Contributions of C1q, bacterial lipopolysaccharide, and porins during attachment and ingestion phases of phagocytosis by murine macrophages

Roles of the alternative complement pathway and C1q during innate immunity to Streptococcus pyogenes

Complement is important for innate immunity to the common bacterial pathogen Streptococcus pyogenes, but the relative importance of the alternative and classical pathways has not been investigated. Using mice and human serum deficient in either C1q, the first component of the classical pathway, or factor B, an important component of the alternative pathway, we have investigated the role of both pathways for innate immunity to S. pyogenes. C3b deposition on four different strains of S. pyogenes was mainly dependent on factor B. As a consequence opsonophagocytosis of S. pyogenes was reduced in serum from factor B-deficient mice, and these mice were very susceptible to S. pyogenes infection. In contrast, C3b deposition was not dependent on C1q for two of the strains investigated, H372 and H305, yet opsonophagocytosis of all four S. pyogenes strains was impaired in serum deficient in C1q. Furthermore, infection in C1q-deficient mice with strain H372 resulted in a rapidly progressive disease associated with large numbers of bacteria in target organs. These results demonstrate the important role of the alternative pathway and C1q for innate immunity to S. pyogenes and suggest that C1q-mediated innate immunity to at least some strains of S. pyogenes may involve mechanisms that are independent of C3b on the bacteria.

Increased susceptibility of C1q-deficient mice to Salmonella enterica serovar Typhimurium infection

The role of the complement system in host defense against Salmonella infection is poorly defined. Bacterial cell wall O-antigen polysaccharide can activate the alternative pathway in vitro. No studies, however, have elucidated the role of the classical pathway in immunity to Salmonella spp. in vivo. C1q-deficient mice (C1qa(-/-)) on a 129/Sv genetic background and strain-matched controls were infected intraperitoneally and intravenously with Salmonella enterica serovar Typhimurium and monitored over a 14-day period. After inoculation by either route, the C1qa(-/-) mice were found to be significantly more susceptible to Salmonella infection. Hepatic and splenic bacterial counts, performed at various time points, showed increased numbers of colonies in complement-deficient mice compared to controls. Analysis of blood clearance showed no difference between the two experimental groups during the first 15 min. However, after 20 min and until 6 h postinfection, numbers of circulating bacteria were significantly higher in complement-deficient mice. In vitro experiments using either resident or thioglycolate-elicited peritoneal macrophages showed a significant increase in the number of bacteria inside C1q-deficient macrophages compared to controls irrespective of the serum used for opsonizing the bacteria. These findings could not be explained either by an increased bacterial uptake, analyzed in vitro and in vivo using green fluorescent protein-tagged salmonellae, or by a defect in the respiratory burst or in NO production. The data presented here suggest the possibility of novel pathways by which C1q may modulate the pathogenesis of infectious diseases caused by intracellular pathogens.

The porin OmpC ofSalmonella typhimuriummediates adherence to macrophages

Macrophages recognize, adhere to, and phagocytose Salmonella typhimurium. The major outer membrane protein OmpC is a candidate ligand for macrophage recognition. To confirm this we used transposon mutagenesis to develop an ompC-deficient mutant in a known virulent strain of S. typhimurium; mutant and wild type were compared in macrophage adherence and association assays. Radiolabeled wild type S. typhimurium bound to macrophages at five-fold higher levels than did the ompC mutant. In association assays, macrophages in monolayers bound and internalized three-fold more wild type than mutant, while macrophages in suspension bound and internalized 40-fold more wild type than mutant. The ompC gene of our test strain of S. typhimurium contains several discrete differences compared with the ompC genes of Salmonella typhi and Escherichia coli. The deduced OmpC amino acid sequence of S. typhimurium shares 77 and 98% identity with OmpC amino acid sequences of E. coli and S. typhi, respectively. Evidence from this study supports a role for the OmpC protein in initial recognition by macrophages and distinguishes regions of this protein that potentially participate in host-cell recognition of bacteria by phagocytic cells.Key words: Salmonella, porin, macrophage, outer membrane protein, DNA sequencing.

Inhibition of murine macrophage protein kinase C activity by nonviable Histoplasma capsulatum

Ingestion of Histoplasma capsulatum yeast cells inhibits the oxidative burst response of murine macrophages (M phi's). Since protein kinase C (PKC) is believed to be an integral part of the signal transduction pathway involved in the production of reactive oxygen intermediates, we investigated the relationship between PKC activity and oxidative burst inhibition in H. capsulatum-containing murine peritoneal M phi's. An inhibitory effect on both basal and phorbol myristate acetate-mobilized membrane PKC activities was observed in M phi's which had ingested H. capsulatum but not latex spheres. These results suggest that one way in which H. capsulatum may disrupt the oxidative burst is through a PCK-dependent mechanism.

Binding of endotoxin to macrophages; interactions of spin-labelled saccharide residues

The molecular mechanisms of endotoxin action are poorly understood. A prerequisite to cellular activation by this agent must be interaction (binding) with the plasma membrane. In this study we have investigated the role of the polysaccharide region of endotoxin (LPS) in binding to macrophages and macrophage-like cell lines. The LPS molecules, from Escherichia coli O111.B4, J5 and the lipid-A, were spin labelled with 2,2,6,6-tetramethylpiperidine-N-oxyl] (Tempo) free radical in their sugar residues, and examined by electron spin resonance spectroscopy. This is the first report of the synthesis of spin-labelled endotoxins. Measurement of the rotational correlation times (Tc) indicated that the saccharide resides do not bind to membrane surface structures and suggests that the binding of LPS to macrophages is mediated by the lipid acyl chains. Anti-sera to LPS from E. coli O111.B4 was effective in binding to the polysaccharide of the same LPS bound to the cell surface.

Studies on the interaction of C1q, a subcomponent of the first component of complement, with porins from Salmonella minnesota incorporated into artificial membranes

Purified outer membrane proteins (OMP) of Salmonella minnesota, Re-form, were incorporated into liposomes. These induced in macrophages a chemiluminescence signal identical to that of the intact Re-form. This signal was abolished by preincubation of porin-containing liposomes with purified C1q. Incorporation of isolated OMP into black lipid membranes (BLM) resulted in channel-formation which could not be inhibited by isolated C1q. Additionally, incubation of OMP-containing liposomes with BLM resulted in pore-formation within the BLM. This was amplified when lipid A was present within the liposomes. Preincubation of OMP-containing liposomes with purified C1q abolished pore-formation within the BLM.

Specific binding of lipopolysaccharides to mouse macrophages--I. Characteristics of the interaction and inefficiency of the polysaccharide region

Tritium-labeled lipopolysaccharide interacted specifically and reversibly with mouse peritoneal macrophages. The binding was higher at 22 degrees C than at 4 degrees C, but was no longer observable at 37 degrees C. The specificity of the interaction (inhibition with unlabeled LPS) was strictly dependent on the presence of serum, and required divalent cations. The binding was saturable. The specific binding sites of peritoneal macrophages were saturated with 6-9 x 10(6) LPS molecules/cell, and those of macrophage-like cell lines with 2-3 x 10(6) molecules/cell. The binding of LPS was not inhibited by ligands of scavenger receptors (maleylated BSA) or complement receptors (zymosan), but was strongly inhibited with dexamethasone, a glucocorticoid which is known to modulate the expression of other surface markers of macrophages. The polysaccharide region of the LPS, as well as 3-deoxy-2-octulosonic acid (KDO) coupled to bovine serum albumin, did not bind to macrophages, whereas a specific binding was observed with a lipid A-BSA conjugate.

Adhesion-promoting receptors on human macrophages recognize Escherichia coli by binding to lipopolysaccharide

We report here that human macrophages bind Escherichia coli by recognizing bacterial lipopolysaccharide (LPS). Purified LPS was inserted into erythrocyte membranes, and the resulting LPS-coated red cells were bound by macrophages with the same temperature and cation dependence as observed for E. coli. When receptors for LPS were withdrawn from the plasma membrane by spreading the macrophages on LPS-coated surfaces, the binding of E. coli was blocked. We have also identified the receptors on macrophages that recognize LPS. Macrophages express three structurally homologous cell surface proteins, CR3, lymphocyte function-associated antigen (LFA-1), and p150,95. We used surface-bound monoclonal antireceptor antibodies to selectively remove these proteins from the apical surface of macrophages. We found that each of these proteins mediated the binding of E. coli to macrophages.