Lipopolysaccharide-related stimuli induce expression of the secretory leukocyte protease inhibitor, a macrophage-derived lipopolysaccharide inhibitor

Mouse secretory leukocyte protease inhibitor (SLPI) was recently characterized as a lipopolysaccharide (LPS)-induced product of macrophages that antagonizes their LPS-induced activation of NF-kappaB and production of NO and tumor necrosis factor (TNF) (F. Y. Jin, C. Nathan, D. Radzioch, and A. Ding, Cell 88:417-426, 1997). To better understand the role of SLPI in innate immune and inflammatory responses, we examined the kinetics of SLPI expression in response to LPS, LPS-induced cytokines, and LPS-mimetic compounds. SLPI mRNA was detectable in macrophages by Northern blot analysis within 30 min of exposure to LPS but levels peaked only at 24 to 36 h and remained elevated at 72 h. Despite the slowly mounting and prolonged response, early expression of SLPI mRNA was cycloheximide resistant. Two LPS-induced proteins-interleukin-10 (IL-10) and IL-6-also induced SLPI, while TNF and IL-1beta did not. The slow attainment of maximal induction of SLPI by LPS in vitro was mimicked by infection with Pseudomonas aeruginosa in vivo, where SLPI expression in the lung peaked at 3 days. Two LPS-mimetic molecules-taxol from yew bark and lipoteichoic acid (LTA) from gram-positive bacterial cell walls-also induced SLPI. Transfection of macrophages with SLPI inhibited their LTA-induced NO production. An anti-inflammatory role for macrophage-derived SLPI seems likely based on SLPI's slowly mounting production in response to constituents of gram-negative and gram-positive bacteria, its induction both as a direct response to LPS and as a response to anti-inflammatory cytokines induced by LPS, and its ability to suppress the production of proinflammatory products by macrophages stimulated with constituents of both gram-positive and gram-negative bacteria.     

Wound healing of intestinal anastomosis after digestive surgery under septic conditions: participation of local interleukin-6 expression

This study aimed to evaluate the integrity of anastomotic wound healing after digestive surgery under septic conditions and to observe local interleukin-6 (IL-6) expression around the anastomotic segment. Experimental animals were separated into lipopolysaccharide (LPS) and control groups. Each was injected with LPS or normal saline solution into the peritoneal cavity 24 hours before transection and anastomosis of the colon. The anastomotic bursting pressure (ABP) and tissue hydroxyproline concentration (HP) were measured as indicators of wound healing. Immunohistochemical staining for IL-6 was performed on tissue samples obtained from the anastomotic segment, lung, liver, and kidney. The reactive cells were counted by light microscopy. The ABP and HP were significantly lower in the LPS group than the control group 7 days after the surgery. In the LPS group, IL-6 expression around the anastomotic segment was enhanced 1 and 6 hours after surgery but suppressed 24 hours afterward. In contrast, IL-6 expression in lung, liver, and kidney was enhanced in the LPS group 24 hours after surgery but not in the control group. It is suggested that anastomotic wound healing is impaired after digestive tract surgery under septic conditions, and local IL-6 expression participates in wound healing.     

Pattern of cytokines and pharmacomodulation in sepsis induced by cecal ligation and puncture compared with that induced by endotoxin

The production of tumor necrosis factor alpha (TNF-alpha), interleukin-1 beta (IL-1 beta), and IL-6 and their pharmacomodulation were evaluated in a model of polymicrobial sepsis induced in mice by cecal ligation and puncture (CLP) and were compared with the effects of endotoxin (lipopolysaccharide [LPS]) treatment. LPS levels rose as early as 1 h after CLP and increased further after 2 and 21 h. TNF-alpha was detectable in serum, spleen, liver, and lungs during the first 4 h, with a peak 2 h after CLP. IL-1 beta was measurable in serum after 24 h, and levels increased significantly in spleen and liver 4 and 8 h after CLP. IL-6 levels increased significantly in serum throughout the first 16 h after CLP. These cytokines were detectable after LPS injection, with kinetics similar to those after CLP but at a significantly higher level. To cast more light on the differences between these two animal models of septic shock, we studied the effects of different reference drugs. Pretreatment with dexamethasone (DEX); ibuprofen (IBU), an inhibitor of cyclooxygenase; and NG-nitro-L-arginine, an inhibitor of nitric oxide synthase, significantly reduced survival, while chlorpromazine (CPZ) and TNF did not affect it. Only the antibiotics and pentoxifylline significantly increased survival in mice with CLP. However, CPZ and DEX protected the mice from LPS mortality. On inhibiting TNF-alpha with DEX, CPZ, or pentoxifylline, survival was reduced, unchanged, and increased, respectively, and on increasing TNF-alpha with IBU and TNF, survival was decreased or unchanged, respectively, suggesting that the modulation of this cytokine does not play a significant role in sepsis induced by CLP, unlike treatment with LPS. The negative effects of IBU and N(G)-nitro-L-arginine suggest a protective role by prostaglandins and nitric oxide in sepsis induced by CLP.     

Hepatocyte growth factor in glycerol-induced acute renal failure

Hepatocyte growth factor (HGF) facilitates the regeneration of injured kidney in acute renal failure (ARF). Here we investigated the HGF production in glycerol-induced ARF rats. HGF mRNA expression levels were elevated in liver, spleen, and lung 6-24 h after glycerol injection. Tissue HGF protein levels determined by an enzyme-linked immunosorbent assay also increased in liver and spleen, whereas they decreased in the injured kidney 24 h after injection. Immunohistochemical studies showed that the number of HGF-producing cells did not increase in the liver. HGF receptor/c-Met mRNA levels were elevated only in the kidney. These results indicate that HGF supplied in an endocrine manner may play an important role in the regenerating process following ARF.     

Production of inflammatory mediators by human macrophages obtained from ascites

Ascites is a readily available source of human macrophages (M phi), which can be used to study M phi functions in vitro. We characterized the mediators of inflammation produced by human peritoneal M phi (hp-M phi) obtained from patients with portal hypertension and ascites. The production of the cytokines interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) was found to be lipopolysaccharide (LPS) concentration dependent (0-10 micrograms/ml) with a maximal production at 10 micrograms/ml and also dependent on the time of exposure to the stimulus (0-36 h). IL-1 beta, IL-6 and TNF-alpha production after LPS administration reached a plateau at 24 h. In vitro stimulation for 24 h with LPS does not influence the eicosanoid production from endogenous arachidonate. 13 min of exposure of the cells to the calcium ionophore A23187 gives a significant increase in eicosanoid production from both exogenous and endogenous arachidonate. The main eicosanoids produced are the 5-lipoxgenase products LTB4 and 5-hydroxyeicosatetraenoic acid (HETE). The increase in production of the other eicosanoids is not significant. The eicosanoid production depends on the stimulus concentration. The optimal A23187 concentration is 1 microM. Oxygen radical production was measured in the M phi by a flowcytometric method. The fluorescence intensity of phorbol 12-myristate 13-acetate stimulated and dihydro-rhodamine 123 loaded hp-M phi increases significantly after 15 min. We conclude that LPS stimulation of hp-M phi from liver disease results in similar production of IL-1 beta, IL-6 and TNF-alpha, but that the profile of the eicosanoid production of these M phi stimulated with LPS and A23187 differs from M phi of other origin and species.     

In vitro IL-1 beta and TNF-alpha release from THP-1 monocytes in response to metal ions

OBJECTIVES: Previous studies have shown that biomaterials can activate macrophages to produce cytokines and promote an inflammatory response. Although the toxicity of many metal ions has been extensively investigated, little is known about the ability of these ions to alter cytokine release from macrophages. Yet the release of these ions from biomaterials has been well documented. Previous studies indicated that alterations in cytokine release might be expected because metal ions alter protein production in macrophages at sub-toxic concentrations. Thus, the hypothesis of this study was that metal ions can alter the secretion of cytokines from macrophages at sub-toxic concentrations. METHODS: The release of interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) from macrophages was investigated when the macrophages were exposed to metal ions, with or without lipopolysaccharide (LPS), a component of dental plaque. Human THP-1 macrophages were exposed to ions of Ag, Au, Cu, Hg, and Ni for 24 h. In half of the cultures, LPS was added for the last 4 h. The release of IL-1 beta and TNF-alpha into the medium was measured using enzyme-linked immunosorbent assays. ANOVA and Tukey multiple comparison intervals were used to compare the various experimental conditions. RESULTS: None of the metal ions elevated the IL-1 beta or TNF-alpha levels after 24 h, but Ni ions significantly elevated the IL-1 beta and TNF-alpha levels after 72 h. With LPS added, Ag, Cu, and Ni significantly amplified the LPS-induced production of IL-1 beta but only Ni amplified the TNF-alpha response. These alterations in cytokine response occurred with metal ion concentrations which have been previously shown to be released from dental alloys in vitro and in vivo. SIGNIFICANCE: It appeared plausible that macrophage-cytokine mediated inflammatory responses may be altered by the presence of some metal ions in tissues, particularly Ni.     

Hypercortisolemia increases plasma interleukin-10 concentrations during human endotoxemia--a clinical research center study

Hypercortisolemia directly before the administration of endotoxin (LPS) to normal humans completely prevents the release of the proinflammatory cytokine tumor necrosis factor, whereas hypercortisolemia 12 h to 7 days before the injection of LPS is associated with enhanced tumor necrosis factor release. To determine the effect of elevated cortisol levels on the secretion of the antiinflammatory cytokine interleukin-10 (IL-10), 23 healthy men were given iv LPS (lot EC-5; 2 ng/kg) alone or in combination with a continuous iv infusion of hydrocortisone (3 micrograms/kg.min) for 6 h immediately before or 6, 12, or 144 h before LPS injection. LPS induced a monophasic increase in plasma IL-10 concentrations that peaked after 2 h (162 +/- 27 pg/mL; P < 0.0001). In subjects who were infused with hydrocortisone directly before LPS administration, IL-10 concentrations were much higher (1784 +/- 331 pg/mL; P < 0.0001 vs. LPS only), whereas hypercortisolemia 6, 12, or 144 h before LPS injection did not influence LPS-induced IL-10 levels. In human whole blood in vitro, hydrocortisone caused a dose-dependent reduction of LPS-induced IL-10 levels. Further, hydrocortisone reversed the increase in IL-10 concentrations by epinephrine in LPS-stimulated whole blood. Stimulation of IL-10 release may contribute to the antiinflammatory properties of glucocorticoids.     

Erratum to "Immunocytochemical detection of cytokines and chemokines in Langerhans cells and in vitro derived dendritic cells"

We have developed a direct immunocytochemical technique to identify cytokine and chemokine production in epidermal Langerhans cells (LC) and in vitro derived CD14-, CD1a+, CD83+, CD40+ dendritic cells (DC) at the single cell level. Formaldehyde fixation combined with saponin permeabilization preserved cellular morphology and generated a characteristic juxtanuclear staining signal due to the accumulation of cytokine to the Golgi organelle. This approach was used for the assessment of TNF-alpha, IL-6, IL-8, IL-10, IL-12, GM-CSF, MIP-1alpha, MIP-1beta and RANTES producing cells. In contrast, a diffuse cytoplasmic staining was evident for IL-1ra, IL-1alpha and IL-1beta production. IL-1ra and IL-1alpha were expressed in 10-25% of unstimulated cultured cells, while all the other cytokines were undetectable. IL-1ra, IL-1alpha and IL-1beta were also the dominating cytokines, expressed in up to 85% of the DC, after 3 h of LPS stimulation. A significantly lower number of cells (0-5%) synthesized TNF-alpha, IL-6, IL-10, IL-12 and GM-CSF. The incidence of chemokine producing cells (IL-8, RANTES, MIP-1alpha, MIP-1beta) peaked 10 h after LPS stimulation in up to 60% of the DC. Both immature CD83- and mature CD83+ DC as well as LC had a similar cytokine production pattern. Thus, in comparison to monocytes, LPS stimulation of DC generated a lower incidence of TNF-alpha, IL-6, IL-10 and IL-12 producing cells while IL-1 was expressed in a comparable number of cells.     

Increase in histamine synthesis by liver macrophages in CCl4-injured mast cell-deficient W/Wv mice

This study set out to examine the possible role of liver macrophages in histamine synthesis in the injured liver. The effects of the hepatotoxins Escherichia coli lipopolysaccharide (LPS) and CCl4 on histamine synthesis in the liver of mice were evaluated. C3H/HeJ mice were resistant to LPS in including histidine decarboxylase (HDC) in the liver compared with C3H/HeN mice and mast cell-deficient W/Wv mice. However, C3H/HeJ mice did respond strongly to another hepatotoxin, CCl4, leading to a significant increase in HDC activity. CCl4 also caused a marked increase in HDC activity and histamine levels in the liver of W/Wv mice. In addition, injection of CCl4 produced a large increase in the activity of HDC in the spleen and lung of W/Wv mice. HDC activity was confined to the nonparenchymal cells, with parenchymal cells expressing essentially no HDC activity. The CCl4-induced increase in HDC activity was confined, at least in part, to the liver macrophages. These results indicate that the macrophages are responsible for the increase in HDC-dependent histamine production in the liver caused by the injection of hepatotoxins. The possible role of histamine in liver regeneration after injury is discussed.     

Insulin-like growth factor-binding proteins: functional diversity or redundancy?

The time course of expression of TNF-alpha in myocardial wound healing following ischemic injury was investigated in the porcine heart. Microembolization was used to induce focal ischemia and necrosis in hearts of 39 adult pigs. The animals were sacrificed after 3, 6, 12, 24 h, 3 and 7 days, and after 4 weeks, and the myocardial tissue was studied by immunofluorescence using specific antibodies. TNF-alpha containing cells were identified as monocytes/macrophages by double staining with a muramidase antibody. Monocytes/macrophages were the only source of TNF-alpha. Microembolization caused multiple necrotic foci with loss of myocytes in the left ventricular myocardium. These foci contained numerous monocytes/macrophages and showed an inflammatory reaction typical of wound healing followed by replacement with scar tissue. The number of TNF-alpha positive cells increased after 24 h, peaked between 3-7 days and slowly decreased thereafter. Expression of TNF-alpha in monocytes/macrophages was significantly reduced after pretreatment of pigs with cyclosporine or dexamethasone. It is concluded that 1.) in myocardial tissue monocytes/macrophages are the only cell type expressing TNF-alpha, 2.) TNF-alpha is involved in wound healing after ischemia, and 3.) synthesis of TNF-alpha and inflammatory angiogenesis can be inhibited be treatment with either cyclosporine or dexamethasone.     

Divergent effects of LPS on expression of IL-1 receptor family members in mononuclear phagocytes in vitro and in vivo

Three molecules, interleukin 1 (IL-1) receptor I (IL-1RI), IL-1 receptor II (IL-1RII or decoy) and IL-1 receptor accessory protein (IL-1R AcP or IL-1RIII), are involved in IL-1 binding and signal transduction. In addition, three homologous genes (T1/ST2, MyD88 and rsc786) have been identified. Expression of the signal transducing type I R and of the decoy type II R in human monocytes is regulated by pro- and anti-inflammatory signals. The present study was designed to evaluate comprehensively how a prototypic pro-inflammatory signal, bacterial lipopolysaccharide (LPS), affects expression of IL-1R family members in mononuclear phagocytes in vitro and in vivo. Resting human monocytes expressed high levels of IL-1RII, IL-1R AcP, MyD88 and rsc786, whereas low levels of IL-1RI and T1/ST2 were present. In vitro exposure to LPS augmented expression of IL-1RI, T1/ST2 and MyD88, whereas it inhibited that of IL-1RII and rsc786. Expression of IL-1R AcP in monocytes was less substantially affected by LPS. The expression of IL-1R family members was also studied in organs of mice given LPS. As expected on the basis of in vitro results, organs (e.g. spleen, lungs and peritoneal exudate cells) from LPS-treated mice showed increased levels of IL-1RI, T1/ST2 and MyD88. Intriguingly, while expression of IL-1RII was inhibited in peritoneal macrophages after LPS, in accordance with in vitro results, increased IL-1RII mRNA was observed in organs such as liver, lungs and spleen. This unexpected effect of LPS was drastically reduced in mice rendered neutropenic by 5-fluorouracil. Therefore, we conclude that the apparent induction of IL-1RII in certain organs of LPS-treated mice is due to recruitment of myeloid cells which express high levels of decoy RII. Therefore, members of IL-1R family are independently and divergently regulated in mononuclear phagocytes exposed to the prototypic pro-inflammatory signal LPS in vitro and in vivo.     

Cytokine induction in murine macrophages infected with virulent and avirulent Rhodococcus equi

To look for a possible correlation between the virulence of Rhodococcus equi and its cytokine-inducing capacity, we evaluated intracellular survival and measured cytokine induction by mouse macrophages infected with a virulent strain containing an 85-kb plasmid and expressing VapA (103+), its avirulent plasmid-cured derivative (103-), and heat-killed 103+ (HK). After incubation with similar numbers of bacteria, macrophages infected with 103- contained significantly more organisms than those infected with 103+ or HK. The number of bacteria in the macrophages infected with 103- and HK decreased progressively, whereas the 103+ numbers remained constant over 48 h. Interleukin 1beta (IL-1beta), IL-6, IL-10, IL-12 p40, and tumor necrosis factor alpha (TNF-alpha) mRNA induction peaked at 4 h and returned to baseline between 12 and 48 h postinfection. IL-1beta, IL-6, IL-10, and TNF-alpha concentrations assessed by enzyme-linked immunosorbent assay generally agreed well with mRNA expression; IL-12 could, however, not be detected. For all the cytokines detected, mean concentrations in the supernatants were consistently higher in the 103(-)-infected monolayers than in those infected with 103+, although, with the exception of IL-1beta, the differences were not statistically significant. R. equi HK was a poor inducer of cytokine production. In conclusion, virulent and avirulent R. equi strains induced similar levels of cytokine synthesis. The slightly greater induction of most cytokines observed following infection with 103- is likely secondary to greater uptake by macrophages rather than to a direct role of VapA or another plasmid-encoded product in downregulating cytokine induction.     

Abnormal development and differentiation of macrophages and dendritic cells in viable motheaten mutant mice deficient in haematopoietic cell phosphatase

In mice homozygous for the 'viable motheaten' (mev) mutation, numbers of macrophage progenitor cells, particularly monocytes, were markedly increased in the bone marrow and spleen. Increased mobilization of these precursor cells to peripheral tissues and their differentiation to macrophages were evidenced by striking increases in macrophage numbers. Immunohistochemical double staining of tissue sections and flow cytometry analyses of single cell suspensions from these mice demonstrated CD5 (Ly-1)-positive macrophages in the peritoneal cavity, spleen and other tissues. Ly-1-positive macrophage precursor cells were demonstrated in the peritoneal cavity of the mev mice and developed in the omental milky spots. The development of marginal metallophilic and marginal zone macrophages was poor in the splenic white pulp and related macrophage populations were absent in the other lymphoid tissues. The numbers of epidermal Langerhans cells in the skin and T cell-associated dendritic cells in the thymic medulla, lymph nodes, and the other peripheral lymphoid tissues were decreased. However, increased numbers of dendritic cells accumulated in the lungs, liver, and kidneys. These abnormalities in development and differentiation of macrophages and dendritic cells may be ascribed to the deficiency in haematopoietic cell SHP-1 tyrosine phosphatase or may be a secondary consequence of abnormal microenvironments, (either constitutive or in response to inflammatory stimuli) in the haematopoietic and lymphopoietic organs and tissues of these mice.     

Role of inducible nitric oxide synthase in endotoxin-induced acute lung injury

The role of nitric oxide (NO) in lung injury remains unclear. Both beneficial and detrimental roles have been proposed. In this study, we used mutant mice lacking the inducible nitric oxide synthase (iNOS) to assess the role of this isoform in sepsis-associated lung injury. Wild-type and iNOS knockout mice were injected with either saline or Escherichia coli endotoxin (LPS) 25 mg/kg and killed 6, 12, and 24 h later. Lung injury was evaluated by measuring lactate dehydrogenase activity in the bronchoalveolar lavage, pulmonary wet/dry ratio, and immunostaining for nitrotyrosine formation. In the wild-type mice, LPS injection elicited more than a 3-fold rise in lactate dehydrogenase activity, a significant rise in lung wet/dry ratio and extensive nitrotyrosine staining in large airway and alveolar epithelium, macrophages, and pulmonary vascular cells. This was accompanied by induction of iNOS protein and increased lung nitric oxide synthase activity. By comparison, LPS injection in iNOS knockout mice elicited no iNOS induction and no significant changes in lung NOS activity, lactate dehydrogenase activity, lung wet/dry ratio, or pulmonary nitrotyrosine staining. These results indicate that mice deficient in iNOS gene are more resistant to LPS-induced acute lung injury than are wild-type mice.     

Physical map of the Clostridium difficile chromosome

The present study was designed to investigate the effect of mercuric chloride administration on copper, zinc, and iron concentrations in the liver, kidney, lung, heart, spleen, and muscle of rats. The results showed that after dose and time exposure to mercuric chloride, the concentration of mercury in the six tissues was significantly elevated. Data showed that there were no interaction between mercury and tissue iron. There was a considerable elevation of the content of copper in the kidney and liver. The most significant changes in the copper concentration took place in the kidneys. About a twofold increase in the copper content of the kidney was noted after exposure to mercuric chloride (3 mg and 5 mg/kg). Only slight elevations in the copper content occurred in the liver especially in high dose and longer exposure time. In the remaining organs, the copper content was not changed significantly (p > 0.05). The most significant changes in the zinc concentration took place in liver, kidney, lung and heart (5 mg/kg). Marked changes in kidney zinc concentrations were observed at any of the specified doses. Zinc concentrations were significantly increased in kidney of rats sacrificed 9-48 h after s.c. injection of HgCl2 (5 mg/kg); in liver obtained from rats at 18, 24 or 48 h after injection; and in lung after 24 or 48 h of treatment. The heart and spleen zinc concentrations were elevated at 24 and 48 h after injection of HgCl2 (5 mg/kg), respectively. The results of this study implicate that effects on copper and zinc concentrations of the target tissues of mercury may play an important role in the pathogenesis of acute mercuric chloride intoxication.     

Effect of anti-macrophage migration inhibitory factor antibody on lipopolysaccharide-induced pulmonary neutrophil accumulation

Macrophage migration inhibitory factor (MIF) is a recently rediscovered pro-inflammatory cytokine that has the unique potential to override the anti-inflammatory action of glucocorticoids. Since recent reports suggest the pivotal role of MIF in acute lung injury, we examined the protective effect of anti-MIF antibody on lipopolysaccharide (LPS)-induced acute lung injury in rats. Rats were injected with LPS (7 mg/kg) intraperitoneally with or without pretreatment with anti-MIF antibody. The anti-MIF antibody significantly attenuated LPS-induced migration of neutrophils to the lungs at 4 and 24 h as demonstrated by observation of the number of neutrophils per alveolus, the activity of myeloperoxidase of the lung tissue, and cell differentiation of neutrophils in bronchoalveolar lavage (BAL) fluid. The increased level of macrophage inflammatory protein-2, a powerful neutrophil chemokine, in BAL fluid was also significantly attenuated by pretreatment with the anti-MIF antibody as compared with the control group. Additionally, positive immunostaining for MIF was observed in bronchial epithelial cells and alveolar macrophages, and Northern blot analysis of lung tissues demonstrated increased MIF mRNA 24 h after LPS injection. These data suggest that the anti-MIF antibody has therapeutic potential for the treatment of acute lung injury by suppressing the level of neutrophil chemokine in the lungs.     

Control of IL-12 and IFN-gamma production in response to live or dead bacteria by TNF and other factors

When mice were infected i.v. with either Listeria monocytogenes or Brucella abortus, bioactive IL-12 was briefly detected in serum and supernatants of spleen homogenates immediately ex vivo. Although the time scale was more prolonged for the more slowly growing B. abortus, in both instances IL-12 production ceased while bacteria still persisted in high numbers. Production of IL-12, detected in serum and spleen, was neither increased nor prolonged by injecting Abs to IL-10 or IL-4. In contrast with live organisms, heat-killed bacteria did not induce detectable IL-12 in vivo and were less efficient when added in vitro to resident peritoneal cells or spleen cells. Mice lacking the receptors for TNF (TNFR-/- mice) were severely deficient in IL-12 production, suggesting a controlling role for TNF, which we have previously shown to be triggered by live, rather than dead, bacteria. Infection in the TNFR-/- mice was exacerbated, although in the Brucella-infected mice splenomegaly, the main indicator of immunopathology, was reduced. Production of NO by macrophages was deficient, but the TNFR-/- mice were not deficient in IFN-gamma production. In addition to being poor inducers of IL-12, killed bacteria actively suppressed IL-12 production in response to live bacteria, by mechanism(s) unknown. The implications of these findings are discussed in light of the fact that only live bacteria satisfactorily induce cell-mediated immunity to infection.