Contribution of a Streptococcus mutans antigen expressed by a Salmonella vector vaccine in dendritic cell activation

Toll-like receptors and Streptococcus mutans: An updated review article

It has been reported that toll-like receptors (TLRs) are the main innate immune receptors that recognize gram-positive pathogen-associated molecular patterns (PAMPs). The molecules can induce expression of the innate immune-related molecules that are essential against the bacteria. Streptococcus mutans (S. mutans) is a potential caries-associated pathogen, and innate immunity plays a key role in inhibiting its development and the progression of inflammatory responses. Recently, the roles played by TLRs against S. mutans and the induction of inflammatory responses were evaluated by several investigations. This review article discusses updated information regarding the roles played by TLRs and their potential therapeutic effects against S. mutans.

Planktonic and Biofilm-Associated Pseudomonas aeruginosa and Staphylococcus epidermidis Elicit Differential Human Peripheral Blood Cell Responses

Despite the considerable progress made in recent years, our understanding of the human immune response to microbial biofilms is still poor. The aim of the present study was to compare the in vitro response of human peripheral blood mononuclear cells (PBMC) to biofilms and planktonic cells of Pseudomonas aeruginosa and Staphylococcus epidermidis, two bacterial species particularly relevant in patients with cystic fibrosis or undergoing endovascular catheterization, respectively. PBMC isolated from healthy donors were co-cultured with 24 h-old biofilms or with exponentially growing cells of both species. Following 24 h of co-culture, the expression of early activation markers and the levels of cytokines in the culture supernatants were assessed by flow cytometry, while biofilm biomass and architecture were evaluated by crystal violet staining, CFU count, and confocal microscopy. Around 20% of PBMC was activated in response to both biofilms and planktonic cells of P. aeruginosa. In contrast, planktonic cells of S. epidermidis induced a statistically higher degree of activation than their biofilm counterpart (25% versus 15%; p < 0.01). P. aeruginosa biofilms stimulated pro-inflammatory (TNF-α, IL-1β, IFN-γ, and IL-6) and anti-inflammatory (IL-10) cytokine production at statistically significant levels higher than its planktonic counterpart, while an opposite trend was observed with S. epidermidis. Differences in the architecture of the biofilms and in the number of PBMC infiltrating the biofilms between the two bacterial species may at least partially explain these findings. Collectively, the results obtained highlighted marked differences in the host–cell response depending on the species and the mode of growth (biofilms versus planktonic cultures), allowing speculations on the different strategies adopted by P. aeruginosa and S. epidermidis to persist in the host during the course of chronic infections.

Binding forces of Streptococcus mutans P1 adhesin

Streptococcus mutans is a Gram-positive oral bacterium that is a primary etiological agent associated with human dental caries. In the oral cavity, S. mutans adheres to immobilized salivary agglutinin (SAG) contained within the salivary pellicle on the tooth surface. Binding to SAG is mediated by cell surface P1, a multifunctional adhesin that is also capable of interacting with extracellular matrix proteins. This may be of particular importance outside of the oral cavity as S. mutans has been associated with infective endocarditis and detected in atherosclerotic plaque. Despite the biomedical importance of P1, its binding mechanisms are not completely understood. In this work, we use atomic force microscopy-based single-molecule and single-cell force spectroscopy to quantify the nanoscale forces driving P1-mediated adhesion. Single-molecule experiments show that full-length P1, as well as fragments containing only the P1 globular head or C-terminal region, binds to SAG with relatively weak forces (∼50 pN). In contrast, single-cell analyses reveal that adhesion of a single S. mutans cell to SAG is mediated by strong (∼500 pN) and long-range (up to 6000 nm) forces. This is likely due to the binding of multiple P1 adhesins to self-associated gp340 glycoproteins. Such a cooperative, long-range character of the S. mutans-SAG interaction would therefore dramatically increase the strength and duration of cell adhesion. We also demonstrate, at single-molecule and single-cell levels, the interaction of P1 with fibronectin and collagen, as well as with hydrophobic, but not hydrophilic, substrates. The binding mechanism (strong forces, cooperativity, broad specificity) of P1 provides a molecular basis for its multifunctional adhesion properties. Our methodology represents a valuable approach to probe the binding forces of bacterial adhesins and offers a tractable methodology to assess anti-adhesion therapy.

Identification of a supramolecular functional architecture of Streptococcus mutans adhesin P1 on the bacterial cell surface

P1 (antigen I/II) is a sucrose-independent adhesin of Streptococcus mutans whose functional architecture on the cell surface is not fully understood. S. mutans cells subjected to mechanical extraction were significantly diminished in adherence to immobilized salivary agglutinin but remained immunoreactive and were readily aggregated by fluid-phase salivary agglutinin. Bacterial adherence was restored by incubation of postextracted cells with P1 fragments that contain each of the two known adhesive domains. In contrast to untreated cells, glutaraldehyde-treated bacteria gained reactivity with anti-C-terminal monoclonal antibodies (mAbs), whereas epitopes recognized by mAbs against other portions of the molecule were masked. Surface plasmon resonance experiments demonstrated the ability of apical and C-terminal fragments of P1 to interact. Binding of several different anti-P1 mAbs to unfixed cells triggered release of a C-terminal fragment from the bacterial surface, suggesting a novel mechanism of action of certain adherence-inhibiting antibodies. We also used atomic force microscopy-based single molecule force spectroscopy with tips bearing various mAbs to elucidate the spatial organization and orientation of P1 on living bacteria. The similar rupture lengths detected using mAbs against the head and C-terminal regions, which are widely separated in the tertiary structure, suggest a higher order architecture in which these domains are in close proximity on the cell surface. Taken together, our results suggest a supramolecular organization in which additional P1 polypeptides, including the C-terminal segment originally identified as antigen II, associate with covalently attached P1 to form the functional adhesive layer.

Antigen Delivery System II

This chapter reviews papers mostly written since 2005 that report results using live attenuated bacterial vectors to deliver after administration through mucosal surfaces, protective antigens, and DNA vaccines, encoding protective antigens to induce immune responses and/or protective immunity to pathogens that colonize on or invade through mucosal surfaces. Papers that report use of such vaccine vector systems for parenteral vaccination or to deal with nonmucosal pathogens or do not address induction of mucosal antibody and/or cellular immune responses are not reviewed.

Dormant bacteria within Staphylococcus epidermidis biofilms have low inflammatory properties and maintain tolerance to vancomycin and penicillin after entering planktonic growth

Staphylococcus epidermidis is the most commonly isolated aetiological agent of nosocomial infections, mainly due to its ability to establish biofilms on indwelling medical devices. Detachment of bacteria from S. epidermidis biofilms and subsequent growth in the planktonic form is a hallmark of the pathogenesis of these infections leading to dissemination. Here we showed that S. epidermidis cells collected from biofilms cultured in conditions that promote cell viability present marked changes in their physiological status upon initiating a planktonic mode of growth. When compared to cells growing in biofilms, they displayed an increased SYBR green I staining intensity, increased transcription of the rpiA gene, decreased transcription of the icaA gene, as well as higher susceptibility to vancomycin and penicillin. When bacteria collected from biofilms with high proportions of dormant cells were subsequently cultured in the planktonic mode, a large proportion of cells maintained a low SYBR green I staining intensity and increased resistance to vancomycin and penicillin, a profile typical of dormant cells. This phenotype further associated with a decreased ability of these biofilm-derived cells to induce the production of pro-inflammatory cytokines by bone marrow-derived dendritic cells in vitro. These results demonstrated that cells detached from the biofilm maintain a dormant cell-like phenotype, having a low pro-inflammatory effect and decreased susceptibility to antibiotics, suggesting these cells may contribute to the recalcitrant nature of biofilm infections.

Co-delivery of ccl19 gene enhances anti-caries DNA vaccine pCIA-P immunogenicity in mice by increasing dendritic cell migration to secondary lymphoid tissues

AIM

To investigate how co-delivery of the gene encoding C-C chemokine ligand-19 (CCL-19) affected the systemic immune responses to an anti-caries DNA vaccine pCIA-P in mice.

METHODS

Plasmid encoding CCL19-GFP fusion protein (pCCL19/GFP) was constructed by inserting murine ccl19 gene into GFP-expressing vector pAcGFP1-N1. Chemotactic effect of the fusion protein on murine dendritic cells (DCs) was assessed in vitro and in vivo using transwell and flow cytometric analysis, respectively. BALB/c mice were administered anti-caries DNA vaccine pCIA-P plus pCCL19/GFP (each 100 μg, im) or pCIA-P alone. Serum level of anti-PAc IgG was assessed with ELISA. Splenocytes from the mice were stimulated with PAc protein for 48 h, and IFN-γ and IL-4 production was measured with ELISA. The presence of pCCL19/GFP in spleen and draining lymph nodes was assessed using PCR. The expression of pCCL19/GFP protein in these tissues was analyzed under microscope and with flow cytometry.

RESULTS

The expression level of CCL19-GFP fusion protein was considerably increased 48 h after transfection of COS-7 cells with pCCL19/GFP plasmids. The fusion protein showed potent chemotactic activity on DCs in vitro. The level of serum PAc-specific IgG was significantly increased from 4 to 14 weeks in the mice vaccinated with pCIA-P plus pCCL19/GFP. Compared to mice vaccinated with pCIA-P alone, the splenocytes from mice vaccinated with pCIA-P plus pCCL19/GFP produced significantly higher level of IFN-γ, but IL-4 production had no significant change. Following intromuscular co-delivery, pCCL19/GFP plasmid and fusion protein were detected in the spleen and draining lymph nodes. Administration of CCL19 gene in mice markedly increased the number of mature DCs in secondary lymphoid tissues.

CONCLUSION

CCL19 serves as an effective adjuvant for anti-caries DNA vaccine by inducing chemotactic migration of DCs to secondary lymphoid tissues.