Specific serum Ig recognizing staphylococcal wall teichoic acid induces complement-mediated opsonophagocytosis against Staphylococcus aureus.

Wall teichoic acid (WTA) of Staphylococcus aureus is a major cell envelope-associated glycopolymer that is a key molecule in promoting colonization during S. aureus infection. The complement system plays a key role in the opsonization and clearance of pathogens. We recently reported that S. aureus WTA functions as a ligand of human serum mannose-binding lectin (MBL), a recognition molecule of the lectin complement pathway. Intriguingly, serum MBL in adults does not bind to WTA because of an inhibitory effect of serum anti-WTA-IgG. In this study, serum anti-WTA-IgG was purified to homogeneity using a purified S. aureus WTA-coupled affinity column to examine the biological function of human anti-WTA-IgG. The purified anti-WTA-IgG contained the IgG2 subclass as a major component and specifically induced C4 and C3 deposition on the S. aureus surface in the anti-WTA-IgG-depleted serum, but not in C1q-deficient serum. Furthermore, the anti-WTA-IgG-dependent C3 deposition induced phagocytosis of S. aureus cells by human polymorphonuclear leukocytes. These results demonstrate that serum anti-WTA-IgG is a real trigger for the induction of classical complement-dependent opsonophagocytosis against S. aureus. Our results also support the fact that a lack of the lectin complement pathway in MBL-deficient adults is compensated by Ag-specific, Ab-mediated adaptive immunity.

L-ficolin and capsular polysaccharide-specific IgG in cord serum contribute synergistically to opsonophagocytic killing of serotype III and V group B streptococci.

Group B streptococci (GBS; Streptococcus agalactiae) are the most common cause of neonatal sepsis and meningitis. Serotype-specific IgG antibody is known to protect neonates against GBS infections by promoting opsonophagocytosis. The L-ficolin-mediated lectin pathway of the complement is also a potential mechanism for opsonization of GBS, because L-ficolin activates the complement after binding to serotype Ib, III, V, VI, and VIII GBS. In the present study, we investigated how L-ficolin and serotype-specific IgG in cord sera contribute to opsonophagocytic killing of GBS. Neither L-ficolin nor serotype-specific IgG concentrations correlated with C3b deposition on serotype Ib and VI GBS, suggesting L-ficolin- and serotype-specific IgG-independent mechanisms of complement activation. The percentage of serotype VIII GBS killed was high regardless of the concentration of L-ficolin and IgG. In contrast, L-ficolin and serotype-specific IgG can each initiate C3b deposition on serotype III and V GBS and promote phagocytosis by polymorphonuclear leukocytes, but L-ficolin and serotype-specific IgG together promote opsonophagocytic killing to a greater extent than does either alone in vitro. This synergy was observed when serotype III-specific IgG concentrations were between 1 and 6 µg/ml and when serotype V-specific IgG concentrations were between 2 and 5 µg/ml. Concentrations of serotype III-specific IgG in cord blood above 7 µg/ml are considered protective for neonates colonized with GBS, but most neonates with IgG levels of less than 7 µg/ml do not develop GBS infections. The data presented here suggest that L-ficolin enhances opsonophagocytosis of serotype III and V GBS when serotype-specific IgG alone is suboptimal for protection.

[Ultra-late onset group B streptococcal disease--a report of two cases].

Group B Streptococcus (GBS) infection in infants aged over 90 days, known as ultra-late onset disease (ULOD), is extremely rare. We present 2 cases of ULOD and investigate etiology from both the host and microbiological aspects. Case 1, 99-day-old girl born in the late preterm, had a history of 6-hour fever. Bacterial meningitis was diagnosed and the patient was treated with intravenous ampicillin for 14 days. The isolate was serotype III GBS. Case 2, a 7-month-old girl with no medically significant history had an intermittent fever for 2 weeks before admission. Serotype Ia GBS was isolated from urine and blood, leading to a diagnosis of urosepsis. Intravenous cefotaxime was administered for 7 days. Both patients were discharged without any sequelae. We examined the host risk factors for ULOD development. (i) One subject had underlying preterm birth and the other had bilateral vesicoureteral reflux. (ii) Both had extremely low serum anti-serotype specific immunoglobulin levels, an important measure of protective immunity. The anti-type Ia antibody concentration was 0.24 microg/mL and the anti-type III IgG antibody concentration was 0.25 microg/mL. We employed multilocus sequence typing (MLST) to determine the genetic background of bacterial isolates. Sequence types (STs) of isolates were ST-335 (one allele variant of ST-19) and ST-23. ST-335 is an epidemic invasive GBS disease strain in Japan and is dominantly correlated with serotype III. ST-23 is highly associated with serotype Ia and is also a common invasive type in Europe, the United States and Japan. Our findings suggest that ULOD likely develops combined with underlying host disease, immunological factors and highly virulent strains. Continuous investigation of large numbers of cases is needed to better understand ULOD etiology.

L-Ficolin/mannose-binding lectin-associated serine protease complexes bind to group B streptococci primarily through N-acetylneuraminic acid of capsular polysaccharide and activate the complement pathway.

Group B streptococci (GBS) are the most common cause of neonatal sepsis and meningitis. Most infants who are colonized with GBS at birth do not develop invasive disease, although many of these uninfected infants lack protective levels of capsular polysaccharide (CPS)-specific antibody. The lectin pathway of complement is a potential mechanism for initiating opsonization of GBS with CPS-specific antibody-deficient serum. In this study, we determined whether mannose-binding lectin (MBL)/MBL-associated serine protease (MASP) complexes and L-ficolin/MASP complexes bind to different strains of GBS to activate the lectin pathway, and we identified the molecules recognized by lectins on the GBS surface. We found that MBL did not bind to any GBS examined, whereas L-ficolin bound to GBS cells of many serotypes. L-ficolin binding to GBS cells correlated with the CPS content in serotypes Ib, III (restriction digestion pattern types III-2 and III-3), and V but not with the group B-specific polysaccharide (GBPS) content or with the lipoteichoic acid (LTA) content. L-ficolin bound to purified CPS and GBPS in a concentration-dependent manner but not to purified LTA. All strains to which L-ficolin/MASP complexes bound consumed C4. When N-acetylneuraminic acid (NeuNAc) was selectively removed from GBS cells by treatment with neuraminidase, the reduction in L-ficolin binding was correlated with the amount of NeuNAc removed. Additionally, L-ficolin was able to bind to wild-type strains but was able to bind only weakly to unencapsulated mutants and a mutant strain in which the CPS lacks NeuNAc. We concluded that L-ficolin/MASP complexes bind to GBS primarily through an interaction with NeuNAc of CPS.

[A case of group B streptococcal occult bacteremia].

We reported an infant with occult bacteremia caused by group B Streptococcus (GBS). An 8-week-old girl, who was uneventfully born to an 18-year-old mother, was hospitalized because of a 4-hour history of fever. On admission, she appeared nontoxic, and the temperature was 39.0 degrees C, and the pulse and respiratory rates were 162/min and 42/min, respectively. Laboratory findings showed a total white blood count of 5,200/microliter with 44% neutrophils and C-reactive protein of 0.7 mg/dl. Cerebrospinal fluid and urine examinations did not disclosed any abnormalities. After a complete evaluation of sepsis including cultures from blood, cerebrospinal fluid, urine, stool, and throat swab, intravenous cefotaxime was administered at 100 mg/kg/day in three fractions. Nine hours after the start of the culture, GBS was isolated from blood, and thereafter from the throat, but not from other culture sites obtained on admission. However, at that time she fed well and her temperature was subsiding. Forty-eight hours after admission, she became afebrile and cefotaxime administration was continued for 7 days. Based on the examinations of minimal inhibitory concentrations of various antibiotics, serotype analysis, and restriction-digestion patterns of genomic DNA, the 3 GBS strains isolated from the patient's blood and throat and the maternal anus were identical, suggesting that the infant was infected by her mother. This is the first report in Japan describing the clinical course of GBS occult bacteremia. According to a case series published in the English literature and our case, there are few clinical and laboratory markers predictive for GBS occult bacteremia, but this condition may develop focal invasive infections. A high index of suspicion is required for correct diagnosis. Further accumulation of such patients is warranted to establish the appropriate treatment.

Role of L-ficolin/mannose-binding lectin-associated serine protease complexes in the opsonophagocytosis of type III group B streptococci.

Serotype III group B streptococci (GBS) are a common cause of neonatal sepsis and meningitis. Although deficiency in maternal capsular polysaccharide (CPS)-specific IgG correlates with susceptibility of neonates to the GBS infection, serum deficient in CPS-specific IgG mediates significant opsonophagocytosis. This IgG-independent opsonophagocytosis requires activation of the complement pathway, a process requiring the presence of both Ca(2+) and Mg(2+), and is significantly reduced by chelating Ca(2+) with EGTA. In these studies, we defined a role of L-ficolin/mannose-binding lectin-associated serine protease (MASP) complexes in Ca(2+)-dependent, Ab-independent opsonophagocytosis of serotype III GBS. Incubation of GBS with affinity-purified L-ficolin/MASP complexes and C1q-depleted serum deficient in CPS-specific Ab supported opsonophagocytic killing, and this killing was inhibited by fluid-phase N-acetylglucosamine, the ligand for L-ficolin. Binding of L-ficolin was proportional to the CPS content of individual strains, and opsonophagocytic killing and C4 activation were inhibited by fluid-phase CPS, suggesting that L-ficolin binds to CPS. Sialic acid is known to inhibit alternative complement pathway activation, and, as expected, the bactericidal index (percentage of bacteria killed) for individual strains was inversely proportional to the sialic acid content of the CPS, and L-ficolin-initiated opsonophagocytic killing was significantly increased by addition of CPS-specific IgG2, which increased activation of the alternative pathway. We conclude that binding of L-ficolin/MASP complexes to the CPS generates C3 convertase C4b2a, which deposits C3b on GBS. C3b deposited by this lectin pathway forms alternative pathway C3 convertase C3bBb whose activity is enhanced by CPS-specific IgG2, leading to increased opsonophagocytic killing by further deposition of C3b on the GBS.

Correlation of phylogenetic lineages of group B Streptococci, identified by analysis of restriction-digestion patterns of genomic DNA, with infB alleles and mobile genetic elements.

Phylogenetic lineages of pathogenic Streptococcus agalactiae (group B streptococci [GBS]) can be identified by analysis of restriction-digestion patterns (RDPs) of chromosomal DNA. The purpose of the present study was to correlate GBS RDP types and (1) alleles of the highly conserved gene encoding translation-initiation factor IF2, infB, and/or (2) the inserted elements IS1548 and GBSi1. Only 1 combination of serotype and infB allele was found within each RDP type. Strains within a particular RDP type also tend to have the same inserted elements in each of 3 loci examined. A novel insertion sequence, designated "IS1563," was found within all RDP type II-2 strains. Most RDP types could be identified by a combination of serotype, infB allele, and inserted elements at each of the loci. These molecular markers can be used to identify GBS populations and to correlate RDP types and phylogenetic lineages identified by different methods.