Contribution of different pneumococcal virulence factors to experimental meningitis in mice

Background

Pneumococcal meningitis (PM) is a life-threatening disease with a high case-fatality rate and elevated risk for serious neurological sequelae. In this study, we investigated the contribution of three major virulence factors of Streptococcus pneumoniae, the capsule, pneumococcal surface protein A (PspA) and C (PspC), to the pathogenesis of experimental PM.

Methods

Mice were challenged by the intracranial route with the serotype 4 TIGR4 strain (wt) and three isogenic mutants devoid of PspA, PspC, and the capsule. Survival, bacterial counts, and brain histology were carried out. To study the interaction between S. pneumoniae mutants and microglia, phagocytosis and survival experiments were performed using the BV2 mouse microglial cell line.

Results

Virulence of the PspC mutant was comparable to that of TIGR4. In contrast, survival of animals challenged with the PspA mutant was significantly increased compared with the wt, and the mutant was also impaired at replicating in the brain and blood of infected mice. Brain histology indicated that all strains, except for the unencapsulated mutant, caused PM. Analysis of inflammation and damage in the brain of mice infected with TIGR4 or its unencapsulated mutant demonstrated that the rough strain was unable to induce inflammation and neuronal injury, even at high challenge doses. Results with BV2 cells showed no differences in phagocytic uptake between wt and mutants. In survival assays, however, the PspA mutant showed significantly reduced survival in microglia compared with the wt.

Conclusions

PspA contributed to PM pathogenesis possibly by interacting with microglia at early infection stages, while PspC had limited importance in the disease. The rough mutant did not cause brain inflammation, neuronal damage or mouse death, strengthening the key role of the capsule in PM.

The factor H-binding fragment of PspC as a vaccine antigen for the induction of protective humoral immunity against experimental pneumococcal sepsis

Pneumococcal surface protein C (PspC) is a major virulence factor of Streptococcus pneumoniae and interferes with complement activity by binding complement factor H (fH). In this study, protection against experimental sepsis caused by pneumococci carrying different PspC variants was evaluated by immunisation with the fH-binding fragment of PspC. The mechanisms of protection mediated by antibodies to PspC were also studied. Mice were immunised with a PspC fragment (PspC(39-261)) from the type 3 strain HB565 and infected intravenously with either strain HB565 (homologous challenge), or strains D39 and TIGR4 (heterologous challenge). Immunisation with PspC(39-261) elicited high titers (>300,000) of PspC-specific serum IgG and conferred protection from challenge with HB565. In contrast, cross-protection was either limited or absent in vaccinated animals infected with D39 and TIGR4, respectively. To correlate protection with reactivity and function of PspC antibodies, pooled sera from vaccinated mice were tested in IgG binding and complement deposition experiments. IgG antibodies efficiently bound to HB565, while binding was lower with D39 and absent with TIGR4. In the presence of mouse post-immune sera, C3 deposition was increased onto HB565, while no effect was observed with D39 and TIGR4. Antibody cross-reactivity and complement deposition progressively declined with reduced amino acid identity between PspC variants. Antibodies to PspC were also found to interfere with fH binding to HB565. Finally, in vitro and ex vivo phagocytosis assays demonstrated that PspC-specific antibodies promoted opsonophagocytic killing of bacteria.

Mucosal Lymphoid Infiltrate Dominates Colonic Pathological Changes in Murine Experimental Shigellosis

BACKGROUND

Shigella species are invasive human pathogens that cause acute rectocolitis by triggering a dysregulated inflammatory reaction in the colonic and rectal mucosa. Because mice are naturally resistant to shigellosis, there is no mouse model that mimics human disease. We explore the susceptibility of intestinal flora-depleted mice to shigellosis after intragastric infection with Shigella strains.

METHODS

Mice given 5 g/L streptomycin as a beverage were infected intragastrically with 1 x 108 cfu of either invasive or noninvasive Shigella strains.

RESULTS

We found that invasive Shigella strains persist up to 30 days in feces, whereas the persistence of noninvasive Shigella strains was reduced. Colonization primarily involves the colon and the cecum and, to a lesser extent, the ileum. The hallmark of inflammation in the intestinal tissue is a dramatic expansion of the lymphoid follicles, in which a high apoptotic index is recorded.

CONCLUSIONS

We provide a murine model in which shigellae are able to reach their natural tissue target: the colon. Moreover, the absence of polymorphonuclear leukocyte recruitment and of epithelial cell lesions reveal some aspects of shigellosis that are usually hidden by the prevalence of this cell population. This novel model may contribute to the identification of new targets for vaccines and therapies.

Antibacterial activity of a competence-stimulating peptide in experimental sepsis caused by Streptococcus pneumoniae

Streptococcus pneumoniae, a major cause of human disease, produces a 17-mer autoinducer peptide pheromone (competence-stimulating peptide [CSP]) for the control of competence for genetic transformation. Due to previous work linking CSP to stress phenotypes, we set up an in vivo sepsis model to assay its effect on virulence. Our data demonstrate a significant increase in the rates of survival of mice, reductions of blood S. pneumoniae counts, and prolonged times to death for mice treated with CSP. In vitro the dose of CSP used in the animal model produced a transitory inhibition of growth. When a mutant with a mutation in the CSP sensor histidine kinase was assayed, no bacteriostatic phenotype was detected in vitro and no change in disease outcome was observed in vivo. The data demonstrate that CSP, which induces in vitro a temporary growth arrest through stimulation of its cognate histidine kinase receptor, is able to block systemic disease in mice. This therapeutic effect is novel, in that the drug-like effect is obtained by stimulation, rather than inhibition, of a bacterial drug target.

Intravenous infection of virulent shigellae causes fulminant hepatitis in mice

Shigella spp. are pathogenic bacteria responsible for bacillary dysentery in humans. The major lesions in colonic mucosa are intense inflammation with apoptosis of macrophages and release of pro-inflammatory cytokines. The study of shigellosis is hindered by the natural resistance of rodents to oral infection with Shigella. Therefore, animal models exploit other routes of infection. Here, we describe a novel murine model in which animals receive shigellae via the caudal vein. Mice infected with 5 x 10(6) (LD(50)) virulent shigellae died at 48 h post infection, whereas animals receiving non-invasive mutants survived. The liver is the main target of infection, where shigellae induce microgranuloma formation. In mice infected with invasive bacteria, high frequency of apoptotic cells is observed within hepatic microgranulomas along with significant levels of mRNA for pro-inflammatory cytokines such as IL-1beta, IL-18, IL-12 and IFN-gamma. Moreover, in the blood of these animals high levels of IL-6 and transaminases are detected. Our results demonstrate the intravenous model is suitable for pathogenicity studies and useful to explore the immune response after Shigella infection.

Pneumococcal surface protein C contributes to sepsis caused by Streptococcus pneumoniae in mice

The role of pneumococcal surface protein C (PspC; also called SpsA, CbpA, and Hic) in sepsis by Streptococcus pneumoniae was investigated in a murine infection model. The pspC gene was deleted in strains D39 (type 2) and A66 (type 3), and the mutants were tested by being injected intravenously into mice. The animals infected with the mutant strains showed a significant increase in survival, with the 50% lethal dose up to 250-fold higher than that for the wild type. Our findings indicate that PspC affords a decisive contribution to sepsis development.

Pneumococcal zinc metalloproteinase ZmpC cleaves human matrix metalloproteinase 9 and is a virulence factor in experimental pneumonia

The ZmpC zinc metalloproteinase of Streptococcus pneumoniae, annotated in the type 4 genome as SP0071, was found to cleave human matrix metalloproteinase 9 (MMP-9). The previously described IgA protease activity was confirmed to be specifically linked to the IgA1-protease/SP1154 zinc metalloproteinase. MMP-9 is a protease cleaving extracellular matrix gelatin and collagen and is activated by proteolytic cleavage like most proteases. MMP-9 is a human protease and is involved in a variety of physiological and pathological matrix degrading processes, including tissue invasion of metastases and opening of the blood-brain barrier. While TIGR4 (serotype 4) and G54 (serotype 19) pneumococcal genome strains have a highly conserved copy of zmpC, the genome of R6 (a derivative of serotype 2 D39 strain) lacks zmpC. Both the analysis for zmpC presence and MMP-9 cleavage activity in various pneumococcal strains showed correlation of ZmpC with MMP-9 cleavage activity. When assaying clinical isolates of S. pneumoniae, the zmpC gene was not found in any of the nasal and conjunctival swab isolates, but it was present in 1 out of 13 meningitis isolates and in 6 out of 11 pneumonia isolates. In a murine pneumonia model, infection with a zmpC-mutant reduced mortality at 3-4 days post-infection by 75%, when compared with infection with wild-type strains. These data indicate that the ZmpC pneumococcal protease may play a role in pneumococcal virulence and pathogenicity in the lung.

The three extra-cellular zinc metalloproteinases of Streptococcus pneumoniae have a different impact on virulence in mice

BACKGROUND

Streptococcus pneumoniae possesses large zinc metalloproteinases on its surface. To analyse the importance in virulence of three of these metalloproteinases, intranasal challenge of MF1 outbred mice was carried out using a range of infecting doses of wild type and knock-out pneumococcal mutant strains, in order to compare mice survival.

RESULTS

Observation of survival percentages over time and detection of LD50s of knock out mutants in the proteinase genes in comparison to the type 4 TIGR4 wild type strain revealed two major aspects: i) Iga and ZmpB, present in all strains of S. pneumoniae, strongly contribute to virulence in mice; (ii) ZmpC, only present in about 25% of pneumococcal strains, has a lower influence on virulence in mice.

CONCLUSIONS

These data suggest Iga, ZmpB and ZmpC as candidate surface proteins responsible for pneumococcal infection and potentially involved in distinct stages of pneumococcal disease.