Isolation and Characterization of Human Monoclonal Antibodies to Pneumococcal Capsular Polysaccharide 3

Pneumococcal surface proteins as targets for next-generation vaccines: Addressing the challenges of serotype variation

Streptococcus pneumoniae is a major global pathogen causing significant morbidity and mortality, particularly among children, the elderly, and immunocompromised populations. While pneumococcal conjugate vaccines (PCVs) have successfully reduced invasive pneumococcal disease (IPD), challenges such as serotype replacement and non-encapsulated strains necessitate serotype-independent vaccine strategies. Pneumococcal surface proteins, including pneumolysin (Ply), choline-binding proteins (CBPs), and histidine triad proteins (PHTs), represent promising universal vaccine targets due to their conserved nature and roles in adhesion, immune evasion, and biofilm formation. Advances in protein engineering, such as detoxified Ply derivatives and multivalent formulations incorporating PhtD and PspA, demonstrate potential in preclinical studies. Novel technologies, including reverse vaccinology and extracellular vesicle-based platforms, further accelerate innovation. This review highlights recent progress in pneumococcal surface protein research, emphasizing their potential to address the limitations of PCVs and mitigate antibiotic-resistant pneumococcal strains, representing a transformative approach to global pneumococcal disease prevention.

Protection induced by Streptococcus pneumoniae extracellular vesicles against nasal colonization and invasive infection in mice and the role of PspA

Diseases caused by Streptococcus pneumoniae (pneumococcus) produce a great impact on public health, killing about one million people annually despite available vaccines. Recent research has revealed that the pneumococcus produces extracellular vesicles (pEVs), which display selective cargo and hold potential for vaccine development. Here, we evaluated the immunogenicity and protective potential of pEVs derived from a non-encapsulated pneumococcal strain (R6) using murine models of pneumococcal colonization and invasive pneumonia. Characterization of the immune response revealed that while pEVs contain multiple virulence determinants, immunization with these nanoparticles only induces antibodies against a subset of them. Specifically, subcutaneous immunization elicits a high antibody response against PspA, a modest response against PrsA, and limited response against Ply, MalX and PsaA. The antibody response was further supported by agglutination studies, showing that sera from pEV immunized mice agglutinate pneumococci and that PspA contributes to this response in a strain-dependent manner. Subcutaneous immunization with pEVs provides protection in the invasive pneumonia model whereas nasal immunization results in one log reduction in pneumococcal colonization of the upper respiratory tract. Finally, PspA is a strong contributor to protection in the invasive model and provides a degree of protection even across heterologous families of PspA. We conclude that pEVs demonstrate potential for vaccine development, protecting across capsular types and providing some degree of protection across heterologous PspA variants.

Serotype 3 Streptococcus pneumoniae Escapes the Immune Responses Induced by PCV13 in Mice With High Susceptibility to Infection

BACKGROUND

Streptococcus pneumoniae (pneumococcus) is a common cause of respiratory and invasive infections in humans. PCV13, a pneumococcal conjugate vaccine used globally, is highly effective against diseases caused by pneumococcal serotypes included in its formulation. However, one of them, the serotype 3 (ST3) is still being relatively commonly isolated from patients, suggesting an escape from vaccine-induced immunity. The thick capsule produced by ST3 facilitates bacterial evasion from the immune system. Additionally, host immune responses may influence the outcome of ST3 infection. Here we evaluated the influence of inflammation in the adaptive immune responses and protection induced by PCV13 against ST3, using two outbred mice lines that were phenotypically selected for high (AIRmax) and low (AIRmin) inflammatory responses.

METHODS

AIRmin and AIRmax mice were immunized with PCV13. Inbred BALB/c mice were used as reference for vaccine efficacy. Induction of IgG against polysaccharides (PS) from pneumococcal serotype 1 (ST1) and ST3 were evaluated by ELISA. Protection was tested against invasive infections with ST1 and ST3 pneumococcal strains. Sera were compared by IgG binding to pneumococcal surface, induction of pneumococcal agglutination and opsonophagocytosis. The phagocytic capacity of mice-derived neutrophils was also evaluated.

RESULTS

Immunization of AIRmin, AIRmax and BALB/c mice with PCV13 induced IgG against PS from ST1 and ST3 pneumococci. Despite vaccination, AIRmin mice were not protected against fatal infection with ST3. Sera from AIRmin mice immunized with PCV13 presented lower levels of anti-PS3 IgG, with reduced capacity to bind to pneumococcal surface. Reduced capacity to induce opsonophagocytosis of ST3 pneumococci in vitro was also observed. Conversely, PCV13 protected AIRmin mice against fatal infection with ST1 and this correlated with the capacity of the sera to induce ST1 opsonophagocytosis.

CONCLUSIONS

Our results show that both host and bacterial features can influence the outcome of protection induced by PCV13 against ST3 pneumococcal infection.

Monoclonal Antibody Therapies for Infectious Diseases

In contrast to therapy in oncology and immune-related diseases, where dozens of monoclonal antibodies (mAbs) have been introduced, often in transformative fashion, the use of mAbs for infectious diseases is generally underdeveloped, with fewer than a dozen mAbs currently licensed for the treatment of microbial diseases. This situation is paradoxical given that antibodies are major products of the immune system for protecting against infectious diseases. The underdevelopment of mAbs for infectious diseases has several causes including the availability of effective therapy against many microbial diseases, the fact that many pathogenic microbes are antigenically diverse and thus all strains are not covered by a single mAb, and the high expense of mAb therapies. Despite these hurdles the number of mAbs licensed for infectious disease indications is slowly increasing and there are numerous opportunities for the development of mAbs in the prevention and treatment of microbial diseases.

PCV15, a pneumococcal conjugate vaccine, for the prevention of invasive pneumococcal disease in infants and children

INTRODUCTION

Streptococcus pneumoniae is a causative agent of pneumonia and acute otitis media (AOM), as well as invasive diseases such as meningitis and bacteremia. PCV15 (V114) is a new 15-valent pneumococcal conjugate vaccine (PCV) approved for use in individuals ≥6 weeks of age for the prevention of pneumonia, AOM, and invasive pneumococcal disease.

AREAS COVERED

This review summarizes the V114 Phase 3 development program leading to approval in infants and children, including pivotal studies, interchangeability and catch-up vaccination studies, and studies in at-risk populations. An integrated safety summary is presented in addition to immunogenicity and concomitant use of V114 with other routine pediatric vaccines.

EXPERT OPINION

Across the development program, V114 demonstrated a safety profile that is comparable to PCV13 in infants and children. Immunogenicity of V114 is comparable to PCV13 for all shared serotypes except serotype 3, where V114 demonstrated superior immunogenicity. Higher immune responses were demonstrated for V114 serotypes 22F and 33F. Results of the ongoing study to evaluate V114 efficacy against vaccine-type pneumococcal AOM and anticipated real-world evidence studies will support assessment of vaccine effectiveness and impact, with an additional question of whether higher serotype 3 immunogenicity translates to better protection against serotype 3 pneumococcal disease.

Liver macrophages and sinusoidal endothelial cells execute vaccine-elicited capture of invasive bacteria

Vaccination has substantially reduced the morbidity and mortality of bacterial diseases, but mechanisms of vaccine-elicited pathogen clearance remain largely undefined. We report that vaccine-elicited immunity against invasive bacteria mainly operates in the liver. In contrast to the current paradigm that migrating phagocytes execute vaccine-elicited immunity against blood-borne pathogens, we found that invasive bacteria are captured and killed in the liver of vaccinated host via various immune mechanisms that depend on the protective potency of the vaccine. Vaccines with relatively lower degrees of protection only activated liver-resident macrophage Kupffer cells (KCs) by inducing pathogen-binding immunoglobulin M (IgM) or low amounts of IgG. IgG-coated pathogens were directly captured by KCs via multiple IgG receptors FcγRs, whereas IgM-opsonized bacteria were indirectly bound to KCs via complement receptors of immunoglobulin superfamily (CRIg) and complement receptor 3 (CR3) after complement C3 activation at the bacterial surface. Conversely, the more potent vaccines engaged both KCs and liver sinusoidal endothelial cells by inducing higher titers of functional IgG antibodies. Endothelial cells (ECs) captured densely IgG-opsonized pathogens by the low-affinity IgG receptor FcγRIIB in a "zipper-like" manner and achieved bacterial killing predominantly in the extracellular milieu via an undefined mechanism. KC- and endothelial cell-based capture of antibody-opsonized bacteria also occurred in FcγR-humanized mice. These vaccine protection mechanisms in the liver not only provide a comprehensive explanation for vaccine-/antibody-boosted immunity against invasive bacteria but also may serve as in vivo functional readouts of vaccine efficacy.

Immunoglobulin repertoire restriction characterizes the serological responses of patients with predominantly antibody deficiency

BACKGROUND

Predominantly antibody deficiency (PAD) is the most common category of inborn errors of immunity and is underpinned by impaired generation of appropriate antibody diversity and quantity. In the clinic, responses are interrogated by assessment of vaccination responses, which is central to many PAD diagnoses. However, the composition of the generated antibody repertoire is concealed from traditional quantitative measures of serological responses. Leveraging modern mass spectrometry-based proteomics (MS-proteomics), it is possible to elaborate the molecular features of specific antibody repertoires, which may address current limitations of diagnostic vaccinology.

OBJECTIVES

We sought to evaluate serum antibody responses in patients with PAD following vaccination with a neo-antigen (severe acute respiratory syndrome coronavirus-2 vaccination) using MS-proteomics.

METHODS

Following severe acute respiratory syndrome coronavirus-2 vaccination, serological responses in individuals with PAD and healthy controls (HCs) were assessed by anti-S1 subunit ELISA and neutralization assays. Purified anti-S1 subunit IgG and IgM was profiled by MS-proteomics for IGHV subfamily usage and somatic hypermutation analysis.

RESULTS

Twelve patients with PAD who were vaccine-responsive were recruited with 11 matched vaccinated HCs. Neutralization and end point anti-S1 titers were lower in PAD. All subjects with PAD demonstrated restricted anti-S1 IgG antibody repertoires, with usage of <5 IGHV subfamilies (median: 3; range 2-4), compared to ≥5 for the 11 HC subjects (P < .001). IGHV3-7 utilization was far less common in patients with PAD than in HCs (2 of 12 vs 10 of 11; P = .001). Amino acid substitutions due to somatic hypermutation per subfamily did not differ between groups. Anti-S1 IgM was present in 64% and 50% of HC and PAD cohorts, respectively, and did not differ significantly between HCs and patients with PAD.

CONCLUSIONS

This study demonstrates the breadth of anti-S1 antibodies elicited by vaccination at the proteome level and identifies stereotypical restriction of IGHV utilization in the IgG repertoire in patients with PAD compared with HC subjects. Despite uniformly pauci-clonal antibody repertoires some patients with PAD generated potent serological responses, highlighting a possible limitation of traditional serological techniques. These findings suggest that IgG repertoire restriction is a key feature of antibody repertoires in PAD.

Promoting Fc-Fc interactions between anti-capsular antibodies provides strong immune protection against Streptococcus pneumoniae

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia and an important cause of childhood mortality. Despite the introduction of successful vaccines, the global spread of both non-vaccine serotypes and antibiotic-resistant strains reinforces the development of alternative therapies against this pathogen. One possible route is the development of monoclonal antibodies (mAbs) that induce killing of bacteria via the immune system. Here, we investigate whether mAbs can be used to induce killing of pneumococcal serotypes for which the current vaccines show unsuccessful protection. Our study demonstrates that when human mAbs against pneumococcal capsule polysaccharides (CPS) have a poor capacity to induce complement activation, a critical process for immune protection against pneumococci, their activity can be strongly improved by hexamerization-enhancing mutations. Our data indicate that anti-capsular antibodies may have a low capacity to form higher-order oligomers (IgG hexamers) that are needed to recruit complement component C1. Indeed, specific point mutations in the IgG-Fc domain that strengthen hexamerization strongly enhance C1 recruitment and downstream complement activation on encapsulated pneumococci. Specifically, hexamerization-enhancing mutations E430G or E345K in CPS6-IgG strongly potentiate complement activation on S. pneumoniae strains that express capsular serotype 6 (CPS6), and the highly invasive serotype 19A strain. Furthermore, these mutations improve complement activation via mAbs recognizing CPS3 and CPS8 strains. Importantly, hexamer-enhancing mutations enable mAbs to induce strong opsonophagocytic killing by human neutrophils. Finally, passive immunization with CPS6-IgG1-E345K protected mice from developing severe pneumonia. Altogether, this work provides an important proof of concept for future optimization of antibody therapies against encapsulated bacteria.

Agglutination of Borreliella burgdorferi by Transmission-Blocking OspA Monoclonal Antibodies and Monovalent Fab Fragments

Lyme disease vaccines based on recombinant Outer surface protein A (OspA) elicit protective antibodies that interfere with tick-to-host transmission of the disease-causing spirochete Borreliella burgdorferi. Another hallmark of OspA antisera and certain OspA monoclonal antibodies (MAbs) is their capacity to induce B. burgdorferi agglutination in vitro, a phenomenon first reported more than 30 years ago but never studied in molecular detail. In this report, we demonstrate that transmission-blocking OspA MAbs, individually and in combination, promote dose-dependent and epitope-specific agglutination of B. burgdorferi. Agglutination occurred within minutes and persisted for hours. Spirochetes in the core of the aggregates exhibited evidence of outer membrane (OM) stress, revealed by propidium iodide uptake. The most potent agglutinator was the mouse MAb LA-2, which targets the OspA C terminus (β-strands 18 to 20). Human MAb 319-44, which also targets the OspA C terminus (β-strand 20), and 857-2, which targets the OspA central β-sheet (strands 8 to 10), were less potent agglutinators, while MAb 221-7, which targets β-strands 10 to 11, had little to no measurable agglutinating activity, even though its affinity for OspA exceeded that of LA-2. Remarkably, monovalent Fab fragments derived from LA-2, and to a lesser degree 319-44, retained the capacity to induce B. burgdorferi aggregation and OM stress, a particularly intriguing observation considering that "LA-2-like" Fabs have been shown to experimentally entrap B. burgdorferi within infected ticks and prevent transmission during feeding to a mammalian host. It is therefore tempting to speculate that B. burgdorferi aggregation triggered by OspA-specific antibodies in vitro may in fact reflect an important biological activity in vivo.

Pneumococcal Surface Proteins as Virulence Factors, Immunogens, and Conserved Vaccine Targets

Streptococcus pneumoniae is an opportunistic pathogen that causes over 1 million deaths annually despite the availability of several multivalent pneumococcal conjugate vaccines (PCVs). Due to the limitations surrounding PCVs along with an evolutionary rise in antibiotic-resistant and unencapsulated strains, conserved immunogenic proteins as vaccine targets continue to be an important field of study for pneumococcal disease prevention. In this review, we provide an overview of multiple classes of conserved surface proteins that have been studied for their contribution to pneumococcal virulence. Furthermore, we discuss the immune responses observed in response to these proteins and their promise as vaccine targets.

Diverse Mechanisms of Protective Anti-Pneumococcal Antibodies

The gram-positive bacterium Streptococcus pneumoniae is a leading cause of pneumonia, otitis media, septicemia, and meningitis in children and adults. Current prevention and treatment efforts are primarily pneumococcal conjugate vaccines that target the bacterial capsule polysaccharide, as well as antibiotics for pathogen clearance. While these methods have been enormously effective at disease prevention and treatment, there has been an emergence of non-vaccine serotypes, termed serotype replacement, and increasing antibiotic resistance among these serotypes. To combat S. pneumoniae, the immune system must deploy an arsenal of antimicrobial functions. However, S. pneumoniae has evolved a repertoire of evasion techniques and is able to modulate the host immune system. Antibodies are a key component of pneumococcal immunity, targeting both the capsule polysaccharide and protein antigens on the surface of the bacterium. These antibodies have been shown to play a variety of roles including increasing opsonophagocytic activity, enzymatic and toxin neutralization, reducing bacterial adherence, and altering bacterial gene expression. In this review, we describe targets of anti-pneumococcal antibodies and describe antibody functions and effectiveness against S. pneumoniae.