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PubMed1978

Phagocytosis by sheep alveolar macrophages: relationship between opsonin concentration and light emission in the presence of luminol.

The observations reported may be applied to determining the effects of various compounds, e.g., environmental pollutants and agricultural chemicals, upon the phagocytic activity of alveolar macrophages, and the method described will aid in detecting compounds which alter Fc receptor activity. A direct linear relationship existed between the concentration of antibody used to opsonize bacterial particles and the quantity of luminol-dependent light emitted by a population of sheep alveolar macrophages exposed to the opsonized particles. The relationship can be illustrated with a Lineweaver-Burk-style double-reciprocal plot. An analogy is suggested between the kinetics of enzyme substrate reactions and the interaction of antibody-coated particles with Fc receptors on cell membranes.

Animals↗
PubMed1979

Strain specificity of opsonins for group B streptococci types II and III.

Strains of types II and III group B streptococci do not appear to be uniformly susceptible to opsonization by antibody-containing human sera, as studied using both a chemiluminescence and a radiolabeled bacterial uptake technique. We could not demonstrate a correlation of serum-sensitive or resistant strains with capsular antigen quantities, although serum absorption studies with whole organisms and HCl, trichloroacetic acid, and saline extracts indicated that the antibody to type-specific capsular polysaccharide is important in opsonizing both serum-resistant and serum-sensitive strains. Since trypsin treatment produced significantly enhanced opsonization of serum-resistant and serum-sensitive strains, proteins present on some group B streptococci may be important antiphagocytic factors.

Antibodies, Bacterial↗
PubMed1979

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.

Bacteria↗
PubMed1982

Characterization of opsonins for Bacteroides fragilis in immune sera collected from experimentally infected mice.

Serum collected from mice experimentally infected with Bacteroides fragilis 23745 (immune serum) was analyzed for its ability to opsonize the in vitro ingestion of this organism by mouse peritoneal macrophages. B. fragilis was shown to be phagocytized most efficiently in the presence of immune serum although normal mouse serum demonstrated reduced, but significant, opsonic activity. Phagocytosis was greater in the presence of serum collected from animals inoculated twice with the organism than in the presence of serum from once-inoculated animals. The increased opsonic activity of serum from twice-inoculated animals compared to singly inoculated animals was associated with increases in immunoglobulin G1(IgG1), IgG2a, and IgG2b but not IgM. Adsorption analysis of immune serum with homologous or heterologous bacterial antigens indicated that both antibody and complement act synergistically in opsonizing B. fragilis, although either alone may effectively opsonize this organism. Further evaluation of antibody-mediated opsonization revealed that prior treatment of heat-inactivated immune serum with the reducing agent 2-mercaptoethanol caused a slight, but significant, decrease in opsonic activity, thus indicating that IgM is a minor opsonizing antibody for B. fragilis. When ingestion of a B. fragilis stock strain (23745) was compared to a recent clinical isolate (C-1), it was observed that the stock strain was more easily phagocytized in the presence of normal mouse serum, thus suggesting a possible anti-opsonic-phagocytic property of the clinical strain. In addition, the clinical isolate was phagocytized to a significantly greater degree in an aerobic than an anaerobic environment. Subsequent analysis of in vitro killing of B. fragilis 23745 by peritoneal macrophages reflected the previous results in that optimal killing occurred in the presence of immune serum, although normal serum promoted phagocytic killing to an intermediate degree. Thus, these studies implicate both antibody and complement, either alone or in combination, in the opsonization of B. fragilis. Moreover, the virulence of clinical B. fragilis strains may relate to their refractoriness to opsonization and phagocytosis.

Animals↗
PubMed1986

Differential contribution of chemotaxins and opsonins to neutrophil-mediated killing of Schistosoma mansoni larvae.

In neutrophil-mediated killing of schistosomula, antibody and serum complement serve two functions: (i) opsonization of the target pathogen and (ii) generation of cell-activating, chemotactic peptides. We studied neutrophil-mediated schistosomulum killing in experiments designed to isolate these functions of antibody and complement in the cell activation response of neutrophils to external pathogens. Schistosomula opsonized by antibody or antibody plus complement induced neutrophil-mediated killing, while complement-opsonized larvae did not. Antibody enhancement of cell-mediated killing correlated with increased cell-to-parasite adherence, while cell-mediated killing in the presence of fresh normal human serum (providing chemotactic factor stimulation and complement opsonization of larvae) demonstrated a dissociation between neutrophil adhesive and killing responses. Exogenous chemotactic stimuli were found to enhance significantly neutrophil-mediated killing of both opsonized and unopsonized larvae. Experiments involving parabiotic chambers confirmed that chemotactic-factor-stimulated neutrophils may cause significant parasite mortality without direct cell-parasite contact. In further analysis of the neutrophil response to surface and fluid-phase stimulation, release of neutrophil cytotoxic mediators was found to vary according to the type of stimulus provided: cell exposure to either larvae opsonized by antibody plus complement or to chemotactic factors provoked low-to-moderate cell release of superoxide anion, granule enzymes, and arginase; when opsonic and chemotactic stimuli were combined, a greater-than-additive secretory response was noted. Under such maximal stimulation, oxidative and nonoxidative mediators were synergistic in effecting neutrophil-mediated parasite killing. The primary function of complement in cell-mediated parasite killing appears to be to promote chemotactic-factor-mediated cellular release of toxic agents and not cell-target linkage, whereas antibody-mediated adherence is associated with concurrent cellular activation. Both neutrophils adherent to the antibody-opsonized schistosomulum and chemotactic-factor-stimulated cells (adherent and not adherent to the parasite) appear to contribute significantly to its demise.

Adhesiveness↗
PubMed1992

Opsonin-independent phagocytosis of group B streptococci: role of complement receptor type three.

The role of complement receptor type 3 (CR3) in nonopsonic recognition of group B streptococci (GBS) by macrophages was investigated. Monoclonal anti-CR3 (anti-Mac-1) inhibited phagocytosis of GBS strains by as much as 50% in serum-free cultures of both mouse peritoneal macrophages and the macrophage cell line PU5-1.8. GBS uptake was unaffected by the presence of anti-C3 or salicylhydroxamate, an inhibitor of the covalent binding reaction of C3. Soluble antibodies to LFA-1 or to the common beta-chain (CD18) of the LFA-1/CR3/p150,95 family of cell adhesion molecules did not inhibit GBS uptake. Down-modulation of surface Mac-1 on macrophages following adherence to anti-Mac-1- or anti-CD18-coated surfaces also inhibited uptake of GBS. Further evidence for GBS interaction with CR3 was demonstrated by reduction of EC3bi rosette formation in macrophages adherent to GBS-coated plates. These studies suggest that GBS can interact with macrophage CR3, promoting phagocytosis in a C3-independent fashion. In the absence of specific immunity in neonates, this recognition mechanism may be a significant virulence determinant for GBS which poorly activate the alternate complement pathway.

Animals↗
PubMed2000

Intracellular trafficking and killing of Streptococcus pneumoniae by human alveolar macrophages are influenced by opsonins.

Human alveolar macrophages (HAM) are the major resident phagocytic cells of the gas-exchanging areas of the lung. Following contact with macrophages, bacteria enter phagosomes, which gradually acquire the characteristics of terminal phagolysosomes, with incorporation of lysosome-associated membrane protein (LAMP). We measured the binding of type 1 Streptococcus pneumoniae to the surface of HAM and then measured subsequent internalization and phagosomal incorporation of LAMP-1 under various opsonic conditions. Opsonization with serum containing immunoglobulin resulted in significantly greater binding of pneumococci to HAM compared with opsonization with immunoglobulin G (IgG)-depleted serum containing complement, which in turn resulted in marginally increased binding over that observed in the absence of opsonization. Internalization of opsonized S. pneumoniae gradually increased to a maximum of 20% of bound bacteria by 120 min of warm incubation, with 20% of internalized pneumococci being localized within LAMP-containing compartments by 80 min. Internalization of opsonized S. pneumoniae by HAM correlated with a reduction of bacterial viability. When inocula were adjusted so that pneumococcal binding under different conditions was equalized, subsequent internalization, trafficking to LAMP-containing compartments, and reduction of bacterial viability were less efficient in the absence of opsonization than that observed following opsonization with adsorbed or IgG-replete adsorbed serum. Once bound to the surface of HAM, pneumococci opsonized with adsorbed serum with or without IgG were internalized, processed, and killed equally well. In conclusion, binding, intracellular trafficking, and killing of S. pneumoniae by HAM are each significantly increased by opsonization with serum containing immunogloblin and/or complement.

Antigens, CD↗
PubMed2001

Antibodies against listerial protein 60 act as an opsonin for phagocytosis of Listeria monocytogenes by human dendritic cells.

Human-monocyte-derived dendritic cells (MoDC) are very efficient in the uptake of Listeria monocytogenes, a gram-positive bacterium which is an important pathogen in humans and animals causing systemic infections with symptoms such as septicemia and meningitis. In this work, we analyzed the influence of blood plasma on the internalization of L. monocytogenes into human MoDC and compared the uptake of L. monocytogenes with that of Salmonella enterica serovar Typhimurium and Yersinia enterocolitica. While human plasma did not significantly influence the uptake of serovar Typhimurium and Y. enterocolitica by human MoDC, the efficiency of the uptake of L. monocytogenes by these phagocytes was strongly enhanced by human plasma. In plasma-free medium the internalization of L. monocytogenes was very low, whereas the addition of pooled human immunoglobulins resulted in the internalization of these bacteria to a degree comparable to the highly efficient uptake observed with human plasma. All human plasma tested contained antibodies against the 60-kDa extracellular protein of L. monocytogenes (p60), and anti-p60 antibodies were also found in the commercially available pooled immunoglobulins. Strikingly, in contrast to L. monocytogenes wild type, an iap deletion mutant (totally deficient in p60) showed only a minor difference in the uptake by human MoDC in the presence or the absence of human plasma. These results support the assumption that antibodies against the listerial p60 protein may play an important role in Fc-receptor-mediated uptake of L. monocytogenes by human MoDC via opsonization of the bacteria. This process may have a major impact in preventing systemic infection in L. monocytogenes in immunocompetent humans.

Antibodies, Bacterial↗
PubMed2006

Collectins: opsonins for apoptotic cells and regulators of inflammation.

The collagenous C-type lectin family (collectins) members are humoral molecules found in the serum and on certain mucosal surfaces. In humans the family of collectins include the mannose-binding lectin, surfactant protein A and surfactant protein D. They demonstrate broad ligand specificity for both pathogenic bacteria and viruses. Over the past 5 years data have emerged indicating that these molecules are able to bind self-derived ligands in the form of apoptotic cells and regulate inflammatory responses. Furthermore, exciting new data from murine models have begun to define the in vivo importance of these molecules as regulators of inflammation and immunity. Here will discuss our current understanding of the process of collectin recognition of dying and damaged cells and its implications for autoimmune and inflammatory diseases.

Animals↗
PubMed1985

Opsonin-independent phagocytosis of Pseudomonas aeruginosa.

Three nonmucoid revertant P. aeruginosa strains from cystic fibrosis patients were phagocytized by human polymorphonuclear leukocytes in the absence of serum. Phagocytosis was inhibited by D-mannose and by mannose-containing sugars. Bacteria killed by heat or ultraviolet irradiation or grown in shaken broth were devoid of pili and resistant to nonopsonic phagocytosis. The mucoid parents of two phagocytosis-susceptible nonmucoid revertant strains were resistant to nonopsonic phagocytosis. The nonmucoid revertants were more hydrophobic in nature than the mucoid parents, but they were comparably piliated. Nonopsonic phagocytosis of P. aeruginosa by human neutrophils appears to be mediated in part by pili. Other factors such as the mucoid coating of certain strains may interfere with this process.

Humans↗
PubMed1987

Role of complement receptor type three and serum opsonins in the neutrophil response to yeast.

Previous studies have suggested that neutrophil complement receptor type three (CR3) has two binding sites: (1) a site for fixed iC3b that does not trigger ingestion or a superoxide (O2-) burst, and (2) a function-triggering site for the beta-glucan component of yeast (Saccharomyces cerevisiae) cell walls. In the present study it was found that yeast (Y) coated with C3b (YC3b) or iC3b (YC3bi), prepared with purified complement in an IgG-free system, were avidly ingested ans stimulated a vigorous O2- burst, whereas sheep erythrocytes (E) bearing C3b or iC3b, were not ingested and did not give an O2- burst. YC3b and YC3bi contained an amount of fixed C3 that was approximately equal to serum-opsonized Y (OY), and produced O2- bursts comparable to OY. Experiments utilizing rabbit F(ab')2 anticomplement receptor type one (anti-CR1) to block fixed C3b binding to CR1, and monoclonal anti-CR3 (MN-41 or OKM1) to block fixed iC3b and Y cell wall binding to CR3, indicated that the O2- burst response to OY was primarily due to fixed iC3b and Y cell wall binding to CR3. Fixed C3b (that represented 33% of the fixed C3 on OY) and IgG anti-Y antibodies that bound to CR1 and Fc receptors, respectively, were found to contribute little to the response. Although YC3b did bind avidly to neutrophil CR1, the results suggested that the O2- burst response to YC3b was triggered after the initial YC3b binding by the secondary attachment of Y cell wall components to CR3. When neutrophils were treated with anti-CR3, 90% of neutrophils bound YC3b (via CR1), but phagocytosis and an O2- burst were completely absent. Similar findings were made with OKM1-treated neutrophils and YC3bi. Responses of OKM1-treated neutrophils were inhibited because only the iC3b-binding site of CR3 was ligated by the YC3bi. Thus, fixed C3b or iC3b on Y mediate avid binding of Y to neutrophils via CR1 or the iC3b-binding site of CR3, respectively, but ingestion and an O2- burst response are only triggered when glucans in the Y cell wall secondarily bind to neutrophils via the beta-glucan binding site of CR3.

Binding Sites↗
PubMed1974

Serum heat-labile opsonins in systemic lupus erythematosus.

To study possible mechanisms responsible for the increased susceptibility to infection of patients with active systemic lupus erythematosus (SLE), a study of the serum heat-labile opsonic capacity (HLOC) in such patients was undertaken. With leukocytes from normal donors, the sera of 12 of 30 patients with active SLE demonstrated decreased HLOC for E. coli 075. The phagocytic activity was partially restored by normal serum, suggesting that decreased HLOC was responsible for the defective phagocytosis. While 8 of 10 patients with active SLE and concomitant infections showed deficient opsonic capacity to E. coli 075, only 4 of 20 such patients without infections showed the defect (P = 0.01). None of 12 patients with inactive disease had deficient opsonic capacity. Similar results were obtained with S. aureus 502A as the test bacterium. In the patients surviving infection, recovery of normal serum opsonic capacity was rapid and usually coincided with an increase of serum complement to normal levels. In three patients with active SLE and infection, the causative microorganisms were isolated and opsonic capacity for these organisms tested with the individual patients' sera. In each case, sera obtained at the onset of the infectious episode had low opsonic capacity when compared with normal sera. Serum C3 proactivator levels were low in 9 of 11 sera with deficient opsonic capacity. However, similar low values were found in other SLE sera with normal HLOC, suggesting that other factors of the opsonic system were also depleted. Addition of the classical complement components C1, C4, C2, C3, and C5 to sera with deficient HLOC failed to restore activity. Addition of pure C3 proactivator also failed to restore activity. However, addition of C3 proactivator together with 50 degrees C-heated normal serum restored activity, indicating that factors active at the early steps of opsonic activation via the alternative pathway of complement were necessary to restore opsonic activity. These findings indicate that in active SLE, a decrease of components of the alternate pathway of complement activation results in an acquired defect of serum HLOC and perhaps other related complement-mediated functions. This defect may be an important factor in the increased susceptibility to infections of patients with active systemic lupus erythematosus.

Complement C2↗