Induction of protective T cells against Listeria monocytogenes in mice by immunization with a listeriolysin O-negative avirulent strain of bacteria and liposome-encapsulated listeriolysin O

Involvement of Reactive Oxygen Intermediate in the Enhanced Expression of Virulence-Associated Genes ofListeria monocytogenesinside Activated Macrophages

Listeriolysin O encoded by 1,587 bp hly is the essential virulence factor of Listeria monocytogenes that replicates in the cytosolic space after escaping from phagosome of macrophages. By using murine macrophage-like J774.1 cells with or without activation by IFN-gamma plus LPS, the expression of both hly and its positive regulator prfA was monitored by means of RT-PCR. In activated J774.1 cells, the level of hly expression was enhanced although the multiplication of bacteria was significantly suppressed. The elevated expression of hly inside activated macrophage was abolished by addition of SOD and catalase, suggesting that reactive oxygen intermediates contribute to the upregulation of prfA and hly transcriptions. Moreover, we found that exposure of L. monocytogenes to H2O2 dramatically enhanced the expression of both prfA and hly mRNAs. Spontaneous ONOO- generator, SIN-1, also promoted the transcription to a certain level. These results suggested that oxygen radicals generated in activated macrophages provide a positive signal for up-regulation of virulence genes in L. monocytogenes.

Protective immunity to Listeria monocytogenes infection mediated by recombinant Listeria innocua harboring the VGC locus

In this study we propose a novel bacterial vaccine strategy where non-pathogenic bacteria are complemented with traits desirable for the induction of protective immunity. To illustrate the proof of principle of this novel vaccination strategy, we use the model organism of intracellular immunity Listeria. We introduced a, low copy number BAC-plasmid harbouring the virulence gene cluster (vgc) of L. monocytogenes (Lm) into the non-pathogenic L. innocua (L.inn) strain and examined for its ability to induce protective cellular immunity. The resulting strain (L.inn::vgc) was attenuated for virulence in vivo and showed a strongly reduced host detrimental inflammatory response compared to Lm. Like Lm, L.inn::vgc induced the production of Type I Interferon's and protection was mediated by Listeria-specific CD8⁺ T cells. Rational vaccine design whereby avirulent strains are equipped with the capabilities to induce protection but lack detrimental inflammatory effects offer great promise towards future studies using non-pathogenic bacteria as vectors for vaccination.

Alpha-galactosylceramide promotes killing of Listeria monocytogenes within the macrophage phagosome through invariant NKT-cell activation

alpha-Galactosylceramide (alpha-GalCer) has been exploited for the treatment of microbial infections. Although amelioration of infection by alpha-GalCer involves invariant natural killer T (iNKT)-cell activation, it remains to be determined whether macrophages (Mphi) participate in the control of microbial pathogens. In the present study, we examined the participation of Mphi in immune intervention in infection by alpha-GalCer using a murine model of listeriosis. Phagocytic and bactericidal activities of peritoneal Mphi from C57BL/6 mice, but not iNKT cell-deficient mice, were enhanced after intraperitoneal injection of alpha-GalCer despite the absence of iNKT cells in the peritoneal cavity. High levels of gamma interferon (IFN-gamma) and nitric oxide (NO) were detected in the peritoneal cavities of mice treated with alpha-GalCer and in culture supernatants of peritoneal Mphi from mice treated with alpha-GalCer, respectively. Although enhanced bactericidal activity of peritoneal Mphi by alpha-GalCer was abrogated by endogenous IFN-gamma neutralization, this was only marginally affected by NO inhibition. Similar results were obtained by using a listeriolysin O-deficient strain of Listeria monocytogenes. Moreover, respiratory burst in Mphi was increased after alpha-GalCer treatment. Our results suggest that amelioration of listeriosis by alpha-GalCer is, in part, caused by enhanced killing of L. monocytogenes within phagosomes of Mphi activated by IFN-gamma from iNKT cells residing in an organ(s) other than the peritoneal cavity.

Multiple effector mechanisms induced by recombinant Listeria monocytogenes anticancer immunotherapeutics

Listeria monocytogenes is a facultative intracellular gram-positive bacterium that naturally infects professional antigen presenting cells (APC) to target antigens to both class I and class II antigen processing pathways. This infection process results in the stimulation of strong innate and adaptive immune responses, which make it an ideal candidate for a vaccine vector to deliver heterologous antigens. This ability of L. monocytogenes has been exploited by several researchers over the past decade to specifically deliver tumor-associated antigens that are poorly immunogenic such as self-antigens. This review describes the preclinical studies that have elucidated the multiple immune responses elicited by this bacterium that direct its ability to influence tumor growth.

Dependency of caspase-1 activation induced in macrophages by Listeria monocytogenes on cytolysin, listeriolysin O, after evasion from phagosome into the cytoplasm

Listeriolysin O (LLO), an hly-encoded cytolysin from Listeria monocytogenes, plays an essential role in the entry of this pathogen into the macrophage cytoplasm and is also a key factor in inducing the production of IFN-gamma during the innate immune stage of infection. In this study, we examined the involvement of LLO in macrophage production of the IFN-gamma-inducing cytokines IL-12 and IL-18. Significant levels of IL-12 and IL-18 were produced by macrophages upon infection with wild-type L. monocytogenes, whereas an LLO-deficient mutant (the L. monocytogenes Deltahly) lacked the ability to induce IL-18 production. Complementation of Deltahly with hly completely restored the ability. However, when Deltahly was complemented with ilo encoding ivanolysin O (ILO), a cytolysin highly homologous with LLO, such a restoration was not observed, although ILO-expressing L. monocytogenes invaded and multiplied in the macrophage cytoplasm similarly as LLO-expressing L. monocytogenes. Induction of IL-18 was diminished when pretreated with a caspase-1 inhibitor or in macrophages from caspase-1-deficient mice, suggesting the activation of caspase-1 as a key event resulting in IL-18 production. Activation of caspase-1 was induced in macrophages infected with LLO-expressing L. monocytogenes but not in those with Deltahly. A complete restoration of such an activity could not be observed even after complementation with the ILO gene. These results show that the LLO molecule is involved in the activation of caspase-1, which is essential for IL-18 production in infected macrophages, and suggest that some sequence unique to LLO is indispensable for some signaling event resulting in the caspase-1 activation induced by L. monocytogenes.

Display of heterologous proteins on the surface of Lactococcus lactis using the H and W domain of PrtB from Lactobacillus delburueckii subsp. bulgaricus as an anchoring matrix

AIMS

The aim of this study was to develop a cell-surface display system for foreign antigens on the surface of a Lactococcus lactis strain using an H and W domain of PrtB from Lactobacillus delburueckii subsp. bulgaricus as an anchoring matrix.

METHODS AND RESULTS

To construct a cell-surface display pACL1 vector, a derivative of pSECE1 vector, we amplified the H and W domain of the cell-surface proteinase Prt B from Lact. bulgaricus using specific primers and then cloned it into a site downstream of the secretion signal sequence in the pSECE1 vector. The new system, designed for cell-surface display of recombinant proteins on L. lactis, was evaluated by the expression and display of the FliC protein of Salmonella enterica serovar Enteritidis as a reporter gene (pALC1:FliC). The expression of the FliC protein by the transformed cells was analysed by Western blot analysis, and the localization of FliC on the cell surface was confirmed by immunofluorescence microscopy and flow cytometry analysis. A specific band corresponding in size (approx. 110 kDa) to FliC plus the anchor residues was detected by anti-FliC antibody in the cell extract of L. lactis H61 harbouring pALC1:FliC, but not L. lactis H61 harbouring pALC1. In addition, flow cytometry and immunofluorescence microscopy revealed FliC-specific positive signals and a significant increase of fluorescence, respectively, in cells harbouring pALC1:FliC compared with that in control cells harbouring the parental pALC1 plasmid. These findings demonstrated that FliC was successfully displayed on the cell surface by the anchor domain of PrtB.

CONCLUSIONS

A pALC1 vector using the H and W domain of PrtB from Lact. bulgaricus as an anchoring matrix can be used to successfully display the FliC protein on the surface of L. lactis.

SIGNIFICANCE AND IMPACT OF THE STUDY

This novel way of displaying heterologous proteins on the cell surface of L. lactis using the PrtB anchor domain should prove useful for the delivery of antigens and other proteins.

Immunisation with anthrolysin O or a genetic toxoid protects against challenge with the toxin but not against Bacillus anthracis

Anthrolysin O (ALO) is a toxin produced by Bacillus anthracis, the causative agent of anthrax. It is a member of the cholesterol-dependent cytolysin (CDC) group of toxins, many of which are potential vaccine candidates that protect against their producing organisms. Pore formation by ALO was studied by transmission electron microscopy and pores were found to be consistent with those formed by other members of this toxin family. We constructed and characterised a novel genetic toxoid of anthrolysin O, Delta6mALO, which was able to bind to cells but was incapable of pore-formation or haemolysis. The capacity of the haemolytic and non-haemolytic forms of ALO to protect against challenge with the toxin or B. anthracis was determined. Immunisation with both active and non-haemolytic forms of ALO elicited protection against lethal i.v. challenge with ALO but neither was protective against B. anthracis in a murine i.p. challenge model. Immunisation with another CDC, pneumolysin, did not confer cross-protection against challenge with ALO. Histopathological investigation following lethal i.v. challenge with ALO revealed acute pathology in the lungs with occlusion of alveolar vessels by fibrin deposits.

Cytolysin-dependent escape of the bacterium from the phagosome is required but not sufficient for induction of the Th1 immune response against Listeria monocytogenes infection: distinct role of Listeriolysin O determined by cytolysin gene replacement

Listeria monocytogenes evades the antimicrobial mechanisms of macrophages by escaping from the phagosome into the cytosolic space via a unique cytolysin that targets the phagosomal membrane, listeriolysin O (LLO), encoded by hly. Gamma interferon (IFN-gamma), which is known to play a pivotal role in the induction of Th1-dependent protective immunity in mice, appears to be produced, depending on the bacterial virulence factor. To determine whether the LLO molecule (the major virulence factor of L. monocytogenes) is indispensable or the escape of bacteria from the phagosome is sufficient to induce IFN-gamma production, we first constructed an hly-deleted mutant of L. monocytogenes and then established isogenic L. monocytogenes mutants expressing LLO or ivanolysin O (ILO), encoded by ilo from Listeria ivanovii. LLO-expressing L. monocytogenes was highly capable of inducing IFN-gamma production and Listeria-specific protective immunity, while the hly-deleted mutant was not. In contrast, the level of IFN-gamma induced by ILO-expressing L. monocytogenes was significantly lower both in vitro and in vivo, despite the ability of this strain to escape the phagosome and the intracellular multiplication at a level equivalent to that of LLO-expressing L. monocytogenes. Only a negligible level of protective immunity was induced in mice against challenge with LLO- and ILO-expressing L. monocytogenes. These results clearly show that escape of the bacterium from the phagosome is a prerequisite but is not sufficient for the IFN-gamma-dependent Th1 response against L. monocytogenes, and some distinct molecular nature of LLO is indispensable for the final induction of IFN-gamma that is essentially required to generate a Th1-dependent immune response.

Listeriolysin O derived from Listeria monocytogenes inhibits the effector phase of an experimental allergic rhinitis induced by ovalbumin in mice

Listeriolysin O (LLO) derived from Listeria monocytogenes is highly capable of inducing interleukin (IL)-12, IL-18 and interferon (IFN)-gamma, and facilitates the generation of Th1 cells. We have recently shown that recombinant LLO (rLLO) inhibits generation of ovalbumin (OVA)-specific Th2 immune response by skewing maturation of antigen-specific T cells into Th1 cells. In the present study, we investigated the effect of rLLO on the effector phase of Th2-dependent allergic rhinitis in BALB/c mice sensitized with OVA. In mice sensitized intraperitoneally and challenged intranasally with OVA, nasal allergic symptoms such as sneezing and nose-scratching were observed at a high frequency. A high titre of anti-OVA IgE antibody was detected in sera and a large number of eosinophils migrated into the nasal tissue. However, rLLO treatment during the intranasal challenge inhibited the allergic symptoms, production of anti-OVA IgE antibody and eosinophil infiltration. Though rLLO did not affect antigen-specific cytokine production from splenic CD4(+) T cells, rLLO significantly suppressed OVA-specific IL-4 and IL-5 production from nasal mononuclear cells. We further found that rLLO inhibited the recruitment of CD4(+) T cells in nasal mucosa, and diminished the transcription and cell surface expression of CCR4 on splenic CD4(+) T cells. Moreover, rLLO was able to inhibit the passive cutaneous anaphylaxis reaction mediated by anaphylactic antibodies (IgE and IgG(1)) and mast cells. Taken together, these data showed that rLLO suppresses the effector phase of allergic rhinitis by inhibition of Th2 cell recruitment to nasal mucosa and type I allergic reaction.

Listeriolysin O, a cytolysin derived from Listeria monocytogenes, inhibits generation of ovalbumin-specific Th2 immune response by skewing maturation of antigen-specific T cells into Th1 cells

Listeriolysin O (LLO), a cholesterol-dependent cytolysin derived from Listeria monocytogenes, is a potent inducer of interleukin (IL)-12, IL-18 and interferon (IFN)-gamma. We have shown that LLO facilitates development of T cells mediating protective immunity against L. monocytogenes through the induction of IFN-gamma production at an early stage. Based on this finding, it is postulated that LLO inhibits differentiation of Th2 cells and the Th2 immune response. By using a murine model of ovalbumin (OVA)-induced allergic rhinitis, we investigated whether LLO has an ability to modulate the Th2-type immune disorder. In mice sensitized intraperitoneally with ovalbumin (OVA)/alum and challenged intranasally with OVA, a large number of eosinophils migrated into the nasal tissue, and high titres of anti-OVA IgE and IgG(1) antibodies were detected in sera. However, LLO treatment during sensitization markedly inhibited the eosinophil infiltration and production of these anti-OVA antibodies. A large number of T cells from mice sensitized and challenged with OVA produced high level of IL-4 and IL-5 but not IFN-gamma after stimulation with OVA. In contrast, OVA-specific IFN-gamma-producing T cells were preferentially induced in mice treated with LLO at the time of sensitization. In the absence of LLO administration, the expression level of GATA-3 and SOCS-3 in CD4(+) T cells was enhanced after sensitization with OVA. LLO treatment resulted in a reduction of GATA-3 and SOCS-3 expressions but induced the transcription of T-bet instead. Taken together, these data show clearly that LLO is capable of inhibiting Th2 immune response by skewing differentiation of antigen-specific T cells into Th1 cells.

Involvement of the virulence gene products of Yersinia enterocolitica in the immune response of infected mice

The virulence of Yersinia enterocolitica is known to be highly dependent on its virulence plasmid. However, it remains unclear whether the virulence plasmid is engaged also in the induction of cell-mediated immunity that is essential for protective immunity in the host. In this study, we have compared the induction of type 1 helper T cell immunity against Y. enterocolitica using a virulent strain (P+) harboring the pYV plasmid and an avirulent strain (P-) harboring no pYV. Spleen cells from both groups of mice immunized with 1/10 LD50 of P+ strain and those with 1/10 LD50 of P- strain produced a high level of gamma interferon (IFN-gamma) upon stimulation with heat-killed bacteria, and CD4+ T cells were exclusively responsible for IFN-gamma production. When crude Yersinia outer proteins (Yops) were used for antigenic stimulation, IFN-gamma response of immune spleen cells against crude Yops was observed only in mice immunized with P+ strain. Flowcytometric analysis revealed a significant level of increase in IFN-gamma-producing CD8+ T cells as well as the increase in IFN-gamma-producing CD4+ T cells against crude Yops. These results suggest that the virulence plasmid of Y. enterocolitica is involved in the induction of Th1-type of possibly protective T cells in infected mice.

Murine pulmonary infection with Listeria monocytogenes: differential susceptibility of BALB/c, C57BL/6 and DBA/2 mice

Murine listeriosis is a paradigm to understand host pathogen interactions. Airway infections with Listeria monocytogenes, although representing a serious problem in early onset neonatal listeriosis, has not been investigated in detail in animal models so far. Here, the susceptibility of BALB/c, DBA/2 and C57BL/6 mice towards an intratracheal (i.t.) infection with virulent L. monocytogenes EGDe and the attenuated variant L. monocytogenes EGD hlyW491A(pERL3-CMVGFP) is reported. The course of infection was characterized by determination of bacterial numbers in the organs and assessment of the health condition of the mice. The distribution and cellular localization of Listeria in the airways was assessed by immunocytochemistry and confocal and electron microscopy. The differential susceptibility of inbred mouse strains to airway infections with L. monocytogenes could be assigned to the major virulence factor listeriolysin O. Resistant C57BL/6 mice were not affected by the two listerial strains. In contrast, BALB/c and DBA/2 mice showed differential susceptibility towards L. monocytogenes EGDe and attenuated bacteria, with all the mice being killed by the wild-type bacteria but rarely by the variant that secretes a listeriolysin of only 10% activity of that of the wild-type toxin. Thus, listeriolysin is a decisive factor for differential susceptibility against Listeria. After i.t. application, bacteria were predominantly localized in the peribronchiolar space and invaded alveolar macrophages but rarely lung epithelial cells. Dissemination from the lung into the deep organs started almost immediately after application, although a pulmonary bacterial reservoir remained during the first 4 days.

Enhanced resistance to Gram-positive bacterium and increased susceptibility to bacterial endotoxin in mice sensitized withPropionibacterium acnes: involvement of Toll-like receptor

Mice sensitized with Propionibacterium acnes showed an enhanced resistance against infection with Listeria monocytogenes in contrast to the increased susceptibility to LPS-induced endotoxin shock. The enhanced protection to L. monocytogenes was mediated by activated innate immunity but not by generation of Listeria-specific acquired immunity. After infection with L. monocytogenes, the elimination of bacteria was observed earlier in accordance with a higher level of endogenous cytokine production in P. acnes-sensitized mice than in control mice. Peritoneal cells from P. acnes-sensitized mice produced a larger amount of IL-12p70 and nitric oxide after stimulation with heat-killed L. monocytogenes or peptidoglycan purified from Staphylococcus aureus. RT-PCR analysis showed that the expression of TLR2 but not TLR1, TLR4 nor TLR6 was induced by injection of P. acnes in peritoneal cells. These results indicated that P. acnes-sensitization could induce the activation of innate immunity against L. monocytogenes through increased recognition of bacterial components by TLR2.

Differences in gamma interferon production induced by listeriolysin O and ivanolysin O result in different levels of protective immunity in mice infected with Listeria monocytogenes and Listeria ivanovii

Two pathogenic species in the genus Listeria, Listeria monocytogenes and Listeria ivanovii, are characterized by the production of hemolysins belonging to cholesterol-dependent cytolysins, listeriolysin O (LLO) and ivanolysin O (ILO), respectively. LLO, produced by L. monocytogenes, is able to induce gamma interferon (IFN-gamma) production and contributes to the generation of Th1-dependent protective immunity. On the other hand, nothing is known about the role of ILO, produced by L. ivanovii, in this regard. In this study, we immunized mice with 0.1 50% lethal dose (LD(50)) of L. monocytogenes and L. ivanovii. Protective immunity against a challenge with 10 LD(50) was generated in mice infected with L. monocytogenes, whereas L. ivanovii infection did not induce protection. After immunization, the level of IFN-gamma in serum samples was increased in mice given L. monocytogenes but not in those given L. ivanovii. To determine the IFN-gamma-inducing activity of cytolysins, recombinant protein was constructed. Recombinant ILO exhibited significantly lower IFN-gamma-inducing activity than LLO. By comparing the IFN-gamma-inducing activity of a chimera incorporating LLO and ILO, it was found that domains 1 to 3 of LLO were critical for IFN-gamma-inducing activity while the counterpart in ILO was unable to induce cytokine production. These results suggested that the weak ability of ILO to induce IFN-gamma production is responsible for the failure of L. ivanovii to generate effective protective immunity.

Enhancing the immunogenicity of bioengineered Listeria monocytogenes by passaging through live animal hosts

Bioengineered Listeria monocytogenes can be used as a recombinant bacterial vaccine vector for the induction of strong cell-mediated immunity to passenger antigens. Listeria loses virulence after undergoing bioengineering techniques, thus decreasing its efficacy as a vaccine vector. We addressed this problem by examining the virulence, and the ability to induce CD8(+) T-cells, of Listeria monocytogenes vaccine strains before and after passaging through mice. We found that two in vivo passages are required to restore the induction of cell-mediated immunity to passenger antigens and maximum virulence to these strains. In addition, we found that after each passage, harvested bacteria must be cloned and checked for expression of the bioengineered gene to counter selection in favor of antigen loss mutants.

Seeligeriolysin O, a cholesterol-dependent cytolysin of Listeria seeligeri, induces gamma interferon from spleen cells of mice

Seeligeriolysin O (LSO), one of the cholesterol-dependent cytolysins produced by Listeria seeligeri, shows 80% homology to listeriolysin O (LLO) produced by Listeria monocytogenes at the amino acid sequence level. In addition to cytolytic activity, LLO has been shown to exhibit cytokine-inducing activity. In order to determine whether LSO is also capable of exhibiting these two different activities, we constructed a recombinant full-length LSO (rLSO530) and a noncytolytic truncated derivative with a C-terminal deletion (rLSO483) and compared these molecules with recombinant LLO. The cytolytic rLSO530 molecule could induce gamma interferon (IFN-gamma) production in spleen cells when the cytolytic activity was blocked by treatment with cholesterol. The noncytolytic truncated rLSO483 molecule also induced IFN-gamma production. Anti-LLO polyclonal antibody inhibited not only LLO-induced IFN-gamma production but also LSO-induced IFN-gamma production. Both NK cells and CD11b(+) cells were required for LSO-induced IFN-gamma production. Among the various cytokines expressed in CD11b(+) cells, interleukin-12 (IL-12) and IL-18 appeared to be essential. We concluded that LSO exhibits the same biological activity as LLO.

Listeriolysin O-liposome-mediated cytosolic delivery of macromolecule antigen in vivo: enhancement of antigen-specific cytotoxic T lymphocyte frequency, activity, and tumor protection

Cytotoxic T lymphocytes (CTLs) are primed by peptide antigens that are endogenously processed in the cytosol and presented in the context of major histocompatibility complex I (MHC I) molecules of antigen-presenting cells (APCs). Exogenous soluble protein antigens do not gain efficient entry into the cytosol of APCs, and therefore requires a special cytosolic delivery method. We have developed such a delivery strategy adopting the well-elucidated cytosol-invading listerial endosomal escape mechanism, and report here an efficient delivery of exogenous whole protein antigen into the cytosol in a mouse model. Co-encapsulation of listeriolysin O (LLO) inside liposome (LLO-liposome) was required for delivery of ovalbumin (OVA) into the cytosol of APCs in primary cultures. LLO-liposome-mediated OVA immunization in mice engendered significantly higher OVA-specific CTL activity and increased antigenic peptide-specific CTL precursor (CTLp) frequency as compared to non-LLO-liposome or soluble OVA immunizations. Interferon-gamma (IFN-gamma) production upon specific stimulation by MHC I-restricted peptide was also significantly stronger by the inclusion of LLO in the liposomes. Rerouting of antigen into the cytosol by LLO-liposomes, however, did not reduce the extent of anti-OVA antibody responses. Moreover, LLO-liposome-antigen vaccination was robust in conferring protection to mice from lethal challenges with antigen-expressing tumor cells. Our study demonstrates a novel delivery system for efficient introduction of exogenous protein into the cytosol in vivo, priming cellular immune responses, which are protective in nature.

Dissociated linkage of cytokine-inducing activity and cytotoxicity to different domains of listeriolysin O from Listeria monocytogenes

Listeriolysin O (LLO), a cholesterol-binding cytolysin of Listeria monocytogenes, exhibits cytokine-inducing and cytolytic activities. Because the cytolytic activity was abolished by cholesterol treatment but the cytokine-inducing activity was not, these activities appeared to be linked to different domains of the LLO molecule. In this study, we constructed recombinant full-length LLO (rLLO529) and various truncated derivatives and examined their cytolytic, cholesterol-binding, and gamma interferon (IFN-gamma)-inducing activities. rLLO529 exhibited both IFN-gamma-inducing and cytolytic activities. Four truncated rLLOs possessing different C termini, which did not exert either cytolytic or cholesterol-binding activity, stimulated IFN-gamma production in normal spleen cells. However, a truncated rLLO corresponding to domain 4 (rLLO416-529) did not exhibit IFN-gamma-inducing activity, whereas it did bind to immobilized cholesterol. In addition, though the hemolysis induced by rLLO529 was inhibited by rLLO416-529, such inhibition was not detected upon rLLO529-induced IFN-gamma production. These data indicated that domain 4 was responsible for binding of LLO to membrane cholesterol followed by oligomerization and pore formation by the entire LLO molecule. In contrast, the other part of LLO, corresponding to domain 1-3, was essential for IFN-gamma-inducing activity. These findings implied a novel aspect of the function of LLO as a bacterial modulin.

Induction of gamma interferon and nitric oxide by truncated pneumolysin that lacks pore-forming activity

Pneumolysin (PLY), an important virulence factor of Streptococcus pneumoniae, is known to exert various effects on the host immune cells, including cytokine induction, in addition to its known cytolytic activity as a member of the thiol-activated cytolysins. It is of interest to determine whether cytolytic activity is involved in triggering the cytokine production. In this study, we constructed full-length recombinant PLY and noncytolytic truncated PLYs with C-terminal deletions to examine the response of spleen cells to these PLY preparations. When cytolytic activity was blocked by treatment with cholesterol, full-length PLY was capable of inducing gamma interferon (IFN-gamma) production. Truncated PLYs that originally exhibited no cytolytic activity were also active in IFN-gamma induction. Therefore, the IFN-gamma-inducing ability of PLY appeared to be independent of the cytolytic activity. Furthermore, IFN-gamma-inducing preparations were also capable of inducing nitric oxide synthase expression and nitric oxide (NO) production, and the addition of neutralizing antibody to IFN-gamma abolished the NO production. These results clearly demonstrated that PLY is capable of inducing IFN-gamma production in spleen cells by a mechanism different from pore formation and that the induced IFN-gamma stimulates NO production. These findings were discussed with reference to the contribution of PLY to the virulence of S. pneumoniae in vivo.

From evil to good: a cytolysin in vaccine development

Current vaccination strategies mainly target antigens into the phagosomal, major histocompatibility complex class II antigen-processing pathway and thus lead predominantly to humoral immune responses. The elicitation of cytotoxic T-cell responses instead requires introduction of antigens into the cytosol of professional antigen-presenting cells (APCs). The intracellular bacterium Listeria monocytogenes gains access to the host cell cytosol by means of a cytolysin, listeriolysin O. Vaccine researchers have successfully employed listeriolysin in novel vaccination approaches to provide access to the cytosol of professional APCs for purified protein antigens, attenuated bacterial vaccine strains, DNA vaccines and liposome contents.