Kinetics and maintenance of acquired resistance in mice to Listeria monocytogenes.

In the mouse system, acquired resistance to Listeria monocytogenes can only be demonstrated after immunization with viable microorganisms. A successful state of immunity cannot be elicited with formalin-killed organisms or bacterial cell-derived products. Viable, serologically cross-reactive organisms (not mouse pathogenic) do not induce a state of immunity as measured by acquired resistance. The duration of immunity, once established, is dose independent, and the absolute interval of its existence is not extended by secondary challenge with large numbers of viable organisms. The decline of immunity in actively immunized animals is not altered by antigenic challenge with formalin-killed cells or cell products. This indicates that the cellular requirements for the development of host resistance are similar for induction as well as maintenance. In vitro measurements of cellular immunity by migration inhibition indicate that formalin-killed organisms as well as cell products were recognized by actively sensitized lymphocytes obtained from immune animals.     

Prolongation of acquired cellular resistance to Listeria monocytogenes

Intracutaneous immunization of mice with 10⁵ or 10⁶ viable listeria resulted in acquired cellular resistance (ACR) of short duration (7 days) and in delayed-type hypersensitivity (DH) lasting at least 27 days. The ACR was partially non-specific, as 50% of the mice were also protected against a lethal challenge with Salmonella enteritidis. The specific element of the ACR could be transferred by non-adherent spleen cells from immune mice to normal recipient mice. Such transfer was not possible with adherent spleen cells from immune mice or with spleen cells from normal mice.

Two systems of multiple immunizations to extend the period during which mice were protected against a challenge with 50 LD₅₀ listeria were used. In the first system, mice were immunized with 10⁶ viable listeria and subsequently challenged with 50 LD₅₀ (= 10⁷) viable listeria. Mice surviving the challenge were actually boosted at the challenge injection for ACR. In the second system mice were immunized and boosted with 10⁸ killed listeria mixed with the adjuvant dimethyl dioctadecyl ammonium bromide (DDA). In the former system after each booster injection with viable listeria the interval during which the mice were protected doubled and reached a maximum of 31 days. In the latter system all intervals between two booster injections were equally long and never exceeded 28 days.

In both systems the existence of immunological memory was suggested. The difference in results obtained after immunization with viable listeria and killed listeria mixed with DDA are discussed.

     

In vitro primary induction of T cells mediating delayed footpad reaction and acquired cellular resistance to Listeria monocytogenes

We established an in vitro system generating L. monocytogenes-specific T cells primarily from unprimed spleen cells of mice. Normal spleen cells were cultured for 5 days in the presence of L. monocytogenes in vitro. Viable cells were harvested and assessed for their capacity to confer acquired cellular resistance (ACR) and delayed footpad reaction (DFR) upon local passive transfer to naive syngeneic recipient mice. When normal spleen cells were stimulated with viable L. monocytogenes, the viable cells that were recovered after 5 days of culture conferred a high level of ACR and DFR. Negative selection revealed that the effector cells obtained in primary in vitro culture were Thy 1+, L3T4+, Lyt2- cells. T cells mediating ACR could not be generated in the culture of normal spleen cells with heat-killed bacteria; however, cells mediating only DFR were generated in the presence of a large number of killed L. monocytogenes. The expression of DFR and ACR by T cells generated in this primary culture system was Listeria-specific; reactions were not observed against unrelated bacterial antigens including S. typhimurium, S. aureus, E. coli and PPD. FACS analysis of the cells in culture showed that L3T4+ and Lyt2- T cells were being enriched during culture. The primary generation of antigen-specific T cells in vitro was also possible with spleen cells from NTx mice but not with cells from nude mice, suggesting the presence of Listeria-specific precursors in NTx mice.     

Enhanced resistance to Listeria monocytogenes in splenectomized mice.

Mice infected with live Listeria monocytogenes intravenously from 1 week to 3 months following splenectomy exhibit greatly enhanced antibacterial resistance to this micro-organism as compared to normal or sham-splenectomized mice. They survive a dose of Listeria 100 times higher than is the LD50 of this parasite for normal mice. Initially, the same number of viable micro-organisms lodge in the livers of splenectomized and normal hosts. However, within 24 h after infection, the number of viable Listeria which can be recovered from the livers of splenectomized animals is significantly reduced in comparison with control mice. This effect of splenectomy is transient and gradually disappears spontaneously within 3 months following splenectomy. Enhancement of anti-listerial resistance in splenectomized mice can be abrogated by the transfer of normal spleen cells. The presence of a normal splenic cell population that controls macrophage activation is postulated.     

Cell-mediated resistance to infection with Listeria monocytogenes in nude mice.

Congenitally dysthymic nude (nu/nu) NMRI mice showed increased resistance to viable Listeria monocytogenes cells during the initial phase of infection as compared with euthymic control mice. The intravenous mean lethal dose (LD50), as determined for euthymic mice after an observation time of 7 and 14 days, amounted consistently to 6 X 10(4) Listeria. The corresponding values determined in nude mice were found to be increased by either 20-fold (1.2 X 10(6) Listeria after an observation time of 7 days) or 4-fold (2.4 X 10(5) Listeria after an observation time of 14 days). The transfer of spleen cells from immune euthymic donor mice into chronically infected nude mice caused almost complete elimination of Listeria within 1 week. The injection of dextran sulfate 24 h before a secondary infection with L. monocytogenes caused loss of antibacterial resistance in both chronically infected nude mice and Listeria-immune euthymic mice, this being expressed by a rapid increase in the numbers of bacteria in the spleens as well as the occurrence of serious signs of illness.     

Interaction of Listeria monocytogenes with mouse dendritic cells.

In this study, the interaction of murine dendritic cells with Listeria monocytogenes was investigated. Dendritic cells are efficient antigen-presenting cells, play a key role in the immune response, and are capable of migrating over substantial distances between sites of infection and lymphoid tissues. L. monocytogenes EGD invaded dendritic cells, escaped from phagosomes into the cytoplasm, and there directed actin nucleation, polymerization, and polarization in a typical fashion, thereby achieving intracellular movement and cell-to-cell spread. The internalization process appears to be independent of the inl locus. Interestingly, an intact microtubular function was essential for efficient uptake, whereas in a previous report, microtubule disruption did not affect bacterial spread in Caco-2 cells. The results obtained also suggest that L. monocytogenes binds to glycosylated receptors of dendritic cells. Uptake of Listeria cells was mediated by a protein kinase-dependent transducing phosphorylation signal that induces the actin polymerization-dependent phagocytic process. To achieve efficient uptake, de novo protein synthesis of eukaryotic and prokaryotic cells is also required. Despite the killing of dendritic cells, wild-type bacteria were found to persist in small numbers in some cells for at least 24 h. When different isogenic mutants of the EGD strain were analyzed for their capability to interact with dendritic cells, it was observed that some virulence-attenuated mutants (i.e., prfA and delta hly) persisted in large numbers for even longer times. Invasion of dendritic cells by L. monocytogenes, which in turn could result in either cell death or persistent infection, might have an important role in the pathogenesis of listeriosis, leading to impaired immune responses with inefficient bacterial clearance and/or promoting bacterial spread.     

Characteristics of resistance to Listeria monocytogenes enhanced by Corynebacterium parvum in mice

In mice pre-treated with Corynebacterium parvum, Listeria monocytogenes was cleared rapidly from the blood and bacterial growth in the liver and spleen was inhibited effectively during the early phase of infection. This enhanced resistance could be transferred with peritoneal exudate cells (PEC) but not with non-adherent spleen cells. In spite of earlier elimination of bacteria, pre-treated mice developed lower levels of delayed-type hypersensitivity (DTH) to bacteria than untreated immune control mice, but the control levels of DTH could be reached by increasing the challenge dose of bacteria in C. parvum-pre-treated mice. Additionally, C. parvum did not inhibit the expression of antibacterial immunity when immune mice were rechallenged. It appeared that the active suppression of the T-cell mediated immune response by C. parvum-activated macrophages was not seen during the course of L. monocytogenes infection, and that the lower levels of DTH seen in mice pre-treated with C. parvum were attributable to an insufficient antigenic stimulus following the accelerated elimination of bacteria by non-specifically activated macrophages during the early phase of infection.     

Delayed-type hypersensitivity and acquired cellular resistance in mice immunized with killed Listeria monocytogenes and adjuvants.

Delayed-type hypersensitivity (DH) and acquired cellular resistance (ARC) to Listeria monocytogenes in mice was studied following immunization with killed bacteria in combination with Freund's complete adjuvant or the adjuvant dimethyldioctadecylammonium bromide (DDA). Intracutaneous or intraperitoneal injections of killed listeria mixed with Freund's complete adjuvant did neither result in DH nor in ACR. Intracutaneous injections of killed listeria and DDA resulted in an antigen-dose dependent DH but not in ACR. Intraperitoneal injections of listeria and DDA, however, induced ACR but no DH. Optimal conditions for the induction of ACR were simultaneous intraperitoneal injection of 15 mg DDA/kg body weight and 10(7) or 10(8) listeria. The optimal interval between immunization and challenge was 7 days. No protection was found against challenge with a lethal dose of Salmonella enteritidis, suggesting that the protection is specific. Intraperitoneal injection of mice with DDA resulted in inhibition of phagosome-lysosome fusion in macrophages harvested 24 h later. Interference with macrophage activity is discussed as one of the possible mechanisms for the adjuvant effect of DDA.     

Toxicity and induction of resistance to Listeria monocytogenes infection by amphotericin B in inbred strains of mice.

Amphotericin B (AmB) treatment before infection with the bacterium Listeria monocytogenes prolonged survival of AKR mice but shortened survival of C57BL/6 mice compared with survival of untreated infected controls. C57BL/6 mice were also more sensitive to the acute toxic effects of AmB than AKR mice, as were (C57BL/6 X AKR)F1 hybrid mice. Spleen cells and erythrocytes (RBCs) from the C57BL/6 and the F1 hybrid mice were both more sensitive to the lytic and lethal effects of AmB than corresponding cells from AKR mice. Biochemical analysis indicated that catalase levels in RBCs from C57BL/6 and F1 hybrid mice were about 60% of those found in RBCs from AKR mice. The lysis by AmB of RBCs from all these strains of mice was inhibited by catalase or incubation in a low-oxygen environment. These findings suggest that (i) the low catalase levels in C57BL/6 and F1 hybrid mice may limit the protection of cells from the oxidant damage involved in AmB action, and (ii) the toxicity which occurs at low concentrations of AmB in the mouse strains with low intracellular catalase levels may interfere with or ablate the AmB-induced increases in mouse resistance to L. monocytogenes infection.     

T-cell co-operation in the mediation of acquired resistance to Listeria monocytogenes.

Monoclonal antibodies were used to select T-cell subsets that mediate delayed-type hypersensitivity (DTH) and acquired cellular resistance (CRI) in rats infected with Listeria monocytogenes. The mediators of DTH were identified as W3/25+ OX8- T cells. The latter comprised a subset distinct from that which could protect recipient rats against a Listeria challenge. The protective T cells had a W3/25- OX8+ phenotype. The T-cell mediators of cellular resistance to infection (TCRI) failed to augment the expression of DTH; however, the mediators of DTH (TDTH) significantly enhanced the protective capacity of TCRI. This property of TDTH correlated with the ability of the cells to promote the focal deployment of TCRI and macrophages at sites of soluble Listeria antigen injection in skin, and in peritoneal exudates induced by killed L. monocytogenes. These findings illustrate the co-operative interaction of activated T cells in acquired resistance to L. monocytogenes, and imply that DTH has a purposeful role in the host defence against infection.     

Impaired resistance to Listeria monocytogenes in mice chronically exposed to cadmium.

It is shown in this work that resistance to Listeria monocytogenes is greatly impaired in C57BL/6 mice chronically exposed to cadmium (Cd) chloride. Animals received 0.5 mg/kg Cd by an intraperitoneal route three times a week during a 4-week period and were then infected with L. monocytogenes. Susceptibility to this pathogenic bacteria was not due to a defect of the specific immune response, since mice developed normal levels of anti-Listeria T cell-mediated immunity and did not show any impairment of macrophage activation. In fact, bacterial growth in organs was rapid in Cd-exposed mice during the early phase of infection, suggesting an impairment of non-specific defence mechanisms. Experimental data indicate that the susceptibility to L. monocytogenes might be due to a defect of macrophage recruitment in sites of infection during the early phase of the host response.     

Recombinant interleukin-12 enhances resistance of mice to Listeria monocytogenes infection

The effect of recombinant murine IL-12 (rIL-12) or anti-IL-12 antibody administration on resistance to murine listeriosis was investigated. Mice given a single 0.5 μg dose of rIL-12 had 1.5 log₁₀ fewer listeriae in their spleens and livers as compared with control infected mice 3 days after L. monocytogenes challenge. Conversely, administration of anti-IL-12 IgG caused an equivalent increase in the cfu of L. monocytogenes recovered from the spleens and livers as compared to control mice. This is the first report of such a protective effect from a single dose of rIL-12. Treatment of uninfected mice with rIL-12 induced IFN-γ mRNA production in their livers. Infection of mice with L. monocytogenes caused a similar increase in IFN-γ mRNA levels that was not increased further by concurrent treatment with rIL-12. Treatment of mice with an anti-IFN-γ MAb eliminated the protective effect of IL-12 on Listeria infection. Expression of TNF-γ, IL-10 and IL-12p40 mRNA in L. monocytogenes-infected mice were not significantly altered by administration of either anti-IL-12 IgG or rIL-12. rIL-12 administration was associated with increased serum AST levels, a measure of liver damage, 1 day after treatment in L. monocytogenes-infected mice. In addition, rIL-12 administration was associated with the increased presence of small inflammatory foci and necrotic hepatocytes in both infected and uninfected mice, suggesting a proinflammatory role for IL-12 in the liver.     

Effects of gonadotrophin on resistance to Listeria monocytogenes infection in mice.

It was demonstrated that exogenous GH suppressed the resistance to L. monocytogenes infection in Listeria resistant C57Bl/6 and susceptible A/J mice. However, different parameters of the immunological reaction to Listeria were affected by GH treatment in these mouse strains. In C57Bl/6 mice GH decreased accumulation of macrophages at the inflammatory site. On the contrary, a depression of anti-listerial activity of the phagocytes and a reduction of DTH reaction to Listeria antigen was demonstrated in GH treated A/J mice.     

Delayed hypersensitivity and acquired cellular resistance in guinea pigs infected with Listeria monocytogenes.

Randomly bred pigs of both sexes were injected intracardially with one-half of a 50% lethal dose of Listeria monocytogenes. When infected animals were skin tested with 30 mug of a water-soluble extract of sonically disrupted Listeria, both males and females had uniformly detectable levels of delayed hypersensitivity (DH) 4 days after infection. In males, cutaneous hypersensitivity to Listeria antigens reached a peak on day 5 or 6 of infection, and high levels of DH persisted through the 7th week. In females, DH reached a peak on day 6 or 7, remained at this level through the 4th week, and then dropped sharply. Cutaneous reactivity was usually higher for males than for females, and differences between the sexes were statistically significant 5, 6, and 7 weeks after infection. Low levels of DH were still present 41 weeks (females) or 46 weeks (males) after infection. Assays to determine the number of viable Listeria present in spleen homogenates indicated that bacterial multiplication occurred only during the first 24 hours of infection. The number of Listeria declined steadily thereafter, and by day 13 no bacteria could be recovered from the spleens of infected animals. Spleen assays indicated that Listeria-infected animals of both sexes were resistant to a small challenge dose of Listeria given 48 hours, 7 days, or 2 weeks after the primary infection. Resistance to re-infection was absent in females challenged at 41 weeks and in males challenged at 46 weeks.     

Suppression of host resistance against Listeria monocytogenes infection by 15-deoxyspergualin in mice

The effects of 15-deoxyspergualin (DSG), an immunosuppressive agent, on host resistance against Listeria monocytogenes were studied in mice. Administration of DSG in the early phase of infection resulted in fatal listeriosis by preventing acquired anti-listerial resistance, even though the infectious dose was sublethal for the untreated controls. In contrast, DSG treatment started after development of the acquired immunity was ineffective. Endogenous production of interferon-gamma (IFN-gamma) and tumour necrosis factor (TNF) in the bloodstreams induced by the infection was normal in DSG-treated mice. Nevertheless, augmentation of macrophage functions such as expression of major histocompatibility complex (MHC) class II antigens, phagocytic activity and listericidal activity induced by the infection was abrogated by DSG treatment. These results suggest that the inhibitory effect of DSG on anti-listerial resistance might be different from cyclosporine A (CsA).     

Human dendritic cells infected by Listeria monocytogenes: induction of maturation, requirements for phagolysosomal escape and antigen presentation capacity

An important feature of microbial infections is the ability of the microorganisms to interfere with and modulate the induction of host immune reactions. However, little is known about the effects of broad host range pathogens such as Listeria monocytogenes on similar cell types in different hosts. Here we examine the effects of the human and animal pathogen L. monocytogenes on human dendritic cells (DC) since this type of cells is essential for the initiation of immune responses. Listeria are phagocytosed efficiently by immature human DC and the bacteria escape from the phagolysosome quickly. Lack of the pore-forming activity of listeriolysin, which was found to be essential for the vacuolar escape of this bacterium in other cell types, retarded but did not prevent egress from the vacuole. Treatment of cultures of immature DC with L. monocytogenes resulted in rapid changes in morphology and cellular constitution followed by maturation of the DC. This could be judged by the appearance of maturation-specific cell surface markers. Antigen presentation to CD4 T cells was apparently not impaired by the infection. These results are in clear contrast to results obtained previously in the mouse system (Guzman et al., Mol. Microbiol. 1996. 20: 119 - 126; Darji et al., Eur. J. Immunol. 1997. 27: 1696 - 1703.).     

Host resistance of CD18 knockout mice against systemic infection with Listeria monocytogenes

Mice with targeted mutations of CD18, the common beta2 subunit of CD11/CD18 integrins, have leukocytosis, impaired transendothelial neutrophil emigration, and reduced host defense to Streptococcus pneumoniae, a gram-positive extracellular bacterium. Previous studies using blocking monoclonal antibodies suggested roles for CD18 and CD11b in hepatic neutrophil recruitment and host innate response to Listeria monocytogenes, a gram-positive intracellular bacterium. We induced systemic listeriosis in CD18 knockout (CD18-ko) and wild-type (WT) mice by tail vein injection with Listeria. By 14 days postinjection (dpi), 8 of 10 WT mice died, compared with 2 of 10 CD18-ko mice (P < 0.01). Quantitative organ culture showed that numbers of Listeria organisms in livers and spleens were similar in both groups at 20 min postinfection. By 3, 5, and 7 dpi, however, numbers of Listeria organisms were significantly lower in livers and spleens of CD18-ko mice than in WT mice. Histopathology showed that following Listeria infection, CD18-ko mice had milder inflammatory and necrotizing lesions in both spleens and livers than did WT mice. Cytokine assays indicated that baseline interleukin-1beta and granulocyte colony-stimulating factor (G-CSF) levels were higher in CD18-ko mice than in WT mice and that CD18-ko splenocytes produced higher levels of interleukin-1beta and G-CSF than WT splenocytes under the same amount of Listeria stimulation. These findings show that CD18 is not an absolute requirement for antilisterial innate immunity or hepatic neutrophil recruitment. We propose that the absence of CD18 in the mice results in the priming of innate immunity, as evidenced by elevated cytokine expression, and neutrophilic leukocytosis, which augments antilisterial defense.     

Transfer of resistance to primary infection of Listeria monocytogenes and early induction of delayed hypersensitivity by sera from L. monocytogenes-infected mice.

We found a new phenomenon which differs from previous reports on experimental listeriosis, that is, failure of passive transfer of serum from Listeria monocytogenes-infected mice to convey resistance to the bacterium. Transfer of immune serum from L. monocytogenes-infected mice markedly augmented resistance to the bacterium, and mechanisms of the transfer of L. monocytogenes-immune serum were investigated. Transfer of immune serum prevented L. monocytogenes lethality. This effect of the immune serum was transferred dose dependently. Augmentation of resistance to L. monocytogenes also appeared in elimination of bacteria from the spleen. The growth of bacteria within 2 days in the spleen was not inhibited. Transfer of the immune serum augmented and accelerated induction of a delayed footpad reaction. Delayed hypersensitivity-dependent accumulation of mononuclear cells, detected by focus formation reaction in the liver, was also augmented. In contrast, polymorphonuclear cell accumulation in the liver was suppressed. Development of delayed hypersensitivity reactions was correlated with the elimination of bacteria in the spleens. These effects of the immune serum were expressed antigen specifically; however, the effector molecule(s) in the immune serum differs from immunoglobulin molecules.     

Influence of cyclophosphamide on delayed hypersensitivity and acquired cellular resistance to Listeria monocytogenes in the mouse.

The effect of a single dose of cyclophosphamide (CY) on delayed type hypersensitivity (DH) and acquired cellular resistance (ACR) to Listeria monocytogenes infection in mice was studied. Intraperitoneal or intracutaneous immunization with L forms of L. monocytogenes did not result in protection against lethal challenge. A positive DH could be observed when CY-treated mice were intracutaneously immunized with 10(8) or more L forms. Intraperitoneal injection of viable L. monocytogenes resulted only in a narrow dose range in survival on immunization and partial protection on challenge. Protection was accompanied by DH. Intracutaneous injection of Listeria in Freund's complete adjuvant permitted the use of even 10(9) viable bacteria for immunization. This figure was reduced to 10(5) or less for CY treated mice. In normal mice protection was afforded on immunization with 10(7) bacteria whereas 10(3) bacteria were sufficient to protect CY treated animals. All protected mice showed a positive DH. These results demonstrate that CY treatment reduces the dose of viable bacteria tolerated for immunization 10(4) times. On the other hand after CY treatment the doses of bacteria effective on immunization for ACR and DH could be reduced in the same order of magnitude. Reduction of the CY dose resulted in a peak DH with 4 mg CY, but the protection was less than that obtained after treatment with 6 mg CY. A dissociation between ACR and DH was observed by varying the interval between immunization and challenge. In normal mice DH was preceded by ACR, with peaks at respectively 10 and 5 days after immunization. CY treatment caused a delay in the onset of the ACR, followed by an enhanced and slightly prolonged response. The effect of CY on DH consisted of enhancement and prolongation.