Nonopsonic binding of Mycobacterium tuberculosis to human complement receptor type 3 expressed in Chinese hamster ovary cells

Phylactic role of anti-lipoarabinomannan IgM directed against mannan core during mycobacterial infection in macrophages

Mycobacteria enter host phagocytes, such as macrophages by binding to several receptors on phagocytes. Several mycobacterial species, including Mycobacterium tuberculosis have evolved systems to evade host bactericidal pathways. Lipoarabinomannan (LAM) is an essential mycobacterial molecule for both binding to phagocytes and escaping from bactericidal pathways. Integrin CD11b plays critical roles as a phagocytic receptor and contributes to host defense by mediating both nonopsonic and opsonic phagocytosis. However, the mechanisms by which CD11b-mediated phagocytosis associates with LAM and drives the phagocytic process of mycobacteria remain to be fully elucidated. We recently identified TMDU3 as anti-LAM IgM antibody against the mannan core of LAM. The present study investigated the roles of CD11b and TMDU3 in macrophage phagocytosis of mycobacteria and subsequent bactericidal lysosomal fusion to phagosomes. CD11b knockout cells generated by a CRISPR/Cas9 system showed significant attenuation of the ability to phagocytose non-opsonized mycobacteria and LAM-conjugated beads. Moreover, recombinant human CD11b protein was found to bind to LAM. TMDU3 markedly inhibited macrophage phagocytosis of non-opsonized mycobacteria. This antibody slightly increased the phagocytosis of mycobacteria under opsonized conditions, whereas it significantly enhanced CD11b-mediated bactericidal functions. Taken together, these results show a novel phylactic role of anti-LAM IgM during mycobacterial infection in macrophages.

Binary and ternary complexes of FLNa-Ig21 with cytosolic tails of αMß2 integrin reveal dual role of filamin mediated regulation

BACKGROUND

Cytoskeletal protein filamin A is critical for the outside-in signaling of integrins. Although molecular mechanisms of filamin-integrin interactions are not fully understood. Mostly, the membrane distal (MD) part of the cytosolic tail (CT) of β subunit of integrin is known to interact with filamin A domain 21 (FLNa-Ig2). However, binary and ternary complexes of full-length CTs of leucocyte specific ß2 integrins with FLNa-Ig21 are yet to be elucidated.

METHODS

Binding interactions of the CTs of integrin αMß2 with FLNa-Ig21 are extensively investigated by NMR, ITC, cell-based functional assays and computational docking.

RESULTS

The αM CT demonstrates interactions with FLNa-Ig21 forming a binary complex. Filamin/αM interface is mediated by sidechain-sidechain interactions among non-polar and aromatic residues involving MP helix of αM and the canonical CD face of FLNa-Ig21. Functional assays delineated an interfacial residue Y1137 of αM CT is critical for in-cell binding to FLNa-Ig2. In addition, full-length ß2 CT occupies two distinct binding sites in complex with FLNa-Ig21. A ternary complex of FLNa-Ig21 with CTs has been characterized. In the ternary complex, αM CT moves away to a distal site of FLNa-Ig21 with fewer interactions.

CONCLUSION

Our findings demonstrate a plausible dual role of filamin in integrin regulation. The molecular interactions of the ternary complex are critical for the resting state of integrins whereas stable FLNa-Ig21/αM CT binary complex perhaps be required for the activated state.

GENERAL SIGNIFICANCE

Filamin binding to both α and β CTs of other integrins could be essential in regulating bidirectional signaling mechanisms.

Rv0180c contributes to Mycobacterium tuberculosis cell shape and to infectivity in mice and macrophages

Mycobacterium tuberculosis, the main causative agent of human tuberculosis, is transmitted from person to person via small droplets containing very few bacteria. Optimizing the chance to seed in the lungs is therefore a major adaptation to favor survival and dissemination in the human population. Here we used TnSeq to identify genes important for the early events leading to bacterial seeding in the lungs. Beside several genes encoding known virulence factors, we found three new candidates not previously described: rv0180c, rv1779c and rv1592c. We focused on the gene, rv0180c, of unknown function. First, we found that deletion of rv0180c in M. tuberculosis substantially reduced the initiation of infection in the lungs of mice. Next, we established that Rv0180c enhances entry into macrophages through the use of complement-receptor 3 (CR3), a major phagocytic receptor for M. tuberculosis. Silencing CR3 or blocking the CR3 lectin site abolished the difference in entry between the wild-type parental strain and the Δrv0180c::km mutant. However, we detected no difference in the production of both CR3-known carbohydrate ligands (glucan, arabinomannan, mannan), CR3-modulating lipids (phthiocerol dimycocerosate), or proteins in the capsule of the Δrv0180c::km mutant in comparison to the wild-type or complemented strains. By contrast, we established that Rv0180c contributes to the functionality of the bacterial cell envelope regarding resistance to toxic molecule attack and cell shape. This alteration of bacterial shape could impair the engagement of membrane receptors that M. tuberculosis uses to invade host cells, and open a new perspective on the modulation of bacterial infectivity.

The epidemic efficiency of tuberculosis bacilli is determined by their capacity to transmit via aerosol. Currently, the bacterial functions that favor Mycobacterium tuberculosis seeding in the lung of naïve host remain mostly unknown. Here we implemented a genome-wide approach to identify M. tuberculosis mutants deficient for seeding and early replication in the lung of mice. In addition to genes known to encode virulence factors, we identified three genes not previously described. We used complementary approaches to characterize the phenotype of a M. tuberculosis mutant with insertion within the rv0180c gene. We found that this mutant is impaired for seeding in the lung of mice and for invasion and replication in human macrophages. In macrophages, the defect relies on a lack of engagement of CR3 receptor. Although we did not detect any difference between the wild type strain and the rv0180c mutant with regard to potential CR3-ligand, we found that the bacterial cell envelope is altered in the rv0180c mutant. Our study provides new insight into bacterial genes required for early interaction of M. tuberculosis with the host and perspective to understand the bacterial functions enhancing infectivity.

Exploring the Use of Medicinal Plants and Their Bioactive Derivatives as Alveolar NLRP3 Inflammasome Regulators during Mycobacterium tuberculosis Infection

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), is a successful intracellular pathogen that is responsible for the highest mortality rate among diseases caused by bacterial infections. During early interaction with the host innate cells, M. tuberculosis cell surface antigens interact with Toll like receptor 4 (TLR4) to activate the nucleotide-binding domain, leucine-rich-repeat containing family, pyrin domain-containing 3 (NLRP3) canonical, and non-canonical inflammasome pathways. NLRP3 inflammasome activation in the alveoli has been reported to contribute to the early inflammatory response that is needed for an effective anti-TB response through production of pro-inflammatory cytokines, including those of the Interleukin 1 (IL1) family. However, overstimulation of the alveolar NLRP3 inflammasomes can induce excessive inflammation that is pathological to the host. Several studies have explored the use of medicinal plants and/or their active derivatives to inhibit excessive stimulation of the inflammasomes and its associated factors, thus reducing immunopathological response in the host. This review describes the molecular mechanism of the NLRP3 inflammasome activation in the alveoli during M. tuberculosis infection. Furthermore, the mechanisms of inflammasome inhibition using medicinal plant and their derivatives will also be explored, thus offering a novel perspective on the alternative control strategies of M. tuberculosis-induced immunopathology.

Complement receptor 3 mediates Aspergillus fumigatus internalization into alveolar epithelial cells with the increase of intracellular phosphatidic acid by activating FAK

Complement receptor 3 (CD11b/CD18) is an important receptor that mediates adhesion, phagocytosis and chemotaxis in various immunocytes. The conidia of the medically-important pathogenic fungus, Aspergillus fumigatus can be internalized into alveolar epithelial cells to disseminate its infection in immunocompromised host; however, the role of CR3 in this process is poorly understood. In the present study, we investigated the potential role of CR3 on A. fumigatus internalization into type II alveolar epithelial cells and its effect on host intracellular PA content induced by A. fumigatus. We found that CR3 is expressed in alveolar epithelial cells and that human serum and bronchoalveolar lavage fluid (BALF) could improve A. fumigatus conidial internalization into A549 type II alveolar epithelial cell line and mouse primary alveolar epithelial cells, which were significantly inhibited by the complement C3 quencher and CD11b-blocking antibody. Serum-opsonization of swollen conidia, but not resting conidia led to the increase of cellular phosphatidic acid (PA) in A549 cells during infection. Moreover, both conidial internalization and induced PA production were interfered by CD11b-blocking antibody and dependent on FAK activity, but not Syk in alveolar epithelial cells. Overall, our results revealed that CR3 is a critical modulator of Aspergillus fumigatus internalization into alveolar epithelial cells.

The Promiscuous Profile of Complement Receptor 3 in Ligand Binding, Immune Modulation, and Pathophysiology

The β2-integrin receptor family has a broad spectrum of physiological functions ranging from leukocyte adhesion, cell migration, activation, and communication to the phagocytic uptake of cells and particles. Among the members of this family, complement receptor 3 (CR3; CD11b/CD18, Mac-1, αMβ2) is particularly promiscuous in its functional profile and ligand selectivity. There are close to 100 reported structurally unrelated ligands for CR3, and while many ligands appear to cluster at the αMI domain, molecular details about binding modes remain largely elusive. The versatility of CR3 is reflected in its functional portfolio, which includes prominent roles in the removal of invaders and cell debris, induction of tolerance and synaptic pruning, and involvement in the pathogenesis of numerous autoimmune and chronic inflammatory pathologies. While CR3 is an interesting therapeutic target for immune modulation due to these known pathophysiological associations, drug development efforts are limited by concerns of potential interference with host defense functions and, most importantly, an insufficient molecular understanding of the interplay between ligand binding and functional impact. Here, we provide a systematic summary of the various interaction partners of CR3 with a focus on binding mechanisms and functional implications. We also discuss the roles of CR3 as an immune receptor in health and disease, as an activation marker in research and diagnostics, and as a therapeutic target.

Innate Immune Pattern Recognition Receptors of Mycobacterium tuberculosis: Nature and Consequences for Pathogenesis of Tuberculosis

Innate immunity against Mycobacterium tuberculosis is a critical early response to prevent the establishment of the infection. Despite recent advances in understanding the host-pathogen dialogue in the early stages of tuberculosis (TB), much has yet to be learnt. The nature and consequences of this dialogue ultimately determine the path of infection: namely, either early clearance of M. tuberculosis, or establishment of M. tuberculosis infection leading to active TB disease and/or latent TB infection. On the frontline in innate immunity are pattern recognition receptors (PRRs), with soluble factors (e.g. collectins and complement) and cell surface factors (e.g. Toll-like receptors and other C-type lectin receptors (Dectin 1/2, Nod-like receptors, DC-SIGN, Mincle, mannose receptor, and MCL) that play a central role in recognising M. tuberculosis and facilitating its clearance. However, in a 'double-edged sword' scenario, these factors can also be involved in enhancement of pathogenesis as well. Furthermore, innate immunity is also a critical bridge in establishing the subsequent adaptive immune response, which is also responsible for granuloma formation that cordons off M. tuberculosis infection, establishing latency and acting as a reservoir for bacterial persistence and dissemination of future disease. This chapter discusses the current understanding of pattern recognition of M. tuberculosis by innate immunity and the role this plays in the pathogenesis and protection against TB.

Phthiocerol Dimycocerosates From Mycobacterium tuberculosis Increase the Membrane Activity of Bacterial Effectors and Host Receptors

Mycobacterium tuberculosis (Mtb) synthesizes a variety of atypical lipids that are exposed at the cell surface and help the bacterium infect macrophages and escape elimination by the cell's immune responses. In the present study, we investigate the mechanism of action of one family of hydrophobic lipids, the phthiocerol dimycocerosates (DIM/PDIM), major lipid virulence factors. DIM are transferred from the envelope of Mtb to host membranes during infection. Using the polarity-sensitive fluorophore C-Laurdan, we visualized that DIM decrease the membrane polarity of a supported lipid bilayer put in contact with mycobacteria, even beyond the site of contact. We observed that DIM activate the complement receptor 3, a predominant receptor for phagocytosis of Mtb by macrophages. DIM also increased the activity of membrane-permeabilizing effectors of Mtb, among which the virulence factor EsxA. This is consistent with previous observations that DIM help Mtb disrupt host cell membranes. Taken together, our data show that transferred DIM spread within the target membrane, modify its physical properties and increase the activity of host cell receptors and bacterial effectors, diverting in a non-specific manner host cell functions. We therefore bring new insight into the molecular mechanisms by which DIM increase Mtb's capability to escape the cell's immune responses.

The Mycobacterium tuberculosis capsule: a cell structure with key implications in pathogenesis

Bacterial capsules have evolved to be at the forefront of the cell envelope, making them an essential element of bacterial biology. Efforts to understand the Mycobacterium tuberculosis (Mtb) capsule began more than 60 years ago, but the relatively recent development of mycobacterial genetics combined with improved chemical and immunological tools have revealed a more refined view of capsule molecular composition. A glycogen-like α-glucan is the major constituent of the capsule, with lower amounts of arabinomannan and mannan, proteins and lipids. The major Mtb capsular components mediate interactions with phagocytes that favor bacterial survival. Vaccination approaches targeting the mycobacterial capsule have proven successful in controlling bacterial replication. Although the Mtb capsule is composed of polysaccharides of relatively low complexity, the concept of antigenic variability associated with this structure has been suggested by some studies. Understanding how Mtb shapes its envelope during its life cycle is key to developing anti-infective strategies targeting this structure at the host-pathogen interface.

Human Properdin Modulates Macrophage: Mycobacterium bovis BCG Interaction via Thrombospondin Repeats 4 and 5

Mycobacterium tuberculosis can proficiently enter macrophages and diminish complement activation on its cell surface. Within macrophages, the mycobacterium can suppress macrophage apoptosis and survive within the intracellular environment. Previously, we have shown that complement regulatory proteins such as factor H may interfere with pathogen–macrophage interactions during tuberculosis infection. In this study, we show that Mycobacterium bovis BCG binds properdin, an upregulator of the complement alternative pathway. TSR4+5, a recombinant form of thrombospondin repeats 4 and 5 of human properdin expressed in tandem, which is an inhibitor of the alternative pathway, was also able to bind to M. bovis BCG. Properdin and TSR4+5 were found to inhibit uptake of M. bovis BCG by THP-1 macrophage cells in a dose-dependent manner. Quantitative real-time PCR revealed elevated pro-inflammatory responses (TNF-α, IL-1β, and IL-6) in the presence of properdin or TSR4+5, which gradually decreased over 6 h. Correspondingly, anti-inflammatory responses (IL-10 and TGF-β) showed suppressed levels of expression in the presence of properdin, which gradually increased over 6 h. Multiplex cytokine array analysis also revealed that properdin and TSR4+5 significantly enhanced the pro-inflammatory response (TNF-α, IL-1β, and IL-1α) at 24 h, which declined at 48 h, whereas the anti-inflammatory response (IL-10) was suppressed. Our results suggest that properdin may interfere with mycobacterial entry into macrophages via TSR4 and TSR5, particularly during the initial stages of infection, thus affecting the extracellular survival of the pathogen. This study offers novel insights into the non-complement related functions of properdin during host–pathogen interactions in tuberculosis.

The impact of impaired macrophage functions in cystic fibrosis disease progression

The underlying cause of morbidity in cystic fibrosis (CF) is the decline in lung function, which results in part from chronic inflammation. Inflammation and infection occur early in infancy in CF and the role of innate immune defense in CF has been highlighted in the last years. Once thought simply to be consumers of bacteria, macrophages have emerged as highly sensitive immune cells that are located at the balance point between inflammation and resolution of this inflammation in CF pathophysiology. In order to assess the potential role of macrophage in CF, we review the evidence that: (1) CF macrophage has a dysregulated inflammatory phenotype; (2) CF macrophage presents altered phagocytosis capacity and bacterial killing; and (3) lipid disorders in CF macrophage affect its function. These alterations of macrophage weaken innate defense of CF patients and may be involved in CF disease progression and lung damage.

Outbreaks of Mycobacterium tuberculosis MDR strains differentially induce neutrophil respiratory burst involving lipid rafts, p38 MAPK and Syk

Background

Neutrophils (PMN) are the first cells to infiltrate the lung after infection, and they play a significant protective role in the elimination of pathogen, by releasing preformed oxidants and proteolytic enzymes from granules and generating ROS, thus limiting inflammation by succumbing to apoptosis. In a previous study, we found marked differences in ROS-induced apoptosis between two Mycobacterium tuberculosis (Mtb) strains, M and Ra, representative of widespread Mtb families in South America, i.e. Haarlem and Latin-American Mediterranean (LAM), being strain M able to generate further drug resistance and to disseminate aggressively.

Methods

In this study we evaluate the nature of bacteria-PMN interaction by assessing ROS production, apoptosis, lipid raft coalescence, and phagocytosis induced by Mtb strains.

Results

Dectin-1 and TLR2 participate in Mtb-induced ROS generation and apoptosis in PMN involving p38 MAPK and Syk activation with the participation of a TLR2-dependent coalescence of lipid rafts. Further, ROS production occurs during the phagocytosis of non-opsonized bacteria and involves α-glucans on the capsule. In contrast, strain M lacks the ability to induce ROS because of: 1) a reduced phagocytosis and 2) a failure in coalescence of lipid raft.

Conclusions

The differences in wall composition could explain the success of some strains which stay unnoticed by the host through inhibition of apoptosis and ROS but making possible its replication inside PMN as a potential evasion mechanism. Innate immune responses elicited by Mtb strain-to-strain variations need to be considered in TB vaccine development.

Impaired functions of macrophage from cystic fibrosis patients: CD11b, TLR-5 decrease and sCD14, inflammatory cytokines increase

BACKGROUND

Early in life, cystic fibrosis (CF) patients are infected with microorganisms. The role of macrophages has largely been underestimated in literature, whereas the focus being mostly on neutrophils and epithelial cells. Macrophages may however play a significant role in the initiating stages of this disease, via an inability to act as a suppressor cell. Yet macrophage dysfunction may be the first step in cascade of events leading to chronic inflammation/infection in CF. Moreover, reports have suggested that CFTR contribute to altered inflammatory response in CF by modification of normal macrophage functions.

OBJECTIVES

In order to highlight possible intrinsic macrophage defects due to impaired CFTR, we have studied inflammatory cytokines secretions, recognition of pathogens and phagocytosis in peripheral blood monocyte-derived macrophages from stable adult CF patients and healthy subjects (non-CF).

RESULTS

In CF macrophage supernatants, concentrations of sCD14, IL-1β, IL-6, TNF-α and IL-10 were strongly raised. Furthermore expression of CD11b and TLR-5 were sorely decreased on CF macrophages. Beside, no difference was observed for mCD14, CD16, CD64, TLR-4 and TLR1/TLR-2 expressions. Moreover, a strong inhibition of phagocytosis was observed for CF macrophages. Elsewhere CFTR inhibition in non-CF macrophages also led to alterations of phagocytosis function as well as CD11b expression.

CONCLUSIONS

Altogether, these findings demonstrate excessive inflammation in CF macrophages, characterized by overproduction of sCD14 and inflammatory cytokines, with decreased expression of CD11b and TLR-5, and impaired phagocytosis. This leads to altered clearance of pathogens and non-resolution of infection by CF macrophages, thereby inducing an exaggerated pro-inflammatory response.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

Molecular cloning, characterization and gene expression of the full length cDNA encoding the porcine CD11b(αM) and chromosomal localization of the porcine CD11a(αL)-CD11b(αM)-CD11b(αD) gene cluster

CD11b (α(M)) is a cell-surface glycoprotein mainly expressed on myeloid cells, required for important interactions during the immune response and involved in the pathogenesis of several diseases. The full length cDNA encoding porcine CD11b protein has been cloned and sequenced. Pig CD11b cDNA sequence contains an ORF of 3459 nucleotides that encodes a deduced polypeptide of 1152 amino acid residues that share with CD11b from other species: High % amino acid identity, common domains (α-I, Ca(++) binding, MIDAS), N-glycosylation sites, and the seven FG-GAP tandem repeats. Real time quantitative PCR expression analysis revealed that CD11b transcripts were highly expressed in neutrophils, showing a lower expression in spleen. The CD11a-CD11b-CD11c gene cluster locates on the porcine chromosome region SSC3p15-17.

Structures and interaction analyses of integrin αMβ2 cytoplasmic tails

Integrins are heterodimeric (α and β subunits) signal transducer proteins involved in cell adhesions and migrations. The cytosolic tails of integrins are essential for transmitting bidirectional signaling and also implicated in maintaining the resting states of the receptors. In addition, cytosolic tails of integrins often undergo post-translation modifications like phosphorylation. However, the consequences of phosphorylation on the structures and interactions are not clear. The leukocyte-specific integrin αMβ2 is essential for myeloid cell adhesion, phagocytosis, and degranulation. In this work, we determined solution structures of the myristoylated cytosolic tail of αM and a Ser phosphorylated variant in dodecylphosphocholine micelles by NMR spectroscopy. Furthermore, the interactions between non-phosphorylated and phosphorylated αM tails with β2 tail were investigated by NMR and fluorescence resonance energy transfer (FRET). The three-dimensional structures of the 24-residue cytosolic tail of αM or phosphorylated αM are characterized by an N-terminal amphipathic helix and a loop at the C terminus. The residues at the loop are involved in packing interactions with the hydrophobic face of the helix. 15N-1H heteronuclear single quantum coherence experiments identified residues of αM and β2 tails that may be involved in the formation of a tail-tail heterocomplex. We further examined interactions between myristoylated β2 tail in dodecylphosphocholine micelles with dansylated αM tail peptides by FRET. These studies revealed enhanced interactions between αM or phosphorylated αM tails with β2 tail with Kd values ∼5.2±0.6 and ∼4.4±0.7 μm, respectively. Docked structures of tail-tail complexes delineated that the αM/β2 interface at the cytosolic region could be sustained by a network of polar interactions, ionic interactions, and/or hydrogen bonds.

Mycobacterium leprae phenolglycolipid-1 expressed by engineered M. bovis BCG modulates early interaction with human phagocytes

The species-specific phenolic glycolipid 1 (PGL-1) is suspected to play a critical role in the pathogenesis of leprosy, a chronic disease of the skin and peripheral nerves caused by Mycobacterium leprae. Based on studies using the purified compound, PGL-1 was proposed to mediate the tropism of M. leprae for the nervous system and to modulate host immune responses. However, deciphering the biological function of this glycolipid has been hampered by the inability to grow M. leprae in vitro and to genetically engineer this bacterium. Here, we identified the M. leprae genes required for the biosynthesis of the species-specific saccharidic domain of PGL-1 and reprogrammed seven enzymatic steps in M. bovis BCG to make it synthesize and display PGL-1 in the context of an M. leprae-like cell envelope. This recombinant strain provides us with a unique tool to address the key questions of the contribution of PGL-1 in the infection process and to study the underlying molecular mechanisms. We found that PGL-1 production endowed recombinant BCG with an increased capacity to exploit complement receptor 3 (CR3) for efficient invasion of human macrophages and evasion of inflammatory responses. PGL-1 production also promoted bacterial uptake by human dendritic cells and dampened their infection-induced maturation. Our results therefore suggest that M. leprae produces PGL-1 for immune-silent invasion of host phagocytic cells.

Integrin alphaMbeta2 clustering triggers phosphorylation and activation of protein kinase C delta that regulates transcription factor Foxp1 expression in monocytes

Integrins are type I membrane and heterodimeric (alphabeta) cell adhesion receptors. Intracellular signals triggered by ligand-bound integrins are important for cell growth, differentiation, and migration. Integrin alpha(M)beta(2) plays key roles in myeloid cell adhesion, phagocytosis, and degranulation. In this study, we show that protein kinase C (PKC) delta is involved in alpha(M)beta(2) signaling. In human monocytic U937 cells and peripheral blood monocytes, alpha(M)beta(2) clustering induced PKCdelta translocation to the plasma membrane, followed by Tyr(311) phosphorylation and activation of PKCdelta by the src family kinases Hck and Lyn. Interestingly, alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation was not mediated by the tyrosine kinase Syk, which is a well reported kinase in beta(2) integrin signaling. Analysis of the beta(2) cytoplasmic tail showed that the sequence Asn(727)-Ser(734) is important in alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation. It has been shown that alpha(M)beta(2) clustering regulates the expression the transcription factor Foxp1 that has a role in monocyte differentiation. We show that Foxp1 expression was reduced in monocytes that were allowed to adhere to human microvascular endothelial cells. However, the expression of Foxp1 was not affected in monocytes that were treated with PKCdelta-targeting small interfering RNA, suggesting that PKCdelta regulates Foxp1 expression. These results demonstrate a role of PKCdelta in alpha(M)beta(2)-mediated Foxp1 regulation in monocytes.

Multiple routes of complement activation by Mycobacterium bovis BCG

Mycobacterium tuberculosis is the leading cause of infectious disease in humans in the world. It evades the host immune system by being phagocytosed by macrophages and residing intracellularly. Complement-dependent opsonisation of extracellular mycobacteria may assist them to enter macrophages. This work examines in detail the mechanisms of complement activation by whole mycobacteria using Mycobacterium bovis BCG as a model organism. M. bovis BCG directly activates the classical, lectin and alternative pathways, resulting in fixation of C3b onto macromolecules of the mycobacterial surface. Investigation into the classical pathway has shown direct binding of human C1q to whole mycobacteria in the absence of antibodies. Most human sera contain IgG and IgM-anti-(M. bovis BCG), and pre-incubation with human immunoglobulin enhances C1q binding to the bacteria. Therefore classical pathway activation is both antibody-independent and dependent. The bacteria also activate the alternative pathway in an antibody-independent manner, but Factor H also binds, suggesting some regulation of amplification by this pathway. For the lectin pathway we have demonstrated direct binding of both MBL and L-ficolin from human serum to whole mycobacteria and subsequent MASP2 activation. H-ficolin binding was not observed. No M. bovis BCG cell surface or secreted protease appears likely to influence complement activation. Together, these data provide a more detailed analysis of the mechanisms by which M. bovis BCG interacts with the complement system.

TREM-2 (triggering receptor expressed on myeloid cells 2) is a phagocytic receptor for bacteria

Phagocytosis, which is essential for the immune response to pathogens, is initiated by specific interactions between pathogens and cell surface receptors expressed by phagocytes. This study identifies triggering receptor expressed on myeloid cells 2 (TREM-2) and its signaling counterpart DAP12 as a molecular complex that promotes phagocytosis of bacteria. Expression of TREM-2-DAP12 enables nonphagocytic Chinese hamster ovary cells to internalize bacteria. This function depends on actin cytoskeleton dynamics and the activity of the small guanosine triphosphatases Rac and Cdc42. Internalization also requires src kinase activity and tyrosine phosphorylation. In bone marrow-derived macrophages, phagocytosis is decreased in the absence of DAP12 and can be restored by expression of TREM-2-DAP12. Depletion of TREM-2 inhibits both binding and uptake of bacteria. Finally, TREM-2-dependent phagocytosis is impaired in Syk-deficient macrophages. This study highlights a novel role for TREM-2-DAP12 in the immune response to bacterial pathogens.

Urokinase-type plasminogen activator receptor induces conformational changes in the integrin alphaMbeta2 headpiece and reorientation of its transmembrane domains

The glycosylphosphatidylinositol-linked urokinase-type plasminogen activator receptor (uPAR) interacts with the heterodimer cell adhesion molecules integrins to modulate cell adhesion and migration. Devoid of a cytoplasmic domain, uPAR triggers intracellular signaling via its associated molecules that contain cytoplasmic domains. Interestingly, uPAR changes the ectodomain conformation of one of its partner molecules, integrin alpha(5)beta(1), and elicits cytoplasmic signaling. The separation or reorientation of integrin transmembrane domains and cytoplasmic tails are required for integrin outside-in signaling. However, there is a lack of direct evidence showing these conformational changes of an integrin that interacts with uPAR. In this investigation we used reporter monoclonal antibodies and fluorescence resonance energy transfer analyses to show conformational changes in the alpha(M)beta(2) headpiece and reorientation of its transmembrane domains when alpha(M)beta(2) interacts with uPAR.

CD11b of Ovis canadensis and Ovis aries: molecular cloning and characterization

Leukotoxin (Lkt) is the primary virulence factor secreted by Mannheimia haemolytica which causes pneumonia in ruminants. Previously, we have shown that CD18, the beta subunit of beta(2) integrins, mediates Lkt-induced cytolysis of ruminant leukocytes. CD18 associates with four distinct alpha subunits giving rise to four beta(2) integrins, CD11a/CD18 (LFA-1), CD11b/CD18 (Mac-1), CD11c/CD18 (CR4), and CD11d/CD18. It is not known whether all the beta(2) integrins serve as a receptor for Lkt. Since PMNs are the leukocyte subset that is most susceptible to Lkt, and Mac-1 expression on PMNs exceeds that of other beta(2) integrins, it is of interest to determine whether Mac-1 serves as a receptor for Lkt which necessitates the cloning of CD11b and CD18. In this study, we cloned and sequenced the cDNA encoding CD11b of Ovis canadensis (bighorn sheep) and Ovis aries (domestic sheep). CD11b cDNA is 3455 nucleotides long encoding a polypeptide of 1152 amino acids. CD11b polypeptides from these two species exhibit 99% identity with each other, and 92% with that of cattle, and 70-80% with that of the non-ruminants analyzed.

Molecular cloning and characterization of cDNA encoding CD11b of cattle

CD18, the common beta subunit of beta2-integrins, associates with four distinct alpha chains to give rise to four different beta2-integrins: CD11a/CD18 (LFA-1), CD11b/CD18 (Mac-1), CD11c/CD18 (CR4), and CD11d/CD18. Previously, we and others showed that CD18 of LFA-1 serves as a receptor for Mannheimia haemolytica leukotoxin (Lkt). Level of expression of Mac-1 is higher than that of LFA-1 and other beta2-integrins on polymorphonuclear leukocytes (PMNs), which constitute the leukocyte subset most susceptible to Lkt. Hence, it is likely that CD18 of Mac-1 also mediates Lkt-induced cytolysis. Co-expression of CD11b and CD18 of cattle on Lkt-resistant cells is necessary to irrefutably demonstrate the role of Mac-1 in Lkt-induced cytolysis. This approach is hindered by lack of availability of complete sequence of cattle CD11b. Therefore, in this study, we cloned and sequenced the full length cDNA encoding cattle CD11b. The 3459 bp cDNA of cattle CD11b encodes a polypeptide of 1152 amino acids. The deduced amino acid sequence of CD11b of cattle exhibits 75% identity to that of humans and chimpanzees, 74% identity to that of dogs, and 70% identity to that of mice and rats. Availability of cattle CD11b cDNA should facilitate the elucidation of Lkt-receptor interactions in cattle and other species.

DC-SIGN induction in alveolar macrophages defines privileged target host cells for mycobacteria in patients with tuberculosis

BACKGROUND

Interplays between Mycobacterium tuberculosis, the etiological agent of tuberculosis (TB) in human and host professional phagocytes, namely macrophages (Mphis) and dendritic cells (DCs), are central to immune protection against TB and to TB pathogenesis. We and others have recently shown that the C-type lectin dendritic cell-specific intercellular adhesion molecule-3 grabbing nonintegrin (DC-SIGN; CD209) mediates important interactions between mycobacteria and human monocyte-derived DCs (MoDCs) in vitro.

METHODS AND FINDINGS

In order to explore the possible role of DC-SIGN in M. tuberculosis infection in vivo, we have analysed DC-SIGN expression in broncho-alveolar lavage (BAL) cells from patients with TB (n = 40) or with other non-mycobacterial lung pathologies, namely asthma (n = 14) and sarcoidosis (n = 11), as well as from control individuals (n = 9). We show that in patients with TB, up to 70% of alveolar Mphis express DC-SIGN. By contrast, the lectin is barely detected in alveolar Mphis from all other individuals. Flow cytometry, RT-PCR, and enzyme-linked immunosorbent assay analyses of BAL-derived fluids and cells indicated that M. tuberculosis infection induces DC-SIGN expression in alveolar Mphis by a mechanism that is independent of Toll-like receptor-4, interleukin (IL)-4, and IL-13. This mechanism most likely relies on the secretion of soluble host and/or mycobacterial factors that have yet to be identified, as both infected and uninfected bystander Mphis were found to express DC-SIGN in the presence of M. tuberculosis. Immunohistochemical examination of lung biopsy samples from patients with TB showed that the bacilli concentrate in pulmonary regions enriched in DC-SIGN-expressing alveolar Mphis in vivo. Ex vivo binding and inhibition of binding experiments further revealed that DC-SIGN-expressing alveolar Mphis constitute preferential target cells for M. tuberculosis, as compared to their DC-SIGN- counterparts. In contrast with what has been reported previously in MoDCs in vitro, ex vivo DC-SIGN ligation by mycobacterial products failed to induce IL-10 secretion by alveolar Mphis, and IL-10 was not detected in BALs from patients with TB.

CONCLUSION

Altogether, our results provide further evidence for an important role of DC-SIGN during TB in humans. DC-SIGN induction in alveolar Mphis may have important consequences on lung colonization by the tubercle bacillus, and on pulmonary inflammatory and immune responses in the infected host.

Cell biology of mycobacterium tuberculosis phagosome

Phagocytosis and phagolysosome biogenesis represent fundamental biological processes essential for proper tissue homeostasis, development, elimination of invading microorganisms, and antigen processing and presentation. Phagosome formation triggers a preprogrammed pathway of maturation into the phagolysosome, a process controlled by Ca2+ and the regulators of organellar trafficking centered around the small GTP-binding proteins Rabs and their downstream effectors, including lipid kinases, organellar tethering molecules, and membrane fusion apparatus. Mycobacterium tuberculosis is a potent human pathogen parasitizing macrophages. It interferes with the Rab-controlled membrane trafficking and arrests the maturing phagosome at a stage where no harm can be done to the pathogen while the delivery of nutrients and membrane to the vacuole harboring the microorganism continues. This process, referred to as the M. tuberculosis phagosome maturation arrest or inhibition of phagosome-lysosome fusion, is critical for M. tuberculosis persistence in human populations. It also provides a general model system for dissecting the phagolysosome biogenesis pathways. Here we review the fundamental trafficking processes targeted by M. tuberculosis and the mycobacterial products that interfere with phagosomal maturation.

Mycobacteria use their surface-exposed glycolipids to infect human macrophages through a receptor-dependent process

Two subfamilies of the polar glycopeptidolipids (GPLs) located on the surface of Mycobacterium smegmatis, along with unknown phospholipids, were recently shown to participate in the nonopsonic phagocytosis of mycobacteria by human macrophages (Villeneuve, C., G. Etienne, V. Abadie, H. Montrozier, C. Bordier, F. Laval, M. Daffe, I. Maridonneau-Parini, and C. Astarie-Dequeker. 2003. Surface-exposed glycopeptidolipids of Mycobacterium smegmatis specifically inhibit the phagocytosis of mycobacteria by human macrophages. Identification of a novel family of glycopeptidolipids. J. Biol. Chem. 278: 51291-51300). As demonstrated herein, a phospholipid mixture that derived from the methanol-insoluble fraction inhibited the phagocytosis of M. smegmatis. Inhibition was essentially attributable to phosphatidylinositol mannosides (PIMs), namely PIM2 and PIM6, because the purified phosphatidylethanolamine, phosphatidylglycerol, and phosphatidylinositol were inactive. This was further confirmed using purified PIM2 and PIM6 from M. bovis BCG that decreased by half the internalization of M. smegmatis. Both compounds also inhibited the uptake of M. tuberculosis and M. avium but had no effect on the internalization of zymosan used as a control particle of the phagocytic process. When coated on latex beads, PIM2 and polar GPL (GPL III) favored the particle entry through complement receptor 3. GPL III, but not PIM2, also directed particle entry through the mannose receptor. Therefore, surface-exposed mycobacterial PIM and polar GPL participate in the receptor-dependent internalization of mycobacteria in human macrophages.

Complement protein C3 binding to Mycobacterium tuberculosis is initiated by the classical pathway in human bronchoalveolar lavage fluid

In high concentrations of fresh nonimmune human serum, Mycobacterium tuberculosis activates the alternative pathway of complement and binds C3 protein, resulting in enhanced phagocytosis by complement receptors on human alveolar macrophages. Yet in the lung, the alternative pathway of complement is relatively inactive compared to the classical pathway. To begin to determine whether C3 opsonophagocytosis of M. tuberculosis by alveolar macrophages can occur in the lung of the immunologically naive host, we characterized the binding of C3 to M. tuberculosis in different concentrations of fresh nonimmune human serum and concentrated human bronchoalveolar lavage fluid. Here we show that in human serum, C3 binding to M. tuberculosis is rapid, initiated by either the alternative pathway or the classical pathway, depending on the concentration of serum, and occurs by covalent linkages between the bacterial surface and the C3 cleavage products, C3b or C3bi. Human bronchoalveolar lavage fluid contains C3 protein and functional classical pathway activity that mediates the binding of C3 to the surface of M. tuberculosis. These studies provide evidence that when M. tuberculosis is first inhaled into the lungs of the human host, the bacterium is opsonized by C3 cleavage via classical pathway activation within the alveolus, providing a C3-dependent entry pathway into resident alveolar macrophages.

Macrophage--Mycobacterium tuberculosis interactions: role of complement receptor 3

Tuberculosis is the leading infectious disease in the world. Mycobacterium tuberculosis, the causal agent of this disease, invades macrophages and can replicate inside them. Because invasion of macrophages is a critical step for establishing a mycobacterial infection, there is much interest in understanding the mechanisms for M. tuberculosis entry into macrophages. Complement receptor 3 (CR3) is a heterodimeric surface receptor with multiple binding sites, which can mediate complement-opsonized as well as nonopsonic entrance of M. tuberculosis into macrophages. Here, we describe and discuss the role of CR3 in macrophage[bond]M. tuberculosis interactions. The actual information suggests that CR3 mediates a substantial amount of M. tuberculosis binding to macrophages, but CR3 is not related to the mechanisms that allow mycobacteria to survive and replicate intracellularly. Understanding the mechanisms of macrophage[bond]M. tuberculosis interaction will help developing more effective methods to prevent and treat tuberculosis in the future.

Role of the lectin domain of Mac-1/CR3 (CD11b/CD18) in regulating intercellular adhesion

Leukocyte diapedesis requires that Mac-1/CR3-dependent adhesion be regulated so that cells can move from one attachment site to another. The high affinity adhesion state of Mac-1/CR3 is generated when it forms a lectin-dependent complex with the receptor for urokinase plasminogen activator (uPAR; CD87). The extensively glycosylated uPAR binds to the same C-terminal lectin domain of CD11b that had previously been shown to prime Mac-1/CR3 for cytotoxic degranulation in response to beta-glucan. uPAR and beta-glucan compete for a lectin site that is near to the CBRM1/23 epitope (residues 943-1047) at the C-terminus of CD11b, and thus the lectin domain is critical to both the adhesion and cytotoxic functions of Mac-1/CR3. Adhesion is reversed when the uPA enzyme is captured by its receptor (uPAR), causing uPAR to bind to CD11b at a second site (residues 424-440) that is in between the N-terminal I-domain and the divalent cation binding region.

Complement receptor 3 (CD11b/CD18) mediates type I and type II phagocytosis during nonopsonic and opsonic phagocytosis, respectively

Two types of opsonic phagocytosis have been defined depending on the receptor engaged: FcgammaRs mediate type I phagocytosis of IgG-coated particles; complement receptor 3 (CR3) mediates type II phagocytosis of complement-coated particles. In addition to opsonic phagocytosis, CR3 also mediates nonopsonic phagocytosis of zymosan (Z) and Mycobacterium kansasii through engagement of distinct sites. Using Chinese hamster ovary cells stably expressing human CR3, we studied CR3-mediated ingestion of nonopsonized particles, Z or M. kansasii, compared with opsonized zymosan (OZ). We show that 1) while OZ sinks into cells, Z is engulfed by pseudopodia as visualized by electron microscopy; 2) in contrast to OZ, nonopsonic phagocytosis of Z and M. kansasii depends on Rac and Cdc42 but not on Rho activity; and 3) CR3-mediated phagocytosis of Z depends on the kinase activity of the Src family tyrosine kinase Hck, while OZ internalization does not. Therefore, CR3 mediates type I phagocytosis under nonopsonic conditions and type II under opsonic conditions. This is the first evidence that a single receptor can mediate both types of phagocytosis depending on the ligand used.

Mycobacterial protein HbhA binds human complement component C3

Mycobacterium tuberculosis and Mycobacterium avium are facultative intracellular pathogens that are able to survive and replicate in mononuclear phagocytes. Human complement component C3 has previously been shown to mediate attachment and phagocytosis of these bacteria by mononuclear phagocytes. In this study, a C3 ligand affinity blot protocol was used to identify a 30-kDa C3-binding protein in M. tuberculosis and Mycobacterium smegmatis and a 31-kDa C3-binding protein in M. avium. The C3-binding proteins in M. tuberculosis and M. avium localized to the cell membrane fraction and partitioned to the detergent fraction during Triton X-114 phase partitioning. The C3-binding protein from M. tuberculosis was partially purified using a cation exchange column and was shown to bind concanavalin A. The N terminus and an internal fragment of the partially purified C3-binding protein were subjected to amino acid sequence analysis. The resulting amino acid sequences matched the M. tuberculosis heparin-binding hemagglutinin (HbhA) protein. Recombinant full-length HbhA and the C terminus of HbhA fused to maltose-binding protein, but not recombinant HbhA lacking the C-terminal region, bound human C3. Recombinant full-length HbhA coated on polystyrene beads, was found to enhance the adherence and/or phagocytosis of the coated beads to J774.A1 cells in both the presence and absence of human serum. The presence of complement-sufficient serum increased the adherence of the HbhA-coated beads to the J774.A1 cells in a C3-dependent manner. If HbhA within the bacterial cell membrane functions similarly to isolated HbhA, this protein may enhance the adherence and phagocytosis of M. tuberculosis and M. avium to mononuclear phagocytes through the binding of C3 and interaction with C3 receptors on mononuclear phagocytes.

Effect of mycobacterial phospholipids on interaction of Mycobacterium tuberculosis with macrophages

This study demonstrates that pretreatment of macrophages with phosphatidylinositol, of either soya bean or mycobacterial origin, results in a down-regulation of the binding and uptake of Mycobacterium tuberculosis by the phagocytes. We also describe the novel observation that cardiolipin induces an increase in the binding and uptake of M. tuberculosis by macrophages. Neither phospholipid interacts with macrophages via the 2F8 epitope of scavenger receptor A, and treatment of macrophages with either phospholipid results in a down-regulation of CR3 function and tumor necrosis factor alpha production by the phagocyte. We have also shown that the ability of macrophages to interact with mycobacteria is greatly affected by an as yet unidentified product from the interaction of chloroform and polypropylene tubes.

Monocyte-derived macrophage cytokine responses induced by M. bovis BCG

SETTING

One important aspect of macrophage function is the production of inflammatory and anti-inflammatory cytokines, which in turn affect the survival of intracellular organisms such as mycobacteria.

OBJECTIVE

To determine the relationship between phagocytosis of mycobacteria and expression of intracellular cytokines.

DESIGN

Phagocytosis and cytokine production were studied simultaneously within human monocyte-derived macrophages (MDMs) from healthy donors using fluorescent labelling of M. bovis BCG and flow cytometry.

RESULTS

At a range of infection ratios (5:1, 1:1, 0.2:1) TNF- alpha, IL-10, IL-6 and IL-12 were all produced in a dose-dependent manner. At an infection ratio representative of the in vivo situation (1:1), cytokine production was induced in both MDMs containing intracellular M. bovis BCG and in uninfected bystander MDMs. Phagocytosis increased over time, but there was considerable donor variation: the proportions of cells containing one or more mycobacterium were 15.4+/-14.8% (mean+/-SD) at 4 h and 32.7+/-21.1% at 24 h (n=19). Analysis of cytokine production by MDMs not containing mycobacteria (bystander cells) at 4 h revealed that these uninfected cells produced 79+/-6.6% of the TNF- alpha, 53.9+/-40.0% of the IL-10 and 64.2+/-12.4% of the IL-12. By 20 h these proportions had decreased to 57+/-13.5%, 30.9+/-7.4% and 45.5+/-13.3% respectively.

CONCLUSION

Both infected and bystander MDMs can be stimulated to produce cytokines in response to M. bovis BCG, indicating that the ability of MDMs to produce cytokines is not necessarily dependent on the ability to phagocytose mycobacteria.

Cellular and molecular mechanisms of internalization of mycobacteria by host cells

Mycobacteria are intracellular pathogens capable of invading mononuclear phagocytes, mucosal epithelial cells (including M cells) and Schwann cells. To enter cells, mycobacteria have been shown to interact with several molecules on macrophage and epithelial cell surfaces. This suggests adaptation to the host environment. In this review we address the strategies used by pathogenic mycobacteria to gain access to the intracellular environment.

Nonopsonic phagocytosis of Mycobacterium kansasii by human neutrophils depends on cholesterol and is mediated by CR3 associated with glycosylphosphatidylinositol-anchored proteins

Receptors involved in the phagocytosis of microorganisms under nonopsonic conditions have been little studied in neutrophils. Complement receptor type 3 (CR3) is a pattern recognition receptor able to internalize zymosan and C3bi-coated particles. We report that Abs directed against CR3 strongly inhibited nonopsonic phagocytosis of Mycobacterium kansasii in human neutrophils. In these cells CR3 has been found associated with several GPI-anchored proteins localized in cholesterol-rich microdomains (rafts) of the plasma membrane. Cholesterol sequestration by nystatin, filipin, or beta-cyclodextrin as well as treatment of neutrophils with phosphatidylinositol phospholipase C to remove GPI-anchored proteins from the cell surface markedly inhibited phagocytosis of M. kansasii, without affecting phagocytosis of zymosan or serum-opsonized M. kansasii. Abs directed against several GPI-anchored proteins inhibited phagocytosis of M. kansasii, but not of zymosan. N:-acetyl-D-glucosamine, which is known to disrupt interactions between CR3 and GPI proteins, also strongly diminished phagocytosis of these mycobacteria. In conclusion, phagocytosis of M. kansasii involved CR3, GPI-anchored receptors, and cholesterol. In contrast, phagocytosis of zymosan or opsonized particles involved CR3, but not cholesterol or GPI proteins. We propose that CR3, when associated with a GPI protein, relocates in cholesterol-rich domains where M. kansasii are internalized. When CR3 is not associated with a GPI protein, it remains outside of these domains and mediates phagocytosis of zymosan and opsonized particles, but not of M. kansasii.

Utilization of CD11b knockout mice to characterize the role of complement receptor 3 (CR3, CD11b/CD18) in the growth of Mycobacterium tuberculosis in macrophages

Using CD11b knockout mice as a source of macrophages (Mphi;), we show that complement receptor 3 (CR3) mediates approximately 40-50% of nonopsonic binding and 50-60% of serum-mediated binding of Mycobacterium tuberculosis to resident Mphi;. We demonstrate that opsonic binding of M. tuberculosis to Mphi; is mediated by an immunoglobulin-independent, heat-labile component of serum, in both the presence and the absence of CD11b. The survival and replication of M. tuberculosis in an in vitro Mphi; model and an in vivo mouse model of infection were not significantly affected by the absence of CD11b, indicating that CR3-mediated uptake of M. tuberculosis is not a major factor in controlling the subsequent intracellular survival of the mycobacteria. However, whether a mycobacterium will gain access to the intracellular environment, and the type of Mφ that the bacterium enters, is significantly affected by the presence or absence of CR3.

Mycobacterium tuberculosis infection in complement receptor 3-deficient mice

Complement receptor type 3 (CR3) present on macrophages is used by Mycobacterium tuberculosis as one of its major phagocytic receptors. In this study, we examined the in vivo significance of CR3-mediated phagocytosis on the pathogenesis of disease caused by M. tuberculosis. The outcome of tuberculous infection in mice deficient in the CD11b subunit of CR3 (CR3-/-) on a mixed 129SV and C57BL background and control wild-type counterparts was comparable with respect to survival, bacterial burden, granulomatous lesion development, and cytokine expression in the spleen and lungs. M. tuberculosis infection was also examined in CR3-/- mice on C57BL/6 and BALB/c backgrounds and was found to be similar. In conclusion, our results suggest that in the absence of CR3, M. tuberculosis is able to gain entry into host cells via alternative phagocytic receptors and establish infection. The data also indicate that absence of CR3 does not alter disease course in either the relatively resistant C57BL/6 or the relatively susceptible BALB/c strains of mice.

Interaction of Mycobacterium tuberculosis with MH-S, an immortalized murine alveolar macrophage cell line: a comparison with primary murine macrophages

We are interested in identifying a suitable model for investigating mycobacteria interactions with alveolar macrophages. MH-S, a murine alveolar macrophage cell line, is a possible candidate.

OBJECTIVE

To compare the receptor mediated interactions of mycobacteria with primary murine macrophages and MH-S.

DESIGN

The association of MH-S monolayers with Mycobacterium tuberculosis (MTB) and other defined particles was compared to that of resident Day 1 peritoneal macrophage (PM) and Day 4 alveolar macrophage (AM) monolayers.

RESULTS

In the absence of serum, the association of MTB with MH-S was comparable to that of AM, with approximately 35% of each macrophage type binding at least one bacterium. In contrast, almost 80% of PM bound at least one bacterium. MTB binding was enhanced for all macrophage types by a heat-labile component of normal mouse serum. Antibodies recognising CR3 inhibited the serum-mediated enhanced binding of MTB by MH-S. Binding of latex, immunoglobulin coated or complement coated SRBC by MH-S, AM and PM was comparable. Binding of zymosan by MH-S was greatly inferior to AM and PM.

CONCLUSION

The receptor expression and particle binding properties of MH-S are similar to AM in many, but not all, ways. MH-S, therefore, has the potential to be used as a model for investigating MTB-macrophage interactions.

Nonopsonic phagocytosis of zymosan and Mycobacterium kansasii by CR3 (CD11b/CD18) involves distinct molecular determinants and is or is not coupled with NADPH oxidase activation

Complement receptor type 3 (CR3) was initially described as an opsonic receptor. Subsequently, CR3-mediated lectin-sugar recognition mechanisms have been shown to play a major role in the nonopsonic phagocytosis of several pathogens, among them Mycobacterium tuberculosis. Little is known about the binding and signal transduction mechanisms operating during nonopsonic ingestion through CR3 of different microorganisms. In the present study, we used CHO cells stably transfected with CR3 to show that CR3 was able to mediate internalization of zymosan and pathogenic mycobacteria (Mycobacterium kansasii and Mycobacterium avium) but not that of nonpathogenic species (Mycobacterium smegmatis and Mycobacterium phlei). A combination of mannan and beta-glucan inhibited the phagocytosis of zymosan but had no effect on M. kansasii ingestion. Among six monoclonal antibodies (MAbs) directed against the CD11b subunit of CR3 that decreased zymosan ingestion, only three inhibited M. kansasii phagocytosis. In particular, MAbs known to block the CR3 lectin site affected only internalization of zymosan. Using U937 macrophages, we observed that zymosan ingestion through CR3 induced superoxide production measured by cytochrome c reduction and by translocation of the NADPH oxidase cytosolic component p47phox to the phagosomal membrane, whereas phagocytosis of viable or heat-killed M. kansasii did not. Furthermore, lack of superoxide anion production during phagocytosis of M. kansasii was not due to inhibition of NADPH oxidase per se or superoxide anion scavenging. Together, our results indicate that (i) nonopsonic phagocytosis of zymosan and M. kansasii by CR3 implicates different molecular mechanisms involving multiple and distinct epitopes of CD11b and (ii) CR3 may transduce different cellular responses depending on the sites mediating nonopsonic phagocytosis.

The capsule of Mycobacterium tuberculosis and its implications for pathogenicity

Mycobacterium tuberculosis, one of the most prevalent causes of death worldwide, is a facultative intracellular parasite that invades and persists within the macrophages. Within host cells, the bacterium is surrounded by a capsule which is electron-transparent in EM sections, outside the bacterial wall and plasma membrane. Although conventional processing of samples for microscopy studies failed to demonstrate this structure around in vitro-grown bacilli, the application of new microscopy techniques to mycobacteria allows the visualization of a thick capsule in specimen from axenic cultures of mycobacteria. Gentle mechanical treatment and detergent extraction remove the outermost components of this capsule which consist primarily of polysaccharide and protein, with small amounts of lipid. Being at the interface between the bacterium and host cells, the capsule and its constituents would be expected to be involved in bacterial pathogenicity and past work supports this concept. Recent studies have identified several capsular substances potentially involved in the key steps of pathogenicity. In this respect, some of the capsular glycans have been shown to mediate the adhesion to and the penetration of bacilli into the host's cells; of related interest, secreted and/or surface-exposed enzymes and transporters probably involved in intracellular multiplication have been characterized in short-term culture filtrates of M. tuberculosis. In addition, the presence of inducible proteases and lipases has been shown. The capsule would also represent a passive barrier by impeding the diffusion of macromolecules towards the inner parts of the envelope; furthermore, secreted enzymes potentially involved in the detoxification of reactive oxygen intermediates have been identified, notably catalase/peroxidase and superoxide dismutase, which may participate to the active resistance of the bacterium to the host's microbicidal mechanisms. Finally, toxic lipids and contact-dependent lytic substances, as well as constituents that inhibit both macrophage-priming and lymphoproliferation, have been found in the capsule, thereby explaining part of the immunopathology of tuberculosis.

Inhibition of Ca(2+) signaling by Mycobacterium tuberculosis is associated with reduced phagosome-lysosome fusion and increased survival within human macrophages

Complement receptor (CR)-mediated phagocytosis of Mycobacterium tuberculosis by macrophages results in intracellular survival, suggesting that M. tuberculosis interferes with macrophage microbicidal mechanisms. As increases in cytosolic Ca(2+) concentration (¿Ca(2+)(c)) promote phagocyte antimicrobial responses, we hypothesized that CR phagocytosis of M. tuberculosis is accompanied by altered Ca(2+) signaling. Whereas the control complement (C)-opsonized particle zymosan (COZ) induced a 4.6-fold increase in ¿Ca(2+)(c) in human macrophages, no change in ¿Ca(2+)(c) occurred upon addition of live, C-opsonized virulent M. tuberculosis. Viability of M. tuberculosis and ingestion via CRs was required for infection of macrophages in the absence of increased ¿Ca(2+)(c), as killed M. tuberculosis or antibody (Ab)-opsonized, live M. tuberculosis induced elevations in ¿Ca(2+)(c) similar to COZ. Increased ¿Ca(2+)(c) induced by Ab-opsonized bacilli was associated with a 76% reduction in intracellular survival, compared with C-opsonized M. tuberculosis. Similarly, reversible elevation of macrophage ¿Ca(2+)(c) with the ionophore A23187 reduced intracellular viability by 50%. Ionophore-mediated elevation of ¿Ca(2+)(c) promoted the maturation of phagosomes containing live C-opsonized bacilli, as evidenced by acidification and accumulation of lysosomal protein markers. These data demonstrate that M. tuberculosis inhibits CR-mediated Ca(2+) signaling and indicate that this alteration of macrophage activation contributes to inhibition of phagosome-lysosome fusion and promotion of intracellular mycobacterial survival.

Activation of human neutrophils by mycobacterial phenolic glycolipids

The interaction between mycobacterial phenolic glycolipids (PGLs) and phagocytes was studied. Human neutrophils were allowed to interact with each of four purified mycobacterial PGLs and the neutrophil production of reactive oxygen metabolites was followed kinetically by luminol-/isoluminol-amplified chemiluminescence. The PGLs from Mycobacterium tuberculosis and Mycobacterium kansasii, respectively, were shown to stimulate the production of oxygen metabolites, while PGLs from Mycobacterium marinum and Mycobacterium bovis BCG, respectively, were unable to induce an oxidative response. Periodate treatment of the M. tuberculosis PGL decreased the production of oxygen radicals, showing the importance of the PGL carbohydrate moiety for the interaction. The activation, however, could not be inhibited by rhamnose or fucose, indicating a complex interaction which probably involves more than one saccharide unit. This is in line with the fact that the activating PGLs from M. tuberculosis and M. kansasii contain tri- and tetrasaccharides, respectively, while the nonactivating PGLs from M. marinum and M. bovis BCG each contain a monosaccharide. The complement receptor 3 (CR3) has earlier been shown to be of importance for the phagocyte binding of mycobacteria, but did not appear to be involved in the activation of neutrophils by PGLs. The subcellular localization of the reactive oxygen metabolites formed was related to the way in which the glycolipids were presented to the cells.

The receptor-mediated uptake, survival, replication, and drug sensitivity of Mycobacterium tuberculosis within the macrophage-like cell line THP-1: a comparison with human monocyte-derived macrophages

We have compared the interaction of Mycobacterium tuberculosis with the human macrophage-like cell line THP-1 and with human monocyte-derived macrophages (MDM). The association of M. tuberculosis with THP-1 and MDM was comparable in both the presence and the absence of serum. For both cells, serum-mediated binding was much greater than nonopsonic binding and was mediated by a heat-labile serum component. Nonopsonic binding of M. tuberculosis to both cells could be inhibited by antibodies recognizing CD11b and by mannan and glucan. Intracellular M. tuberculosis grew progressively in infected MDM and THP-1 cells. Treatment of the infected MDM and THP-1 cells with the anti-mycobacterial isoniazid resulted in the rapid killing of the intracellular mycobacteria. Differentiated, adherent THP-1 cells bound IgG and complement-coated particles at levels similar to those of MDM. However, binding of zymosan by THP-1 cells was significantly lower than that seen for MDM.

Enterococcus faecalis aggregation substance promotes opsonin-independent binding to human neutrophils via a complement receptor type 3-mediated mechanism

Enterococcus faecalis aggregation substance (AS) mediates efficient adhesion between bacteria, thereby facilitating plasmid exchange as an integral part of a bacterial sex pheromone system. We examined the interaction of AS-bearing E. faecalis with human neutrophils (PMNs), an important component of the host defense system. AS promoted a markedly increased opsonin-independent bacterial binding to PMNs. Adhesion was dependent on the expression of the enterococcal Asc10 protein, which contains two Arg-Gly-Asp (RGD) sequences, and addition of exogenous RGD-containing peptides inhibited AS-mediated binding by 66%. AS-mediated adhesion was inhibited by 85% by anti-human complement receptor type 3 (CR3) monoclonal antibodies or by use of PMNs from a patient with leukocyte adhesion deficiency. However, AS-bearing E. faecalis cells were unable to bind to CHO-Mac-1 cells, expressing functionally active CR3, suggesting the potential need for additional PMN surface receptors for bacterial adhesion. Monoclonal antibodies against integrin-associated protein (CD47) and L-selectin, both of which may interact with CR3 and bind to ligands on E. faecalis, also inhibited AS-dependent binding. The non-opsonic binding of E. faecalis to PMNs may play an important role in this organism's pathogenesis.

Role of complement receptors in uptake of Mycobacterium avium by macrophages in vivo: evidence from studies using CD18-deficient mice

Mycobacterium avium is an intracellular pathogen that has been shown to invade macrophages by using complement receptors in vitro, but mycobacteria released from one cell can enter a second macrophage by using receptors different from complement receptors. Infection of CD18 (beta(2) integrin) knockout mice and the C57 BL/6 control mice led to comparable levels of tissue infection at 1 day, 2 days, 1 week, and 3 weeks following administration of bacteria. A histopathological study revealed similar granulomatous lesions in the two mouse strains, with comparable numbers of organisms. In addition, transmission electron microscopy of spleen tissues from both strains of mice showed bacteria inside macrophages. Our in vivo findings support the hypothesis that M. avium in the host is likely to use receptors other than CR3 and CR4 receptors to enter macrophages with increased efficiency.

Generation of multinucleated giant cells in vitro by culture of human monocytes with Mycobacterium bovis BCG in combination with cytokine-containing supernatants

Multinucleated giant cells (MGC), a characteristic feature of tuberculous granulomas, form by fusion of monocytes or macrophages, but little is known about the mechanism of the fusion process itself. Several studies report an indirect effect of mycobacteria, i.e., induction of a soluble lymphocyte-derived fusion factor following stimulation by mycobacteria or mycobacterial products. The aim of our study was to determine whether contact with mycobacteria can induce MGC formation from human monocytes in vitro. Stimulation of monocytes with Mycobacterium bovis bacillus Calmette-Guérin (BCG) in combination with cytokine-containing supernatants of herpesvirus saimiri-transformed human T-cell clones (T-SN) led to MGC formation with fusion rates of about 27%. In contrast, stimulation with one component alone induced only low fusion rates of up to 10%. Heat-killed BCG in combination with T-SN induced monocyte fusion to the same extent as live mycobacteria. BCG culture supernatant, BCG lysate, or inert particles in combination with T-SN did not induce MGC formation. Experiments using transwell plates containing a semipermeable membrane revealed that induction of the fusion process is dependent on direct contact of monocytes and mycobacteria. MGC formation induced by BCG plus T-SN could be inhibited by addition of monoclonal antibodies to gamma interferon (but not tumor necrosis factor alpha) as well as to the beta chain (CD18) of beta2-integrins. These results demonstrate that contact with mycobacteria in combination with cytokine-containing supernatants is able to induce human monocytes to form MGC and that membrane-bound molecules of mycobacteria and monocytes are involved in the fusion process.