Toll-like receptor 4 mediates intracellular signaling without TNF-alpha release in response to Cryptococcus neoformans polysaccharide capsule

Immunity to fungal infections

The topic of immunity to fungal infections is of interest to a wide range of disciplines, from microbiology to immunology. It is of particular interest in terms of therapy of HIV-infected individuals, and patients with cancer or individuals who have received transplants. Understanding the nature and function of the immune response to fungi is an exciting challenge that might set the stage for new approaches to the treatment of fungal diseases, from immunotherapy to vaccines. The past decade has witnessed the development of a wide range of new approaches to elucidate events that occur at the host-fungus interface.

Cellular recognition of tri-/di-palmitoylated peptides is independent from a domain encompassing the N-terminal seven leucine-rich repeat (LRR)/LRR-like motifs of TLR2

Toll-like receptors (TLRs) mediate microbial pattern recognition in vertebrates. A broad variety of agonists has been attributed to TLR2 and three TLRs, TLR4, TLR2, and TLR5, have been demonstrated to bind microbial products. Distinct agonists might interact with different subdomains of the TLR2 extracellular domain. The TLR2 extracellular domain sequence includes 10 canonical leucine-rich repeat (LRR) motifs and 8-10 additional and potentially functionally relevant LRR-like motifs. Thus, the transfection of TLR2 LRR/LRR-like motif deletion constructs in human embryonic kidney 293 cells and primary TLR2-deficient mouse fibroblasts was performed for analysis of the role of the regarding domains in specific pattern recognition. Preparations applied as agonists were highly purified soluble peptidoglycan, lipoteichoic acid, outer surface protein A from Borrelia burgdorferi, synthetic mycoplasmal macrophage-activating lipoprotein-2, tripalmitoyl-cysteinyl-seryl-(lysyl)3-lysine (P3CSK4), dipalmitoyl-CSK4 (P2-CSK4), and monopalmitoyl-CSK4 (PCSK4) as well as lipopolysaccharide and inactivated bacteria. We found that a block of the N-terminal seven LRR/LRR-like motifs was not involved in TLR2-mediated cell activation by P3CSK4 and P2CSK4 ligands mimicking triacylated and diacylated bacterial polypeptides, respectively. In contrast, the integrity of the TLR2 holoprotein was compulsory for effective cellular recognition of other TLR2 agonists applied, including PCSK4. The formation of a functionally relevant subdomain by a region including the N-terminal seven LRR/LRR-like motifs rather than by single LRRs is suggested by our results. They further imply that TLR2 contains multiple binding domains for ligands that may contribute to the characterization of its promiscuous molecular pattern recognition.

The antifungal drug amphotericin B promotes inflammatory cytokine release by a Toll-like receptor- and CD14-dependent mechanism

Amphotericin B is the most effective drug for treating many life-threatening fungal infections. Amphotericin B administration is limited by infusion-related toxicity, including fever and chills, an effect postulated to result from proinflammatory cytokine production by innate immune cells. Because amphotericin B is a microbial product, we hypothesized that it stimulates immune cells via Toll-like receptors (TLRs) and CD14. We show here that amphotericin B induces signal transduction and inflammatory cytokine release from cells expressing TLR2 and CD14. Primary murine macrophages and human cell lines expressing TLR2, CD14, and the adapter protein MyD88 responded to amphotericin B with NF-kappaB-dependent reporter activity and cytokine release, whereas cells deficient in any of these failed to respond. Cells mutated in TLR4 were less responsive to amphotericin B stimulation than cells expressing normal TLR4. These data demonstrate that TLR2 and CD14 are required for amphotericin B-dependent inflammatory stimulation of innate immune cells and that TLR4 may also provide stimulation of these cells. Our results provide a putative molecular basis for inflammatory responses elicited by amphotericin B and suggest strategies to eliminate the acute toxicity of this drug.

Toll-like receptor 4-mediated signaling by epithelial surfaces: necessity or threat?

Recent data suggest that the lipopolysaccharide receptor Toll-like receptor (TLR) 4 is expressed by epithelial cells and might play a role in the mucosal host defense against Gram-negative bacteria. However, since many body surfaces are colonized by the physiological microflora, activation of epithelial TLRs must be tightly controlled to avoid unintended stimulation and mucosal inflammation. The present review summarizes the current understanding of TLR4-mediated recognition and addresses specific questions on microbial recognition on mucosal surfaces, with particular emphasis on the gastrointestinal and urinary tract.

Differential role of MyD88 in macrophage-mediated responses to opportunistic fungal pathogens

Toll-like receptors mediate macrophage recognition of microbial ligands, inducing expression of microbicidal molecules and cytokines via the adapter protein MyD88. We investigated the role of MyD88 in regulating murine macrophage responses to a pathogenic yeast (Candida albicans) and mold (Aspergillus fumigatus). Macrophages derived from bone marrow of MyD88-deficient mice (MyD88(-/-)) demonstrated impaired phagocytosis and intracellular killing of C. albicans compared to wild-type (MyD88(+/+)) macrophages. In contrast, ingestion and killing of A. fumigatus conidia was MyD88 independent. Cytokine production by MyD88(-/-) macrophages in response to C. albicans yeasts and hyphae was substantially decreased, but responses to A. fumigatus hyphae were preserved. These results provide evidence that MyD88 signaling is involved in phagocytosis and killing of live C. albicans, but not A. fumigatus. The differential role of MyD88 may represent one mechanism by which macrophages regulate innate responses specific to different pathogenic fungi.

Toll-like receptor (TLR) 2 and TLR4 are essential for Aspergillus-induced activation of murine macrophages

Aspergillus fumigatius is a ubiquitous saprophytic fungus that has become the most prevalent airborne fungal pathogen for immunocompromised patients during the last two decades. In this report we have analysed how macrophages recognize this microorganism. Using transfected human HEK 293 cells we demonstrate that NF-kappaB-dependent promoter activation triggered by A. fumigatus is mediated by Toll-like receptors TLR2 and TLR4, whereas no activation was observed in cells overexpressing other distinct TLR proteins (TLR1, TLR3, TLR5-10). Using macrophages derived from mice lacking TLR2 expression, expressing defective TLR4 or both we found that A. fumigatus conidia and hyphae induce NF-kappaB translocation, release of pro-inflammatory molecules, like TNFalpha, and the chemoattractant MIP-2 in a TLR2- and TLR4-dependent manner. Recognition of A. niger and A. fumigatus, was similar in terms of the parameters analysed, suggesting that pathogenic and non-pathogenic aspergilli are sensed by macrophages in a similar fashion. Finally, we found that recruitment of neutrophils is severely impaired in mice lacking both functional TLR2 and TLR4, but is less impaired in single TLR2- or TLR4-deficient mice, providing evidence that both receptors are required for an optimal immune response to Aspergillus in vivo.

CpG DNA in the prevention and treatment of infections

Microbial infection is sensed by Toll-like receptors (TLRs) on innate immune cells. Among the ten so far defined TLRs, TLR9 and its ligand are peculiar. TLR9 recognises bacterial DNA characterised by the abundance of unmethylated CpG dinucleotides, which distinguish bacterial DNA (CpG DNA) from mammalian DNA. Moreover, TLR9 shows a restricted cellular and subcellular pattern of expression. In contrast to other TLR agonists, CpG DNA is superior in activation of dendritic dells and induction of costimulatory cytokines such as interleukin (IL)-12 and IL-18. This qualifies CpG DNA as a Th1-promoting adjuvant. During infection, recognition of CpG DNA of intracellular pathogens skews and fine-tunes the ongoing immune response and induces long-lasting Th1 milieus. Thus, CpG DNA might play an important role in driving the immune system to a Th1 profile, preventing undesired Th2 milieus that might favour induction of allergic responses. Since CpG DNA can be synthesised with high purity and sequence fidelity, synthetic CpG DNA will become an important agent for Th1 instruction and be an effective adjuvant during vaccination.

Humoral response against Cryptococcus neoformans mannoprotein antigens in HIV-infected patients

Twenty-four sera from healthy donors, 18 from HIV-positive patients (< 200 CD4+/mm3) and 18 sera collected before and during cryptococcosis from HIV-positive patients were analysed for the presence of humoral response to C. neoformans mannoproteins. Our results show that samples from healthy subjects and from HIV-positive patients had one of three antibody response profiles: (i) presence of reactive antibodies against both 105 and 80 kilodalton mannoproteins; (ii) presence of reactive antibodies against one of the two mannoproteins; or (iii) absence of reactive antibodies. Importantly the percentage of unreactive sera increased 6-fold in HIV-positive patients and more than 10-fold in patients with cryptococcosis. In addition, in the latter patients no variation of humoral response before and during cryptococcosis was observed. These results suggest that HIV-positive patients show a marked difficulty in mounting or maintaining antibody response to mannoprotein and this could contribute to predisposition to cryptococcosis.

The role of Toll-like receptor 2 in microbial disease and immunity

Cells expressing Toll-like receptor (TLR), TLR2 in association with TLR1, TLR6 or some other unknown co-receptor can respond upon interaction with a large variety of microbial ligands. The variety of TLR2 ligands is the greatest among all the TLRs and this is due to the heterodimerization needed for TLR2 mediated responses. Like other TLRs, TLR2 signaling induces antigen presenting cell activation, pro-inflammatory cytokine production and increased expression of co-stimulatory ligand expression. These events are important for induction of innate immune responses and improved acquired immunity. There is strong suggestive evidence that alteration or lack of TLR2 function in vivo may correlate to decreased immune protection from pathogens that contain TLR2 ligands, but more work needs to be performed to strengthen this correlation.

Role of innate immune factors in the adjuvant activity of monophosphoryl lipid A

Monophosphoryl lipid A (MPL) is a nontoxic derivative of lipopolysaccharide (LPS) that exhibits adjuvant properties similar to those of the parent LPS molecule. However, the mechanism by which MPL initiates its immunostimulatory properties remains unclear. Due to the involvement of Toll-like receptors in recognizing and transducing intracellular signals in response to LPS, the aim of the present study was to determine the ability of MPL to utilize the Toll-like receptor 2 (TLR2) and TLR4. We provide evidence that MPL differentially utilizes TLR2 and TLR4 for the induction of tumor necrosis factor alpha, interleukin 10 (IL-10), and IL-12 by purified human monocytes as well as by human peripheral blood mononuclear cells. Assessment of NF-kappa B activity demonstrated that MPL utilized TLR2 and especially TLR4 for the activation of NF-kappa B p65 by human monocytes. In addition, stimulation of human monocytes by MPL led to an up-regulation of the costimulatory molecules CD80 and CD86, an effect that could be reduced by pretreatment of cells with a monoclonal antibody to TLR2 or TLR4. Analysis of MPL-induced activation of the extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein (MAP) kinases revealed that MPL utilized both TLR2 and TLR4 for the phosphorylation of ERK1/2, while TLR4 was the predominant receptor involved in the ability of MPL to phosphorylate p38. Moreover, using selective inhibitors for MAP kinase kinase (PD98059) and p38 (SB203580), we show that ERK1/2 exhibited differential effects on production of TNF-alpha and IL-12 p40 by human monocytes, whereas MPL-induced activation of p38 appeared to be predominantly involved in production of IL-10 and IL-12 p40 by MPL-stimulated monocytes. Taken together, these findings aid in understanding the cellular mechanisms by which MPL induces host cell activation and subsequent adjuvant properties.

Antigenic and biological characteristics of mutant strains of Cryptococcus neoformans lacking capsular O acetylation or xylosyl side chains

Cryptococcus neoformans is surrounded by an antiphagocytic polysaccharide capsule whose primary constituent is glucuronoxylomannan (GXM). Three prominent structural features of GXM are single xylosyl and glucuronosyl side chains and O acetylation of the mannose backbone. Isogenic pairs of O-acetyl-positive and O-acetyl-negative strains (cas1 Delta) as well as xylose-positive and xylose-negative strains (uxs1 Delta) of serotype D have been reported. The cas1 Delta strains were hypervirulent, and the uxs1 Delta strains were avirulent. The goal of this study was to examine the effects of the cas1 Delta and uxs1 Delta mutations on the following: (i) binding of anti-GXM monoclonal antibodies (MAbs) in capsular quellung reactions, (ii) activation of the complement system and binding of C3, (iii) phagocytosis by neutrophils, and (iv) clearance of GXM in vivo. The results showed that loss of O acetylation produced dramatic changes in the reactivities of five of seven anti-GXM MAbs. In contrast, loss of xylosylation produced a substantive alteration in the binding behavior of only one MAb. O-acetyl-negative strains showed no alteration in activation and binding of C3 from normal serum. Xylose-negative strains exhibited accelerated kinetics for C3 deposition. Loss of O acetylation or xylosylation had no effect on phagocytosis of serum-opsonized yeast cells by human neutrophils. Finally, loss of O acetylation or xylosylation altered the kinetics for clearance of GXM from serum and accumulation of GXM in the liver and spleen. These results show that O acetylation and/or xylosylation are important for binding of anti-GXM MAbs, for complement activation, and for tissue accumulation of GXM but do not impact phagocytosis by neutrophils.

Toll-like receptors and their role in experimental models of microbial infection

Effective host defense against microbial infection depends upon prompt recognition of pathogens, activation of immediate containment measures, and ultimately the generation of a specific and definitive adaptive immune response. The innate immune system of the host is responsible for providing constant surveillance against infection; when confronted by pathogens it deploys a series of rapidly acting antimicrobial effectors while simultaneously instructing the adaptive immune system as to the nature and context of the infectious threat. Pathogen recognition and activation of innate immunity is mediated by members of the Toll-like receptor (TLR) family through detection of conserved microbial structures that are absent from the host. Experimental models of infection using TLR-deficient mice, as well as limited human studies, have clearly demonstrated the critical role of TLRs in host defense against most major groups of mammalian pathogens.

Synthesis of endotoxin receptor CD14 protein in Kupffer cells and its role in alcohol-induced liver disease

AIM

To observe the synthesis of endotoxin receptor CD14 protein and its mRNA expression in Kupffer cells (KCs), and evaluate the role of CD14 in the pathogenesis of liver injury in rats with alcohol-induced liver disease (ALD).

METHODS

Twenty-eight Wistar rats were divided into two groups: ethanol-fed group and control group. Ethanol-fed group was fed ethanol (dose of 5g-12 g/kg/d) and control group received dextrose instead of ethanol. Two groups were sacrificed at 4 wk and 8 wk, respectively. KCs were isolated and the synthesis of CD14 protein and its mRNA expression in KCs were determined by flow cytometric analysis (FCM) or the reverse transcription polymerase chain reaction (RT-PCR) analysis. The levels of plasma endotoxin and alanine transaminase (ALT) were measured by Limulus Amebocyte Lysate assay and standard enzymatic procedures respectively, and the levels of plasma tumor necosis factor (TNF)-alpha and interleukin (IL)-6 were both determined by ELISA. The liver pathology change was observed under light and electric microscopy.

RESULTS

In ethanol-fed group, the percentages of FITC-CD14 positive cells were 76.23 % and 89.42 % at 4 wk and 8 wk, respectively. Compared with control group (4.45 % and 5.38 %), the difference was significant (P<0.05). The expressions of CD14 mRNA were 7.56+/-1.02 and 8.74+/-1.37 at 4 wk and 8 wk, respectively, which were significantly higher compared with the control group (1.77+/-0.21 and 1.98+/-0.23) (P<0.05). Plasma endotoxin levels at 4 wk and 8 wk increased significantly in ethanol-fed group (129+/-21 ng/L and 187+/-35 ng/L) than those in control rats (48+/-9 ng/L and 53+/-11 ng/L)(P<0.05). Mean values of plasma ALT levels increased dramatically in ethanol-fed rats (112+/-15 IU/L and 147+/-22 IU/L) than those in the control animals (31+/-12 IU/L and 33+/-9 IU/L) (P<0.05). In ethanol-fed rats, the levels of TNF-alpha were 326+/-42 ng/L and 402+/-51 ng/L at 4 wk and 8 wk, respectively which were significantly higher than those in control group (86+/-12 ng/L and 97+/-13 ng/L) (P<0.05). The levels of IL-6 were 387+/-46 ng/L and 413+/-51 ng/L, which were also higher than control group (78+/-11 ng/Land 73+/-10 ng/L) (P<0.05). In liver section from ethanol-fed rats, there were marked pathological changes including steatosis, cell infiltration and necrosis. No marked pathological changes were seen in control group.

CONCLUSION

Ethanol administration led to a significant synthesis of endotoxin receptor CD14 protein and its gene expression in KCs, which maybe result in the pathological changes of liver tissue and hepatic functional damages.

Counteracting interactions between lipopolysaccharide molecules with differential activation of toll-like receptors

We investigated counteracting interactions between the lipopolysaccharides (LPS) from Escherichia coli (Ec-LPS) and Porphyromonas gingivalis (Pg-LPS), which induce cellular activation through Toll-like receptor 4 (TLR4) and TLR2, respectively. We found that Ec-LPS induced tolerance in THP-1 cells to subsequent tumor necrosis factor alpha (TNF-alpha) and interleukin 1 beta (IL-1beta) induction by Pg-LPS, though the reverse was not true, and looked for explanatory differential effects on the signal transduction pathway. Cells exposed to Pg-LPS, but not to Ec-LPS, displayed persisting expression of IL-1 receptor-associated kinase without apparent degradation, presumably allowing prolonged relay of downstream signals. Accordingly, cells pretreated with Pg-LPS, but not with Ec-LPS, were effectively activated in response to subsequent exposure to either LPS molecule, as evidenced by assessing nuclear factor (NF)-kappaB activity. In fact, Pg-LPS primed THP-1 cells for enhanced NF-kappaB activation and TNF-alpha release upon restimulation with the same LPS. This was a dose-dependent effect and correlated with upregulation of surface TLR2 expression. Furthermore, we observed inhibition of NF-kappaB-dependent transcription in a reporter cell line pretreated with Ec-LPS and restimulated with Pg-LPS (compared to cells pretreated with medium only and restimulated with Pg-LPS), but not when the reverse treatment was made. Although Pg-LPS could not make cells tolerant to subsequent activation by Ec-LPS, Pg-LPS inhibited Ec-LPS-induced TNF-alpha and IL-6 release when the two molecules were added simultaneously into THP-1 cell cultures. Pg-LPS also suppressed P. gingivalis FimA protein-induced NF-kappaB-dependent transcription in the 3E10/huTLR4 reporter cell line, which does not express TLR2. This rules out competition for common signaling intermediates, suggesting that Pg-LPS may block component(s) of the TLR4 receptor complex. Interactions between TLR2 and TLR4 agonists may be important in the regulation of inflammatory reactions.

Involvement of microbial components and toll-like receptors 2 and 4 in cytokine responses to air pollution particles

Inhalation of particulate matter (PM) may result in exacerbation of inflammatory airways disease, including asthma. Results from this laboratory have shown that the coarse inhalable particle fraction (PM(2.5-10)) is responsible for most of the PM effects on human airway macrophages (AM), including induction of cytokine production. Endotoxins associated with these particles account for a large part of their potency, as activity of PM can be inhibited by polymixin B and an activating moiety bound by lipopolysaccharide (LPS)-binding protein (LBP). The hypothesis behind the present study was that not only particle-bound LPS, but also Gram-negative (Gram-) and Gram-positive (Gram+) bacteria are responsible for PM-induced stimulation of AM, and therefore that PM are likely to activate receptors involved in recognition of microbes. Low level contamination of model pollution particles with environmental Staphyloccocus, Streptococcus, and Pseudomonas species was found to confer cytokine-inducing activity on inactive particles. Only one Gram- bacterium was sufficient for significant stimulatation of 100 AM, whereas at least three times more Gram+ bacteria were required for a similar level of response. Cytokine responses induced by PM as well as Gram+ and Gram- bacteria were inhibited by anti-CD14 antibody and required the presence of LBP-containing serum. The involvement of Toll-like receptor (TLR) 2 and 4 in recognition of PM(2.5-10) was investigated in transfected Chinese hamster ovary cells expressing CD14 and TLR2 or TLR4. TLR4 was found to be involved in PM(2.5-10) and Pseudomonas-induced activation, whereas TLR2 activation was induced by both Gram+ and Gram- bacteria and by PM. The synthetic lipid A analog E5531 fully inhibited the response to purified LPS and partially inhibited the response to PM and Pseudomonas. In contrast, E5531 had no effect on the response to Staphylococcus. Taken together, these results implicate microbial components as important players in AM-dependent inflammatory responses to PM.

Toll-like receptor (TLR) signaling in response to Aspergillus fumigatus

Aspergillus fumigatus causes life-threatening infections in patients with qualitative and quantitative defects in phagocytic function. Here, we examined the contribution of Toll-like receptor (TLR)-2, TLR4, the adapter protein MyD88, and CD14 to signaling in response to the three forms of A. fumigatus encountered during human disease: resting conidia (RC), swollen conidia (SC), and hyphae (H). Compared with elicited peritoneal macrophages obtained from wild-type and heterozygous mice, TLR2(-/-) and MyD88(-/-) macrophages produced significantly less tumor necrosis factor-alpha (TNFalpha) following A. fumigatus stimulation. In contrast, following stimulation with RC, SC, and H, TLR4(-/-) and CD14(-/-) macrophages exhibited no defects in tumor necrosis factor-alpha release. TLR2(-/-), TLR4(-/-), MyD88(-/-), and CD14(-/-) macrophages bound similar numbers of RC and SC compared with wild-type macrophages. RC, SC, and H stimulated greater activation of a nuclear factor kappa B (NFkappaB)-dependent reporter gene and greater release of tumor necrosis factor-alpha from the human monocytic THP-1 cell line stably transfected with CD14 compared with control cells stably transfected with empty vector. A. fumigatus stimulated NFkappaB-dependent reporter gene activity in the human embryonic kidney cell line, HEK293, only if the cells were transfected with TLR2. Moreover, activity increased when TLR2 and CD14 were co-transfected. Taken together, these data suggest that optimal signaling responses to A. fumigatus require TLR2 in both mouse and human cells. In contrast, a role for CD14 was found only in the human cells. MyD88 acts as a central adapter protein mediating signaling responses following stimulation with RC, SC, and H.

Involvement of toll-like receptor (TLR) 2 and TLR4 in cell activation by mannuronic acid polymers

The alginate capsule produced by the human pathogen Pseudomonas aeruginosa is composed mainly of mannuronic acid polymers (poly-M) that have immunostimulating properties. Poly-M shares with lipopolysaccharide the ability to stimulate cytokine production from human monocytes in a CD14-dependent manner. In the present study we examined the role of Toll-like receptor (TLR) 2 and TLR4 in responses to poly-M. Blocking antibodies to TLR2 and TLR4 partly inhibited tumor necrosis factor production induced by poly-M in human monocytes, and further inhibition was obtained by combining the antibodies. By transiently transfecting HEK293 cells, we found that membrane CD14 together with either TLR2 or TLR4/MD-2 could mediate activation by poly-M. Transfection of HEK293 cells with TLR2 and fluorescently labeled TLR4 followed by co-patching of TLR2 with an antibody revealed no association of these molecules on the plasma membrane. However, macrophages from the Tlr4 mutant C3H/HeJ mice and TLR4 knockout mice were completely non-responsive to poly-M, whereas the tumor necrosis factor release from TLR2 knockout macrophages was half of that seen with wild type cells. Taken together the results suggest that both TLR2 and TLR4 are involved in cell activation by poly-M and that TLR4 may be required in primary murine macrophages.

Interactions of fungi with phagocytes

Recent advances have broadened our knowledge of the unique role that dendritic cells, macrophages and neutrophils play in protecting the host against fungal infections and the mechanisms by which fungal pathogens attempt to subvert phagocytic defenses. In this article, the interplay between phagocytes and fungi is reviewed.

Mammalian toll-like receptors

OBJECTIVE

To review the role of mammalian Toll-like receptors (TLRs) in host defense.

DATE SOURCES

MEDLINE search and current literature.

RESULTS

First, TLRs participate in the recognition of molecular patterns present on microorganisms. Second, TLRs are expressed at the interface with the environment, the site of microbial invasion. Third, activation of TLRs induces expression of costimulatory molecules and the release of cytokines that instruct the adaptive immune response. Fourth, activation of TLRs leads to direct antimicrobial effector pathways that can result in elimination of the foreign invader.

CONCLUSIONS

Maintained throughout evolution, mammalian TLRs are proteins that participate in innate immunity to microbial pathogens. Insights into TLR activation pathways provide new therapeutic targets for intervention in infectious and immunologic disease.

Fc-dependent and Fc-independent opsonization of Cryptococcus neoformans by anticapsular monoclonal antibodies: importance of epitope specificity

Monoclonal antibodies (MAbs) reactive with glucuronoxylomannan (GXM), the major capsular polysaccharide of the yeast Cryptococcus neoformans, produce distinct capsular reactions when viewed by differential interference contrast microscopy. These reactions depend on the epitope specificity of the antibody. Opsonic activities of immunoglobulin G1 (IgG1) MAbs that produce patterns termed rim and puffy were examined. Rim-pattern MAbs are reactive with an epitope shared by GXM serotypes A, B, C, and D. Puffy-pattern MAbs are reactive only with serotypes A and D. In phagocytosis assays, using serotype A cells and resident murine peritoneal macrophages, rim-pattern MAbs were markedly more opsonic than puffy-pattern MAbs. F(ab')(2) fragments of rim-pattern MAbs were synergistic with heat-labile factors in normal human serum for opsonization of the yeast. F(ab')(2) fragments of puffy-pattern MAbs were also synergistic with normal serum in opsonization but at a much lower level than fragments of rim-pattern MAbs. Normal serum alone was not opsonic. F(ab')(2) fragments of rim-pattern MAbs, but not puffy-pattern MAbs, stimulated phagocytosis of encapsulated cryptococci in the absence of serum. This serum-independent opsonic action of F(ab')(2) fragments was abrogated by pretreatment of macrophages with purified GXM, suggesting the involvement of a phagocyte GXM receptor. The results indicate that (i) there are multiple mechanisms by which anticapsular IgG MAbs facilitate phagocytosis of encapsulated cryptococci, (ii) some anti-GXM antibodies are opsonic in an Fc-independent manner, and (iii) opsonic activity correlates with the capsular reaction and occurs in an epitope-specific manner.

BAD1, an essential virulence factor of Blastomyces dermatitidis, suppresses host TNF-alpha production through TGF-beta-dependent and -independent mechanisms

We investigated how BAD1, an adhesin and virulence factor of Blastomyces dermatitidis, suppresses phagocyte proinflammatory responses. Wild-type yeast cocultured with murine neutrophils or macrophages prompted release of a soluble factor into conditioned supernatant that abolished TNF-alpha production in response to the fungus; isogenic, attenuated BAD1 knockout yeast did not have this effect. Phagocytes released 4- to 5-fold more TGF-beta in vitro in response to wild-type yeast vs BAD1 knockout yeast. Treatment of inhibitory, conditioned supernatant with anti-TGF-beta mAb neutralized detectable TGF-beta and restored phagocyte TNF-alpha production. Similarly, addition of anti-TGF-beta mAb into cultures of phagocytes and wild-type yeast reversed BAD1 inhibition of TNF-alpha production. Conversely, TGF-beta treatment of phagocytes cultured with knockout yeast suppressed TNF-alpha production. Hence, TGF-beta mediates BAD1 suppression of TNF-alpha by wild-type B. dermatitidis cultured in vitro with phagocytes. In contrast to these findings, neutralization of elevated TGF-beta levels during experimental pulmonary blastomycosis did not restore BAD1-suppressed TNF-alpha levels in the lung or ameliorate disease. Soluble BAD1 was found to accumulate in the alveoli of infected mice at levels that suppressed TNF-alpha production by phagocytes. However, in contrast to yeast cell surface BAD1, which induced TGF-beta, soluble BAD1 failed to do so and TNF-alpha suppression mediated by soluble BAD1 was unaffected by neutralization of TGF-beta. Thus, BAD1 of B. dermatitidis induces suppression of TNF-alpha and progressive infection by both TGF-beta-dependent and -independent mechanisms.

Inhibition of mitogen-activated protein kinase signaling by chloroquine

Previously, we demonstrated that the anti-inflammatory drug chloroquine (CQ) inhibited LPS-induced TNF-alpha transcription. To define further the mechanism of CQ, we studied the effect of this drug on mitogen-activated protein kinase signaling pathways involved in regulation of TNF production. CQ interfered with phosphorylation of extracellular signal-regulated kinases (ERK)1/2 and the ERK-activating kinases mitogen-activating protein/ERK kinase (MEK)1/2. Both CQ and PD98059, a MEK1 inhibitor, reduced luciferase reporter activity driven by human TNF promoter sequences. However, CQ appeared to mediate these effects by deactivating Raf, the upstream activator of MEK. These findings were supported by functional data demonstrating that CQ and PD98059 interfered with TNF expression in several human and murine cell types while neither inhibitor blocked TNF production in murine RAW264.7 macrophages, a cell line that does not require MEK-ERK signaling for TNF production. Finally, we evaluated whether CQ could sensitize HeLa cells to undergo anti-Fas-mediated apoptosis, an effect observed when ERK activation is interrupted in this cell line. CQ rendered HeLa cells sensitive to anti-Fas treatment in a manner similar to PD98059. Taken together, these data argue that therapeutic concentrations of CQ interfere with ERK activation by a novel mechanism, an effect that could be responsible, at least in part, for the potent anti-inflammatory effects of this drug.

Dependence of bacterial protein adhesins on toll-like receptors for proinflammatory cytokine induction

Toll-like receptors (TLRs) are important signal transducers that mediate inflammatory reactions induced by microbes through pattern recognition of virulence molecules such as lipopolysaccharide (LPS) and lipoproteins. We investigated whether proinflammatory cytokine responses induced by certain bacterial protein adhesins may also depend on TLRs. In differentiated THP-1 mononuclear cells stimulated by LPS-free recombinant fimbrillin (rFimA) from Porphyromonas gingivalis, cytokine release was abrogated by monoclonal antibodies (MAbs) to CD14 and TLR4 but not to TLR2. Similar experiments using anti-beta2 integrin MAbs suggested that beta2 integrins (CD11/CD18) also play a role in cytokine induction by rFimA or native fimbriae. Minor fimbriae (distinct from the fimA-encoded major fimbriae) of P. gingivalis induced proinflammatory cytokine release in a CD14- and TLR2-dependent mode. Cytokine induction by BspA, a leucine-rich repeat protein from Bacteroides forsythus, depended heavily on CD14 and TLR2. We also found that the ability of the streptococcal protein AgI/II to stimulate cytokine release depended partially on CD14 and TLR4, and the AgI/II segment that possibly interacts with these receptors was identified as its N-terminal saliva-binding region. When THP-1 cells were exposed to rFimA for 24 h, surface expression of CD14 and CD18 was decreased and the cells became hyporesponsive to cytokine induction by a second challenge with rFimA. However, tolerance induction was abolished when the THP-1 cells were pretreated with rFimA in the presence of either anti-CD14 MAb or anti-TLR4 MAb. Induction of cross-tolerance between rFimA and LPS correlated with downregulation of the pattern recognition receptors involved. Our data suggest that the CD14-TLR2/4 system is involved in cytokine production and tolerance induction upon interaction with certain proinflammatory bacterial protein adhesins.

Toll-like receptors

The ability of a host to sense invasion by pathogenic organisms and to respond appropriately to control infection is paramount to survival. In the case of sepsis and septic shock, however, an exaggerated systemic response may, in fact, contribute to the morbidity and mortality associated with overwhelming infections. The innate immune system has evolved as the first line of defense against invading microorganisms. The Toll-like receptors (TLRs) are a part of this innate immune defense, recognizing conserved patterns on microorganisms. These TLRs and their signaling pathways are represented in such diverse creatures as mammals, fruit flies, and plants. Ten members of the TLR family have been identified in humans, and several of them appear to recognize specific microbial products, including lipopolysaccharide, bacterial lipoproteins, peptidoglycan, and bacterial DNA. Signals initiated by the interaction of TLRs with specific microbial patterns direct the subsequent inflammatory response. Thus, TLR signaling represents a key component of the innate immune response to microbial infection.

Differential induction of endotoxin tolerance by lipopolysaccharides derived from Porphyromonas gingivalis and Escherichia coli

Exposure of mononuclear phagocytes to enterobacterial LPS induces a state of transient hyporesponsiveness to subsequent LPS exposure, termed endotoxin tolerance. In the present study, LPS derived from the oral periodontal pathogen, Porphyromonas gingivalis, was compared with that derived from the enterobacterium, Escherichia coli, for the ability to induce endotoxin tolerance. Pretreatment of the human macrophage cell line, THP-1, with E. coli LPS resulted in a severe reduction in the levels of IL-1beta, IL-6, and TNF-alpha upon secondary stimulation. In contrast, pretreatment of THP-1 cells with P. gingivalis LPS resulted in a mitigation of IL-1beta, but not IL-6 and TNF-alpha production upon subsequent exposure to P. gingivalis LPS: primary or secondary stimulation with < or =100 ng/ml P. gingivalis LPS resulted in comparable levels of IL-6 and TNF-alpha, while stimulation of THP-1 cells with > or =1 microg/ml P. gingivalis LPS induced a significant enhancement in IL-6 and TNF-alpha levels upon secondary exposure. To identify possible mechanisms for these differences, changes in the expression of molecules involved in the LPS-signaling pathway were assessed. Pretreatment of THP-1 cells with E. coli LPS resulted in a significant reduction in surface Toll-like receptor 4 (TLR4) expression and an inability to degrade I-kappaB-alpha or I-kappaB-beta proteins upon secondary stimulation. In contrast, pretreatment of THP-1 cells with P. gingivalis LPS resulted in a significant enhancement of both CD14 and TLR2, while maintaining the ability to degrade I-kappaB-beta only upon secondary stimulation. Thus, E. coli and P. gingivalis LPS differentially affect CD14 and TLR expression as well as secondary LPS-associated responses.

Toll-like receptors: cellular signal transducers for exogenous molecular patterns causing immune responses

Innate immunity initiates protection of the host organism against invasion and subsequent multiplication of microbes by specific recognition. Germ line-encoded receptors have been identified for microbial products such as mannan, lipopeptide, peptidoglycan (PGN), lipoteichoic acid (LTA), lipopolysaccharide (LPS), and CpG-DNA. The Drosophila Toll protein has been shown to be involved in innate immune response of the adult fruitfly. Members of the family of Toll-like receptors (TLRs) in vertebrates have been implicated as pattern recognition receptors (PRRs). Ten TLRs are known and six of these have been demonstrated to mediate cellular activation by distinct microbial products. TLR4 has been implicated as activator of adaptive immunity, and analysis of systemic LPS responses in mice led to the identification of LPS-resistant strains instrumental in its identification as a transmembrane LPS signal transducer. Structural similarities between TLRs and receptor molecules involved in immune responses such as CD14 and the IL-1 receptors (IL-1Rs), as well as functional analysis qualified TLR2 as candidate receptor for LPS and other microbial products. Targeted disruption of the TLR9 gene in mice led to identification of TLR9 as CpG-DNA signal transducer. Involvement of TLR5 in cell activation by bacterial flagellin has been demonstrated. Further understanding of recognition and cellular signaling activated through the ancient host defense system represented by Toll will eventually lead to means for its therapeutic modulation.