The mannose receptor mediates uptake of pathogenic and nonpathogenic mycobacteria and bypasses bactericidal responses in human macrophages

The impact of the absence of glycopeptidolipids on the ultrastructure, cell surface and cell wall properties, and phagocytosis of Mycobacterium smegmatis

Glycopeptidolipids (GPLs) are a class of species- or type-specific mycobacterial lipids and major constituents of the cell envelopes of many non-tuberculous mycobacteria. To determine the function of GPLs in the physiology of these bacteria, a mutant of Mycobacterium smegmatis in which the gene encoding a mycobacterial nonribosomal peptide synthetase has been inactivated by transposon mutagenesis was analysed. Labelling experiments indicated that half of the bacterial GPLs were located on the cell surface and represented 85% of the surface-exposed lipids of the parent strain whereas the mutant was defective in the production of the GPLs. Compared to the parent smooth morphotype strain, the GPL-deficient mutant strain exhibited a rough colony morphology, an increase of the cell hydrophobicity and formed huge aggregates. As a consequence, the mutant cells were no longer able to bind ruthenium red, as observed by transmission electron microscopy. The altered surface properties of the mutant cells also affected the phagocytosis of individual bacilli by human monocyte-derived macrophages since mutant cells were internalized more rapidly than cells from the parent strain. Nevertheless, no specific release of surface constituents into the culture broth of the mutant was observed, indicating that the cell surface is composed of substances other than GPLs and that these are essential for maintaining the architecture of the outermost layer of the cell envelope. Importantly, the absence of these major extractable lipids of M. smegmatis from the mutant strain has a profound effect on the uptake of the hydrophobic chenodeoxycholate by cells, indicating that GPLs are involved in the cell wall permeability barrier of M. smegmatis. Altogether, these data showed that, in addition to being distinctive markers of numerous mycobacterial species, GPLs play a role in the bacterial phenotype, surface properties and cell wall permeability.

A novel lipoarabinomannan from the equine pathogen Rhodococcus equi. Structure and effect on macrophage cytokine production

Rhodococcus equi is a major cause of foal morbidity and mortality. We have investigated the presence of lipoglycan in this organism as closely related bacteria, notably Mycobacterium tuberculosis, produce lipoarabinomannans (LAM) that may play multiple roles as virulence determinants. The lipoglycan was structurally characterized by gas chromatography-mass spectrometry following permethylation, capillary electrophoresis after chemical degradation, and (1)H and (31)P and two-dimensional heteronuclear nuclear magnetic resonance studies. Key structural features of the lipoglycan are a linear alpha-1,6-mannan with side chains containing one 2-linked alpha-d-Manp residue. This polysaccharidic backbone is linked to a phosphatidylinositol mannosyl anchor. In contrast to mycobacterial LAM, there are no extensive arabinan domains but single terminal alpha-d-Araf residue capping the 2-linked alpha-d-Manp. The lipoglycan binds concanavalin A and mannose-binding protein consistent with the presence of t-alpha-d-Manp residues. We studied the ability of the lipoglycans to induce cytokines from equine macrophages, in comparison to whole cells of R. equi. These data revealed patterns of cytokine mRNA induction that suggest that the lipoglycan is involved in much of the early macrophage cytokine response to R. equi infection. These studies identify a novel LAM variant that may contribute to the pathogenesis of disease caused by R. equi.

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.

Role of Yops and adhesins in resistance of Yersinia enterocolitica to phagocytosis

Yersinia enterocolitica is a pathogen endowed with two adhesins, Inv and YadA, and with the Ysc type III secretion system, which allows extracellular adherent bacteria to inject Yop effectors into the cytosol of animal target cells. We tested the influence of all of these virulence determinants on opsonic and nonopsonic phagocytosis by PU5-1.8 and J774 mouse macrophages, as well as by human polymorphonuclear leukocytes (PMNs). The adhesins contributed to phagocytosis in the absence of opsonins but not in the presence of opsonins. In agreement with previous results, YadA counteracted opsonization. In every instance, the Ysc-Yop system conferred a significant level of resistance to phagocytosis. Nonopsonized single-mutant bacteria lacking either YopE, -H, -T, or -O were phagocytosed significantly more by J774 cells and by PMNs. Opsonized bacteria were phagocytosed more than nonopsonized bacteria, and mutant bacteria lacking either YopH, -T, or -O were phagocytosed significantly more by J774 cells and by PMNs than were wild-type (WT) bacteria. Opsonized mutants lacking only YopE were phagocytosed significantly more than were WT bacteria by PMNs but not by J774 cells. Thus, YopH, -T, and -O were involved in all of the phagocytic processes studied here but YopE did not play a clear role in guarding against opsonic phagocytosis by J774. Mutants lacking YopP and YopM were, in every instance, as resistant as WT bacteria. Overexpression of YopE, -H, -T, or -O alone did not confer resistance to phagocytosis, although it affected the cytoskeleton. These results show that YopH, YopT, YopO, and, in some instances, YopE act synergistically to increase the resistance of Y. enterocolitica to phagocytosis by macrophages and PMNs.

Lipid-restricted recognition of mycobacterial lipoglycans by human pulmonary surfactant protein A: a surface-plasmon-resonance study

The human pulmonary surfactant protein A (hSP-A), a member of the mammalian collectin family, is thought to play a key defensive role against airborne invading pulmonary pathogens, among which is Mycobacterium tuberculosis, the aetiologic agent of tuberculosis. hSP-A has been shown to promote the uptake and the phagocytosis of pathogenic bacilli through the recognition and the binding of carbohydrate motifs on the invading pathogen surface. Recently we identified lipomannan and mannosylated lipoarabinomannan (ManLAM), two major mycobacterial cell-wall lipoglycans, as potential ligands for binding of hSP-A. We demonstrated that both the terminal mannose residues and the fatty acids are critical for binding, whereas the inner arabinosyl and mannosyl domains do not participate. In the present study we developed a surface-plasmon-resonance assay to analyse the molecular basis for the recognition of ManLAM by hSP-A and to try to define further the role of the lipidic aglycone moiety. Binding of ManLAM to immobilized hSP-A was consistent with the simplest one-to-one interaction model involving a single class of carbohydrate-binding site. This observation strongly suggests that the lipid moiety of ManLAM does not directly interact with hSP-A, but is rather responsible for the macromolecular organization of the lipoglycan, which may be necessary for efficient recognition of the terminal mannosyl epitopes. The indirect, structural role of the lipoglycan lipidic component is further supported by the complete lack of interaction with hSP-A in the presence of a low concentration of mild detergent.

p59Hck isoform induces F-actin reorganization to form protrusions of the plasma membrane in a Cdc42- and Rac-dependent manner

Hck is a protein kinase of the Src family specifically expressed in phagocytes as two isoforms, p59Hck and p61Hck, localized at the plasma membrane and lysosomes, respectively. Their individual involvement in functions ascribed to Hck, phagocytosis, cell migration, and lysosome mobilization, is still unclarified. To investigate the specific role of p59Hck, a constitutively active variant in fusion with green fluorescent protein (p59Hck(ca)) was expressed in HeLa cells. p59Hck(ca) was found at focal adhesion sites and triggered reorganization of the actin cytoskeleton, leading to plasma membrane protrusions where it co-localized with F-actin. Similarly, microinjection of p59Hck(ca) cDNA in J774.A1 macrophages induced membrane protrusions. Whereas kinase activity and membrane association of p59Hck were dispensable for location at focal adhesions, p59Hck-induced membrane protrusions were dependent on kinase activity, plasma membrane association, and Src homology 2 but not Src homology 3 domain and were inhibited by dominant-negative forms of Cdc42 or Rac but not by blocking Rho activity. A dominant negative form of p59Hck inhibited the Cdc42- and Rac-dependent FcgammaRIIa-mediated phagocytosis. Expression of the Cdc42/Rac-interacting domain of p21-activated kinase in macrophages abolished the p59Hck(ca)-induced morphological changes. Therefore, p59Hck-triggered remodeling of the actin cytoskeleton depends upon the activity of Cdc42 and Rac to promote formation of membrane protrusions necessary for phagocytosis and cell migration.

Phagocytosis of microbes: complexity in action

The phagocytic response of innate immune cells such as macrophages is defined by the activation of complex signaling networks that are stimulated by microbial contact. Many individual proteins have been demonstrated to participate in phagocytosis, and the application of high-throughput tools has indicated that many more remain to be described. In this review, we examine this complexity and describe how during recognition, multiple receptors are simultaneously engaged to mediate internalization, activate microbial killing, and induce the production of inflammatory cytokines and chemokines. Many signaling molecules perform multiple functions during phagocytosis, and these molecules are likely to be key regulators of the process. Indeed, pathogenic microorganisms target many of these molecules in their attempts to evade destruction.

Innate immunity to Mycobacterium tuberculosis

The different manifestations of infection with Mycobacterium tuberculosis reflect the balance between the bacillus and host defense mechanisms. Traditionally, protective immunity to tuberculosis has been ascribed to T-cell-mediated immunity, with CD4(+) T cells playing a crucial role. Recent immunological and genetic studies support the long-standing notion that innate immunity is also relevant in tuberculosis. In this review, emphasis is on these natural, innate host defense mechanisms, referring to experimental data (e.g., studies in gene knockout mice) and epidemiological, immunological, and genetic studies in human tuberculosis. The first step in the innate host defense is cellular uptake of M. tuberculosis, which involves different cellular receptors and humoral factors. Toll-like receptors seem to play a crucial role in immune recognition of M. tuberculosis, which is the next step. The subsequent inflammatory response is regulated by production of pro- and anti-inflammatory cytokines and chemokines. Different natural effector mechanisms for killing of M. tuberculosis have now been identified. Finally, the innate host response is necessary for induction of adaptive immunity to M. tuberculosis. These basic mechanisms augment our understanding of disease pathogenesis and clinical course and will be of help in designing adjunctive treatment strategies.

M. tuberculosis induction of matrix metalloproteinase-9: the role of mannose and receptor-mediated mechanisms

Mycobacterium tuberculosis (Mtb) infection induces the expression of matrix metalloproteinase-9 (MMP-9) in mouse lungs. In cultured human monocytic cells, Mtb bacilli and the cell wall glycolipid lipoarabinomannan (LAM) stimulate high levels of MMP-9 activity. Here, we explore the cellular mechanisms involved in the induction of MMP-9 by Mtb. We show that infection of THP-1 cells with Mtb caused a fivefold increase in MMP-9 mRNA that was associated with increased MMP-9 activity. MMP-9 induction was dependent on microtubule polymerization and protein kinase activation and was associated with increased DNA binding by the transcription factor activator protein-1 (AP-1), which appeared to be important for MMP-9 expression. We then explored the surface molecules potentially involved in Mtb induction of MMP-9, focusing on ligands of the mannose and beta-glucan receptors. MMP-9 activity was induced by the mannose receptor ligands mannan, zymosan, and LAM, whereas the beta-glucan receptor ligand laminarin was not effective. The most active inducers of MMP-9 activity were the particulate ligand zymosan and LAM. Pretreatment of cells with an anti-mannose receptor monoclonal antibody, but not anti-complement receptor 3, decreased the induction of MMP-9 activity by Mtb bacilli. Together, these results suggest that MMP-9 induction by Mtb occurs by receptor-mediated signaling mechanisms involving the binding of mannosylated ligands to mannose receptors, the modulation by cytoskeletal elements such as microtubules, the activation of protein kinases, and transcriptional activation by AP-1.

Lack of fusion of azurophil granules with phagosomes during phagocytosis of Mycobacterium smegmatis by human neutrophils is not actively controlled by the bacterium

Biogenesis of phagolysosomes is a very rapid event in neutrophils which takes place with nascent unclosed phagosomes, leading to the release of lysosomal enzymes such as beta-glucuronidase in the extracellular medium. We have previously shown that, under nonopsonic conditions, both pathogenic and nonpathogenic mycobacteria uncouple phagocytosis from fusion of azurophil granules (specialized secretory lysosomes) with phagosomes. In the present study we questioned whether they actively act on neutrophils to block this process or use phagocytic receptors that negatively control the biogenesis of phagolysosomes. As for live unicellular Mycobacterium smegmatis, we observed that nonopsonic phagocytosis of heat-killed mycobacteria did not induce the release of beta-glucuronidase, indicating that M. smegmatis does not actively act on the fusion process in neutrophils. In contrast, phagocytosis of unicellular M. smegmatis opsonized in immune serum or that of small nonopsonized mycobacterial aggregates restored the biogenesis of phagolysosomes. Aggregates were internalized in a CR3- and cholesterol-dependent manner as unicellular mycobacteria. However, aggregates but not unicellular bacteria triggered F-actin and Hck recruitment at the phagosomes, events that have been associated with lysosome fusion. Thus, we propose that M. smegmatis does not actively control the fusion of azurophil granules at early time points postinfection and that mycobacterial aggregates recruit large clusters of receptors at the neutrophil surface which could trap proteins implicated in the biogenesis of phagolysosomes.

Macrophage functional maturation and cytokine production are impaired in C/EBP epsilon-deficient mice

Members of the CCAAT/enhancer-binding protein (C/EBP) family are involved in the regulation of cellular differentiation and function of many tissues. Unlike the other members of the family, C/EBP epsilon expression is restricted to granulocytes, macrophages, and lymphocytes. C/EBP epsilon is highly conserved between human and rodents and is essential for terminal granulopoiesis in both species. To study the role that C/EBP epsilon plays in macrophages, wild-type and C/EBP epsilon-deficient (-/-) murine macrophages obtained from thioglycollate-elicited peritoneal lavages and differentiated bone marrow cells were compared. Although macrophage development occurred in both types of mice, the C/EBP epsilon -/- cells had a lower expression of macrophage markers and a morphologic and ultrastructural appearance of immaturity. Phagocytic function, measured by calculating the percentage of internalized opsonized fluorescein isothiocyanate (FITC)-labeled yeast, was significantly impaired in the C/EBP epsilon -/- macrophages compared with their wild-type counterparts. Furthermore, the differential expression of 26 macrophage-specific genes between wild-type and C/EBP-/- mice was analyzed. A subset of genes involved in differentiation, immune, and inflammatory responses was found down-regulated in the C/EBP-/- macrophages. Taken together, this study implicates the C/EBP epsilon gene as an important transcription factor required for normal function and development of macrophages.

A new assay to monitor the degranulation process in phagocytizing human neutrophils

oxidation, we set up a method for measuring MPO intraphagosomal release in human neutrophils. The method is based on the passive engulfment of DAB together with the phagocytosable particle. Inside the vacuole, this substrate is oxidized by MPO released from the azurophilic granules. The colorimetrical evaluation of the amount of DAB oxidized allows for cheap, rapid quantification of MPO intraphagosomal secretion in whole cells. Using this method, we show that the degranulation process, involving azurophilic granules, can be monitored carefully during phagocytosis. It takes place after the ingestion of zymosan particles opsonized with normal human serum, as well as during IgG-mediated phagocytosis and under conditions where beta2 integrins are blocked. However our findings also show that the extent of intraphagosomal secretion depends on either the extent of opsonization or the type of receptor engaged during the phagocytic event.

The protein tyrosine kinase Hck is located on lysosomal vesicles that are physically and functionally distinct from CD63-positive lysosomes in human macrophages

In macrophages, lysosomes are suspected to have a heterogenous population of vesicles. This study was thus undertaken to identify and to characterize lysosomal compartments in human macrophages. Hck is a Src-family tyrosine kinase associated with secretory lysosomes in neutrophils and with cytoplasmic vesicles in macrophages that fuse with phagosomes. We identified these Hck-positive vesicles and compared them to CD63-positive, M6PR-negative vesicles known as classical lysosomes. Hck vesicles exhibited lysosomal features. Indeed, Hck-positive vesicles could be loaded with rhodamine-dextran, which has been shown to accumulate in lysosomal compartments. Hck was delivered to zymosan-containing phagosomes at a late stage of the maturation process, which occurs after the fusion with CD63-positive lysosomes. Finally, when mycobacteria were used to prevent phagolysosome biogenesis, Hck was not recruited to phagosomes. Moreover, Hck lysosomes were physically and functionally distinct from CD63-lysosomes. For instance, sucrose induced swelling of CD63-lysosomes without affecting Hck-positive ones. Only CD63-lysosomes fused with phagosomes in a microtubule-dependent manner. Entry of particles through the mannose receptor and Fcγ receptors drove the phagosome towards a fusion with CD63-lysosomes, whereas only Fcγ receptors induced the mobilisation of Hck-lysosomes. This study provides further evidence for the existence of sub-populations of lysosomes in macrophages: one stained by CD63 and another one characterized by the presence of Hck. Therefore, Hck represents a new tool to study the fusion dynamics of lysosomal compartments and their subversion by several intracellular pathogens.

How does Mycobacterium avium subsp. paratuberculosis resist intracellular degradation?

Paratuberculosis is a chronic, progressive disease of mainly ruminants caused by the facultative intracellular bacterium, Mycobacterium avium subsp. paratuberculosis. Infection usually occurs in young animals through oral uptake of food contaminated with the organisms. The ingested bacteria are transcytosed through M-cells overlying the Peyer's patches and are released in the stroma, where they are taken up by macrophages. Inside the macrophage, the mycobacteria resist enzymatic and toxic degradation and multiply until the infected macrophage ruptures. The thick, lipid-rich cell envelope is mainly responsible for micobacterial resistance. In addition to its barrier effect, which provides protections, the mycobacterial cell wall also contains several biologically active components that down-regulate the bactericidal function of macrophages. The basic survival strategy of pathogenic mycobacteria can be viewed at three levels: selective use of relatively safe entry pathways that do not trigger oxidative attack, modification of the intracellular trafficking of mycobacteria-containing phagosomes, and modulation of the cooperation between the innate and specific immunity. In doing so, pathogenic mycobacteria are successful intracellular organisms that survive and multiply inside macrophages. Current understanding about the survival strategies of M. a. paratuberculosis and its implications in the epidemiology, diagnosis, and control of the disease are discussed.

Opsonophagocytosis versus lectinophagocytosis in human milk macrophages

Some important immunoprotective effects of human breast milk have been attributed to the presence of macrophages. We investigated the generation of superoxide anion (O2-) by monocytes and human milk macrophages after stimulation with opsonized and unopsonized zymosan in the absence and presence of mannose as an inhibitor to investigate lectinophagocytic and opsonophagocytic properties. Peripheral blood monocytes generated more O2- than human milk macrophages (417,4 + 79,1 nmol O2-/mg protein vs. 216,1 +/-15,1 nmol O2-/mg protein, p<0,05) after stimulation with opsonized zymosan. When unopsonized zymosan was used as a serum-independent stimulus monocytes generated slightly less O2- in comparison to human milk macrophages (150,8 +/- 34,5 nmol/mg protein vs. 176,1 +/- 18 nmol O2-/mg protein, p<0,05). These findings demonstrate that the proportion of opsonin-independent phagocytosis in human milk macrophages is higher than in monocytes (82% vs. 36%). When mannose was used as an inhibitor a significantly higher reduction of O2- generation occurred in human milk macrophages compared to monocytes stimulated with opsonized zymosan, whereas no difference was found when unopsonized zymosan was used. These results indicate that human milk macrophages are stimulated to a greater extent by opsonin-independent mechanisms than blood borne monocytes. As the colostrum and the intestinal environment of the neonate offers only a little amount of opsonins like complement and immunoglobulin G, such a differentiation to lectinophagocytic properties could bear a great advantage for protective functions of human milk macrophages.

Fusion of human neutrophil phagosomes with lysosomes in vitro: involvement of tyrosine kinases of the Src family and inhibition by mycobacteria

The intracellular killing of microorganisms in phagocytes involves the fusion of lysosomes containing bactericidal factors with phagosomes, and several intracellular pathogens are able to inhibit this fusion event. In this study, we report the reconstitution of phagosome-lysosome fusion in vitro, using an assay based on resonance energy transfer between fluorescent phospholipid analogues that were inserted into whole human NB4-neutrophil membranes from liposomes containing positively charged lipids. Cytosol was required for fusion, and fusion was stimulated 3-fold if this cytosol had been prepared from neutrophils activated by using opsonized zymosan or a combination of the calcium ionophore (A23187) and phorbol myristate acetate (PMA). Fusion was inhibited by the addition of PP1, an inhibitor of Src family protein kinases, or GTPgammaS. We have previously reported that the biogenesis of phagolysosomes in human neutrophils is inhibited by mycobacteria. Here we show that cytosol from cells having internalized live (not heat-killed) Mycobacterium smegmatis or cytosol simply incubated with mycobacteria inhibited fusion, indicating that soluble factors are involved in mycobacterial inhibition of phagosome-lysosome fusion.

Mycobacteria-induced TNF-alpha and IL-10 formation by human macrophages is differentially regulated at the level of mitogen-activated protein kinase activity

The clinical course of mycobacterial infections is linked to the capacity of pathogenic strains to modulate the initial antimycobacterial response of the macrophage. To elucidate some of the mechanisms involved, we studied early signal transduction events leading to cytokine formation by human monocyte-derived macrophages (MDM) in response to clinical isolates of Mycobacterium avium. TNF-alpha production induced by M. avium was inhibited by anti-CD14 mAbs, but not by Abs against the macrophage mannose receptor. Analysis of mitogen-activated protein (MAP) kinase activation (extracellular signal-regulated kinase 1/2, p38, and c-Jun NH(2)-terminal kinase) showed a rapid phosphorylation of all three subfamilies in response to M. avium, which was inhibited by anti-CD14 Abs. Using highly specific inhibitors of p38 (SB203580) and MAP kinase kinase-1 (PD98059), we found that activation of the extracellular signal-regulated kinase pathway, but not of p38, was essential for the M. avium-induced TNF-alpha formation. In contrast, IL-10 production was abrogated by the p38 inhibitor, but not by the MAP kinase kinase-1 inhibitor. In conclusion, M. avium-induced secretion of TNF-alpha and IL-10 by human macrophages is differentially regulated at the level of MAP kinase activity.

Mannosylated lipoarabinomannans inhibit IL-12 production by human dendritic cells: evidence for a negative signal delivered through the mannose receptor

IL-12 is a key cytokine in directing the development of type 1 Th cells, which are critical to eradicate intracellular pathogens such as Mycobacterium tuberculosis. Here, we report that mannose-capped lipoarabinomannans (ManLAMs) from Mycobacterium bovis bacillus Calmette-Guérin and Mycobacterium tuberculosis inhibited, in a dose-dependent manner, the LPS-induced IL-12 production by human dendritic cells. The inhibitory activity was abolished by the loss of the mannose caps or the GPI acyl residues. Mannan, which is a ligand for the mannose receptor (MR) as well as an mAb specific for the MR, also inhibited the LPS-induced IL-12 production by dendritic cells. Our results indicate that ManLAMs may act as virulence factors that contribute to the persistence of M. bovis bacillus Calmette-Guérin and M. tuberculosis within phagocytic cells by suppressing IL-12 responses. Our data also suggest that engagement of the MR by ManLAMs delivers a negative signal that interferes with the LPS-induced positive signals delivered by the Toll-like receptors.

M. tuberculosis: immunology and vaccination

Tuberculosis is increasing. Current treatment regimens require at least 6 months, because latent or stationary phase organisms are difficult to kill. Such regimens do not achieve full compliance, and "directly observed therapy short course" (DOTS) is having less impact than expected. This worrying situation is aggravated by coinfection with human immunodeficiency virus (HIV), and by the increase in drug-resistant strains. We need new insights that lead to more rapid therapies and immunotherapies, and more reliable vaccines. Recent insights have come from: understanding of the relationship between Mycobacterium tuberculosis and macrophages; the multiple T cell types that recognise mycobacterial peptides, lipids and glycolipids; the critical role of interferon-gamma (IFNgamma) and interleukin-12 (IL-12) in human mycobacterial infection revealed by genetically defective children; quantitation of the presence and importance of Th2 lymphocyte activation in human tuberculosis; the role of local conversion of inactive cortisone to active cortisol in the lesions; the recognition that some effective prophylactic vaccines also work as immumotherapeutics whereas others do not. In the longer term the recent sequencing of the M. tuberculosis genome will lead to further advances. In the short term, effective immunotherapy remains the most accessible breakthrough in the management of tuberculosis. The types of practical advance that will result from sequencing the genome are discussed speculatively, but cannot yet be predicted with certainty.

Superoxide anion generation in human milk macrophages: opsonin-dependent versus opsonin-independent stimulation compared with blood monocytes

Macrophages are believed to play an important role within the immunoprotective effects of human breast milk. It was the purpose of this study to evaluate the capability of human milk macrophages (MMPhi) to generate superoxide anions (O2(-)) in comparison with peripheral blood monocytes (BMo) after stimulation with opsonized and unopsonized zymosan. Potential inhibitors of attachment and phagocytosis such as mannose and cytochalasin B were used. Expression of the mannose receptor on MMPhi was demonstrated by staining with MAb. BMo generated more O2(-) than MMPhi (417 +/- 79 versus 216 +/- 15 nmol O2(-)/mg protein, p < 0.05) after stimulation with opsonized zymosan. When unopsonized zymosan was used as a serum-independent stimulus, BMo generated slightly less O2(-) in comparison with MMPhi (150 +/- 34 versus 176 +/- 18 nmol O2(-)/mg protein, p < 0.05). These findings imply a higher proportion of opsonin-independent phagocytosis in MMPhi than in BMo (82 versus 36 %). Preincubation with mannose resulted in a significantly higher reduction of O2(-) generation in MMPhi compared with BMo stimulated with opsonized zymosan, whereas no difference was found when unopsonized zymosan was used. After addition of cytochalasin B, equal inhibition of O2(-) generation was observed regardless of the cell type or stimulus used. Thus, MMPhi are stimulated to a greater extent by serum-independent mechanisms than BMo. As opsonins like complement or IgG are rare in the colostrum and the neonatal intestinal environment, such a differentiation toward serum-independent phagocytic abilities could play an important role for protective functions of human MMPhi. Possible involvement of the mannose receptor and the beta-glucan receptor in this specialization are discussed.

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.

Lack of palmitoylation redirects p59Hck from the plasma membrane to p61Hck-positive lysosomes

Hck, a protein-tyrosine kinase of phagocytes, is the unique member of the Src family expressed under two alternatively translated isoforms differing in their N-terminal site of acylation: p61(Hck) has an additional 21-amino acid sequence comprising a single myristoylation motif, whereas p59(Hck) N terminus has myristoylation and palmitoylation sites. To identify the molecular determinants involved in the targeting of each isoform, they were fused to GFP and expressed in HeLa and CHO cells. p61(Hck) was associated with lysosomal vesicles, whereas p59(Hck) was found at the plasma membrane and to a low extent associated with lysosomes. Their unique N-terminal domains were sufficient to target GFP to the corresponding intracellular compartments. Mutation of the palmitoylation site of p59(Hck) redirected this isoform to lysosomes, indicating that the palmitoylation state governs the association of p59(Hck) with the plasma membrane or with lysosomes. In addition, both isoforms and the nonpalmitoylated p59(Hck) mutant were found on the Golgi apparatus, suggesting a role of this organelle in the subcellular sorting of Hck isoforms. Regarding their subcellular localizations, we propose that bi-acylated p59(Hck) might transduce plasma membrane receptor signals, whereas p61(Hck) and the nonpalmitoylated p59(Hck) might control the biogenesis of phagolysosomes, two functions yet proposed for Hck in phagocytes.

Dermal microvascular endothelial cells express the 180-kDa macrophage mannose receptor in situ and in vitro

Expression of the 180-kDa mannose receptor (MR) is mainly found on cells of the macrophage lineage. MR mediates the uptake of micro-organisms and host-derived glycoproteins. We demonstrate that endothelium of the human skin in situ and dermal microvascular endothelial cells (DMEC) in vitro expressed MR at both the protein and mRNA levels. In contrast, HUVEC were consistently negative for MR expression. DMEC internalized dextran as well as Escherichia coli by the way of MR into acidic phagosomes, only a few of which fused with CD63- and lysosomal-associated membrane glycoprotein-2-positive lysosomes. This contrasts with the situation in monocyte-derived dendritic cells, where almost all of the MR-Ag complexes reached CD63- and lysosomal-associated membrane glycoprotein-2-positive compartments, indicating differences in the phagolysosomal fusion rate between DMEC and dendritic cells. In conclusion, DMEC express functional MR, a finding that corroborates a role of skin endothelium in Ag capture/clearing.

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.

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.

Phagocytosis and killing of intracellular pathogens: interaction between cytokines and antibiotics

Phagocytosis and bacterial killing are the primary functions of macrophages. Among the mechanisms involved in the phagocytic process, cytokines, especially those of T-helper 1 profile, appear to influence considerably the internalization and the intracellular fate of the pathogen within the macrophage. In particular, the evidence for a cooperation of cytokines with antibiotics in intracellular infection could provide new therapeutic approaches to intracellular infectious diseases in the future.

Macrophage lectins in host defence

Macrophage lectins contribute to host defence by a variety of mechanisms. The best characterised, mannose receptor (MR) and complement receptor three (CR3), are both able to mediate phagocytosis of pathogenic microbes and induce intracellular killing mechanisms. The regulation of the effector functions induced via MR is complex, and may involve both host and microbial factors. Therefore, MR is likely to play a dynamic role in the response to infection; it may act as a classical pattern recognition receptor in phagocytosis, whereas other poorly characterised factors may make a more decisive contribution to its function in physiologic settings. In contrast, the lectin site of CR3 appears to lack host-derived ligands and may be a true pattern recognition receptor. Further studies are required to evaluate the roles of other macrophage lectins in recognition of and responses to microbes.

Clearance and organ distribution of Mycobacterium tuberculosis lipoarabinomannan (LAM) in the presence and absence of LAM-binding immunoglobulin M

Lipoarabinomannan (LAM) is a component of the mycobacterial surface which has been associated with a variety of deleterious effects on immune system function. Despite the importance of LAM to the pathogenesis of mycobacterial infection, there is no information available on its fate in vivo. In this study, we determined the pharmacokinetics and tissue distribution of exogenously administered LAM in mice. For measurements of serum and tissue LAM concentrations, we developed an enzyme-linked immunosorbent assay which used monoclonal antibodies of different isotypes to capture and detect LAM at concentrations of >/=0.4 microg/ml. Intravenous administration of LAM to mice resulted in transient serum levels with organ deposition in the spleen and in the liver. Immunohistochemical studies localized LAM to the spleen marginal zone macrophages and, to a lesser degree, to liver macrophages. When LAM was administered to mice previously given a LAM-binding immunoglobulin M (IgM), LAM was very rapidly cleared from circulation. In those mice, deposition of LAM in the spleen was significantly reduced while LAM deposition in the liver increased. Administration of LAM-binding IgM resulted in significant levels of IgM to LAM in bile consistent with an increased hepatobiliary excretion of LAM in the presence of specific antibody. Bile, liver extracts, and bile salts were found to rapidly inactivate the immunoreactivity of LAM. The results indicate that serum clearance and organ deposition of LAM in mice are affected by the presence of LAM-binding antibody and suggest a mechanism by which antibody could modify the course of mycobacterial infection.