Transcriptional regulation, occurrence and putative role of the Pht family of Streptococcus pneumoniae

Time-resolved RNA-seq analysis to unravel the in vivo competence induction by Streptococcus pneumoniae during pneumonia-derived sepsis

Competence development in Streptococcus pneumoniae (pneumococcus) is tightly intertwined with virulence. In addition to genes encoding genetic transformation machinery, the competence regulon also regulates the expression of allolytic factors, bacteriocins, and cytotoxins. Pneumococcal competence system has been extensively interrogated in vitro where the short transient competent state upregulates the expression of three distinct phases of "early," "late," and "delayed" genes. Recently, we have demonstrated that the pneumococcal competent state develops naturally in mouse models of pneumonia-derived sepsis. To unravel the underlying adaptive mechanisms driving the development of the competent state, we conducted a time-resolved transcriptomic analysis guided by the spatiotemporal live in vivo imaging system of competence induction during pneumonia-derived sepsis. Mouse lungs infected by the serotype 2 strain D39 expressing a competent state-specific reporter gene (D39-ssbB-luc) were subjected to RNA sequencing guided by monitoring the competence development at 0, 12, 24, and, at the moribund state, >40 hours post-infection (hpi). Transcriptomic analysis revealed that the competence-specific gene expression patterns in vivo were distinct from those under in vitro conditions. There was significant upregulation of early, late, and some delayed phase competence-specific genes as early as 12 hpi, suggesting that the pneumococcal competence regulon is important for adaptation to the lung environment. Additionally, members of the histidine triad (pht) gene family were sharply upregulated at 12 hpi followed by a steep decline throughout the rest of the infection cycle, suggesting that Pht proteins participate in the early adaptation to the lung environment. Further analysis revealed that Pht proteins execute a metal ion-dependent regulatory role in competence induction.IMPORTANCEThe induction of pneumococcal competence for genetic transformation has been extensively studied in vitro but poorly understood during lung infection. We utilized a combination of live imaging and RNA sequencing to monitor the development of a competent state during acute pneumonia. Upregulation of competence-specific genes was observed as early as 12 hour post-infection, suggesting that the pneumococcal competence regulon plays an important role in adapting pneumococcus to the stressful lung environment. Among others, we report novel finding that the pneumococcal histidine triad (pht) family of genes participates in the adaptation to the lung environment and regulates pneumococcal competence induction.

Design, development, and assessment of a novel multi-peptide vaccine targeting PspC, PsaA, and PhtD proteins of Streptococcus pneumoniae

Pneumococcus is the top cause of diseases such as pneumonia/meningitis, and of secondary infections after viral respiratory diseases like COVID-19/flu. Pneumococcal protein-based vaccines consisting of proteins with various functions in virulence might provide a qualified alternative for present vaccines. In this project, PspC, PsaA, and PhtD proteins were considered to anticipate B/T-cell epitopes using immunoinformatics to develop 4 multi-peptide constructs (C, A, and D individual constructs, and a fusion construct CAD). We tested whether vaccination with CAD is able to elicit more efficient protective responses against infection than vaccination with the individual constructs or combination of C + A + D. Based on the in silico results, the constructs were predicted to be antigenic, soluble, non-toxic, and stable, and also be able to provoke humoral/cellular immune reactions. When mice were immunized with the fusion protein, significantly higher levels of IgG and cytokines were induced in serum. The IgG in the fusion group had an effective bioactivity for pneumococcus clearance utilizing the complement pathway. The mice immunized with fusion protein were the most protected from challenge. This report for the first time presents a novel multi-peptide vaccine composed of immunodominant peptides of PspC, PsaA, and PhtD. In general, the experimental results supported the immunoinformatics predictions.

Recent progress in pneumococcal protein vaccines

Pneumococcal infections continue to pose a significant global health concern, necessitating the development of effective vaccines. Despite the progress shown by pneumococcal polysaccharide and conjugate vaccines, their limited coverage and the emergence of non-vaccine serotypes have highlighted the need for alternative approaches. Protein-based pneumococcal vaccines, targeting conserved surface proteins of Streptococcus pneumoniae, have emerged as a promising strategy. In this review, we provide an overview of the advancements made in the development of pneumococcal protein vaccines. We discuss the key protein vaccine candidates, highlight their vaccination results in animal studies, and explore the challenges and future directions in protein-based pneumococcal vaccine.

In silico designing of a novel epitope-based candidate vaccine against Streptococcus pneumoniae with introduction of a new domain of PepO as adjuvant

BACKGROUND

Streptococcus pneumoniae is the leading reason for invasive diseases including pneumonia and meningitis, and also secondary infections following viral respiratory diseases such as flu and COVID-19. Currently, serotype-dependent vaccines, which have several insufficiency and limitations, are the only way to prevent pneumococcal infections. Hence, it is plain to need an alternative effective strategy for prevention of this organism. Protein-based vaccine involving conserved pneumococcal protein antigens with different roles in virulence could provide an eligible alternative to existing vaccines.

METHODS

In this study, PspC, PhtD and PsaA antigens from pneumococcus were taken to account to predict B-cell and helper T-cell epitopes, and epitope-rich regions were chosen to build the construct. To enhance the immunogenicity of the epitope-based vaccine, a truncated N-terminal fragment of pneumococcal endopeptidase O (PepO) was used as a potential TLR2/4 agonist which was identified by molecular docking studies. The ultimate construct was consisted of the chosen epitope-rich regions, along with the adjuvant role (truncated N-PepO) and suitable linkers.

RESULTS

The epitope-based vaccine was assessed as regards physicochemical properties, allergenicity, antigenicity, and toxicity. The 3D structure of the engineered construct was modeled, refined, and validated. Molecular docking and simulation of molecular dynamics (MD) indicated the proper and stable interactions between the vaccine and TLR2/4 throughout the simulation periods.

CONCLUSIONS

For the first time this work presents a novel vaccine consisting of epitopes of PspC, PhtD, and PsaA antigens which is adjuvanted with a new truncated domain of PepO. The computational outcomes revealed that the suggested vaccine could be deemed an efficient therapeutic vaccine for S. pneumoniae; nevertheless, in vitro and in vivo examinations should be performed to prove the potency of the candidate vaccine.

Protective role of PhtD and its amino and carboxyl fragments against pneumococcal sepsis

The implementation of polysaccharide-based vaccines has massively reduced the incidence of invasive pneumococcal diseases. However, there is great concern regarding serotype replacement and the increase in antibiotic resistant strains expressing non-vaccine capsular types. In addition, conjugate vaccines have high production costs, a limiting factor for their implementation in mass immunization programs in developing countries. These limitations have prompted the development of novel vaccine strategies for prevention of Streptococcus pneumoniae infections. The use of conserved pneumococcal antigens such as recombinant proteins or protein fragments presents an interesting serotype-independent alternative. Pht is a family of surface-exposed proteins which have been evaluated as potential vaccine candidates with encouraging results. The present work investigated the immune responses elicited by subcutaneous immunization of mice with the polyhistidine triad protein D (PhtD) and its amino and carboxyl terminal fragments. The proteins were immunogenic and protective against pneumococcal sepsis in mice. Antibodies raised against PhtD increased complement C3b deposition on the pneumococcal surface, mainly mediated by the alternative pathway. Sera from mice immunized with PhtD and PhtD_Cter promoted an increase in bacterial uptake by mouse phagocytes. The interaction of PhtD with the complement system regulator factor H was investigated in silico and in vitro by ELISA and western blot, confirming PhtD as a factor-H binding protein. Our results support the inclusion of PhtD and more specifically, its C-terminal fragment in a multicomponent serotype independent vaccine and suggests a role for the complement system in PhtD-mediated protection.

Broadly Reactive Human Monoclonal Antibodies Targeting the Pneumococcal Histidine Triad Protein Protect against Fatal Pneumococcal Infection

Streptococcus pneumoniae remains a leading cause of bacterial pneumonia despite the widespread use of vaccines. While vaccines are effective at reducing the incidence of most serotypes included in vaccines, a rise in infection due to nonvaccine serotypes and moderate efficacy against some vaccine serotypes have contributed to high disease incidence.

Streptococcus pneumoniae remains a leading cause of bacterial pneumonia despite the widespread use of vaccines. While vaccines are effective at reducing the incidence of most serotypes included in vaccines, a rise in infection due to nonvaccine serotypes and moderate efficacy against some vaccine serotypes have contributed to high disease incidence. Additionally, numerous isolates of S. pneumoniae are antibiotic or multidrug resistant. Several conserved pneumococcal proteins prevalent in the majority of serotypes have been examined for their potential as vaccines in preclinical and clinical trials. An additional, yet-unexplored tool for disease prevention and treatment is the use of human monoclonal antibodies (MAbs) targeting conserved pneumococcal proteins. Here, we isolated the first human MAbs (PhtD3, PhtD6, PhtD7, PhtD8, and PspA16) against the pneumococcal histidine triad protein (PhtD) and the pneumococcal surface protein A (PspA), two conserved and protective antigens. MAbs to PhtD target diverse epitopes on PhtD, and MAb PspA16 targets the N-terminal segment of PspA. The PhtD-specific MAbs bind to multiple serotypes, while MAb PspA16 serotype breadth is limited. MAbs PhtD3 and PhtD8 prolong the survival of mice infected with pneumococcal serotype 3. Furthermore, MAb PhtD3 prolongs the survival of mice in intranasal and intravenous infection models with pneumococcal serotype 4 and in mice infected with pneumococcal serotype 3 when administered 24 h after pneumococcal infection. All PhtD and PspA MAbs demonstrate opsonophagocytic activity, suggesting a potential mechanism of protection. Our results identify new human MAbs for pneumococcal disease prevention and treatment and identify epitopes on PhtD and PspA recognized by human B cells.

Understanding host immune responses to pneumococcal proteins in the upper respiratory tract to develop serotype-independent pneumococcal vaccines

Introduction: Nasopharyngeal colonization is a precondition for mucosal and invasive pneumococcal disease. Prevention of colonization may reduce pneumococcal transmission and disease incidence. Therefore, several protein-based pneumococcal vaccines are currently under investigation. Areas covered: We aimed to better understand the host immune responses to pneumococcal proteins in the upper respiratory tract (URT) that could facilitate the development of serotype-independent pneumococcal vaccines. English peer-reviewed papers reporting immunological mechanisms involved in host immune response to pneumococcal proteins in the URT were retrieved through a PubMed search using the terms 'pneumococcal proteins,' 'nasopharyngeal colonization' and/or 'cellular/humoral host immune response.' Expert opinion: Although pneumococcal protein antigens induce humoral immune responses, as well as IL-17A-mediated immunity, none of them, when used as single antigen, is sufficient to control and broadly protect against pneumococcal colonization. Novel vaccines should contain multiple conserved protein antigens to activate both arms of the immune system and evoke protection against the whole spectrum of pneumococcal variants by reducing, rather than eradicating, pneumococcal carriage. The highest efficacy would likely be achieved when the vaccine is intranasally applied, inducing mucosal immunity and enhancing the first line of defense by restricting pneumococcal density in the URT, which in turn will lead to reduced transmission and protection against disease.

Immunization with PhtD truncated fragments reduces nasopharyngeal colonization by Streptococcus pneumoniae

Despite the undeniable success of polysaccharide vaccines against Streptococcus pneumoniae infections, there is a consensus on the scientific field that this approach should be revised in order to overpass the problems related with these formulations, such as serotype replacement and high production costs. The study of conserved pneumococcal proteins or its truncated fragments has emerged as a serotype independent alternative. In this work, we have characterized the immune response elicited by systemic immunization of mice with the Histidine triad protein D (PhtD) and its' amino and carboxyl terminal fragments. The proteins were shown to be immunogenic and protective against pneumococcal colonization, with increased IL-17 production, and induction of antibodies able to limit pneumococcal adhesion to human respiratory cells. Antiserum against PhtD_Nter, but not C_ter or PhtD, promoted an increase in bacterial phagocytosis in vitro. Interestingly, antibodies against the PhtD_Nter displayed cross-reactivity with two other pneumococcal proteins, PspA and PspC, due to sequence similarities in the proline rich region of the molecules. On a whole, our results support the inclusion of PhtD, and more specifically, its N-terminal fragment, in a multicomponent serotype independent vaccine.

Evaluation of protective immunity responses against pneumococcal PhtD and its C-terminal in combination with outer-membrane vesicles as adjuvants

Introduction. Streptococcus pneumoniae is a significant bacterial pathogen in humans. Currently, there are two types of pneumococcal vaccines, but there are concerns regarding their application.

Aim. Since many pneumococcal proteins are serotype-independent, polyhistidine triad protein D (PhtD) has been selected as a vaccine candidate.

Methodology. We prepared recombinant PhtD and its C-terminal fragment (PhtD-C) using alum and outer-membrane vesicles (OMVs) as adjuvants. The combinations were injected intraperitoneally into mice, and then total immunoglobulin G (IgG) and specific IgG, IgG1 and IgG2a were measured. A serum bactericidal assay and opsonophagocytosis were also performed as complementary tests. Meningococcal OMVs were used as an adjuvant.

Results. The levels of specific IgG and IgG1 against combinations of PhtD and its C-terminal with OMVs and alum as adjuvants increased at the time of the third mouse immunization on day 35. Forty per cent and 60% of S. pneumoniae ATCC 6303 (serotype 3) as a virulent pneumococcal strain, respectively, were killed in the opsonophagocytosis test and these results could also be observed in the serum bactericidal assay. Mice mmunized iwith PhtD and its C-terminal with OMVs and alum as adjuvants survived after 10 days of pneumococcal challenge.

Conclusion. The combination of PhtD and PhtD-C with alum produced optimal results, but the combination of PhtD and PhtD-C with OMVs produced minimal results by comparison. The survival rates were also measured, and these corresponded with the results of the immunological assessments. Our findings showed that mice receiving PhtD and PhtD-C plus OMV and alum had higher survival rates than the mice in the other groups.

Efficacy, safety and immunogenicity of a pneumococcal protein-based vaccine co-administered with 13-valent pneumococcal conjugate vaccine against acute otitis media in young children: A phase IIb randomized study

BACKGROUND

Native American populations experience a substantial burden of pneumococcal disease despite use of highly effective pneumococcal conjugate vaccines (PCVs). Protein-based pneumococcal vaccines may extend protection beyond the serotype-specific protection elicited by PCVs.

METHODS

In this phase IIb, double-blind, controlled trial, 6-12 weeks-old Native American infants randomized 1:1, received either a protein-based pneumococcal vaccine (dPly/PhtD) containing pneumolysin toxoid (dPly, 10 µg) and pneumococcal histidine triad protein D (PhtD, 10 µg) or placebo, administered along with 13-valent PCV (PCV13) at ages 2, 4, 6 and 12-15 months. Other pediatric vaccines were given per the routine immunization schedule. We assessed vaccine efficacy (VE) against acute otitis media (AOM) and acute lower respiratory tract infection (ALRI) endpoints. Immunogenicity, reactogenicity and unsolicited adverse events were assessed in a sub-cohort and serious adverse events were assessed in all children.

RESULTS

1803 infants were randomized (900 dPly/PhtD; 903 Control). VE against all episodes of American Academy of Pediatrics (AAP)-defined AOM was 3.8% (95% confidence interval: -11.4, 16.9). Point estimates of VE against other AOM outcomes ranged between 2.9% (-9.5, 14.0) and 5.2% (-8.0, 16.8). Point estimates of VE against ALRI outcomes ranged between -4.4% (-39.2, 21.8) and 2.0% (-18.3, 18.8). Point estimates of VE tended to be higher against first than all episodes but the confidence intervals included zero. dPly/PhtD vaccine was immunogenic and had an acceptable reactogenicity and safety profile after primary and booster vaccination in Native American infants.

CONCLUSIONS

The dPly/PhtD vaccine was immunogenic and well tolerated, however, incremental efficacy in preventing AAP-AOM over PCV13 was not demonstrated.

CLINICAL TRIALS REGISTRATION

NCT01545375 (www.clinicaltrials.gov).

Prevalence of Various Vaccine Candidate Proteins in Clinical Isolates of Streptococcus pneumoniae: Characterization of the Novel Pht Fusion Proteins PhtA/B and PhtA/D

Pneumococcal proteins unrelated to serotypes are considered to be candidates of antigens in next-generation vaccines. In the present study, the prevalence of vaccine candidate protein genes, along with serotypes and antimicrobial resistance determinants, was investigated in a total of 57 isolates obtained from a tertiary care hospital in Japan. All of the pediatric isolates and 76.6% of the adult isolates did not belong to PCV13 (a 13-valent pneumococcal conjugate vaccine) serotypes, and 70.2% of all isolates showed multidrug resistance. All of the isolates had ply, pavA, nanA, and nanB, and high prevalence was noted for the pspA and pspC genes (96.5% and 78.9%, respectively). Detection rates for the pneumococcal histidine triad protein (Pht) genes phtA, phtB, phtD, and phtE were 49.1%, 26.3%, 61.4%, and 100%, respectively. Two fusion-type genes, phtA/B and phtA/D, were identified, with a prevalence of 36.9% and 14.0%, respectively. These fusion types showed 78.1–90.0% nucleotide sequence identity with phtA, phtB, and phtD. The most prevalent pht profile was phtA + phtD + phtE (26.3%), followed by phtA/B + phtE (19.3%) and phtA/B + phtD + phtE (17.5%), while pht profiles including phtD and/or phtA/phtD were found in 71.9% of isolates. The present study revealed the presence of two fusion types of Pht and their unexpectedly high prevalence. These fusion types, as well as PhtA and PhtB, contained sequences similar to the B cell epitopes that have been previously reported for PhtD.

Deep genome annotation of the opportunistic human pathogen Streptococcus pneumoniae D39

A precise understanding of the genomic organization into transcriptional units and their regulation is essential for our comprehension of opportunistic human pathogens and how they cause disease. Using single-molecule real-time (PacBio) sequencing we unambiguously determined the genome sequence of Streptococcus pneumoniae strain D39 and revealed several inversions previously undetected by short-read sequencing. Significantly, a chromosomal inversion results in antigenic variation of PhtD, an important surface-exposed virulence factor. We generated a new genome annotation using automated tools, followed by manual curation, reflecting the current knowledge in the field. By combining sequence-driven terminator prediction, deep paired-end transcriptome sequencing and enrichment of primary transcripts by Cappable-Seq, we mapped 1015 transcriptional start sites and 748 termination sites. We show that the pneumococcal transcriptional landscape is complex and includes many secondary, antisense and internal promoters. Using this new genomic map, we identified several new small RNAs (sRNAs), RNA switches (including sixteen previously misidentified as sRNAs), and antisense RNAs. In total, we annotated 89 new protein-encoding genes, 34 sRNAs and 165 pseudogenes, bringing the S. pneumoniae D39 repertoire to 2146 genetic elements. We report operon structures and observed that 9% of operons are leaderless. The genome data are accessible in an online resource called PneumoBrowse (https://veeninglab.com/pneumobrowse) providing one of the most complete inventories of a bacterial genome to date. PneumoBrowse will accelerate pneumococcal research and the development of new prevention and treatment strategies.

Th17 responses to pneumococcus in blood and adenoidal cells in children

Pneumococcal infections cause a large global health burden, and the search for serotype-independent vaccines continues. Existing conjugate vaccines reduce nasopharyngeal colonization by target serotypes. Such mucosal effects of novel antigens may similarly be important. CD4+ Th17 cell-dependent, antibody-independent reductions in colonization and enhanced clearance have been described in mice. Here we describe the evaluation of T helper type 17 (Th17) cytokine responses to candidate pneumococcal protein vaccine antigens in human cell culture, using adenoidal and peripheral blood mononuclear cells. Optimal detection of interleukin (IL)-17A was at day 7, and of IL-22 at day 11, in these primary cell cultures. Removal of CD45RO+ memory T cells abolished these responses. Age-associated increases in magnitude of responses were evident for IL-17A, but not IL-22, in adenoidal cells. There was a strong correlation between individual IL-17A and IL-22 responses after pneumococcal antigen stimulation (P < 0·015). Intracellular cytokine staining following phorbol myristate acetate (PMA)/ionomycin stimulation demonstrated that  > 30% CD4+ T cells positive for IL-22 express the innate markers γδT cell receptor and/or CD56, with much lower proportions for IL-17A+ cells (P < 0·001). Responses to several vaccine candidate antigens were observed but were consistently absent, particularly in blood, to PhtD (P < 0·0001), an antigen recently shown not to impact colonization in a clinical trial of a PhtD-containing conjugate vaccine in infants. The data presented and approach discussed have the potential to assist in the identification of novel vaccine antigens aimed at reducing pneumococcal carriage and transmission, thus improving the design of empirical clinical trials.

Novel Protein-Based Pneumococcal Vaccines: Assessing the Use of Distinct Protein Fragments Instead of Full-Length Proteins as Vaccine Antigens

Non-serotype-specific protein-based pneumococcal vaccines have received extensive research focus due to the limitations of polysaccharide-based vaccines. Pneumococcal proteins (PnPs), universally expressed among serotypes, may induce broader immune responses, stimulating humoral and cellular immunity, while being easier to manufacture and less expensive. Such an approach has raised issues mainly associated with sequence/level of expression variability, chemical instability, as well as possible undesirable reactogenicity and autoimmune properties. A step forward employs the identification of highly-conserved antigenic regions within PnPs with the potential to retain the benefits of protein antigens. Besides, their low-cost and stable construction facilitates the combination of several antigenic regions or peptides that may impair different stages of pneumococcal disease offering even wider serotype coverage and more efficient protection. This review discusses the up-to-date progress on PnPs that are currently under clinical evaluation and the challenges for their licensure. Focus is given on the progress on the identification of antigenic regions/peptides within PnPs and their evaluation as vaccine candidates, accessing their potential to overcome the issues associated with full-length protein antigens. Particular mention is given of the use of newer delivery system technologies including conjugation to Toll-like receptors (TLRs) and reformulation into nanoparticles to enhance the poor immunogenicity of such antigens.

Structural characterisation of the HT3 motif of the polyhistidine triad protein D from Streptococcus pneumoniae

The bacterium Streptococcus pneumoniae (the pneumococcus) is a major human pathogen that requires Zn²⁺ for its survival and virulence in the host environment. Polyhistidine triad protein D (PhtD) has a known role in pneumococcal Zn²⁺ homeostasis. However, the mechanistic basis of PhtD function remains unclear, partly due to a lack of structural information. Here, we determined the crystal structure of the fragment PhtD^269-339 , containing the third Zn²⁺ -binding histidine triad (HT) motif of the protein. Analysis of the structure suggests that Zn²⁺ binding occurs at the surface of the protein and that all five HT motifs in the protein bind Zn²⁺ and share similar structures. These new structural insights aid in our understanding of how the Pht proteins facilitate pneumococcal Zn²⁺ acquisition.

Identification of SP1683 as a pneumococcal protein that is protective against nasopharyngeal colonization

Serotype-independent protein-based pneumococcal vaccines represent attractive alternatives to capsular polysaccharide-based vaccines. The aim of this study was to identify novel immunogenic proteins from Streptococcus pneumoniae that may be used in protein-based pneumococcal vaccine. An immunoproteomics approach and a humanized severe combined immunodeficient mouse model were used to identify S. pneumoniae proteins that are immunogenic for the human immune system. Among the several proteins identified, SP1683 was selected, recombinantly produced, and infection and colonization murine models were used to evaluate the capacity of SP1683 to elicit protective responses, in comparison to known pneumococcal immunogenic proteins (PhtD and detoxified pneumolysin, dPly). Immunisation with SP1683 elicited a weaker antibody response than immunisation with PhtD and did not provide protection in the model of invasive disease. However, similar to PhtD, it was able to significantly reduce colonization in the mouse model of nasopharyngeal carriage. Treatment with anti-IL17A and anti-IL17F antibodies abolished the protection against colonization elicited by SP1683 or PhtD + dPly, which indicated that the protection afforded in this model was Th17-dependent. In conclusion, intranasal immunization with the pneumococcal protein SP1683 conferred IL17-dependent protection against nasopharyngeal carriage in mice, but systemic immunization did not protect against invasive disease. These results do not support the use of SP1683 as an isolated pneumococcal vaccine antigen. Nevertheless, SP1683 could be used as a first line of defence in formulations combining several proteins.

Pneumococcal histidine triads – involved not only in Zn2+, but also Ni2+ binding?

Polyhistidine triad proteins (with multiple copies of the HxxHxH sequence), which participate in Zn²⁺ uptake in Streptococcus pneumoniae, are also efficient Ni²⁺ binders.

A bivalent pneumococcal histidine triad protein D-choline-binding protein A vaccine elicits functional antibodies that passively protect mice from Streptococcus pneumoniae challenge

Vaccines based on conserved pneumococcal proteins are being investigated because serotype coverage by pneumococcal polysaccharide and polysaccharide conjugate vaccines is incomplete and may eventually decrease due to serotype replacement. Here, we examined the functionality of human antibodies induced by a candidate bivalent choline-binding protein A- pneumococcal histidine triad protein D (PcpA-PhtD) vaccine. Pre- and post-immune sera from subjects who had been vaccinated with the PcpA-PhtD candidate vaccine were tested in an established passive protection model in which mice were challenged by intravenous injection with Streptococcus pneumoniae serotype 3 strain A66.1. Serum antibody concentrations were determined by enzyme-linked immunosorbent assay (ELISA). Bacterial surface binding by serum antibodies was determined by a flow cytometry-based assay. Sera from 20 subjects were selected based on low activity of pre-immune samples in the passive protection model. Bacterial surface binding correlated more strongly with anti-PcpA (0.87; p < 0.0001) than with anti-PhtD (0.71; p < 0.0001). The odds ratio for predicting survival in the passive protection assay was higher for the anti-PcpA concentration (470 [95% confidence interval (CI), 46.8 to >999.9]) than for the anti-PhtD concentration (3.4 [95% CI, 1.9 to 5.6]) or bacterial surface binding (9.4 [95% CI, 3.6 to 24.3]). Pooled post-immune serum also protected mice against a challenge with S. pneumoniae serotype 3 strain WU2. Both anti-PcpA and anti-PhtD antibodies induced by the bivalent candidate vaccine mediate protection against S. pneumoniae. The results also showed that the ELISA titer might be useful as a surrogate for estimating the functional activity of antibodies induced by pneumococcal protein vaccines.

Role of Streptococcus pneumoniae Proteins in Evasion of Complement-Mediated Immunity

The complement system plays a central role in immune defense against Streptococcus pneumoniae. In order to evade complement attack, pneumococci have evolved a number of mechanisms that limit complement mediated opsonization and subsequent phagocytosis. This review focuses on the strategies employed by pneumococci to circumvent complement mediated immunity, both in vitro and in vivo. At last, since many of the proteins involved in interactions with complement components are vaccine candidates in different stages of validation, we explore the use of these antigens alone or in combination, as potential vaccine approaches that aim at elimination or drastic reduction in the ability of this bacterium to evade complement.

Protection against Streptococcus pneumoniae Invasive Pathogenesis by a Protein-Based Vaccine Is Achieved by Suppression of Nasopharyngeal Bacterial Density during Influenza A Virus Coinfection

An increase in Streptococcus pneumoniae nasopharynx (NP) colonization density during a viral coinfection initiates pathogenesis. To mimic natural S. pneumoniae pathogenesis, we commensally colonized the NPs of adult C57BL/6 mice with S. pneumoniae serotype (ST) 6A or 8 and then coinfected them with mouse-adapted H1N1 influenza A virus (PR/8/34). S. pneumoniae established effective commensal colonization, and influenza virus coinfection caused S. pneumoniae NP density to increase, resulting in bacteremia and mortality. We then studied histidine triad protein D (PhtD), an S. pneumoniae adhesin vaccine candidate, for its ability to prevent invasive S. pneumoniae disease in adult and infant mice. In adult mice, the efficacy of PhtD vaccination was compared with that of PCV13. Vaccination with PCV13 led to a greater reduction of S. pneumoniae NP density (>2.5 log units) than PhtD vaccination (∼1-log-unit reduction). However, no significant difference was observed with regard to the prevention of S. pneumoniae bacteremia, and there was no difference in mortality. Depletion of CD4⁺ T cells in PhtD-vaccinated adult mice, but not PCV13-vaccinated mice, caused a loss of vaccine-induced protection. In infant mice, passive transfer of antisera or CD4⁺ T cells from PhtD-vaccinated adult mice led to a nonsignificant reduction in NP colonization density, whereas passive transfer of antisera and CD4⁺ T cells was needed to cause a significant reduction in NP colonization density. For the first time, these data show an outcome with regard to prevention of invasive S. pneumoniae pathogenesis with a protein vaccine similar to that which occurs with a glycoconjugate vaccine despite a less robust reduction in NP bacterial density.

In vivo screen of genetically conserved Streptococcus pneumoniae proteins for protective immunogenicity

We evaluated 52 different E. coli expressed pneumococcal proteins as immunogens in a BALB/c mouse model of S. pneumoniae lung infection. Proteins were selected based on genetic conservation across disease-causing serotypes and bioinformatic prediction of antibody binding to the target antigen. Seven proteins induced protective responses, in terms of reduced lung burdens of the serotype 3 pneumococci. Three of the protective proteins were histidine triad protein family members (PhtB, PhtD and PhtE). Four other proteins, all bearing LPXTG linkage domains, also had activity in this model (PrtA, NanA, PavB and Eng). PrtA, NanA and Eng were also protective in a CBA/N mouse model of lethal pneumococcal infection. Despite data inferring widespread genomic conservation, flow-cytometer based antisera binding studies confirmed variable levels of antigen expression across a panel of pneumococcal serotypes. Finally, BALB/c mice were immunized and intranasally challenged with a viulent serotype 8 strain, to help understand the breadth of protection. Those mouse studies reaffirmed the effectiveness of the histidine triad protein grouping and a single LPXTG protein, PrtA.

Preclinical evaluation of a chemically detoxified pneumolysin as pneumococcal vaccine antigen

The use of protein antigens able to protect against the majority of Streptococcus pneumoniae serotypes is envisaged as stand-alone and/or complement to the current capsular polysaccharide-based pneumococcal vaccines. Pneumolysin (Ply) is a key virulence factor that is highly conserved in amino acid sequence across pneumococcal serotypes, and therefore may be considered as a vaccine target. However, native Ply cannot be used in vaccines due to its intrinsic cytolytic activity. In the present work a completely, irreversibly detoxified pneumolysin (dPly) has been generated using an optimized formaldehyde treatment. Detoxi-fication was confirmed by dPly challenge in mice and histological analysis of the injection site in rats. Immunization with dPly elicited Ply-specific functional antibodies that were able to inhibit Ply activity in a hemolysis assay. In addition, immunization with dPly protected mice against lethal intranasal challenge with Ply, and intranasal immunization inhibited nasopharyngeal colonization after intranasal challenge with homologous or heterologous pneumococcal strain. Our findings supported dPly as a valid candidate antigen for further pneumococcal vaccine development.

A Novel Typing Method for Streptococcus pneumoniae Using Selected Surface Proteins

The diverse pneumococcal diseases are associated with different pneumococcal lineages, or clonal complexes. Nevertheless, intra-clonal genomic variability, which influences pathogenicity, has been reported for surface virulence factors. These factors constitute the communication interface between the pathogen and its host and their corresponding genes are subjected to strong selective pressures affecting functionality and immunogenicity. First, the presence and allelic dispersion of 97 outer protein families were screened in 19 complete pneumococcal genomes. Seventeen families were deemed variable and were then examined in 216 draft genomes. This procedure allowed the generation of binary vectors with 17 positions and the classification of strains into surfotypes. They represent the outer protein subsets with the highest inter-strain discriminative power. A total of 116 non-redundant surfotypes were identified. Those sharing a critical number of common protein features were hierarchically clustered into 18 surfogroups. Most clonal complexes with comparable epidemiological characteristics belonged to the same or similar surfogroups. However, the very large CC156 clonal complex was dispersed over several surfogroups. In order to establish a relationship between surfogroup and pathogenicity, the surfotypes of 95 clinical isolates with different serogroup/serotype combinations were analyzed. We found a significant correlation between surfogroup and type of pathogenic behavior (primary invasive, opportunistic invasive, and non-invasive). We conclude that the virulent behavior of S. pneumoniae is related to the activity of collections of, rather than individual, surface virulence factors. Since surfotypes evolve faster than MLSTs and directly reflect virulence potential, this novel typing protocol is appropriate for the identification of emerging clones.

Next generation protein based Streptococcus pneumoniae vaccines

All currently available Streptococcus pneumoniae (Spn) vaccines have limitations due to their capsular serotype composition. Both the 23-valent Spn polysaccharide vaccine (PPV) and 7, 10, or 13-valent Spn conjugate vaccines (PCV-7, 10, -13) are serotype-based vaccines and therefore they elicit only serotype-specific immunity. Emergence of replacement Spn strains expressing other serotypes has consistently occurred following introduction of capsular serotype based Spn vaccines. Furthermore, capsular polysaccharide vaccines are less effective in protection against non-bacteremic pneumonia and acute otitis media (AOM) than against invasive pneumococcal disease (IPD). These shortcomings of capsular polysaccharide-based Spn vaccines have created high interest in development of non-serotype specific protein-based vaccines that could be effective in preventing both IPD and non-IPD infections. This review discusses the progress to date on development of Spn protein vaccine candidates that are highly conserved by all Spn strains, are highly conserved, exhibit maximal antigenicity and minimal reactogenicity to replace or complement the current capsule-based vaccines. Key to development of a protein based Spn vaccine is an understanding of Spn pathogenesis. Based on pathogenesis, a protein-based Spn vaccine should include one or more ingredients that reduce NP colonization below a pathogenic inoculum. Elimination of all Spn colonization may not be achievable or even advisable. The level of expression of a target protein antigen during pathogenesis is another key to the success of protein based vaccines.. As with virtually all currently licensed vaccines, production of a serum antibody response in response to protein based vaccines is anticipated to provide protection from Spn infections. A significant advantage that protein vaccine formulations can offer over capsule based vaccination is their potential benefits associated with natural priming and boosting to all strains of Spn. One of the most universal and comprehensive approaches of identifying novel vaccine candidates is the investigation of human sera from different disease stages of natural infections. Antigens that are robustly reactive in preliminary human serum screening constitute a pathogen-specific antigenome. This strategy has identified a number of Spn protein vaccine candidates that are moving forward in human clinical trials.

The First Histidine Triad Motif of PhtD Is Critical for Zinc Homeostasis in Streptococcus pneumoniae

Streptococcus pneumoniae is the world's foremost human pathogen. Acquisition of the first row transition metal ion zinc is essential for pneumococcal colonization and disease. Zinc is acquired via the ATP-binding cassette transporter AdcCB and two zinc-binding proteins, AdcA and AdcAII. We have previously shown that AdcAII is reliant upon the polyhistidine triad (Pht) proteins to aid in zinc recruitment. Pht proteins generally contain five histidine (His) triad motifs that are believed to facilitate zinc binding and therefore play a significant role in pneumococcal metal ion homeostasis. However, the importance and potential redundancy of these motifs have not been addressed. We examined the effects of mutating each of the five His triad motifs of PhtD. The combination of in vitro growth assays, active zinc uptake, and PhtD expression studies show that the His triad closest to the protein's amino terminus is the most important for zinc acquisition. Intriguingly, in vivo competitive infection studies investigating the amino- and carboxyl-terminal His triad mutants indicate that the motifs have similar importance in colonization. Collectively, our new insights into the contributions of the individual His triad motifs of PhtD, and by extension the other Pht proteins, highlight the crucial role of the first His triad site in zinc acquisition. This study also suggests that the Pht proteins likely play a role beyond zinc acquisition in pneumococcal virulence.

Streptococcus pneumoniae proteomics: determinants of pathogenesis and vaccine development

Streptococcus pneumoniae is a major pathogen that is responsible for a variety of invasive diseases. The bacteria gain entry initially by establishing a carriage state in the nasopharynx from where they migrate to other sites in the body. The worldwide distribution of the bacteria and the severity of the diseases have led to a significant level of interest in the development of vaccines against the bacteria. Current vaccines, based on the bacterial polysaccharide, have a number of limitations including poor immunogenicity and limited effectiveness against all pneumococcal serotypes. There are many challenges in developing vaccines that will be effective against the diverse range of isolates and serotypes for this highly variable bacterial pathogen. This review considers how proteomic technologies have extended our understanding of the pathogenic mechanisms of nasopharyngeal colonization and disease development as well as the critical areas in developing protein-based vaccines.

The host immune dynamics of pneumococcal colonization: implications for novel vaccine development

The human nasopharynx (NP) microbiota is complex and diverse and Streptococcus pneumoniae (pneumococcus) is a frequent member. In the first few years of life, children experience maturation of their immune system thereby conferring homeostatic balance in which pneumococci are typically rendered as harmless colonizers in the upper respiratory environment. Pneumococcal carriage declines in many children before they acquire capsular-specific antibodies, suggesting a capsule antibody-independent mechanism of natural protection against pneumococcal carriage in early childhood. A child's immune system in the first few years of life is Th2-skewed so as to avoid inflammation-induced immunopathology. Understanding Th1/Th2 and Th17 ontogeny in early life and how adjuvant vaccine formulations shift the balance of T helper-cell differentiation, may facilitate the development of new protein-based pneumococcal vaccines. This article will discuss the immune dynamics of pneumococcal colonization in infants. The discussion aims to benefit the design and improvement of protein subunit-based next-generation pneumococcal vaccines.

Discovery of Immunodominant B Cell Epitopes within Surface Pneumococcal Virulence Proteins in Pediatric Patients with Invasive Pneumococcal Disease

The identification of immunodominant B cell epitopes within surface pneumococcal virulence proteins in pediatric patients with invasive pneumococcal disease (IPD) is a valuable approach to define novel vaccine candidates. To this aim, we evaluated sera from children with IPD and age-matched controls against 141 20-mer synthetic peptides covering the entire sequence of major antigenic fragments within pneumococcal virulence proteins; namely, choline-binding protein D (CbpD), pneumococcal histidine triad proteins (PhtD and PhtE), pneumococcal surface protein A (PspA), plasminogen and fibronectin binding protein B (PfbB), and zinc metalloproteinase B (ZmpB). Ten immunodominant B cell epitopes were identified: CbpD-pep4 (amino acids (aa) 291-310), PhtD-pep11 (aa 88-107), PhtD-pep17 (aa 172-191), PhtD-pep19 (aa 200-219), PhtE-pep32 (aa 300-319), PhtE-pep40 (aa 79-98), PfbB-pep76 (aa 180-199), PfbB-pep79 (aa 222-241), PfbB-pep90 (aa 484-503), and ZmpB-pep125 (aa 431-450). All epitopes were highly conserved among different pneumococcal serotypes, and four of them were located within the functional zinc-binding domain of the histidine triad proteins PhtD and PhtE. Peptides CbpD-pep4, PhtD-pep19, and PhtE-pep40 were broadly recognized by IPD patient sera with prevalences of 96.4%, 92.9%, and 71.4%, respectively, whereas control sera exhibited only minor reactivities (<10.7%). Their specificities for IPD were 93.3%, 95%, and 96.7%; their sensitivities were 96.4%, 92.9%, and 71.4% and their positivity likelihood ratios for IPD were 14.5, 18.6, and 21.4, respectively. Furthermore, purified antibodies against CbpD-pep4, PhtD-pep19, and PhtE-pep40 readily bound on the surfaces of different pneumococcal serotypes, as assessed by FACS and immunofluorescence analysis. The identified immunodominant B cell epitopes provide a better understanding of immune response in IPD and are worth evaluation in additional studies as potential vaccine candidates.

Diversity of Pneumolysin and Pneumococcal Histidine Triad Protein D of Streptococcus pneumoniae Isolated from Invasive Diseases in Korean Children

Pneumolysin (Ply) and pneumococcal histidine triad protein D (PhtD) are candidate proteins for a next-generation pneumococcal vaccine. We aimed to analyze the genetic diversity and antigenic heterogeneity of Ply and PhtD for 173 pneumococci isolated from invasive diseases in Korean children. Allele was designated based on the variation of amino acid sequence. Antigenicity was predicted by the amino acid hydrophobicity of the region. There were seven and 39 allele types for the ply and phtD genes, respectively. The nucleotide sequence identity was 97.2%-99.9% for ply and 91.4%-98.0% for phtD gene. Only minor variations in hydrophobicity were noted among the antigenicity plots of Ply and PhtD. Overall, the allele types of the ply and phtD genes were remarkably homogeneous, and the antigenic diversity of the corresponding proteins was very limited. The Ply and PhtD could be useful antigens for universal pneumococcal vaccines.

Virulence Factors of Streptococcus pneumoniae. Comparison between African and French Invasive Isolates and Implication for Future Vaccines

Background

Many surface proteins thought to promote Streptocococcus pneumoniae virulence have recently been discovered and are currently being considered as future vaccine targets. We assessed the prevalence of 16 virulence genes among 435 S. pneumoniae invasive isolates from France and the “African meningitis belt” region, with particular focus on serotype 1 (Sp1), to compare their geographical distribution, assess their association with site of infection and evaluate their potential interest as new vaccine candidates.

Methods

Detection by PCR of pspA (+families), pspC (+pspC.4), pavA, lytA, phtA,B,D,E, nanA,B,C, rrgA (Pilus-1), sipA (Pilus-2), pcpA and psrp was performed on all isolates, as well as antibiotic resistance testing and MLVA typing (+MLST on 54 representative strains). Determination of ply alleles was performed by sequencing (Sp1 isolates).

Results

MLVA and virulence genes profiles segregated Sp1 isolates into 2 groups that followed continent distribution. The ply allele 5 and most of the genes that were variable (nanC, Pilus-2, psrp, pcpA, phtD) were present in the French Sp1 isolates (PMEN clone Sweden¹-28, ST306) but absent from the African ones. Whereas all African Sp1 isolates clustered into a single MLST CC (CC217), MLVA distinguished two CCs that followed temporal evolution. Pilus-2 and psrp were more prevalent in bacteraemic pneumonia yielded isolates and phtB in meningitis-related isolates. Considering vaccine candidates, phtD was less prevalent than anticipated (50%) and pcpA varied importantly between France and Africa (98% versus 34%). Pilus-1 was carried by 7-11% of isolates and associated with β-lactams resistance.

Conclusions

Most virulence genes were carried by the European ST306 clone but were lacking on Sp1 isolates circulating in the African meningitis belt, where a more serious pattern of infection is observed. While virulence proteins are now considered as vaccine targets, the geographical differences in their prevalence could affect the efficacy expected from future vaccines.

Trivalent pneumococcal protein recombinant vaccine protects against lethal Streptococcus pneumoniae pneumonia and correlates with phagocytosis by neutrophils during early pathogenesis

OBJECTIVE

Due to the fact that current polysaccharide-based pneumococcal vaccines have limited serotype coverage, protein-based vaccine candidates have been sought for over a decade to replace or complement current vaccines. We previously reported that a trivalent Pneumococcal Protein recombinant Vaccine (PPrV), showed protection against pneumonia and sepsis in an infant murine model. Here we investigated immunological correlates of protection of PPrV in the same model.

METHODS

C57BL/6J infant mice were intramuscularly vaccinated at age 1-3 weeks with 3 doses of PPrV, containing pneumococcal histidine triad protein D (PhtD), pneumococcal choline binding protein A (PcpA), and detoxified pneumolysin mutant PlyD1. 3-4 weeks after last vaccination, serum and lung antibody levels to PPrV components were measured, and mice were intranasally challenged with a lethal dose of Streptococcus pneumoniae (Spn) serotype 6A. Lung Spn bacterial burden, number of neutrophils and alveolar macrophages, phagocytosed Spn by granulocytes, and levels of cytokines and chemokines were determined at 6, 12, 24, and 48h after challenge.

RESULTS

PPrV vaccination conferred 83% protection against Spn challenge. Vaccinated mice had significantly elevated serum and lung antibody levels to three PPrV components. In the first stage of pathogenesis of Spn induced pneumonia (6-24h after challenge), vaccinated mice had lower Spn bacterial lung burdens and more phagocytosed Spn in the granulocytes. PPrV vaccination led to lower levels of pro-inflammatory cytokines IL-6, IL-1β, and TFN-α, and other cytokines and chemokines (IL-12, IL-17, IFN-γ, MIP-1b, MIP-2 and KC, and G-CSF), presumably due to a lower lung bacterial burden.

CONCLUSION

Trivalent PPrV vaccination results in increased serum and lung antibody levels to the vaccine components, a reduction in Spn induced lethality, enhanced early clearance of Spn in lungs due to more rapid and thorough phagocytosis of Spn by neutrophils, and correspondingly a reduction in lung inflammation and tissue damage.

Overlapping functionality of the Pht proteins in zinc homeostasis of Streptococcus pneumoniae

Streptococcus pneumoniae is a globally significant pathogen that causes a range of diseases, including pneumonia, sepsis, meningitis, and otitis media. Its ability to cause disease depends upon the acquisition of nutrients from its environment, including transition metal ions such as zinc. The pneumococcus employs a number of surface proteins to achieve this, among which are four highly similar polyhistidine triad (Pht) proteins. It has previously been established that these proteins collectively aid in the delivery of zinc to the ABC transporter substrate-binding protein AdcAII. Here we have investigated the contribution of each individual Pht protein to pneumococcal zinc homeostasis by analyzing mutant strains expressing only one of the four pht genes. Under conditions of low zinc availability, each of these mutants showed superior growth and zinc accumulation profiles relative to a mutant strain lacking all four genes, indicating that any of the four Pht proteins are able to facilitate delivery of zinc to AdcAII. However, optimal growth and zinc accumulation in vitro and pneumococcal survival and proliferation in vivo required production of all four Pht proteins, indicating that, despite their overlapping functionality, the proteins are not dispensable without incurring a fitness cost. We also show that surface-attached forms of the Pht proteins are required for zinc recruitment and that they do not contribute to defense against extracellular zinc stress.

Role of Pht proteins in attachment of Streptococcus pneumoniae to respiratory epithelial cells

Pneumococcal adherence to mucosal surfaces is a critical step in nasopharyngeal colonization, but so far few pneumococcal adhesins involved in the interaction with host cells have been identified. PhtA, PhtB, PhtD, and PhtE are conserved pneumococcal surface proteins that have proven promising as vaccine candidates. One suggested virulence function of Pht proteins is to mediate adherence at the respiratory mucosa. In this study, we assessed the role of Pht proteins in pneumococcal binding to respiratory epithelial cells. Pneumococci were incubated with human nasopharyngeal epithelial cells (Detroit-562) and lung epithelial cells (A549 and NCI-H292), and the proportion of bound bacteria was measured by plating viable counts. Strains R36A (unencapsulated), D39 (serotype 2), 43 (serotype 3), 4-CDC (serotype 4), and 2737 (serotype 19F) with one or more of the four homologous Pht proteins deleted were compared with their wild-type counterparts. Also, the effect of anti-PhtD antibodies on the adherence of strain 2737 to the respiratory epithelial cells was studied. Our results suggest that Pht proteins play a role in pneumococcal adhesion to the respiratory epithelium. We also found that antibody to PhtD is able to inhibit bacterial attachment to the cells, suggesting that antibodies against PhtD present at mucosal surfaces might protect from pneumococcal attachment and subsequent colonization. However, the relative significance of Pht proteins to the ability of pneumococci to bind in vitro to epithelial cells depends on the genetic background and the capsular serotype of the strain.

The roles of transition metals in the physiology and pathogenesis of Streptococcus pneumoniae

For bacterial pathogens whose sole environmental reservoir is the human host, the acquisition of essential nutrients, particularly transition metals, is a critical aspect of survival due to tight sequestration and limitation strategies deployed to curtail pathogen outgrowth. As such, these bacteria have developed diverse, specialized acquisition mechanisms to obtain these metals from the niches of the body in which they reside. To oppose the spread of infection, the human host has evolved multiple mechanisms to counter bacterial invasion, including sequestering essential metals away from bacteria and exposing bacteria to lethal concentrations of metals. Hence, to maintain homeostasis within the host, pathogens must be able to acquire necessary metals from host proteins and to export such metals when concentrations become detrimental. Furthermore, this acquisition and efflux equilibrium must occur in a tissue-specific manner because the concentration of metals varies greatly within the various microenvironments of the human body. In this review, we examine the functional roles of the metal import and export systems of the Gram-positive pathogen Streptococcus pneumoniae in both signaling and pathogenesis.

New insights into histidine triad proteins: solution structure of a Streptococcus pneumoniae PhtD domain and zinc transfer to AdcAII

Zinc (Zn²⁺) homeostasis is critical for pathogen host colonization and invasion. Polyhistidine triad (Pht) proteins, located at the surface of various streptococci, have been proposed to be involved in Zn²⁺ homeostasis. The phtD gene, coding for a Zn²⁺-binding protein, is organized in an operon with adcAII coding for the extracellular part of a Zn²⁺ transporter. In the present work, we investigate the relationship between PhtD and AdcAII using biochemical and structural biology approaches. Immuno-precipitation experiments on purified membranes of Streptococcus pneumoniae (S. pneumoniae) demonstrate that native PhtD and AdcAII interact in vivo confirming our previous in vitro observations. NMR was used to demonstrate Zn²⁺ transfer from the Zn²⁺-bound form of a 137 amino acid N-terminal domain of PhtD (t-PhtD) to AdcAII. The high resolution NMR structure of t-PhtD shows that Zn²⁺ is bound in a tetrahedral site by histidines 83, 86, and 88 as well as by glutamate 63. Comparison of the NMR parameters measured for apo- and Zn²⁺-t-PhtD shows that the loss of Zn²⁺ leads to a diminished helical propensity at the C-terminus and increases the local dynamics and overall molecular volume. Structural comparison with the crystal structure of a 55-long fragment of PhtA suggests that Pht proteins are built from short repetitive units formed by three β-strands containing the conserved HxxHxH motif. Taken together, these results support a role for S. pneumoniae PhtD as a Zn²⁺ scavenger for later release to the surface transporter AdcAII, leading to Zn²⁺ uptake.

Vaccination against Streptococcus pneumoniae using truncated derivatives of polyhistidine triad protein D

Polyhistidine triad protein D (PhtD) has been described as a promising vaccine candidate for use against Streptococcus pneumoniae, but there has been a lack of examination of its structure and of which region(s) of the protein are targeted by protective immune responses. In this study, we purified recombinant truncated derivatives of PhtD and examined their secondary structural composition, as well as their capacity to bind antibodies from polyclonal murine serum generated against the full length protein. This allowed the identification of a particularly immunogenic fragment of PhtD, which was also purified and characterised. The truncated derivatives were tested as vaccine antigens in mouse models of pneumococcal sepsis and colonisation, using alum and E. coli heat labile toxin B subunit respectively as adjuvants. These experiments revealed that whilst the immunogenic region identified may be a promising candidate to protect against sepsis, the full length PhtD was ineffective at conferring significant protective immunity. These results are significant for the potential for PhtD to be used in novel vaccines, which are currently being tested in clinical trials.

Safety, immunogenicity, and antibody persistence following an investigational Streptococcus pneumoniae and Haemophilus influenzae triple-protein vaccine in a phase 1 randomized controlled study in healthy adults

We investigated a protein-based nontypeable Haemophilus influenzae (NTHi) and pneumococcal (HiP) vaccine containing pneumococcal histidine triad D (PhtD), detoxified pneumolysin (dPly), and NTHi protein D (PD) in adults. In a phase I study, 40 healthy 18- to 40-year-old subjects were randomized (2:2:1) to receive two HiP doses administered 60 days apart, with or without AS03 adjuvant (HiP-AS and HiP groups, respectively), or Engerix B (GlaxoSmithKline, Belgium) as a control. Safety, antibodies, and antigen-specific CD4(+) T-cell immune responses were assessed before and until 480 days after vaccination. No serious adverse events were reported, and no subject withdrew due to an adverse event. Local and systemic symptoms were reported more frequently in the HiP-AS group than in the other two groups. The frequency and intensity of local and systemic symptoms appeared to increase after the second dose of HiP-AS or HiP but not Engerix B. Antibody geometric mean concentrations (GMCs) for PhtD, dPly, and PD increased after each dose of HiP-AS or HiP, with higher GMCs being observed in the HiP-AS group (statistically significant for anti-PD after dose 1 and anti-Ply after dose 2). GMCs remained higher at day 420 than prior to vaccination in both the HiP-AS and HiP groups. Antigen-specific CD4(+) T cells increased after each dose but were unmeasurable by day 480. Two doses of an investigational PhtD-dPly-PD protein vaccine induced humoral immunity and antigen-specific CD4(+) T-cell responses after each dose, with generally higher responses when the vaccine was administered with AS03. HiP combined with AS03 appeared to be more reactogenic than the antigens alone. (This study has been registered at ClinicalTrials.gov under registration no. NCT00814489.).

CD4 T cell memory and antibody responses directed against the pneumococcal histidine triad proteins PhtD and PhtE following nasopharyngeal colonization and immunization and their role in protection against pneumococcal colonization in mice

The present study was undertaken to understand the role of vaccine candidates PhtD and PhtE in pneumococcal nasopharyngeal (NP) colonization, their ability to induce CD4 T cell memory and antibody responses following primary NP colonization, and their contribution to protection against secondary pneumococcal colonization in mice. The study was also aimed at understanding the potential of immunization with PhtD and PhtE in eliciting qualitative CD4 T cell memory responses and protection against pneumococcal NP colonization in mice. PhtD and PhtE isogenic mutants in a TIGR4 background (TIGR4 ΔPhtD and TIGR4 ΔPhtE) were constructed and found to have a significantly reduced colonization density over time in the nasopharynges of mice compared to those of mice colonized with wild-type TIGR4. Mice with primary colonization by wild-type TIGR4, TIGR4 ΔPhtD, or TIGR4 ΔPhtE were protected against secondary colonization by wild-type TIGR4; nonetheless, the clearance of secondary colonization was slower in mice with primary colonization by either TIGR4 ΔPhtD or TIGR4 ΔPhtE than in mice with primary colonization by wild-type TIGR4. Colonization was found to be an immunizing event for PhtD and PhtE antigens (antibody response); however, we failed to detect any antigen (PhtD or PhtE)-specific CD4 T cell responses in any of the colonized groups of mice. Intranasal immunization with either PhtD or PhtE protein generated robust serum antibody and CD4 Th1-biased immune memory and conferred protection against pneumococcal colonization in mice. We conclude that PhtD and PhtE show promise as components in next-generation pneumococcal vaccine formulations.

Surface association of Pht proteins of Streptococcus pneumoniae

Streptococcus pneumoniae is a major human pathogen responsible for massive global morbidity and mortality. The pneumococcus attaches a variety of proteins to its cell surface, many of which contribute to virulence; one such family are the polyhistidine triad (Pht) proteins PhtA, PhtB, PhtD, and PhtE. In this study, we have examined the mechanism of Pht surface attachment using PhtD as a model. Analysis of deletion and point mutants identified a three-amino-acid region of PhtD (Q27-H28-R29) that is critical for the process. The analogous region in PhtE was also necessary for its attachment to the cell surface. Furthermore, we show that a large proportion of the total amount of each Pht protein is released into bacterial culture supernatants. Other surface proteins were also released, albeit to lesser extents, and this was not due to pneumococcal autolysis. The extent of release of surface proteins was strain dependent and was not affected by the capsule. Lastly, we compared the fitness of wild-type and ΔphtABDE pneumococci in vivo in a mouse coinfection model. Release of Pht proteins by the wild type did not complement the mutant strain, consistent with surface-attached rather than soluble forms of the Pht proteins playing the major role in virulence. The significant degree of release of Pht proteins from intact bacteria may have implications for the use of these proteins in novel vaccines.

Metal binding is critical for the folding and function of laminin binding protein, Lmb of Streptococcus agalactiae

Lmb is a 34 kDa laminin binding surface adhesin of Streptococcus agalactiae. The structure of Lmb reported by us recently has shown that it consists of a metal binding crevice, in which a zinc ion is coordinated to three highly conserved histidines. To elucidate the structural and functional significance of the metal ion in Lmb, these histidines have been mutated to alanine and single, double and triple mutants were generated. These mutations resulted in insolubility of the protein and revealed altered secondary and tertiary structures, as evidenced by circular dichroism and fluorescence spectroscopy studies. The mutations also significantly decreased the binding affinity of Lmb to laminin, implicating the role played by the metal binding residues in maintaining the correct conformation of the protein for its binding to laminin. A highly disordered loop, proposed to be crucial for metal acquisition in homologous structures, was deleted in Lmb by mutation (ΔLmb) and its crystal structure was solved at 2.6 Å. The ΔLmb structure was identical to the native Lmb structure with a bound zinc ion and exhibited laminin binding activity similar to wild type protein, suggesting that the loop might not have an important role in metal acquisition or adhesion in Lmb. Targeted mutations of histidine residues confirmed the importance of the zinc binding crevice for the structure and function of the Lmb adhesin.

Panel 6: Vaccines

OBJECTIVE

To update progress on the effectiveness of vaccine for prevention of acute otitis media (AOM) and identification of promising candidate antigens against Streptococcus pneumoniae, nontypeable Haemophilus influenzae, and Moraxella catarrhalis.

REVIEW METHODS

Literature searches were performed in OvidSP and PubMed restricted to articles published between June 2007 and September 2011. Search terms included otitis media, vaccines, vaccine antigens, and each of the otitis pathogens and candidate antigens identified in the ninth conference report.

CONCLUSIONS

The current report provides further evidence for the effectiveness of pneumococcal conjugate vaccines (PCVs) in the prevention of otitis media. Observational studies demonstrate a greater decline in AOM episodes than reported in clinical efficacy trials. Unmet challenges include extending protection to additional serotypes and additional pathogens, the need to prevent early episodes, the development of correlates of protection for protein antigens, and the need to define where an otitis media vaccine strategy fits with priorities for child health.

IMPLICATIONS FOR PRACTICE

Acute otitis media continues to be a burden on children and families, especially those who suffer from frequent recurrences. The 7-valent PCV (PCV7) has reduced the burden of disease as well as shifted the pneumococcal serotypes and the distribution of otopathogens currently reported in children with AOM. Antibiotic resistance remains an ongoing challenge. Multiple candidate antigens have demonstrated the necessary requirements of conservation, surface exposure, immunogenicity, and protection in animal models. Further research on the role of each antigen in pathogenesis, in the development of correlates of protection in animal models, and in new adjuvants to elicit responses in the youngest infants is likely to be productive and permit more antigens to move into human clinical trials.

Insight into the evolution of the histidine triad protein (HTP) family in Streptococcus

The Histidine Triad Proteins (HTPs), also known as Pht proteins in Streptococcus pneumoniae, constitute a family of surface-exposed proteins that exist in many pathogenic streptococcal species. Although many studies have revealed the importance of HTPs in streptococcal physiology and pathogenicity, little is known about their origin and evolution. In this study, after identifying all htp homologs from 105 streptococcal genomes representing 38 different species/subspecies, we analyzed their domain structures, positions in genome, and most importantly, their evolutionary histories. By further projecting this information onto the streptococcal phylogeny, we made several major findings. First, htp genes originated earlier than the Streptococcus genus and gene-loss events have occurred among three streptococcal groups, resulting in the absence of the htp gene in the Bovis, Mutans and Salivarius groups. Second, the copy number of htp genes in other groups of Streptococcus is variable, ranging from one to four functional copies. Third, both phylogenetic evidence and domain structure analyses support the division of two htp subfamilies, designated as htp I and htp II. Although present mainly in the pyogenic group and in Streptococcus suis, htp II members are distinct from htp I due to the presence of an additional leucine-rich-repeat domain at the C-terminus. Finally, htp genes exhibit a faster nucleotide substitution rate than do housekeeping genes. Specifically, the regions outside the HTP domains are under strong positive selection. This distinct evolutionary pattern likely helped Streptococcus to easily escape from recognition by host immunity.

Polyhistidine triad proteins of pathogenic streptococci

The polyhistidine triad (Pht) proteins are an intriguing family of proteins found on the surface of members of the genus Streptococcus. Their defining feature is the presence of multiple copies of the eponymous His triad motif HxxHxH. This review focuses on the Pht proteins of Streptococcus pneumoniae, which contribute to virulence and are leading candidates for inclusion in protein-based pneumococcal vaccines. They appear to have multiple functions, including metal ion homeostasis, evasion of complement deposition and adherence of bacteria to host cells. Across the streptococci, there are many Pht homologs, which can be grouped according to structural features. Critically, there is considerable potential to use members of the Pht protein family as components of vaccines targeted at other streptococci.

Vaccine candidates PhtD and PhtE of Streptococcus pneumoniae are adhesins that elicit functional antibodies in humans

We evaluated the role of vaccine candidate surface proteins, PhtD and PhtE as antigens with functional importance for Streptococcus pneumoniae (pneumococci) in adherence to nasopharyngeal (D562) and lung (A549) epithelial cell lines. Comparing TIGR4 to PhtD and PhtE- isogenic mutants, a 40% (p=0.001) and 42% (p=0.002) drop in the number of epithelial cells with adherent pneumococci was observed to both cells lines with the mutants, as quantitated using flow cytometry. We expressed PhtD and PhtE on the surface of Escherichia coli and demonstrated that when PhtD and PhtE were surface expressed on E. coli, adherence increased to D562 and A549 cells, compared with the E. coli parent strain (p=0.005, 0.013 for D562 and p=0.034, p=0.035 for A549). Using flow cytometry and confocal microscopy we found that pneumococci aggregated in the presence of human serum IgG, leading to a non-specific drop in adherence. Therefore IgG Fab fragments were prepared to study the functional role of PhtD and PhtE-specific Fabs in blocking adherence. The addition of 1μg of IgG Fab from adult sera led to a 34% reduction (p=0.002) and from children a 20% (p=0.023) reduction in D562 epithelial cells with adherent pneumococci. In purified IgG from adult sera, the depletion of PhtD and PhtE specific Fab from total IgG Fab resulted in a significant increase in the number of D562 epithelial cells with adherent pneumococci (p=0.005 for PhtD and p=0.024 for PhtE). We conclude that antibody directed to PhtD and PhtE adhesins of pneumococci, if raised by vaccination, may function to prevent pneumococcal adherence to human airway epithelial cells.

Regulation of arginine acquisition and virulence gene expression in the human pathogen Streptococcus pneumoniae by transcription regulators ArgR1 and AhrC

In this study, we investigated for the first time the transcriptional response of the human pathogen Streptococcus pneumoniae to fluctuating concentrations of arginine, an essential amino acid for this bacterium. By means of DNA microarray analyses, several operons and genes were found, the expression of which was affected by the concentration of arginine in the medium. Five of the identified operons were demonstrated to be directly repressed in the presence of high arginine concentrations via the concerted action of the ArgR-type regulators ArgR1 and AhrC. These ArgR1/AhrC targets encompass the putative amino acid transport genes artPQ, abpA, abpB, and aapA; the arginine biosynthetic genes argGH; and the virulence genes aliB and lmB/adcAII-phtD encoding an oligopeptide-binding lipoprotein and cell surface Zn(2+)-scavenging units, respectively. In addition, the data indicate that three of the amino acid transport genes encode an arginine ATP-binding cassette transporter unit required for efficient growth during arginine limitation. Instead of regulating arginine biosynthetic and catabolic genes as has been reported for other Gram-positive bacteria, our findings suggest that the physiological function of ArgR1/AhrC in S. pneumoniae is to ensure optimal uptake of arginine from the surrounding milieu.