Polymyxin B stimulates production of complement components and cytokines in human monocytes

Klebsiella pneumoniae Lipopolysaccharides Serotype O2afg Induce Poor Inflammatory Immune Responses Ex Vivo

Currently, Klebsiella pneumoniae is a pathogen of clinical relevance due to its plastic ability of acquiring resistance genes to multiple antibiotics. During K. pneumoniae infections, lipopolysaccharides (LPS) play an ambiguous role as they both activate immune responses but can also play a role in immune evasion. The LPS O2a and LPS O2afg serotypes are prevalent in most multidrug resistant K. pneumoniae strains. Thus, we sought to understand if those two particular LPS serotypes were involved in a mechanism of immune evasion. We have extracted LPS (serotypes O1, O2a and O2afg) from K. pneumoniae strains and, using human monocytes ex vivo, we assessed the ability of those LPS antigens to induce the production of pro-inflammatory cytokines and chemokines. We observed that, when human monocytes are incubated with LPS serotypes O1, O2a or O2afg strains, O2afg and, to a lesser extent, O2a but not O1 failed to elicit the production of pro-inflammatory cytokines and chemokines, which suggests a role in immune evasion. Our preliminary data also shows that nuclear translocation of NF-κB, a process which regulates an immune response against infections, occurs in monocytes incubated with LPS O1 and, to a smaller extent, with LPS O2a, but not with the LPS serotype O2afg. Our results indicate that multidrug resistant K. pneumoniae expressing LPS O2afg serotypes avoid an initial inflammatory immune response and, consequently, are able to systematically spread inside the host unharmed, which results in the several pathologies associated with this bacterium.

Polymyxin-B Stimulates Tumor Necrosis Factor-α Production by Human Peripheral Blood Mononuclear Cells

Gram-negative bacterial lipopolysaccharide (LPS) is a well known stimulus for cytokine production, particularly interleukin-1 (IL-1) and tumor necrosis tactor alpha (TNFα). Polymyxin B (PMX-B) is a cationic polypeptide that binds to LPS, neutralizing its biological effects. PMX-B also disrupts gram-negative bacterial cell membrane phospholipids but is highly toxic to mammalian cells, therefore is of limited use. PMX-B is used as additive to media, as a way to handle LPS contamination. To derive benefit from the ability of PMX-B to neutralize lipid A in vivo while avoiding its systemic toxicity, PMX-B was covalently bound to polystyrene-derivative fibers, creating a hemoperfusion column (PMX-F) for the selective removal of circulating ET In vitro PMX-F hemoperfusion studies have demonstrated effective ET removal, using either the Limulus amebocyte lysate assay or TNFα production by peripheral blood mononuclear cells (PBMC) as an index of ET removal. However, the question whether PMX-B itself could stimulate human PBMC to produce cytokines has not been adequately addressed. We examined the effect of increasing concentrations of PMX-B on cytokine production by PBMC in vitro. PBMC harvested from healthy volunteers were incubated for 24 hours at 37°C with control (tissue culture media RPMI), or 5 µg/ml, 10 µg/ml, 20 µg/ml or 100 µg/ml PMX-B. At the end of 24 hours, PBMC were subjected to three freeze-thaw cycles, and total TNFα production (pg/2.5x106 PBMC) was measured by radioimmunoassay. Total TNFα production by PBMC was 163 ± 3 pg, 171 ± 9 pg, 164 ± 4 pg, 323 ± 63 pg and 331 ± 58 pg, in the control, PMX-B 5 µg/ml, 10 µg/ml, 20 µg/ml and 100 µg/ml conditions, respectively. Compared to controls (RPMI), the percentage increase in TNFα production by PBMC was 5 ± 6% (P=0.23), 1 ± 3% (P=0.45), 99 ± 40% (P=0.03) and 103 ± 36% (P=0.02) in the presence of 5 µg/ml, 10 µg/ml, 20 µg/ml and 100 µg/ml of PMX-B, respectively. Furthermore, total TNFα production correlated significantly with increasing concentrations of PMX-B (R=0.53, P=0.007). We conclude that the use of PMX-B in in vitro studies as an LPS-neutralizing agent, or in the experimental treatment of endotoxic or septic shock can lead to erroneous interpretations of cytokine production by PBMC, and should be used cautiously in in vitro systems at high concentrations.

Role of inflammation in the aging bones

Chronic inflammation in aging is characterized by increased inflammatory cytokines, bone loss, decreased adaptation, and defective tissue repair in response to injury. Aging leads to inherent changes in mesenchymal stem cell (MSC) differentiation, resulting in impaired osteoblastogenesis. Also, the pro-inflammatory cytokines increase with aging, leading to enhanced myelopoiesis and osteoclastogenesis. Bone marrow macrophages (BMMs) play pivotal roles in osteoblast differentiation, the maintenance of hematopoietic stem cells (HSCs), and subsequent bone repair. However, during aging, little is known about the role of macrophages in the differentiation and function of MSC and HSC. Aged mammals have higher circulating pro-inflammatory cytokines than young adults, supporting the hypothesis of increased inflammation with aging. This review will aid in the understanding of the potential role(s) of pro-inflammatory (M1) and anti-inflammatory (M2) macrophages in differentiation and function of osteoblasts and osteoclasts in relation to aging.

Monocyte cytokine synthesis in response to extracellular cell stress proteins suggests these proteins exhibit network behaviour

Human peripheral blood monocytes were exposed to single or pairs of cell stress proteins (CSPs), specifically Hsp10, Hsp27, Hsp60 and Hsp70-the former two having anti-inflammatory actions while the latter pair being assumed to be pro-inflammatory in activity. This study was to test if these proteins exhibited any network behaviour. To control for possible lipopolysaccharide contamination, polymyxin B was used. Surprisingly, at concentrations higher than 1 μg/ml, polymyxin B itself could induce cytokine synthesis. A number of commercial sources of the molecular chaperones were tested, and marked variations in monocyte cytokine synthesis were found. All four CSPs stimulated the same profile of IL-1/IL-6 synthesis and IL-10/TNF-α synthesis although the kinetics of production of these two pairs of cytokines were very different. A key question was whether extracellular molecular chaperones exhibited network behaviour. To test this, monocytes were cultured with suboptimal concentrations of single CSP and pairs of CSP to look for additive, synergistic or antagonistic cell responses. The major finding was that pairs of molecular chaperones, including chaperones thought to stimulate monocyte cytokine synthesis, could produce significant antagonistic cellular responses. This demonstrates that extracellular CSPs constitute an additional potent layer within the complex cytokine network and furthermore suggests that monocytes have evolved to dampen their immune responses upon exposure to extracellular networks of CSPs-perhaps as a mechanism for protecting cells against detrimental cellular stress responses.

Different cytokine production and Toll-like receptor expression induced by heat-killed invasive and carrier strains of Neisseria meningitidis

Neisseria meningitidis may cause severe invasive disease. The carriage state of the pathogen is common, and the reasons underlying why the infection becomes invasive are not fully understood. The aim of this study was to compare the differences between invasive and carrier strains in the activation of innate immunity. The monocyte expression of TLR2, TLR4, CD14, and HLA-DR, cytokine production, and the granulocyte oxidative burst were analyzed after in vitro stimulation by heat-killed invasive (n = 14) and carrier (n = 9) strains of N. meningitidis. The expression of the cell surface markers in monocytes, the oxidative burst, and cytokine concentrations were measured using flow cytometry. Carrier strains stimulated a higher production of inflammatory cytokines and oxidative burst in granulocytes than invasive strains (all p < 0.001), whereas invasive strains significantly up-regulated TLR2, TLR4 (p < 0.001), and CD14 (p < 0.01) expression on monocytes. Conversely, the monocyte expression of HLA-DR was higher after the stimulation by carrier strains (p < 0.05) in comparison to invasive strains. The LPS inhibitor polymyxin B abolished the differences between the strains. Our findings indicate different immunostimulatory potencies of invasive strains of N. meningitidis compared with carrier strains.

Mitogen-activated protein kinases-dependent induction of hepatocyte growth factor production in human dermal fibroblasts by the antibiotic polymyxin B

Hepatocyte growth factor (HGF) stimulates migration and proliferation of keratinocytes and has been suggested to be involved in wound healing. The cationic antibiotic polymyxin B (PMB) is commonly used as a topical antibiotic for wound care. If PMB possesses an HGF-inducing activity, the antibiotic is potentially beneficial for wound healing in addition to minimizing chances of infection. In this study, we found that PMB markedly induced HGF production from various types of cells including human dermal fibroblasts. Its effect was stronger than the effects of epidermal growth factor and cholera toxin and was comparable to the effect of 8-bromo-cAMP. Among the polymyxin family and polymyxin derivatives, colistin was also effective, whereas colistin methanesulfonate had only a marginal effect and PMB nonapeptide was ineffective. The stimulatory effect of PMB was accompanied by upregulation of HGF gene expression. Increase in phosphorylation of extracellular signal-regulated protein kinase (ERK) and c-Jun N-terminal kinase (JNK) was observed from 0.25 h to 6h after the addition of PMB, while increase in phosphorylation of p38 mitogen-activated protein kinase (MAPK) was detected from 24h to 60 h after PMB addition. The MAPK/ERK kinase inhibitor PD98059, the JNK inhibitor SP600125 and the p38 MAPK inhibitor SB203580 all potently inhibited PMB-induced HGF production. Lastly, proliferation of human dermal fibroblasts was significantly stimulated by PMB. These results indicate that PMB-induced HGF production and proliferation of human dermal fibroblasts and suggest that activation of MAPKs is involved in the induction of HGF production.

"Dirty little secrets"--endotoxin contamination of recombinant proteins

The identification of Toll-like receptors has revolutionised our understanding of innate immunity. TLR4 transduces the LPS signal and that of a number of structurally and functionally unrelated agonists. However, recent evidence adds to longstanding concerns that endotoxin contamination of bacterially derived recombinant TLR4 agonists is responsible for effects attributed to these molecules. We highlight key factors in differentiating specific agonist effects from those of endotoxin and emphasize why conventional methods of detecting and eliminating LPS may lead to erroneous results. We propose that considerable caution is needed in the investigation of TLR4 agonists, particularly when using proteins produced in a bacterium that also houses the most ideal TLR4 agonist, LPS.

Interference of antibacterial agents with phagocyte functions: immunomodulation or "immuno-fairy tales"?

Professional phagocytes (polymorphonuclear neutrophils and monocytes/macrophages) are a main component of the immune system. These cells are involved in both host defenses and various pathological settings characterized by excessive inflammation. Accordingly, they are key targets for immunomodulatory drugs, among which antibacterial agents are promising candidates. The basic and historical concepts of immunomodulation will first be briefly reviewed. Phagocyte complexity will then be unravelled (at least in terms of what we know about the origin, subsets, ambivalent roles, functional capacities, and transductional pathways of this cell and how to explore them). The core subject of this review will be the many possible interactions between antibacterial agents and phagocytes, classified according to demonstrated or potential clinical relevance (e.g., neutropenia, intracellular accumulation, and modulation of bacterial virulence). A detailed review of direct in vitro effects will be provided for the various antibacterial drug families, followed by a discussion of the clinical relevance of these effects in two particular settings: immune deficiency and inflammatory diseases. The prophylactic and therapeutic use of immunomodulatory antibiotics will be considered before conclusions are drawn about the emerging (optimistic) vision of future therapeutic prospects to deal with largely unknown new diseases and new pathogens by using new agents, new techniques, and a better understanding of the phagocyte in particular and the immune system in general.

Interference of antibacterial agents with phagocyte functions: immunomodulation or "immuno-fairy tales"?

Professional phagocytes (polymorphonuclear neutrophils and monocytes/macrophages) are a main component of the immune system. These cells are involved in both host defenses and various pathological settings characterized by excessive inflammation. Accordingly, they are key targets for immunomodulatory drugs, among which antibacterial agents are promising candidates. The basic and historical concepts of immunomodulation will first be briefly reviewed. Phagocyte complexity will then be unravelled (at least in terms of what we know about the origin, subsets, ambivalent roles, functional capacities, and transductional pathways of this cell and how to explore them). The core subject of this review will be the many possible interactions between antibacterial agents and phagocytes, classified according to demonstrated or potential clinical relevance (e.g., neutropenia, intracellular accumulation, and modulation of bacterial virulence). A detailed review of direct in vitro effects will be provided for the various antibacterial drug families, followed by a discussion of the clinical relevance of these effects in two particular settings: immune deficiency and inflammatory diseases. The prophylactic and therapeutic use of immunomodulatory antibiotics will be considered before conclusions are drawn about the emerging (optimistic) vision of future therapeutic prospects to deal with largely unknown new diseases and new pathogens by using new agents, new techniques, and a better understanding of the phagocyte in particular and the immune system in general.

Human leptin stimulates proliferation and activation of human circulating monocytes

The Ob gene product, leptin, is an adipocyte-secreted hormone that centrally regulates weight control. However, leptin receptor is expressed not only in the central nervous system, but also in other systems such as reproductive and hematopoietic tissues. Human leptin has previously been shown to enhance cytokine production by murine peritoneal macrophages. In this paper we show that human leptin stimulates proliferation in a dose-dependent manner and functionally activates human circulating monocytes in vitro, by inducing the production of cytokines such as TNF-alpha and IL-6. Proliferation was assessed both by [3H]thymidine and bromodeoxyuridine incorporation at 48 h. We also checked the leptin stimulated monocyte expression of activation markers by flow cytometry: CD25, HLA-DR, CD38, CD71, CD11b, and CD11c expression increased after 72 h. Moreover, leptin increases the expression of the early activation marker CD69 in monocytes but not in lymphocytes. The stimulation produced by leptin is comparable to that produced by endotoxin [lipopolysaccharide (LPS)]. In addition, leptin can potentiate the stimulatory effect of LPS or PMA. Furthermore, we studied cytokine production (TNF-alpha and IL-6) simultaneously by flow cytometric detection of intracellular cytokines in the activated monocytes. Leptin produced a dose-dependent increase in the number of activated monocytes producing cytokines. These data indicate that leptin is a potent stimulatory hormone on human peripheral blood monocytes and suggest that it may have a role as a proinflammatory cytokine.

Secretion of the C3 component of complement by peritoneal cells cultured with encapsulated Cryptococcus neoformans

Two isolates of Cryptococcus neoformans were identified as being widely divergent in pathogenic potential after intratracheal infection of mice. These isolates differed in their ability to upregulate capsule synthesis when grown under tissue culture conditions, and this property correlated with virulence. We postulated that differential capsule synthesis may cause differential stimulation of macrophages to produce products such as complement components. To test this hypothesis, heat-killed yeast cells were incubated with normal mouse peritoneal cells (PC) before the level of C3 secreted was determined. Cryptococcal stimulants were grown on mycological agar, which does not promote capsule synthesis, or in RPMI 1640 at 37 degrees C in an atmosphere of 5% CO2, which stimulates capsule synthesis, to determine the role that the capsule plays in the induction of C3 secretion. C3 levels were elevated in cultures containing cryptococci grown in RPMI 1640 at 37 degrees C in an atmosphere of 5% CO2, and the level of C3 detected was correlated with the amount of capsule expressed by the yeast cell stimulant. Nonencapsulated mutants of C. neoformans did not stimulate C3 secretion. Purified capsular polysaccharide (glucuronoxylomannan [GXM]) also stimulated the PC to secrete C3. Two signals were required before GXM stimulated C3 secretion. The second signal was identified as endotoxin present in small amounts (0.06 ng per ml) in tissue medium. Endotoxin may provide a priming stimulus for PC to express receptors or other cytokines needed for effective stimulation of C3. These experiments show that enhancement of C3 secretion by C. neoformans is due to GXM and is correlated with the virulence of the cryptococcal isolate.