Human antibody response towards the pneumococcal surface proteins PspA and PspC during invasive pneumococcal infection

The diversity of the proline-rich domain of pneumococcal surface protein A (PspA): Potential relevance to a broad-spectrum vaccine

Pneumococcal surface protein A (PspA) is a surface exposed, highly immunogenic protein of Streptococcus pneumoniae. Its N-terminal α-helical domain (αHD) elicits protective antibody in humans and animals that can protect mice from fatal infections with pneumococci and can be detected in vitro with opsonophagocytosis assays. The proline-rich domain (PRD) in the center of the PspA sequence can also elicit protection. This study revealed that although the sequence of PRD was diverse, PRD from different pneumococcal isolates contained many shared elements. The inferred amino acid sequences of 123 such PRDs, which were analyzed by assembly and alignment-free (AAF) approaches, formed three PRD groups. Of these sequences, 45 were classified as Group 1, 19 were classified as Group 2, and 59 were classified as Group 3. All Group 3 sequences contained a highly conserved 22-amino acid non-proline block (NPB). A significant polymorphism was observed, however, at a single amino acid position within NPB. Each of the three PRD groups had characteristic patterns of short amino acid repeats, with most of the repeats being found in more than one PRD group. One of these repeats, PKPEQP as well as the NPB were previously shown to elicit protective antibodies in mice. In this study, we found that sera from 12 healthy human adult volunteers contained antibodies to all three PRD groups. This suggested that a PspA-containing vaccine containing carefully selected PRDs and αHDs could redundantly cover the known diversity of PspA. Such an approach might reduce the chances of PspA variants escaping a PspA vaccine's immunity.

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

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

Cord blood Streptococcus pneumoniae-specific cellular immune responses predict early pneumococcal carriage in high-risk infants in Papua New Guinea

In areas where Streptococcus pneumoniae is highly endemic, infants experience very early pneumococcal colonization of the upper respiratory tract, with carriage often persisting into adulthood. We aimed to explore whether newborns in high‐risk areas have pre‐existing pneumococcal‐specific cellular immune responses that may affect early pneumococcal acquisition. Cord blood mononuclear cells (CBMC) of 84 Papua New Guinean (PNG; high endemic) and 33 Australian (AUS; low endemic) newborns were stimulated in vitro with detoxified pneumolysin (dPly) or pneumococcal surface protein A (PspA; families 1 and 2) and compared for cytokine responses. Within the PNG cohort, associations between CBMC dPly and PspA‐induced responses and pneumococcal colonization within the first month of life were studied. Significantly higher PspA‐specific interferon (IFN)‐γ, tumour necrosis factor (TNF)‐α, interleukin (IL)‐5, IL‐6, IL‐10 and IL‐13 responses, and lower dPly‐IL‐6 responses were produced in CBMC cultures of PNG compared to AUS newborns. Higher CBMC PspA‐IL‐5 and PspA‐IL‐13 responses correlated with a higher proportion of cord CD4 T cells, and higher dPly‐IL‐6 responses with a higher frequency of cord antigen‐presenting cells. In the PNG cohort, higher PspA‐specific IL‐5 and IL‐6 CBMC responses were associated independently and significantly with increased risk of earlier pneumococcal colonization, while a significant protective effect was found for higher PspA‐IL‐10 CBMC responses. Pneumococcus‐specific cellular immune responses differ between children born in pneumococcal high versus low endemic settings, which may contribute to the higher risk of infants in high endemic settings for early pneumococcal colonization, and hence disease.

Improving diagnosis of pneumococcal disease by multiparameter testing and micro/nanotechnologies

The diagnosis and management of pneumococcal disease remains challenging, in particular in children who often are asymptomatic carriers, and in low-income countries with a high morbidity and mortality from febrile illnesses where the broad range of bacterial, viral and parasitic cases are in contrast to limited, diagnostic resources. Integration of multiple markers into a single, rapid test is desirable in such situations. Likewise, the development of multiparameter tests for relevant arrays of pathogens is important to avoid overtreatment of febrile syndromes with antibiotics. Miniaturization of tests through use of micro- and nanotechnologies combines several advantages: miniaturization reduces sample requirements, reduces the use of consumables and reagents leading to a reduction in costs, facilitates parallelization, enables point-of-care use of diagnostic equipment and even reduces the amount of potentially infectious disposables, characteristics that are highly desirable in most healthcare settings. This critical review emphasizes our vision on the importance of multiparametric testing for diagnosing pneumococcal infections in patients with fever and examines recent relevant developments in micro/nanotechnologies to achieve this goal.

Towards nano-diagnostics for bacterial infections

Sensitive, specific and rapid diagnosis of infectious diseases is essential for effective and economic medical care. Focused medical treatment of the patient enabled by pathogen-specific diagnosis may benefit the patient, may reduce cost, and may minimize the risk of drug resistance development. The rapid progress in micro and nanotechnologies contributes to the development of novel diagnostic methods. This critical review assesses emerging nanotechnologies for diagnosis of bacterial infection in developed and developing countries on the background of the current state of the art and includes particular challenges and pitfalls posed by a number of specific pathogens.

Preventing pneumococcal disease in the elderly: recent advances in vaccines and implications for clinical practice

Streptococcus pneumoniae remains a major cause of morbidity and mortality throughout the world. To date, after the introduction of routine childhood immunization, elderly people (i.e., persons aged 65 years or older) suffer the greatest burden of pneumococcal disease in developed countries. At present, two anti-pneumococcal vaccines are available for use in adults: the 23-valent pneumococcal polysaccharide vaccine (PPV23) and the 13-valent protein-polysaccharide conjugate vaccine (PCV13). This article reviews current data about the burden of pneumococcal disease in the elderly, as well as evidence for immunogenicity, clinical efficacy, and possible cost-effectiveness of both vaccines. The main advantage of PCV13 is that it may be more effective than PPV23, but a major limitation is that it is directed against strains that are likely to be greatly reduced in the population since its introduction in childhood immunization. The main disadvantage of PPV23 is that it may be less effective than PCV13 against vaccine-type infections but a major advantage is that it may provide protection against ten additional serotypes. To date, expert committees have not changed recommendations for pneumococcal vaccination in adults. However, at present, they are evaluating different alternatives (basically, maintaining PPV23, changing from PPV23 to PCV13 in some groups, or adding PCV13 for all or some target adult population subgroups). Critical data (clinical efficacy reported in ongoing trials and magnitude of indirect effects of pediatric PCV13 programs) needed to make a well-informed decision could be available during 2013. Considering all concerns over indirect effects and replacement strains following the use of polysaccharide-based vaccines, efforts should be directed toward developing vaccines, such as protein-based pneumococcal vaccines, with potential serotype-independent protection. Meanwhile, according to current recommendations, PPV23 should continue to be used for high-risk adults and all elderly people (with and without additional high-risk conditions). Although it is only moderately effective, it has a considerable serotype coverage and at-risk persons can benefit from the vaccination. High-risk individuals could also obtain a benefit from adding PCV13, but more data are needed before a universal recommendation can be made.

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.

Th17/Th1 biased immunity to the pneumococcal proteins PcsB, StkP and PsaA in adults of different age

Streptococcus pneumoniae is a major human pathogen, causing high morbidity and mortality in children, and also in the elderly, who are particularly susceptible to S. pneumoniae infections due to the dysregulated function of the aged immune system. As the current generation of polysaccharide vaccines do not provide sufficient protection for elderly, new vaccination strategies are urgently needed. To learn whether pneumococcal proteins are able to induce adaptive immune responses in adults in different age groups, we determined serum IgG antibody titers and T cell immunity (IFN-γ, IL-17A and IL-5 production) to three pneumococcal antigens, PcsB, StkP and PsaA, that are components of an investigational protein-based pneumococcal vaccine, IC47. Therefore, sera and PBMCs of 108 healthy adults in three different age groups (young, middle-aged and elderly) were analyzed by ELISA and ELISpot, respectively. We found naturally acquired antibodies to all three proteins in all age groups against all three antigens. However, elderly individuals had significantly lower IgG levels to PcsB and PsaA compared to those of younger donors. There was no significant age-related difference in the overall rate of T cell immunity for the three pneumococcal proteins. We found that the Th17 response was dominant in all age groups and was frequently combined with a Th1 or Th2 response in young and middle-aged subjects. However, in elderly persons there was a lower percentage of PBMC samples producing more than one cytokine upon antigenic stimulation. The narrow cytokine secretion pattern was the most striking difference between elderly and younger adult age groups. Our results demonstrate that in the majority of adults there is a naturally acquired humoral and cellular immune response to the three pneumococcal proteins tested. The dominance of the Th17 response is especially interesting in the light of new insights regarding the role of Th17 cells in mucosal protection against this pathogen.

Discovery of a novel class of highly conserved vaccine antigens using genomic scale antigenic fingerprinting of pneumococcus with human antibodies

Pneumococcus is one of the most important human pathogens that causes life-threatening invasive diseases, especially at the extremities of age. Capsular polysaccharides (CPSs) are known to induce protective antibodies; however, it is not feasible to develop CPS-based vaccines that cover all of the 90 disease-causing serotypes. We applied a genomic approach and described the antibody repertoire for pneumococcal proteins using display libraries expressing 15–150 amino acid fragments of the pathogen's proteome. Serum antibodies of exposed, but not infected, individuals and convalescing patients identified the ANTIGENome of pneumococcus consisting of ∼140 antigens, many of them surface exposed. Based on several in vitro assays, 18 novel candidates were preselected for animal studies, and 4 of them showed significant protection against lethal sepsis. Two lead vaccine candidates, protein required for cell wall separation of group B streptococcus (PcsB) and serine/threonine protein kinase (StkP), were found to be exceptionally conserved among clinical isolates (>99.5% identity) and cross-protective against four different serotypes in lethal sepsis and pneumonia models, and have important nonredundant functions in bacterial multiplication based on gene deletion studies. We describe for the first time opsonophagocytic killing activity for pneumococcal protein antigens. A vaccine containing PcsB and StkP is intended for the prevention of infections caused by all serotypes of pneumococcus in the elderly and in children.

Factor H binding to PspC of Streptococcus pneumoniae increases adherence to human cell lines in vitro and enhances invasion of mouse lungs in vivo

Pneumococcal surface protein C (PspC) binds to both human secretory immunoglobulin A (sIgA) and complement factor H (FH). FH, a regulator of the alternative pathway of complement, can also mediate adherence of different host cells. Since PspC contributes to adherence and invasion of host cells, we hypothesized that the interaction of PspC with FH may also mediate adherence of pneumococci to human cells. In this study, we investigated FH- and sIgA-mediated pneumococcal adherence to human cell lines in vitro. Adherence assays demonstrated that preincubation of Streptococcus pneumoniae D39 with FH increased adherence to human umbilical vein endothelial cells (HUVEC) 5-fold and to lung epithelial cells (SK-MES-1) 18-fold, relative to that of D39 without FH on the surface. The presence of sIgA enhanced adherence to SK-MES-1 6-fold and to pharyngeal epithelial cells (Detroit 562) 14-fold. Furthermore, sIgA had an additive effect on adherence to HUVEC; specifically, preincubation of D39 with both FH and sIgA led to a 21-fold increase in adherence. Finally, using a mouse model, we examined the significance of the FH-PspC interaction in pneumococcal nasal colonization and lung invasion. Mice intranasally infected with D39 preincubated with FH had increased bacteremia and lung invasion, but they had similar levels of nasopharyngeal colonization compared to that of mice challenged with D39 without FH.

Advances in Pneumococcal Vaccines

Streptococcus pneumoniae causes considerable morbidity and mortality in the elderly. There are three established approaches to pneumococcal vaccination: polysaccharide vaccines, protein-polysaccharide conjugate vaccines and protein-based vaccines. This article reviews advances in anti-pneumococcal vaccines, with reference to advantages and shortcomings for the elderly in particular. The 23-valent polysaccharide pneumococcal vaccine (PPV) is currently recommended for high-risk patients and the general elderly population. Although the effectiveness of PPV against pneumonia is unclear, recent studies point to significant protective effects in preventing pneumococcal pneumonia and reducing the severity of disease in vaccinated elderly patients. PPV offers high serotype coverage and, although it is poorly immunogenic in some individuals, provides approximately 60% protection against invasive disease in the general elderly population. PPV vaccination appears cost effective for elderly patients although the vaccine might only be effective in preventing invasive disease. Additional benefits could mean a greater level of vaccine cost effectiveness. However, it is important to understand that PPV provides incomplete protection, especially in those with underlying high-risk conditions, and development of more effective pneumococcal vaccination strategies for elderly patients is still needed. In recent years, the most important advance in the prevention of pneumococcal infections in the elderly has been the introduction of a 7-valent conjugate pneumococcal vaccine (CPV) as a routine vaccination for infants. In addition to dramatically reducing invasive infection in children, CPV has been observed to have a considerable indirect protective effect in parents and grandparents. While the possibility of using CPV in elderly patients has been suggested, currently there are only limited immunogenicity data and no efficacy data in adults. The low serotype coverage is an important shortcoming and if CPV were to be used in the elderly, it would need to be given sequentially with the PPV. New CPVs covering more serotypes are currently under investigation, and these could be an alternative for use in all groups in the future. Numerous protein-based vaccine candidates offer the potential advantage of prevention against infections caused by all pneumococcal serotypes. Several are in various stages of development in animal models, but none can be expected to be available in clinical practice for several years at least. To date, the 23-valent PPV is still the best anti-pneumococcal vaccine option in the management of elderly persons. Introduction of the 7-valent CPV as a routine vaccine for children has provided considerable indirect benefits for older adults via herd immunity, but this vaccine has limited serotype coverage in elderly individuals. New CPVs including more serotypes (various CPVs are in different phases of pre-licensure studies) could prove to be good options in the future for all age groups. Several protein-based pneumococcal vaccine candidates (currently under investigation in animal models) offer the potential advantage of serotype independent protection, but none can be expected to be available in clinical practice in the near future.