Are anticapsular antibodies the primary mechanism of protection against invasive pneumococcal disease?

Serum antibody responses to pneumococcal colonization in the first 2 years of life: results from an SE Asian longitudinal cohort study

Assessment of antibody responses to pneumococcal colonization in early childhood may aid our understanding of protection and inform vaccine antigen selection. Serum samples were collected from mother-infant pairs during a longitudinal pneumococcal colonization study in Burmese refugees. Maternal and cord sera were collected at birth and infants were bled monthly (1–24 months of age). Nasopharyngeal swabs were taken monthly to detect colonization. Serum IgG titres to 27 pneumococcal protein antigens were measured in 2624 sera and IgG to dominant serotypes (6B, 14, 19F, 19A and 23F) were quantified in 864 infant sera. Antibodies to all protein antigens were detect ablein maternal sera. Titres to four proteins (LytB, PcpA, PhtD and PhtE) were significantly higher in mothers colonized by pneumococci at delivery. Maternally-derived antibodies to PiuA and Spr0096 were associated with delayed pneumococcal acquisition in infants in univariate,but not multivariate models. Controlling for infant age and previous homologous serotype exposure, nasopharyngeal acquisition of serotypes 19A, 23F, 14 or 19F was associated significantly with a ≥2-fold antibody response to the homologous capsule (OR 12.84, 7.52,6.52, 5.33; p <0.05). Acquisition of pneumococcal serotypes in the nasopharynx of infants was not significantly associated with a ≥2-fold rise in antibodies to any of the protein antigens studied. In conclusion, nasopharyngeal colonization in young children resulted in demonstrable serum IgG responses to pneumococcal capsules and surface/virulence proteins. However, the relationship between serum IgG and the prevention of, or response to, pneumococcal nasopharyngeal colonization remains complex. Mechanisms other than serum IgG are likely to have a role but are currently poorly understood.

Adjuvant system AS02V enhances humoral and cellular immune responses to pneumococcal protein PhtD vaccine in healthy young and older adults: randomised, controlled trials

BACKGROUND

The protection elicited by polysaccharide pneumococcal vaccines against community-acquired pneumonia in older adults remains debatable. Alternative vaccine targets include well-conserved pneumococcal protein antigens, such as pneumococcal histidine triad protein D (PhtD).

OBJECTIVE

To evaluate humoral and cellular immune responses and safety/reactogenicity following immunisation with PhtD vaccine with or without adjuvant (alum or AS02V) in older (≥65 years) and young (18-45 years) healthy adults.

METHODS

Two phase I/II, single-blind, parallel-group studies were conducted in 150 older and 147 young adults. Participants were randomised to receive 2 doses (months 0 and 2) of PhtD 30 μg, PhtD 10 μg plus alum, PhtD 30 μg plus alum, PhtD 10 μg plus AS02V or PhtD 30 μg plus AS02V, or the 23-valent polysaccharide pneumococcal vaccine (23PPV) at month 0 with placebo (saline solution) at month 2. Safety/reactogenicity was assessed. PhtD-specific antibody, T cell and memory B cell responses were evaluated.

RESULTS

Solicited adverse events were more common in young participants and with adjuvanted vaccines. No vaccine-related serious adverse events were reported. Although anti-PhtD geometric mean antibody concentrations (GMCs) were consistently lower in the older adult cohort than in young adults, GMCs in the older cohort following PhtD 30 μg plus AS02V were comparable to those induced by plain PhtD or PhtD 30 μg plus alum in the young cohort. Compared with alum adjuvant, AS02V adjuvant system was associated with an increased frequency of PhtD-specific CD4 cells in both cohorts and a significantly higher specific memory B cell response in the older cohort, similar to responses obtained in the young cohort.

CONCLUSION

The improved immune response to PhtD vaccine containing the AS02V adjuvant system in comparison to alum suggests that the reduced immune response to vaccines in older adults can be partially restored to the response level observed in young adults. ClinicalTrials.gov identifiers: NCT00307528/NCT01767402.

A mathematical model of serotype replacement in pneumococcal carriage following vaccination

A number of childhood vaccination programmes have recently introduced vaccination against Streptococcus pneumoniae, the pneumococcus, a major cause of pneumonia and meningitis. The pneumococcal conjugate vaccines (PCVs) that are currently in use only protect against some serotypes of the bacterium, and there is now strong evidence that those serotypes not included in the vaccine increase in prevalence among most vaccinated populations. We present a mathematical model for the dynamics of nasopharyngeal carriage of S. pneumoniae that allows for carriage with multiple serotypes. The model is used to predict the prevalence of vaccine type (VT) and non-VT (NVT) serotypes following the introduction of PCV. Parameter estimates for the model are obtained by maximum likelihood using pre-vaccination data from The Gambia. The model predicts that low (1, 6A and 9V) and medium (4, 5, 7F, 14, 18C, 19A and 19F) prevalence serotypes can be eliminated through vaccination, but that the overall prevalence of carriage will be reduced only slightly because of an increase in the prevalence of NVT serotypes. Serotype replacement will be sequential, with high and medium prevalence NVT serotypes dominating initially, followed by an increase of serotypes of low prevalence. We examine the impact of a hypothetical vaccine that provides partial protection against all serotypes, and find that this reduces overall carriage, but is unable to eliminate low or medium prevalence serotypes.

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.

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.

Controlled human infection and rechallenge with Streptococcus pneumoniae reveals the protective efficacy of carriage in healthy adults

RATIONALE

The immunological and protective role of pneumococcal carriage in healthy adults is not known, but high rates of disease and death in the elderly are associated with low carriage prevalence.

OBJECTIVES

We employed an experimental human pneumococcal carriage model to investigate the immunizing effect of a single carriage episode.

METHODS

Seventy healthy adults were challenged, and of those with carriage, 10 were rechallenged intranasally with live 6B Streptococcus pneumoniae up to 11 months after clearance of the first carriage episode. Serum and nasal wash antibody responses were measured before and after each challenge.

MEASUREMENTS AND MAIN RESULTS

A total of 29 subjects were experimentally colonized. No subjects were colonized by experimental rechallenge, demonstrating the protective effect of initial carriage against subsequent infection. Carriage increased both mucosal and serum IgG levels to pneumococcal proteins and polysaccharide, resulting in a fourfold increase in opsonophagocytic activity. Importantly, passive transfer of postcarriage sera from colonized subjects conferred 70% protection against lethal challenge by a heterologous strain in a murine model of invasive pneumococcal pneumonia. These levels were significantly higher than the protection conferred by either precarriage sera (30%) or saline (10%).

CONCLUSIONS

Experimental human carriage resulted in mucosal and systemic immunological responses that conferred protection against recolonization and invasive pneumococcal disease. These data suggest that mucosal pneumococcal vaccination strategies may be important for vulnerable patient groups, particularly the elderly, who do not sustain carriage.

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.

The overlap of bronchiectasis and immunodeficiency with asthma

Bronchiectasis should be considered as a differential diagnosis for, as well as a comorbidity in, patients with asthma, especially severe or long-standing asthma. Chronic airway inflammation is thought to be the primary cause, as with chronic or recurrent pulmonary infection and autoimmune conditions that involve the airways. Consequently, immunodeficiencies with associated increased susceptibility to respiratory tract infections or chronic inflammatory airways also increase the risk of developing bronchiectasis. Chronic bronchiectasis is associated with impaired mucociliary clearance and increased bronchial secretions, leading to airway obstruction and airflow limitation, which can lead to exacerbation of underlying asthma or increased asthma symptoms.

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.

Interactions between Streptococcus pneumoniae and influenza virus: a mutually beneficial relationship?

Historically, most research on infectious diseases has focused on infections with single pathogens. However, infections with pathogens often occur in the context of pre-existing viral and bacterial infections. Clinically, this is of particular relevance for coinfections with Streptococcus pneumoniae and influenza virus, which together are an important cause of global morbidity and mortality. In recent years new evidence has emerged regarding the underlying mechanisms of influenza virus-induced susceptibility to secondary pneumococcal infections, in particular regarding the sustained suppression of innate recognition of S. pneumoniae. Conversely, it is also increasingly being recognized that there is not a unidirectional effect of the virus on S. pneumoniae, but that asymptomatic pneumococcal carriage may also affect subsequent influenza virus infection and the clinical outcome. Here, we will review both aspects of pneumococcal influenza virus infection, with a particular focus on the age-related differences in pneumococcal colonization rates and invasive pneumococcal disease.

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.

Human nasal challenge with Streptococcus pneumoniae is immunising in the absence of carriage

Infectious challenge of the human nasal mucosa elicits immune responses that determine the fate of the host-bacterial interaction; leading either to clearance, colonisation and/or disease. Persistent antigenic exposure from pneumococcal colonisation can induce both humoral and cellular defences that are protective against carriage and disease. We challenged healthy adults intra-nasally with live 23F or 6B Streptococcus pneumoniae in two sequential cohorts and collected nasal wash, bronchoalveolar lavage (BAL) and blood before and 6 weeks after challenge. We hypothesised that both cohorts would successfully become colonised but this did not occur except for one volunteer. The effect of bacterial challenge without colonisation in healthy adults has not been previously assessed. We measured the antigen-specific humoral and cellular immune responses in challenged but not colonised volunteers by ELISA and Flow Cytometry. Antigen-specific responses were seen in each compartment both before and after bacterial challenge for both cohorts. Antigen-specific IgG and IgA levels were significantly elevated in nasal wash 6 weeks after challenge compared to baseline. Immunoglobulin responses to pneumococci were directed towards various protein targets but not capsular polysaccharide. 23F but not 6B challenge elevated IgG anti-PspA in BAL. Serum immunoglobulins did not increase in response to challenge. In neither challenge cohort was there any alteration in the frequencies of TNF, IL-17 or IFNγ producing CD4 T cells before or after challenge in BAL or blood. We show that simple, low dose mucosal exposure with pneumococci may immunise mucosal surfaces by augmenting anti-protein immunoglobulin responses; but not capsular or cellular responses. We hypothesise that mucosal exposure alone may not replicate the systemic immunising effect of experimental or natural carriage in humans.

Exposure to respiratory pathogens such as Streptococcus pneumoniae (pneumococcus) is a frequent event that can result in immediate clearance, nasal colonisation or disease for the host. Human and mouse studies have shown that natural colonisation is an immunising event. Colonisation is prevalent in children but rare in human adults (<10%), suggesting that despite high pneumococcal exposure adult mucosal defences are sufficient to prevent colonisation. We exposed healthy adults to pneumococci in the nose in order to achieve colonisation and mimic a natural colonisation event. In most volunteers, however, we were not able to obtain colonisation using this protocol. In exposed but not colonised volunteers we measured antibody and cellular responses in nose, lung and blood samples. The mucosal defences elicited during acute pneumococcal exposure are poorly described but these data will shed light on the mechanisms that prevent colonisation in healthy adults and inform future vaccine design. Live bacterial exposure increases specific antibody and innate responses at mucosal surfaces such as the nose and lung. Systemic responses were not increased. These data suggest that acute bacterial exposure per se augments mucosal but not systemic defences. Natural or experimental colonisation may be required for systemic immunisation.

Serotype-independent pneumococcal experimental vaccines that induce cellular as well as humoral immunity

For prevention of Streptococcus pneumoniae (pneumococcus) infections in infancy, protein-conjugated capsular polysaccharide vaccines provide serotype-specific, antibody-mediated immunity but do not cover all of the 90+ capsule serotypes. Therefore, microbiologists have sought protective noncapsular antigens common to all strains. Alternatively, we investigated killed cells of a noncapsulated strain, which expose many such common antigens. Given to mice intranasally, this vaccine elicits antibody-independent, CD4+ T lymphocyte-dependent accelerated clearance of pneumococci of various serotypes from the nasopharynx mediated by the cytokine IL-17A. Such immunity may reproduce the natural resistance that develops in infants before capsular antibodies arise. Given by injection, the killed cell vaccine induces bifunctional immunity: plasma antibodies protective against fatal pneumonia challenge, as well as IL-17A-mediated nasopharyngeal clearance. Human testing of this inexpensive candidate vaccine by intramuscular injection is planned. Bacterial cellular vaccines are complex--a challenge for reproducibility. However, when several known protective antigens were deleted, the killed pneumococcal vaccine was still protective. This antigenic redundancy may prevent vaccine escape variants by recombinational loss, which is frequent in pneumococcus. Biochemically defined immunogens with bifunctional activity have also been devised. These immunogens are three-component conjugates in which cell wall teichoic acid (a common antigen capable of T cell activation) is coupled to a genetic fusion of two common pneumococcal proteins: a protective surface antigen and a derivative of pneumolysin, which provides TLR4 agonist activity and induces antitoxic immunity. Such constructs induce accelerated clearance when given intranasally and induce both immune mechanisms when injected. The defined composition permits analysis of structure-function activity.

What is different about serotype 1 pneumococci?

Serotype 1 Streptococcus pneumoniae is among the most commonly isolated serotype in invasive pneumococcal disease but is rarely found causing asymptomatic nasopharyngeal colonization. Compared to infection by other serotypes, infection caused by serotype 1 is more likely to be identified in young patients without comorbidities but is generally associated with a lower mortality. Empyema and extrapulmonary manifestations are common. Outbreaks of serotype 1 disease have been reported in closed communities and epidemics are particularly common in sub-Saharan Africa. The serotype 1 capsular polysaccharide is a zwitterionic structure that enables it to function as a T-cell dependent antigen under some circumstances, in contrast to other pneumococcal capsular polysaccharides that are T-cell independent antigens. There are also differences in the key virulence factor pneumolysin in some serotype 1 isolates. The clinical significance of these differences remains to be determined.

Acquisition of pneumococci specific effector and regulatory Cd4+ T cells localising within human upper respiratory-tract mucosal lymphoid tissue

The upper respiratory tract mucosa is the location for commensal Streptococcus (S.) pneumoniae colonization and therefore represents a major site of contact between host and bacteria. The CD4⁺ T cell response to pneumococcus is increasingly recognised as an important mediator of immunity that protects against invasive disease, with data suggesting a critical role for Th17 cells in mucosal clearance. By assessing CD4 T cell proliferative responses we demonstrate age-related sequestration of Th1 and Th17 CD4⁺ T cells reactive to pneumococcal protein antigens within mucosal lymphoid tissue. CD25^hi T cell depletion and utilisation of pneumococcal specific MHCII tetramers revealed the presence of antigen specific Tregs that utilised CTLA-4 and PDL-1 surface molecules to suppress these responses. The balance between mucosal effector and regulatory CD4⁺ T cell immunity is likely to be critical to pneumococcal commensalism and the prevention of unwanted pathology associated with carriage. However, if dysregulated, such responses may render the host more susceptible to invasive pneumococcal infection and adversely affect the successful implementation of both polysaccharide-conjugate and novel protein-based pneumococcal vaccines.

The S. pneumoniae bacteria is a major cause of disease (e.g. pneumonia and meningitis) particularly affecting infants. In most cases bacteria can colonise the nose without causing harm, however colonisation is thought to be a prerequisite of disease. With increasing age colonization and disease, rates gradually decrease which is likely due to the development of immunity to the pneumococcus with age. The CD4 T cells of the immune system may contribute to the defence against bacterial colonisation by producing factors that promote pneumococcal killing. Herein, we show that CD4 T cells reactive to pneumococci are found in greater numbers at the site of colonisation and gradually increase in their levels from infancy. However, at the peak of CD4 T cell responses from late teens, we detected the presence of regulatory T cells (Tregs) which suppressed anti-pneumococci CD4 T cell activity greatly. Our finding shows that pneumococcal reactive CD4 T cells selectively populate colonisation sites and increase with age as a result of ongoing bacterial exposure throughout life, inversely correlating with colonisation and disease rates. As factors that utilise CD4 T cells become increasingly advocated as potential preventative strategies against pneumococcal carriage and disease, the observed effect of Tregs must be considered.

Direct ex-vivo evaluation of pneumococcal specific T-cells in healthy adults

Streptococcus pneumoniae is an encapsulated bacterium that causes significant global morbidity and mortality. The nasopharynxes of children are believed to be the natural reservoir of pneumococcus and by adulthood nasopharyngeal carriage is infrequent; such infrequency may be due to demonstrable pneumococcal specific T and B-cell responses. HLA Class 2 tetrameric complexes have been used to characterise antigen specific T-cell responses in a variety of models of infection. We therefore sought to determine the frequency and phenotype of pneumococcal specific T-cells, using a novel HLA-DRB1*1501 tetramer complex incorporating a recently defined T-cell epitope derived from the conserved pneumococcal serine/threonine kinase (StkP). We were able to detect direct ex-vivo StkP_446–60-tetramer binding in HLA-DRB1*1501 adults. These StkP_446–60-tetramer binding T-cells had increased CD38 expression and were enriched in CCR7- CD45RA+ expression indicating recent and on-going activation and differentiation. Furthermore, these StkP_446–60-tetramer binding T-cells demonstrated rapid effector function by secreting interferon-gamma on stimulation with recombinant StkP. This is the first study to directly enumerate and characterise pneumococcal specific T-cells using HLA class 2 tetrameric complexes. We found that ex-vivo pneumococcal-specific T cells were detectable in healthy adults and that they were enriched with cell surface markers associated with recent antigen exposure and later stages of antigen-driven differentiation. It is likely that these activated pneumococcal specific T-cells reflect recent immunostimulatory pneumococcal exposure in the nasopharynx and it is possible that they may be preventing subsequent colonisation and disease.

Protective contributions against invasive Streptococcus pneumoniae pneumonia of antibody and Th17-cell responses to nasopharyngeal colonisation

The nasopharyngeal commensal bacteria Streptococcus pneumoniae is also a frequent cause of serious infections. Nasopharyngeal colonisation with S. pneumoniae inhibits subsequent re-colonisation by inducing Th17-cell adaptive responses, whereas vaccination prevents invasive infections by inducing antibodies to S. pneumoniae capsular polysaccharides. In contrast, protection against invasive infection after nasopharyngeal colonisation with mutant S. pneumoniae strains was associated with antibody responses to protein antigens. The role of colonisation-induced Th17-cell responses during subsequent invasive infections is unknown. Using mouse models, we show that previous colonisation with S. pneumoniae protects against subsequent lethal pneumonia mainly by preventing bacteraemia with a more modest effect on local control of infection within the lung. Previous colonisation resulted in CD4-dependent increased levels of Th17-cell cytokines during subsequent infectious challenge. However, mice depleted of CD4 cells prior to challenge remained protected against bacteraemia, whereas no protection was seen in antibody deficient mice and similar protection could be achieved through passive transfer of serum. Serum from colonised mice but not antibody deficient mice promoted phagocytosis of S. pneumoniae, and previously colonised mice were able to rapidly clear S. pneumoniae from the blood after intravenous inoculation. Thus, despite priming for a Th17-cell response during subsequent infection, the protective effects of prior colonisation in this model was not dependent on CD4 cells but on rapid clearance of bacteria from the blood by antibody-mediated phagocytosis. These data suggest that whilst nasopharyngeal colonisation induces a range of immune responses, the effective protective responses depend upon the site of subsequent infection.

Impaired CD4 T cell memory response to Streptococcus pneumoniae precedes CD4 T cell depletion in HIV-infected Malawian adults

OBJECTIVE

Invasive pneumococcal disease (IPD) is a leading cause of morbidity and mortality in HIV-infected African adults. CD4 T cell depletion may partially explain this high disease burden but those with relatively preserved T cell numbers are still at increased risk of IPD. This study evaluated the extent of pneumococcal-specific T cell memory dysfunction in asymptomatic HIV infection early on in the evolution of the disease.

METHODS

Peripheral blood mononuclear cells were isolated from asymptomatic HIV-infected and HIV-uninfected Malawian adults and stained to characterize the underlying degree of CD4 T cell immune activation, senescence and regulation. Pneumococcal-specific T cell proliferation, IFN-γ, IL-17 production and CD154 expression was assessed using flow cytometry and ELISpot.

RESULTS

We find that in asymptomatic HIV-infected Malawian adults, there is considerable immune disruption with an increase in activated and senescent CD4+CD38+PD-1+ and CD4+CD25(high)Foxp3+ Treg cells. In the context of high pneumococcal exposure and therefore immune stimulation, show a failure in pneumococcal-specific memory T cell proliferation, skewing of T cell cytokine production with preservation of interleukin-17 but decreased interferon-gamma responses, and failure of activated T cells to express the co-stimulatory molecule CD154.

CONCLUSION

Asymptomatic HIV-infected Malawian adults show early signs of pneumococcal- specific immune dysregulation with a shift in the balance of CD4 memory, T helper 17 cells and Treg. Together these data offer a mechanistic understanding of how antigen-specific T cell dysfunction occurs prior to T cell depletion and may explain the early susceptibility to IPD in those with relatively preserved CD4 T cell numbers.

Failure to elicit seroresponses to pneumococcal surface proteins (pneumococcal histidine triad D, pneumococcal choline-binding protein A, and serine proteinase precursor A) in children with pneumococcal bacteraemia

Pneumococcal surface proteins (PSPs) elicit antibody responses in infants and young children exposed to Streptococcus pneumoniae. These seroresponses could contribute to the aetiological diagnosis of pneumococcal disease, e.g. during the clinical development of novel PSP-based vaccines. In this study, we assessed the kinetics of antibody responses to three highly conserved and immunogenic PSPs (pneumococcal histidine triad D (PhtD), pneumococcal choline-binding protein A (PcpA), and serine proteinase precursor A (PrtA)) in 106 children (median age, 21.3 months; males, 58.5%) admitted for pneumococcal bacteraemia. Anti-PhtD, anti-PcpA and anti-PrtA antibodies were measured by ELISA, and compared in 61 pairs of acute (≤7 days) and convalescent (>14 days of admission) serum samples. Acute serum titres were similar to those observed in healthy children, and were unaffected by the acid dissociation of circulating immune complexes. Despite proven bacteraemia, seroresponses (≥2-fold increase in anti-PSP antibody concentrations) were only identified in 31 of 61 children (50.8%), directed against PrtA (n = 23, 37.7%), PcpA (n = 19, 31.1%), and PhtD (n = 16, 26.2%), or several PSPs (two PSPs, n = 13, 21.3%; three PSPs, n = 7, 11.5%). Certain seroresponses were very strong (maximal fold-increases: PhtD, 26; PcpA, 72; PrtA, 12). However, anti-PSP antibody concentrations failed to increase in the convalescent sera of 30 of 61 (49.2%) bacteraemic children, and even declined (≥2 fold) in 13 of 61 (21.3%), mostly infants aged <6 months (8/13, 61.5%), possibly through consumption of maternal antibodies. Thus, pneumococcal bacteraemia may fail to elicit antibody responses, and may even have an antibody-depleting effect in infants. This novel observation identifies an important limitation of serology-based studies for the identification of bacteraemic children.

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.

Evicting the pneumococcus from its nasopharyngeal lodgings

Nasopharyngeal colonization by Streptococcus pneumoniae precedes invasive disease and mediates community transmission of the pathogen. In this issue, Moffitt et al. (2011) used proteomic analysis to identify conserved pneumococcal protein vaccine antigens that elicit T(H)17-dependent responses capable of preventing such colonization.

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.

Molecular identification of Coccidioides spp. in soil samples from Brazil

Background

Since 1991 several outbreaks of acute coccidioidomycosis (CM) were diagnosed in the semi-arid Northeast of Brazil, mainly related to disturbance of armadillo burrows caused by hunters while digging them for the capture of these animals. This activity causes dust contaminated with arthroconidia of Coccidioides posadasii, which, once inhaled, cause the mycosis. We report on the identification of C. posadasii in soil samples related to outbreaks of CM.

Results

Twenty four soil samples had their DNA extracted and subsequently submitted to a semi-nested PCR technique using specific primers. While only 6 (25%) soil samples were positive for C. posadasii by mice inoculation, all (100%) were positive by the molecular tool.

Conclusion

This methodology represents a simple, sensitive and specific molecular technique to determine the environmental distribution of Coccidioides spp. in endemic areas, but cannot distinguish the species. Moreover, it may be useful to identify culture isolates. Key-words: 1. Coccidioidomycosis. 2. Coccidioides spp. 3. C. posadasii. 4. Semi-arid. 5. Semi-nested PCR

Next generation pneumococcal vaccines

Currently licensed pneumococcal vaccines are based on the generation of antibodies to the pneumococcal polysaccharide, of which there are more than 90 different types. While these vaccines are highly effective against the serotypes included, their high cost and limited serotype coverage limit their usefulness worldwide, particularly in low resource areas. Thus alternative or adjunctive options are being actively pursued. This review will present these various approaches, including variations of the polysaccharide-protein conjugate strategy, protein-based strategies, and whole cell pneumococcal vaccines. The immunological basis for these different approaches is discussed as well.

Serotype specific invasive capacity and persistent reduction in invasive pneumococcal disease

Defining the propensity of Streptococcus pneumoniae (SP) serotypes to invade sterile body sites following nasopharyngeal (NP) acquisition has the potential to inform about how much invasive pneumococcal disease (IPD) may occur in a typical population with a given distribution of carriage serotypes. Data from enhanced surveillance for IPD in Massachusetts children ≤7 years in 2003/04, 2006/07 and 2008/09 seasons and surveillance of SP NP carriage during the corresponding respiratory seasons in 16 Massachusetts communities in 2003/04 and 8 of the 16 communities in both 2006/07 and 2008/09 were used to compute a serotype specific "invasive capacity (IC)" by dividing the incidence of IPD due to serotype x by the carriage prevalence of that same serotype in children of the same age. A total of 206 IPD and 806 NP isolates of SP were collected during the study period. An approximate 50-fold variation in the point estimates between the serotypes having the highest (18C, 33F, 7F, 19A, 3 and 22F) and lowest (6C, 23A, 35F, 11A, 35B, 19F, 15A, and 15BC) IC was observed. Point estimates of IC for most of the common serotypes currently colonizing children in Massachusetts were low and likely explain the continued reduction in IPD from the pre-PCV era in the absence of specific protection against these serotypes. Invasive capacity differs among serotypes and as new pneumococcal conjugate vaccines are introduced, ongoing surveillance will be essential to monitor whether serotypes with high invasive capacity emerge (e.g. 33F, 22F) as successful colonizers resulting in increased IPD incidence due to replacement serotypes.

Enhanced in vivo activity of cefditoren in pre-immunized mice against penicillin-resistant S. pneumoniae (serotypes 6B, 19F and 23F) in a sepsis model

BACKGROUND

Specific antibodies are likely to be present before S. pneumoniae infection. We explored cefditoren (CDN) total and free values of serum concentrations exceeding the MIC (t>MIC) related to efficacy in a mice sepsis model, and the effect of specific gammaglobulins on in-vitro phagocytosis and in-vivo efficacy.

METHODOLOGY/PRINCIPAL FINDINGS

We used three pneumococcal isolates (serotype, MIC OF CDN): Strain 1 (6B, 1 microg/ml), Strain 2 (19F, 2 microg/ml) and Strain 3 (23F, 4 microg/ml). Hyperimmune serum (HS) was obtained from mice immunized with heat-inactivated strains. In-vitro, phagocytosis by HS diluted 1/10 in presence/absence of sub-inhibitory concentrations was measured by flow cytometry including fluorescent bacteria and a neutrophil cell line. In-vivo dose-ranging experiments with HS (dilutions 1/2-1/16) and CDN (6.25 mg/kg-100 mg/kg tid for 48 h) were performed to determine the minimal protective dilution/dose (highest survival) and the non-protective highest dilution/dose (highest mortality: HS-np dilution and CDN-np dose) over 7 days. Efficacy of CDN-np in animals pre-immunized with HS-np (combined strategy) was explored and blood bacterial clearance determined. The CDN measured protein binding was 86.9%. In-vitro, CDN significantly increased phagocytosis (vs. HS 1/10). In non pre-immunized animals, t>MIC values for CDN of approximately 35% (total) and approximately 19% (free) were associated with 100% survival. Significant differences in survival were found between HS-np alone (< or = 20%) or CDN-np alone (< or = 20%) vs. the combined strategy (90%, 60% and 60% for Stains 1, 2 and 3), with t>MIC (total/free) of 22.8%/14.3%, 26.8%/16.0%, and 22.4%/12.7% for Strains 1, 2 and 3, respectively. Prior to the second dose (8 h), median bacterial counts were significantly lower in animals surviving vs. dead at day 7.

CONCLUSIONS/SIGNIFICANCE

In mice (CDN protein binding similar to humans) total t>MIC values of approximately 35% (approximately 19% free) were efficacious, with a decrease in the required values in pre-immunized animals. This reinforces that immunoprotection to overcome resistance may provide lifesaving strategies.

The dynamics of nasopharyngeal streptococcus pneumoniae carriage among rural Gambian mother-infant pairs

Background

Streptococcus pneumoniae is an important cause of community acquired pneumonia, sepsis, meningitis and otitis media globally and has been incriminated as a major cause of serious childhood bacterial infections in The Gambia. Better understanding of the dynamics of transmission and carriage will inform control strategies.

Methods

This study was conducted among 196 mother-infant pairs recruited at birth from six villages in the West Kiang region of The Gambia. Nasopharyngeal swabs were collected from mother-infant pairs at birth (within 12 hours of delivery), 2, 5 and 12 months. Standard techniques of culture were used to identify carriage and serotype S. pneumoniae.

Results

Of 46 serotypes identified, the 6 most common, 6A, 6B, 14, 15, 19F and 23F, accounted for 67.3% of the isolates from infants. Carriage of any serotype among infants rose from 1.5% at birth to plateau at approximately 80% by 2 m (prevalence at 2 m = 77%; 5 m = 86%; 12 m = 78%). Likewise, maternal carriage almost doubled in the first 2 months post-partum and remained elevated for the next 10 m (prevalence at birth = 13%; 2 m = 24%; 5 m = 22%; 12 m = 21%). Carriage was significantly seasonal in both infants and mothers with a peak in December and lowest transmission in August. The total number of different serotypes we isolated from each infant varied and less than would be expected had the serotypes assorted independently. In contrast, this variability was much as expected among mothers. The half-life of a serotype colony was estimated to be 1.90 m (CI_95%: 1.66-2.21) in infants and 0.75 m (CI_95%: 0.55-1.19) in mothers. While the odds for a serotype to be isolated from an infant increased by 9-fold if it had also been isolated from the mother, the population attributable fraction (PAF) of pneumococcal carriage in infants due to maternal carriage was only 9.5%. Some marked differences in dynamics were observed between vaccine and non-vaccine serotypes.

Conclusions

Colonisation of the nasopharynx in Gambian infants by S. pneumoniae is rapid and highly dynamic. Immunity or inter-serotype competition may play a role in the dynamics. Reducing mother-infant transmission would have a minimal effect on infant carriage.

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.

Cellular effectors mediating Th17-dependent clearance of pneumococcal colonization in mice

Microbial colonization of mucosal surfaces may be an initial event in the progression to disease, and it is often a transient process. For the extracellular pathogen Streptococcus pneumoniae studied in a mouse model, nasopharyngeal carriage is eliminated over a period of weeks and requires cellular rather than humoral immunity. Here, we demonstrate that primary infection led to TLR2-dependent recruitment of monocyte/macrophages into the upper airway lumen, where they engulfed pneumococci. Pharmacologic depletion of luminal monocyte/macrophages by intranasal instillation of liposomal clodronate diminished pneumococcal clearance. Efficient clearance of colonization required TLR2 signaling to generate a population of pneumococcal-specific IL-17-expressing CD4+ T cells. Depletion of either IL-17A or CD4+ T cells was sufficient to block the recruitment of monocyte/macrophages that allowed for effective late pneumococcal clearance. In contrast with naive mice, previously colonized mice showed enhanced early clearance that correlated with a more robust influx of luminal neutrophils. As for primary colonization, these cellular responses required Th17 immunity. Our findings demonstrate that monocyte/macrophages and neutrophils recruited to the mucosal surface are key effectors in clearing primary and secondary bacterial colonization, respectively.

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.

Pneumococcal vaccination of elderly adults: new paradigms for protection

Pneumococcal polysaccharide vaccine has been licensed for use in the United States for >30 years, and two-thirds of the elderly population in the United States have received this vaccine. Observational studies have demonstrated that pneumococcal polysaccharide vaccine reduces the risk of invasive pneumococcal disease in immunocompetent elderly individuals, but neither observational studies nor clinical trials have demonstrated consistent evidence for a reduction in the incidence of pneumonia in vaccinated older adults. The introduction of pneumococcal protein conjugate vaccine among children has led to a herd immunity effect that has resulted in a 38% decrease in the rate of invasive pneumococcal disease among elderly adults. The high efficacy of pneumococcal protein conjugate vaccine in children has renewed interest in evaluating pneumococcal protein conjugate vaccines in adults for prevention of invasive pneumococcal disease and pneumonia. Moreover, the recognition of the presence and function of noncapsular pneumococcal protein antigens and the increasing availability of adjuvants highlight the promise of new vaccination strategies to decrease the burden of pneumococcal infection in this high-risk population.

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.

[Why are pediatric empyemas on the increase in Spain?]

There is a widespread perception among Spanish pediatricians that the incidence of empyema has significantly increased in the last few years, even though the objective information available is limited, and there is no specific active epidemiological surveillance system for this condition. In the present article, we review the situation of empyema in Spain, and discuss the main hypotheses put forward in the international literature to explain this increase, as well as the limitations of the sources available. Despite the scarcity of information, we draw the following conclusions: 1) the incidence of pediatric empyema is increasing in Spain, both generally and when caused by pneumococcus in particular; 2) the reason for this increase remains unknown, and to date no firm link has been established between this phenomenon and the heptavalent conjugate pneumococcal vaccine; and 3) this situation justifies the establishment of prospective systems for the surveillance and control of empyema and, once again, highlights the importance of developing active surveillance systems for pneumococcal disease.

Impaired opsonization with C3b and phagocytosis of Streptococcus pneumoniae in sera from subjects with defects in the classical complement pathway

Results from studies using mice deficient in specific complement factors and clinical data on patients with an inherited deficiency of the classical complement pathway component C2 suggest that the classical pathway is vital for immunity to Streptococcus pneumoniae. However, the consequences of defects in classical pathway activity for opsonization with C3b and the phagocytosis of different S. pneumoniae serotypes in human serum are not known, and there has not been a systematic analysis of the abilities of sera from subjects with a C2 deficiency to opsonize S. pneumoniae. Hence, to investigate the role of the classical pathway in immunity to S. pneumoniae in more detail, flow cytometry assays of opsonization with C3b and the phagocytosis of three capsular serotypes of S. pneumoniae were performed using human sera depleted of the complement factor C1q or B or sera obtained from C2-deficient subjects. The results demonstrate that, in human serum, the classical pathway is vital for C3b-iC3b deposition onto cells of all three serotypes of S. pneumoniae and seems to be more important than the alternative pathway for phagocytosis. Compared to the results for sera from normal subjects, C3b-iC3b deposition and total anti-S. pneumoniae antibody activity levels in sera obtained from C2(-/-) subjects were reduced and the efficiency of phagocytosis of all three S. pneumoniae strains was impaired. Anticapsular antibody levels did not correlate with phagocytosis or C3b-iC3b deposition. These data confirm that the classical pathway is vital for complement-mediated phagocytosis of S. pneumoniae and demonstrate why subjects with a C2 deficiency have a marked increase in susceptibility to S. pneumoniae infections.

The role of Streptococcus pneumoniae virulence factors in host respiratory colonization and disease

Streptococcus pneumoniae is a Gram-positive bacterial pathogen that colonizes the mucosal surfaces of the host nasopharynx and upper airway. Through a combination of virulence-factor activity and an ability to evade the early components of the host immune response, this organism can spread from the upper respiratory tract to the sterile regions of the lower respiratory tract, which leads to pneumonia. In this Review, we describe how S. pneumoniae uses its armamentarium of virulence factors to colonize the upper and lower respiratory tracts of the host and cause disease.

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.

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.

Streptococcus pneumoniae virulence factors and their clinical impact: An update

The morbidity and mortality rates associated with Streptococcus pneumoniae remain very high worldwide. The virulence of this bacterium is largely dependent on its polysaccharide capsule, which is quite heterogeneous and represents a serious obstacle for designing effective vaccines. However, it has been demonstrated that numerous protein virulence factors are involved in the pathogenesis of pneumococcal disease. An important related finding from experimental animal models is that non-capsulated strains of pneumococci are protective against capsulated ones. Hence, new vaccine designs are focused on the surface proteins (e. g., PspA and PspC) and on the cytolysin, pneumolysin. Moreover, several virulence factors have potential value for pneumococcal diagnosis by urinalysis. In this paper, we review the virulence factors involved in bacteria-host interactions, and the new developments in vaccines and diagnostic methods.

Anticapsular serum antibody concentration and protection against pneumococcal colonization among children vaccinated with 7-valent pneumococcal conjugate vaccine

BACKGROUND

Pneumococcal conjugate vaccines prevent invasive and noninvasive disease due to infection with vaccine serotypes. Pneumococcal conjugate vaccines also prevent nasopharyngeal acquisition of vaccine serotypes, although the mechanism is incompletely understood.

METHODS

An efficacy trial of a 7-valent pneumococcal conjugate vaccine was conducted on the Navajo and White Mountain Apache reservations, located in the Southwestern United States; group C meningococcal conjugate vaccine was the control vaccine. Infants were randomized to receive 7-valent pneumococcal conjugate vaccine or group C meningococcal conjugate vaccine at 2, 4, 6, and 12 months of age. Immunogenicity and nasopharyngeal colonization studies were nested in the efficacy trial. We analyzed the correlation between serotype-specific serum IgG concentration at 7 and 13 months of age and nasopharyngeal acquisition of disease at 12 and 18 months of age, respectively. We adjusted for potential confounders using multivariate logistic regression.

RESULTS

Among 203 subjects, we observed 60 acquisitions of vaccine-type pneumococci, including 19 acquisitions of serotype 19F (31.7%), and 17 acquisitions of serotype 23F (28.3%). Among recipients of 7-valent pneumococcal conjugate vaccine, increased serotype-specific serum IgG was associated with a reduction in nasopharyngeal acquisition of serotype 23F (relative risk, 0.53; 95% confidence interval, 0.31-0.93) but was not associated with a reduction in acquisition of serotype 19F (relative risk, 1.07; 95% confidence interval, 0.57-2.03). Among group C meningococcal conjugate vaccine recipients, serotype-specific serum IgG was not associated with a reduction in nasopharyngeal acquisition for either serotype.

CONCLUSION

An increase in serum antibody concentration was associated with reduced acquisition of serotype 23F pneumococcus (but not with reduced acquisition of serotype 19F pneumococcus) among recipients of 7-valent pneumococcal conjugate vaccine. Differences in antibody concentration, in the functional characteristics of antibody, or in antibody kinetics during infancy may account for differences in carriage protection.

Live attenuated Streptococcus pneumoniae strains induce serotype-independent mucosal and systemic protection in mice

Streptococcus pneumoniae is an important human pathogen causing both mucosal (otitis media and pneumonia) and systemic (sepsis and meningitis) diseases. Due to increasing rates of antibiotic resistance, there is an urgent need to improve prevention of pneumococcal disease. Two currently licensed vaccines have been successful in reducing pneumococcal disease, but there are limitations with their use and effectiveness. Another approach for prevention is the use of live attenuated vaccines. Here we investigate the safety and protection induced by live attenuated strains of S. pneumoniae containing combinations of deletions in genes encoding three of its major virulence determinants: capsular polysaccharide (cps), pneumolysin (ply), and pneumococcal surface protein A (pspA). Both the cps and ply/pspA mutants of a virulent type 6A isolate were significantly attenuated in a mouse model of sepsis. These attenuated strains retained the ability to colonize the upper respiratory tract. A single intranasal administration of live attenuated vaccine without adjuvant was sufficient to induce both systemic and mucosal protection from challenge with a high dose of the parent strain. Immunization with cps mutants demonstrated cross-protective immunity following challenge with a distantly related isolate. Serum and mucosal antibody titers were significantly increased in mice immunized with the vaccine strains, and this antibody is required for full protection, as microMT mice, which do not make functional, specific antibody, were not protected by immunization with vaccine strains. Thus, colonization by live attenuated S. pneumoniae is a potentially safe and less complex vaccine strategy that may offer broad protection.

How mathematical models have helped to improve understanding the epidemiology of infection

Mathematical models have proved to be useful tools for addressing questions about the process of infection. This is because the model allows the investigation of the mathematical link between invisible events (transmission of infection between individuals) and more visible ones (incidence of clinical infection, seroprevalence data etc.). A useful model is often one that is simple enough to provide answers that are applicable to as general a context as possible, but also complex enough so that it can address the most relevant questions. Thus, relatively simple models of viral (or hospital acquired-) infection were used to establish the existence of critical thresholds of immunisation (or antibiotic usage) coverage, although more complex models were needed for the investigation of the impact of infection control practices within more specific contexts. Mathematical models have the potential to elucidate key determinants of the epidemiology of infectious disease arising in infants nursed in neonatal intensive care units.

The roles of pneumococcal serotypes 1 and 5 in paediatric invasive disease

A small number of serotypes are responsible for most invasive pneumococcal disease in children worldwide. Serotypes 1 and 5 were among the first pneumococcal serotypes to be identified, reflecting their great prominence as causes of disease in the early part of the 20th century. In the antibiotic era, these serotypes persist as important causes of disease in children in most countries even though they, unlike some other common serotypes, remain sensitive to antimicrobial agents. These types also exhibit a high level of virulence, cause disease clusters or outbreaks, infect all age groups, and cause severe disease. Future vaccine formulations that protect against these types are important for developed and developing countries alike.

Age- and serogroup-related differences in observed durations of nasopharyngeal carriage of penicillin-resistant pneumococci

Using data from an ongoing Swedish intervention project, the observed durations of nasopharyngeal carriage of penicillin-nonsusceptible Streptococcus pneumoniae (PNSP) (MIC of penicillin G of >or=0.5 microg/ml) stratified by both pneumococcal serogroup and age of the carrier were compared. The means and 95% confidence intervals (CIs) were estimated by fitting a gamma distribution to the observed duration of carriage for each age and serogroup stratum. The mean observed duration of carriage for all cases was 37 days (95% CI, 35 to 38 days). Children below the age of 5 years carried PNSP for significantly longer periods (43 days; 95% CI, 41 to 45 days) compared with older individuals (25 days; 95% CI, 24 to 27 days). There were also differences within the group of cases below the age of 5 years, as the duration of carriage became significantly shorter for each increasing age step: <1, 1 to 2, and 3 to 4 years. In addition, patients <5 years of age carried serogroups 9 and 14 for significantly shorter periods than groups 6 and 23. Serogroup 9 was also carried for significantly shorter periods than group 19. For patients aged 5 years or older, no significant difference in carriage duration for different ages or serogroups could be noted. As young children have the longest duration of PNSP carriage, interventions aiming to reduce the prevalence in this group are of great importance. The results highlight the importance of taking both serogroup and age of the carriers into account when studying the dynamics of pneumococcal transmission in young children.