Antibodies to Streptococcus pneumoniae capsular polysaccharide enhance pneumococcal quorum sensing

Monoclonal antibodies against lipopolysaccharide protect against Pseudomonas aeruginosa challenge in mice

Pseudomonas aeruginosa is a common cause of hospital-acquired infections, including central line-associated bloodstream infections and ventilator-associated pneumonia. Unfortunately, effective control of these infections can be difficult, in part due to the prevalence of multi-drug resistant strains of P. aeruginosa. There remains a need for novel therapeutic interventions against P. aeruginosa, and the use of monoclonal antibodies (mAb) is a promising alternative strategy to current standard of care treatments such as antibiotics. To develop mAbs against P. aeruginosa, we utilized ammonium metavanadate, which induces cell envelope stress responses and upregulates polysaccharide expression. Mice were immunized with P. aeruginosa grown with ammonium metavanadate and we developed two IgG2b mAbs, WVDC-0357 and WVDC-0496, directed against the O-antigen lipopolysaccharide of P. aeruginosa. Functional assays revealed that WVDC-0357 and WVDC-0496 directly reduced the viability of P. aeruginosa and mediated bacterial agglutination. In a lethal sepsis model of infection, prophylactic treatment of mice with WVDC-0357 and WVDC-0496 at doses as low as 15 mg/kg conferred 100% survival against challenge. In both sepsis and acute pneumonia models of infection, treatment with WVDC-0357 and WVDC-0496 significantly reduced bacterial burden and inflammatory cytokine production post-challenge. Furthermore, histopathological examination of the lungs revealed that WVDC-0357 and WVDC-0496 reduced inflammatory cell infiltration. Overall, our results indicate that mAbs directed against lipopolysaccharide are a promising therapy for the treatment and prevention of P. aeruginosa infections.

Targeting Peptide-Based Quorum Sensing Systems for the Treatment of Gram-Positive Bacterial Infections

Bacteria utilize a cell density-dependent communication system called quorum sensing (QS) to coordinate group behaviors. In Gram-positive bacteria, QS involves the production of and response to auto-inducing peptide (AIP) signaling molecules to modulate group phenotypes, including pathogenicity. As such, this bacterial communication system has been identified as a potential therapeutic target against bacterial infections. More specifically, developing synthetic modulators derived from the native peptide signal paves a new way to selectively block the pathogenic behaviors associated with this signaling system. Moreover, rational design and development of potent synthetic peptide modulators allows in depth understanding of the molecular mechanisms that drive QS circuits in diverse bacterial species. Overall, studies aimed at understanding the role of QS in microbial social behavior could result in the accumulation of significant knowledge of microbial interactions, and consequently lead to the development of alternative therapeutic agents to treat bacterial infectivity. In this review, we discuss recent advances in the development of peptide-based modulators to target QS systems in Gram-positive pathogens, with a focus on evaluating the therapeutic potential associated with these bacterial signaling pathways.

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.

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

The current pneumococcal capsular polysaccharide (PPS) conjugate vaccine (PCV13) is less effective against Streptococcus pneumoniae serotype 3 (ST3), which remains a major cause of pneumococcal disease and mortality. Therefore, dissecting structure-function relationships of human ST3 pneumococcal capsular polysaccharide (PPS3) antibodies may reveal characteristics of protective antibodies. Using flow cytometry, we isolated PPS3-binding memory B cells from pneumococcal vaccine recipients and generated seven PPS3-specific human monoclonal antibodies (humAbs). Five humAbs displayed ST3 opsonophagocytic activity, four induced ST3 agglutination in vitro, and four mediated both activities. Two humAbs, namely, C10 and C27, that used the same variable heavy (VH) and light (VL) chain domains (VH3-9*01/VL2-14*03) both altered ST3 gene expression in vitro; however, C10 had fewer VL somatic mutations, higher PPS3 affinity, and promoted in vitro ST3 opsonophagocytic and agglutinating activity, whereas C27 did not. In C57BL/6 mice, both humAbs reduced nasopharyngeal colonization with ST3 A66 and a clinical strain, B2, and prolonged survival following lethal A66 intraperitoneal infection, but only C10 protected against lethal intranasal infection with the clinical strain. After performing VL swaps, C10VH/C27VL exhibited reduced ST3 binding and agglutination, but C27VH/C10VL binding was unchanged. However, both humAbs lost the ability to reduce colonization in vivo when their light chains were replaced. Our findings associate the ability of PPS3-specific humAbs to reduce colonization with ST3 agglutination and opsonophagocytic activity, and reveal an unexpected role for the VL in their functional activity in vitro and in vivo. These findings also provide insights that may inform antibody-based therapy and identification of surrogates of vaccine efficacy against ST3. IMPORTANCE Despite the global success of vaccination with pneumococcal conjugate vaccines, serotype 3 (ST3) pneumococcus remains a leading cause of morbidity and mortality. In comparison to other vaccine-included serotypes, the ST3 pneumococcal capsular polysaccharide (PPS3) induces a weaker opsonophagocytic response, which is considered a correlate of vaccine efficacy. Previous studies of mouse PPS3 monoclonal antibodies identified ST3 agglutination as a correlate of reduced ST3 nasopharyngeal colonization in mice; however, neither the agglutinating ability of human vaccine-elicited PPS3 antibodies nor their ability to prevent experimental murine nasopharyngeal colonization has been studied. We generated and analyzed the functional and in vivo efficacy of human vaccine-elicited PPS3 monoclonal antibodies and found that ST3 agglutination associated with antibody affinity, protection in vivo, and limited somatic mutations in the light chain variable region. These findings provide new insights that may inform the development of antibody-based therapies and next-generation vaccines for ST3.

How Basic Immunological Principles May Instruct the Design of a Successful HIV-Type 1 Vaccine

This article is dedicated to Dr. Peter Doherty. While Peter continues to make groundbreaking discoveries in the field of immunology, he also provides outstanding scientific mentorship to his trainees. Here we contemplate our past training with Peter, Peter's teachings of basic immunological principles, and how basic principles may instruct the design of a successful human immunodeficiency virus-type 1 vaccine.

Noncanonical Functions of Antibodies

The typical functions of antibodies are based on linking the process of antigen recognition with initiation of innate immune reactions. With the introduction of modern research technologies and the use of sophisticated model systems, recent years have witnessed the discovery of a number of noncanonical functions of antibodies. These functions encompass either untypical strategies for neutralization of pathogens or exertion of activities that are characteristic for other proteins (cytokines, chaperones, or enzymes). Here, we provide an overview of the noncanonical functions of antibodies and discuss their mechanisms and implications in immune regulation and defense. A better comprehension of these functions will enrich our knowledge of the adaptive immune response and shall inspire the development of novel therapeutics.

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

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

Methamphetamine Impairs IgG1-Mediated Phagocytosis and Killing of Cryptococcus neoformans by J774.16 Macrophage- and NR-9640 Microglia-Like Cells

The prevalence of methamphetamine (METH) use is estimated at ∼35 million people worldwide, with over 10 million users in the United States. Chronic METH abuse and dependence predispose the users to participate in risky behaviors that may result in the acquisition of HIV and AIDS-related infections. Cryptococcus neoformans is an encapsulated fungus that causes cryptococcosis, an opportunistic infection that has recently been associated with drug users. METH enhances C. neoformans pulmonary infection, facilitating its dissemination and penetration into the central nervous system in mice. C. neoformans is a facultative intracellular microorganism and an excellent model to study host-pathogen interactions. METH compromises phagocyte effector functions, which might have deleterious consequences on infection control. In this study, we investigated the role of METH in phagocytosis and antigen processing by J774.16 macrophage- and NR-9460 microglia-like cells in the presence of a specific IgG1 to C. neoformans capsular polysaccharide. METH inhibits antibody-mediated phagocytosis of cryptococci by macrophages and microglia, likely due to reduced expression of membrane-bound Fcγ receptors. METH interferes with phagocytic cells' phagosomal maturation, resulting in impaired fungal control. Phagocytic cell reduction in nitric oxide production during interactions with cryptococci was associated with decreased levels of tumor necrosis factor alpha (TNF-α) and lowered expression of Fcγ receptors. Importantly, pharmacological levels of METH in human blood and organs are cytotoxic to ∼20% of the phagocytes. Our findings suggest that METH abrogates immune cellular and molecular functions and may be deadly to phagocytic cells, which may result in increased susceptibility of users to acquire infectious diseases.

The Damage-Response Framework as a Tool for the Physician-Scientist to Understand the Pathogenesis of Infectious Diseases

The Damage-Response Framework (DRF) is a powerful tool to inform research in infectious diseases. It can integrate clinical observation with microbiology and immunology to incorporate the role of the host response into the outcome of microbial pathogenesis. Although the role that microbial factors may play in the pathogenesis of infectious diseases is well recognized, the DRF brings the indispensable role of the host response to the fore. For example, inflammation may induce microbial control, but it can also produce host damage. On the other hand, insufficient inflammation may fail to induce sufficient microbial control. Each scenario may lead to the diagnosis of an infectious disease. Given the central role that the host response plays in the pathogenesis of infectious diseases, new strategies for treatment need to consider the nature of the host response as well as microbial factors.

A Capsular Polysaccharide-Specific Antibody Alters Streptococcus pneumoniae Gene Expression during Nasopharyngeal Colonization of Mice

Pneumococcal conjugate vaccines (PCV) elicit opsonophagocytic (opsonic) antibodies to pneumococcal capsular polysaccharides (PPS) and reduce nasopharyngeal (NP) colonization by vaccine-included Streptococcus pneumoniae serotypes. However, nonopsonic antibodies may also be important for protection against pneumococcal disease. For example, 1E2, a mouse IgG1 monoclonal antibody (MAb) to the serotype 3 (ST3) PPS (PPS3), reduced ST3 NP colonization in mice and altered ST3 gene expression in vitro Here, we determined whether 1E2 affects ST3 gene expression in vivo during colonization of mice by performing RNA sequencing on NP lavage fluid from ST3-infected mice treated with 1E2, a control MAb, or phosphate-buffered saline. Compared to the results for the controls, 1E2 significantly altered the expression of over 50 genes. It increased the expression of the piuBCDA operon, which encodes an iron uptake system, and decreased the expression of dpr, which encodes a protein critical for resistance to oxidative stress. 1E2-mediated effects on ST3 in vivo required divalent binding, as Fab fragments did not reduce NP colonization or alter ST3 gene expression. In vitro, 1E2 induced dose-dependent ST3 growth arrest and altered piuB and dpr expression, whereas an opsonic PPS3 MAb, 5F6, did not. 1E2-treated bacteria were more sensitive to hydrogen peroxide and the iron-requiring antibiotic streptonigrin, suggesting that 1E2 may increase iron import and enhance sensitivity to oxidative stress. Finally, 1E2 also induced rapid capsule shedding in vitro, suggesting that this may initiate 1E2-induced changes in sensitivity to oxidative stress and gene expression. Our data reveal a novel mechanism of direct, antibody-mediated antibacterial activity that could inform new directions in antipneumococcal therapy and vaccine development.

Development of an Efficacious, Semisynthetic Glycoconjugate Vaccine Candidate against Streptococcus pneumoniae Serotype 1

Infections with Streptococcus pneumoniae are a major health burden. Glycoconjugate vaccines based on capsular polysaccharides (CPSs) successfully protect from infection, but not all pneumococcal serotypes are covered with equal potency. Marketed glycoconjugate vaccines induce low levels of functional antibodies against the highly invasive serotype 1 (ST1), presumably due to the obscuring of protective epitopes during chemical activation and conjugation to carrier proteins. Synthetic oligosaccharide antigens can be designed to carry linkers for site-selective protein conjugation while keeping protective epitopes intact. Here, we developed an efficacious semisynthetic ST1 glycoconjugate vaccine candidate. A panel of synthetic oligosaccharides served to reveal a critical role of the rare aminosugar, 2-acetamido-4-amino-2,4,6-trideoxy-d-galactose (d-AAT), for ST1 immune recognition. A monovalent ST1 trisaccharide carrying d-AAT at the nonreducing end induced a strong antibacterial immune response in rabbits and outperformed the ST1 component of the multivalent blockbuster vaccine Prevenar 13, paving the way for a more efficacious vaccine.

Enhanced control of Mycobacterium tuberculosis extrapulmonary dissemination in mice by an arabinomannan-protein conjugate vaccine

Currently there are a dozen or so of new vaccine candidates in clinical trials for prevention of tuberculosis (TB) and each formulation attempts to elicit protection by enhancement of cell-mediated immunity (CMI). In contrast, most approved vaccines against other bacterial pathogens are believed to mediate protection by eliciting antibody responses. However, it has been difficult to apply this formula to TB because of the difficulty in reliably eliciting protective antibodies. Here, we developed capsular polysaccharide conjugates by linking mycobacterial capsular arabinomannan (AM) to either Mtb Ag85b or B. anthracis protective antigen (PA). Further, we studied their immunogenicity by ELISA and AM glycan microarrays and protection efficacy in mice. Immunization with either Abg85b-AM or PA-AM conjugates elicited an AM-specific antibody response in mice. AM binding antibodies stimulated transcriptional changes in Mtb. Sera from AM conjugate immunized mice reacted against a broad spectrum of AM structural variants and specifically recognized arabinan fragments. Conjugate vaccine immunized mice infected with Mtb had lower bacterial numbers in lungs and spleen, and lived longer than control mice. These findings provide additional evidence that humoral immunity can contribute to protection against Mtb.

Vaccine design in the TB field has been driven by the imperative of attempting to elicit strong cell-mediated responses. However, in recent decades evidence has accumulated that humoral immunity can protect against many intracellular pathogens through numerous mechanisms. In this work, we demonstrate that immunization with mycobacterial capsular arabinomannan (AM) conjugates elicited responses that contributed to protection against Mtb infection. We developed two different conjugates including capsular AM linked to the Mtb related protein Ag85b or the Mtb unrelated PA from B. anthracis and found that immunization with AM conjugates elicited antibody populations with different specificities. These surface-specific antibodies could directly modify the transcriptional profile and metabolism of mycobacteria. In addition, we observed a prolonged survival and a reduction in bacterial numbers in lungs and spleen in mice immunized with Ag85b-AM conjugates after infection with Mtb and that the presence of AM-binding antibodies was associated with modest prolongation in survival and a marked reduction in mycobacterial dissemination. Finally, we show that AM is antigenically variable and could potentially form the basis for a serological characterization of mycobacteria based on serotypes.

Unveiling Unexpected Immune Activities Induced by Your Pneumococcal Vaccine

In modern-day vaccine design, a good pneumococcal capsular polysaccharide vaccine is measured by its ability to induce opsonic antibodies. These antibodies label bacteria for phagocytosis by neutrophils and thereby overcome the capsule’s barrier function. Doyle and Pirofski have raised a serious challenge to the current paradigm by describing anti-capsular antibodies that are highly protective but nonopsonic [C.R. Doyle and L. Pirofski, mBio 7(1):e02260-15, 2016, doi:10.1128/mBio.02260-15]. In fact, some functions are not related to neutrophils or phagocytosis at all. An increased awareness of these activities is critical not only for accurate comparisons of vaccine candidates but also for improvements in vaccination outcomes in settings of neutropenia. When vaccine developers select a single gatekeeper assay (e.g., an opsonophagocytic assay for bacteria or a neutralization assay for viruses), promising vaccine candidates may be missed. Doyle and Pirofski stress that multiple functions, not just one, should be investigated to enhance discovery of antibody mechanisms and to best assess vaccine-induced correlates of immune protection.

Coordinated Bacteriocin Expression and Competence in Streptococcus pneumoniae Contributes to Genetic Adaptation through Neighbor Predation

Streptococcus pneumoniae (pneumococcus) has remained a persistent cause of invasive and mucosal disease in humans despite the widespread use of antibiotics and vaccines. The resilience of this organism is due to its capacity for adaptation through the uptake and incorporation of new genetic material from the surrounding microbial community. DNA uptake and recombination is controlled by a tightly regulated quorum sensing system that is triggered by the extracellular accumulation of competence stimulating peptide (CSP). In this study, we demonstrate that CSP can stimulate the production of a diverse array of blp bacteriocins. This cross stimulation occurs through increased production and secretion of the bacteriocin pheromone, BlpC, and requires a functional competence regulatory system. We show that a highly conserved motif in the promoter of the operon encoding BlpC and its transporter mediates the upregulation by CSP. The accumulation of BlpC following CSP stimulation results in augmented activation of the entire blp locus. Using biofilm-grown organisms as a model for competition and genetic exchange on the mucosal surface, we demonstrate that DNA exchange is enhanced by bacteriocin secretion suggesting that co-stimulation of bacteriocins with competence provides an adaptive advantage. The blp and com regulatory pathways are believed to have diverged and specialized in a remote ancestor of pneumococcus. Despite this, the two systems have maintained a regulatory connection that promotes competition and adaptation by targeting for lysis a wide array of potential competitors while simultaneously providing the means for incorporation of their DNA.

Reduction of Streptococcus pneumoniae Colonization and Dissemination by a Nonopsonic Capsular Polysaccharide Antibody

Streptococcus pneumoniae colonization of the nasopharynx (NP) is a prerequisite for invasive pneumococcal disease (IPD). The marked reduction in IPD that followed the routine use of pneumococcal polysaccharide conjugate vaccines (PCVs) has been linked to reduced NP colonization with vaccine-included serotypes (STs), with the caveat that PCVs are less effective against pneumonia than against IPD. Although PCV-elicited opsonic antibodies that enhance phagocytic killing of the homologous ST are considered a key correlate of PCV-mediated protection, recent studies question this relationship for some STs, including ST3. Studies with monoclonal antibodies (MAbs) to the pneumococcal capsular polysaccharide (PPS) of ST3 (PPS3) have shown that nonopsonic, as well as opsonic, antibodies can each protect mice against pneumonia and sepsis, but the effect of these types of MAbs on NP colonization is unknown. In this study, we determined the effects of protective opsonic and nonopsonic PPS3 MAbs on ST3 NP colonization in mice. Our results show that a nonopsonic MAb reduced early NP colonization and prevented ST3 dissemination to the lungs and blood, but an opsonic MAb did not. Moreover, the opsonic MAb induced a proinflammatory NP cytokine response, but the nonopsonic MAb had an antiinflammatory effect. The effect of the nonopsonic MAb on colonization did not require its Fc region, but its antiinflammatory effect did. Our findings challenge the paradigm that opsonic MAbs are required to prevent NP colonization and suggest that further studies of the activity of nonopsonic antibodies could advance our understanding of mechanisms of PCV efficacy and provide novel correlates of protection.

Pneumococcal conjugate vaccines (PCVs) have markedly reduced the incidence of invasive pneumococcal disease (IPD). Vaccine-elicited pneumococcal polysaccharide (PPS) antibodies that enhance in vitro phagocyte killing of vaccine-included serotypes (STs) (opsonic antibodies) have been considered correlates of vaccine protection and are thought to exert their effect at the initial site of infection, the nasopharynx (NP). However, the data presented here show that this is not the necessarily the case. A nonopsonic PPS monoclonal antibody (MAb) reduced pneumococcal colonization and dissemination of its homologous ST in mice, but surprisingly, an opsonic PPS MAb to the same ST did not. These results reveal that PPS antibodies can work in different ways than previously thought, challenge the paradigm that opsonic antibodies are required to prevent IPD, and provide new insights into PCV efficacy that could lead to novel correlates of vaccine protection.

A Flagellar Glycan-Specific Protein Encoded by Campylobacter Phages Inhibits Host Cell Growth

We previously characterized a carbohydrate binding protein, Gp047, derived from lytic Campylobacter phage NCTC 12673, as a promising diagnostic tool for the identification of Campylobacter jejuni and Campylobacter coli. We also demonstrated that this protein binds specifically to acetamidino-modified pseudaminic acid residues on host flagella, but the role of this protein in the phage lifecycle remains unknown. Here, we report that Gp047 is capable of inhibiting C. jejuni growth both on solid and liquid media, an activity, which we found to be bacteriostatic. The Gp047 domain responsible for bacterial growth inhibition is localized to the C-terminal quarter of the protein, and this activity is both contact- and dose-dependent. Gp047 gene homologues are present in all Campylobacter phages sequenced to date, and the resulting protein is not part of the phage particle. Therefore, these results suggest that either phages of this pathogen have evolved an effector protein capable of host-specific growth inhibition, or that Campylobacter cells have developed a mechanism of regulating their growth upon sensing an impending phage threat.

B cells and antibodies in the defense against Mycobacterium tuberculosis infection

Better understanding of the immunological components and their interactions necessary to prevent or control Mycobacterium tuberculosis (Mtb) infection in humans is critical for tuberculosis (TB) vaccine development strategies. Although the contributory role of humoral immunity in the protection against Mtb infection and disease is less defined than the role of T cells, it has been well-established for many other intracellular pathogens. Here we update and discuss the increasing evidence and the mechanisms of B cells and antibodies in the defense against Mtb infection. We posit that B cells and antibodies have a variety of potential protective roles at each stage of Mtb infection and postulate that such roles should be considered in the development strategies for TB vaccines and other immune-based interventions.

Quorum quenching: role in nature and applied developments

Quorum sensing (QS) refers to the capacity of bacteria to monitor their population density and regulate gene expression accordingly: the QS-regulated processes deal with multicellular behaviors (e.g. growth and development of biofilm), horizontal gene transfer and host-microbe (symbiosis and pathogenesis) and microbe-microbe interactions. QS signaling requires the synthesis, exchange and perception of bacterial compounds, called autoinducers or QS signals (e.g. N-acylhomoserine lactones). The disruption of QS signaling, also termed quorum quenching (QQ), encompasses very diverse phenomena and mechanisms which are presented and discussed in this review. First, we surveyed the QS-signal diversity and QS-associated responses for a better understanding of the targets of the QQ phenomena that organisms have naturally evolved and are currently actively investigated in applied perspectives. Next the mechanisms, targets and molecular actors associated with QS interference are presented, with a special emphasis on the description of natural QQ enzymes and chemicals acting as QS inhibitors. Selected QQ paradigms are detailed to exemplify the mechanisms and biological roles of QS inhibition in microbe-microbe and host-microbe interactions. Finally, some QQ strategies are presented as promising tools in different fields such as medicine, aquaculture, crop production and anti-biofouling area.

Passive Immunization

Whereas active immunity refers to the process of exposing the individual to an antigen to generate an adaptive immune response, passive immunity refers to the transfer of antibodies from one individual to another. Passive immunity provides immediate but short-lived protection, lasting several weeks up to 3 or 4 months. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta or from breast milk to the gut of the infant. It can also be produced artificially, when antibody preparations derived from sera or secretions of immunized donors or, more recently, different antibody producing platforms are transferred via systemic or mucosal route to nonimmune individuals. Passive immunization has recently become an attractive approach because of the emergence of new and drug-resistant microorganisms, diseases that are unresponsive to drug therapy and individuals with an impaired immune system who are unable to respond to conventional vaccines. This chapter addresses the contributions of natural and artificial acquired passive immunity in understanding the concept of passive immunization. We will mainly focus on administration of antibodies for protection against various infectious agents entering through mucosal surfaces.

Role of B cells and antibodies in acquired immunity against Mycobacterium tuberculosis

Accumulating evidence has documented a role for B cells and antibodies (Abs) in the immunity against Mycobacterium tuberculosis (Mtb). Passive transfer studies with monoclonal antibodies (mAbs) against mycobacterial antigens have shown protection against the tubercle bacillus. B cells and Abs are believed to contribute to an enhanced immune response against Mtb by modulating various immunological components in the infected host including the T-cell compartment. Nevertheless, the extent and contribution of B cells and Abs to protection against Mtb remains uncertain. In this article we summarize the most relevant findings supporting the role of B cells and Abs in the defense against Mtb and discuss the potential mechanisms of protection.

Current concepts in host-microbe interaction leading to pneumococcal pneumonia

PURPOSE OF REVIEW

Infection with Streptococcus pneumoniae (pneumococcus) results in colonization, which can lead to local or invasive disease, of which pneumonia is the most common manifestation. Despite the availability of pneumococcal vaccines, pneumococcal pneumonia is the leading cause of community and inhospital pneumonia in the United States and globally. This article discusses new insights into the pathogenesis of pneumococcal disease.

RECENT FINDINGS

The host-microbe interactions that determine whether pneumococcal colonization will result in clearance or invasive disease are highly complex. This article focuses on new information in three areas that bear on the pathogenesis of pneumococcal disease: factors that govern colonization, the prelude to invasive disease, including effects on colonization and invasion of capsular serotype, pneumolysin, surface proteins that regulate complement deposition, biofilm formation and agglutination; the effect of coinfection with other bacteria and viruses on pneumococcal growth and virulence, including the synergistic effect of influenza virus; and the contribution of the host inflammatory response to the pathogenesis of pneumococcal pneumonia, including the effects of pattern recognition molecules, cells that enhance and modulate inflammation, and therapies that modulate inflammation, such as statins.

SUMMARY

Recent research on pneumococcal pathogenesis reveals new mechanisms by which microbial factors govern the ability of pneumococcus to progress from the state of colonization to disease and host inflammatory responses contribute to the development of pneumonia. These mechanisms suggest that therapies which modulate the inflammatory response could hold promise for ameliorating damage stemming from the host inflammatory response in pneumococcal disease.

Antibody-mediated immunity against tuberculosis: implications for vaccine development

There is an urgent need for new and better vaccines against tuberculosis (TB). Current vaccine design strategies are generally focused on the enhancement of cell-mediated immunity. Antibody-based approaches are not being considered, mostly due to the paradigm that humoral immunity plays little role in the protection against intracellular pathogens. Here, we reappraise and update the increasing evidence for antibody-mediated immunity against Mycobacterium tuberculosis, discuss the complexity of antibody responses to mycobacteria, and address mechanism of protection. Based on these findings and discussions, we challenge the common belief that immunity against M. tuberculosis relies solely on cellular defense mechanisms, and posit that induction of antibody-mediated immunity should be included in TB vaccine development strategies.

Staphylococcal Enterotoxin B-specific monoclonal antibody 20B1 successfully treats diverse Staphylococcus aureus infections

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) has become a major health threat in the United States. Staphylococcal enterotoxin B (SEB) is a potent superantigen that contributes to its virulence. High mortality and frequent failure of therapy despite available antibiotics have stimulated research efforts to develop adjunctive therapies.

METHODS

Treatment benefits of SEB-specific monoclonal antibody (mAb) 20B1 were investigated in mice in sepsis, superficial skin, and deep-tissue infection models.

RESULTS

Mice challenged with a SEB-producing MRSA strain developed fatal sepsis, extensive tissue skin infection, and abscess-forming deep-seeded thigh muscle infection. Animals preimmunized against SEB or treated passively with mAb 20B1 exhibited enhanced survival in the sepsis model, whereas decrease of bacterial burden was observed in the superficial skin and deep-tissue models. mAb 20B1 bound to SEB in the infected tissue and decreased abscess formation and proinflammatory cytokine levels, lymphocyte proliferation, and neutrophil recruitment.

CONCLUSIONS

mAb 20B1, an SEB-neutralizing mAb, is effective against MRSA infection. mAb 20B1 protects against lethal sepsis and reduces skin tissue invasion and deep-abscess formation. The mAb penetrates well into the abscess and binds to SEB. It affects the outcome of S. aureus infection by modulating the host's proinflammatory immune response.

Targeting agr- and agr-Like quorum sensing systems for development of common therapeutics to treat multiple gram-positive bacterial infections

Invasive infection by the Gram-positive pathogen Staphylococcus aureus is controlled by a four gene operon, agr that encodes a quorum sensing system for the regulation of virulence. While agr has been well studied in S. aureus, the contribution of agr homologues and analogues in other Gram-positive pathogens is just beginning to be understood. Intriguingly, other significant human pathogens, including Clostridium perfringens, Listeria monocytogenes, and Enterococcus faecalis contain agr or analogues linked to virulence. Moreover, other significant human Gram-positive pathogens use peptide based quorum sensing systems to establish or maintain infection. The potential for commonality in aspects of these signaling systems across different species raises the prospect of identifying therapeutics that could target multiple pathogens. Here, we review the status of research into these agr homologues, analogues, and other peptide based quorum sensing systems in Gram-positive pathogens as well as the potential for identifying common pathways and signaling mechanisms for therapeutic discovery.

Fungal cell wall dynamics and infection site microenvironments: signal integration and infection outcome

Upon entrance into the host, fungi encounter a myriad of host effector products and microenvironments that they sense and adapt to for survival. Alterations of the structure and composition of the cell wall is a major fungal adaptation mechanism to evade these environments. Here we discuss recent findings of host-microenvironmental induced fungal cell wall changes, including structure, composition, and protein content, and their effects on host immune responses. A take home message from these recent studies is an emerging understanding of how integration of multiple signals, of both fungal and host responses to dynamic infection site microenvironments, determines outcomes of infection. A challenge moving forward is to further understand these mechanisms and harness them for therapeutic benefit.

Antibody binding to Cryptococcus neoformans impairs budding by altering capsular mechanical properties

Abs to microbial capsules are critical for host defense against encapsulated pathogens, but very little is known about the effects of Ab binding on the capsule, apart from producing qualitative capsular reactions ("quellung" effects). A problem in studying Ab-capsule interactions is the lack of experimental methodology, given that capsules are fragile, highly hydrated structures. In this study, we pioneered the use of optical tweezers microscopy to study Ab-capsule interactions. Binding of protective mAbs to the capsule of the fungal pathogen Cryptococcus neoformans impaired yeast budding by trapping newly emerging buds inside the parental capsule. This effect is due to profound mAb-mediated changes in capsular mechanical properties, demonstrated by a concentration-dependent increase in capsule stiffness. This increase involved mAb-mediated cross-linking of capsular polysaccharide molecules. These results provide new insights into Ab-mediated immunity, while suggesting a new nonclassical mechanism of Ab function, which may apply to other encapsulated pathogens. Our findings add to the growing body of evidence that Abs have direct antimicrobial functions independent of other components of the immune system.

Antibiotic resistance: from Darwin to Lederberg to Keynes

The emergence and spread of antibiotic-resistant bacteria reflects both, a gradual, completely Darwinian evolution, which mostly yields slight decreases in antibiotic susceptibility, along with phenotypes that are not precisely characterized as "resistance"; and sudden changes, from full susceptibility to full resistance, which are driven by a vast array of horizontal gene transfer mechanisms. Antibiotics select for more than just antibiotic resistance (i.e., increased virulence and enhanced gene exchange abilities); and many non-antibiotic agents or conditions select for or maintain antibiotic resistance traits as a result of a complex network of underlying and often overlapping mechanisms. Thus, the development of new antibiotics and thoughtful, integrated anti-infective strategies is needed to address the immediate and long-term threat of antibiotic resistance. Since the biology of resistance is complex, these new drugs and strategies will not come from free-market forces, or from "incentives" for pharmaceutical companies.

Strain-related differences in antibody-mediated changes in gene expression are associated with differences in capsule and location of binding

We recently established that antibody (Ab)-binding can induce gene expression changes in a serotype A strain (H99) of the pathogenic yeast, Cryptococcus neoformans. That study showed that monoclonal antibodies (mAbs) differing in epitope specificity and protective efficacy elicited differences in gene expression. Because many mAbs bind to serotypes A and D strains differently, we now investigate the binding of one mAb to two strains representing these serotypes. Cells of the serotype A strain H99 and the serotype D strain 24067 were incubated with near saturating concentrations of the IgG1 capsule-binding mAb 18B7 or MOPC, an irrelevant mAb matched control. Comparative immunofluorescence analysis of mAb 18B7 binding revealed that it bound closer to the cell wall in H99 than 24067, where it was associated with decreased or increased cell diameter, respectively. A comparison of encapsulated cell compressibility showed that strain 24067 was more compressible than that of strain H99. RNA was extracted and used for gene expression analysis using the C. neoformans JEC21 genomic microarray. After 1h incubation with mAb 18B7, there were just 2 gene expression changes observed with strain 24067 or strain JEC21, unlike the 43 seen with strain H99. After 4h incubation with mAb 18B7, there were 14 and 140 gene expression changes observed with strain 24067 and JEC21, respectively. Thus, C. neoformans strains differ both in the response and the time of response to mAb binding and these differences may reflect differences in the location of Ab binding, Ab-mediated changes in cell diameter and compressibility of the capsular polysaccharide.

A new synthesis for antibody-mediated immunity

The view that immunoglobulins function largely by potentiating neutralization, cytotoxicity or phagocytosis is being replaced by a new synthesis whereby antibodies participate in all aspects of the immune response, from protecting the host at the earliest time of encounter with a microbe to later challenges. Perhaps the most transformative concept is that immunoglobulins manifest emergent properties, from their structure and function as individual molecules to their interactions with microbial targets and the host immune system. Given that emergent properties are neither reducible to first principles nor predictable, there is a need for new conceptual approaches for understanding antibody function and mechanisms of antibody immunity.

New antibody weapons against an old foe

Antibodies have been used in a diagnostic capacity for many diseases and for identifying serotypes within single species of pathogens, notably between the multiple capsular polysaccharide serotypes of Streptococcus pneumoniae. For many years, the functions of antibodies in infection were thought to be limited to the opsonization of microorganisms followed by phagocytosis and to the fixing of complement. The thought that antibodies could have other functions has emerged only recently. The study by Yano and coworkers from the laboratory of Liise-anne Pirofski published in mBio [M. Yano, S. Gohil, J. R. Coleman, C. Manix, and L.-A. Pirofski, mBio 2(5):e00176-11, 2011] identifies one mechanism whereby nonopsonic antibodies enhance the transformation competence of two S. pneumoniae serotypes, which leads to an increase in genetic exchange and bacterial variability with a resulting population reduction through fratricide. These new and revealing antibody functions will add another chapter to the burgeoning story of the diversity and versatility of the immune response to bacteria.