Induction by killed Listeria monocytogenes of effector T cells mediating delayed-type hypersensitivity but not protection in mice.

Using a local passive transfer system, we found that effector T cells mediating delayed-type hypersensitivity (DTH) but not acquired cellular resistance (ACR) to Listeria monocytogenes (strain EGD) were generated in mice immunized with killed Listeria, although immunized mice did not express DTH or ACR. When non-adherent cells of peritoneal, lymph node, or spleen cells from mice immunized with killed Listeria were transferred into the footpad of naive recipient mice along with eliciting antigen, positive delayed footpad reaction (DFR) was elicited. However, there was no evident protection against challenge at the site of the local transfer. Cells from mice immunized with viable Listeria conferred significant degrees of DFR and ACR on the recipients. DFR transferred by cells immunized with killed Listeria was mediated by L3T4+ T cells in an antigen-specific manner. The antigen-specific proliferative response of T cells from mice immunized with killed Listeria was much lower than that of T cells from mice immunized with viable Listeria. The production of macrophage chemotactic factor (MCF) by cells from killed Listeria-immune mice was much the same as that by cells from viable Listeria-immune mice. In contrast, the production of interleukin-2 (IL-2) and macrophage activating factor (MAF) was much lower in cells from killed Listeria-immune mice. The elimination of L. monocytogenes (strain L461), a strain of low virulence, was enhanced at the site of DFR transferred with cells from killed Listeria-immune mice. These results suggest that stimulation with killed bacteria is effective for the generation of DTH-mediating effector T cells, and that different effector T cells mediating DTH or ACR are involved in cell-mediated immunity to L. monocytogenes.     

Endogenous tumor necrosis factor alpha is required for enhanced antimicrobial activity against Toxoplasma gondii and Listeria monocytogenes in recombinant gamma interferon-treated mice.

In vitro studies have shown that macrophages stimulated with recombinant gamma interferon (rIFN-gamma) produce tumor necrosis factor alpha (TNF-alpha), which in an autocrine fashion activates these cells. The aim of the present study was to determine whether endogenously formed TNF-alpha also is required for rIFN-gamma-induced macrophage activation and enhanced antimicrobial activity in vivo. After an intraperitoneal injection of rIFN-gamma into CBA/J mice, their peritoneal macrophages released enhanced amounts of NO2- and inhibited the intracellular proliferation of Toxoplasma gondii. Injection of neutralizing antibodies against TNF-alpha simultaneously with the rIFN-gamma completely inhibited both the release of NO2- by macrophages and their toxoplasmastatic activity. Similar results were observed after intraperitoneal injection of a competitive inhibitor of L-arginine, NG-monomethyl-L-arginine, together with rIFN-gamma, demonstrating that in vivo L-arginine-derived reactive nitrogen intermediates are essential for the induction of toxoplasmastatic activity. Intravenous injection of rIFN-gamma inhibited the growth of Listeria monocytogenes in the livers and spleens of mice; this effect was abrogated by antibodies against TNF-alpha. Intravenous injection of a large dose of rTNF-alpha resulted in a decrease in the number of bacteria in the liver and spleen, but an injection of rIFN-gamma and rTNF-alpha did not result in enhanced inhibition of the proliferation of L. monocytogenes. Together, the results of the present study are the first to demonstrate that endogenous TNF-alpha is required in vivo for the expression of macrophage activation with respect to the release of reactive nitrogen intermediates and toxoplasmastatic activity and for enhanced listericidal activity in the livers and spleens of mice stimulated with rIFN-gamma.     

Separate and combined effects of recombinant interleukin-1 alpha and gamma interferon on antibacterial resistance.

Our laboratory has previously reported that administration of murine recombinant interleukin 1 alpha (rIL-1 alpha) substantially enhanced the resistance of mice to Listeria monocytogenes infection. Other investigators have reported that gamma interferon (IFN-gamma) plays a pivotal role in antilisteria resistance. In the present study, we have defined doses of human rIL-1 alpha that enhanced the antilisteria resistance of mice. We then addressed the possibility that combined immunotherapy with rIL-1 alpha and recombinant IFN-gamma (rIFN-gamma) might result in an additive or synergistic enhancement of antibacterial resistance. Simultaneous administration of rIL-1 alpha and rIFN-gamma enhanced antilisteria resistance (at 3 days after infection) to a greater extent than did either cytokine alone, although the results did not imply a synergistic action between the two cytokines. Experiments which examined the effects of the timing of cytokine administration indicated that maximal protection was observed when rIL-1 alpha and rIFN-gamma were administered together concomitantly with the L. monocytogenes challenge. When we compared the separate and combined protective effects of rIL-1 alpha and rIFN-gamma throughout the course of a primary L. monocytogenes infection, we observed an additive effect of the two cytokines only at 3 days after challenge, the time at which the peak bacterial burden occurs in the spleens and livers of infected mice. Histopathological comparisons of livers and spleens from cytokine-treated and control listeria-infected mice verified that cytokine treatment reduced the severity of tissue damage in cytokine-treated listeria-infected mice. In an attempt to provide a potential mechanism for the protective effects of rIL-1 alpha and rIFN-gamma administration, we compared levels of colony-stimulating activity in sera from cytokine-treated and control listeria-infected mice. The highest levels of colony-stimulating activity were detected in sera from control listeria-infected mice; somewhat lower levels were found in sera from listeria-infected mice that received rIL-1 alpha and rIFN-gamma either alone or in combination.     

Identity between human interferon-gamma and "macrophage-activating factor" produced by human T lymphocytes

Human peripheral blood monocytes purified by counterflow elutriation were activated in vitro by human natural or recombinant interferon-gamma (IFN-gamma) as shown by enhanced killing of Listeria monocytogenes and increased production of H2O2 in response to phorbol myristate acetate. Half-maximal stimulation for macrophage activation (MAF) was observed with 10-20 antiviral U/ml of purified recombinant IFN-gamma. These MAF activities were found to correlate with the antiviral activity dependent on IFN-gamma under several experimental conditions. Both activities were recovered together from supernatants of concanavalin A-stimulated peripheral blood mononuclear cells and in the media of a large number of T cell clones of different specificities. The parallelism between the two activities was also observed upon fractionation of culture media from producing cells and upon treatment of such preparations with low pH and high temperature. Finally, three antibodies with different specificities were found to abrogate the MAF and antiviral activities from lymphocyte culture supernatant. These results indicate that MAF released by stimulated lymphocytes is identical to IFN-gamma.     

Recombinant gamma interferon causes neutrophil migration mediated by the release of a macrophage neutrophil chemotactic factor

A dose-dependent neutrophil migration was observed following the injection of purified (Hu IFN-gamma) or recombinant (rIFN-gamma) human gamma interferon into rat peritoneal cavities. This finding contrasts with their inability to cause chemotaxis in vitro in the Boyden chamber. Neutrophil migration into peritoneal cavities and subcutaneous air pouches induced by both preparations of interferon was abolished by pretreatment of the animals with dexamethasone. IFN-gamma-induced neutrophil migration was enhanced when the macrophage population of the peritoneal cavities was increased by previous injection of thioglycollate and reduced by peritoneal lavage. Macrophage monolayers pretreated either with rIFN-gamma or with lipopolysaccharide from E. coli release into the supernatant a factor that stimulates neutrophil recruitment in animals treated with dexamethasone. Dexamethasone blocked this release but did not affect the neutrophil recruitment induced by this factor. These results suggest that IFN-gamma-induced neutrophil migration in vivo may be mediated by the release from resident macrophages of a neutrophil chemotactic factor and that dexamethasone blockade of neutrophil recruitment by IFN-gamma is due to inhibition of the release of this factor.     

Difference in the functional maturation of T cells against Listeria monocytogenes in lymph nodes and spleen.

After subcutaneous immunization of mice with viable Listeria monocytogenes (LM), we evaluated the function of T cells induced in draining lymph nodes (LN) and spleen as determined by the local transfer of delayed-type hypersensitivity (DTH), acquired cellular resistance (ACR) and in vitro lymphokine production. LN cells could transfer specifically DTH but not ACR. In contrast, spleen cells from the same donor mice evoked the DTH response as well as bacterial clearance at the reaction site. Comparison of bacterial counts in spleen and in LN upon subcutaneous inoculation of mice with LM suggested that the lack of bacterial proliferation in LN underlay the failure to induce protective T cells in this lymphoid tissue. Spleen and LN T cells expressed CD4 and CD8 surface antigens equally and DTH response was solely dependent on CD4+ cells. Another major difference between LN and spleen cells was the profile of lymphokines produced in vitro. Upon the in vitro restimulation with killed Listeria, immune spleen cells produced macrophage chemotactic factor (MCF), interleukin-2 (IL-2) and interferon-gamma (IFN-gamma). In contrast, LN cells could produce all of the measured lymphokines but not IFN-gamma. The results provided strong evidence for the dissociation of DTH and ACR. Listerial growth appeared to be the requirement for full maturation of anti-listerial immunity that may coincide with the generation of IFN-gamma-producing T cells.     

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.     

Intravenous injection of interferon-gamma inhibits the proliferation of Listeria monocytogenes in the liver but not in the spleen and peritoneal cavity.

In the present study the effects of intravenous administration of recombinant interferon-gamma (IFN-gamma) on both the proliferation of Listeria monocytogenes in the liver and spleen of mice and the listericidal activity of their peritoneal macrophages were investigated. A single intravenous injection of 1 x 10(6) U or three injections of 2 x 10(5) U recombinant IFN-gamma (rIFN-gamma) induced optimal activation of resident and exudate peritoneal macrophages, as judged by their ability to inhibit the intracellular proliferation of Toxoplasma gondii and their enhanced release of H2O2 and NO2-. The rate of intracellular killing of L. monocytogenes by the rIFN-gamma-activated resident and exudate macrophages was not higher than that by resident macrophages. Addition of 10 ng lipopolysaccharides (LPS) to the rIFN-gamma also did not enhance the bactericidal activity of the activated peritoneal macrophages. The decrease in the number of L. monocytogenes in the peritoneal cavity of mice that had received an i.p. injection of 1 x 10(4) U rIFN-gamma was similar to that in control mice. Intravenous administration of 1 x 10(5) rIFN-gamma activated cells in the liver, as indicated by the increased expression of Ia antigen, and reduced the rate of proliferation of L. monocytogenes in the liver relative to that in control mice when 0.1 LD50 or 1 LD50 L. monocytogenes were injected. However, when 10 LD50 L. monocytogenes were administered there was no effect on their proliferation. The number of L. monocytogenes found initially in the spleen of rIFN-gamma-treated mice was 20-30% of that in the spleen of control mice, but the rate of proliferation of L. monocytogenes was not reduced. These divergent results for the proliferation of L. monocytogenes in the liver, spleen and peritoneal cavity indicate that cells other than macrophages and/or as yet unknown local factors play an important role in the listericidal activity.     

Transient control of interleukin-4-producing natural killer T cells in the livers of Listeria monocytogenes-infected mice by interleukin-12.

Unconstrained development of gamma interferon (IFN-gamma)-secreting natural killer (NK) cells and T helper (Th) 1 cells is central to protection against Listeria monocytogenes. In contrast, interleukin 4 (IL-4) is considered harmful. IL-12 produced by infected macrophages promotes, and IL-4 interferes with, protective antilisterial immunity. The liver NK T lymphocytes, which are a potent source of IL-4, are downregulated at an intermediate stage of listeriosis. Here we demonstrate that endogenous IL-12 participates in the control of IL-4-producing liver NK T lymphocytes during listeriosis. The effects of L. monocytogenes infection on IL-4-producing liver NK T lymphocytes were reversed by antibody neutralization of IL-12 but not of IFN-gamma or tumor necrosis factor alpha (TNF-alpha). IL-4 production by liver NK T lymphocytes was virtually unaffected by heat-killed L. monocytogenes (HKL). Viable L. monocytogenes markedly increased the numbers of IL-12 producers in livers in parallel with an increase in macrophage numbers, whereas HKL failed to do so with similar efficiency. These results indicate that in the liver endogenous IL-12 improves protective immunity against listeriosis by downregulating IL-4-producing NK T lymphocytes. Moreover, our findings that HKL have a low level of IL-12-inducing activity and fail to control IL-4-producing NK T lymphocytes in the liver are consistent with the lesser protective capacity of HKL compared to that of live listeriae.     

Requirement of the initial production of gamma interferon in the generation of protective immunity of mice against Listeria monocytogenes.

Protective immunity of mice against Listeria monocytogenes, which is mediated mainly by gamma interferon (IFN-gamma)-producing T cells, was induced by immunization with viable bacteria but not with killed bacteria. By comparing mice immunized with either viable or killed L. monocytogenes, it was found that IFN-gamma was produced at the initial stage only after immunization with viable bacteria. This finding prompted us to investigate the effect of neutralizing the IFN-gamma on the final generation of protective T cells against L. monocytogenes. When endogenous IFN-gamma was neutralized by administration of anti-IFN-gamma monoclonal antibody for the initial 2 days in mice immunized with viable bacteria, the generation of protective T cells on day 6 was completely blocked, as revealed by T-cell adoptive transfer. The generation of listeria-specific IFN-gamma-producing T cells was also abolished. These results clearly demonstrated that endogenous IFN-gamma, which is produced at the initial stage of immunization, actually plays a critical role in the generation of protective T cells against L. monocytogenes in vivo. Moreover, this study suggested that the lack of IFN-gamma-inducing ability is responsible for the inability of killed L. monocytogenes to induce protective T cells in mice.     

Ubiquicidin, a novel murine microbicidal protein present in the cytosolic fraction of macrophages.

Previously we have identified and characterized three murine microbicidal proteins purified from the granule fraction of cells from the murine macrophage cell line RAW264.7. During these studies evidence was obtained for the presence of an additional antimicrobial protein in the cytosolic fraction of RAW264.7 cells that had been activated with interferon-gamma (IFN-gamma). In this study we have purified this protein, designated ubiquicidin, to apparent homogeneity and demonstrated that it is a cationic, small (Mr 6654) protein. Ubiquicidin displayed marked antimicrobial activity against Listeria monocytogenes and Salmonella typhimurium. Using a gel overlay procedure evidence was obtained that the protein also displays activity against Escherichia coli, Staphylococcus aureus, and an avirulent strain of Yersinia enterocolitica. Aminoterminal amino acid sequencing and mass spectrometry analysis of purified ubiquicidin indicated that it is most likely identical to the ribosomal protein S30. This protein is produced by posttranslational processing of the Fau protein, a 133-amino-acid fusion protein consisting of S30 linked to an unusual peptide with significant homology to ubiquitin. The fau gene has been reported to be expressed in a variety of tissues in humans and various animal species. The presence of ubiquicidin in the cytosol of macrophages may serve to restrict the intracellular growth of microorganisms. In addition, because macrophage disintegration will likely lead to release of ubiquicidin into the extracellular environment, it may contribute to host defense after macrophage death.     

The role of gammadelta T cells in induction of bacterial antigen-specific protective CD8+ cytotoxic T cells in immune response against the intracellular bacteria Listeria monocytogenes.

The role of T-cell receptor (TCR) gammadelta T cells in the induction of protective TCR alphabeta T cells against infection by the intracellular bacteria Listeria monocytogenes was analysed. We found that depletion of gammadelta T cells by anti-TCR delta monoclonal antibody treatment before intravenous immunization of mice with a sublethal dose of viable L. monocytogenes resulted in reduction of protection against secondary challenge infection in the immunized mice. The gammadelta T-cell depletion also reduced induction of protective alphabeta T cells capable of transferring the protection against challenge infection of L. monocytogenes into naive mice. Furthermore, the protective T cells that were affected by the gammadelta T-cell depletion were suggested to be CD8+ cytotoxic T cells rather than CD4+ T cells by the following observations. First, induction of cytotoxic T lymphocytes specific to a L. monocytogenes-derived H-2Kd-restricted peptide (listeriolysin O 91-99) was significantly suppressed by gammadelta T-cell depletion before immunization. Second, gammadelta T-cell depletion did not affect cytokine production and proliferation of T cells from immunized mice in response to in vitro stimulation with heat-killed Listeria which preferentially stimulates CD4+ T cells. Third, CD8+ alphabeta T cells from control immunized mice transferred protection against infection of L. monocytogenes into naive mice but only a limited degree of protection was transferred by CD8+ T cells from the gammadelta T-cell-depleted immunized mice; and fourth, CD4+ alphabeta T cells from the gammadelta T-cell-depleted mice transferred a similar level of protection as those from the control immunized mice. All these results suggest that gammadelta T cells participate in establishment of protective immunity against intracellular bacteria by supporting priming of bacterial antigen-specific CD8+ cytotoxic T cells.     

Interactions between endogenous gamma interferon and tumor necrosis factor in host resistance against primary and secondary Listeria monocytogenes infections.

Intravenous injection of rat anti-mouse gamma interferon (IFN-gamma) monoclonal antibody as well as rabbit anti-mouse tumor necrosis factor (TNF) antibody into mice which had received a sublethal infection with Listeria monocytogenes cells resulted in acceleration of listeriosis. Endogenous IFN-gamma seemed to be produced early in infection, because suppression of antilisterial resistance was significant when a single injection of anti-IFN-gamma monoclonal antibody was given on day 0 or day 1 of infection. Production of TNF but not of IFN-gamma in the bloodstream early in infection was inhibited by administration of anti-IFN-gamma monoclonal antibody. The suppressive effect of anti-IFN-gamma and anti-TNF antibodies on antilisterial resistance was not augmented by simultaneous administration of these antibodies. On the other hand, in the secondary infection, simultaneous administration of anti-IFN-gamma and anti-TNF antibodies, but not of either of these antibodies alone, into L. monocytogenes-immune mice resulted in high mortality and explosive multiplication of bacterial cells in the spleens and livers. These results suggest that endogenously produced IFN-gamma and TNF are both essential to the host defense against L. monocytogenes infection and that these cytokines might act by different modes between the primary infection and the secondary infection.     

Enhancement of host resistance against Listeria infection by Lactobacillus casei: role of macrophages.

Among the 10 species of the genus Lactobacillus, L. casei showed the strongest protective action against Listeria monocytogenes infection in mice. The activity of L. casei differed with regard to the dose of administration. The anti-L. monocytogenes resistance in mice intravenously administered 5.5 X 10(7), 2.8 X 10(8), or 1.1 X 10(9) L. casei cells was most manifest at ca. 2, 2 and 13, and 3 to 21 days after its administration, respectively. The growth of L. monocytogenes in the liver of mice injected with L. casei (10(7), 10(8), or 10(9) cells) 48 h after infection was suppressed, particularly when 10(8) or 10(9) L. casei cells were given 2 or 13 days before the induced infection, respectively. This suppression of L. monocytogenes growth was overcome by carrageenan treatment or X-ray irradiation. [3H]thymidine incorporation into the liver DNA increased 13 days after administration of L. casei, and augmentation of [3H]thymidine incorporation during 6 to 48 h after infection was dependent on the dose of L. casei. Peritoneal macrophage accumulation observed 1 to 5 days after intraperitoneal injection of UV-killed L. monocytogenes was markedly enhanced when the mice were treated with L. casei cells 13 days before macrophage elicitation. Therefore, the enhanced host resistance by L. casei to L. monocytogenes infection may be mediated by macrophages migrating from the blood stream to the reticuloendothelial system in response to L. casei injection before or after L. monocytogenes infection.     

Endogenous tumor necrosis factor (cachectin) is essential to host resistance against Listeria monocytogenes infection.

During a sublethal murine infection with Listeria monocytogenes cells, tumor necrosis factor (TNF) activity was detectable in neither sera nor spleen homogenates at any stage of the infection when a bioassay with L-929 cells (less than 4 U/ml) was used. However, injecting the mice with an immunoglobulin fraction obtained from a rabbit hyperimmunized with recombinant murine TNF-alpha resulted in acceleration of listeriosis. When 1 mg of anti-TNF antibody was injected per mouse, all the mice died from listeriosis, even though the infectious dose was sublethal for the untreated controls. The antigen-specific elimination of the bacterium from the spleens and livers of anti-TNF antibody-treated mice was delayed, depending on the dose of the antibody injected. Endogenous TNF seemed to be produced early in infection, because suppression of antilisterial resistance was significant when a single injection of anti-TNF antibody was given between day zero and day 2 of infection. The effect of endogenous TNF on antilisterial resistance was due to neither regulation of alpha interferon (IFN-alpha) and IFN-gamma production nor induction of IFN-beta subtype 1 (IFN-beta 1), because anti-TNF antibody treated-mice produced normal levels of IFN-alpha and IFN-gamma in the bloodstream during infection and administration of monoclonal anti-murine IFN-beta 1 antibody had no effect on the development of listeriosis. Alternatively, the listericidal activity of peritoneal macrophages of L. monocytogenes-infected mice could be abrogated by injection of anti-TNF antibody in vivo. These results suggest that the lower level of TNF is produced endogenously in mice that received L. monocytogenes infection and that it plays an essential role in the host defense against L. monocytogenes infection.     

Dissociated development of T cells mediating delayed-type hypersensitivity and protective T cells against Listeria monocytogenes and their functional difference in lymphokine production.

CD4+ T cells mediating both delayed-type hypersensitivity (DTH) and acquired cellular resistance (ACR) were generated in mice after immunization with viable Listeria monocytogenes. In contrast, CD4+ T cells from mice immunized with killed L. monocytogenes in complete Freund's adjuvant were capable of mediating only DTH but not ACR. To determine the functional difference between T cells mediating DTH and T cells mediating ACR, we examined two different populations of T cells for profiles of lymphokine production after stimulation with a specific antigen in vitro. The production of interleukin-2 (IL-2) and IL-3 but not IL-4 was observed in both T cells mediating only DTH and those mediating DTH and ACR. In this respect, both types of T cells could be categorized into the TH1 population, and they produced macrophage chemotactic factor equally well. However, the production of gamma interferon (IFN-gamma) was observed only in T cells capable of mediating both DTH and ACR. This result was confirmed not only by an enzyme immunoassay specific for murine IFN-gamma but also by Northern (RNA) analysis for the detection of IFN-gamma mRNA. These results suggested that the TH1 population may be subdivided further into two distinct subsets and that the ineffectiveness of the killed bacterial vaccine may be partly explained by the dissociated development of T cell function.     

In vitro propagation of antigen-specific T lymphocytes that adoptively transfer resistance to Listeria monocytogenes

Murine T cells generated against heat-killed Listeria monocytogenes or Listeria intracellular product (LIP) were propagated in a source of Interleukin 2. Both T-cell cultures were greater than 98% Lyt 1+, 2/3- and proliferated specifically against LIP and L. monocytogenes crude whole-cell antigen in vitro. Proliferation of both T-cell cultures required the presence of antigen and accessory cells syngeneic to the T cells at the left end of the major histocompatibility complex. The ability of these cultures to adoptively transfer protection against challenge with viable Listeria cells was dramatically different. As few as 10(6) LIP-specific T cells conferred significant protection against a lethal challenge of Listeria cells, whereas cultures induced against crude whole-cell antigen showed little or no protective function. The resistance conferred by LIP-specific T cells was specific in that the cells did not reduce the mortality seen after challenge with Salmonella typhimurium.     

IL-12 is dispensable for innate and adaptive immunity against low doses of Listeria monocytogenes

We have studied IL-12p35-deficient (IL-12p35(-/-)) mice to evaluate the role of IL-12 in resistance against Listeria monocytogenes. In the absence of bioactive IL-12p75, mutant mice acquired higher bacterial organ burden than wild-type mice and died during the first week following infection with normally sublethal doses of Listeria. Moreover, blood IFN-gamma levels were strikingly reduced in mutant mice at day 2 post-infection. These results suggest that in IL-12p35-deficient mice impaired production of IFN-gamma which is crucial for activation of listericidal effector functions of macrophages leads to defective innate immunity against Listeria. In contrast to mice deficient for IFN-gamma or IFN-gamma receptor which are unable to resist very low infection doses of Listeria, IL-12p35(-/-) mice resisted up to 1000 c.f.u. and were able to eliminate Listeria. Spleen cells from mutant mice re-stimulated with heat-killed Listeria produced considerable amounts of IFN-gamma, suggesting that at low dose infection sufficient IFN-gamma is produced independently of IL-12. Subsequent challenge of these immunized mice with high doses of L. monocytogenes resulted in sterile elimination demonstrating efficient memory responses. These results demonstrate for the first time that at low doses of Listeria IL-12 is neither critical for innate immunity nor for the development of protective T cell-dependent acquired immunity.     

Effect of macrophage activation on killing of Listeria monocytogenes. Roles of reactive oxygen or nitrogen intermediates, rate of phagocytosis, and retention of bacteria in endosomes.

The role of macrophage activation in the killing of L. monocytogenes is unclear. Some studies suggest that activation for enhanced production of reactive oxygen and nitrogen intermediates may not be of central importance. Recent data have indicated an important role for interferon-gamma (IFN-gamma) induced retention of L. monocytogenes in endosomes. Data from the present study indicate that proteose peptone-elicited macrophages from DBA2/J, CD-1, and C3H/HeN mice are listericidal. Activation of these cells in vitro for 20 h by IFN-gamma (20 or 500 U/ml) increased H2O2 or nitrite production, but did not increase the number of L. monocytogenes killed during a subsequent 6-h or 7-h culture. Incubation of macrophages with IFN-gamma plus lipopolysaccharide (LPS) caused greater activation and increased the number of Listeria killed during a 6-h or 7-h culture. However, this seems primarily attributable to enhanced phagocytosis. Proteose peptone-elicited macrophages were significantly more effective than resident macrophages in preventing the escape of L. monocytogenes from endosomes into the cytoplasm. This capability was not significantly enhanced by IFN-gamma in vitro, but was enhanced by IFN-gamma plus LPS. This correlates well with the effects of these activation stimuli on killing of L. monocytogenes by proteose peptone-elicited macrophages. These results indicate that enhanced retention of L. monocytogenes in endosomes is induced by proteose peptone elicitation and that further macrophage activation in vitro by IFN-gamma does not improve listericidal activity.