Role of complement in mouse macrophage binding of Haemophilus influenzae type b

Protection by natural human immunoglobulin M antibody to meningococcal serogroup B capsular polysaccharide in the infant rat protection assay is independent of complement-mediated bacterial lysis

Neisseria meningitidis, an important cause of bacterial meningitis and septicemia worldwide, is associated with high mortality and serious sequelae. Natural immunity against meningococcal disease develops with age, but the specificity and functional activity of natural antibodies associated with protection are poorly understood. We addressed this question by using a selected subset of prevaccination sera (n = 26) with convergent or discrepant serum bactericidal activity (SBA) and infant rat protective activity (IRPA) against the serogroup B meningococcal strain 44/76-SL (B:15:P1.7,16) from Icelandic teenagers. The sera were analyzed by opsonophagocytic activity (OPA) assay, immunoblotting, immunoglobulin G (IgG) quantitation against live meningococcal cells by flow cytometry, and enzyme immunosorbent assay (EIA). High levels of SBA and OPA were reflected in distinct IgG binding to major outer membrane proteins and/or lipopolysaccharide in immunoblots. However, we could not detect any specific antibody patterns on blots that could explain IRPA. Only IgM antibody to group B capsular polysaccharide (B-PS), measured by EIA, correlated positively (r = 0.76, P < 0.001) with IRPA. Normal human sera (NHS; n = 20) from healthy Finnish children of different ages (7, 14, and 24 months and 10 years) supported this finding and showed an age-related increase in IRPA that coincided with the acquisition of B-PS specific IgM antibody. The protection was independent of complement-mediated bacterial lysis, as detected by the inability of NHS to augment SBA in the presence of human or infant rat complement and the equal protective activity of NHS in rat strains with fully functional or C6-deficient complement.

Inactivation of deoxyadenosine methyltransferase (dam) attenuates Haemophilus influenzae virulence

Mutants in deoxyadenosine methyltransferase (dam) from many Gram-negative pathogens suggest multiple roles for Dam methylase: directing post-replicative DNA mismatch repair to the correct strand, guiding the temporal control of DNA replication and regulating the expression of multiple genes (including virulence factors) by differential promoter methylation. Dam methylase (HI0209) in strain Rd KW20 was inactivated in Haemophilus influenzae strains Rd KW20, Strain 12 and INT-1; restriction with Dam methylation-sensitive enzymes DpnI and DpnII confirmed the absence of Dam methylation, which was restored by complementation with a single copy of dam ectopically expressed in cis. Despite the lack of increased mutation frequency, the dam mutants had a 2-aminopurine-susceptible phenotype that could be suppressed by secondary mutations in mutS, suggesting a role for Dam in H. influenzae DNA mismatch repair. Invasion of human brain microvascular endothelial cells (HBMECs) and human respiratory epithelial cells (NCI-H292) by the dam mutants was significantly attenuated in all strains, suggesting the absence of a Dam-regulated event necessary for uptake or invasion of host cells. Intracellular replication was inhibited only in the Strain 12 dam mutant, whereas in the infant rat model of infection, the INT-1 dam mutant was less virulent. Dam activity appears to be necessary for both in vitro and in vivo virulence in a strain-dependent fashion and may function as a regulator of gene expression including virulence factors.

Role for complement in development of Helicobacter-induced gastritis in interleukin-10-deficient mice

The mechanisms by which the immune response can eradicate gastric Helicobacter infection are unknown. We hypothesized that Helicobacter-induced activation of the complement system could promote both inflammation and eradication of Helicobacter from the stomach. In vitro studies demonstrated that Helicobacter felis activates complement in normal mouse serum but not in serum from Rag2(-/-) mice, indicating that H. felis activates complement through the classical pathway. Next, we infected complement-depleted wild-type control and interleukin-10-deficient (IL-10(-/-)) mice with H. felis. Helicobacter infection of wild-type mice elicited a mild, focal gastritis and did not alter serum complement levels. Infection of IL-10(-/-) mice with H. felis elicited severe gastritis. After the initial colonization, the IL-10(-/-) mice completely cleared Helicobacter from the stomach by day 8. In contrast to wild-type mice, H. felis-infected IL-10(-/-) mice had a marked increase in serum complement levels. Complement depletion of wild-type mice did not affect the intensity of gastric inflammation or the extent of Helicobacter colonization compared to that for the wild-type control mice. In contrast, complement depletion of Helicobacter-infected IL-10(-/-) mice decreased the severity of gastritis, decreased the Helicobacter-induced infiltration of neutrophils into the stomach, and delayed the clearance of bacteria. In vitro studies of stimulated splenocytes and neutrophils from IL-10(-/-) mice produced a twofold increase in complement production compared to that for wild-type mice. Pretreatment with IL-10 inhibited this increase. These studies identify a role for complement in the local immune response to gastric Helicobacter in IL-10(-/-) mice and suggest a role for IL-10 in the regulation of complement production.

Two-component systems in Haemophilus influenzae: a regulatory role for ArcA in serum resistance

Knockout mutations were constructed in the arcA gene of a virulent type b strain of Haemophilus influenzae, and the behavior of the resulting mutants was investigated in a number of conditions that mimicked distinct steps in the natural infection pathway. In arcA mutants, synthesis of capsule and lipooligosaccharide (LOS) and growth in synthetic media were unaltered compared to synthesis of capsule and LOS and growth in synthetic media in the wild-type H. influenzae type b parent strain. However, the virulence of the arcA mutants for BALB/c mice was significantly reduced. Upon exposure to human blood or serum, the arcA mutants showed markedly reduced survival compared with the survival of its wild-type parent. Serum resistance could be fully restored by complementation in cis with the H. influenzae arcA gene but not by complementation in cis with the homologous gene from Escherichia coli. The proteomes of wild-type and mutant bacteria were markedly different, especially under anaerobic conditions, underscoring the global regulatory role of ArcAB in H. influenzae. Evaluation of antibody titers and classical complement activities in various serum samples pointed to complement-mediated bactericidal activity as the factor that distinguishes between the arcA mutant and wild-type phenotypes. Comparative analysis of the membrane fractions of the arcA mutants and the wild-type strain revealed several ArcA-regulated proteins, some of which may be implicated in the serum hypersensitivity phenotype.

Streptococcus suis interactions with the murine macrophage cell line J774: adhesion and cytotoxicity

Streptococcus suis capsular type 2 is an important etiological agent of swine meningitis, and it is also a zoonotic agent. Since one hypothesis of the pathogenesis of S. suis infection is that bacteria enter the bloodstream and invade the meninges and other tissues in close association with mononuclear phagocytes, the objective of the present study was to evaluate the capacity of S. suis type 2 to adhere to macrophages. An enzyme-linked immunosorbent assay technique was standardized to simply and accurately measure the rate of bacterial attachment to phagocytic cells. Results were confirmed by plate counting. Adhesion was dependent on bacterial concentration and incubation time and was not affected by cytochalasin pretreatment of macrophages. Inhibition studies showed that the sialic acid moiety of the S. suis capsule would be, at least in part, responsible for bacterial recognition by macrophages. Serum preopsonization of bacteria increased adhesion levels. Complement would be partially implicated in the serum-enhanced binding of S. suis to cells. Adhesion varied among different S. suis type 2 isolates. However, high bacterial concentrations of several isolates were cytotoxic for cells, and these cytotoxic effects correlated with suilysin production. Indeed, hemolytic strain supernatants, as well as purified suilysin, reproduced cytotoxic effects observed with live bacteria, and these effects were inhibited by cholesterol pretreatment. Bacterial adhesion and cytotoxicity were confirmed by scanning and transmission electron microscopy. We hypothesize that attachment of bacteria to phagocytes could play an important role in the pathogenesis of S. suis infection by allowing bacterial dissemination and causing a bacteremia and/or septicemia. This interaction could also be related to the activation of the host inflammatory response observed during meningitis.

Characterization of mannose receptor-dependent phagocytosis mediated by Mycobacterium tuberculosis lipoarabinomannan

The macrophage mannose receptor (MR) along with complement receptors mediates phagocytosis of the M. tuberculosis virulent strains Erdman and H37Rv. We have determined that the terminal mannosyl units of the M. tuberculosis surface lipoglycan, lipoarabinomannan (LAM), from the Erdman strain serve as ligands for the MR. The biology of the MR (receptor binding and trafficking) in response to phagocytic stimuli is not well characterized. This study analyzes the MR-dependent phagocytosis mediated by Erdman LAM presented on a 1-micron-diameter phagocytic particle. Erdman LAM microspheres exhibited a time- and dose-dependent rapid increase in attachment and internalization by human monocyte-derived macrophages (MDMs). In contrast, internalization of LAM microspheres by monocytes was minimal. Microsphere internalization by MDMs was visualized and quantitated by immunofluorescence and confocal and electron microscopy and resembled conventional phagocytosis. Phagocytosis of LAM microspheres by MDMs was energy, cytoskeleton, and calcium dependent and was mannan inhibitable. Trypsin treatment of MDMs at 37 degrees C, which depleted surface and recycling intracellular pools of the MR, reduced the subsequent attachment of LAM microspheres. Trypsin treatment at 4 degrees C allowed for subsequent recovery of LAM microsphere phagocytosis at 37 degrees C by recycled MRs. Pretreatment of MDMs with cycloheximide influenced LAM microsphere phagocytosis to only a small extent, indicating that MR-dependent phagocytosis of the microspheres was occurring primarily by preformed recycled receptors. This study characterizes the requirements for macrophage phagocytosis of a LAM-coated particle mediated by the MR. This model will be useful in further characterization of the intracellular pathway taken by phagocytic particles coated with different LAM types in macrophages following ingestion.

Effect of amplification of the Cap b locus on complement-mediated bacteriolysis and opsonization of type b Haemophilus influenzae

Amplification of the Cap b locus of Haemophilus influenzae occurs frequently in clinical isolates and has been proposed to be a mechanism by which this organism evades host defense. To determine if amplification of this locus affected complement fixation, in vitro studies to determine complement-mediated bacteriolysis and complement-mediated opsonization of an isogenic set of organisms containing 2, 3, and 4 copies of the Cap b locus were performed. Organisms containing 4 copies of the Cap b locus were significantly more resistant to antibody-dependent, classical complement pathway-directed bacteriolysis than were organisms containing 2 copies. Organisms containing 3 copies of this locus exhibited intermediate susceptibility to lysis. Complement-mediated opsonization of these organisms was assessed by determining the degree of binding of bacteria to murine or human macrophages or to nonphagocytic cells transfected with the genes for human Mac-1, the complement receptor type 3. In all three assay systems, organisms containing 4 copies of the Cap b locus bound less well than did organisms containing 2 copies of this locus. Consistent with their decreased susceptibility to lysis and opsonization, organisms with 4 copies of the Cap b locus fixed less C3 than did organisms containing 2 copies. These data demonstrate that amplification of the Cap b locus is associated with decreased susceptibility to complement-mediated lysis and decreased complement-mediated opsonization and suggest that amplification is used by these pathogens to increase their resistance to complement-dependent host defense mechanisms [correction of mecanisms].

Leishmania major-human macrophage interactions: cooperation between Mac-1 (CD11b/CD18) and complement receptor type 1 (CD35) in promastigote adhesion

It has been suggested that the developmental maturation of Leishmania major promastigotes can affect their interaction with human complement receptors. To study this, we measured the adhesion of metacyclic and logarithmic-phase L. major promastigotes to complement receptors expressed on primary macrophages, to recombinant receptors expressed on transfected cells, or to purified complement receptors in a cell-free system. We demonstrate that complement-opsonized promastigotes can bind to both Mac-1 and complement receptor type 1 (CR1) and that the transition of promastigotes from the noninfectious logarithmic phase of growth to the infectious metacyclic stage does not affect this interaction. Furthermore, we show that Mac-1 and CR1 can cooperate to mediate the efficient adhesion of complement-opsonized metacyclic promastigotes to cells expressing both receptors. On human monocyte-derived macrophages, Mac-1 appears to make a quantitatively greater contribution to this adhesion than does CR1, since blocking macrophage Mac-1 diminishes metacyclic promastigote adhesion to a greater extent than does blocking CR1. In addition, bovine monocytes lacking Mac-1 exhibit a dramatic decrease in complement-dependent promastigote adhesion, relative to normal monocytes. The predominance of Mac-1 in these interactions is due, at least in part, to the factor I cofactor activity of CR1, which facilitates the conversion of C3b to iC3b. The stable adhesion of complement-opsonized metacyclic promastigotes to Mac-1 is a prerequisite for phagocytosis by human monocyte-derived macrophages. Blocking Mac-1 on macrophages abrogates the majority of the complement-dependent phagocytosis of promastigotes, whereas blocking CR1 has no detectable effect on phagocytosis. In addition, bovine monocytes lacking Mac-1 exhibit a dramatic reduction in promastigote phagocytosis relative to normal bovine monocytes. We conclude, therefore, that the two complement receptors, Mac-1 and CR1, can cooperate to mediate the initial complement-dependent adhesion of metacyclic promastigotes to human monocyte-derived macrophages and that Mac-1 is the predominant complement receptor responsible for the phagocytosis of complement-opsonized metacyclic promastigotes.

Antibodies to Haemophilus influenzae type b polysaccharide affect bacterial adherence and multiplication

Since immunization of infants with conjugated Haemophilus influenzae type b (Hib) capsular polysaccharide (PS) vaccines results in a reduction of colonization, we determined the inhibitory effect of anti-Hib PS on two steps in the colonization, i.e., adherence of H. influenzae to nasopharyngeal epithelium and bacterial growth. Monoclonal antibody (MAb) E117-5 specific for Hib PS inhibited at a concentration of at least 80 microg/ml in vitro the adherence of Hib strain 770235f+b+ to oropharyngeal epithelial cells by 50% (P <, 0.02), but this MAb and sera from children immunized with Hib PS conjugate vaccine (n = 10) were not inhibitory in final dilutions containing up to 20 microg of anti-Hib PS per ml. The growth of Hib strain 770235f+b+ did completely stop in the presence of 5 microg of anti-Hib PS MAb E117-5 per ml and human sera with an anti-Hib PS concentration of 2 microg/ml or more, in contrast to the growth of the nonencapsular variant strain 770235f+b0.

Survival and replication of Rhodococcus equi in macrophages

Rhodococcus equi is a facultative intracellular bacterium of macrophages that can cause serious pneumonia in both young horses and immunocompromised people. Essential to understanding rhodococcus pathogenesis is a quantitative documentation of the intracellular events that follow macrophage phagocytosis of the organism. By using a bacterial immunofluorescence staining assay, we verified the intracellular survival and replicative potential of R. equi in both murine peritoneal macrophages and equine alveolar macrophages in vitro. Following an initial lag period of 6 to 12 h, the intracellular numbers of R. equi begin to rise, often reaching macrophage-compromising levels by 48 h. A quantitative determination of bacterial growth by a novel image analysis cytometry technique confirmed our fluorescence microscopic results. By 48 h postinfection, bacterial numbers had increased by more than fivefold, and the majority of infected macrophages in the monolayer contained 10 or more bacteria per cell. The intracellular organisms were viable, as evidenced by the ability to incorporate radiolabeled uracil. The use of these techniques has identified differences in the in vitro replicative capacities of a virulent strain and an avirulent strain of R. equi. A clinical isolate of R. equi expressing a 17-kDa virulence-associated plasmid-encoded antigen was able to survive and replicate within macrophages, whereas an avirulent, non-plasmid-containing strain replicated poorly. These results suggest that plasmid-encoded bacterial virulence factors may contribute to the ability of R. equi to replicate within its host cell, the macrophage.

Mechanisms of binding of Brucella abortus to mononuclear phagocytes from cows naturally resistant or susceptible to brucellosis

During the course of bovine brucellosis, Brucella abortus adheres to and infects cells of the mononuclear phagocyte system. Potential mechanisms of binding, as measured by numbers of phagocytosed bacteria, were studied in two populations of cattle genetically resistant (R) or susceptible (S) to infection with B. abortus. Live B. abortus gained entry into cultured bovine macrophages without organism-specific opsonization. Bacterial entry into macrophages from R was inhibited by the peptide RGDS, outer membrane-peptidoglycan complex from B. abortus strain RB51, anti-LFA-1 monoclonal antibody, anti-C3 antiserum, fibronectin, purified O-antigen from B. abortus lipopolysaccharide, mannan and heat-aggregated IgG. Bacterial entry into macrophages from S was inhibited by outer membrane-peptidoglycan complex, anti-LFA-1 monoclonal antibody, O-antigen and heat-aggregated IgG. The peptide RGES did not inhibit entry into macrophages from R or S. These data support the existence of organism-related receptors on monocyte-derived macrophages for B. abortus which mediate binding in the absence of serum. Secondly, there are demonstrable differences in mechanisms of binding of B. abortus to cells from cattle genetically resistant or susceptible to infection by this organism. These findings further substantiate the importance of phagocytosis and clearance functions of the mononuclear phagocyte system in resistance to bovine brucellosis. Perpetuation of infection in susceptible cattle may occur by establishing an intracellular reservoir of viable organisms. Further studies are necessary to investigate receptor affinities, and the potential for an alternate receptor for this organism in S cattle.

Complement defense mechanisms

The susceptibility of complement-deficient individuals to various severe infections, and studies of the effector mechanisms involved in the destruction of infectious agents, demonstrate the importance of complement in providing an effective host defense system. It is also becoming increasingly apparent that complement not only plays a role in 'natural' defenses against infection and in enhancing the antibody-mediated effector mechanisms, but also influences adaptive immune responses directly.

An assay to quantitate the binding of Rhodococcus equi to macrophages

A Rhodococcus equi radiobinding assay has been developed using organisms labeled with 3H-uracil. These labeled organisms resemble their unlabeled counterparts with respect to colony morphology, viability, and buoyant density. Bacteria routinely incorporate between 5 x 10(-3) and 5 x 10(-2) counts per minute per colony forming unit (cfu) which in this assay allows the detection of fewer than 0.2 cfu per macrophage. Once incorporated, greater than 90% of the label remains bacterial associated for at least 4 h postlabeling. The majority of the label is trichloroacetic acid precipitable, partitions into the aqueous phase following phenol/chloroform extraction and is ethanol precipitable. RNAse treatment of the ethanol precipitate abolishes label trichloroacetic acid precipitation. This radiolabeling technique has been used to quantitate the attachment of R. equi to both murine peritoneal and equine alveolar macrophages adherent to 13 mm glass coverslips. R. equi binding is dose dependent, saturable, and specific to macrophages. Further, binding is enhanced in the presence of fresh serum. Inhibition of radiolabeled bacterial binding can be obtained by competition with cold R. equi. This radiolabeled binding assay represents a crucial step in identifying the receptors on macrophages involved in the recognition of R. equi and may help to provide information on how macrophages recognize intracellular bacteria in general.

Macrophage phagocytosis: use of fluorescence microscopy to distinguish between extracellular and intracellular bacteria

One of the challenges of phagocytosis research is to differentiate bacteria adherent to a host cell from bacteria which the cell has internalized. To address this question, various techniques such as fluorescence microscopy, electron microscopy, and flow cytometry have been used. We have adapted a flow cytometric method (Fattorossi et al., 1989) to use fluorescence microscopy for studying phagocytosis of fluorescein-labeled Listeria by inflammatory mouse peritoneal macrophages. In this assay, ethidium bromide is used as a quenching agent and is added to cells after they have phagocytosed labeled bacteria. Ethidium bromide causes extracellular FITC-labeled Listeria to fluoresce red-orange, whereas intracellular bacteria are not exposed to the dye and remain green. This process allows distinction between intracellular and extracellular bacteria by simultaneous visualization of both populations.

Roles of complement and complement receptor type 3 in phagocytosis of Listeria monocytogenes by inflammatory mouse peritoneal macrophages

Listeria monocytogenes is a facultative intracellular bacterium that is phagocytosed by and can proliferate within cells of the mononuclear phagocyte system. However, the receptors used by macrophages to internalize this organism have not been identified. In the experiments described here, the contributions of serum complement component C3 and macrophage complement receptor type 3 (CR3) to opsonization and phagocytosis of L. monocytogenes by mouse inflammatory peritoneal macrophages were studied. An assay which allowed the distinction of adherent versus internalized bacteria was used to show that following mixing of L. monocytogenes with inflammatory macrophages, greater than 95% of the bacteria bound were internalized by these phagocytes. When immunofluorescent antibodies to C3 and immunoglobulin were used, C3 but not immunoglobulin was detected on L. monocytogenes following incubation in normal serum or ethylene glycol-bis(beta-aminoethyl ether)-N,N'-tetracetic acid-Mg(2+)-chelated serum. When macrophages were incubated with 5% serum and L. monocytogenes in a standard assay, approximately 80% of the phagocytosis was inhibited by heat-inactivated serum or by the addition of F(ab')2 anti-C3 antibody. The role of macrophage CR3 was demonstrated by the ability of anti-CR3 monoclonal antibody M1/70 to decrease phagocytosis to the same levels as those seen with heat-inactivated serum. These experiments indicated that in the presence of normal serum, L. monocytogenes is phagocytosed by inflammatory macrophages primarily because CR3 on these cells binds to C3 deposited on the bacterial surface.

Integrin-mediated localization of Bordetella pertussis within macrophages: role in pulmonary colonization

The adherence of Bordetella pertussis to human respiratory cilia is critical to the pathogenesis of whooping cough but the significance of bacterial attachment to macrophages has not been determined. Adherence to cilia and macrophages is mediated by two large, nonfimbrial bacterial proteins, filamentous hemagglutinin (FHA), and pertussis toxin (PT). PT and FHA both recognize carbohydrates on cilia and macrophages; FHA also contains an Arg-Gly-Asp (RGD) sequence which promotes bacterial association with the macrophage integrin complement receptor 3 (CR3). We determined that virulent B. pertussis enter and survive in mammalian macrophages in vitro and that CR3 is important for this uptake process. We then determined the relative contribution of CR3 versus carbohydrate-dependent interactions to in vivo pulmonary colonization using a rabbit model. B. pertussis colonized the lung as two approximately equal populations, one extracellular population attached to ciliary and macrophage surface glycoconjugates and another population within pulmonary macrophages. Loss of the CR3 interaction, either by mutation of FHA or treatment with antibody to CR3, disrupted accumulation of viable intracellular bacteria but did not prevent lung pathology. In contrast, elimination of carbohydrate-bound bacteria, either by a competitive receptor analogue or an anti-receptor antibody, was sufficient to prevent pulmonary edema. We propose that CR3-dependent localization of B. pertussis within macrophages promotes persistence of bacteria in the lung without pulmonary injury. On the other hand, the presence of extracellular bacteria adherent to cilia and macrophages in carbohydrate-dependent interactions is associated with pulmonary pathology.