Evidence for the importance of O antigen specific antibodies in mouse-protective Salmonella outer membrane protein (porin) antisera

Deletions in guaBA and htrA but not clpX or rfaL constitute a live-attenuated vaccine strain of Salmonella Newport to protect against serogroup C2-C3 Salmonella in mice

Non-typhoidal Salmonella (NTS) are a leading cause of foodborne infections worldwide, and serogroups B, C₁, C_2-C₃ and D are the most common serogroups associated with human disease. While live vaccine candidates that protect against S. Typhimurium (serogroup B) and S. Enteritidis (serogroup D) have been described by us and others, far less effort has been directed towards vaccines that target either serogroup C₁ or C_2-C₃Salmonella. Here we describe a Salmonella Newport-based live-attenuated vaccine (serogroup C_2-C₃). Deletion of the genes clpX or rfaL, previously used in live vaccines to attenuate S. Typhimurium and/or S. Enteritidis, failed to attenuate S. Newport. However, we found that deletion of either guaBA or htrA raised the 50% lethal dose of S. Newport in an intraperitoneal infection model in BALB/c mice. Our live-attenuated vaccine candidate CVD 1966 (S. Newport ΔguaBA ΔhtrA) elicited strong antibody responses against COPS, flagellin and outer membrane proteins when administered intraperitoneally or orally. Following lethal challenge with the parental virulent strain of S. Newport, we observed vaccine efficacies of 53% for immunization via the intraperitoneal route and 47% for immunization via the oral route. Following intraperiteonal immunization, the vaccine also significantly reduced the bacterial burden of challenge organisms in the liver and spleen. Interestingly, reducing the LPS chain length by deleting rfaL did not induce a stronger immune response towards surface antigens, and failed to elicit any protection against lethal homologous challenge. In conclusion, we have developed a live-attenuated Salmonella serogroup C_2-C₃ vaccine that we are further evaluating.

Review: Evidence against the hypothesis that antibodies to the inner core of lipopolysaccharides in antisera raised by immunization with enterobacterial deep-rough mutants confer broad-spectrum protection during Gram-negative bacterial sepsis

Antisera to rough enterobacterial mutants of chemotypes Ra, Rc, and Re have been reported to confer broad-spectrum protection against wild-type smooth strains. It has been hypothesized that binding and neutralization of lipopolysaccharides (LPS) by antibodies to common core epitopes underlies such protection. This review summarizes experiments by our laboratory and others that do not confirm this concept and proposes reasons for the divergent results. Studies indicating broad-spectrum protection by rough-mutant antisera often had defects in experimental design or methodology. These include the failure: (i) to use matched pre- and postimmune sera from the same donors to control for variable protective activity of normal sera; (ii) to exclude the role of natural and polyclonally stimulated antibodies with proven protective activity against the infecting bacterial strain (e.g. O-specific, capsular, Pseudomonas exotoxin A); (iii) to exclude protective effects of acute-phase serum factors; (iv) to exclude protective effects of endotoxin contamination after adsorption or fractionation of antibody preparations; (v) to use non-boiled bacteria and LPS not subjected to acid-hydrolysis or gel-fractionation, and to exclude nonspecific adsorption, to demonstrate physiologically meaningful binding of rough-mutant antibodies to smooth enterobacteria and their LPS.

Extracellular antigens from Salmonella enteritidis induce effective immune response in mice after oral vaccination

We have studied polyacryl starch microparticles as an adjuvant in oral vaccination in mice. Secreted antigens from Salmonella enterica serovar Enteritidis were administered covalently conjugated to microparticles, or as free antigens, orally or intramuscularly and evaluated for their immunogenicity and ability to elicit protective immune response against an oral challenge with live serovar Enteritidis. The highest immunoglobulin M (IgM)-plus-IgG titers were obtained in the groups immunized with antigen-conjugated microparticles. The subclass profile switched to a stronger Th1 influence in the oral groups after booster, while the intramuscular group showed a constant Th1/Th2 profile. A strong specific IgA response was seen in feces in the oral groups, which was further confirmed in an enzyme-linked immunospot assay. The delayed-type hypersensitivity test, as a measure of the cellular response, showed a significant increase in ear thickness in all the immunized groups, except for the group that received free antigen orally, compared to the nonimmunized group. The cytokines released from in vitro-stimulated spleens showed a strong gamma interferon response in all immunized groups. A significant reduction in CFU in liver and spleen was seen in the orally immunized groups compared to the nonimmunized group after oral challenge with serovar Enteritidis. Western blotting analysis with both sera and feces revealed that antibodies against three bands, 53, 56, and 60 kDa, dominated the oral groups, and an electrospray-mass spectroscopy analysis of these bands showed amino acid sequences coinciding with those of phase-1 flagellin and hook-associated protein 2.

Immune recognition of porin and lipopolysaccharide epitopes of Salmonella typhimurium in mice

We investigated the antigenic specificity of the humoral immune response to infection by Salmonella typhimurium, by competitive inhibition enzyme-linked immunosorbent assay and Western immunoblots. A panel of eight murine monoclonal antibodies, raised to OmpC and OmpD porins and lipopolysaccharide (LPS)-O antigens, was used to define the specificity of the polyclonal immune response in mice. The monoclonal antibody panel recognized five distinct epitopes; these were localized to surface-exposed loops of OmpC and OmpD porin, to the "eye-let" forming loop L3 of OmpC/OmpD, and to LPS-O4 and O5 factors. The immune mouse serum raised to infections with S. typhimurium LT-2 strain WB600 (wild-type) competitively inhibited the binding of biotin-labelled monoclonal antibodies to the epitopes that they recognize, indicating that all five epitopes were targets of the host immune response to natural infection. However, only two epitopes, one within a surface-exposed loop of OmpC porin, and the other in the LPS-O4 factor, were immunodominant. Furthermore, the bacterial LPS core and O-antigen structure influenced the immune response to the porins. Surface epitopes of porins were dominant in the rough strain SH5014 (rfa), whereas the immune recognition of LPS epitopes was predominant in mice infected with the smooth, wild-type strain (WB600). Finally, the immune response to LPS epitopes O4 and O5 was more pronounced in mice immunized with heat-killed cells than those infected with live S. typhimurium.

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.

Th1 and Th2 cell involvement in immune response to Salmonella typhimurium porins

In understanding the regulation of the specific immune response to Salmonella typhimurium, the role of a surface major component (porins) was studied. In this study we demonstrate that purified porins are able to induce a different response to that induced by the porins present on the S. typhimurium cell surface. Porin-treated or orally infected mice show anti-porin antibodies with bactericidal activity. The complete adoptive transfer of resistance to S. typhimurium is achieved only using splenic T cells from survivor mice after experimental infection. After stimulation with specific antigen in vitro CD4+ cells from porin-immunized mice released large amounts of interleukin-4 (IL-4), at a time when CD4+ cells from S. typhimurium-infected mice predominantly secreted interferon-gamma (IFN-gamma). Limiting dilution analysis showed that infection resulted in a higher precursor frequency of IFN-gamma-producing CD4+ T cells and a lower precursor frequency of IL-4-producing CD4+ T cells, while immunization with porins resulted in a higher precursor frequency of IL-4-producing cells and a low frequency of IFN-gamma-producing cells. Analysis of polymerase chain reaction-amplified cDNA from the spleens of infected mice revealed that IFN-gamma, IL-2 and IL-12 p40 mRNA were found 5 days after in vitro challenge and increased after 15 days; IL-10 expression was barely present after both 5 and 15 days, while IL-4 mRNA expression was not detected. In immunized mice, the IL-4 mRNA expression increased after 15 days, IFN-gamma mRNA expression disappeared entirely after 15 days, while IL-2, IL-10 and IL-12 mRNA remained relatively unchanged.

Salmonella typhi O-polysaccharide--tetanus toxoid conjugated vaccine

A Salmonella typhi conjugated vaccine was prepared by covalently linking the antigenic O-polysaccharide, selectively activated by periodate oxidation, to tetanus toxoid via reductive amination. The immunogenicity of the conjugate (O-TT) was examined by injecting Balb/c mice with 5 micrograms of the conjugate and Alhydrogel as adjuvant, boosting 14 and 28 days after the primary immunization, and quantification of the development of anti-polysaccharide and anti-tetanus toxoid antibodies by enzyme-linked immunosorbent assay. Mean anti-O-chain titres after the first and second boost were 129 and 502, respectively, while anti-tetanus toxoid titres were 159 and 1000, respectively. Anti-O-polysaccharide antibodies exhibited complement-mediated bactericidal activity against S. typhi. Immunized mice were fully protected against challenge with 10 LD50 of S. typhi Ty2 (p < 0.001) and partially protected against challenge with 100 LD50 of S. typhi Ty2 (p < 0.04).

Role of gamma interferon in late stages of murine salmonellosis

Gamma interferon (IFN-gamma) is an important mediator of natural resistance of mice to Salmonella species during the first week of infection, when it restricts the rate of intracellular growth of the bacteria but does not lead to their killing (A. Muotiala and P. H. Mäkelä, Microb. Pathog. 8:135-141, 1990). We used the experimental mouse salmonellosis model to investigate the role of IFN-gamma in the later stages of a sublethal infection and the ensuing specific immunity. When anti-IFN-gamma was administered starting 6 days after challenge, it did not prevent the cessation of intracellular bacterial growth and the formation of the plateau stage in the second week of infection. In addition, anti-IFN-gamma given 14 and 16 days after challenge did not alter the elimination of the bacteria in the clearance stage in the third week of infection. When mice immunized 2 months previously with live vaccine were infected with virulent salmonellae, depletion of IFN-gamma enhanced the early growth of the bacteria in the same manner as that seen in naive mice. However, when the immunized mice were infected with attenuated aroA bacteria, their clearance started immediately and was unaffected by IFN-gamma depletion, demonstrating that IFN-gamma is not required for the clearance. We conclude that IFN-gamma restricts the rate of intracellular bacterial multiplication in the first week of Salmonella infection in both naive and immune mice but is not a mediator of bacterial clearance in either naive or immunized mice.

Salmonella typhi O:9,12 polysaccharide-protein conjugates: characterization and immunoreactivity with pooled and individual normal human sera, sera from patients with paratyphoid A and B and typhoid fever, and animal sera

Polysaccharide of O:9,12 specificity purified from Salmonella typhi was conjugated to tetanus toxoid or bovine serum albumin in order to obtain defined antigenic material that would contain O chain free of other S. typhi antigens and that would be suitable for characterizing host humoral response to only S. typhi O-chain antigens. These artificial conjugates were strongly reactive in immunodots with 18 pooled and 3 individual serum samples from patients with typhoid fever and with rabbit anti-Salmonella O antiserum (group D, factors 1, 9, and 12). They reacted weakly with one serum sample from one human with paratyphoid A. These results suggest that the periodate oxidation and the reductive amination used in the conjugation conserved the immunogenicity of the O chain and allowed its absorption to nitrocellulose. They also suggest that the bovine serum albumin conjugate could be used in the diagnosis of S. typhi infections as normal sera may react with the protein molecule of the tetanus toxoid conjugate.

Lipopolysaccharide-independent radioimmunoprecipitation and identification of structural and in vivo induced immunogenic surface proteins of Salmonella typhi in typhoid fever

The humoral response to Salmonella typhi is important for protective immunity against typhoid fever, as indicated by the protection obtained with killed cell vaccines and component vaccines (outer membrane proteins, Vi antigen) in animals and human beings. Nonetheless, analysis and interpretation of host humoral immune response to S. typhi surface antigens have been difficult because of the complex structure of the S. typhi envelope and the lack of purified reagents for detection of immune response to individual surface components. Normal and convalescent human sera from typhoid fever patients were absorbed with S. typhi lipopolysaccharide. These sera were used in radioimmunoprecipitation assays of whole S. typhi cells and S. typhi membranes labelled with either 125I or 35S-methionine. This strategy has permitted the unequivocal identification of a humoral immune response to structural and in vivo induced outer membrane proteins of S. typhi. In this manner, we have identified the porins, lipoprotein, the iron-starvation-induced proteins, and three proteins of 30, 18.5 and 15 kDa as surface-exposed immunogens of S. typhi in patients with typhoid fever. These studies suggest that further experimental work is needed to characterize the relevance of both anti-S. typhi outer membrane protein and antilipopolysaccharide antibodies in recovery from S. typhi infections and protective immunity.

Structural relatedness of enteric bacterial porins assessed with monoclonal antibodies to Salmonella typhimurium OmpD and OmpC

The immunochemistry and structure of enteric bacterial porins are critical to the understanding of the immune response to bacterial infection. We raised 41 monoclonal antibodies (MAbs) to Salmonella typhimurium OmpD and OmpC porin trimers and monomers. Enzyme-linked immunosorbent assays, immunoprecipitations, and/or Western immunoblot techniques indicated that 39 MAbs (11 anti-trimer and 28 anti-monomer) in the panel are porin specific and one binds to the lipopolysaccharide; the specificity of the remaining MAb probably lies in the porin-lipopolysaccharide complex. Among the porin-specific MAbs, 10 bound cell-surface-exposed epitopes, one reacted with a periplasmic epitope, and the remaining 28 recognized determinants that are buried within the outer membrane bilayer. Many of the MAbs reacting with surface-exposed epitopes were highly specific, recognizing only the homologous porin trimers; this suggests that the cell-surface-exposed regions of porins tends to be quite different among S. typhimurium OmpF, OmpC, and OmpD porins. Immunological cross-reaction showed that S. typhimurium OmpD was very closely related to Escherichia coli NmpC and to the Lc porin of bacteriophage PA-2. Immunologically, E. coli OmpG and protein K also appear to belong to the family of closely related porins including E. coli OmpF, OmpC, PhoE, and NmpC and S. typhimurium OmpF, OmpC, and OmpD. It appears, however, that S. typhimurium "PhoE" is not closely related to this group. Finally, about one-third of the MAbs that presumably recognize buried epitopes reacted with porin domains that are widely conserved in 13 species of the family Enterobacteriaceae, but apparently not in the seven nonenterobacterial species tested. These data are evaluated in relation to host immune response to infection by gram-negative bacteria.

Protective immunity induced by porin in experimental mouse salmonellosis

The induction of protective immunity to mouse salmonellosis by porin from Salmonella typhimurium LT2 was studied. The immunization with porin induced a high level of protective immunity to salmonellosis in BALB/c mice. Mice immunized with porin exhibited significant levels of delayed-type hypersensitivity response and interleukin-2 production, indicating that porin was capable of inducing cell-mediated immunity (CMI). Furthermore, we found that both T cells and sera taken from the porin-immunized mice could transfer the protection against salmonellosis into nonimmunized mice. These observations suggested that a high level of the protection to salmonellosis obtained by the porin immunization resulted from the induction of CMI in addition to humoral immunity.

Antibody production to a meningococcal outer membrane protein cloned into liv Salmonella typhimurium aroA vaccine strain

We cloned a 28 kDa outer membrane protein (OMP) of Neisseria meningitidis group B into a live Salmonella typhimurium aroA vaccine strain SL3261. The cloned 28 kDa protein was produced in large amounts in the S. typhimurium transformant SH8182 and located in the outer membrane. A mouse-passaged derivative of SH8182 was used as a live vaccine to immunize mice; with antibiotic pressure the strain survived in the mice as well as the parent strain SL3261 and maintained the plasmid carrying the gene encoding the 28 kDa OMP. The mice produced a high titer of antibodies to the 28 kDa OMP, showing that it had been effectively presented to the immune system. The hyperimmune mouse serum bound in an enzyme immunoassay to whole cells of E. coli and group B meningococci expressing the 28 kDa OMP, but its bactericidal activity towards the meningococci was marginal. In a passive protection study, the antiserum did not protect infant rats from meningococcal infection. The results indicate that the antibodies elicited did not bind to intact meningococcal cells, possibly because of inaccessibility of the 28 kDa OMP.

Protective immunity in mouse salmonellosis: comparison of smooth and rough live and killed vaccines

Protective immunity against Salmonella infection was studied in a mouse model. To study specificity of protection we used smooth (O-4,5,12) and rough vaccines; live and killed vaccines of both types were compared. The protection was assessed by enumerating the number of bacteria in the livers, and by following survival of the mice after intravenous challenge with smooth O-4,5,12 bacteria. Passively transferred antibodies induced by the smooth vaccines had a small protective effect and those induced by the rough vaccines no protective effect in this model. Both live vaccines induced long-lasting protective immunity which was much stronger than that mediated by antibodies. Since the live rough vaccine induced protective immunity and contained no O antigen we conclude that the protective immunity induced by it was mainly cell-mediated and directed to other antigens than the O antigen, the target of protective antibodies. Both killed vaccines also induced protective immunity, but this was weaker than that induced by the corresponding live vaccine.

Salmonella, complement and mouse macrophages

The structure of the polysaccharide chains that constitute the O antigen on the surface of Salmonella bacteria determines the rate of complement activation and C3b deposition on the bacteria. A fast-activating O antigen causes rapid C3-dependent opsonization of the bacteria injected intraperitoneally; as a consequence, the bacteria are taken up and killed by the resident peritoneal macrophages, and their virulence is low. A slow-activating O antigen protects the bacteria from opsonization in the peritoneal cavity, and is associated with higher virulence. However, if injected intravenously bacteria with either O-antigenic type are equally virulent; in the high complement concentration of the blood they become opsonized and taken up by macrophages in the liver and spleen, which are unable to kill them but instead provide a protected site for multiplication.

Effectiveness of natural and synthetic complexes of porin and O polysaccharide as vaccines against Brucella abortus in mice

A single vaccination of mice with a complex of porin and smooth lipopolysaccharide (porin-S-LPS) extracted from virulent Brucella abortus 2308 provided significant protection (P less than 0.01 to P less than 0.001) against challenge with the same strain, equivalent to that achieved by vaccination with living attenuated B. abortus 19. The porin-S-LPS vaccine given without adjuvant or in several adjuvants (trehalose dimycolate and muramyl dipeptide; the pluronic polymer L-121 and muramyl dipeptide; or complexed with Quil A in immunostimulating complexes) provided equivalent protection. In contrast, one vaccination with porin complexed with rough LPS (porin-R-LPS) from a rough mutant of strain 2308 provided no protection with any adjuvant tested. In one experiment, two inoculations with the porin-R-LPS resulted in a low level of protection, probably owing to priming of the animals for production of O-polysaccharide-specific antibodies. However, one vaccination with rough-strain porin covalently bound to purified O polysaccharide conferred protection equal to that obtained with natural complexes of porin-S-LPS or with living strain 19. A synthetic vaccine containing long chains of O polysaccharide was more effective than one prepared with short chains. Protective vaccines caused the formation of increased concentrations of circulating O-polysaccharide-specific antibodies, although there were individual exceptions to the quantitative association between O-polysaccharide-specific antibodies and protection. Antibodies specific for porin or R-LPS were found in negligible quantities in vaccinated mice. These results provide additional evidence that the O polysaccharide will constitute an essential component of an effective subcellular vaccine against B. abortus and that O-polysaccharide-specific antibodies play an important role in protective immunity in brucellosis.

Protective ability of antibodies against 78- and 40-kilodalton outer membrane antigens of Haemophilus somnus

The ability of concentrated antibody against the 78- or 40-kilodalton (kDa) outer membrane protein (OMP) of Haemophilus somnus to passively protect calves against H. somnus-induced pneumonia was determined. The 78- and 40-kDa OMPs were evaluated in passive protection experiments, because results of previous studies demonstrated their (i) immunogenicity for cattle, (ii) intense reactivity with convalescent-phase sera which passively protected calves against experimental H. somnus pneumonia, (iii) surface location and accessibility to antibody, and (iv) conservation among a wide range of H. somnus isolates obtained from animals with different diseases and from different geographic locations. The specificity of the two antisera evaluated in this study was verified by (i) immunoblots in which reactivity against the 78- or 40-kDa OMP was present in postimmunization but not preimmunization serum and (ii) immunoblots in which affinity-purified, surface-reactive antibodies in each antisera were used, which demonstrated that essentially only antibody to the 78- or 40-kDa OMP was reactive with the surface of H. somnus. In enzyme-linked immunosorbent assays, the antiserum against the 40-kDa OMP contained immunoglobulin G1 (IgG1), IgG2, and IgM against H. somnus, while the antiserum against the 78-kDa OMP contained IgG1 and IgM but no IgG2 against H. somnus. The antiserum against the 40-kDa OMP contained IgG1 and IgG2 specific for the 40-kDa OMP, as determined by Western blot analysis. Slight reactivity against H. somnus lipopolysaccharide was detected by enzyme-linked immunosorbent assay but not by Western blot analysis. In passive protection experiments, preincubation of bacteria with antibody against the 40-kDa OMP protected calves (P less than 0.025) against H. somnus pneumonia, while antibody against the 78-kDa OMP failed to protect calves against H. somnus pneumonia. Determination of the potential protective capacity of the 78-kDa OMP awaits resolution of the role of anti-78-kDa IgG2 in protection against H. somnus pneumonia. The 40-kDa OMP is, however, a good candidate antigen for evaluation of protective ability against H. somnus pneumonia following active immunization.

Role of antibodies in the opsonization of Yersinia spp

We have determined the opsonic capacity of specific antibodies in patient sera obtained after Yersinia infection. The results indicate that Yersinia antibodies lead to complement activation through the classical pathway, thus overcoming the inhibition of complement-mediated opsonization in the absence of specific antibodies provided by the virulence plasmid in Yersinia enterocolitica and Yersinia pseudotuberculosis. Further, antibodies against plasmid-encoded structures, the Yersinia outer membrane proteins (YOPs), are not necessary in this effect. This is indicated by two facts. (i) Monoclonal antibodies directed against the O polysaccharide of Y. enterocolitica O:3 are capable of opsonizing the plasmid-containing bacteria through C1q binding. (ii) Rabbit antisera show opsonic activity when obtained by immunization both with plasmid-containing Y. enterocolitica expressing the YOPs and a plasmid-cured variant not expressing these proteins.

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.

Cloning, expression and characterization of the 36 KDal Salmonella typhi porin gene in Escherichia coli

A recombinant plasmid containing the gene for the 36 KDal porin of Salmonella typhi has been identified in a cosmid library of S. typhi propagated in Escherichia coli. The recombinant clone was identified by its ability to endow E. coli with susceptibility to porin specific phages, and by the appearance in the outer membrane of E. coli containing the clone of a new protein of 36 KDal. While the porin confers upon a porinless mutant of E. coli an increased susceptibility to beta-lactam antibiotics, it does not react with serum from patients with typhoid fever in immunoblotting assays.

The roles of influenza virus haemagglutinin and nucleoprotein in protection: analysis using vaccinia virus recombinants

Vaccinia virus recombinants expressing haemagglutinin (HA) or nucleoprotein (NP) from influenza virus A/PR/8/34 were used to investigate protective immunity in mice, with two protocols. Protection was assessed by mortality and morbidity rates and by lung virus titres after infection intranasally with A/PR/8/34. In the first protocol, mice immunized with vaccinia-HA recombinant virus and infected intranasally with A/PR/8/34 were almost totally protected, but mice immunized with vaccinia-NP virus were very poorly protected. In the second protocol, the recombinant viruses were used to stimulate in vitro T cells that are specific for HA and NP; both populations of T cells, when transferred to A/PR/8/34-infected mice, afforded good protection. The results indicate that an immune response specific for just HA provided protection that was almost indistinguishable from that provided by whole A/PR/8/34. On the other hand, immunization with vaccinia-NP provided poor protective immunity, despite the fact that transferred NP-specific T cells were very effective and vaccinia-NP immunization has previously been shown to stimulate cytotoxic T cells. These results demonstrate that a single viral antigen, delivered by live vaccinia virus, can provide effective protection, but that immunization for cross-protection against heterologous influenza virus remains elusive.