Artificial Salmonella vaccines: Salmonella typhimurium O-antigen-specific oligosaccharide-protein conjugates elicit opsonizing antibodies that enhance phagocytosis

Development in the Concept of Bacterial Polysaccharide Repeating Unit-Based Antibacterial Conjugate Vaccines

The surface of cells is coated with a dense layer of glycans, known as the cell glycocalyx. The complex glycans in the glycocalyx are involved in various biological events, such as bacterial pathogenesis, protection of bacteria from environmental stresses, etc. Polysaccharides on the bacterial cell surface are highly conserved and accessible molecules, and thus they are excellent immunological targets. Consequently, bacterial polysaccharides and their repeating units have been extensively studied as antigens for the development of antibacterial vaccines. This Review surveys the recent developments in the synthetic and immunological investigations of bacterial polysaccharide repeating unit-based conjugate vaccines against several human pathogenic bacteria. The major challenges associated with the development of functional carbohydrate-based antibacterial conjugate vaccines are also considered.

The effect of O-antigen length determinant wzz on the immunogenicity of Salmonella Typhimurium for Escherichia coli O2 O-polysaccharides delivery

Attenuated Salmonella Typhimurium is a promising antigen delivery system for live vaccines such as polysaccharides. The length of polysaccharides is a well-known key factor in modulating the immune response induced by glycoconjugates. However, the relationship between the length of Lipopolysaccharide (LPS) O-antigen (OAg) and the immunogenicity of S. Typhimurium remains unclear. In this study, we assessed the effect of OAg length determined by wzz_ST on Salmonella colonization, cell membrane permeability, antimicrobial activity, and immunogenicity by comparing the S. Typhimurium wild-type ATCC14028 strain to those with various OAg lengths of the Δwzz_ST mutant and Δwzz_ST::wzz_ECO2. The analysis of the OAg length distribution revealed that, except for the very long OAg, the short OAg length of 2-7 repeat units (RUs) was obtained from the Δwzz_ST mutant, the intermediate OAg length of 13-21 RUs was gained from Δwzz_ST::wzz_ECO2, and the long OAg length of over 20 RUs was gained from the wild-type. In addition, we found that the OAg length affected Salmonella colonization, cell permeability, and antibiotic resistance. Immunization of mice revealed that shortening the OAg length by altering wzz_ST had an effect on serum bactericidal ability, complement deposition, and humoral immune response. S. Typhimurium mutant strain Δwzz_ST::wzz_ECO2 possessed good immunogenicity and was the optimum option for delivering E. coli O2 O-polysaccharides. Furthermore, the attenuated strain ATCC14028 ΔasdΔcrpΔcyaΔrfbPΔwzz_ST::wzz_ECO2-delivered E. coli O2 OAg gene cluster outperforms the ATCC14028 ΔasdΔcrpΔcyaΔrfbP in terms of IgG eliciting, cytokine expression, and immune protection in chickens. This study sheds light on the role of OAg length in Salmonella characteristics, which may have a potential application in optimizing the efficacy of delivered polysaccharide vaccines.

Synthetic Glycans to Improve Current Glycoconjugate Vaccines and Fight Antimicrobial Resistance

Antimicrobial resistance (AMR) is emerging as the next potential pandemic. Different microorganisms, including the bacteria Acinetobacter baumannii, Clostridioides difficile, Escherichia coli, Enterococcus faecium, Klebsiella pneumoniae, Neisseria gonorrhoeae, Pseudomonas aeruginosa, non-typhoidal Salmonella, and Staphylococcus aureus, and the fungus Candida auris, have been identified by the WHO and CDC as urgent or serious AMR threats. Others, such as group A and B Streptococci, are classified as concerning threats. Glycoconjugate vaccines have been demonstrated to be an efficacious and cost-effective measure to combat infections against Haemophilus influenzae, Neisseria meningitis, Streptococcus pneumoniae, and, more recently, Salmonella typhi. Recent times have seen enormous progress in methodologies for the assembly of complex glycans and glycoconjugates, with developments in synthetic, chemoenzymatic, and glycoengineering methodologies. This review analyzes the advancement of glycoconjugate vaccines based on synthetic carbohydrates to improve existing vaccines and identify novel candidates to combat AMR. Through this literature survey we built an overview of structure-immunogenicity relationships from available data and identify gaps and areas for further research to better exploit the peculiar role of carbohydrates as vaccine targets and create the next generation of synthetic carbohydrate-based vaccines.

Advances in the development of Salmonella-based vaccine strategies for protection against Salmonellosis in humans

Salmonella spp. are important human pathogens globally causing millions of cases of typhoid fever and non‐typhoidal salmonellosis annually. There are only a few vaccines licensed for use in humans which all target Salmonella enterica serovar Typhi. Vaccine development is hampered by antigenic diversity between the thousands of serovars capable of causing infection in humans. However, a number of attenuated candidate vaccine strains are currently being developed. As facultative intracellular pathogens with multiple systems for transporting effector proteins to host cells, attenuated Salmonella strains can also serve as ideal tools for the delivery of foreign antigens to create multivalent live carrier vaccines for simultaneous immunization against several unrelated pathogens. Further, the ease with which Salmonella can be genetically modified and the extensive knowledge of the virulence mechanisms of this pathogen means that this bacterium has often served as a model organism to test new approaches. In this review we focus on (1) recent advances in live attenuated Salmonella vaccine development, (2) improvements in expression of foreign antigens in carrier vaccines and (3) adaptation of attenuated strains as sources of purified antigens and vesicles that can be used for subunit and conjugate vaccines or together with attenuated vaccine strains in heterologous prime‐boosting immunization strategies. These advances have led to the development of new vaccines against Salmonella which have or will soon be tested in clinical trials.

Effect of O-Antigen Chain Length Regulation on the Immunogenicity of Shigella and Salmonella Generalized Modules for Membrane Antigens (GMMA)

Recently, generalized modules for membrane antigens (GMMA) technology has been proposed as an alternative approach to traditional glycoconjugate vaccines for O-antigen delivery. Saccharide length is a well-known parameter that can impact the immune response induced by glycoconjugates both in terms of magnitude and quality. However, the criticality of O-antigen length on the immune response induced by GMMA-based vaccines has not been fully elucidated. Here, Shigella and Salmonella GMMA-producing strains were further mutated in order to display homogeneous polysaccharide populations of different sizes on a GMMA surface. Resulting GMMA were compared in mice immunization studies. Athymic nude mice were also used to investigate the involvement of T-cells in the immune response elicited. In contrast with what has been reported for traditional glycoconjugate vaccines and independent of the pathogen and the sugar structural characteristics, O-antigen length did not result in being a critical parameter for GMMA immunogenicity. This work supports the identification of critical quality attributes to optimize GMMA vaccine design and improve vaccine efficacy and gives insights on the nature of the immune response induced by GMMA.

Immunogenicity and Induction of Functional Antibodies in Rabbits Immunized with a Trivalent Typhoid-Invasive Nontyphoidal Salmonella Glycoconjugate Formulation

Typhoid fever due to Salmonella Typhi and invasive nontyphoidal Salmonella (iNTS) infections caused by serovars Enteritidis (SE) and Typhimurium (STm) are major pediatric health problems in sub-Saharan Africa. Typhoid has high complication rates, and iNTS infections have high case fatality rates; moreover, emerging antimicrobial resistance is diminishing treatment options. Vi capsule-based typhoid conjugate vaccine (Typbar-TCV™), licensed in India and pre-qualified by the World Health Organization, elicits durable immunity when administered to infants, but no iNTS vaccines are licensed or imminent. We have developed monovalent SE and STm glycoconjugate vaccines based on coupling lipopolysaccharide-derived core-O polysaccharide (COPS) to phase 1 flagellin protein (FliC) from the homologous serovar. Herein, we report the immunogenicity of multivalent formulations of iNTS COPS:FliC conjugates with Typbar-TCV™. Rabbits immunized with the trivalent typhoid-iNTS glycoconjugate vaccine generated high titers of serum IgG antibody to all three polysaccharide antigens for which anti-COPS IgG antibodies were directed primarily against serogroup-specific OPS epitopes. Responses to SE and STm FliC were lower relative to anti-COPS titers. Post-vaccination rabbit sera mediated bactericidal activity in-vitro, and protected mice after passive transfer against challenge with virulent SE or STm Malian blood isolates. These results support accelerated progression to clinical trials.

Assessing Bacterial Interactions Using Carbohydrate-Based Microarrays

Carbohydrates play a crucial role in host-microorganism interactions and many host glycoconjugates are receptors or co-receptors for microbial binding. Host glycosylation varies with species and location in the body, and this contributes to species specificity and tropism of commensal and pathogenic bacteria. Additionally, bacterial glycosylation is often the first bacterial molecular species encountered and responded to by the host system. Accordingly, characterising and identifying the exact structures involved in these critical interactions is an important priority in deciphering microbial pathogenesis. Carbohydrate-based microarray platforms have been an underused tool for screening bacterial interactions with specific carbohydrate structures, but they are growing in popularity in recent years. In this review, we discuss carbohydrate-based microarrays that have been profiled with whole bacteria, recombinantly expressed adhesins or serum antibodies. Three main types of carbohydrate-based microarray platform are considered; (i) conventional carbohydrate or glycan microarrays; (ii) whole mucin microarrays; and (iii) microarrays constructed from bacterial polysaccharides or their components. Determining the nature of the interactions between bacteria and host can help clarify the molecular mechanisms of carbohydrate-mediated interactions in microbial pathogenesis, infectious disease and host immune response and may lead to new strategies to boost therapeutic treatments.

Design of glycoconjugate vaccines against invasive African Salmonella enterica serovar Typhimurium

Nontyphoidal salmonellae, particularly Salmonella enterica serovar Typhimurium, are a major cause of invasive disease in Africa, affecting mainly young children and HIV-infected individuals. Glycoconjugate vaccines provide a safe and reliable strategy against invasive polysaccharide-encapsulated pathogens, and lipopolysaccharide (LPS) is a target of protective immune responses. With the aim of designing an effective vaccine against S. Typhimurium, we have synthesized different glycoconjugates, by linking O-antigen and core sugars (OAg) of LPS to the nontoxic mutant of diphtheria toxin (CRM(197)). The OAg-CRM(197) conjugates varied in (i) OAg source, with three S. Typhimurium strains used for OAg extraction, producing OAg with differences in structural specificities, (ii) OAg chain length, and (iii) OAg/CRM(197) ratio. All glycoconjugates were compared for immunogenicity and ability to induce serum bactericidal activity in mice. In vivo enhancement of bacterial clearance was assessed for a selected S. Typhimurium glycoconjugate by challenge with live Salmonella. We found that the largest anti-OAg antibody responses were elicited by (i) vaccines synthesized from OAg with the highest glucosylation levels, (ii) OAg composed of mixed- or medium-molecular-weight populations, and (iii) a lower OAg/CRM(197) ratio. In addition, we found that bactericidal activity can be influenced by S. Typhimurium OAg strain, most likely as a result of differences in OAg O-acetylation and glucosylation. Finally, we confirmed that mice immunized with the selected OAg-conjugate were protected against S. Typhimurium colonization of the spleen and liver. In conclusion, our findings indicate that differences in the design of OAg-based glycoconjugate vaccines against invasive African S. Typhimurium can have profound effects on immunogenicity and therefore optimal vaccine design requires careful consideration.

Glycoconjugate vaccine strategies for protection against invasive Salmonella infections

Salmonella enterica serovars Typhi and Paratyphi A and B and certain non-typhoidal Salmonella enterica (NTS) serovars are important causes of invasive Salmonella disease worldwide. NTS serovars Typhimurium and Enteritidis typically cause gastroenteritis in healthy children and adults in industrialized countries but in certain hosts (e.g., young infants, the elderly, immunocompromised individuals) they also cause invasive infections. These two serovars also cause invasive disease in infants and young children in sub-Saharan Africa. Whereas Salmonella surface polysaccharides are poor immunogens in animal models and do not generate immunologic memory, conjugation with carrier proteins overcomes these limitations. S. Typhi expresses a Vi polysaccharide capsule; Vi either alone or as a glycoconjugate protects humans from typhoid fever. In contrast, S. Paratyphi A and B and NTS (with rare exceptions) do not express capsular polysaccharides. Rather, their surface polysaccharides are the O polysaccharide (OPS) of lipopolysaccharide. In animal studies, immunization with Salmonella COPS (core polysaccharide-OPS) conjugated with carrier proteins generates functional immunity and protects against fatal Salmonella challenge. Conjugating to Salmonella proteins (flagellin, porins) may extend immune responses to another relevant target for antibody generation and enhance the glyconjugate's efficacy.

Salmonella enterica serovar enteritidis core O polysaccharide conjugated to H:g,m flagellin as a candidate vaccine for protection against invasive infection with S. enteritidis

Nontyphoidal Salmonella enterica serovars Enteritidis and Typhimurium are a common cause of gastroenteritis but also cause invasive infections and enteric fever in certain hosts (young children in sub-Saharan Africa, the elderly, and immunocompromised individuals). Salmonella O polysaccharides (OPS) and flagellar proteins are virulence factors and protective antigens. The surface polysaccharides of Salmonella are poorly immunogenic and do not confer immunologic memory, limitations overcome by covalently attaching them to carrier proteins. We conjugated core polysaccharide-OPS (COPS) of Salmonella Enteritidis lipopolysaccharide (LPS) to flagellin protein from the homologous strain. COPS and flagellin were purified from a genetically attenuated (ΔguaBA) "reagent strain" (derived from an isolate from a patient with clinical bacteremia) engineered for increased flagellin production (ΔclpPX). Conjugates were constructed by linking flagellin monomers or polymers at random COPS hydroxyls with various polysaccharide/protein ratios by 1-cyano-4-dimethylaminopyridinium tetrafluoroborate (CDAP) or at the 3-deoxy-d-manno-octulosonic acid (KDO) terminus by thioether chemistry. Mice immunized on days 0, 28, and 56 with COPS-flagellin conjugates mounted higher anti-LPS IgG levels than mice receiving unconjugated COPS and exhibited high antiflagellin IgG; anti-LPS and antiflagellin IgG levels increased following booster doses. Antibodies generated by COPS-flagellin conjugates mediated opsonophagocytosis of S. Enteritidis cells into mouse macrophages. Mice immunized with flagellin alone, COPS-CRM₁₉₇, or COPS-flagellin conjugates were significantly protected from lethal challenge with wild-type S. Enteritidis (80 to 100% vaccine efficacy).

Mouse models to assess the efficacy of non-typhoidal Salmonella vaccines: revisiting the role of host innate susceptibility and routes of challenge

Non-typhoidal Salmonella enterica (NTS) serovars Typhimurium and Enteritidis are important causes of bacterial gastroenteritis in the USA and worldwide. In sub-Saharan Africa these two serovars are emerging as agents associated with lethal invasive disease (e.g., bacteremia, meningitis). The development of NTS vaccines, based on mucosally administered live attenuated strains and parenteral non-living antigens, could diminish the NTS disease burden globally. Mouse models of S. Typhimurium and S. Enteritidis invasive disease can accelerate the development of NTS vaccines. Live attenuated NTS vaccines elicit both cellular and humoral immunity in mice and their efficacy is well established. In contrast, non-living vaccines that primarily elicit humoral immunity have demonstrated variable efficacy. An analysis of the reported studies with non-living vaccines against S. Typhimurium and S. Enteritidis reveals that efficacy is influenced by two important independent variables: (1) the innate susceptibility to NTS infection that differs dramatically between commonly used mouse strains and (2) the virulence of the NTS strain used for challenge. Protection by non-living vaccines has generally been seen only in host-pathogen interactions where a sub-lethal infection results, such as challenging resistant mice with either highly virulent or weakly virulent strains or susceptible mice with weakly virulent strains. The immunologic basis of this discrepancy and the implications for human NTS vaccine development are reviewed herein.

Pathogen specific carbohydrate antigen microarrays: a chip for detection of Salmonella O-antigen specific antibodies

A Salmonella O-antigen microarray was developed by covalent coupling of oligosaccharide antigens specific for serogroups Salmonella enterica sv. Paratyphi (group A), Typhimurium (group B) and Enteritidis (group D). Antibodies were correctly detected in sera from patients with culture verified salmonellosis. High serogroup-specificity was seen with the disaccharide antigens. With the larger antigens, containing the backbone sequence Manalpha1-2Rhaalpha1-2Gal (MRG), common backbone-specific antibodies (O-antigen 12) were also detected. This is "proof of principle" that pathogen-specific carbohydrate antigen microarrays constitute a novel technology for rapid and specific serological diagnosis in either individual patients or larger sero-epidemiological and vaccine studies.

Vaccines based on the cell surface carbohydrates of pathogenic bacteria

Glycoconjugate vaccines, in which a cell surface carbohydrate from a micro-organism is covalently attached to an appropriate carrier protein are proving to be the most effective means to generate protective immune responses to prevent a wide range of diseases. The technology appears to be generic and applicable to a wide range of pathogens, as long as antibodies against surface carbohydrates help protect against infection. Three such vaccines, against Haemophilus influenzae type b, Neisseria meningitidis Group C and seven serotypes of Streptococcus pneumoniae, have already been licensed and many others are in development. This article discusses the rationale for the development and use of glycoconjugate vaccines, the mechanisms by which they elicit T cell-dependent immune responses and the implications of this for vaccine development, the role of physicochemical methods in the characterisation and quality control of these vaccines, and the novel products which are under development.

Recognition specificity of monoclonal antibodies which protect mice against Salmonella typhimurium infection

We used enzyme-linked immunosorbent assay (ELISA), competitive inhibition ELISA, flow cytometry and western immunoblots to study the antigenic specificity of two monoclonal antibodies (mAbs) raised against the cell surface antigens of Salmonella typhimurium. These mAbs (SH6.11 and WB60.4) protect CAF1 (Ity(r)) mice against endotoxemia and mouse typhoid. We found that SH6.11 and WB60.4 recognize Salmonella serogroup B-specific lipopolysaccharide O4 and O5 factors, respectively. These mAbs did not bind to Salmonella serotypes that belong to serogroup A, D1, E4, G2, or R and did not cross-react with other enteric and nonenteric bacterial species.

Electron beam fragmentation of bacterial polysaccharides as a method of producing oligosaccharides for the preparation of conjugate vaccines

End-group mediated conjugation of bacterial polysaccharides (PSs) to carrier proteins containing T-helper cell epitopes renders such polysaccharides immunogenic also in young infants. Optimal construction of such conjugate vaccines requires fragmentation of the PS prior to the coupling reaction. In the present study a general simple and inexpensive method for the fragmentation of PSs is presented. It is based on the irradiation of isolated PSs in an electron beam accelerator. Exposure of isolated pneumococcal capsular polysaccharides (PnPSs) to ionizing radiation resulted in their partial depolymerization in a radiation dose-dependent manner. Radiation, unlike sonication, generated PnPS fragments of molecular size lower than 50 kDa and as small as 1.5 kDa when high radiation doses were used. These PnPS fragments have terminal reducing groups that can be easily used for chemical activation and subsequent coupling to any chosen carrier protein. The radiation-produced PnPS fragments retained their antigenic epitopes, when compared to native, full-size PnPSs as determined by enzyme-linked immunoassay.

Relationship of serotype, leukotoxin gene type and lysogeny in Actinobacillus actinomycetemcomitans to periodontal disease status

Actinobacillus actinomycetemcomitans has been associated with different forms of periodontitis, particularly with localized juvenile periodontitis (LJP). The bacterium possesses several virulence factors which have been shown to interact with the host immune system. Among these factors, leukotoxin, surface antigens (serotype) and bacteriophages have been suggested directly or indirectly to influence the course of infection. However, few studies have been able to show associations between these factors and periodontal disease, alone or in combination. Thus, the purpose of the present study was to investigate possible correlations between periodontal disease status and selected virulence factors (serotype, presence of bacteriophages, and the presence of a 530 bp deletion in the promoter region of the leukotoxin gene). 36 subjects took part in the study. Serotype c was the most frequently found serotype among periodontally affected subjects, although serotypes a and b were also present. 27 out of 36 strains harbored bacteriophages, and there was strong evidence that some of the bacteriophages were different from the previously characterized phi Aa phage. A. actinomycetemcomitans containing the F-fragment phage were more frequently associated with periodontal disease. Five strains, all serotype b, 3 from LJP patients and 2 from healthy subjects, showed a 530-bp deletion in the promoter region of the leukotoxin gene.

Glycoconjugate vaccines. What next?

The principle that infants can be protected from invasive diseases caused by encapsulated organisms has been proved with the introduction of Haemophilus influenzae type b conjugate vaccines. The use of glycoconjugates to implement some of the goals of the Children's Vaccine Initiative requires a clear delineation of the chemical and immunological specifications for optimal vaccines.

Characterization of novel fucosyl- and tyvelosyl-containing glycoconjugates from Trichinella spiralis muscle stage larvae

The monosaccharide composition of an affinity-purified family of antigenically-related Trichinella spiralis larval glycoproteins was determined by gas chromatography/mass spectrometry. This group of 6 major glycoproteins, designated TSL-1, originates in the muscle stage (L1) larval stichosome. They are present on the L1 surface and in excretory/secretory products of L1 larvae, are stage-specific, and are highly immunodominant. The glycosyl composition of the TSL-1 antigens was remarkable in 2 respects: (1) fucose accounted for 36 molar percent of the glycosyl residues; and (2) a 3,6-dideoxyhexose was identified, which accounted for at least 24 molar percent of the glycosyl residues. Previously, 3,6-dideoxyhexoses have been found only in certain Gram-negative bacterial lipopolysaccharides and in ascaroside alcohols (ascarylose) of Ascaris eggs. The 3,6-dideoxyhexose found in the TSL-1 antigens also was found in ES. This Trichinella sugar has been chemically identified as a 3,6-dideoxyarabinohexose, the same as found in Ascaris eggs. However, the absolute configuration of the TSL-1 sugar is D-(tyvelose), not L-(ascarylose) as is found in Ascaris eggs. Methylation analysis indicated that the TSL-1 3,6-dideoxy-D-arabinohexose was present entirely as non-reducing terminal residues. Approximately 83% of the fucose was also present as non-reducing terminal residues, with the remaining fucose found as 3,4-linked branched residues.

Mechanism of protective immunity induced by porin-lipopolysaccharide against murine salmonellosis

Investigations were undertaken to characterize the protective immunity induced by porin-lipopolysaccharide (LPS) against Salmonella typhimurium infection in mice. Mice immunized with porin-LPS showed higher levels of antiporin immunoglobulin G than mice which received porin alone. Further, T cells from porin-LPS-immunized mice showed an augmented proliferative response to porin in vitro compared with the response of T cells from porin-injected animals. The passive transfer of anti-LPS antibodies conferred significant protection (17%), while antiporin serum failed to protect mice against lethal challenge, indicating the protective ability of anti-LPS antibodies. However, the transfer of serum obtained from porin-LPS-immunized mice resulted in better protection (30%) than did anti-LPS or antiporin antibodies alone. In contrast to LPS, monophosphoryl lipid A completely failed to induce protection against lethal infection. However, comparable to the effect of LPS, injection of porin with monophosphoryl lipid A enhanced antibody response and the protective ability of porin (81.25%). The transfer of T cells from porin-LPS-immunized mice provided higher levels of protection (47%) against lethal challenge than did T cells from porin-immunized mice (23%). The combination of T cells and serum from porin-immunized mice transferred 36% protection. However, a combination of T cells and serum from porin-LPS-immunized mice conferred the highest level of protection (92%), which was reflected by the number of survivors (100%) in the porin-LPS-immunized group. These results demonstrate that besides the protective effect of anti-LPS antibodies, the ability of LPS to augment humoral and cell-mediated immune responses to porin confers effective protection against Salmonella infection.

Mechanism of the protective immunity against murine typhoid: persistence of Salmonella L forms in the liver after immunization with live-cell vaccines

Live-cell vaccines of Salmonella typhimurium, either a sub-lethal dose of a wild-type (strain LT2) or a high dose of its two-heptose Rd1 mutant (strain SL1004), induced acquired resistance to murine typhoid, which remained 180 days after immunization. Growth of S. typhimurium as a bacillary form ceased between days 30 and 60 of immunization, but L forms of this bacterium colonized the liver (the mean number of L forms in the liver: 600 L-forming units) even at 180 days post-immunization. In contrast, a high inoculum of either a Ra mutant (strain TV148) of strain LT2 or S. schottmülleri 8006 sharing the same O antigenic components with those of S. typhimurium induced only a short-lived protection in proportion to the number of L forms in the liver, and the protective immunity was lost before day 180. However, there was no significant difference in the salmonella-specific T-cell responses among groups of immunized mice on day 180 of immunization. A lethal infection with strain LT2 in mice which had been immunized 75 days previously with living cells of strain SL1004 resulted in a rapid clearance of the challenge inoculum, together with a rapid elevation of anti-S. typhimurium antibody responses. Thus, the present data suggest that the long-lived immunity conferred upon live S. typhimurium vaccines is attributable to the colonization of this bacterium in the liver as L forms and the ability to colonize the liver as L forms is independent of the chain length of salmonella O-antigens.

Pathogenesis and immunity in murine salmonellosis

Salmonella is traditionally described as a facultative intracellular parasite, and host macrophages are regarded as the primary effector cells in both native and acquired immunity in mouse typhoid. This concept has not been unanimously accepted in the literature. Based on cell culture experiments and electron microscopic examinations of infected tissues, we observed that virulent Salmonella typhimurium is killed within polymorphs and macrophages of guinea pigs and mice. In a systemic disease, the organism propagates primarily in the extracellular locations of sinusoids and tissue lesions and within hepatocytes. Hence, it is more likely to be an extracellular pathogen and its virulence is directly related to its antiphagocytic property. The conspicuous absence of macrophages in the primary lesions of murine salmonellosis disputes the likelihood of their significant role in native resistance to the disease. Acquired cellular immunity is expressed as an enhanced antibacterial activity of macrophages facilitated by cytophilic antibodies rather than as an altered antibacterial action of immune macrophages. It is proposed that acquired immunity in murine salmonellosis is a synergistic manifestation of the innate capacity of polymorphs and macrophages to destroy ingested salmonellae, the activated antibacterial functions of macrophages mediated by cytophilic antibodies, the opsonic and agglutinating actions of antiserum, and the accelerated inflammation associated with delayed hypersensitivity to bacterial antigens. Unlike live attenuated vaccines, nonviable vaccines offer a significant, though not a solid, protection against subsequent challenges.

Synthesis and physicochemical and immunological characterization of pneumococcus type 12F polysaccharide-diphtheria toxoid conjugates

A scheme for the synthesis and purification of conjugates, composed of the type 12F capsular polysaccharide of Streptococcus pneumoniae (Pn12F) and diphtheria toxoid, is described. The scheme is a modification of that described previously for the Vi capsular polysaccharide of Salmonella typhi, a linear homopolymer of N-acetylgalactoseaminouronic acid (S. C. Szu, A. L. Stone, J. D. Robbins, R. Schneerson, and J. B. Robbins, J. Exp. Med. 166:1510-1524, 1986). Pn12F is a branched-chain copolymer composed of a hexasaccharide repeating unit containing an aminouronic acid, N-acetylmannoseaminouronic acid (K. Leontein, B. Lindberg, and J. Lonngren, Can. J. Chem. 59:2081-2085, 1981). Sulfhydryl groups were introduced into Pn12F by forming an amide bond between cystamine and carboxyl groups of N-acetylmannoseaminouronic acid in the presence of a carbodiimide. The disulfide moiety of cystamine was reduced to form the cysteamine derivative of Pn12F which was, in turn, covalently bound to diphtheria toxoid by using the heterobifunctional linker N-succinimidyl-3-(2-pyridylthio)propionate. Unbound, high-molecular-weight Pn12F was removed from the conjugate by hydrophobic interaction chromatography through octyl Sepharose by using n-octyl-beta-D-glucopyranoside as the eluent. In young outbred mice, Pn12F did not elicit detectable serum antibodies. Pn12F-diphtheria toxoid, in contrast, elicited antibodies after two injections and had T-cell-dependent properties as evidenced by a response to priming and by its ability to elicit booster responses. This scheme seems applicable to the synthesis of conjugates with other capsular polysaccharides containing aminouronic acids. Clinical evaluation of Pn12F-diphtheria toxoid conjugates in healthy and in immunocompromised hosts is planned.

Vi capsular polysaccharide-protein conjugates for prevention of typhoid fever. Preparation, characterization, and immunogenicity in laboratory animals

The Vi has proven to be a protective antigen in two double masked, controlled clinical trials in areas with high rates of typhoid fever (approximately 1% per annum). In both studies the protective efficacy of the Vi was approximately 70%. Approximately 75% of subjects in these areas responded with a fourfold or greater rise of serum Vi antibodies. In contrast, the Vi elicited a fourfold or greater rise in 95-100% of young adults in France and the United States. Methods were devised, therefore, to synthesize Vi-protein conjugates in order to both enhance the antibody response and confer T-dependent properties to the Vi (and theoretically increase its protective action in populations at high risk for typhoid fever). We settled on a method that used the heterobifunctional crosslinking reagent, N-succinimidyl-3-(2-pyridyldithio)-propionate (SPDP), to bind thiol derivatives of the Vi to proteins. This synthetic scheme was reproducible, provided high yields of Vi-protein conjugates, and was applicable to several medically relevant proteins such as diphtheria and tetanus toxoids. The resultant conjugates were more immunogenic in mice and juvenile Rhesus monkeys than the Vi alone. In contrast to the T-independent properties of the Vi, conjugates of this polysaccharide with several medically relevant proteins induced booster responses in mice and in juvenile Rhesus monkeys. Clinical studies with Vi-protein conjugates are planned. This scheme is also applicable to synthesize protein conjugates with other polysaccharides that have carboxyl functions.

Coprecipitation of lipopolysaccharide and the 39,000-molecular-weight major outer membrane protein of Haemophilus influenzae type b by lipopolysaccharide-directed monoclonal antibody

The major outer membrane protein of Haemophilus influenzae type b (Hib) with an apparent molecular weight of 39,000 (39K) was purified from three different Hib strains and was shown to be free from detectable contamination with other proteins. However, these purified 39K protein preparations were found to contain Hib lipopolysaccharide (LPS). Immunization of rats with these 39K protein preparations resulted in the production of antisera containing both 39K protein-directed and LPS-directed antibodies, as determined by Western blot analysis. The reactivity pattern of the LPS-directed serum antibodies with different Hib strains was identical to the reactivity of these Hib strains with a set of monoclonal antibodies (mabs) previously shown to immunoprecipitate the 39K protein in a radioimmunoprecipitation (RIP) system. Examination of the antigenic specificities of the 39K protein-immunoprecipitating mabs by using Western blot analysis showed that these mabs were actually directed against Hib LPS. RIP analysis of 125I-labeled Hib cells and 32P-labeled Hib cells revealed that the 39K protein and LPS existed as a complex in a RIP system, which resulted in the coprecipitation of both antigens by LPS-directed mabs. The interaction between LPS and the 39K protein was highly selective for this protein and did not involve other outer membrane proteins. The LPS/39K protein complex could be reconstituted by mixing purified LPS and purified 39K protein; it could also be reconstituted with 39K protein from one Hib strain and LPS from another Hib strain. These findings have necessitated the reinterpretation of previous studies involving the 39K protein-immunoprecipitating mabs. Of primary importance is the fact that the demonstrated immunoprotective ability of a 39K protein-immunoprecipitating mab (E. J. Hansen, S. M. Robertson, P. A. Gulig, C. F. Frisch, and E. J. Haanes, Lancet i:366-368, 1982) must now be regarded as evidence that antibody directed against Hib LPS can be protective against experimental Hib disease.

Antibody response in rabbits to gonococcal lipopolysaccharide-bovine serum albumin conjugates

The elicitation of antibodies to gonococcal lipopolysaccharide (LPS) was studied. Rabbits were immunized with whole cells of gonococci, purified LPS, or LPS linked to bovine serum albumin with glutaraldehyde. Purified LPS was not immunogenic. Anti-LPS antibodies were produced by rabbits receiving the LPS-BSA conjugate. These animals showed an earlier IgG anti-LPS antibody response than animals receiving whole bacterial cells. Antiserum to the LPS-BSA conjugate gave rise to a single precipitation line against ultrasonically disrupted gonococci, and agglutinated heat-treated cells of the bacteria.

31P- and 13C-n.m.r.-spectral and chemical characterization of the end-group and repeating-unit components of oligosaccharides derived by acid hydrolysis of Haemophilus influenzae type b capsular polysaccharide

Haemophilus influenzae type b capsular polysaccharide [repeating unit, leads to 3)-beta-D-Ribf-(1 leads to 1)-D-Ribol-5-(PO2H leads to] was partially hydrolyzed with HCl to give oligosaccharides that were isolated by size-exclusion chromatography, and then characterized by 31P- and 13C-n.m.r.-spectral and chemical methods, in order to determine the end-group composition and, hence, the number-average chain-length (L). The ratio (approximately 17:8) of monophosphate end-groups to D-ribofuranose end-groups revealed the relative rates of hydrolysis of the phosphoric diester linkage and the glycosidic linkage in the repeating-unit structure. Cleavage of the phosphoric diester linkage was approximately 92% regioselective, as indicated by the approximately 12:1 ratio of D-ribofuranose monophosphate end-groups to D-ribitol monophosphate end-groups. The n.m.r. spectra of the oligosaccharide repeating-unit provided evidence for partial stereomutation (approximately 3-8%) that involved rearrangement of the D-ribofuranose phosphoric diester linkage and anomerization at C-1 of D-ribofuranose. Variously sized oligosaccharides (L = 4, 7, and 12), that had D-ribofuranose end-groups reacted with bovine serum albumin that had an average of approximately 9 adipyl hydrazide functionalities, to give, within experimental error, quantitative yields of the corresponding hydrazone-linked, oligosaccharide-protein conjugates.

Artificial Salmonella vaccines: Salmonella typhimurium O-antigen-specific oligosaccharide-protein conjugates elicit protective antibodies in rabbits and mice

Several saccharides representative of the O-antigenic polysaccharide chain of Salmonella typhimurium (O antigens 4 and 12) were used as haptenic groups covalently linked to bovine serum albumin. The disaccharide abequose 1 --> 3 D-mannose was synthesized, and the [Formula: see text] tetra-, octa- and dodecasaccharides were isolated after cleavage of isolated S. typhimurium O-polysaccharide chains by using bacteriophage endo-glycosidases. Rabbits immunized with the saccharide-protein conjugates suspended in Freund complete adjuvant readily responded with O4 antibody titers as high, or almost as high, as those elicited by heat-killed bacteria. The octa- and dodecasaccharide conjugates also gave rise to O12 antibody titers. The antibody response in mice differed in two ways from that seen in rabbits: mice did not respond with measurable antibody production against the disaccharide haptens, and the highest S. typhimurium lipopolysaccharide antibody response elicited by the saccharide haptens was still approximately 50-fold lower than that elicited by heat-killed bacteria. The latter difference may be a consequence of the fact that the mouse preferentially produces antibodies against the galactose residue which is terminal in the hapten but not in the native O-antigenic polysaccharide chain. Antibodies elicited in rabbits against all saccharide-protein conjugates protected passively transferred mice against intraperitoneal challenge with 100 times the 50% lethal dose of S. typhimurium SH 2201 (O4, 12) but not against challenge with S. enteritidis SH 2204 (O9, 12). The antibodies elicited by the saccharide-protein conjugates protected as well as antibodies elicited by heat-killed bacteria.