Effect of specific immune mouse serum on the growth of Salmonella enteritidis in mice preimmunized with living or ethyl alcohol-killed vaccines

In silico designing and characterization of outer membrane protein (OmpC) gene from Salmonella enterica and its expression in Nicotiana tabacum for developing a plant-based vaccine against salmonellosis

Salmonella, a gram-negative bacteria, is the leading cause of foodborne illness globally. Two serovars of Salmonella, S. enteritidis and S. typhimurium are responsible for the majority of human salmonellosis. Prolonged salmonellosis caused by Salmonella species leads to the development of colon cancer, which is 3rd most common cancer in the world. Porins in the outer membrane of Salmonella can be used to elicit immune response. The production of plant-based vaccine against salmonellosis and the subsequent colon cancer using outer membrane proteins can be helpful for the people of developing countries. In this study, OmpC protein from Salmonella enteritidis was subjected to various bioinformatics tools which exhibited OmpC vaccine construct to be sufficiently immunogenic, non-allergenic, non-toxic and non-homologous to human proteins. Docking analysis showed strong interaction of OmpC vaccine model with TLR-4. After in silico analysis, this vaccine construct was expressed in tobacco plants via Agrobacterium-mediated transformation. Gateway® cloning was used to clone OmpC gene. Transformation and integration of transgene within tobacco plants was confirmed through conventional PCR. qRT-PCR was done for expression analysis and copy number calculated was 2. The expressed OmpC protein accumulated up to 0.42 % of total soluble protein. Immunization of mice with total soluble protein (TSP) and purified OmpC protein generated significant level of anti-OmpC antibodies. The vaccine candidate also demonstrated significant protective effect in mice upon challenging with Salmonella typhimurium. To the best of our knowledge, this is the first study reporting the expression of OmpC antigen in plants for potential use as vaccine against salmonellosis.

Navigating ESKAPE Pathogens: Considerations and Caveats for Animal Infection Models Development

The misuse of antibiotics has led to the global spread of drug-resistant bacteria, especially multi-drug-resistant (MDR) ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species). These opportunistic bacteria pose a significant threat, in particular within hospitals, where they cause nosocomial infections, leading to substantial morbidity and mortality. To comprehensively explore ESKAPE pathogenesis, virulence, host immune response, diagnostics, and therapeutics, researchers increasingly rely on necessitate suitable animal infection models. However, no single model can fully replicate all aspects of infectious diseases. Notably when studying opportunistic pathogens in immunocompetent hosts, rapid clearance by the host immune system can limit the expression of characteristic disease symptoms. In this study, we examine the critical role of animal infection models in understanding ESKAPE pathogens, addressing limitations and research gaps. We discuss applications and highlight key considerations for effective models. Thoughtful decisions on disease replication, parameter monitoring, and data collection are crucial for model reliability. By meticulously replicating human diseases and addressing limitations, researchers maximize the potential of animal infection models. This aids in targeted therapeutic development, bridges knowledge gaps, and helps combat MDR ESKAPE pathogens, safeguarding public health.

The effects of vaccination and immunity on bacterial infection dynamics in vivo

Salmonella enterica infections are a significant global health issue, and development of vaccines against these bacteria requires an improved understanding of how vaccination affects the growth and spread of the bacteria within the host. We have combined in vivo tracking of molecularly tagged bacterial subpopulations with mathematical modelling to gain a novel insight into how different classes of vaccines and branches of the immune response protect against secondary Salmonella enterica infections of the mouse. We have found that a live Salmonella vaccine significantly reduced bacteraemia during a secondary challenge and restrained inter-organ spread of the bacteria in the systemic organs. Further, fitting mechanistic models to the data indicated that live vaccine immunisation enhanced both the bacterial killing in the very early stages of the infection and bacteriostatic control over the first day post-challenge. T-cell immunity induced by this vaccine is not necessary for the enhanced bacteriostasis but is required for subsequent bactericidal clearance of Salmonella in the blood and tissues. Conversely, a non-living vaccine while able to enhance initial blood clearance and killing of virulent secondary challenge bacteria, was unable to alter the subsequent bacterial growth rate in the systemic organs, did not prevent the resurgence of extensive bacteraemia and failed to control the spread of the bacteria in the body.

The bacterium Salmonella enterica causes gastroenteritis and the severe systemic diseases typhoid, paratyphoid fever and non-typhoidal septicaemia (NTS). Treatment of systemic disease with antibiotics is becoming increasingly difficult due to the acquisition of resistance. Licensed vaccines are available for the prevention of typhoid, but not paratyphoid fever or NTS. Vaccines can be either living (attenuated strains) or non-living (e.g. inactivated whole cells or surface polysaccharides) and these different classes potentially activate different components of the host immune system. Improvements in vaccine design require a better understanding of how different vaccine types differ in their ability to control a subsequent infection. We have improved a previously developed experimental system and mathematical model to investigate how these different vaccine types act. We show that the inactivated vaccine can only control bacterial numbers by a transient increase in bactericidal activity whereas the living vaccine is superior as it can induce an immune response that rapidly kills, then restrains the growth and spread of infecting bacteria.

The in vivo extracellular life of facultative intracellular bacterial parasites: role in pathogenesis

Classically labeled facultative intracellular pathogens are characterized by the ability to have an intracellular phase in the host, which is required for pathogenicity, while capable of extracellular growth in vitro. The ability of these bacteria to replicate in cell-free conditions is usually assessed by culture in artificial bacteriological media. However, the extracellular growth ability of these pathogens may also be expressed by a phase of extracellular infection in the natural setting of the host with pathologic consequences, an ability that adds to the pathogenic potential of the infectious agent. This infective capability to grow in the extracellular sites of the host represents an additional virulence attribute of those pathogens which may lead to severe outcomes. Here we discuss examples of infectious diseases where the in vivo infective extracellular life is well documented, including infections by Francisella tularensis, Yersinia pestis, Burkholderia pseudomallei, Burkholderia cenocepacia, Salmonella enterica serovar Typhimurium and Edwardsiella tarda. The occurrence of a phase of systemic dissemination with extracellular multiplication during progressive infections by facultative intracellular bacterial pathogens has been underappreciated, with most studies exclusively centered on the intracellular phase of the infections. The investigation of the occurrence of a dual lifestyle in the host among bacterial pathogens in general should be extended and likely will reveal more cases of infectious diseases with a dual infective intracellular/extracellular pattern.

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.

Oral Immunization in Experimental Salmonellosis II. Characteristics of the Immune Response to Temperature-Sensitive Mutants Given by Oral and Parenteral Routes

A temperature-sensitive mutant of Salmonella enteritidis, selected because of its inability to proliferate normally at 37 C, has been used as a living vaccine in mice. When given parenterally or orally, it confers a high degree of resistance against otherwise lethal S. enteritidis infections given intraperitoneally or by mouth. In contrast to most other effective living Salmonella vaccines, the temperature-sensitive mutant survives for only short periods in mouse tissues. Although the vaccine provides protection against S. typhimurium infection, possibly because of antigenic relationships between the immunizing and challenge organisms, it is ineffective against the intracellular infection caused by Listeria monocytogenes. A study of the kinetics of S. enteritidis infection in the liver and spleen of normal and immunized mice has suggested that immunity is dependent upon development of a secondary immunological response which arises approximately 7 days after introduction of the challenge infection. Although humoral antibody production forms part of this secondary response, it is not necessarily responsible for control of the infection.

Immunity to enteric infection in mice

Specific pathogen-free CD-1 mice infected orally with sublethal doses (10(4) to 10(6) viable organisms) of Salmonella enteritidis rapidly developed extensive bacterial populations in the liver, spleen, and mesenteric lymph nodes. Although the pathogen did not multiply extensively in the gut, the infection persisted in the intestine at between 10(4) and 10(5) viable organisms throughout the experiment. S. gallinarum was less invasive than S. enteritidis when given by mouth; S. pullorum failed to survive in the intestine or to invade the tissues of orally infected mice. Vaccination with a sublethal dose of living S. enteritidis, either orally or intravenously, completely prevented the establishment of liver and spleen populations of a drug-resistant, virulent strain of S. enteritidis. Vaccination with an ethyl alcohol-killed vaccine given by various routes delayed the spread of the orally introduced challenge population to the liver and spleen by 1 to 2 days but was unable to prevent the subsequent growth of the pathogen in vivo, although the vaccinated mice survived the infection. The importance of these findings in relation to vaccination against typhoid fever in man is discussed.

Cutting edge: role of B lymphocytes in protective immunity against Salmonella typhimurium infection

Infection of mice with Salmonella typhimurium gives rise to a disease similar to human typhoid fever caused by S. typhi. Since S. typhimurium is a facultative intracellular bacterium, the requirement of B cells in the immune response against S. typhimurium is a longstanding matter of debate. By infecting mice on a susceptible background and deficient in B cells (Igmu-/- mice) with different strains of S. typhimurium, we could for the first time formally clarify the role of B cells in the response against S. typhimurium. Compared with Igmu+/+ mice, LD50 values in Igmu-/- mice were reduced during primary, and particularly secondary, oral infection with virulent S. typhimurium. After systemic infection, Igmu-/- mice cleared attenuated aroA- S. typhimurium, but vaccine-induced protection against systemic infection with virulent S. typhimurium involved both B cell-dependent and -independent effector mechanisms. Thus, B cell-mediated immunity plays a distinct role in control of S. typhimurium in susceptible mice.

Construction of an aroA mutant of Salmonella serotype Gallinarum: its effectiveness in immunization against experimental fowl typhoid

An aroA mutant was produced from a virulent strain of Salmonella Gallinarum, the causative agent of fowl typhoid in poultry, by Tn10 insertional inactivation and deletion. The mutant was highly attenuated for chickens. A single intramuscular immunization of 2-week-old chickens with 10(7) mutant organisms reduced mortality following oral challenge with 10(8) organisms of the parent strain from 63 to 30%. In a second experiment multiple immunizations with 10(8) mutant organisms reduced mortality after challenge with 10(7) organisms of the virulent strain from 77 to 0%. By oral inoculation the mutant was not able to immunize chickens against oral challenge.

Construction of a recombinant oral vaccine against Salmonella typhi and Salmonella typhimurium

The viaB gene coding for the Vi antigen of Salmonella typhi Ty2 was subcloned into expression vector pYA248. The recombinant plasmid was termed SMM202 and transformed into Salmonella typhimurium chi 4072, an attenuated delta cya delta crp mutant. Recombinant S. typimurium Vi4072 had the ability to produce Vi capsular polysaccharide and also to invade and colonize the small intestine, mesenteric lymph nodes, and spleen of BALB/c mice. Mice orally immunized with Vi4072 developed serum and secretory antibody responses to the Vi antigen, as measured by a passive hemagglutination assay. Mice developed a delayed-type hypersensitivity following a footpad injection with Vi antigen after being sensitized orally with a suitable dose of Vi4072. Immunization of mice with Vi4072 afforded complete protection against fatal infection with virulent S. typhi Ty2. All data indicate that this route of antigen delivery is effective for stimulating antibody-mediated immunity and for inducing a cell-mediated immune response in BALB/c mice. Thus, S. typhimurium Vi4072 may serve as a vaccine for protection against typhoid fever and salmonellosis caused by S. typhimurium.

Method for identifying microbial antigens that stimulate specific lymphocyte responses: application to Salmonella

Vaccine development and understanding of cellular immune stimulatory mechanisms have been impeded by the paucity of data on microbial antigens that stimulate protective immunity. We describe here a general method for identifying and isolating peptide antigens that specifically stimulate sensitized lymphocytes. First, Salmonella typhimurium C5 genomic DNA fragments were subcloned into Escherichia coli by use of the lambda gt11 expression vector. Next, antigens expressed by recombinant phage from this genomic library were tested for their capacity to stimulate proliferative responses in pooled lymphocytes obtained from BALB/c mice infected 14 days earlier with S. typhimurium. Of 2000 recombinant phages tested, 5 stimulated a polypeptide-antigen-specific proliferative response. Physical analyses of these 5 recombinant phages revealed cloned inserts of 0.5-2.4 kilobase pairs representing nonoverlapping regions of the C5 chromosome. Four of the five insert DNAs hybridized at high stringency to both S. typhimurium and Salmonella typhi total chromosomal DNA, suggesting that these pathogens of different host specificity share several antigenic determinants. Use of sensitized primary polyclonal lymphocytes provides a rapid and simple method for screening recombinant DNA libraries for clones that stimulate specific immune responses and avoids the use of cloned lymphocyte cell lines. This approach should be generally applicable to similar studies in different hosts of many other microbial pathogens.

Immune responses in BALB/c mice following immunization with aromatic compound or purine-dependent Salmonella typhimurium strains

Two near isogenic strains of Salmonella typhimurium HWSH, stably mutated in either the aroA gene affecting the biosynthesis of aromatic compounds, or the purA gene affecting the biosynthesis of purines, were administered intravenously as live attenuated vaccines to BALB/c mice. HWSH aroA-immunized mice were well protected against intravenous (i.v.) challenge with wild-type virulent HWSH for at least 10 weeks, whereas HWSH purA-immunized mice were unprotected. Furthermore, HWSH aroA-immunized mice could also control a heterologous challenge with virulent Listeria monocytogenes at 7 and 14 days post-immunization, whereas mice receiving a similar dose of HWSH purA could not. Increasing the i.v. dose of HWSH purA compared to HWSH aroA induced some resistance to L. monocytogenes. Induction of early anti-S. typhimurium resistance by HWSH aroA immunization appeared slightly later than the anti-L. monocytogenes resistance. Mice immunized with either vaccine were able to mount S. typhimurium-specific T-cell proliferative responses and produced anti-S. typhimurium humoral antibodies in their serum. The antibody titre was greater in those mice immunized with the aroA mutant.

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.

Route-related variation in the immunogenicity of killed Salmonella enteritidis vaccine: role of antigen presenting cells

In order to assess the role of the route of immunization on the immunogenicity of killed Salmonella vaccine, mice were immunized with killed S. enteritidis by intraperitoneal (i.p.) and intradermal (i.d.) routes. Whereas the former was non-immunogenic, the i.d. immunization generated an excellent delayed-type hypersensitivity response; further, i.p. immunization could even suppress the subsequent i.d. immunization. Since the peritoneal macrophages (MO) are known to be particularly low in Ia or MHC-class II antigens, so essential for antigen presentation, the non-immunogenicity by i.p. route was thought to be due to their poor presentation efficiency. Poly I: poly C, an interferon inducer, is known to enhance the MHC-class II expression; hence effect of poly I: poly C treatment on the immunogenicity of the killed vaccine by i.p. route was tested and indeed the non-immunogenicity was corrected. Poor efficiency of presentation of S. enteritidis antigen by peritoneal cells and its improvement by prior poly I: poly C treatment was further confirmed by in vitro lymphocyte transformation test using primed T cells and peritoneal cells from normal and poly I: poly C treated mice. Poly I: poly C treatment also enhanced expression of Ia antigens on peritoneal cells.

Novel virulence properties of the Salmonella typhimurium virulence-associated plasmid: immune suppression and stimulation of splenomegaly

Mice infected subcutaneously with wild-type Salmonella typhimurium, SR11, developed a significant splenomegaly when compared with mice infected with an equal number of a plasmid-cured strain. Further, the bacterial load in the spleen at 14 days after infection, measured as colony-forming units per gram tissue, was significantly higher in mice infected with the parent strain than in mice infected with the plasmid-cured strain. These data confirm the previously reported plasmid-associated ability of Salmonella to multiply within the spleen. In addition, lymph node cells (LNC) from mice infected with the parent strain had a significantly reduced ability to proliferate in response to concanavalin A, a T-cell mitogen, and to heat-killed S. typhimurium cells when compared with LNC isolated from mice infected with the plasmid-cured strain. Finally, reintroduction of a functional Tn5-tagged 90-kb plasmid into a plasmid-free strain restored its capacity to cause a marked splenomegaly and to suppress lymph node cell proliferation in BALB/c mice. These data demonstrate that the 90-kb plasmid of highly virulent S. typhimurium strains mediates several novel pathogenic properties in infected mice: (1) enhancement of the ability of Salmonella to multiply within the spleen; (2) stimulation of a splenic inflammatory response as displayed by marked splenomegaly; and (3) a general suppression of lymphocyte responsiveness to both T-cell mitogens and specific Salmonella antigens.

Determinants of immunity to murine salmonellosis: studies involving immunization with lipopolysaccharide-lipid A-associated protein complexes in C3H/HeJ mice

We have earlier demonstrated that the C3H/HeJ Salmonella hypersusceptible mouse can be protected against infection with this organism by prior immunization with lipopolysaccharide (LPS)-lipid A-associated protein (LAP) complexes, but not with LPS alone. In the current studies, protection has been shown to correlate with the induction of LPS-specific antibody in immunized mice. LPS was demonstrated to be a relevant target antigen for Salmonella immunity since C3H/HeJ mice were afforded higher survival rates when they were challenged with Salmonella that shared the same LPS O-antigen as the vaccine. Although low levels of LPS-specific antibody can be detected 14 days after immunization with LAP-LPS, significant antibody is present only after 21-28 days. In addition, anti-LAP specific antibodies can be detected after 14 days of immunization with LAP-LPS. Adoptive transfer of either day 28 anti-LAP-LPS immune serum or day 28 LAP-LPS immune splenocytes alone to naive recipients affords mice minimal, if any, survival against lethal S. typhimurium LT2 challenge. In contrast, transfer of day 28 anti-LAP-LPS immune serum and day 28 LAP-LPS immune splenocytes together is able to transfer Salmonella immunity to naive C3H/HeJ mice. Further, equivalent transfer of only day 28 anti-LAP-LPS immune serum to C3H/HeJ mice immunized 7 days previously with LAP-LPS provides protection similar to that found in mice adoptively transferred with immune cells and serum. These results suggest that a host cellular factor or factors responsive to LAP-LPS, in addition to day 28 anti-LAP-LPS immune serum, may contribute to the protection afforded C3H/HeJ mice following immunization with LAP-LPS.

Protection of C3H/HeJ mice from lethal Salmonella typhimurium LT2 infection by immunization with lipopolysaccharide-lipid A-associated protein complexes

C3H/HeJ mice were immunized intraperitoneally (i.p.) with lipopolysaccharide (LPS)-lipid A-associated protein (LAP) complexes or with purified protein-free LPS prior to lethal i.p. or intravenous Salmonella typhimurium LT2 challenge. Our results demonstrated that these Salmonella-hypersusceptible mice can be effectively protected against 1,000 100% lethal doses of S. typhimurium LT2 (i.e., 1,000 viable bacteria) administered by intravenous challenge when previously immunized with LAP-LPS complexes. In contrast to these results, immunization with LPS afforded markedly less protection regardless of the route of challenge, thus suggesting that the LAP portion of LAP-LPS complexes may be necessary for inducing protection against Salmonella infections. For most experiments, antigens were emulsified in complete Freund adjuvant (CFA); however, the CFA portion of the vaccine was suggested not to be an essential component for the induction of immunity to Salmonella infections, since equivalent levels of protection were obtained when it was omitted from the vaccine. The induction of immunity to murine salmonellosis by prior immunization with CFA-LAP-LPS was demonstrated not to be a transient phenomenon, since C3H/HeJ mice were still protected against lethal S. typhimurium LT2 challenge as late as 225 days postimmunization.

Immunization against experimental murine salmonellosis with liposome-associated O-antigen

Partially delipidated Salmonella typhimurium (O-1,4,5,12) lipopolysaccharide was incorporated into small multilamellar liposomes composed of either naturally occurring or synthetic phospholipids. Vaccination of mice with the liposome-lipopolysaccharide complexes induced a cellular response specific for O-1,4,5,12 determinants, as determined by the development of a delayed-type hypersensitivity reaction. The liposome-lipopolysaccharide vaccines were significantly more effective, compared with other nonviable vaccines tested, in protecting mice against a lethal intravenous challenge infection with virulent S. typhimurium. Protection afforded by the liposome-lipopolysaccharide vaccines was comparable to that conferred by a live S. typhimurium vaccine. Results suggest that liposome-induced modulation of the host immune response in favor of cell-mediated immunity may be more efficacious in preventing diseases in which cell-mediated immunity is of prime importance.

Immunity to Salmonella infection

The foregoing literature review and data presentation have been set forth in the hope of clarifying some complex and confusing issues in regard to Salmonella infection. From a practical point of view, the information presented has implications for the direction to take with regard to improving the current typhoid vaccine, as the presently used acetone-killed cell preparation has considerable toxicity. The issues are important from a theoretical standpoint, because they have bearing on the nature of the concepts researchers and clinicians carry as working hypothesis with regard to the mechanisms of immunity to Salmonella infection. An incomplete appreciation of the literature seems to have led many scientists to believe that only cellular immunity can protect a mouse, and by analogy a human, against Salmonella. The logical deduction from such a premise is that only live vaccines will be effective in humans againsT S. typhi. Such a conclusion would appear unfounded, as documented in this review, for killed vaccines have been shown to be highly effective in vaccinating many mouse strains, as well as humans, against enteric fever.

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

Outbred NMRI mice and rabbits were vaccinated with different artificial Salmonella typhimurium immunogens and the specificity and activity of elicited antibodies were studied in in vivo and in vitro phagocytosis assays. The Salmonella immunogens used were: (i) the synthetic disaccharide, abequose (formula see text) D-mannose, representative of Salmonella O antigen 4, covalently linked to bovine serum albumin (BSA); (ii) the octa- and dodecasaccharides, (formula see text) covalently linked to BSA; and (iii) whole heat-killed Salmonella. Rabbit antibodies passively administered to mice significantly enhanced the clearance of intravenously injected S. typhimurium challenge bacteria from the bloodstream. The clearance rate and the titer of anti-O-antigen-specific antibodies correlated. The clearance rate of an S. thompson (O6,7) strain, which has a different O antigen, was the same irrespective of the rabbit serum given. NMRI mice actively immunized with the various oligosaccharide-BSA conjugates had a significantly increased clearance rate of S. typhimurium only. In the in vitro assay, mouse antioligosaccharide-BSA sera promoted phagocytosis of S. typhimurium, but not S. thompson, when incubated with complement and mouse peritoneal exudate cells activated with Freund complete adjuvant.

Antigenic modification, rosette-forming cells, and Salmonella typhimurium resistance in outbred and inbred mice

To assess the separate contributions of host T cells and the physical state of the antigen in the development of effective. Salmonella resistance, glutaraldehyde-treated and untreated protein- and ribonucleic acid-rich extracts (E-RNA extracts) of virulent Salmonella typhimurium SR-11 or attenuated S. typhimurium RIA were used to immunize Salmonella-resistant Salmonella-susceptible strains of mice for the purpose of determining whether antigen-specific T-cell or B-cell responses were formed and, if so, which responses predominated. The resistance imparted to each mouse strain after vaccination with S. typhimurium RIA was used as the standard for comparison. The inbred mouse strains C57BL/6 and DBA/2 and their F(1) hybrid (strain BDF(1)), outbred ICR Swiss mice, and endotoxin-resistant C3H/HeJ mice were examined for the capacity to develop resistance to lethal Salmonella infections, as well as the ability to generate antigen-reactive T cells. Only the BDF(1), C3H/HeJ, and ICR Swiss mice were able to develop resistance to challenge infections mediated by the virulent SR-11 strain of S. typhimurium after vaccination with the living, attenuated RIA strain of S. typhimurium or immunization with E-RNA extracts. We developed an assay to identify the antigen-reactive rosette-forming lymphocytes present in lymph nodes and spleens of immunized mice. Levels of 0.2% or higher of theta antigen-bearing, antigen-reactive rosette-forming cells were found in the lymph nodes or spleens or both of only the BDF(1), C3H/HeJ, and ICR Swiss mice (i.e., in the "Salmonella responder" strains). Mouse strains C57BL/6 and DBA/2, which failed to develop resistance to lethal infections after immunization with the S. typhimurium RIA vaccine or with the E-RNA extracts, lacked effective numbers of antitheta antigen-sensitive rosette-forming cells. Modification of the effective E-RNA extracts by polymerization with glutaraldehyde resulted in a marked diminution in their abilities to induce resistance to salmonellosis in the two responder mouse strains tested (BDF(1) and ICR Swiss), even though detectable levels of antibody were induced.

Antigenic modification: its relation to protective host resistance in murine salmonellosis

Both the physical state of the immunogen and the route of immunization were found to be extremely important in inducing effective host resistance against salmonellosis.

Correlation of the duration and magnitude of protection against Salmonella infection afforded by various vaccines with antibody titers

Groups of mice were immunized with optimal doses of the following vaccines of Salmonella typhimurium W118-2: acetone-killed cells, lipopolysaccharide, ribosomes, and live cells. At 3 weeks, 1 month, 2 months, 4 months, or 6 months postimmunization, sera were collected from control and vaccinated animals, and the anti-lipopolysaccharide and whole-cell agglutination titers of the sera were determined. Other groups of similarly vaccinated mice were tested for resistance to infection by challenging with live W118-2 and scoring the number of survivors 30 days postinfection. It was found that only ribosomes and live cells afforded significant protection 6 months after immunization. Thus, in duration of protection ribosomes were superior to the other nonviable vaccines tested. At all time intervals tested, purified lipopolysaccharide was the least effective vaccine. Protection afforded by the acetone-killed cell and ribosomal vaccines correlated better with the whole-cell agglutination titers than with the anti-lipopolysaccharide titers. However, the longer duration of protection afforded by the ribosomal vaccine, as compared with the acetone-killed vaccine, could not be accounted for by differences in whole-cell agglutination titers. These studies show that ribosomal vaccines are equal in all parameters to acetone-killed cells and have the advantage of providing longer-lasting immunity.

Cellular antimicrobial immunity

Acquired resistance to infectious disease may be expressed by a predominantly humoral or a cellular mechanism or, more frequently, by a combination of the two. The cellular interactions which are responsible for the induction of the immune response in the skin, lung, intestinal mucosa, genitourinary tract, conjunctiva, and peritoneal cavity are discussed and the role of living or dead vaccines in the induction of acquired resistance is outlined. The host response involves three different cell types: the phagocytic cell (polymorphs or macrophages), the thymus-dependent (T) lymphocyte, and the thymus-independent (B) lymphocyte-plasma cell line. The normal unstimulated phagocytic cell is capable of killing most nonpathogenic bacteria that gain entry to the tissues. However, the presence of opsonic antibodies and activated macrophages is required to eliminate the pathogenic intracellular parasites. Such immunological activation involves the presence of sensitized T-lymphocytes in the lesion. The cellular response is also characterized by the simultaneous development of a state of delayed-type hypersensitivity (DTH), along with the antimicrobial CMI response. A rising humoral response normally develops subsequently. Killed bacterial cells (except when incorporated into Freund's complete adjuvant) induce the humoral response without the CMI reaction so that such vaccines are not able to fully protect the host against the naturally acquired disease. With the development of cell fractionation methods as well as the identification of distinctive cell surface markers, suspensions of B- and T-cells and macrophages can now be prepared for use in increasingly sophisticated transfer and reconstitution studies. The role of the different cell types in the expression of humoral and cellular immunity has been determined, and the effect of various immunopotentiating and immunosuppressive regimens on the immune system as a whole has been evaluated quantitatively. These studies have led to an appreciation of the role played by suppressor B- and T-cells in the interplay of both humoral and cellular components of the host defense system during the development of immune tolerance, desensitization, anergy, autoimmunity, and the expression of an anamnestic immune response following reinfection.

Protective effects of a supernatant factor from Salmonella typhimurium on Salmonella typhimurium infection of inbred mice

A supernatant factor prepared from 48-h cultures of Salmonella typhimurium has been used to immunize mice against subsequent challenge with normally lethal doses of S. typhimurium. The mouse strains used, C57BL and BALB/c, were sensitive to S. typhimurium with 50% lethal doses of less than 50 organisms. Two doses of supernatant factor, given intraperitoneally 20 days apart, protected mice against a subcutaneous challenge dose 10 days later of 100 50% lethal doses of S. typhimurium, resulting in 50 to 80% survival. The viable counts were reduced initially in organs of immunized mice compared with controls, and the multiplication of bacteria was delayed, although the final levels found in the organs would normally have been lethal. Protection obtained was specific for S. typhimurium in that no increased survival was shown after Salmonella enteritidis challenge of immunized mice. Although lipopolysaccharide was demonstrated in the supernatant factor, lipopolysaccharide alone did not protect challenged mice. Supernatant factor produced delayed-type hypersensitivity reactions in mice sensitized with nonlethal doses of Salmonella. The nature of the active factor, found to be partially protein, has yet to be elucidated.

Evaluation of experimentally induced Fusobacterium necrophorum infections in mice

Two strains of mice, Swiss Webster and DBA/2Cr, were injected intraperitoneally or intravenously with varying dosages of Fusobacterium necrophorum. The ability to eliminate the infection was assessed by quantitative enumeration of the organisms present in the blood, liver, and spleen, Three- to 4-week-old DBA/2Cr mice were highly resistant to both routes of injection. The intraperitoneal injection of older mice failed to demonstrate a dose-effect relationship whereas an intravenous injection of as few as 10(4) cells of F. necrophorum produced progressively necrotic leg abscesses, apparently involving the lymphonodus ischiadicus which filters the site of injection. Mortality was increased with sensitization by a previous sublethal injection. Also, an ethanol-killed cell vaccine delayed the onset of lethal infection, whereas repeated sublethal live cell injections provided nonspecific protection since mice vaccinated with the growth medium were equally protected. The development of leg abscesses after intravenous injection visibly demonstrated the pathogenicity of F. necrophorum and may provide a suitable model for the evaluation of vaccines and the effectiveness of antibiotics.

Cellular and humoral aspects of host resistance in murine salmonellosis

Mice were challenged with a highly virulent strain of Salmonella typhimurium by intraperitoneal injections. At relatively low infecting doses, immunizations with either viable attenuated or heat killed Salm. typhimurium were found to be equally protective against otherwise fatal infections. Pre-opsonization of virulent salmonellae significantly increased the survival rate of mice infected with small numbers of the pathogen. By a cell culture method, peritoneal macrophages of mice were shown to be innately capable of destroying the ingested virulent Salm. typhimurium. Macrophages from previously infected mice did not appear to have any significant increase in their bactericidal activity against salmonellae, but they possessed cytophilic antibodies specific against the H and the O antigens of Salm. typhimurium. It is believed that humoral elements play an important role in acquired immunity in murine salmonellosis by opsonization of the pathogen.

Protection against Toxoplasma gondii in mice immunized with Toxoplasma cell fractions, RNA and synthetic polyribonucleotides

Mice immunized with fractions obtained by centrifugation of disrupted Toxoplasma gondii trophozoites as well as with 200 μg of Toxoplasma ribonucleic acid (RNA) were resistant (as measured by time to death and total mortality) to challenge with Toxoplasma 30 days later. When mice were challenged at 15 days no protection was noted. A dose of 50 μg of Toxoplasma RNA was effective in protecting mice against lethal challenge only when incorporated into Freund's incomplete adjuvant. In studies performed to determine the specificity of the resistance observed, resistance was also noted in mice immunized with 200 μg of RNA extracted from normal mouse peritoneal macrophages, as well as in mice immunized with 100 μg of the synthetic polyribonucleotide polycytidylic acid. Polyadenylicuridylic acid conferred protection only when incorporated into Freund's incomplete adjuvant and polyinosinic—cytidylic acid had no effect. The protection induced by Toxoplasma RNA was eliminated by prior treatment of the preparation with ribonuclease but not by treatment with pronase, suggesting that the moiety responsible for the protective effect was RNA. In experiments designed to explore the mechanism of resistance in the vaccinated mice, macrophages harvested from mice which had been injected with Toxoplasma RNA 15 days earlier were found to be activated in that they resisted challenge with Listeria monocytogenes.

Effect of Corynebacterium parvum treatment on the growth of Salmonella enteritidis in mice

The growth of Salmonella enteritidis in mice pretreated with 700 mug of killed Corynebacterium parvum was less than that seen in normal CD-1 mice. In treated mice, there was an early increased inactivation of the blood, liver, and spleen bacterial populations, followed by a prolonged period of slow but continuous bacterial growth. The treated mice failed to develop significant delayed hypersensitivity and did not show the characteristic antibacterial immune response seen in untreated infected animals. Eventually sufficient resistance did develop in most of the treated animals to protect them against the lethal effects of the challenge infection. The peak C. parvum effect was seen when S. enteritidis was injected 7 to 14 days later. Injection of C. parvum 24 h after the bacterial challenge actually potentiated the Salmonella infection. There was no evidence of an increased specific humoral response by the C. parvum-treated mice, suggesting that the slower growth of the S. enteritidis was due to the continued enhanced killing of the bacterial population by the nonspecifically stimulated cells of the reticuloendothelial system, rather than to any specific augmentation of the host immune response.

Immunogenicity of living and heat-killed Salmonella pullorum vaccines

Specific pathogen-free CD-1 and C57Bl mice were infected in a hind footpad with 5 x 10(4) viable Salmonella enteritidis cells or 10(7) viable S. pullorum cells. The resulting bacterial growth within the footpad, the draining lymph nodes, and the liver and spleen was followed for 14 days. Mice vaccinated with live S. enteritidis rapidly developed an effective antibacterial resistance to both intravenous and intragastric challenge with S. enteritidis SM(R). The viable inoculum of S. pullorum was rapidly eliminated from the normal mouse tissues and failed to induce a detectable anti-Salmonella resistance to parenteral or oral challenge with S. enteritidis. Heat-killed saline suspensions (200 mug, dry wt) of S. enteritidis or S. pullorum were unable to induce an effective antimicrobial resistance against a subsequent virulent Salmonella challenge. However, when the organisms were suspended in Freund complete adjuvant, both vaccines induced an antibacterial resistance to intravenous and intragastric challenge. Reduction of the antigenic dose from 200 to 40 mug did not greatly affect the protective value of the two killed vaccines against an intravenous challenge, but the level of protection observed with two 40-mug doses of S. pullorum was considerably reduced when the animals were infected intragastrically, suggesting that some quantitative differences existed between the sensitizing antigenic contents of the two test organisms.

Comparative immunogenicity of heat-killed and living oral Salmonella vaccines

CD-1 mice were vaccinated intragastrically or intramuscularly with one or two doses of 200 mug of heat-killed Salmonella enteritidis 5694. Control mice were vaccinated with sublethal doses of living S. enteritidis Se795. The mice were challenged intragastrically with approximately 10(6)S. enteritidis 5694 SM(R) 7 to 14 days later, and the growth of the challenge population in the liver, spleen, mesenteric lymph nodes, lungs, and intestine was measured quantitatively. Mice receiving two doses of heat-killed vaccine by mouth were able to delay the systemic emergence of a gastrically introduced salmonella infection by 1 to 2 days. The corresponding liver and spleen populations were slightly lower than those seen in the normal controls. On the other hand, mice receiving the living, attenuated vaccine (either intravenously or intragastrically) developed an effective anti-salmonella immunity against subsequent reinfection.

Immunity in experimental salmonellosis. 3. Comparative immunization with viable and heat-inactivated cells of Salmonella typhimurium

Vaccination with viable cells of an avirulent Salmonella typhimurium galE mutant provides mice with solid specific immunity against subsequent infection with a virulent smooth strain. Such a live vaccine is markedly more potent than one prepared from inactivated cells of the virulent smooth strain. The superiority of the live vaccine is particularly well demonstrated when the oral route of application is used. The protective capacity of the galE mutant is based on its ability to synthesize complete smooth-like cell wall lipopolysaccharide in vivo.

Degree of immunity induced by killed vaccines to experimental salmonellosis in mice

Killed vaccines, deoxycholate-extracted or heated, were shown to induce an effective degree of immunity which protected against death (100%), prevented extensive multiplication, and left the mice with low residual salmonella populations in spleen and liver after intravenous (iv) or intraperitoneal (ip) challenge with virulent Salmonella typhimurium. Protection was most effective against the ip challenge route and less effective against the iv route. A study of the kinetics of the population of bacteria in the spleens and livers of immunized animals showed that after ip challenge there was an initial reduction of 99% at 6 hr after challenge, maintenance of levels of less than 10(3) bacteria per organ, and a final population of 10(2) to 10(3) per organ at 21 days. With iv challenge, after an initial reduction of 90% at 6 hr, growth ensued to levels above 10(6) bacteria per organ until 8 days, followed by a steady decline yielding residual populations of 10(3) to 10(4) in some cases. Organ hypertrophy correlated with bacterial population. Morbidity was prevented (as measured by gain in body weight) by immunization against ip challenge but not against iv challenge. Killed vaccines protected by their ability to induce an immune state which reduced the initial challenge population, prevented extensive multiplication, yet allowed "cellular immunity" to develop due to response to the living challenge infection itself. The consequence was a low-level carrier state similar to that induced by recovery from sublethal virulent infection.

Immunity in experimental salmonellosis. II. Basis for the avirulence and protective capacity of gal E mutants of Salmonella typhimurium

Salmonella typhimurium strains which are deficient in uridine diphosphate (UDP)-galactose-4-epimerase (gal E mutants) owe their outstanding protective capacity when used as live vaccine to the fact that when galactose is supplied exogenously, such as occurs in vivo, smooth cell wall lipopolysaccharides are synthesized. The mutants lose most of their protective capacity when this phenotypic curing is prevented by a second mutation of the kind found in strains LT(2)M(1)A (deficient in galactokinase) or E(32) (deficient in UDP-galactose-lipopolysaccharide transferase). Despite such phenotypic reversion, the gal E mutants are rendered avirulent as a result of galactose-induced bacteriolysis. Secondary mutants have been isolated which differ from each other with respect to the extent of galactose-induced lysis. The differences in galactose sensitivity are attributable to different activities of the other Leoloir pathway enzymes, namely, galactokinase and galactose-1-phosphate-uridyl transferase. The influence of these enzymes on lipopolysaccharide composition and galactose sensitivity and thus on virulence and immunogenicity of gal E mutants has been studied.

Serum-mediated resistance induced with immunogenic preparations of Salmonella typhimurium

Serum obtained from animals immunized with attenuated Salmonella typhimurium strain RIA, heat-killed bacterial suspensions, or immunogenic ribosomal preparations was capable of passively conferring protection on normal recipient mice to challenge infection with virulent S. typhimurium strain SR-11. Protection was measured by the ability of a recipient animal to reduce the total number of challenge organisms significantly below that found in challenged control mice. Intraperitoneal administration of 0.1 ml of pooled sera was more effective than 0.05 ml in transferring resistance. The transfer of equivalent amounts of immune serum subcutaneously did not result in demonstrable resistance. In all cases in which serum transfer was effective, resistance was maximal at 5 days, greatly diminished by 10 days, and gone by 15 days post-transfer. Pooled serum obtained from normal donor mice or 0.2 ml of serum from mice hyperimmunized with immunogenic ribonucleic acid preparations did not possess the capacity to confer demonstrable resistance on normal recipients.

Isolation and partial characterization of an immunogenic moiety obtained from Salmonella typhimurium

Ribosomal preparations obtained from Salmonella typhimurium by differential centrifugation and sodium dodecyl sulfate (SDS) treatment of the bacillary lysate were found to be immunogenic in F(1) hybrid (C(3)H/HeJ x DBA/2J) and albino Swiss mice, as determined by progressive host survival. The immunity obtained was independent of the need for adjuvant and dependent on the dosage of immunogen given. Immunizations with the ribosomal preparations induced an immune response comparable to that obtained by vaccination with living organisms and significantly greater than that obtained by immunization with heat-killed salmonellae, purified lipopolysaccharide, or crude and SDS-treated endotoxin preparations. No effect on the immunogenicity of the ribosomal fraction was observed by enzymatic treatment with trypsin, Pronase, deoxyribonuclease, and pancreatic ribonuclease. Linear sucrose density gradient resolution of the preparations showed that the immunogenicity of the ribosomal fraction was not unique to any one of its subcomponents. Ethyl alcohol-precipitated, crude ribonucleic acid preparations obtained from the ribosomal and sucrose density-resolved ribosomal preparations were found to induce an immune response comparable to that obtained by immunization with the entire ribosomal fraction. Dialysis in doubly distilled demineralized water slightly reduced the immunogenicity of the preparation; however, comparable dialysis in 10(-4)m MgCl(2)-phosphate buffer did not. Chemical assays of the preparations found to be immunogenic were performed.

Effect of specific immune mouse serum on the growth of Salmonella enteritidis in nonvaccinated mice challenged by various routes

Salmonella enteritidis was injected intravenously, intraperitoneally, or subcutaneously into specific pathogen-free mice. The number of organisms in the blood, liver, spleen, peritoneal cavity, and draining inguinal lymph node was determined by daily enumeration. Opsonization of the organism with hyperimmune serum increased the rate of phagocytosis, resulting in rapid blood clearance together with an alteration in the relative numbers of organisms accumulating in the liver and spleen. Serum treatment also brought about a substantial increase in the number of bacteria killed during the first 60 min of the infection. However, the survivors of this initial period of inactivation then multiplied rapidly in the liver and spleen, ultimately resulting in the death of the animal from a generalized infection. Attempts to passively protect mice with hyperimmune serum were uniformly negative. The effects of treatment of the virulent S. enteritidis with hyperimmune serum were consistent with the general thesis that cellular rather than humoral factors play the major role in the expression of an effective antibacterial immunity against salmonella infections.