Binding of Legionella pneumophila to macrophages increases cellular cytokine mRNA

Coxiella burnetii Infects Primary Bovine Macrophages and Limits Their Host Cell Response

Although domestic ruminants have long been recognized as the main source of human Q fever, little is known about the lifestyle that the obligate intracellular Gram-negative bacterium Coxiella burnetii adopts in its animal host. Because macrophages are considered natural target cells of the pathogen, we established primary bovine monocyte-derived macrophages (MDM) as an in vitro infection model to study reservoir host-pathogen interactions at the cellular level. In addition, bovine alveolar macrophages were included to take cell type peculiarities at a host entry site into account. Cell cultures were inoculated with the virulent strain Nine Mile I (NMI; phase I) or the avirulent strain Nine Mile II (NMII; phase II). Macrophages from both sources internalized NMI and NMII. MDM were particularly permissive for NMI internalization, but NMI and NMII replicated with similar kinetics in these cells. MDM responded to inoculation with a general upregulation of Th1-related cytokines such as interleukin-1β (IL-1β), IL-12, and tumor necrosis factor alpha (TNF-α) early on (3 h postinfection). However, inflammatory responses rapidly declined when C. burnetii replication started. C. burnetii infection inhibited translation and release of IL-1β and vastly failed to stimulate increased expression of activation markers, such as CD40, CD80, CD86, and major histocompatibility complex (MHC) molecules. Such capability of limiting proinflammatory responses may help Coxiella to protect itself from clearance by the host immune system. The findings provide the first detailed insight into C. burnetii-macrophage interactions in ruminants and may serve as a basis for assessing the virulence and the host adaptation of C. burnetii strains.

Immunological variation between inbred laboratory mouse strains: points to consider in phenotyping genetically immunomodified mice

Inbred laboratory mouse strains are highly divergent in their immune response patterns as a result of genetic mutations and polymorphisms. The generation of genetically engineered mice (GEM) has, in the past, used embryonic stem (ES) cells for gene targeting from various 129 substrains followed by backcrossing into more fecund mouse strains. Although common inbred mice are considered "immune competent," many have variations in their immune system-some of which have been described-that may affect the phenotype. Recognition of these immune variations among commonly used inbred mouse strains is essential for the accurate interpretation of expected phenotypes or those that may arise unexpectedly. In GEM developed to study specific components of the immune system, accurate evaluation of immune responses must take into consideration not only the gene of interest but also how the background strain and microbial milieu contribute to the manifestation of findings in these mice. This article discusses points to consider regarding immunological differences between the common inbred laboratory mouse strains, particularly in their use as background strains in GEM.

Chlamydia pneumoniae stimulates the proliferation of HUVEC through the induction of VEGF by THP-1

Chlamydia pneumoniae, a gram-negative bacterium, is an important human intracellular pathogen; studies of C. pneumoniae pathogenesis have shown that the organism can infect many cell types associated with both respiratory and vascular sites, including arterial smooth muscle cells, macrophages and vascular endothelium. Recently, the recognition of atherosclerosis as an inflammatory disease in its genesis, progression and ultimate clinical manifestations has created an interesting area of vascular research. We assessed the hypothesis that growth factors from THP-1 macrophages infected with C. pneumoniae are involved in the regulation of cell proliferation in HUVEC. The induction of these factors were dependent on time of infection, as medium harvested 48 h after infection had a greater activity than media harvested at 12 or 24 h after infection. Heat-killed C. pneumoniae produced similar results to those of live bacteria. In addition, conditioned medium filtered sterile from infected macrophages induced the proliferation of HUVEC, thus demonstrating its angiogenic potential. Moreover, pretreatment of macrophages with cytochalasin D, a phagocytosis inhibitor, yielded almost comparable results, suggesting that bacterium cell-attachment is sufficient for VEGF, IL-1beta and IL-8 induction. Further studies are necessary to elucidate the biological role of chlamydial involvement in the complex and mutifactored processes of angiogenesis and possibly contribute to the development of therapeutic strategies.

Mycoplasma pneumoniae subtype-independent induction of proinflammatory cytokines in THP-1 cells

Mycoplasma pneumoniae can be divided into two main subtypes depending on the amino acid sequences of the P1 adhesin and the P65 protein, both located in the attachment organelle. Differences between these subtypes in infectivity, virulence and interaction with host cells have not been extensively studied. Using ELISA to measure released protein and real-time PCR to quantify mRNA, we have demonstrated that both M. pneumoniae subtypes significantly increased tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6) and interleukin-8 (IL-8) at comparable levels in THP-1 cells over a 72 h period of time. However, subtype 2 induced a statistically significant increase (P<0.001) in the release of interleukin-1beta at 24 h post-infection compared to subtype 1. These data provide evidence that the induction of proinflammatory cytokine gene and protein expression by M. pneumoniae is not dependent on the infecting subtype.

Ehrlichia chaffeensis downregulates surface Toll-like receptors 2/4, CD14 and transcription factors PU.1 and inhibits lipopolysaccharide activation of NF-kappa B, ERK 1/2 and p38 MAPK in host monocytes

Microbial ligands, such as lipopolysaccharide (LPS), activate Toll-like receptors (TLRs) of mononuclear phagocytes, thus activating transcription factors including NF-kappa B and inducing antimicrobial activity. Ehrlichia chaffeensis, an obligatory intramonocytic Gram-negative bacterium, causes human monocytic ehrlichiosis. In the present study, we found that E. chaffeensis-infected human monocytes became progressively less responsive to Escherichia coli lipopolysaccharide (LPS) in activating NF-kappa B and mobilizing ehrlichiacidal activities. E. chaffeensis infection caused downregulation of the expression of several pattern recognition receptors, such as CD14, TLR2 and TLR4, as revealed by flow cytometry and/or reverse transcription polymerase chain reaction analysis. Electrophoretic mobility shift assay revealed that the activity of a transcription factor PU.1 was also downregulated by E. chaffeensis infection. ERK 1/2 and p38 MAPK were slightly activated at the early stage of E. chaffeensis infection; however, the activations of ERK 1/2 and p38 MAPK by LPS treatment were subsequently reduced in E. chaffeensis-infected monocytes compared with those in uninfected monocytes. Like E. chaffeensis, the p38 MAPK-specific inhibitor SB 203580 downregulated PU.1 activity and the expression of TLR2, TLR4 and CD14 in human monocytes, suggesting that the inhibition of p38 MAPK by E. chaffeensis is involved in the suppression of several downstream signalling pathways. These data point to a novel mechanism by which E. chaffeensis can survive by inhibiting critical signalling in monocyte activation pathways linked to pattern recognition receptors.

Role and regulation of CXC-chemokines in acute experimental keratitis

The aim of this study was to elucidate the expression of chemokines, their role and regulation in bacterial corneal infection using three bacterial strains (Pseudomonas. aeruginosa- invasive, cytotoxic and contact lens induced acute red eye strains) which have been shown to produce three distinct patterns of corneal disease in the mouse. The predominant chemokine expressed in response to all three strains was MIP-2. Prolonged expression of high levels of MIP-2 was associated with increased severity of corneal inflammation. Significantly reduced disease severity upon administration of anti-MIP-2 antibodies suggested that MIP-2 may play an important role in the pathogenesis of Pseudomonas keratitis at least in part by being a major chemoattractant for polymorphonuclear leukocytes (PMN) recruitment. Interestingly, the numbers of bacteria in eyes with neutralized MIP-2 activity did not decrease even though the severity of the disease was decreased. This implies PMNs as the major destructive factor in microbial keratitis. Further, neutralization of IL-1beta activity alone using monoclonal antibodies resulted in significant reduction of both MIP-2 and KC activity indicating that chemokine levels were regulated by IL-1beta. These studies demonstrate that the regulation of MIP-2 activity may be beneficial in reducing corneal damage during microbial keratitis in rodents and perhaps that regulation of the human homologue of MIP-2, IL-8, may be useful for controlling keratitis in humans.

Induction of a potential paracrine angiogenic loop between human THP-1 macrophages and human microvascular endothelial cells during Bartonella henselae infection

Bartonella henselae is responsible for various disease syndromes that loosely correlate with the immune status of the host. In the immunocompromised individual, B. henselae-induced angiogenesis, or bacillary angiomatosis, is characterized by vascular proliferative lesions similar to those in Kaposi's sarcoma. We hypothesize that B. henselae-mediated interaction with immune cells, namely, macrophages, induces potential angiogenic growth factors and cytokines which contribute in a paracrine manner to the proliferation of endothelial cells. Vascular endothelial growth factor (VEGF), a direct inducer of angiogenesis, and interleukin-1beta (IL-1beta), a potentiator of VEGF, were detected within 12 and 6 h, respectively, in supernatants from phorbol 12-myristate 13-acetate-differentiated human THP-1 macrophages exposed to live B. henselae. Pretreatment of macrophages with cytochalasin D, a phagocytosis inhibitor, yielded comparable results, suggesting that bacterium-cell attachment is sufficient for VEGF and IL-1beta induction. IL-8, an angiogenic cytokine with chemotactic properties, was induced in human microvascular endothelial cells (HMEC-1) within 6 h of infection, whereas no IL-8 induction was observed in infected THP-1 cells. In addition, conditioned medium from infected macrophages induced the proliferation of HMEC-1, thus demonstrating angiogenic potential. These data suggest that Bartonella modulation of host or target cell cytokines and growth factors, rather than a direct role of the bacterium as an endothelial cell mitogen, is the predominant mechanism responsible for angiogenesis. B. henselae induction of VEGF, IL-1beta, and IL-8 outlines a broader potential paracrine angiogenic loop whereby macrophages play the predominant role as the effector cell and endothelial cells are the final target cell, resulting in their proliferation.

Alveolar macrophage cell line MH-S is valuable as an in vitro model for Legionella pneumophila infection

Alveolar macrophages are the preferential site for growth of Legionella pneumophila (Lp) during infection. However, the study of Lp infection in alveolar macrophages is difficult due to the limitation of available primary alveolar macrophages. In the present study, we established an in vitro Lp infection model in alveolar macrophages using a continuous cell line of murine alveolar macrophages designated MH-S. Infection of both MH-S cells and primary mouse alveolar macrophages obtained by alveolar lavage with virulent L. pneumophila (Lp-V) showed vigorous growth of the bacteria, but infection with avirulent L. pneumophila (Lp-Av) resulted in only minimum growth. Cytokine message expression determination in the MH-S cells after infection showed strong induction of interleukin (IL)-6, IL-10, and tumor necrosis factor-alpha messages induced by Lp-V but minimal induction of these cytokines by Lp-Av infection. IL-1 alpha protein secretion and the message levels for IL-1 alpha were also analyzed, and remarkable induction of IL-1 alpha was evident in both macrophage types when infected with Lp-V. Analysis of IL-12 p40 responses of both macrophage types to Lp-V infection assessed by reverse transcriptase/polymerase chain reaction revealed induction of increased message levels, but significant levels were induced only slowly. Determination of IL-12 protein secretion by enzyme-linked immunosorbent assay of culture supernatants from both macrophage types infected with either Lp-V or Lp-Av showed only minimum production. Thus, MH-S alveolar macrophages showed a similar response to Lp infection compared with primary alveolar macrophages and can be a useful in vitro model system to study Lp infection. The study also revealed the restricted IL-12 protein secretion of alveolar macrophages by Lp infection.

Differential expression of IL-1 and TNF receptors in murine macrophages infected with virulent vs. avirulentLegionella pneumophila

Infection of macrophages from genetically susceptible A/J mice with Legionella pneumophila induces high levels of various cytokines in serum as well as in cultures of spleen or peritoneal cells from the mice. However, modulation of receptor expression for these cytokines during infection has not been studied in detail, even though these receptors on macrophages have a critical role in inflammatory responses during the infection. In the present study, the differential expression of mRNA for TNF and IL-1 receptors as well as receptor antigens during infection of macrophages with virulent vs. avirulent L. pneumophila was investigated. Mouse thioglycollate-elicited peritoneal macrophages showed by RT-PCR constitutive steady-state levels of mRNA for TNF-type I and -type II receptors as well as IL-1 type I receptor. However, IL-1 type II receptor mRNA was not expressed in thioglycollate-elicited macrophages. Infection of macrophages with virulent bacteria caused an upregulation of IL-1 type I and TNF type I receptor mRNA, but had no effect on TNF type II receptor message. Avirulent L. pneumophila infection caused much less induction of these receptor mRNAs. The amount of receptor antigen of IL-1 type I on the surface of macrophages was also increased by infection with virulent L. pneumophila determined by flow cytometric analysis. These results indicate that L. pneumophila infection not only causes induction of various cytokines, but also modulation of certain cytokine receptors, which may regulate the susceptibility to infection.Key words: Legionella pneumophila, cytokine receptors, macrophages.

alpha(v)beta(3) integrin and bacterial lipopolysaccharide are involved in Coxiella burnetii-stimulated production of tumor necrosis factor by human monocytes

Coxiella burnetii, the agent of Q fever, enters human monocytes through alpha(v)beta(3) integrin and survives inside host cells. In addition, C. burnetii stimulates the synthesis of inflammatory cytokines including tumor necrosis factor (TNF) by monocytes. We studied the role of the interaction of C. burnetii with THP-1 monocytes in TNF production. TNF transcripts and TNF release reached maximum values within 4 h. Almost all monocytes bound C. burnetii after 4 h, while the percentage of phagocytosing monocytes did not exceed 20%. Cytochalasin D, which prevented the uptake of C. burnetii without interfering with its binding, did not affect the expression of TNF mRNA. Thus, bacterial adherence, but not phagocytosis, is necessary for TNF production by monocytes. The monocyte alpha(v)beta(3) integrin was involved in TNF synthesis since peptides containing RGD sequences and blocking antibodies against alpha(v)beta(3) integrin inhibited TNF transcripts induced by C. burnetii. Nevertheless, the cross-linking of alpha(v)beta(3) integrin by specific antibodies was not sufficient to induce TNF synthesis. The signal delivered by C. burnetii was triggered by bacterial lipopolysaccharide (LPS). Polymyxin B inhibited the TNF production stimulated by C. burnetii, and soluble LPS isolated from C. burnetii largely mimicked viable bacteria. On the other hand, avirulent variants of C. burnetii induced TNF production through an increased binding to monocytes rather than through the potency of their LPS. We suggest that the adherence of C. burnetii to monocytes via alpha(v)beta(3) integrin enables surface LPS to stimulate TNF production in THP-1 monocytes.

Heat-killed Streptococcus suis capsular type 2 strains stimulate tumor necrosis factor alpha and interleukin-6 production by murine macrophages

Streptococcus suis capsular type 2 is an important etiological agent of swine meningitis, and it is also a zoonotic agent. Since mononuclear phagocytes have been suggested to play a central role in the pathogenesis of meningitis, the objective of the present study was to evaluate the capacity of whole killed S. suis type 2 organisms to induce the release of the proinflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interleukin-6 (IL-6) by murine macrophages. Induction of cytokines was evaluated in the presence or absence of phorbol ester (phorbol 12-myristate 13-acetate [PMA]) costimulation. Results showed that S. suis type 2 stimulated the production of both cytokines in a concentration- and time-dependent fashion. Although large doses of bacteria were required for maximal cytokine release, titers were similar to those obtained with the lipopolysaccharide (LPS) positive control. An increase in cytokine release was observed with both S. suis and LPS with PMA costimulation. Experiments with cytochalasin-treated macrophages showed that the stimulation of cytokine production was phagocytosis independent. When macrophages were stimulated with an unencapsulated mutant, an increase in TNF production was observed, but the absence of the capsule had no effect on IL-6 production. In fact, whereas purified capsular polysaccharide of S. suis failed to induce cytokine release, purified S. suis cell wall induced both TNF and, to a lesser extent, IL-6. IL-6 secretion probably requires some distinct stimuli which differ from those of TNF. Finally, the S. suis putative virulence factors suilysin and extracellular protein EF showed no cytokine-stimulating activity. The ability of S. suis to trigger macrophages to produce proinflammatory cytokines may have an important role in the initiation and development of meningitis caused by this microorganism.

Multiplication of Legionella pneumophila in HeLa cells in the presence of cytoskeleton and metabolic inhibitors

A study has been carried out on the action of cytoskeleton and metabolic inhibitors on intracellular multiplication in HeLa cells of a virulent strain of Legionella pneumophila serogroup 6. The effects of the substances were separately tested on both penetration and intracellular multiplication of L. pneumophila. Only cytochalasin A and 2-deoxy-D-glucose (2dG) affected bacterial internalisation, whereas intracellular multiplication was inhibited by cytochalasins A, B, C, D and J (D being the most active) and by 2dG with a dose-response effect. The action of 2dG was counteracted by 50 mM glucose. Experiments carried out with cytochalasin D and a rhodamine-phalloidin conjugate showed the involvement of cytoskeletal elements in intracellular multiplication of Legionella; compounds acting on microtubules had no effect.

Involvement of mannose receptor in cytokine interleukin-1beta (IL-1beta), IL-6, and granulocyte-macrophage colony-stimulating factor responses, but not in chemokine macrophage inflammatory protein 1beta (MIP-1beta), MIP-2, and KC responses, caused by attachment of Candida albicans to macrophages

The production of chemotactic cytokines (chemokines) and other cytokines by macrophages in response to fungal infection is thought to be critical during the course of candidiasis. However, the mechanism of cytokine synthesis by macrophages in response to fungi is not well understood. Therefore, the response of macrophages to Candida albicans was examined in terms of receptor-mediated chemokine and other cytokine mRNA induction. Attachment of C. albicans to murine thioglycollate-elicited peritoneal macrophages induced increased mRNA levels of the cytokines interleukin-1beta (IL-1beta), IL-6, and granulocyte-macrophage colony-stimulating factor (GM-CSF) and the chemokines macrophage inflammatory protein 1beta (MIP-1beta), MIP-2, and KC (a member of the platelet factor 4 neutrophil chemoattractant family), as determined by quantitative reverse transcription-PCR. However, treatment of macrophages with alpha-methyl-D-mannoside significantly reduced the cytokine GM-CSF response to C. albicans but did not affect the chemokine MIP-2 response. Antisense oligodeoxynucleotide (ODN) to mannose receptor (MR) mRNA inhibited the expression and function of MR in macrophages as determined by Western blot analysis and 125I-labeled mannose-bovine serum albumin (BSA) binding, and also inhibited the elevation of cytokine IL-1beta, IL-6, and GM-CSF mRNA levels induced by C. albicans attachment. Elevation of chemokine MIP-1beta, MIP-2, and KC mRNA levels induced by C. albicans was not affected in macrophages whose MR expression was suppressed by antisense ODN treatment. Furthermore, IL-4 treatment of macrophages, which up-regulated MR expression as determined by Western blot analysis and fluorescein isothiocyanate-labeled mannose-BSA uptake, enhanced the level of cytokine GM-CSF mRNA induced by C. albicans but not the level of the chemokine MIP-2 mRNA. These results indicate that selected cytokine responses of macrophages to C. albicans are mediated by MR, while some chemokine responses may be mediated by other receptors.

Cytokine and adhesion molecule expression in human monocytes and endothelial cells stimulated with bacterial heat shock proteins

Bacterial heat shock proteins (HSPs) from Escherichia coli (GroES, GroEL, and DNAk) were tested for their ability to induce by themselves the expression and release of interleukin-6 (IL-6), tumor necrosis factor alpha (TNF-alpha), and granulocyte-monocyte colony-stimulating factor (GM-CSF) by human monocytes and GM-CSF, IL-6, E-selectin, intercellular adhesion molecule-1 (ICAM-1), and vascular cell adhesion molecule-1 (VCAM-1) by human umbilical vein endothelial cells (HUVEC). Our study demonstrated that treatment of monocytes with DNAk increased IL-6, TNF-alpha, and GM-CSF release in a dose-dependent manner. The same effect was elicited by GroEL but at a lower rate. Treatment of HUVEC cultures with DNAk and GroEL also increased GM-CSF, IL-6, E-selectin, ICAM-1, and VCAM-1 release in a dose-dependent fashion. In any case, the greatest release was obtained by using DNAk and GroEL at a concentration of 1 microg/ml. DNAk and GroEL were also able to up-regulate the surface expression of E-selectin, ICAM-1, and VCAM-1. As detected by reverse transcription-PCR analysis, DNAk and GroEL also increased the steady-state levels of cytokines and adhesion molecules in human monocytes and endothelial cells. In our study GroES showed a significant activity only on the release, surface expression, and mRNA transcription of E-selectin. Adhesion molecule expression seems to be a direct effect of HSPs and not via cytokines. Furthermore, these effects are due to HSPs properties because they are inhibited by specific monoclonal antibodies. These findings support the potential role of HSPs in modulating cell interactions during immunological and inflammatory responses.

Legionella pneumophila heat-shock protein-induced increase of interleukin-1 beta mRNA involves protein kinase C signalling in macrophages

Heat-shock proteins (hsp) are chaperon molecules important in protein folding and assembly. Furthermore, they may have functions in immunoregulatory processes, like T-cell stimulation and antigen presentation, which are not yet fully understood. It has been shown that several hsp of various species and family derivations modulate functions in macrophage immunity by directly increasing cytokine production. In the present study we showed that the 60,000 MW hsp of Legionella pneumophila (Lp-hsp 60) increased cellular steady-state levels of interleukin-1 beta (IL-1 beta) mRNA measured by quantitative reverse transcription-polymerase chain reaction and Northern blotting as well as IL-1 secretion, when added to cultures of thioglycollate-elicited mouse peritoneal macrophages in vitro. The level of mRNA increased in a dose-dependent manner with a minimum effective concentration of 0.5 microgram/ml and peaked 3 hr after stimulation. Lp-hsp 60-coated latex beads also increased IL-1 beta mRNA levels in the presence of cytochalasin D, which inhibits bead uptake but permits binding, indicating that binding to the macrophage surface was sufficient for induction. Accumulation of IL-1 beta mRNA was completely blocked by pretreatment with the protein kinase C (PKC) inhibitor, H7, but not decreased by prior treatment with cycloheximide. The cell lysates of macrophages stimulated with Lp-hsp 60 showed an increased PKC activity measured by phosphorylation of PKC pseudosubstrate. The IL-1 bioactivity in culture supernatants after 24 hr of stimulation with Lp-hsp 60 was increased in a dose-dependent manner but at hsp concentrations in excess of those needed to increase mRNA. Thus, the present study demonstrates that Lp-hsp 60 rapidly increases the steady-state level of IL-1 beta mRNA, possibly through a cell surface receptor system involving a PKC-dependent signalling pathway.

Absence of tumor necrosis factor alpha, interleukin-6 (IL-6), and granulocyte-macrophage colony-stimulating factor expression but presence of IL-1beta, IL-8, and IL-10 expression in human monocytes exposed to viable or killed Ehrlichia chaffeensis

Ehrlichia chaffeensis is a recently isolated minute gram-negative obligatory intracellular bacterium of monocytes/macrophages and is the etiologic agent of human monocytic ehrlichiosis. It is not known how macrophages respond when they encounter ehrlichiae in terms of cytokine production. In this study, we examined cytokine mRNA expression by incubating E. chaffeensis with THP-1 cells and performing competitive reverse transcription-PCR (RT-PCR). At 2 h postinfection, the levels of interleukin-1beta (IL-1beta), IL-8, and IL-10 mRNAs were significant but lower than those following Escherichia coli lipopolysaccharide (LPS) stimulation. Unlike the situation with E. coli LPS stimulation, however, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), and tumor necrosis factor alpha (TNF-alpha) mRNAs were not induced. Time course and dose-response studies confirmed the absence of IL-6, GM-CSF, and TNF-alpha mRNA induction with E. chaffeensis. Viable E. chaffeensis organisms were not required for IL-1beta IO, IL-8, and IL-10 mRNA induction, since heat-killed E. chaffeensis induced identical time course responses. IL-1beta, IL-8, and IL-10 mRNAs were detected for up to 21, 21, and 24 h postexposure with E. chaffeensis, respectively, which were shut off more rapidly than with LPS stimulation. Although heat treatment of E. chaffeensis had no effect, periodate treatment completely abolished the ability of E. chaffeensis to induce IL-1beta, IL-8, and IL-10 mRNAs. The capture enzyme-linked immunosorbent assay result corresponded with the RT-PCR results, showing that viable and heat-killed E. chaffeensis produced and secreted the same levels of IL-1beta and IL-8. IL-10 production was significantly reduced by heat treatment. Periodate-treated ehrlichiae did not induce production of any of the cytokines tested. Anti-CD14 monoclonal antibody and polymyxin B did not inhibit IL-1beta mRNA expression upon exposure to E. chaffeensis. The absence of TNF-alpha, IL-6, and GM-CSF mRNA induction may delay the development of a protective immune response, thereby allowing E. chaffeensis to set up residence in macrophages.

Cytokines and bacterial infections

During the recent 10-15 years a growing amount of knowledge has been accumulated on the role of cytokines in the pathogenesis and resistance to infections caused by nonviral agents, including a wide range of bacteria. Cytokines can be major mediators of the pathogenic effect in some diseases, and represent important defense mechanisms in others. Detailed knowledge on the role of the growing number of recognised cytokines is important, because it may represent means to combat and to prevent diseases caused by such infections.

Induction of cytokine granulocyte-macrophage colony-stimulating factor and chemokine macrophage inflammatory protein 2 mRNAs in macrophages by Legionella pneumophila or Salmonella typhimurium attachment requires different ligand-receptor systems

The attachment of bacteria to macrophages is mediated by different ligands and receptors and induces various intracellular molecular responses. In the present study, induction of cytokines and chemokines, especially granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage inflammatory protein 2 (MIP-2), was examined, following bacterial attachment, with regard to the ligand-receptor systems involved. Attachment of Legionella pneumophila or Salmonella typhimurium to cultured mouse peritoneal macrophages increased the steady-state levels of cellular mRNAs for the cytokines interleukin 1beta (IL-1beta), IL-6, and GM-CSF as well as the chemokines MIP-1beta, MIP-2, and KC. However, when macrophages were treated with alpha-methyl-D-mannoside (alphaMM), a competitor of glycopeptide ligands, induction of cytokine mRNAs was inhibited, but the levels of chemokine mRNAs were not. Pretreatment of the bacteria with fresh mouse serum enhanced the level of GM-CSF mRNA but not the level of MIP-2 mRNA. In addition, serum treatment reduced the inhibitory effect of alphaMM on GM-CSF mRNA. These results indicate that bacterial attachment increases the steady-state levels of the cytokine and chemokine mRNAs tested by at least two distinct receptor-ligand systems, namely, one linked to cytokine induction and involving mannose or other sugar residues and the other linked to chemokine induction and relatively alphaMM insensitive. Furthermore, opsonization with serum engages other pathways in the cytokine response which are relatively independent of the alphaMM-sensitive system. Regarding bacterial surface ligands involved in cytokine mRNA induction, evidence is presented that the flagellum may be important in stimulating cytokine GM-CSF message but not chemokine MIP-2 message. Analysis of cytokine GM-CSF and chemokine MIP-2 signaling pathways with protein kinase inhibitors revealed the involvement of calmodulin and myosin light-chain kinase in GM-CSF but not MIP-2 mRNA induction, adding further evidence that several distinct receptor systems are engaged during the process of bacterial attachment and induction of cytokines and chemokines, such as GM-CSF and MIP-2, respectively.

Bacterial modulins: a novel class of virulence factors which cause host tissue pathology by inducing cytokine synthesis

Cytokines are a diverse group of proteins and glycoproteins which have potent and wide-ranging effects on eukaryotic cell function and are now recognized as important mediators of tissue pathology in infectious diseases. It is increasingly recognized that for many bacterial species, cytokine induction is a major virulence mechanism. Until recent years, the only bacterial component known to stimulate cytokine synthesis was lipopolysaccharide (LPS). It is only within the past decade that it has been clearly shown that many components associated with the bacterial cell wall, including proteins, glycoproteins, lipoproteins, carbohydrates, and lipids, have the capacity to stimulate mammalian cells to produce a diverse array of cytokines. It has been established that many of these cytokine-inducing molecules act by mechanisms distinct from that of LPS, and thus their activities are not due to LPS contamination. Bacteria produce a wide range of virulence factors which cause host tissue pathology, and these diverse factors have been grouped into four families: adhesins, aggressins, impedins, and invasins. We suggest that the array of bacterial cytokine-inducing molecules represents a new class of bacterial virulence factor, and, by analogy with the known virulence families, we suggest the term "modulin" to describe these molecules, because the action of cytokines is to modulate eukaryotic cell behavior. This review summarizes our current understanding of cytokine biology in relation to tissue homeostasis and disease and concisely reviews the current literature on the cytokine-inducing molecules produced by gram-negative and gram-positive bacteria, with an emphasis on the cellular mechanisms responsible for cytokine induction. We propose that modulins, by controlling the host immune and inflammatory responses, maintain the large commensal flora that all multicellular organisms support.

Porcine interferon-gamma inhibits the growth of Legionella pneumophila in WiREF cells in vitro

The intracellular growth of Legionella pneumophila in WiREF (Wistar rat embryonal fibroblast) cells was inhibited by porcine interferon-gamma. The effect was compared with that of different human interferons (alpha and gamma). The growth inhibition was dose-dependent and required the pretreatment of WiREF cells with interferon. The development of an antibacterial state of the cells was observed. When interferon was added together with bacteria or 1 d after the infection there was no inhibition. Also, there was no direct antibacterial effect of the interferon. In addition, cell pretreatment with a combination of interferon and antibiotics failed to show a synergistic effect.

In vivo regulation of replicative Legionella pneumophila lung infection by endogenous tumor necrosis factor alpha and nitric oxide

The in vivo role of endogenous tumor necrosis factor alpha (TNF-alpha) and reactive nitrogen intermediates (RNIs) in modulation of growth of Legionella pneumophila in the lung was assessed using a murine model of replicative L. pneumophila lung infection. Intratracheal inoculation of mice with L. pneumophila resulted in induction of endogenous TNF-alpha, which preceded clearance of L. pneumophila from the lung. Inhibition of endogenous TNF-alpha activity, via in vivo administration of TNF-alpha neutralizing antibody, or inhibition of endogenous RNIs, via administration of the nitric oxide (NO) synthetase inhibitor N-monomethyl-L-arginine (NMMA), resulted in enhanced growth of L. pneumophila in the lung at > or = 3 days postinfection (when compared with untreated L. pneumophila-infected mice). Because of the similar kinetics of enhanced pulmonary growth of L. pneumophila in mice treated in vivo with either anti-TNF-alpha antibody or NMMA, the immunomodulatory effect of NO on endogenous TNF-alpha activity in the lung was assessed. Administration of NMMA to L. pneumophila-infected mice resulted in a significant decrease in endogenous TNF-alpha activity in the lung during replicative L. pneumophila infections in vivo. However, administration of exogenous TNF-alpha to NMMA-treated mice failed to significantly enhance clearance of L. pneumophila from the lung. Results of these studies indicate that both endogenous NO and TNF-alpha facilitate resolution of replicative L. pneumophila lung infections and that regulation of L. pneumophila replication by TNF-alpha is mediated, at least in part, by NO.

Quantitative reverse transcription-PCR analysis of Legionella pneumophila-induced cytokine mRNA in different macrophage populations by high-performance liquid chromatography

Cytokine production in macrophages infected by bacteria is critical for the course of infection. However, it is not known how infection of macrophages with opportunistic bacteria leads to cytokine production in different populations of cells. Since it is possible that cytokine genes may be differentially regulated by attachment rather than by active infection, the levels of various cytokine mRNAs were measured in alveolar macrophages (AMs), peritoneal resident macrophages (RMs), and peritoneally elicited macrophages (EMs) interacting with Legionella pneumophila by using cytochalasin D-treated macrophages and a newly developed quantitative reverse transcription-PCR procedure with high-performance liquid chromatographic analysis to determine cytokine mRNA formation. Increased levels of interleukin-1 beta (IL-1 beta), IL-6, tumor necrosis factor alpha, granulocyte-macrophage colony-stimulating factor, and macrophage inflammatory protein 2 mRNAs were quantitated in the macrophages responding to L. pneumophila attachment in vitro. Using this technique, we showed that the three different macrophage populations responded differently to bacterial attachment. We found that the levels of IL-6 and granulocyte-macrophage colony-stimulating factor mRNAs induced by the attachment of L. pneumophila to AMs were significantly lower than the levels in RMs but similar to the levels in EMs. Furthermore, the levels of MIP-2 mRNA in the AMs were found to be higher than those in the RMs, but similar levels were found in EMs. IL-1 beta mRNA levels were higher in both AMs and RMs than in EMs, but tumor necrosis factor alpha levels were not different among the three macrophage populations examined. Thus, the responses of macrophages to bacterial attachment in terms of cytokine mRNA levels were readily quantitated by the reverse transcription-PCR assay. However, the results obtained showed different levels of responsiveness of distinct macrophage populations to L. pneumophila attachment, and this could be related to the characteristic nature of the macrophage type examined.

Bacterial heat shock proteins directly induce cytokine mRNA and interleukin-1 secretion in macrophage cultures

Bacterial heat shock proteins (hsp) have been shown to be important immunogens stimulating both T cells and B cells. However, little is known concerning the direct interactions between hsp and macrophages. In this study, we demonstrated that treatment of macrophage cultures with purified bacterial hsp, including Legionella pneumophila hsp60, Escherichia coli GroEL, Mycobacterium tuberculosis hsp70, Mycobacterium leprae hsp65, and Mycobacterium bovis BCG hsp65, increased the steady-state levels of cytokine mRNA for interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-6, tumor necrosis factor alpha, and granulocyte-macrophage colony-stimulating factor as well as supernatant IL-1 secretion. This effect was shown not to be due to contamination of the hsp preparations with bacterial lipopolysaccharide. However, not all hsp induced cytokines; M. tuberculosis hsp10 showed minimal activity in our study. These results suggest that bacterial hsp might modulate immunity by rapidly and directly increasing cytokine production in macrophages.