Electron microscopic study showing antibody-independent binding of C1q, a subcomponent of the first component of complement, to serum-sensitive salmonellae

The Prolonged Treatment of Salmonella enterica Strains with Human Serum Effects in Phenotype Related to Virulence

Salmonella enterica as common pathogens of humans and animals are good model organisms to conduct research on bacterial biology. Because these bacteria can multiply in both the external environments and in the living hosts, they prove their wide adaptability. It has been previously demonstrated that prolonged exposition of Salmonella serotype O48 cells to normal human serum led to an increase in resistance to sera in connection with the synthesis of very long O-antigen. In this work, we have studied the phenotype connected to virulence of Salmonella enterica strains that were subjected to consecutive passages in 50% human serum from platelet-poor plasma (SPPP). We found that eight passages in SPPP may not be enough for the bacteria to become serum-resistant (S. Typhimurium ATCC 14028, S. Senftenberg). Moreover, C1q and C3c complement components bound to Salmonellae (S. Typhimurium ATCC 14028, S. Hammonia) membrane proteins, which composition has been changed after passaging in sera. Interestingly, passages in SPPP generated genetic changes within gene fljB, which translated to cells’ motility (S. Typhimurium ATCC 14028, S. Erlangen). One strain, S. Hammonia exposed to a serum developed a multi-drug resistance (MDR) phenotype and two S. Isaszeg and S. Erlangen tolerance to disinfectants containing quaternary ammonium salts (QAS). Furthermore, colonial morphotypes of the serum adaptants were similar to those produced by starter cultures. These observations suggest that overcoming stressful conditions is manifested on many levels. Despite great phenotypic diversity occurring after prolonged exposition to SPPP, morphotypes of colonies remained unchanged in basic media. This work is an example in which stable morphotypes distinguished by altered virulence can be confusing during laboratory work with life-threatening strains.

The crystal structure of a complement-1q family protein suggests an evolutionary link to tumor necrosis factor

ACRP30--adipocyte complement-related protein of 30 kDa or AdipoQ--is an abundant serum protein, secreted exclusively from fat cells, which is implicated in energy homeostasis and obesity [1,2]. ACRP30 is a close homologue of the complement protein C1q, which is involved in the recognition of microbial surfaces [3-5] and antibody-antigen complexes [6,7] in the classical pathway of complement. We have determined the crystal structure of a homotrimeric fragment from ACRP30 at 2.1 A resolution. The structure reveals an unexpected homology to the tumor necrosis factor (TNF) family. Identical folding topologies, key residue conservations, and similarity of trimer interfaces and intron positions firmly establish an evolutionary link between the TNF and C1q families. We suggest that TNFs--which control many aspects of inflammation, adaptive immunity, apoptosis and energy homeostasis--arose by divergence from a primordial recognition molecule of the innate immune system. The evolutionary connection between C1q-like proteins and TNFs illuminates the shared functions of these two important groups of proteins.

Solid-phase C1q-directed bacterial capture followed by PCR for detection of Chlamydia trachomatis in clinical specimens

An antigen capture system based on the binding of bacteria to solid-phase immobilized complement C1q followed by PCR for detection of Chlamydia trachomatis in clinical samples was developed and clinically evaluated. Comparison of C1q-directed antigen capture PCR with cell culture and direct PCR on 71 consecutive clinical specimens revealed an identical sensitivity. In this group, all 11 cell culture-positive samples were positive by direct PCR and C1q-directed antigen capture PCR. In addition, two samples found negative by cell culture were found positive by both direct PCR and C1q-directed antigen capture PCR. To further assess the sensitivity of C1q-directed antigen capture PCR, 20 clinical samples with one to five inclusions in cell culture and 20 clinical samples with 6 to 20 inclusions in cell culture were tested. Results obtained showed sensitivities of 95 and 90% for clinical samples with 6 to 20 and 1 to 5 inclusions in cell culture, respectively. Using C1q-coated solid phases, C1q-binding Chlamydia particles can be concentrated from large volumes with concomitant removal of inhibitors of PCR, allowing the use of large volumes of clinical samples for clinical testing. Since C1q has been shown to bind to a range of gram-negative bacteria, the newly developed technique has utility for a broad range of bacteria.

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.

Role of C1q in phagocytosis of Salmonella minnesota by pulmonary endothelial cells

The Re mutant of Salmonella minnesota adheres in much greater numbers than the wild type to endothelial cells derived from the bovine pulmonary artery. Since the Re mutant is distinguished from wild-type S. minnesota by its ability to bind C1q and since endothelial cells possess receptors for C1q, we examined the role of C1q in the phagocytosis of the S. minnesota Re mutant. First, preincubating endothelial cells with C1q-enriched medium resulted in increased adherence of the Re mutant (17.9 x 10(4) versus 6.6 x 10(4]. Second, preincubating the Re mutant with C1q-enriched medium resulted in increased numbers of adherent bacteria (62.1 x 10(4) versus 6.6 x 10(4]. Preincubation of both endothelial cells and bacteria with C1q-enriched medium resulted in increased adherence above control levels but less adherence than when either cells or bacteria were preincubated separately in C1q-enriched medium. If serum depleted of C1q was used for preincubation of endothelial cells or bacteria, adherence was reduced below control levels. Thus, C1q plays an important role in the initial steps (recognition, binding, and ingestion) of phagocytosis. Next, the role of C1q was investigated in the respiratory burst response. Levels of superoxide anion released from endothelial cells 15 min after phagocytosis of the Re mutant (100 bacteria per endothelial cell) were assayed by measurement of the superoxide dismutase-inhibitable reduction of ferricytochrome c. Superoxide anion release was increased during phagocytosis of the Re mutant (35 nmol of O2- per 3 x 10(6) endothelial cells) and was also elevated above control values by incubation with soluble C1q (10 nmol of O2- per 3 x 10(6) endothelial cells). These results indicate a role for C1q in both the ingestion and the response of endothelial cells to the S. minnesota Re mutant.

Antibody-independent interactions of fibronectin, C1q, and human neutrophils with Treponema pallidum

Although recent evidence suggests that fibronectin may be involved in the attachment of treponemes to mammalian cells, its possible role in promoting phagocytosis of Treponema pallidum has not been investigated. In the present study, we examined the antibody-independent interactions of fibronectin, C1q, and human polymorphonuclear leukocytes with T. pallidum. Binding of [125I]fibronectin was specific and saturable with an affinity constant of approximately 2 X 10(7) M-1. The number of binding sites per treponeme at 37 degrees C, irrespective of the mammalian source of fibronectin, was between 2,500 and 7,500, with a mean of approximately 4,700. Binding of [125I]C1q to T. pallidum, in the absence of antibodies to the organism, also was saturable and specific. Pretreatment of treponemes with C1q enhanced binding of soluble [125I]fibronectin two- to threefold and also increased attachment of 125I-surface-labeled treponemes to fibronectin-coated surfaces. Treatment of 125I-labeled T. pallidum with fibronectin alone, or together with C1q, however, did not enhance surface phagocytosis by neutrophils.

Lipopolysaccharides as complement inhibitors by complex formation with the purified third complement component (C3)

Lipopolysaccharides (LPS) from different bacteria in smooth or rough form (Y. enterocolitica, Y. pseudotuberculosis, E. coli, S. typhimurium, S. marcescens) strongly inhibited hemolytic C3 in incubation mixtures with purified C3. LPS from a core deficient mutant was still reactive, whereas lipid A no longer affected C3 activity. The physical state of LPS was critical for its effect on C3. Strand-like LPS structures formed by Ca++-induced aggregation of solubilized LPS, as shown by electron microscopy, demonstrated the highest reactivity with C3. Inhibition of hemolytic C3 was found to be due to complex formation between LPS and C3 by a hydrophobic reaction. The binding capacity of 1 microgram LPS-R and LPS-S was as high as 125 ng C3 and 56 ng C3, respectively. The C3b fragment required different reaction conditions for maximal binding. The strong binding capacity of LPS for the complement component C3 raises the possibility that LPS act as inhibitors of complement by interruption of the reaction cascade at local infectious sites with gram-negative bacteria.

C1q, a subunit of the first component of complement, enhances binding of plasma fibronectin to bacteria

The interaction of plasma fibronectin with C1q of the complement system has been demonstrated in the past several years. In addition, the antibody-independent binding of C1q to bacteria, as well as the binding of plasma fibronectin to bacteria, is well documented. This study examines whether the binding of C1q to bacteria enhances the interaction of C1q and bacteria with plasma fibronectin. Highly purified 125I-C1q bound to several species of bacteria in the absence of antibody. The binding of 125I-C1q to bacteria was saturable and specific since the addition of unlabeled C1q inhibited binding while the presence of bovine serum albumin did not. Bacteria which had been pretreated with either buffer or unlabeled C1q were tested for their ability to bind 125I-fibronectin. When bacteria were preincubated with buffer, Staphylococcus aureus bound fivefold more 125I-fibronectin than did Escherichia coli. However, preincubation of E. coli with C1q increased the binding of 125I-fibronectin by up to 20-fold, whereas pretreatment of S. aureus with C1q increased fibronectin binding by only twofold. These results were confirmed by immunoblotting studies which demonstrated the presence of C1q, as well as an increase in fibronectin antigens on the C1q-treated bacteria as compared with the level of fibronectin on buffer-treated bacteria. In addition, preincubation of 3H-labeled bacteria with C1q enhanced their attachment to fibronectin-coated surfaces but not to albumin-coated surfaces. The biological consequences of these observations are discussed.