Antibodies to conserved pneumococcal antigens correlate with, but are not required for, protection against pneumococcal colonization induced by prior exposure in a mouse model

Genomic and panproteomic analysis of the development of infant immune responses to antigenically-diverse pneumococci

Streptococcus pneumoniae (pneumococcus) is a nasopharyngeal commensal and respiratory pathogen. This study characterises the immunoglobulin G (IgG) repertoire recognising pneumococci from birth to 24 months old (mo) in a prospectively-sampled cohort of 63 children using a panproteome array. IgG levels are highest at birth, due to transplacental transmission of maternal antibodies. The subsequent emergence of responses to individual antigens exhibit distinct kinetics across the cohort. Stable differences in the strength of individuals' responses, correlating with maternal IgG concentrations, are established by 6 mo. By 12 mo, children develop unique antibody profiles that are boosted by re-exposure. However, some proteins only stimulate substantial responses in adults. Integrating genomic data on nasopharyngeal colonisation demonstrates rare pneumococcal antigens can elicit strong IgG levels post-exposure. Quantifying such responses to the diverse core loci (DCL) proteins is complicated by cross-immunity between variants. In particular, the conserved N terminus of DCL protein zinc metalloprotease B provokes the strongest early IgG responses. DCL proteins' ability to inhibit mucosal immunity likely explains continued pneumococcal carriage despite hosts' polyvalent antibody repertoire. Yet higher IgG levels are associated with reduced incidence, and severity, of pneumonia, demonstrating the importance of the heterogeneity in response strength and kinetics across antigens and individuals.

Effect of colonisation with Neisseria lactamica on cross-reactive anti-meningococcal B-cell responses: a randomised, controlled, human infection trial

BACKGROUND

Pharyngeal colonisation by the commensal bacterium Neisseria lactamica inhibits colonisation by Neisseria meningitidis and has an inverse epidemiological association with meningococcal disease. The mechanisms that underpin this relationship are unclear, but could involve the induction of cross-reactive immunity. In this study, we aimed to evaluate whether colonisation with N lactamica induces N lactamica-specific B-cell responses that are cross-reactive with N meningitidis.

METHODS

In this randomised, placebo-controlled, human infection trial at University Hospital Southampton Clinical Research Facility (Southampton, UK), healthy adults aged 18-45 years were randomly assigned (2:1) to receive intranasal inoculation with either 105 colony-forming units of N lactamica in 1 mL phosphate-buffered saline (PBS) or 1 mL PBS alone. Participants and researchers conducting participant sampling and immunological assays were masked to allocation. The primary endpoint was the frequency of circulating N lactamica-specific plasma cells and memory B cells after N lactamica inoculation (day 7-28) compared with baseline values (day 0), measured using enzyme-linked immunospot assays. The secondary endpoint was to measure the frequency of N meningitidis-specific B cells. In a second study, we measured the effect of duration of N lactamica colonisation on seroconversion by terminating carriage at either 4 days or 14 days with single-dose oral ciprofloxacin. The studies are now closed to participants. The trials are registered with ClinicalTrials.gov, NCT03633474 and NCT03549325.

FINDINGS

Of 50 participants assessed for eligibility between Sept 5, 2018, and March 3, 2019, 31 were randomly assigned (n=20 N lactamica, n=11 PBS). Among the 17 participants who were colonised with N lactamica, the median baselines compared with peak post-colonisation N lactamica-specific plasma-cell frequencies (per 105 peripheral blood mononuclear cells) were 0·0 (IQR 0·0-0·0) versus 5·0 (1·5-10·5) for IgA-secreting plasma cells (p<0·0001), and 0·0 (0·0-0·0) versus 3·0 (1·5-9·5) for IgG-secreting plasma cells (p<0·0001). Median N lactamica-specific IgG memory-B-cell frequencies (percentage of total IgG memory B cells) increased from 0·0024% (0·0000-0·0097) at baseline to 0·0384% (0·0275-0·0649) at day 28 (p<0·0001). The frequency of N meningitidis-specific IgA-secreting and IgG-secreting plasma cells and memory B cells also increased signficantly in participants who were colonised with N lactamica. Upper respiratory tract symptoms were reported in ten (50%) of 20 participants who were inoculated with N lactamica and six (55%) of 11 participants who were inoculated with PBS (p>0·99). Three additional adverse events (two in the N lactamica group and one in the PBS group) and no serious adverse events were reported. In the second study, anti-N lactamica and anti-N meningitidis serum IgG titres increased only in participants who were colonised with N lactamica for 14 days.

INTERPRETATION

Natural immunity to N meningitidis after colonisation with N lactamica might be due to cross-reactive adaptive responses. Exploitation of this microbial mechanism with a genetically modified live vector could protect against N meningitidis colonisation and disease.

FUNDING

Wellcome Trust, Medical Research Council, and NIHR Southampton Biomedical Research Centre.

Pneumococcal Vaccines: Past Findings, Present Work, and Future Strategies

The importance of Streptococcus pneumoniae has been well established. These bacteria can colonize infants and adults without symptoms, but in some cases can spread, invade other tissues and cause disease with high morbidity and mortality. The development of pneumococcal conjugate vaccines (PCV) caused an enormous impact in invasive pneumococcal disease and protected unvaccinated people by herd effect. However, serotype replacement is a well-known phenomenon that has occurred after the introduction of the 7-valent pneumococcal conjugate vaccine (PCV7) and has also been reported for other PCVs. Therefore, it is possible that serotype replacement will continue to occur even with higher valence formulations, but the development of serotype-independent vaccines might overcome this problem. Alternative vaccines are under development in order to improve cost effectiveness, either using proteins or the pneumococcal whole cell. These approaches can be used as a stand-alone strategy or together with polysaccharide vaccines. Looking ahead, the next generation of pneumococcal vaccines can be impacted by the new technologies recently approved for human use, such as mRNA vaccines and viral vectors. In this paper, we will review the advantages and disadvantages of the addition of new polysaccharides in the current PCVs, mainly for low- and middle-income countries, and we will also address future perspectives.

The emerging and diverse roles of the SLy/SASH1-protein family in health and disease-Overview of three multifunctional proteins

Intracellular adaptor proteins are indispensable for the transduction of receptor-derived signals, as they recruit and connect essential downstream effectors. The SLy/SASH1-adaptor family comprises three highly homologous proteins, all of them sharing conserved structural motifs. The initial characterization of the first member SLy1/SASH3 (SH3 protein expressed in lymphocytes 1) in 2001 was rapidly followed by identification of SLy2/HACS1 (hematopoietic adaptor containing SH3 and SAM domains 1) and SASH1/SLy3 (SAM and SH3 domain containing 1). Based on their pronounced sequence similarity, they were subsequently classified as one family of intracellular scaffold proteins. Despite their obvious homology, the three SLy/SASH1-members fundamentally differ with regard to their expression and function in intracellular signaling. On the contrary, growing evidence clearly demonstrates an important role of all three proteins in human health and disease. In this review, we systematically summarize what is known about the SLy/SASH1-adaptors in the field of molecular cell biology and immunology. To this end, we recapitulate current research about SLy1/SASH3, SLy2/HACS1, and SASH1/SLy3, with an emphasis on their similarities and differences.

The epidemiologic and biologic basis for classifying older age as a high-risk, immunocompromising condition for pneumococcal vaccine policy

INTRODUCTION

Immunosenescence is a normal biologic process involving deterioration of protective immune responses. Consequently, older adults experience increased risk of infectious diseases, particularly pneumonia, and its leading bacterial cause, Streptococcus pneumoniae. Pneumococcal vaccine recommendations are often limited to adults with specific medical conditions despite similar disease risks among older adults due to immunosenescence.

AREAS COVERED

This article reviews epidemiologic, biologic, and clinical evidence supporting the consideration of older age due to immunosenescence as an immunocompromising condition for the purpose of pneumococcal vaccine policy and the role vaccination can play in healthy aging.

EXPERT OPINION

Epidemiologic and biologic evidence suggest that pneumococcal disease risk increases with age and is comparable for healthy older adults and younger adults with immunocompromising conditions. Because immunocompromising conditions are already indicated for pneumococcal conjugate vaccines (PCVs), a comprehensive public health strategy would also recognize immunosenescence. Moreover, older persons should be vaccinated before reaching the highest risk ages, consistent with the approach for other immunocompromising conditions. To facilitate PCV use among older adults, vaccine technical committees (VTCs) could classify older age as an immunocompromising condition based on the process of immunosenescence. With global aging, VTCs will need to consider immunosenescence and vaccine use during healthy aging.

SLy2-overexpression impairs B-cell development in the bone marrow and the IgG response towards pneumococcal conjugate-vaccine

Background

Infections with Streptococcus pneumoniae can cause severe diseases in humans including pneumonia. Although guidelines for vaccination have been established, S. pneumoniae is still responsible for a serious burden of disease around the globe. Currently, two pneumococcal immunizations are available, namely the pure polysaccharide vaccine Pneumovax23 (P23) and the conjugate‐vaccine Prevenar13 (PCV13). We recently reported impaired thymus‐independent antibody responses towards P23 in mice overexpressing the immunoinhibitory adapter SLy2. The purpose of this study was to evaluate adaptive B‐cell responses towards the thymus‐dependent vaccine PCV13 in SLy2‐overexpressing mice and to study their survival rate during pneumococcal lung infection. Moreover, we investigated B‐cell developmental stages within the bone marrow (BM) in the context of excessive SLy2‐expression.

Methods

B‐cell subsets and their surface immune globulins were investigated by flow cytometry. For class‐switch assays, isolated splenic B cells were stimulated in vitro with lipopolysaccharide and interleukin‐4 and antibody secretion was quantified via LEGENDplex. To study PCV13‐specific responses, mice were immunized and serum antibody titers (immunoglobulin M, immunoglobulins IgG₁, IgG₂, and IgG₃) were examined by enzyme‐linked immunosorbent assay. Survival rates of mice were assessed within 7 days upon intranasal challenge with S. pneumoniae.

Results

Our data demonstrate impaired IgG₁ and IgG₃ antibody responses towards the pneumococcal conjugate‐vaccine PCV13 in SLy2‐overexpressing mice. This was accompanied by reduced frequencies and numbers of BM‐resident plasmablasts. In addition, we found drastically reduced counts of B‐cell precursors in the BM of SLy2‐Tg mice. The survival rate upon intranasal challenge with S. pneumoniae was mostly comparable between the genotypes.

Conclusion

Our findings demonstrate an important role of the adapter protein SLy2 in the context of adaptive antibody responses against pneumococcal conjugate‐vaccine. Interestingly, deficits in humoral immunity seemed to be compensated by cellular immune effectors upon bacterial challenge. Our study further shows a novel relevance of SLy2 for plasmablasts and B‐cell progenitors in the BM.

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.

Opening the OPK Assay Gatekeeper: Harnessing Multi-Modal Protection by Pneumococcal Vaccines

Pneumococcal vaccine development is driven by the achievement of high activity in a single gatekeeper assay: the bacterial opsonophagocytic killing (OPK) assay. New evidence challenges the dogma that anti-capsular antibodies have only a single function that predicts success. The emerging concept of multi-modal protection presents an array of questions that are fundamental to adopting a new vaccine design process. If antibodies have hidden non-opsonic functions that are protective, should these be optimized for better vaccines? What would protein antigens add to protective activity? Are cellular immune functions additive to antibodies for success? Do different organs benefit from different modes of protection? Can vaccine activities beyond OPK protect the immunocompromised host? This commentary raises these issues at a time when capsule-only OPK assay-based vaccines are increasingly seen as a limiting strategy.

Enhanced safety and immunogenicity of a pneumococcal surface antigen A mutant whole-cell inactivated pneumococcal vaccine

Existing capsular polysaccharide-based vaccines against pneumococcal disease are highly effective against vaccine-included serotypes, but they are unable to combat serotype replacement. We have developed a novel pneumococcal vaccine that confers serotype-independent protection, and could therefore constitute a "universal" vaccine formulation. This preparation is comprised of whole un-encapsulated pneumococci inactivated with gamma irradiation (γ-PN), and we have previously reported induction of cross-reactive immunity after nonadjuvanted intranasal vaccination. To further enhance vaccine immunogenicity and safety, we modified the pneumococcal vaccine strain to induce a stressed state during growth. Specifically, the substrate binding component of the psaBCA operon for manganese import was mutated to create a pneumococcal surface antigen A (psaA) defective vaccine strain. psaA mutation severely attenuated the growth of the vaccine strain in vitro without negatively affecting pneumococcal morphology, thereby enhancing vaccine safety. In addition, antibodies raised against vaccine preparations based on the modified strain [γ-PN(ΔPsaA)] showed more diversified reactivity to wild-type pneumococcal challenge strains compared to those induced by the original formulation. The modified vaccine also induced comparable protective T_H 17 responses in the lung, and conferred greater protection against lethal heterologous pneumococcal challenge. Overall, the current study demonstrates successful refinement of a serotype-independent pneumococcal vaccine candidate to enhance safety and immunogenicity.

Mechanisms of Naturally Acquired Immunity to Streptococcus pneumoniae

In this review we give an update on the mechanisms of naturally acquired immunity against Streptococcus pneumoniae, one of the major human bacterial pathogens that is a common cause of pneumonia, septicaemia, and meningitis. A clear understanding of the natural mechanisms of immunity to S. pneumoniae is necessary to help define why the very young and elderly are at high risk of disease, and for devising new prevention strategies. Recent data has shown that nasopharynx colonization by S. pneumoniae induces antibody responses to protein and capsular antigens in both mice and humans, and also induces Th17 CD4+ cellular immune responses in mice and increases pre-existing responses in humans. These responses are protective, demonstrating that colonization is an immunizing event. We discuss the data from animal models and humans on the relative importance of naturally acquired antibody and Th17 cells on immunity to S. pneumoniae at three different anatomical sites of infection, the nasopharynx (the site of natural asymptomatic carriage), the lung (site of pneumonia), and the blood (site of sepsis). Mouse data suggest that CD4+ Th17 cells prevent both primary and secondary nasopharyngeal carriage with no role for antibody induced by previous colonization. In contrast, antibody is necessary for prevention of sepsis but CD4+ cellular responses are not. Protection against pneumonia requires a combination of both antibody and Th17 cells, in both cases targeting protein rather than capsular antigen. Proof of which immune component prevents human infection is less easily available, but two recent papers demonstrate that human IgG targeting S. pneumoniae protein antigens is highly protective against septicaemia. The role of CD4+ responses to prior nasopharyngeal colonization for protective immunity in humans is unclear. The evidence that there is significant naturally-acquired immunity to S. pneumoniae independent of anti-capsular polysaccharide has clinical implications for the detection of subjects at risk of S. pneumoniae infections, and the data showing the importance of protein antigens as targets for antibody and Th17 mediated immunity should aid the development of new vaccine 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.

Nasopharyngeal Exposure to Streptococcus pneumoniae Induces Extended Age-Dependent Protection against Pulmonary Infection Mediated by Antibodies and CD138+ Cells

Streptococcus pneumoniae commonly resides asymptomatically in the nasopharyngeal (NP) cavity of healthy individuals but can cause life-threatening pulmonary and systemic infections, particularly in the elderly. NP colonization results in a robust immune response that protects against invasive infections. However, the duration, mechanism, and cellular component of such responses are poorly understood. In this study, we found that repeated NP exposure of mice to S. pneumoniae TIGR4 strain results in pneumococcal-specific Ab responses that protect against lethal lung challenge. Abs were necessary and sufficient for protection because Ab-deficient μMT mice did not develop postexposure protection, only becoming resistant to lung infection after transfer of immune sera from NP-exposed mice. T cells contributed to immunity at the time of NP exposure, but neither CD4⁺ nor CD8⁺ T cells were required. The protective activity was detectable 20 wk after exposure and was maintained in irradiated mice, suggesting involvement of long-lived Ab-secreting cells (ASC), which are radioresistant and secrete Abs for extended periods of time in the absence of T cells or persistent Ag. CD138⁺ bone marrow cells, likely corresponding to long-lived ASC, were sufficient to confer protection. NP exposure of aged mice failed to protect against subsequent lung infection despite eliciting a robust Ab response. Furthermore, transfer of CD138⁺ bone marrow cells or sera from NP-exposed old mice failed to protect naive young mice. These findings suggest that NP exposure elicits extended protection against pneumococcal lung infection by generating long-lived CD138⁺ ASC and that the protective efficacy of these responses declines with age.

Subcutaneous Immunization with Fusion Protein DnaJ-ΔA146Ply without Additional Adjuvants Induces both Humoral and Cellular Immunity against Pneumococcal Infection Partially Depending on TLR4

Subunit vaccines that are poorly immunogenic are often combined with adjuvants for immunization. Our previous research identified a pneumolysin variant (ΔA146Ply), a Toll-like receptor 4 agonist, that was an effective adjuvant in the protection of fusion protein DnaJ-ΔA146Ply against mucosal Streptococcus pneumoniae infections. For pneumococcal vaccines, World Health Organization recommend injection as a regular vaccination approach. Subcutaneous immunization is a common and effective method of injection, so we explored the immunity mechanism of subcutaneous immunization with DnaJ-ΔA146Ply. We found that mice immunized subcutaneously with fusion proteins ΔA146Ply-DnaJ and DnaJ-ΔA146Ply produced a higher anti-DnaJ IgG titer than when DnaJ alone was administered. DnaJ-ΔA146Ply induced both B-cell and T-cell-dependent protection against both colonization and lethal pneumococcal infections. Levels of IFN-γ, IL-4, and IL-17A were also elevated in DnaJ-ΔA146Ply immunized mice. However, all these effects were negated in TLR4^-/- mice compared to WT mice immunized with DnaJ-ΔA146Ply. B-cell-deficient μMT mice, nude mice, IFN-γ^-/-, and IL-4^-/- mice immunized with DnaJ-ΔA146Ply could not resist infection with pneumococci. IL-17A^-/- and TLR4^-/- mice did not benefit from DnaJ-ΔPly immunization in colonization experiments although their survival was not impaired compared with WT mice. Collectively, our data indicated that ΔA146Ply can be a potential subcutaneous adjuvant, and the DnaJ-ΔA146Ply fusion protein induces both humoral and cellular immune response to resist S. pneumoniae infection. The protective effect of colonization also depends on TLR4.

Pneumococcal Capsular Polysaccharide Immunity in the Elderly

Immunity to pneumococcal infections is impaired in older people, and current vaccines are poorly protective against pneumococcal disease in this population. Naturally acquired immunity to pneumococcal capsular polysaccharides develops during childhood and is robust in young adults but deteriorates with advanced age. In particular, antibody levels and function are reduced in older people. Pneumococcal vaccines are recommended for people >65 years old. However, the benefits of polysaccharide and protein-conjugated vaccines in this population are small, because of both serotype replacement and incomplete protection against vaccine serotype pneumococcal disease. In this review, we overview the immune mechanisms by which naturally acquired and vaccine-induced pneumococcal capsular polysaccharide immunity declines with age, including altered colonization dynamics, reduced opsonic activity of antibodies (particularly IgM), and impaired mucosal immunity.

Association of Pneumococcal Protein Antigen Serology With Age and Antigenic Profile of Colonizing Isolates

Background

Several Streptococcus pneumoniae proteins play a role in pathogenesis and are being investigated as vaccine targets. It is largely unknown whether naturally acquired antibodies reduce the risk of colonization with strains expressing a particular antigenic variant.

Methods

Serum immunoglobulin G (IgG) titers to 28 pneumococcal protein antigens were measured among 242 individuals aged <6 months-78 years in Native American communities between 2007 and 2009. Nasopharyngeal swabs were collected >- 30 days after serum collection, and the antigen variant in each pneumococcal isolate was determined using genomic data. We assessed the association between preexisting variant-specific antibody titers and subsequent carriage of pneumococcus expressing a particular antigen variant.

Results

Antibody titers often increased across pediatric groups before decreasing among adults. Individuals with low titers against group 3 pneumococcal surface protein C (PspC) variants were more likely to be colonized with pneumococci expressing those variants. For other antigens, variant-specific IgG titers do not predict colonization.

Conclusion

We observed an inverse association between variant-specific antibody concentration and homologous pneumococcal colonization for only 1 protein. Further assessment of antibody repertoires may elucidate the nature of antipneumococcal antibody-mediated mucosal immunity while informing vaccine development.

Nasopharyngeal colonization with Streptococcus pneumoniae triggers dendritic cell dependent antibody responses against invasive disease in mice

Nasopharyngeal colonization with Streptococcus pneumoniae (Spn) is an important precondition for the development of pneumococcal pneumonia. At the same time, nasopharyngeal colonization with Spn has been shown to mount adaptive immune responses against Spn in mice and humans. Cellular responses of the nasopharyngeal compartment, including the nasal-associated lymphoid tissue, to pneumococcal colonization and their importance for developing adaptive immune responses are poorly defined. We show that nasopharyngeal colonization with S. pneumoniae led to substantial expansion of dendritic cells (DCs) both in nasopharyngeal tissue and nasal-associated lymphoid tissue of mice. Depletion of DCs achieved by either diphtheria toxin (DT) treatment of chimeric zDC^+/DTR mice, or by use of FMS-like tyrosine kinase 3 ligand (Flt3L) KO mice exhibiting congenitally reduced DC pool sizes, significantly diminished antibody responses after colonization with Spn, along with impaired protective immunity against invasive pneumococcal disease. Collectively, the data show that classical DCs contribute to pneumococcal colonization induced adaptive immune responses against invasive pneumococcal disease in two different mouse models. These data may be useful for future nasopharyngeal vaccination strategies against pneumococcal diseases in humans.

Delayed reconstitution of B cell immunity to pneumococcus in HIV-infected Malawian children on antiretroviral therapy

OBJECTIVE

Despite CD4(+) count restoration and viral load suppression with antiretroviral therapy (ART), HIV-infected children remain at increased risk of life-threatening infections including invasive pneumococcal disease (IPD). We therefore investigated whether persistent susceptibility to IPD following ART is associated with incomplete recovery of B-cell function.

METHODS

41 HIV-infected Malawian children commencing ART were followed-up for a 1 year period during which time blood samples were collected at 0, 3, 6 and 12 months for comprehensive immunophenotyping and pneumomococcal-specific Memory B-cell Enzyme-Linked Immunospot assays. In addition, nasopharyngeal swab samples were cultured to determine pneumococcal carriage rates.

RESULTS

Normalization of major lymphocyte subsets such as CD4(+) percentages was evident following 3 months of ART. The proportions of mature naïve B cells (CD19(+) CD10(-) CD27(-) CD21(hi)) and resting memory B cells (CD19(+) CD27(+) CD21(hi)) increased and apoptosis-prone mature activated B cells (CD19(+) CD21(lo) CD10(-)) decreased markedly by 12 months. However, in the context of high nasopharyngeal pneumococcal carriage rates (83%), restoration of pneumococcal protein antigen-specific B-cell memory was more delayed.

CONCLUSIONS

These data show that, in chronically HIV-infected children receiving ART, improvement in B-cell memory profiles and function is slower than CD4(+) T-cells. This supports early initiation of ART and informs research into optimal timing of immunization with pneumococcal vaccines.

Mucosal immunization with the live attenuated vaccine SPY1 induces humoral and Th2-Th17-regulatory T cell cellular immunity and protects against pneumococcal infection

Mucosal immunization with attenuated vaccine can protect against pneumococcal invasion infection, but the mechanism was unknown. Our study found that mucosal delivery with the live attenuated SPY1 vaccine strain can confer T cell- and B cell-dependent protection against pneumococcal colonization and invasive infection; yet it is still unclear which cell subsets contribute to the protection, and their roles in pneumococcal colonization and invasion remain elusive. Adoptive transfer of anti-SPY1 antibody conferred protection to naive μMT mice, and immune T cells were indispensable to protection examined in nude mice. A critical role of interleukin 17A (IL-17A) in colonization was demonstrated in mice lacking IL-17A, and a vaccine-specific Th2 immune subset was necessary for systemic protection. Of note, we found that SPY1 could stimulate an immunoregulatory response and that SPY1-elicited regulatory T cells participated in protection against colonization and lethal infection. The data presented here aid our understanding of how live attenuated strains are able to function as effective vaccines and may contribute to a more comprehensive evaluation of live vaccines and other mucosal vaccines.

Characterization of the inflammatory infiltrate in Streptococcus pneumoniae pneumonia in young and elderly patients

There is an increased susceptibility and mortality in the elderly due to pneumonia caused by Streptococcus pneumoniae. We aimed to assess the inflammatory cell composition with respect to age in pneumococcal pneumonia patients. Neutrophilic granulocytes and various lymphocyte and macrophage subpopulations were immunohistochemically quantified on lung tissue specimens of young (n = 5; mean age 8.4 years), middle-aged (n = 8; mean age 55.9 years) and elderly (n = 9; mean age 86.6 years) pneumonia patients with microbiologically proven S. pneumoniae pneumonia. We discovered a higher percentage of neutrophilic granulocytes in elderly as opposed to young patients (95 vs. 75%, p = 0.012). Conversely, young patients versus elderly patients had more alveolar macrophages (CD11c+: 20 vs. 9%, p = 0.029) and M1 macrophages (CD14+: 30 vs. 10%, p = 0.012 and HLA-DR+: 52 vs. 11%, p = 0.019). There was no significant difference concerning M2 macrophages and lymphocytes. Comparison of young patients with middle-aged patients showed similar significant results for alveolar macrophages (p = 0.019) and subsignificant results for M1 macrophages and neutrophilic granulocytes (p < 0.08). This is the first study characterizing the inflammatory infiltrate of pneumococcal pneumonia in situ. Our observations improve the understanding of the innate immune mechanisms of pneumococcal lung infection and point at the potential of therapies for restoring macrophage function and decreasing neutrophilic influx in order to help prevent or cure pneumonia.

Immunosenescence and pneumococcal disease: an imbalance in host-pathogen interactions

Respiratory infections are among the most important causes of morbidity and mortality from infectious diseases worldwide. The most common causative bacterium, Streptococcus pneumoniae, frequently colonises the upper respiratory tract, where it resides mostly asymptomatically. Occasionally, however, S pneumoniae can cause severe disease such as pneumonia. Local host immunity is essential to control colonising pathogens by preventing overgrowth, spread, and invasion. However, age-related immune deficits in elderly people, known as immunosenescence, might contribute to increased disease burden. We review present knowledge about immunosenescence in the respiratory tract against Gram-positive bacteria, particularly S pneumoniae. We discuss the possible underdetection of pneumococcal colonisation in elderly people, and suggest changes to present surveillance methods to improve understanding of the relation between colonisation and disease. We conclude that present knowledge about alteration of host-pathogen interactions by immunosenescence in the respiratory tract is insufficient, and that research is needed to enable improved measures for prevention.

Serotype-independent pneumococcal vaccines

Streptococcus pneumoniae remains an important cause of disease with high mortality and morbidity, especially in children and in the elderly. The widespread use of the polysaccharide conjugate vaccines in some countries has led to a significant decrease in invasive disease caused by vaccine serotypes, but an increase in disease caused by non-vaccine serotypes has impacted on the overall efficacy of these vaccines on pneumococcal disease. The obvious solution to overcome such shortcomings would be the development of new formulations that provide serotype-independent immunity. This review focuses on the most promising approaches, including protein antigens, whole cell pneumococcal vaccines, and recombinant bacteria expressing pneumococcal antigens. The protective capacity of these vaccine candidates against the different stages of pneumococcal infection, including colonization, mucosal disease, and invasive disease in animal models is reviewed. Some of the human trials that have already been performed or that are currently ongoing are presented. Finally, the feasibility and the possible shortcomings of these candidates in relation to an ideal vaccine against pneumococcal infections are discussed.

Immune response to Streptococcus pneumoniae in asthma patients: comparison between stable situation and exacerbation

In Argentina, more than 3 million people suffer from asthma, with numbers rising. When asthma patients acquire viral infections which, in turn, trigger the asthmatic response, they may develop subsequent bacterial infections, mainly by Streptococcus (S.) pneumoniae. This encapsulated Gram(+) bacterium has been considered historically a T cell-independent antigen. Nevertheless, several papers describe the role of T cells in the immune response to S. pneumoniae. We evaluated the response to S. pneumoniae and compared it to the response to Mycobacterium (M.) tuberculosis, a different type of bacterium that requires a T helper type 1 (Th1) response, in cells from atopic asthmatic children, to compare parameters for the same individual under exacerbation and in a stable situation whenever possible. We studied asthma patients and a control group of age-matched children, evaluating cell populations, activation markers and cytokine production by flow cytometry, and cytokine concentration in serum and cell culture supernatants by enzyme-linked immunosorbent assay (ELISA). No differences were observed in γδ T cells for the same patient in either situation, and a tendency to lower percentages of CD4(+) CD25(hi) T cells was observed under stability. A significantly lower production of tumour necrosis factor (TNF)-α and a significantly higher production of interleukin (IL)-5 was observed in asthma patients compared to healthy individuals, but no differences could be observed for IL-4, IL-13 or IL-10. A greater early activation response against M. tuberculosis, compared to S. pneumoniae, was observed in the asthmatic patients' cells. This may contribute to explaining why these patients frequently acquire infections caused by the latter bacterium and not the former.

Defective pneumococcal-specific Th1 responses in HIV-infected adults precedes a loss of control of pneumococcal colonization

BACKGROUND

African adults infected with human immunodeficiency virus (HIV) have high rates of pneumococcal colonization and invasive disease. Here we have investigated the possibility that HIV disrupts the normal balance of pneumococcal-specific helper T cell (Th) 1/Th17 immunity to colonization, resulting in a more permissive nasopharyngeal niche.

METHODS

One hundred thirty-six HIV-infected and -uninfected Malawian adults were enrolled in the study. Changes in rates and composition of nasopharyngeal pneumococcal colonization were analyzed using microarray. The underlying pneumococcal-specific Th1/Th17 responses associated with altered pneumococcal colonization were investigated using flow cytometry.

RESULTS

We find that pneumococcal carriage is only modestly increased in asymptomatic HIV-infected Malawian adults but that colonization rates rise dramatically during symptomatic disease (HIV(neg) 13%, HIV(asy) 19%, and HIV(sym) 38%). These rates remain high in subjects established on antiretroviral therapy (ART): 33% (at 6-12 months) and 52% (at 18 months), with HIV-infected individuals carrying a broader range of invasive and noninvasive serotypes compared with HIV-negative controls. The frequency of multiple serotype carriage (>1 serotype HIV(neg) 26%, HIV(asy) 30%, HIV(sym) 31%, HIV(ART) 31%) is not affected. These changes in colonization are associated with generalized CD4 T-cell depletion, impaired antigen-specific proliferation, and a defect in pneumococcal-specific T-cell interferon-γ but not interleukin 17 production.

CONCLUSIONS

These data reveal the persistently poor control of pneumococcal colonization in HIV-infected adults following immune ART-mediated reconstitution, highlighting a potential reservoir for person-to-person spread and vaccine escape. Novel approaches to control colonization either through vaccination or through improvements in the quality of immune reconstitution are required.

Regulation of naturally acquired mucosal immunity to Streptococcus pneumoniae in healthy Malawian adults and children

Worldwide, invasive pneumococcal disease caused by Streptococcus pneumoniae is most common in young children. In adults, disease rates decline following intermittent colonization and the acquisition of naturally acquired immunity. We characterized mucosal and systemic pneumococcal-specific T-cell responses in African children and adults who contend with intense rates of colonization, up to 100% and 60% respectively. We find most Malawian children have high pneumococcal-specific T-cell responses in tonsil tissue and peripheral blood. In addition, frequent commensalism generates CD25(hi) (Tregs) which modulate mucosal pneumococcal-specific T-cell responses in some children and ≥50% of adults. We propose that immune regulation may prolong pneumococcal colonization and predispose vulnerable individuals to disease.

Distinct effects on diversifying selection by two mechanisms of immunity against Streptococcus pneumoniae

Antigenic variation to evade host immunity has long been assumed to be a driving force of diversifying selection in pathogens. Colonization by Streptococcus pneumoniae, which is central to the organism's transmission and therefore evolution, is limited by two arms of the immune system: antibody- and T cell- mediated immunity. In particular, the effector activity of CD4⁺ T_H17 cell mediated immunity has been shown to act in trans, clearing co-colonizing pneumococci that do not bear the relevant antigen. It is thus unclear whether T_H17 cell immunity allows benefit of antigenic variation and contributes to diversifying selection. Here we show that antigen-specific CD4⁺ T_H17 cell immunity almost equally reduces colonization by both an antigen-positive strain and a co-colonized, antigen-negative strain in a mouse model of pneumococcal carriage, thus potentially minimizing the advantage of escape from this type of immunity. Using a proteomic screening approach, we identified a list of candidate human CD4⁺ T_H17 cell antigens. Using this list and a previously published list of pneumococcal Antibody antigens, we bioinformatically assessed the signals of diversifying selection among the identified antigens compared to non-antigens. We found that Antibody antigen genes were significantly more likely to be under diversifying selection than the T_H17 cell antigen genes, which were indistinguishable from non-antigens. Within the Antibody antigens, epitopes recognized by human antibodies showed stronger evidence of diversifying selection. Taken together, the data suggest that T_H17 cell-mediated immunity, one form of T cell immunity that is important to limit carriage of antigen-positive pneumococcus, favors little diversifying selection in the targeted antigen. The results could provide new insight into pneumococcal vaccine design.

Streptococcus pneumoniae, or pneumococcus, is a leading cause of morbidity and mortality in young children and elderly persons worldwide. Current pneumococcus vaccines target a limited number of clinically important serotypes, while strains with serotypes not targeted by current vaccines are increasing in importance in both carriage and invasive disease. As a result, there has been a substantial interest to develop novel, cost-effective vaccines based on protein antigens from pneumococcus. To this end, it is critical to understand how the human immune system exerts selection pressures on the targeted antigens. Two immune mechanisms targeting pneumococcal protein antigens have been documented, mediated by antibody and T cells, respectively. In this study, we screened for pneumococcal antigens that are commonly recognized by human CD4⁺ T_H17 cells. Using a mouse model of pneumococcal colonization, we demonstrate that T_H17 cell-based immunity almost equally reduces colonization by both an antigen-positive strain and a co-colonizing, antigen-negative strain. Furthermore, we demonstrate that the DNA sequences of T_H17 cell antigens demonstrate no detectable signs of being under selective pressure, unlike pneumococcal antigens known to be strong antibody targets. Thus, one form of the T cell-mediated immunity that is important to limit carriage of antigen-positive pneumococcus favors little diversifying selection in the targeted antigen. These results suggest evolution of escape from T_H17 -based vaccines may be slower than from antibody-based vaccines.

The fundamental link between pneumococcal carriage and disease

Streptococcus pneumoniae (pneumococcus) is a major cause of worldwide mortality and morbidity, and to a large extent is vaccine-preventable. Nasopharyngeal carriage of pneumococcus precedes disease and is the source of pneumococcal spread between people. The use of vaccine effect on carriage as part of the vaccine licensure and post-vaccine introduction evaluation could facilitate and expand the licensure of new, life-saving pneumococcal vaccines and enable a comprehensive estimate of population effects after vaccine introduction. The authors provide a review of the evidence supporting pneumococcal carriage at the individual level as an immediate and necessary precursor to pneumococcal disease. Based on such a causal link between carriage and disease, the authors emphasize the role of information on pneumococcal carriage in vaccine trials and in public health decision-making.

Immune response and protective efficacy of S9 ribosomal protein of Streptococcus pneumoniae in a model of sepsis

Vaccination is the most promising strategy to reduce the incidence of pneumococcal infection. Although there are vaccines available, all of them are based on polysaccharide antigens (conjugated or not). In addition to their high cost, those vaccines do not cover all serotypes. To overcome these hindrances, we evaluated the immunogenicity and the protective efficacy of the S9 ribosomal protein of Streptococcus pneumoniae with the aim of developing a protein-based vaccine in the future. The gene encoding the S9 ribosomal protein was cloned in pET21-a expression vector, and the recombinant S9 protein was used to immunize mice. Significantly higher levels of anti-S9 immunoglobulin G were achieved (with predominance of immunoglobulin G1) in comparison with the control. Antibodies elicited against S. pneumoniae protein extract in rabbit recognized the recombinant S9 protein by Western blot, thus demonstrating its immunogenicity. Moreover, mice immunized with recombinant S9 protein and challenged with a virulent strain of S. pneumoniae presented a significant reduction of bacteremia after 24 h of infection as compared with the control. However, in the S9-immunized mice the onset of death was insignificantly delayed, but all of them died by the fourth day postinfection.

Characterization of Th17 responses to Streptococcus pneumoniae in humans: comparisons between adults and children in a developed and a developing country

Intranasal exposure to Streptococcus pneumoniae as well as mucosal or parenteral immunization with a recently developed killed pneumococcal whole cell vaccine, confer Th17-mediated protection against subsequent S. pneumoniae colonization in mice. Given our interest in the function of Th17 cells and the ongoing efforts to develop this vaccine for use in infants and children in developing countries, we analyzed Th17 responses to the whole cell antigen (WCA) and individual pneumococcal antigens in healthy individuals and patients with pneumococcal disease and compared responses in children and adults from Sweden and Bangladesh. Peripheral blood mononuclear cells (PBMCs) isolated from Swedish adults produced IL-17A after stimulation with WCA, with the pneumolysoid PdT and with the protein required for cell separation in group B streptococci (PcsB). IL-22 and IFN-γ responses were also detected, but these cytokines originated from separate CD4+ T cell subsets. PBMCs from Swedish children produced lower levels of IL-17A in response to WCA compared to adults, whereas no such difference was noted from the samples from Bangladesh, where responses by children and adults were both significantly higher than those in Sweden. High IL-17A responses to stimulation with WCA were also observed in children with proven or probable pneumococcal pneumonia. Our results thus demonstrate the presence of Th17-type T cells that are specific for pneumococcus in both children and adults. The different levels of Th17 responses to pneumococci in children and adults in developing and developed countries, which may at least partly be due to differences in exposure to pneumococci, are important factors to consider in the evaluation of candidate pneumococcal protein-based vaccines in human trials.

Protection against Streptococcus pneumoniae serotype 1 acute infection shows a signature of Th17- and IFN-γ-mediated immunity

Acute pneumonia caused by Streptococcus pneumoniae is a major cause of child mortality. Antibodies are considered the main effectors of protection in this clinical presentation of pneumococcal invasive disease. To get new insights into the mechanisms involved in the protective immunity, we established a murine experimental model of protection against acute pneumococcal pneumonia and then evaluated the transcriptional, humoral and cellular responses in protected and non-protected animals. We found that intranasal inoculation of a sublethal dose of S. pneumoniae serotype 1 conferred complete protection against a subsequent challenge with a lethal dose of the same strain. Sublethal infection elicited a strong IgM and IgG antibody response against the capsular polysaccharide, as assessed one week later, and an exacerbated influx of neutrophils into the lungs immediately after the lethal challenge. Genome-wide microarray-based transcriptional analysis of whole lungs showed 149 differentially expressed genes among which we found upregulation of Il17a, Ifng and several IL-17A- and IFN-γ-related genes in protected versus non-protected mice. Kinetics analysis showed higher expression levels of Il17a in protected animals at all time points whereas Ifng was upregulated early in the protected mice and later in the non-protected animals. Intracelluar cytokine staining demonstrated that CD4(+) T cells account for a great proportion of the IL-17A produced in the lungs of protected animals. Overall, these results showed that an upregulation of IL-17A- and a timely regulation of IFN-γ-related gene expression, together with development of a Th17 response, are relevant characteristics of the protective immunity against S. pneumoniae acute pneumonia.

Association of serotype-specific antibody concentrations and functional antibody titers with subsequent pneumococcal carriage in toddlers immunized with a 9-valent pneumococcal conjugate vaccine

Association of pneumococcal nasopharyngeal carriage with the concentration and opsonophagocytic activity (OPA) of serum serotype-specific antibodies was determined for toddlers 1 month after immunization with a 9-valent pneumococcal conjugate vaccine. Higher anti-serotype 14 and anti-serotype 19F IgG and anti-serotype 14 IgM correlated with a lowered probability of pneumococcal acquisition. Postvaccination OPA did not correlate with pneumococcal carriage.

Novel protein-based pneumococcal vaccines administered with the Th1-promoting adjuvant IC31 induce protective immunity against pneumococcal disease in neonatal mice

Streptococcus pneumoniae is responsible for many vaccine-preventable deaths, annually causing around 1 million deaths in children younger than 5 years of age. A new generation of pneumococcal vaccines based on conserved proteins is being developed. We evaluated the immunogenicities and protective efficacies of four pneumococcal protein vaccine candidates, PcsB, StkP, PsaA, and PspA, in a neonatal mouse model. Mice were immunized three times and challenged intranasally with virulent pneumococci. All four proteins were immunogenic in neonatal mice, and antibody (Ab) responses were significantly enhanced by the novel adjuvant IC31, which consists of an antibacterial peptide (KLKL5KLK) and a synthetic oligodeoxynucleotide, ODN1a, that signals through Toll-like receptor 9 (TLR9). Two single proteins, StkP and PspA, combined with IC31 significantly reduced pneumococcal bacteremia but had no effects on lung infection. Three proteins, PcsB, StkP, and PsaA, were evaluated with alum or IC31. IC31 enhanced Ab responses and avidity to all three proteins, whereas alum enhanced Ab responses and avidity to StkP and PsaA only. Mice receiving the trivalent protein formulation with IC31 had significantly reduced bacteremia and lung infection compared to unvaccinated mice, but the level of protection was dependent on the dose of IC31. When PspA was added to the trivalent protein formulation, the dose of IC31 needed to obtain protective immunity could be reduced. These results demonstrate that a novel pneumococcal protein-based vaccine is immunogenic at an early age of mice and emphasize the benefits of using a combination of conserved proteins and an effective adjuvant to elicit potent protective immunity against invasive pneumococcal disease.

Human monocytes promote Th1 and Th17 responses to Streptococcus pneumoniae

Streptococcus pneumoniae is a leading cause of bacterial pneumonia, meningitis, and sepsis in children. Human immunity to pneumococcal infections has been assumed to depend on anticapsular antibodies. However, recent findings from murine models suggest that alternative mechanisms, dependent on T helper cells, are also involved. Although the immunological events in which T helper cells contribute to acquired immunity have been studied in mice, little is known about how these responses are generated in humans. Therefore, we examined bacterial and host factors involved in the induction of Th1 and Th17 responses, using a coculture model of human monocytes and CD4(+) T cells. We show that monocytes promote effector cytokine production by memory T helper cells, leading to a mixed Th1/Th17 (gamma interferon [IFN-γ]/interleukin-17 [IL-17]) profile. Both T helper cytokines were triggered by purified pneumococcal peptidoglycan; however, the balance between the two immune effector arms depended on bacterial viability. Accordingly, live pneumococci triggered a Th1-biased response via monocyte production of IL-12p40, whereas heat-killed pneumococci triggered a Th17 response through TLR2 signaling. An increased understanding of human T helper responses is essential for the development of novel pneumococcal vaccines designed to elicit cell-mediated immunity.

T(H)17-based vaccine design for prevention of Streptococcus pneumoniae colonization

Streptococcus pneumoniae is a leading cause of mortality in young children. While successful conjugate polysaccharide vaccines exist, a less expensive serotype-independent protein-based pneumococcal vaccine offers a major advancement for preventing life-threatening pneumococcal infections, particularly in developing nations. IL-17A-secreting CD4+ T cells (T(H)17) mediate resistance to mucosal colonization by multiple pathogens including S. pneumoniae. Screening an expression library containing >96% of predicted pneumococcal proteins, we identified antigens recognized by T(H)17 cells from mice immune to pneumococcal colonization. The identified antigens also elicited IL-17A secretion from colonized mouse splenocytes and human PBMCs suggesting that similar responses are primed during natural exposure. Immunization of two mouse strains with identified antigens provided protection from pneumococcal colonization that was significantly diminished in animals treated with blocking CD4 or IL-17A antibodies. This work demonstrates the potential of proteomic screening approaches to identify specific antigens for the design of subunit vaccines against mucosal pathogens via harnessing T(H)17-mediated immunity.

Nasal immunization of mice with Lactobacillus casei expressing the pneumococcal surface protein C primes the immune system and decreases pneumococcal nasopharyngeal colonization in mice

Streptococcus pneumoniae colonizes the upper respiratory tract of healthy individuals, from where it can be transmitted to the community. Occasionally, bacteria invade sterile niches, causing diseases. The pneumococcal surface protein C (PspC) is a virulence factor that is important during colonization and the systemic phases of the diseases. Here, we have evaluated the effect of nasal or sublingual immunization of mice with Lactobacillus casei expressing PspC, as well as prime-boosting protocols using recombinant PspC, on nasopharyngeal pneumococcal colonization. None of the protocols tested was able to elicit significant levels of anti-PspC antibodies before challenge. However, a significant decrease in pneumococcal recovery from the nasopharynx was observed in animals immunized through the nasal route with L. casei-PspC. Immune responses evaluated after colonization challenge in this group of mice were characterized by an increase in mucosal anti-PspC immunoglobulin A (IgA) 5 days later, a time point in which the pneumococcal loads were already low. A negative correlation between the concentrations of anti-PspC IgA and pneumococcal recovery from the nasopharynx was observed, with animals with the lowest colonization levels having higher IgA concentrations. These results show that nasal immunization with L. casei-PspC primes the immune system of mice, prompting faster immune responses that result in a decrease in pneumococcal colonization.

Serine protease PrtA from Streptococcus pneumoniae plays a role in the killing of S. pneumoniae by apolactoferrin

It is known that apolactoferrin, the iron-free form of human lactoferrin, can kill many species of bacteria, including Streptococcus pneumoniae. Lactoferricin, an N-terminal peptide of apolactoferrin, and fragments of it are even more bactericidal than apolactoferrin. In this study we found that apolactoferrin must be cleaved by a serine protease in order for it to kill pneumococci. The serine protease inhibitors were able to block killing by apolactoferrin but did not block killing by a lactoferrin-derived peptide. Thus, the killing of pneumococci by apolactoferrin appears to require a protease to release a lactoferricin-like peptide(s). Incubation of apolactoferrin with growing pneumococci resulted in a 12-kDa reduction in its molecular mass, of which about 7 to 8 kDa of the reduction was protease dependent. Capsular type 2 and 19F strains with mutations in the gene encoding the major cell wall-associated serine protease, prtA, lost much of their ability to degrade apolactoferrin and were relatively resistant to killing by apolactoferrin (P < 0.001). Recombinant PrtA was also able to cleave apolactoferrin, reducing its mass by about 8 kDa, and greatly enhance the killing activity of the solution containing the apolactoferrin and its cleavage products. Mass spectroscopy revealed that PrtA makes a major cut between amino acids 78 and 79 of human lactoferrin, removing the N-terminal end of the molecule (about 8.6 kDa). The simplest interpretation of these data is that the mechanism by which apolactoferrin kills Streptococcus pneumoniae requires the release of a lactoferricin-like peptide(s) and that it is this peptide(s), and not the intact apolactoferrin, which kills pneumococci.

Inhibition of T cells provides protection against early invasive pneumococcal disease

Infections caused by Streptococcus pneumoniae are major causes of morbidity and mortality, which are in part mediated by immune cell-dependent mechanisms. Yet, the specific contributions of individual cell types to immunopathology are only partially understood. T cells are well characterized with respect to their function in protective humoral immune responses; however, their roles during early stages of infection and invasive pneumococcal disease (IPD) are less well defined. Using a mouse model of pneumococcal sepsis, we found that CD4(+) T cells were recruited to the lung as early as 12 h after intranasal infection. Recruitment was accompanied by upregulation of CD69 and B7-H1, reflecting T-cell activation. Unexpectedly, major histocompatibility complex (MHC) class II-deficient mice, which lack CD4(+) T cells, displayed an increased survival despite comparable bacterial titers in the blood, spleen, and lung. The higher survival correlated with a lower cytokine and chemokine response upon S. pneumoniae challenge in MHC class II-deficient mice, suggesting that inflammation may contribute to the mortality of IPD. Comparable to the case for MHC class II-deficient mice, antibody-mediated depletion of CD4(+) T cells and drug-induced inhibition of T-cell function with cyclosporine, or interference with T-cell activation using CTLA4-immunoglobulin (Abatacept), led to significant increases in survival during IPD. Our results reveal an important and adverse role of CD4(+) T cells in the pathogenesis of IPD and suggest that modulation of T-cell activation during early phases of S. pneumoniae invasive infection may provide a therapeutic option.

Protection against nasal colonization with Streptococcus pneumoniae by parenteral immunization with a DNA vaccine encoding PspA (Pneumococcal surface protein A)

Pneumococcal surface protein A (PspA) is an important candidate for a cost-effective vaccine with broad coverage against Streptococcus pneumoniae. We have previously shown that intramuscular immunization with PspA as a DNA vaccine induces an immune response characterized by the induction of a balanced IgG1/IgG2a antibody response in BALB/c mice, which was able to efficiently mediate complement deposition onto intact bacteria and to induce protection against an intraperitoneal challenge. We now confirm the results in C57BL/6 mice and further show that the response induced by the DNA vaccine expressing PspA is able to mediate protection against colonization of the nasopharyngeal mucosa even though immunization was given parenterally. Moreover, a positive correlation was observed between IgG1 and the numbers of CFU recovered, whereas an inverse correlation was observed between nasal CFU levels and IgG2a. A positive correlation was also found for IgG1/IgG2a antibody ratios with CFU recovered from the nasopharynx. Therefore, reduction of nasal colonization was strongly associated with increased levels of serum IgG2a complement fixing antibody and low levels of IgG1 antibody which has much less complement fixing activity. Passive transfer of serum from animals immunized with the DNA vaccine expressing PspA was also able to reduce the fraction of mice with high density of colonization of the nasopharynx. Secretion of IFN-gamma, but not IL-17, was observed in splenocytes from mice immunized with the DNA vaccine.

Antibody and cell-mediated immunity to Streptococcus pneumoniae: implications for vaccine development

It has long been assumed that children develop natural immunity to pneumococci via the acquisition of anticapsular antibodies, which confers serotype-specific immunity to the organism. This view has been further reinforced by the recent success of capsular polysaccharide conjugate vaccines in children in reducing colonization and disease caused by vaccine-type strains. Less clear, however, is whether this mechanism is responsible for the age-related gradual increased resistance to pneumococcal carriage and disease. Recent epidemiologic and experimental evidence point to the possibility that another mechanism may be involved. Here, an alternative possibility is presented, whereby it is proposed that acquired immunity to this common human pathogen is derived not only from natural acquisition of antibodies (capsular and noncapsular) that provides protection against invasive disease but also from the development of pneumococcus-specific CD4+ T(H)17 cells that reduces the duration of carriage and may also impact mucosal disease. This review focuses on the experimental and clinical evidence in support of this hypothesis. The implications for future vaccine development against Streptococcus pneumoniae are also discussed.

The immunising effect of pneumococcal nasopharyngeal colonisation; protection against future colonisation and fatal invasive disease

The human nasopharynx is an important ecological niche for Streptococcus pneumoniae, and asymptomatic nasopharyngeal carriage is a common precursor to invasive disease. However, knowledge of the immunological events, which occur during carriage, both on a cellular and humoral level, remains limited. Here, we present a long-term stable model of asymptomatic nasopharyngeal carriage using outbred naïve mice, in which we have investigated the effect of previous nasopharyngeal exposure to pneumococci, in the prevention of subsequent carriage and invasive disease. Carriage of D39 wildtype pneumococci restricted to the nasopharynx could be detected for at least 28 days post-infection, whereas nasopharyngeal carriage of a pneumolysin negative isogenic mutant (PLN-A) was cleared in 7-14 days. Both carriage events induced total and capsule specific IgA mucosal antibodies and increased levels of systemic antibodies (IgG against pneumococcal surface protein A (PspA) and IgM capsular polysaccharide), which increased over time and correlated to reduced nasopharyngeal pneumococcal numbers. Prior nasopharyngeal colonisation with PLN-A significantly reduced the duration of subsequent D39 wildtype carriage, and significantly increased survival following invasive pneumococcal challenge. In this case systemic anti-PspA and anti-capsular antibody IgM concentrations showed a strong correlation with reduced bacterial numbers in the lungs and nasopharynx, respectively and also with increased levels of IL17A and CD4+ T cells in lungs of pre-colonised mice. Prior nasopharyngeal colonisation with PLN-A also resulted in significant cross-serotype protection with mice protected from invasive disease with serotype 3 strain (A66) after pre-colonisation with a serotype 2 strain (D39). Our results suggest that both mucosal and systemic antibody as well as cellular host factors have a role in long-term protection against both colonisation and invasive pneumococcal challenge.

Mechanisms in the serotype-independent pneumococcal immunity induced in mice by intranasal vaccination with the cell wall polysaccharide

We previously reported that cell wall polysaccharide (CWPS) given to mice intranasally with adjuvant induces serotype-independent immunity to pneumococci. Some strains make CWPS with one phosphocholine group (CWPS/1), but most express two per tetrasaccharide repeat unit (CWPS/2). Here, CWPS/1 and CWPS/2 were equally protective against colonization by CWPS/2-type pneumococci, but the related Streptococcus mitis polymer lacking phosphocholine was non-protective. Previously the protection was shown to be CD4+T cell-dependent, abrogated by antiserum to interleukin (IL)-17A, and demonstrable in antibody-defective mice. Here, CWPS failed to protect IL-17A receptor knockout mice, further indicating IL-17A-dependence. When commercial CWPS/1 was size-fractionated preparatively, the larger exceeded the smaller molecules in their capacity to prime for IL-17A responses, and only the larger protected against pneumococcal colonization. However, a CWPS-tetanus toxoid conjugate - despite raising high titers of phosphocholine antibody - was non-protective, confirming the irrelevance of humoral immunity in this model. The results strengthen the concept that IL-17A-mediated T cell immunity is inducible by zwitterionic polysaccharides with sufficient chain length to provide coiled secondary structure. Coupling CWPS to protein, which paradoxically prevents protection, may occlude this regular linear conformation. We suggest that mucosal immunization with CWPS primes T(H)17 cells, which - upon contact with the phosphocholine of colonizing pneumococci - elaborate IL-17A, enhancing phagocytosis.

Pneumococcal carriage and acute otitis media induce serum antibodies to pneumococcal surface proteins CbpA and PhtD in children

We assessed the development and role of serum anti-CbpA and -PhtD in early childhood in relation to pneumococcal exposure. Serum IgG concentrations to CbpA and PhtD were measured with enzyme immunoassay in serum samples collected at the ages of 6, 12, 18, and 24 months from 50 healthy children and from 50 adults. Furthermore, antibodies to CbpA, PhtD and the C-terminal fragment of PhtD (PhtD C) were measured in serum samples collected at 12 (N=286) and 18 months (N=259) to evaluate the risk of subsequent pneumococcal acute otitis media (AOM) in relation to antibody concentrations. The increase in anti-CbpA and -PhtD concentrations was related to prior pneumococcal exposure. At 12 and 18 months, in the risk model of pneumococcal AOM adjusted for prior pneumococcal AOM, higher concentrations of anti-CbpA, but not anti-PhtD, were associated with a lowered risk of subsequent pneumococcal AOM. In conclusion, pneumococcal exposure induces the development of serum anti-CbpA and -PhtD in early childhood. Anti-CbpA antibodies may play a role in the prevention of subsequent pneumococcal AOM during the second year of life.

Antibodies to pneumococcal proteins PhtD, CbpA, and LytC in Filipino pregnant women and their infants in relation to pneumococcal carriage

This study focuses on the immunogenicity of the following three pneumococcal vaccine candidate proteins in Filipino infants, all inducing protection in animal models: pneumococcal histidine triad protein D (PhtD), choline binding protein A (CbpA), and the lysozyme LytC. The immunoglobulin G antibody concentrations to PhtD, its putative, protective, and exposed C-terminal fragment (PhtD C), CbpA, and LytC were measured by enzyme immunoassay in 52 serum samples from pregnant women, 39 cord blood samples, and consecutive serum samples (n = 263) from 52 newborns between 6 weeks and 10 months of age scheduled to be taken at six time points. A nasopharyngeal swab to detect pneumococcal carriage was taken parallel to the serum samples. The antibody concentrations in the cord blood samples were similar to those in the samples from the mothers. In infant sera, the geometric mean antibody concentrations (GMCs) for all three proteins decreased until the age of 18 weeks and started to increase after that age, suggesting that the infants' own antibody production started close to the age of 4 to 5 months. The increase in GMCs by age, most clear-cut for CbpA, was associated with pneumococcal carriage. Anti-PhtD concentrations were higher than anti-PhtD C concentrations but correlated well (r of 0.89 at 10.5 months), suggesting that antibodies are directed to the supposedly exposed and protective C-terminal part of PhtD. Our results show that young children are able to develop an antibody response to PhtD, CbpA, and LytC and encourage the development of pneumococcal protein vaccines for this age group.

Impaired innate and adaptive immunity to Streptococcus pneumoniae and its effect on colonization in an infant mouse model

Streptococcus pneumoniae colonization and invasive disease peak around the third and first birthdays, respectively, and decline thereafter. While these declines are attributable in part to immunity acquired via natural exposure, maturation of innate immune responses may also be involved. A mucosally administered candidate whole-cell pneumococcal vaccine (WCV) containing killed pneumococcal antigen (WCA) plus a cholera toxin adjuvant protects against intranasal carriage of pneumococci by a mechanism that is antibody independent and CD4(+) TH17 cell dependent. Because infants and children are a key target population for this vaccine, we sought to evaluate the immune responses of neonatal and infant mice to S. pneumoniae and to assess whether the WCV would be effective in these mice. Like human infants, infant mice showed impaired clearance of nasopharyngeal colonization with S. pneumoniae. Macrophages from neonatal and infant mice stimulated with killed pneumococci in vitro showed significantly reduced cytokine production, including that of KC, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, macrophage chemoattractant protein 1, interleukin-6 (IL-6), IL-1alpha, tumor necrosis factor alpha, and gamma interferon, whereas IL-10 expression was significantly increased compared to that in macrophages from adult mice. IL-17A production from adult immune CD4(+) T cells was significantly delayed when neonatal macrophages instead of adult macrophages were used as antigen-presenting cells. Moreover, whole blood from mice immunized as neonates with WCV produced significantly less IL-17A after stimulation with WCA than did blood from mice immunized as adults. Nonetheless, a single immunization of neonatal mice with WCV significantly reduced colonization density. Overall, our data suggest an impairment of both innate and acquired cellular immune responses in neonatal and infant mice. However, WCV confers a significant reduction in colonization following pneumococcal challenge, suggesting that it may still be effective in the setting of immature immune responses.

Interleukin-17A mediates acquired immunity to pneumococcal colonization

Although anticapsular antibodies confer serotype-specific immunity to pneumococci, children increase their ability to clear colonization before these antibodies appear, suggesting involvement of other mechanisms. We previously reported that intranasal immunization of mice with pneumococci confers CD4+ T cell–dependent, antibody- and serotype-independent protection against colonization. Here we show that this immunity, rather than preventing initiation of carriage, accelerates clearance over several days, accompanied by neutrophilic infiltration of the nasopharyngeal mucosa. Adoptive transfer of immune CD4+ T cells was sufficient to confer immunity to naïve RAG1^−/− mice. A critical role of interleukin (IL)-17A was demonstrated: mice lacking interferon-γ or IL-4 were protected, but not mice lacking IL-17A receptor or mice with neutrophil depletion. In vitro expression of IL-17A in response to pneumococci was assayed: lymphoid tissue from vaccinated mice expressed significantly more IL-17A than controls, and IL-17A expression from peripheral blood samples from immunized mice predicted protection in vivo. IL-17A was elicited by pneumococcal stimulation of tonsillar cells of children or adult blood but not cord blood. IL-17A increased pneumococcal killing by human neutrophils both in the absence and in the presence of antibodies and complement. We conclude that IL-17A mediates pneumococcal immunity in mice and probably in humans; its elicitation in vitro could help in the development of candidate pneumococcal vaccines.

The bacterium Streptococcus pneumoniae (pneumococcus) causes serious disease in children and the elderly, including pneumonia and meningitis (inflammation of the brain). Carriage of pneumococcus in the nose is a necessary first step for most infections. As children age, they carry pneumococcus for shorter periods of time and their risk of disease decreases also. The mechanisms underlying this age-related decrease of carriage are not well understood. A deeper understanding of resistance to colonization would enable us to develop better pneumococcal vaccines. Using experimental mouse models, we show that repeated exposure to pneumococci leads to a subsequent reduction in duration of pneumococcal carriage, similar to what is observed in humans. We identify the immune cells that are responsible for this process, so-called T_H17 cells, which release a factor that enables human blood cells to kill pneumococcus more efficiently. We show that these T_H17 cells exist in adults and children, but not in newborn babies, which suggests that they may arise as a consequence of humans being exposed to pneumococcus. We describe an assay for the measurement of these cells in humans. Such an assay could facilitate the development of novel vaccines directed against pneumococcal carriage.

Epidemiologic evidence for serotype-specific acquired immunity to pneumococcal carriage

Nasopharyngeal carriage of Streptococcus pneumoniae is required for transmission of the bacteria and for invasive disease. There have been conflicting reports as to whether protection against carriage is serotype specific and which immune mechanisms drive carriage. Analyzing longitudinal carriage data from Israeli toddlers in day care, we found a lower risk of colonization with types 6A, 14, and 23F after previous exposure to the homologous type. Nonsignificant trends suggesting possible protection derived from prior exposure were found for types 19A and 23A. Furthermore, we found that, for types 14 and 23F, this specific protection correlated with increased serotype-specific antibody concentration. We found no evidence of specific protection for type 6B, group 15, or type 19F. Our findings imply that at least some serotypes generate anti-capsular antibodies that can reduce the risk of carriage in unimmunized toddlers.

Protection against nasopharyngeal colonization by Streptococcus pneumoniae is mediated by antigen-specific CD4+ T cells

CD4(+) T-cell-dependent acquired immunity confers antibody-independent protection against pneumococcal colonization. Since this mechanism is poorly understood for extracellular bacteria, we assessed the antigen specificity of the induction and recall of this immune response by using BALB/c DO11.10Rag(-/-) mice, which lack mature B and T cells except for CD4(+) T cells specific for the OVA(323-339) peptide derived from ovalbumin. Serotype 6B Streptococcus pneumoniae strain 603S and unencapsulated strain Rx1Delta lytA were modified to express OVA(323-339) as a fusion protein with surface protein A (PspA) (strains 603OVA(1) and Rx1Delta lytAOVA(1)) or with PspA, neuraminidase A, and pneumolysin (Rx1Delta lytAOVA(3)). Whole-cell vaccines (WCV) were made of ethanol-killed cells of Rx1Delta lytA plus cholera toxin (CT) adjuvant, of Rx1Delta lytAOVA(1) + CT (WCV-OVA(1)), and of Rx1Delta lytAOVA(3) + CT (WCV-OVA(3)). Mice intranasally immunized with WCV-OVA(1), but not with WCV or CT alone, were protected against intranasal challenge with 603OVA(1). There was no protection against strain 603S in mice immunized with WCV-OVA(1). These results indicate antigen specificity of both immune induction and the recall response. Effector action was not restricted to antigen-bearing bacteria since colonization by 603S was reduced in animals immunized with vaccines made of OVA-expressing strains when ovalbumin or killed Rx1Delta lytAOVA(3) antigen was administered around the time of challenge. CD4(+) T-cell-mediated protection against pneumococcal colonization can be induced in an antigen-specific fashion and requires specific antigen for effective bacterial clearance, but this activity may extend beyond antigen-expressing bacteria. These results are consistent with the recruitment and/or activation of phagocytic or other nonspecific effectors by antigen-specific CD4(+) T cells.

Mouse models for the study of mucosal vaccination against otitis media

Otitis media (OM) is one of the most common infectious diseases in humans. The pathogenesis of OM involves nasopharyngeal (NP) colonization and retrograde ascension of the pathogen up the Eustachian tube into the middle ear (ME). Due to increasing rates of antibiotic resistance, there is an urgent need for vaccines to prevent infections caused by the most common causes of bacterial OM, including nontypeable Haemophilus influenzae, Streptococcus pneumoniae and Moraxella catarrhalis. Current vaccine strategies aim to diminish bacterial NP carriage, thereby reducing the likelihood of developing acute OM. To be effective, vaccination should induce local mucosal immunity both in the ME and in the NP. Studies in animal models have demonstrated that the intranasal route of vaccination is particularly effective at inducing immune responses in the nasal passage and ME for protection against OM. The mouse is increasingly used in these models, because of the availability of murine reagents and the existence of technology to manipulate murine models of disease immunologically and genetically. Previous studies confirmed the suitability of the mouse as a model for inflammatory processes in acute OM. Here, we discuss various murine models of OM and review the applicability of these models to assess the efficacy of mucosal vaccination and the mechanisms responsible for protection. In addition, we discuss various mucosal vaccine antigens, mucosal adjuvants and mucosal delivery systems.

Patterns of antigenic diversity and the mechanisms that maintain them

Many of the remaining challenges in infectious disease control involve pathogens that fail to elicit long-lasting immunity in their hosts. Antigenic variation is a common reason for this failure and a contributor to the complexity of vaccine design. Diversifying selection by the host immune system is commonly, and often correctly, invoked to explain antigenic variability in pathogens. However, there is a wide variety of patterns of antigenic variation across space and time, and within and between hosts, and we do not yet understand the determinants of these different patterns. This review describes five such patterns, taking as examples two bacteria (Streptococcus pneumoniae and Neisseria meningitidis), two viruses (influenza A and HIV-1), as well as the pathogens (taken as a group) for which antigenic variation is negligible. Pathogen-specific explanations for these patterns of diversity are critically evaluated, and the patterns are compared against predictions of theoretical models for antigenic diversity. Major remaining challenges are highlighted, including the identification of key protective antigens in bacteria, the design of vaccines to combat antigenic variability for viruses and the development of more systematic explanations for patterns of antigenic variation.