Fluorescein-conjugated, monovalent antiserum specific to heavy chains of human immunoglobulin A (IgA), IgG, or IgM and to complement C3 were used. All strains of P. aeruginosa recovered from the sputum specimens of patients with cystic fibrosis were found to be coated with antibodies of IgA, IgG, and IgM classes and with C3. The specificity of the antibody coating was determined. The fluorescence was most intense with IgA and was followed in intensity by IgG, IgM, and C3. No difference was noted between rough and mucoid strains of P. aeruginosa. When the subcultured P. aeruginosa was incubated with the sputum eluates, a similar pattern of fluorescence was demonstrated, indicating that these humoral factors are present in the sputum and that the coating process can take place in the lower respiratory tract of the patients. By single radial immunodiffusion, significant quantities of the humoral factors in the sputum eluates were detected. These findings suggest that P. aeruginosa is opsonized in sputum of patients with cystic fibrosis.

[The importance of serum components, particularly complement factors, properdin and transferrin, in inhibition of bacterial growth by human serum (author's transl)]

In this study human serum was broken down of column chromatography into its protein components, which were than examined singly and in comnination for antibacterial activity. The bactericidal effect of serum was retained to an apperciable extent in only one of six fractions. Antibacterial inhibitory activity could be restored fully by combining the fractions however. There was no correlation with the specific antibody titers. Complement had an essential part in this unspecific defence. In the absence of C'9 all other combinations of the remanining complement components exhibited no appreciable inhibitory effect. Not all complement componets are required, however, for unspcific bacterial growth inhibition. In contrast to this all complement components are necessary in the antigen-antibody reaction. Futhermore, properdin displays bactericidal activity which is related to the presence of complements. A great deal of the antibacterial activity of human serum thus arises only from the combined action of several components. These do not include proteins with a molecular weight between 100,000 and 70,000, whereby transferrin plays a part.

Protective role of smooth lipopolysaccharide in the serum bactericidal reaction

The mechanism by which smooth lipopolysaccharide (LPS) protects Salmonella typhimurium C5 from the lethal action of specific antibody and complement has been investigated. Tris(hydroxymethyl)aminomethane (Tris) and ethylenediaminetetraacetate (EDTA), which sensitize S. typhimurium C5 to the bactericidal action of immune serum, were shown to cause appreciable structural modification of the smooth LPS associated with these cells. Mg(2+) was shown to reverse the sensitization of C5 by Tris, but this divalent cation was unable to reverse the sensitizing effect of Tris plus EDTA, which, unlike Tris, caused relatively large amounts of O-antigenic components to be released from the C5 cell wall. The possibility that smooth LPS blocks the interaction of activated complement components with receptors on the cell wall is discussed.

The virulence for mice of strains of Escherichia coli related to the effects of K antigens on their resistance to phagocytosis and killing by complement

Strains of Escherichia coli with sufficient K antigen to resist killing by complement were poorly phagocytosed when injected intravenously into mice. Phagocytosis was markedly increased by anti-OK but not by anti-O sera. In contrast anti-K sera had little or no effect on the bactericidal reaction. This was not because K antigenic sites were scarce but may have been because their position was such that complement was activated at a distance from its substrate. Red cells coated with K antigen were poorly lysed by complement and anti-K serum, suggesting that the K antibody did not activate complement very effectively although again the sites may have been too superficial. The effect of K antigens on phagocytosis and complement killing or lysis could all be explained by their ability to impair protein binding.

Strains of E. coli rich in K antigen were resistant to phagocytosis and complement killing and were virulent for mice on intracerebral injection. The significance of K antigens in animal and human infections is discussed.