Infection by Mycobacterium tuberculosis promotes human alveolar macrophage apoptosis

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.

Induction of TNF in human alveolar macrophages as a potential evasion mechanism of virulent Mycobacterium tuberculosis

The ability of macrophages to release cytokines is crucial to the host response to intracellular infection. In particular, macrophage-derived TNF plays an important role in the host response to infection with the intracellular pathogen Mycobacterium tuberculosis. In mice, TNF is indispensable for the formation of tuberculous granulomas, which serve to demarcate the virulent bacterium. TNF is also implicated in many of the immunopathological features of tuberculosis. To investigate the role of TNF in the local immune response, we infected human alveolar macrophages with virulent and attenuated mycobacteria. Infection with virulent strains induced the secretion of significantly higher levels of bioactive TNF than attenuated strains correlating with their ability to multiply intracellularly. Treatment of infected macrophages with neutralizing anti-TNF Abs reduced the growth rate of intracellular bacteria, whereas bacterial replication was augmented by addition of exogenous TNF. Infected and uninfected macrophages contributed to cytokine production as determined by double-staining of M. tuberculosis and intracellular TNF. The induction of TNF by human alveolar macrophages at the site of infection permits the multiplication of intracellular bacteria and may therefore present an evasion mechanism of human pathogens.

Increased expression of fas ligand in human tuberculosis and leprosy lesions: a potential novel mechanism of immune evasion in mycobacterial infection

To study the location and mechanism of apoptosis within the human tuberculosis (TB) and leprosy lesions, parallel sections were analyzed for mycobacterial antigens (M.Ag), Fas ligand (FasL), Fas, CD68 and Mac387 by immunohistochemistry, and apoptotic cells by the terminal deoxynucleotidyl-transferase-mediated dUTP-digoxigenin nick end labelling method. Cutaneous leishmaniasis and foreign body granulomas were analyzed for comparison. The heavily infected macrophages in multibacillary TB and leprosy granulomas very strongly expressed FasL, indicating that a mycobacterial infection can induce an increased expression of FasL in a population of infected macrophages, which may protect them from the attack of Fas-expressing lymphocytes. However, macrophages with high levels of leishmania amastigotes did not selectively express FasL, suggesting that this phenomenon is specific for the mycobacteria. Interestingly, in the well-formed TB granulomas, 84% of the multinucleated giant cells strongly expressed FasL. The expression of Fas was weak (34%) or absent. A higher number (33%) of epithelioid cells expressed FasL than Fas (23%). Lymphocytes were scanty among the epithelioid cells. The frequency of apoptotic cells was higher in the epithelioid cells (0.25%) than the mononuclear cells in the mantle zone (0.14%). Thus, the epithelioid cells and the multinucleated giant cells by virtue of the increased expression of FasL may make these granulomas an immune privileged site for mycobacteria.

Tuberculosis associated with infliximab, a tumor necrosis factor alpha-neutralizing agent

BACKGROUND

Infliximab is a humanized antibody against tumor necrosis factor alpha (TNF-alpha) that is used in the treatment of Crohn's disease and rheumatoid arthritis. Approximately 147,000 patients throughout the world have received infliximab. Excess TNF-alpha in association with tuberculosis may cause weight loss and night sweats, yet in animal models it has a protective role in the host response to tuberculosis. There is no direct evidence of a protective role of TNF-alpha in patients with tuberculosis.

METHODS

We analyzed all reports of tuberculosis after infliximab therapy that had been received as of May 29, 2001, through the MedWatch spontaneous reporting system of the Food and Drug Administration.

RESULTS

There were 70 reported cases of tuberculosis after treatment with infliximab, for a median of 12 weeks. In 48 patients, tuberculosis developed after three or fewer infusions. Forty of the patients had extrapulmonary disease (17 had disseminated disease, 11 lymph node disease, 4 peritoneal disease, 2 pleural disease, and 1 each meningeal, enteric, paravertebral, bone, genital, and bladder disease). The diagnosis was confirmed by a biopsy in 33 patients. Of the 70 reports, 64 were from countries with a low incidence of tuberculosis. The reported frequency of tuberculosis in association with infliximab therapy was much higher than the reported frequency of other opportunistic infections associated with this drug. In addition, the rate of reported cases of tuberculosis among patients treated with infliximab was higher than the available background rates.

CONCLUSIONS

Active tuberculosis may develop soon after the initiation of treatment with infliximab. Before prescribing the drug, physicians should screen patients for latent tuberculosis infection or disease.

Immunology of tuberculosis

The resurgence of tuberculosis worldwide has intensified research efforts directed at examining the host defense and pathogenic mechanisms operative in Mycobacterium tuberculosis infection. This review summarizes our current understanding of the host immune response, with emphasis on the roles of macrophages, T cells, and the cytokine/chemokine network in engendering protective immunity. Specifically, we summarize studies addressing the ability of the organism to survive within macrophages by controlling phagolysosome fusion. The recent studies on Toll-like receptors and the impact on the innate response to M. tuberculosis are discussed. We also focus on the induction, specificity, and effector functions of CD4(+) and CD8(+) T cells, and the roles of cytokines and chemokines in the induction and effector functions of the immune response. Presentation of mycobacterial antigens by MHC class I, class II, and CD1 as well as the implications of these molecules sampling various compartments of the cell for presentation to T cells are discussed. Increased attention to this disease and the integration of animal models and human studies have afforded us a greater understanding of tuberculosis and the steps necessary to combat this infection. The pace of this research must be maintained if we are to realize an effective vaccine in the next decades.

Cytosolic phospholipase A2 participates with TNF-alpha in the induction of apoptosis of human macrophages infected with Mycobacterium tuberculosis H37Ra

Macrophage (MPhi) apoptosis, an important innate microbial defense mechanism induced by Mycobacterium tuberculosis (Mtb) H37Ra, depends on the induction of TNF-alpha synthesis. When protein synthesis is blocked, both infection with Mtb and addition of TNF-alpha are required to induce caspase 9 activation, caspase 3 activation and apoptosis. In this study, we show that the second protein synthesis-independent signal involves activation of group IV cytosolic phospholipase A2 (cPLA2). Apoptosis of Mtb-infected MPhi and concomitant arachidonic acid release are abrogated by group IV cPLA2 inhibitors (methyl arachidonyl fluorophosphate and methyl trifluoromethyl ketone), but not by inhibitors of group VI Ca2+-independent (iPLA2; bromoenol lactone) or of secretory low molecular mass PLA2. In MPhi homogenates, the predominant PLA2 activity showed the same inhibitor sensitivity pattern and preferred arachidonic acid over palmitic acid in substrates, also indicating the presence of one or more group IV cPLA2 enzymes. In concordance with these findings, MPhi lysates contained transcripts and protein for group IV cPLA2-alpha and cPLA2-gamma. Importantly, group IV cPLA2 inhibitors significantly reduced MPhi antimycobacterial activity and addition of arachidonic acid, the major product of group IV cPLA2, to infected MPhi treated with cPLA2 inhibitors completely restored the antimycobacterial activity. Importantly, addition of arachidonic acid alone to infected MPhi significantly reduced the mycobacterial burden. These findings indicate that Mtb induces MPhi apoptosis by independent signaling through at least two pathways, TNF-alpha and cPLA2, which are both also critical for antimycobacterial defense of the MPhi.

Protein tyrosine kinase regulates FAS-mediated apoptosis in human BCG-infected monocytes

Apoptosis of monocytes/macrophages has emerged as a central regulatory event in the defense against mycobacterial infections. The involvement of protein tyrosine kinases (PTK) in Fas-mediated apoptosis in T cells is well established, but the possible role of PTK in Fas-dependent death of human bacillus Calmette-Guerin (BCG)-infected monocytes remains unclear. Here, we first examined the expression and function of Fas on BCG-infected human monocytes by flow cytometry. The results demonstrated that BCG-infected monocytes expressed significant Fas protein levels. In addition, engagement of the Fas antigen with its agonistic antibody (Ab) resulted in apoptosis of monocytes, as monitored by DNA analysis and fluorescence-activated cell sorter (FACS) analysis. The apoptotic action of Fas was suppressed significantly by genistein, indicating a role for PTK in this death process. Consistent with this observation, herbimycin A and tyrphostin, two selective tyrosine kinase inhibitors with different mechanisms of action, effectively inhibited Fas-mediated apoptosis of BCG-infected monocytes, as demonstrated by DNA content analysis. Moreover, we confirmed the effect of genistein, herbimycin A, and tyrphostin by examining apoptosis with the terminal transferase dUTP nick endlabeling (TUNEL) assay. Collectively, these data demonstrate that Fas-induced apoptosis may represent an important mechanism for eliminating BCG-activated human monocytes and that this apoptosis is due, at least in part, to signaling via a PTK pathway.

Depletion of alveolar macrophages exerts protective effects in pulmonary tuberculosis in mice

Mycobacterium tuberculosis bacilli are intracellular organisms that reside in phagosomes of alveolar macrophages (AMs). To determine the in vivo role of AM depletion in host defense against M. tuberculosis infection, mice with pulmonary tuberculosis induced by intranasal administration of virulent M. tuberculosis were treated intranasally with either liposome-encapsulated dichloromethylene diphosphonate (AM(-) mice), liposomes, or saline (AM(+) mice). AM(-) mice were completely protected against lethality, which was associated with a reduced outgrowth of mycobacteria in lungs and liver, and a polarized production of type 1 cytokines in lung tissue, and by splenocytes stimulated ex vivo. AM(-) mice displayed deficient granuloma formation, but were more capable of attraction and activation of T cells into the lung and had increased numbers of pulmonary polymorphonuclear cells. These data demonstrate that depletion of AMs is protective during pulmonary tuberculosis.

Requirement of A1-a for bacillus Calmette-Guérin-mediated protection of macrophages against nitric oxide-induced apoptosis

The role of apoptosis in regulating the course of intracellular microbial infection is not well understood. We studied the relationship between apoptotic regulation and bacillus Calmette-Guérin (BCG) treatment in murine peritoneal exudate macrophages (PEM) and the J774 macrophage cell line. In both PEM and J774 cells, mRNA expression of the anti-apoptotic gene, A1, was selectively induced by BCG treatment as compared with other bcl2 family members (bcl-w, bcl-2, bcl-xl, bcl-xs, bax, bak, bad). In PEM, A1 expression was maximal by 8 h postinfection and was abrogated by the proteasomal inhibitor MG-132. The induction was independent of protein synthesis as well as the p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways and did not require live organism. Three genes encoding closely related isoforms of A1 were all expressed; however, the A1-a isoform displayed the greatest fold induction in PEM. BCG-induced A1 expression was associated with protection of host macrophages from NO-mediated apoptosis in both PEM and J774 cells. BCG-mediated protection was abrogated in PEM derived from A1-a(-/-) mice, indicating a requirement of A1-a for survival of inflammatory macrophages.

Differential effects of a Toll-like receptor antagonist on Mycobacterium tuberculosis-induced macrophage responses

We previously showed that viable Mycobacterium tuberculosis (Mtb) bacilli contain distinct ligands that activate cells via the mammalian Toll-like receptor (TLR) proteins TLR2 and TLR4. We now demonstrate that expression of a dominant negative TLR2 or TLR4 proteins in RAW 264.7 macrophages partially blocked Mtb-induced NF-kappa B activation. Coexpression of both dominant negative proteins blocked virtually all Mtb-induced NF-kappa B activation. The role of the TLR4 coreceptor MD-2 was also examined. Unlike LPS, Mtb-induced macrophage activation was not augmented by overexpression of ectopic MD-2. Moreover, cells expressing an LPS-unresponsive MD-2 mutant responded normally to Mtb. We also observed that the lipid A-like antagonist E5531 specifically inhibited TLR4-dependent Mtb-induced cellular responses. E5531 could substantially block LPS- and Mtb-induced TNF-alpha production in both RAW 264.7 cells and primary human alveolar macrophages (AM phi). E5531 inhibited Mtb-induced AM phi apoptosis in vitro, an effect that was a consequence of the inhibition of TNF-alpha production by E5531. In contrast, E5531 did not inhibit Mtb-induced NO production in RAW 264.7 cells and AM phi. Mtb-stimulated peritoneal macrophages from TLR2- and TLR4-deficient animals produced similar amounts of NO compared with control animals, demonstrating that these TLR proteins are not required for Mtb-induced NO production. Lastly, we demonstrated that a dominant negative MyD88 mutant could block Mtb-induced activation of the TNF-alpha promoter, but not the inducible NO synthase promoter, in murine macrophages. Together, these data suggest that Mtb-induced TNF-alpha production is largely dependent on TLR signaling. In contrast, Mtb-induced NO production may be either TLR independent or mediated by TLR proteins in a MyD88-independent manner.

Fas (CD95)-Fas ligand interactions are responsible for monocyte apoptosis occurring as a result of phagocytosis and killing of Staphylococcus aureus

Human peripheral blood monocytes become apoptotic following phagocytosis of Staphylococcus aureus. In this study, we investigated the mechanisms involved in this phenomenon. Cells exposed to bacteria were examined for the surface expression of Fas and Fas ligand (FasL). The level of soluble form of FasL was also measured in the culture supernatants. As Fas-mediated apoptosis involves the activation of caspases, the activities of caspase-8 and caspase-3 were determined. Finally, the involvement of oxidative stress in apoptosis of infected monocytes was investigated. The data indicated that as a consequence of phagocytosis of S. aureus, FasL is released from the monocyte surface and induces apoptosis of phagocytic monocytes and to some extent the bystander cells. The importance of this mechanism was confirmed by demonstrating that blockage of CD95 prevents S. aureus-induced apoptosis of monocytes. Cell death occurring after phagocytosis of S. aureus involves the activation of caspase-3-like proteases, as the specific caspase-3 inhibitor suppressed apoptosis of infected cells. The generation of reactive oxygen intermediates by phagocytic monocytes by itself is not sufficient as a death signal but rather acts in up-regulating FasL shedding and possibly in modulating caspase activity.

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.

Use of Hoechst 33342 staining to detect apoptotic changes in bovine mononuclear phagocytes infected with Mycobacterium avium subsp. paratuberculosis

Mycobacterium avium subsp. paratuberculosis is an intracellular pathogen of macrophages that causes a chronic enteritis (Johne's disease) in ruminants. The purpose of this study was to determine whether M. avium subsp. paratuberculosis infection causes apoptosis in bovine monocytes. Using Hoechst 33342 staining, we observed increased numbers of apoptotic monocytes within 6 h of infection with M. avium subsp. paratuberculosis, and these numbers increased further at 24 and 48 h. This effect appeared to require viable bacilli, because monocytes infected with heat-killed M. avium subsp. paratuberculosis did not exhibit a significant increase in apoptosis. Preincubation of monocytes with bovine growth hormone prior to infection with M. avium subsp. paratuberculosis did not significantly alter the number of apoptotic cells.

Effects of tumor necrosis factor alpha on host immune response in chronic persistent tuberculosis: possible role for limiting pathology

Reactivation of latent tuberculosis contributes significantly to the incidence of disease caused by Mycobacterium tuberculosis. The mechanisms involved in the containment of latent tuberculosis are poorly understood. Using the low-dose model of persistent murine tuberculosis in conjunction with MP6-XT22, a monoclonal antibody that functionally neutralizes tumor necrosis factor alpha (TNF-alpha), we examined the effects of TNF-alpha on the immunological response of the host in both persistent and reactivated tuberculous infections. The results confirm an essential role for TNF-alpha in the containment of persistent tuberculosis. TNF-alpha neutralization resulted in fatal reactivation of persistent tuberculosis characterized by a moderately increased tissue bacillary burden and severe pulmonic histopathological deterioration that was associated with changes indicative of squamous metaplasia and fluid accumulation in the alveolar space. Analysis of pulmonic gene and protein expression of mice in the low-dose model revealed that nitric oxide synthase was attenuated during MP6-XT22-induced reactivation, but was not totally suppressed. Interleukin-12p40 and gamma interferon gene expression in TNF-alpha-neutralized mice was similar to that in control mice. In contrast, interleukin-10 expression was augmented in the TNF-alpha-neutralized mice. In summary, results of this study suggest that TNF-alpha plays an essential role in preventing reactivation of persistent tuberculosis, modulates the pulmonic expression of specific immunologic factors, and limits the pathological response of the host.

A model to predict cell-mediated immune regulatory mechanisms during human infection with Mycobacterium tuberculosis

A key issue for the study of tuberculosis infection (TB) is to understand why individuals infected with Mycobacterium tuberculosis experience different clinical outcomes. Elaborating the immune mechanisms that determine whether an infected individual will suffer active TB or latent infection can aid in developing treatment and prevention strategies. To better understand the dynamics of M. tuberculosis infection and immunity, we have developed a virtual human model that qualitatively and quantitatively characterizes the cellular and cytokine control network operational during TB infection. Using this model, we identify key regulatory elements in the host response. In particular, factors affecting cell functions, such as macrophage activation and bactericidal capabilities, and effector T cell functions such as cytotoxicity and cytokine production can each be determinative. The model indicates, however, that even if latency is achieved, it may come at the expense of tissue damage if the response is not properly regulated. A balance in Th1 and Th2 immune responses governed by IFN-gamma, IL-10, and IL-4 facilitate this down-regulation. These results are further explored through virtual deletion and depletion experiments.

Lipoarabinomannan from Mycobacterium tuberculosis promotes macrophage survival by phosphorylating Bad through a phosphatidylinositol 3-kinase/Akt pathway

Efforts in prevention and control of tuberculosis suffer from the lack of detailed knowledge of the mechanisms used by pathogenic mycobacteria for survival within host cell macrophages. The exploitation of host cell signaling pathways to the benefit of the pathogen is a phenomenon that deserves to be looked into in detail. We have tested the hypothesis that lipoarabinomannan (LAM) from the virulent species of Mycobacterium tuberculosis possesses the ability to modulate signaling pathways linked to cell survival. The Bcl-2 family member Bad is a proapoptotic protein. Phosphorylation of Bad promotes cell survival in many cell types. We demonstrate that man-LAM stimulates Bad phosphorylation in a phosphatidylinositol 3-kinase (PI-3K)-dependent pathway in THP-1 cells. Man-LAM activated PI-3K. LAM-stimulated phosphorylation of Bad was abrogated in cells transfected with a dominant-negative mutant of PI-3K (Delta p85), indicating that activation of PI-3K is sufficient to trigger phosphorylation of Bad by LAM. Since phosphorylation of Bad occurred at serine 136, the target of the serine/threonine kinase Akt, the effect of LAM on Akt kinase activity was tested. Man-LAM could activate Akt as evidenced from phosphorylation of Akt at Thr(308) and by the phosphorylation of the exogenous substrate histone 2B. Akt activation was abrogated in cells transfected with Deltap85. The phosphorylation of Bad by man-LAM was abrogated in cells transfected with a kinase-dead mutant of Akt. These results establish that LAM-mediated Bad phosphorylation occurs in a PI-3K/Akt-dependent manner. It is therefore the first demonstration of the ability of a mycobacterial virulence factor to up-regulate a signaling pathway involved in cell survival. This is likely to be one of a number of virulence-associated mechanisms by which bacilli control host cell apoptosis.

Osteopontin: a key cytokine in cell-mediated and granulomatous inflammation

Osteopontin (Opn) is a secreted adhesive, glycosylated phosphoprotein that contains the arginine-glycine-aspartic acid (RGD) cell-binding sequence that is found in many extracellular matrix (ECM) proteins (for a review of Opn see References Denhardt & Guo 1993; Patarca et al. 1993; Rittling & Denhardt 1999). Since its initial description in 1979 as a secreted protein associated with malignant transformation, Opn has been independently discovered by investigators from diverse scientific disciplines, and has been associated with a remarkable range of pathologic responses. Opn is an important bone matrix protein, where it is thought to mediate adhesion of osteoclasts to resorbing bone. However, studies from the past decade have identified an alternative role for Opn as a key cytokine regulating tissue repair and inflammation. Recent work by our laboratory and that of others has underlined the importance of Opn as a pivotal cytokine in the cellular immune response. Despite this Opn is not well known to the immunologist. In this review we will focus on studies that pertain to the role of Opn in cell-mediated and granulomatous inflammation.

Necrosis of lung epithelial cells during infection with Mycobacterium tuberculosis is preceded by cell permeation

Mycobacterium tuberculosis establishes infection, progresses towards disease, and is transmitted from the alveolus of the lung. However, the role of the alveolar epithelium in any of these pathogenic processes of tuberculosis is unclear. In this study, lung epithelial cells (A549) were used as a model in which to examine cytotoxicity during infection with either virulent or avirulent mycobacteria in order to further establish the role of the lung epithelium during tuberculosis. Infection of A549 cells with M. tuberculosis strains Erdman and CDC1551 demonstrated significant cell monolayer clearing, whereas infection with either Mycobacterium bovis BCG or Mycobacterium smegmatis LR222 did not. Clearing of M. tuberculosis-infected A549 cells correlated to necrosis, not apoptosis. Treatment of M. tuberculosis-infected A549 cells with streptomycin, but not cycloheximide, demonstrated a significant reduction in the necrosis of A549 cell monolayers. This mycobacterium-induced A549 necrosis did not correlate to higher levels of intracellular or extracellular growth by the mycobacteria during infection. Staining of infected cells with propidium iodide demonstrated that M. tuberculosis induced increased permeation of A549 cell membranes within 24 h postinfection. Quantitation of lactate dehydrogenase (LDH) release from infected cells further demonstrated that cell permeation was specific to M. tuberculosis infection and correlated to A549 cellular necrosis. Inactivated M. tuberculosis or its subcellular fractions did not result in A549 necrosis or LDH release. These studies demonstrate that lung epithelial cell cytotoxicity is specific to infection by virulent mycobacteria and is caused by cellular necrosis. This necrosis is not a direct correlate of mycobacterial growth or of the expression of host cell factors, but is preceded by permeation of the A549 cell membrane and requires infection with live bacilli.

Hijacking of apoptotic pathwaysby bacterial pathogens

Increasing evidence indicates that apoptosis of the host cell may constitute a defense mechanism to confine the infection by bacterial pathogens. Certain pathogens have developed elegant mechanisms to modulate the fate of the host cell, which include induction or blockage of apoptosis. These studies will promote our understanding of the pathogenesis of infectious diseases and aid the development of means for therapeutic intervention.

Evidence for the expression of native Mycobacterium tuberculosis phospholipase C: recognition by immune sera and detection of promoter activity

The genome of Mycobacterium tuberculosis H37Rv contains three contiguous genes (plc-a, plc-b and plc-c) which are similar to the Pseudomonas aeruginosa phospholipase C (PLC) genes. Expression of mycobacterial PLC-a and PLC-b in E. coli and M. smegmatis has been reported, whereas expression of the native proteins in M. tuberculosis H37Rv has not been demonstrated. The objective of the present study was to demonstrate that native PLC-a is expressed in M. tuberculosis H37Rv. Sera from mice immunized with recombinant PLC-a expressed in E. coli were used in immunoblots to evaluate PLC-a expression. The immune serum recognized a 49-kDa protein in immunoblots against M. tuberculosis extracts. No bands were visible in M. tuberculosis culture supernatants or extracts from M. avium, M. bovis and M. smegmatis. A 550-bp DNA fragment upstream of plc-a was cloned in the pJEM12 vector and the existence of a functional promoter was evaluated by detection of beta-galactosidase activity. beta-Galactosidase activity was detected in M. smegmatis transformed with recombinant pJEM12 grown in vitro and inside macrophages. The putative promoter was active both in vitro and in vivo, suggesting that expression is constitutive. In conclusion, expression of non-secreted native PLC-a was demonstrated in M. tuberculosis.

Group B Streptococcus induces apoptosis in macrophages

Group B Streptococcus (GBS) is a pathogen that has developed some strategies to resist host immune defenses. Because phagocytic killing is an important pathogenetic mechanism for bacteria, we investigated whether GBS induces apoptosis in murine macrophages. GBS type III strain COH31 r/s (GBS-III) first causes a defect in cell membrane permeability, then at 24 h, apoptosis. Apoptosis was confirmed by several techniques based on morphological changes and DNA fragmentation. Cytochalasin D does not affect apoptosis, suggesting that GBS-III needs not be within the macrophage cytoplasm to promote apoptosis. Inhibition of host protein synthesis prevents apoptosis, whereas inhibition of caspase-1 or -3, does not. Therefore, GBS can trigger an apoptotic pathway independent of caspase-1 and -3, but dependent on protein synthesis. Inhibition of apoptosis by EGTA and PMA, and enhancement of apoptosis by calphostin C and GF109203X suggests that an increase in the cytosolic calcium level and protein kinase C activity status are important in GBS-induced apoptosis. Neither alteration of plasma membrane permeability nor apoptosis were induced by GBS grown in conditions impeding hemolysin expression or when we used dipalmitoylphosphatidylcholine, which inhibited GBS beta-hemolytic activity, suggesting that GBS beta-hemolysin could be involved in apoptosis. beta-Hemolysin, by causing membrane permeability defects, could allow calcium influx, which initiates macrophage apoptosis. GBS also induces apoptosis in human monocytes but not in tumor lines demonstrating the specificity of its activity. This study suggests that induction of macrophage apoptosis by GBS is a novel strategy to overcome host immune defenses.

Induction of MMP-9 mediated gelatinolytic activity in human monocytic cells by cell wall components of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) infection induces the expression of host matrix metalloIproteinases (MMPs) capable of tissue degradation. We show that infection of mice with Mtb results in differential expression of MMPs in the lung. MMP-9 activity increased by week 1 post-infection, while MMP-2 activity increased after week 2. RT-PCR analysis for gene expression of gelatinases and their respective inhibitors showed: a small increase in MMP-9 by week 1, no change in TIMP-1 and MMP-2, and a significant decrease in TIMP-2 by week 4. The increase in MMP-2 could be due to a decrease in TIMP-2 expression. Addition of 4-aminophenylmercuric acid to lung extracts increased MMP-9 activity, suggesting that its regulation could be due to endogenous activation by proteases. In vitro, attenuated and virulent Mtb strains equally induced MMP-9 expression in U937 monocytes. The inducer of MMP-9 in Mtb was present in culture filtrates, and was active after paraformaldehyde fixation. LAM stimulated MMP-9 expression in THP-1 cells, but not U937 cells. However, LAM-free extracts also induced MMP-9 activity in THP-1 cells. Fractionation of Mtb extracts by chromatography revealed fractions of 17 and 156 kDa with MMP-9 inducing activity. In conclusion, LAM and other components of Mtb induce the expression of MMP-9.

A macrophage invasion mechanism for mycobacteria implicating the extracellular domain of CD43

We studied the role of CD43 (leukosialin/sialophorin), the negatively charged sialoglycoprotein of leukocytes, in the binding of mycobacteria to host cells. CD43-transfected HeLa cells bound Mycobacterium avium, but not Salmonella typhimurium or Shigella flexneri. Quantitative bacteriology showed that macrophages (M(phi)) from wild-type mice (CD43(+/+)) bound M. avium, Mycobacterium bovis (bacillus Calmette-Guérin), and Mycobacterium tuberculosis (strain H37Rv), whereas M(phi) from CD43 knockout mice (CD43(-/)-) did not. Fluorescence microscopy demonstrated that the associated M. avium had been ingested by the CD43(+/+) M(phi). The inability of CD43(-/)- M(phi) to bind M. avium could be restored by addition of galactoglycoprotein (Galgp), the extracellular mucin portion of CD43. The effect of Galgp is not due to opsonization of the bacteria, but required its interaction with the M(phi) other mucins had no effect. CD43 expression by the M(phi) was also required for optimal induction by M. avium of tumor necrosis factor (TNF)-alpha production, which likewise could be reconstituted by Galgp. In contrast, interleukin (IL)-10 production by M. avium-infected M(phi) was CD43 independent, demonstrating discordant regulation of TNF-alpha and IL-10. These findings describe a novel role of CD43 in promoting stable interaction of mycobacteria with receptors on the M(phi) enabling the cells to respond specifically with TNF-alpha production.

Flow cytometry as a tool to identify Mycobacterium tuberculosis interaction with the immune system and drug susceptibility

Flow cytometric analysis is a useful and widely employed tool to identify immunological alterations caused by different microorganisms, including Mycobacterium tuberculosis. However, this tool can be used for several others analysis. We will discuss some applications for flow cytometry to the study of M. tuberculosis, mainly on cell surface antigens, mycobacterial secreted proteins, their interaction with the immune system using inflammatory cells recovered from peripheral blood, alveolar and pleura spaces and the influence of M. tuberculosis on apoptosis, and finally the rapid determination of drug susceptibility. All of these examples highlight the usefulness of flow cytometry in the study of M. tuberculosis infection.

The modulation of host cell apoptosis by intracellular bacterial pathogens

Recent years have witnessed significant advances in unraveling the elegant mechanisms by which intracellular bacterial pathogens induce and/or block apoptosis, which can influence disease progression. This intriguing aspect of the host-pathogen interaction adds another fascinating dimension to our understanding of the exploitation of host cell biology by intracellular bacterial pathogens.

Induction of apoptosis in bacillus Calmette-Guérin-activated T cells by transforming growth factor-beta

In view of the critical role played by bacillus Calmette-Guérin (BCG) in the development and functional activation of protective T cells against tuberculosis, it has become important to understand the mechanisms by which cytokines regulate BCG-mediated immune responses. There is evidence that cytokine-mediated suppression of T cell function by mechanisms, including apoptosis, may reduce host resistance in tuberculosis. However, it is unclear whether cytokine-mediated suppression of antigen-responsive T cells through apoptotic mechanisms may be operating during human cellular activation induced by BCG. Here we present evidence, for the first time, that treatment of BCG-activated T cells with transforming growth factor-beta (TGF-beta) induces cellular apoptosis. These results were further supported by the fact that treatment of cells with a blocking mAb directed to TGF-beta significantly inhibited the percentage of apoptosis induced by TGF-beta. Interestingly, TGF-beta-mediated death of BCG-activated T cells in cultures containing interleukin (IL)-12 was observed. Moreover, our results demonstrated the induction of apoptosis by TGF-beta in BCG-activated T cells cultured in the presence of exogenous IL-12. In addition, our data indicated that TGF-beta significantly inhibited both BCG-induced cell growth determined by thymidine uptake and BCG-induced IFN-gamma secretion. Finally, TGF-beta-induced apoptosis in BCG-activated T cells correlated inversely with BCG-induced IFN-gamma secretion. Taken together, these findings indicate that TGF-beta induces apoptosis in human T cells activated with BCG and at the same time suggest that loss of BCG-reactive T cells through apoptotic mechanisms could contribute to an increased susceptibility to Mycobacterium tuberculosis infection.

Lung infections: role of apoptosis in host defense and pathogenesis of disease

Apoptosis is a form of cell death that has gained enormous attention during the past few years, and its mechanisms, important to biology and medicine, are being unraveled at an accelerating pace. Apoptosis of lung cells occurs during lung infections and may be either a host defense mechanism or reflect the pathogenesis of the infection. In the first part of this review, the biochemistry and physiology of apoptotic pathways and its regulators are discussed. This is followed by an overview of apoptotic mechanisms in selected lung infections. The implications of apoptosis in host immunity, pathogenesis, and treatment of pulmonary infections will be discussed in this context.

Implications of early apoptosis of infected cells as an important host defense

Apoptosis is widely recognized as being a host defense against viral infections, since viruses require live cells. There has been increasing acceptance of the view that apoptosis is also a defense against other intracellular pathogens and even against pathogens that adhere to host cells. An implication of apoptosis being a host defense is a need to reassess to what extent the cell death at infection sites may constitute a protective host response. A concept stressed here is that infected cells are a hazard to other cells and to the individual, so the benefits of early apoptosis are emphasized. Therefore, promoting the survival of infected cells, even though still functional, may carry risks. A further consideration is the possibility that the apoptotic stimulus of nutrient restriction may be acting in infection-induced anorexia to promote apoptosis of infected cells, thereby serving as a non-specific host defense.

Effect of Chlamydia trachomatis infection and subsequent tumor necrosis factor alpha secretion on apoptosis in the murine genital tract

The pathology observed during Chlamydia infection is due initially to localized tissue damage caused by the infection itself, followed by deleterious host inflammatory responses that lead to permanent scarring. We have recently reported that the infection by Chlamydia in vitro results in apoptosis of epithelial cells and macrophages and that infected monocytes secrete the proinflammatory cytokine interleukin-1beta. At the same time, proinflammatory cytokines such as tumor necrosis factor alpha (TNF-alpha) can also trigger apoptosis of susceptible cells. To study the possible relationship between Chlamydia trachomatis infection and apoptosis in vivo, we used the terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling technique to determine whether infection may cause apoptosis in the genital tract of mice and, conversely, whether cytokines produced during the inflammatory response may modulate the level of apoptosis. Our results demonstrate that infected cells in the endocervix at day 2 or 7 after infection are sometimes apoptotic, although there was not a statistically significant change in the number of apoptotic cells in the endocervix. However, large clumps of apoptotic infected cells were observed in the lumen, suggesting that apoptotic cells may be shed from the endocervix. Moreover, there was a large increase in the number of apoptotic cells in the uterine horns and oviducts after 2 or 7 days of infection, which was accompanied by obvious signs of upper tract pathology. Interestingly, depletion of TNF-alpha led to a decrease in the level of apoptosis in the uterine horns and oviducts of animals infected for 7 days, suggesting that the inflammatory cytokines may exert part of their pathological effect via apoptosis in infected tissues.

Quantitative analysis of apoptotic cell death in granulomatous inflammation induced by intravenous challenge with Cryptococcus neoformans and bacillus Calmette-Guérin vaccine

Apoptotic cell death of macrophage has become recognized as a significant mechanism responsible for the resolution of inflammation. The purpose of this study was to examine how the apoptotic cell death involves the formation and resolution of granulomas in rats intravenously inoculated with Cryptococcus neoformans (Cr. neoformans) and Mycobacterium bovis-derived bacillus Calmette-Guérin (BCG) vaccine. The number and size of granulomas in the livers obtained on days 5, 10, 15, 20 and 25 after inoculation were examined by morphometric image analysis, as well as the occurrence of apoptotic cell death quantitatively analyzed by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end-labeling (TUNEL) procedure on tissue sections. In both groups the number and size of granulomas were maximized on day 10, then the granulomas were almost resolved until day 25 when the inoculated Cr. neoformans and BCG almost disappeared. From the induction to the resolving stages of granulomatous inflammation, TUNEL-positive cells constantly appeared in granulomas, and the highest frequency of apoptotic cells in granulomas was observed in the earlier stage of granuloma formation. These results indicate that the maintenance and resolution of infectious granulomas are regulated by the balance between the influx of newly recruited macrophages and the apoptotic elimination of granuloma macrophages. The apoptosis of granuloma macrophages actively involves the cellular turnover in both granuloma formation and resolution.

Apoptosis of human keratinocytes after bacterial invasion

In this study, we examined the invasive capacity of Staphylococcus aureus and Salmonella typhi in human keratinocytes and monitored the number of viable intracellular bacteria at different post-infection times. The strains tested entered keratinocytes; both S. typhi and S. aureus were internalized within 30 min to 2 h after infection. No intracellular multiplication was observed, but S. typhi and S. aureus remained viable 72 h after infection. We also demonstrated that keratinocyte death following S. typhi and S. aureus invasion occurs by apoptosis as shown by DNA fragmentation. After 24 h of infection with S. typhi, the number of cells undergoing apoptosis were higher compared to infection with S. aureus. For prolonged infection times (48 h, 72 h) with both bacteria, there was no significant change in the number of cells undergoing apoptosis. The results demonstrated that viable intracellular S. typhi and S. aureus induced apoptosis in keratinocyte cells.

Virulent Mycobacterium tuberculosis strains evade apoptosis of infected alveolar macrophages

Human alveolar macrophages (AMphi) undergo apoptosis following infection with Mycobacterium tuberculosis in vitro. Apoptosis of cells infected with intracellular pathogens may benefit the host by eliminating a supportive environment for bacterial growth. The present study compared AMphi apoptosis following infection by M. tuberculosis complex strains of differing virulence and by Mycobacterium kansasii. Avirulent or attenuated bacilli (M. tuberculosis H37Ra, Mycobacterium bovis bacillus Calmette-Guérin, and M. kansasii) induced significantly more AMphi apoptosis than virulent strains (M. tuberculosis H37Rv, Erdman, M. tuberculosis clinical isolate BMC 96.1, and M. bovis wild type). Increased apoptosis was not due to greater intracellular bacterial replication because virulent strains grew more rapidly in AMphi than attenuated strains despite causing less apoptosis. These findings suggest the existence of mycobacterial virulence determinants that modulate the apoptotic response of AMphi to intracellular infection and support the hypothesis that macrophage apoptosis contributes to innate host defense in tuberculosis.

Infection of the macrophage cell line NR8383 with Mycobacterium tuberculosis (H37Ra) leads to an increase in oligodeoxynucleotide accumulation

Mycobacterium tuberculosis infection continues to be a daunting clinical challenge. Although it may well be one of the most studied bacteria in history, several aspects of its pathology remain a mystery. The resurgence of drug-resistant M. tuberculosis strains and with its unusual pathology have promoted a renewed basic and clinical research interest in developing new therapies to combat this pathogen. The primary localization site for M. tuberculosis is within alveolar macrophages. Drug delivery strategies and novel therapeutic agents designed to target alveolar macrophages may lead to efficient destruction of M. tuberculosis. Oligodeoxynucleotides (ODN) are short segments of nucleic acids that can interfere with transcription and translation processes. In this report, a monocyte-macrophage cell line was characterized in regard to ODN transport in the presence or absence of M. tuberculosis infection. The cells accumulated ODN in a time-dependent and concentration-dependent manner, regardless of the presence of serum. After 4 hours of incubation with M. tuberculosis (multiplicity of infection [MOI] 10:1), infected NR8383 cells demonstrated 1.5-7-fold increase in fluorescein isothiocyanate (FITC)-labeled phosphorothioate ODN accumulation as measured by flow cytometry. The increase in uptake was associated only with fluorescent-labeled ODN and not labeled markers of fluid phase endocytosis (e.g., tetramethylrhodamine isothiocyanate [TRITC], FITC-labeled dextran). NR8383 cells activated by phytohemagglutinin (PHA) did not demonstrate a significant increase in the uptake of either FITC-labeled dextran or FITC-labeled ODN. These studies demonstrate that NR8383 cells that have been infected with M. tuberculosis can specifically accumulate ODN, and this route of accumulation may lead to a means of drug targeting to mycobacteria-containing cells.

In vitro Brucella suis infection prevents the programmed cell death of human monocytic cells

During the complex interaction between an infectious agent and a host organism, the pathogen can interfere with the host cell's programmed death to its own benefit. Induction or prevention of host cell apoptosis appears to be a critical step for determining the infection outcome. Members of the gram-negative bacterial genus Brucella are intracellular pathogens which preferentially invade monocytic cells and develop within these cells. We investigated the effect of Brucella suis infection on apoptosis of human monocytic phagocytes. The present study provides evidence that Brucella infection inhibited spontaneously occurring apoptosis in human monocytes. Prevention of monocyte apoptosis was not mediated by Brucella lipopolysaccharide and required bacterial survival within infected cells. Both invaded and noninvaded cells were protected, indicating that soluble mediators released during infection were involved in the phenomenon. Analysis of Brucella-infected monocytes revealed specific overexpression of the A1 gene, a member of the bcl-2 family implicated in the survival of hematopoietic cells. Brucella infection also rendered macrophage-like cells resistant to Fas ligand- or gamma interferon-induced apoptosis, suggesting that Brucella infection protected host cells from several cytotoxic processes occurring at different steps of the immune response. The present data clearly show that Brucella suis modulated the monocyte/macrophage's apoptotic response to the advantage of the pathogen, thus preventing host cell elimination. This might represent a strategy for Brucella development in infected hosts.

Increased expression of Fas ligand on Mycobacterium tuberculosis infected macrophages: a potential novel mechanism of immune evasion by Mycobacterium tuberculosis?

We have studied the location and mechanism of apoptosis within the granulomas in the lungs at various stages of slowly progressive primary murine Mycobacterium tuberculosis infection. Parallel sections were analyzed for detection of mycobacterial antigens, Fas, and Fas ligand (FasL) by immunohistochemistry, and for apoptotic cells by terminal deoxynucleotidyl-transferase-mediated dUTP-digoxigenin nick end labeling (TUNEL) method. The frequency of apoptosis was high in the macrophage aggregates as compared to the lymphocyte aggregates and at the interface between them. Five to seven percent of the vacuolated macrophages in the granulomas expressed FasL intensely. These cells contained large amounts of mycobacterial antigens. These findings suggest that M. tuberculosis infection can induce increased expression of FasL in a population of infected macrophages. As a consequence the infected macrophages will be protected from the attack of cytotoxic T cells and activation of bactericidal mechanisms by Th1 type lymphocytes. This constitutes a novel evasion mechanism for M. tuberculosis possibly explaining the chronic course of infection.

The induction of apoptosis by bacterial pathogens

Apoptosis is a highly regulated process of cell death that is required for the development and homeostasis of multicellular organisms. In contrast to necrosis, apoptosis eliminates individual cells without inducing an inflammatory response. Activation or prevention of cell death could be a critical factor in the outcome of an infection. Programmed cell death has been observed as a response to infection by a wide range of animal and plant pathogens and is mediated by an array of pathogen-encoded virulence determinants. Pathogen-induced modulation of the host cell-death pathway may serve to eliminate key immune cells or evade host defenses that can act to limit the infection. Alternatively, suppression of the death pathway may facilitate the proliferation of intracellular pathogens.

Apoptosis of murine peritoneal macrophages induced by an avian pathogenic strain of Escherichia coli

The mechanisms used by avian strains of Escherichia coli to invade the respiratory epithelia, leading to septicemia in poultry, are not well-established. In this work, we show that resident murine peritoneal macrophages infected in vitro with an avian strain of E. coli underwent apoptosis 4 h after infection (55.6% of apoptosis in infected cells versus 3.5% in non-infected cells). Heat-inactivated bacteria did not induce apoptosis and the inhibition of phagocytosis by pretreatment of cells with cytochalasin D reduced the number of apoptotic cells from 55.6 to 13.9% (P<0.05), showing that the bacteria must be intracellularly located and viable to induce apoptosis. Therefore, these data suggest that induction of macrophage apoptosis may be a pathogenic mechanism employed by avian E. coli to circumvent the host defences and invade the respiratory epithelia.

Group A Streptococcus induces apoptosis in human epithelial cells

Internalization of group A streptococcus (GAS) by epithelial cells may have a role in causing invasive diseases. The purpose of this study was to examine the fate of GAS-infected epithelial cells. GAS has the ability to invade A-549 and HEp-2 cells. Both A-549 and HEp-2 cells were killed by infection with GAS. Epithelial cell death mediated by GAS was at least in part through apoptosis, as shown by changes in cellular morphology, DNA fragmentation laddering, and propidium iodide staining for hypodiploid cells. A total of 20% of A-549 cells and 11 to 13% of HEp-2 cells underwent apoptosis after 20 h of GAS infection, whereas only 1 to 2% of these cells exhibited spontaneous apoptosis. We further examined whether streptococcal pyrogenic exotoxin B (SPE B), a cysteine protease produced by GAS, was involved in the apoptosis of epithelial cells. The speB isogenic mutants had less ability to induce cell death than wild-type strains. When A-549 cells were cocultured with the mutant and SPE B for 2 h, the percentage of apoptotic cells did not increase although the number of intracellular bacteria increased to the level of wild-type strains. In addition, apoptosis was blocked by cytochalasin D treatment, which interfered with cytoskeleton function. The caspase inhibitors Z-VAD.FMK, Ac-YVAD.CMK, and Ac-DEVD.FMK inhibited GAS-induced apoptosis. These results demonstrate for the first time that GAS induces apoptosis of epithelial cells and internalization is required for apoptosis. The caspase pathway is involved in GAS-induced apoptosis, and the expression of SPE B in the cells enhances apoptosis.

Roles of lipoarabinomannan in the pathogenesis of tuberculosis

Tuberculosis is a worldwide public health threat caused by Mycobacterium tuberculosis. All mycobacteria express a unique cell envelope glycolipid, lipoarabinomannan, which can be released at sites of infection. Lipoarabinomannan is a potential virulence factor which can bind to leukocytes and modulate immune responses. Here, we provide an overview of the interactions of mycobacteria and lipoarabinomannan with immune cells.

TNF-α and IL-10 Modulate the Induction of Apoptosis by Virulent Mycobacterium tuberculosis in Murine Macrophages

The Bcg/Nramp1 gene controls early resistance and susceptibility of macrophages to mycobacterial infections. We previously reported that Mycobacterium tuberculosis-infected (Mtb) B10R (Bcgr) and B10S (Bcgs) macrophages differentially produce nitric oxide (NO−), leading to macrophage apoptosis. Since TNF-α and IL-10 have opposite effects on many macrophage functions, we determined the number of cells producing TNF-α and IL-10 in Mtb-infected or purified protein derivative-stimulated B10R and B10S macrophages lines, and Nramp1+/+ and Nramp1−/− peritoneal macrophages and correlated them with Mtb-mediated apoptosis. Mtb infection and purified protein derivative treatment induced more TNF-α+Nramp1+/+ and B10R, and more IL-10+Nramp1−/− and B10S cells. Treatment with mannosylated lipoarabinomannan, which rescues macrophages from Mtb-induced apoptosis, augmented the number of IL-10 B10R+ cells. Anti-TNF-α inhibited apoptosis, diminished NO− production, p53, and caspase 1 activation and increased Bcl-2 expression. In contrast, anti-IL-10 increased caspase 1 activation, p53 expression, and apoptosis, although there was no increment in NO− production. Murine rTNF-α induced apoptosis in noninfected B10R and B10S macrophages that was reversed by murine rIL-10 in a dose-dependent manner with concomitant inhibition of NO− production and caspase 1 activation. NO− and caspase 1 seem to be independently activated in that aminoguanidine did not affect caspase 1 activation and the inhibitor of caspase 1, Tyr-Val-Ala-Asp-acylooxymethylketone, did not block NO− production; however, both treatments inhibited apoptosis. These results show that Mtb activates TNF-α- and IL-10-dependent opposite signals in the induction of macrophage apoptosis and suggest that the TNF-α-IL-10 ratio is controlled by the Nramp1 background of resistance/susceptibility and may account for the balance between apoptosis and macrophage survival.

The regulation of apoptosis by microbial pathogens

In the past few years, there has been remarkable progress unraveling the mechanism and significance of eukaryotic programmed cell death (PCD), or apoptosis. Not surprisingly, it has been discovered that numerous, unrelated microbial pathogens engage or circumvent the host's apoptotic program. In this chapter, we briefly summarize apoptosis, emphasizing those studies which assist the reader in understanding the subsequent discussion on PCD and pathogens. We then examine the relationship between virulent bacteria and apoptosis. This section is organized to reflect both common and diverse mechanisms employed by bacteria to induce PCD. A short discussion of parasites and fungi is followed by a detailed description of the interaction of viral pathogens with the apoptotic machinery. Throughout the review, apoptosis is considered within the broader contexts of pathogenesis, virulence, and host defense. Our goals are to update the reader on this rapidly expanding field and identify topics in the current literature which demand further investigation.

Apoptosis of Mycobacterium avium-infected macrophages is mediated by both tumour necrosis factor (TNF) and Fas, and involves the activation of caspases

Mycobacterium avium causes disseminated infection in AIDS patients and several forms of infection in immunocompetent hosts. Recent studies have shown that M. avium infection of macrophages in vitro leads to apoptosis of significant numbers of infected cells. Several strains of M. avium used to infect human macrophages for 5 days (multiplicity of infection of 10) triggered 28-46% higher levels of apoptosis than observed with uninfected macrophages at the same time points. Mycobacterium avium strains unable to replicate intracellularly (rep-) resulted in a 15% rate of apoptosis, while M. smegmatis-infected monolayers showed the same percentage of apoptotic cells as the uninfected macrophage control. The presence of anti-TNF-alpha antibody reduced apoptosis to 17% and the presence of anti-Fas antibody reduced apoptosis to 10%. When both antibodies were used together, the apoptosis level was 5% above the control. Treatment with TGF-beta also reduced the number of apoptotic cells in infected monolayers. If intracellular growth was inhibited, apoptosis of macrophages decreased significantly. It was also shown that apoptosis was associated with IL-1 beta-converting enzyme (ICE) activation and was significantly reduced by a caspase inhibitor. Gaining understanding of the mechanisms of M. avium-associated apoptosis of macrophages will provide important insight into M. avium pathogenesis.

Mycobacterium tuberculosis phagosome

The arrest of Mycobacterium tuberculosis phagosome maturation in infected macrophages is a phenomenon of dual significance both for the pathogenesis of tuberculosis and as a model system to study interference of microbes with membrane trafficking and organelle biogenesis in host cells. Among other factors, compartment-specialized regulators of vesicular trafficking and other parts of membrane fusion machinery are likely to play a role in these processes. Here we summarize the emerging view of mycobacterial phagosome maturation arrest in the context of the dynamic processes of intracellular membrane trafficking.

In vivo apoptosis of hepatocytes in guinea pigs infected with Leptospira interrogans serovar icterohaemorrhagiae

To investigate the contribution of the previously demonstrated in vitro apoptosis to the pathogenesis of leptospirosis, guinea pigs were infected with Leptospira interrogans serovar icterohaemorrhagiae strain Verdun and sequentially killed to collect target organs involved in the natural history of the disease (liver, kidneys, lungs, spleen and heart). The combination of histopathological procedures and a specific TUNEL assay showed a significant Leptospira-induced programmed cell death of hepatocytes with a peak at 48 h post inoculation. Hepatocyte nuclei showed morphological changes including fragmented and condensed nuclei. This phenomenon occurred early in the course of the disease at a time where infecting leptospires were present at a low density between the liver parenchyma cells.

Cryptosporidium parvum is cytopathic for cultured human biliary epithelia via an apoptotic mechanism

While the clinical features of sclerosing cholangitis secondary to opportunistic infections of the biliary tree in patients with acquired immunodeficiency syndrome (AIDS) are well known, the mechanisms by which microbial pathogens such as Cryptosporidium parvum associated with this syndrome actually cause disease are obscure. We established an in vitro model of biliary cryptosporidiosis employing a human biliary epithelial cell line. Using morphological and biochemical techniques, we examined the interaction of C. parvum with cultured human cholangiocytes. When the apical plasma membrane of polarized, confluent monolayers of human biliary epithelial cells was exposed to C. parvum oocysts that had been excysted in vitro, sporozoites attached to and invaded the cells in a time-, dose-, temperature-, and pH-dependent manner. The infectious process was both plasma membrane domain- and cell-specific, because no attachment or invasion occurred when the basolateral membrane of cholangiocytes was exposed to the parasite, or when a human hepatocyte cell line (HepG2) was used. Time-lapse video microscopy and scanning electron microscopy (SEM) showed that sporozoite attachment was rapid, involved extensive cholangiocyte membrane ruffling, and culminated in parasite penetration into a tight-fitting vacuole formed by invagination of the plasma membrane similar to those found in naturally occurring infection in vivo. Transmission electron microscopy (TEM) showed that C. parvum organisms formed parasitophorus vacuoles and were able to undergo a complete reproductive cycle, forming both asexual and sexual reproductive stages. Unexpectedly, direct cytopathic effects were noted in infected monolayers, with widespread programmed cell death (i.e., apoptosis) of biliary epithelial cells as assessed both morphologically and biochemically beginning within hours after exposure to the organism. The novel finding of specific cytopathic invasion of biliary epithelia by C. parvum may be relevant to the pathogenesis and possible therapy of the secondary sclerosing cholangitis seen in AIDS patients with biliary cryptosporidiosis.

Pathogenic Mycobacterium tuberculosis Evades Apoptosis of Host Macrophages by Release of TNF-R2, Resulting in Inactivation of TNF-α

Infection by Mycobacterium tuberculosis (MTB) induces human alveolar macrophage (AMφ) apoptosis by a TNF-α-dependent mechanism. The apoptotic response is postulated to be a defense mechanism, limiting the growth of this intracellular pathogen. Consistent with that model, recent studies showed that the virulent MTB strain H37Rv induces substantially less AMφ apoptosis than the attenuated strain H37Ra. We now report that AMφ infection with either H37Rv or H37Ra induces comparable levels of TNF-α measured by ELISA but that TNF-α bioactivity is reduced in supernatants of H37Rv-infected AMφ. Differential release of soluble TNFR2 (sTNFR2), with formation of inactive TNF-α-TNFR2 complexes accounted for the difference in TNF-α bioactivity in these cultures. Release of sTNFR2 by H37Rv-infected AMφ was IL-10 dependent since it was inhibited by neutralizing anti-IL-10 Ab. Thus, the effect of TNF-α produced by AMφ following infection can be modulated by virulent MTB, using IL-10 as an upstream mediator.