Pretreatment with a heat-killed probiotic modulates monocyte chemoattractant protein-1 and reduces the pathogenicity of influenza and enterovirus 71 infections

It has been proposed that inactivated probiotics may modulate the host immune system and contribute to mitigation of viral infections. This study demonstrated that administration of heat-killed Enterococcus faecalis, a widely used probiotic, can protect host animals against viral infections. The influenza-mediated morbidity and lung inflammation in E. faecalis-treated mice decreased significantly compared with those of the control mice. Furthermore, we found that the protection is associated with production of monocyte chemoattractant protein-1 (MCP-1). The intratracheal injection of a recombinant mouse MCP-1 protein abrogated the antiviral effects elicited by pretreatment with E. faecalis. CC chemokine receptor 2 (CCR2) is a receptor for MCP-1, and the intraperitoneal administration of a CCR2 antagonist effectively inhibited viral pathogenicity. The reduced pathogenicity was also observed in CCR2-deficient mice. Finally, E. faecalis significantly attenuated neuropathogenicity induced by another RNA virus, enterovirus 71. This study demonstrates that killed probiotics can reduce viral disease severity and identify that the MCP-1 pathway might act as a key mediator in the improved antiviral immune response. Our findings suggest that MCP-1 and its related signaling pathway can serve as critical therapeutic targets for development of new antiviral strategies.

A Dual Role for P2X7 Receptor during Porphyromonas gingivalis Infection

Emerging evidence suggests a role for purinergic signaling in the activation of multiprotein intracellular complexes called inflammasomes, which control the release of potent inflammatory cytokines, such as interleukin (IL) -1β and -18. Porphyromonas gingivalis is intimately associated with periodontitis and is currently considered one of the pathogens that can subvert the immune system by limiting the activation of the NLRP3 inflammasome. We recently showed that P. gingivalis can dampen eATP-induced IL-1β secretion by means of its fimbriae in a purinergic P2X7 receptor-dependent manner. Here, we further explore the role of this purinergic receptor during eATP-induced IL-1β processing and secretion by P. gingivalis-infected macrophages. We found that NLRP3 was necessary for eATP-induced IL-1β secretion as well as for caspase 1 activation irrespective of P. gingivalis fimbriae. Additionally, although the secretion of IL-1β from P. gingivalis-infected macrophages was dependent on NLRP3, its adaptor protein ASC, or caspase 1, the cleavage of intracellular pro-IL-1β to the mature form was found to occur independently of NLRP3, its adaptor protein ASC, or caspase 1. Our in vitro findings revealed that P2X7 receptor has a dual role, being critical not only for eATP-induced IL-1β secretion but also for intracellular pro-IL-1β processing. These results were relevant in vivo since P2X7 receptor expression was upregulated in a P. gingivalis oral infection model, and reduced IFN-γ and IL-17 were detected in draining lymph node cells from P2rx7(-/-) mice. Furthermore, we demonstrated that P2X7 receptor and NLRP3 transcription were modulated in human chronic periodontitis. Overall, we conclude that the P2X7 receptor has a role in periodontal immunopathogenesis and suggest that targeting of the P2X7/NLRP3 pathway should be considered in future therapeutic interventions in periodontitis.

Porphyromonas gingivalis infection sequesters pro-apoptotic Bad through Akt in primary gingival epithelial cells

Porphyromonas gingivalis, a self-limiting oral pathogen, can colonize and replicate in gingival epithelial cells (GECs). P. gingivalis-infected GECs are protected from mitochondrion-dependent apoptosis, partially through activation of phosphatidyl inositol 3-kinase/Akt signaling. Biochemical events associated with P. gingivalis-induced inhibition of apoptosis include the blocking of mitochondrial membrane permeability and cytochrome-c release. We studied functional importance of Akt and the status of associated key mitochondrial molecules, pro-apoptotic Bad and caspase-9, during infection of GECs. We found that P. gingivalis infection caused significant phosphorylation of Bad progressively, while messenger RNA levels for Bad slowly decreased. Fluorescence microscopy showed translocation of the mitochondrial Bad to the cytosol post-infection. Conversely, P. gingivalis lost the ability to promote phosphorylation and translocation of Bad in Akt-deficient GECs. Caspase-9 activation induced by a chemical inducer of apoptosis was significantly inhibited by infection over time. However, Akt depletion by small interfering RNA did not reverse inhibition of caspase-9 activation by infection. Hence, P. gingivalis inactivates pro-apoptotic Bad through Akt. The inhibition of caspase-9 activation appears to be independent of Akt. Overall, our findings suggest that Akt is a key component of anti-apoptotic pathways stimulated by P. gingivalis. The P. gingivalis uses other mitochondrial pathways to protect host cells from cell-death and to ensure its survival in gingival epithelium.

Characterization of a monoclonal antibody reacting with antigen-4 domain of gp900 in Cryptosporidium parvum invasive stages

Cryptosporidium parvum (Protozoa, Apicomplexa) infects the apical surface of intestinal epithelial cells, where it grows and divides within a membrane-bound parasitophorous vacuole. gp900, an abundant glycoprotein of C. parvum merozoites and sporozoites, is localized in micronemes and at the surface of invasive stages and participates in the invasion process. Here, we describe a new monoclonal antibody (mAb) against gp900. As shown by immunofluorescence of excysted parasites and immunoelectron microscopy of infected tissues, the mAb reacted with micronemes present in the apical pole of invasive stages. In immunoprecipitation experiments, the mAb was shown to react with a high molecular weight antigen co-migrating with gp900. Finally, three reactive clones were selected upon screening of a C. parvum genomic expression library with the mAb; and sequencing of the insert from one of them showed a 596 bp sequence identical to the DNA region encoding a domain of gp900 identified as antigen 4.

Modulation of P2Z/P2X(7) receptor activity in macrophages infected with Chlamydia psittaci

Given the role that extracellular ATP (ATP(o))-mediated apoptosis may play in inflammatory responses and in controlling mycobacterial growth in macrophages, we investigated whether ATP(o) has any effect on the viability of chlamydiae in macrophages and, conversely, whether the infection has any effect on susceptibility to ATP(o)-induced killing via P2Z/P2X(7) purinergic receptors. Apoptosis of J774 macrophages could be selectively triggered by ATP(o), because other purine/pyrimidine nucleotides were ineffective, and it was inhibited by oxidized ATP, which irreversibly inhibits P2Z/P2X(7) purinergic receptors. Incubation with ATP(o) but not other extracellular nucleotides inhibits the growth of intracellular chlamydiae, consistent with previous observations on ATP(o) effects on growth of intracellular mycobacteria. However, chlamydial infection for 1 day also inhibits ATP(o)-mediated apoptosis, which may be a mechanism to partially protect infected cells against the immune response. Infection by Chlamydia appears to protect cells by decreasing the ability of ATP(o) to permeabilize macrophages to small molecules and by abrogating a sustained Ca(2+) influx previously associated with ATP(o)-induced apoptosis.

'Infectious web'

A comprehensive list of all known bacterial pathogens of humans is now available at various web-sites on the internet. The sites contain hyperlinks to original scientific literature, along with general information on laboratory testing, antibiotic resistance and clinical treatment. More specific sites highlight the fungus Pneumocystic carinii, arguably the main cause of pneumonia in immunosuppressed individuals.

'Infectious web'

Exhaustive information on the Epstein-Barr virus, a member of the herpes family, is described at the International Herpes Management Forum web-site. Cervical cancer associations, AIDS treatment projects, and the Los Alamos National Laboratories provide useful information on papillomavirus infections, as well as hyperlinks to recent international papillomavirus conferences. A private pharmaceutical company, in collaboration with the National Institutes of Health, has launched a lively web-site covering different aspects of microbial infections for the general public.

'Infectious web'

Infections by Helicobacter pylori are responsible for duodenal and gastric ulcers and are a significant risk factor for the development of gastric adenocarcinoma. H. pylori was discovered in 1983, but many institutes in Canada, Europe, and the United States are already involved in programs to understand and treat the infections, as reflected by the growing number of internet sites devoted to this bacterium. Most AIDS patients and about 20% of children with acute lymphoblastic leukemia develop Pneumocystis carinii pneumoniae. Information on clinical symptoms and treatment, as well as the P. carinii genome sequencing project, are described at several web sites. Students and researchers wishing to understand the correlation between telomere length and AIDS may turn to web sites of the University of Colorado and Washington University School of Medicine for the latest on telomeres and telomerase, and their function in aging and cancer.

Fluorescent labelling of intracellular bacteria in living host cells

The fluorescent reagent, CellTracker, labels metabolically-active cells and was used here to label Chlamydia in vivo during their exponential phase of growth in infected cells. HeLa cells infected with C. psittaci were labelled with the CellTracker reagents between 15 and 48 h post-infection. The fluorescent label accumulated in the host-cell membrane compartment (inclusion) within which Chlamydia reside and replicate, and was also incorporated by the bacteria. Labelling with the CellTracker affected neither the growth nor the differentiation of the chlamydiae, and labelled chlamydiae isolated from infected cells were infectious. Our results demonstrate that the CellTracker could become a valuable tool for in vivo labelling of obligate intracellular parasites for which no genetic tools exist.

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.

'Infectious web'

This issue of 'Infectious Web' includes web-sites related to AIDS/HIV, pathogenic characteristics and resistance to Staphylococcus spp., diagnostic and clinical aspects of arthritis, and comprehensive information resources on malaria, cystic fibrosis and biological weapons.

Caspase-dependent apoptosis during infection with Cryptosporidium parvum

The protozoan parasite Cryptosporidium parvum causes persistent diarrhea and malnutrition in children and the diarrhea-wasting syndrome in AIDS. No therapy exists for eliminating the parasite in the absence of a healthy immune response. Although it had been reported that infection of intestinal cell lines with C. parvum leads to host cell death, the mechanisms of cytolysis have not been characterized. We show here that infection with C. parvum leads to typical apoptotic nuclear condensation and DNA fragmentation in host cells. Both nuclear condensation and DNA fragmentation are inhibited by a caspase inhibitor, showing that caspases are involved in this type of apoptosis. Finally, blocking apoptosis with the caspase inhibitor increases the percentage of infected cells, suggesting that parasites may use apoptosis to exit from the infected cell or that the infected cells may eliminate the parasite through apoptosis. These results suggest that apoptosis could be involved in the pathogenesis of C. parvum infections in vivo, and raise the possibility that therapeutic interference with host cell death could alter the course of the pathology in vivo.

'Infectious web'

The infection-related web-sites discussed in this issue include sites on the Campylobacter jejuni genome, Toxoplasma-expressed sequence tags (ESTs), Salmonella infections, the history and biology of tuberculosis, both DNA and RNA vaccines, and interactions between cytomegalovirus and antibodies.

Chlamydia infection of epithelial cells expressing dynamin and Eps15 mutants: clathrin-independent entry into cells and dynamin-dependent productive growth

Chlamydiae enter epithelial cells via a mechanism that still remains to be fully elucidated. In this study we investigated the pathway of entry of C. psittaci GPIC and C. trachomatis LGV/L2 into HeLa cells and demonstrated that it does not depend on clathrin coated vesicle formation. We used mutant cell lines defective in clathrin-mediated endocytosis due to overexpression of dominant negative mutants of either dynamin I or Eps15 proteins. When clathrin-dependent endocytosis was inhibited by overexpression of the dynK44A mutant of dynamin I (defective in GTPase activity), Chlamydia entry was not affected. However, in these cells there was a dramatic inhibition in the proliferation of Chlamydia and the growth of the chlamydia vacuole (inclusion). When clathrin-dependent endocytosis was inhibited by overexpression of an Eps15 dominant negative mutant, the entry and growth of Chlamydia was unaltered. These results indicate that the effect on the growth of Chlamydia in the dynK44A cells was not simply due to a deprivation of nutrients taken up by endocytosis. Instead, the dominant-negative mutant of dynamin most likely affects the vesicular traffic between the Chlamydia inclusion and intracellular membrane compartments. In addition, cytochalasin D inhibited Chlamydia entry by more than 90%, indicating that chlamydiae enter epithelial cells by an actin-dependent mechanism resembling phagocytosis. Finally, dynamin is apparently not involved in the formation of phagocytic vesicles containing Chlamydia.

P2Z/P2X7 receptor-dependent apoptosis of dendritic cells

Macrophages and thymocytes express P2Z/P2X7 nucleotide receptors that bind extracellular ATP. These receptors play a role in immune development and control of microbial infections, but their presence on dendritic cells has not been reported. We investigated whether extracellular ATP could trigger P2Z/P2X7 receptor-dependent apoptosis of dendritic cells. Apoptosis could be selectively triggered by tetrabasic ATP, since other purine/pyrimidine nucleotides were ineffective, and it was mimicked by the P2Z receptor agonist, benzoylbenzoyl ATP, and blocked by magnesium and the irreversible antagonist, oxidized ATP. RT-PCR analysis confirmed the mRNA expression of the P2Z/P2X7 receptor and the absence of P2X1. Caspase inhibitors and cycloheximide had only a partial effect on the apoptosis, suggesting that a caspase-independent mechanism may also be operative. Brief treatment with ATP led to an increase in the intracellular calcium concentration and permeabilization of the plasma membrane to Lucifer yellow, which diffused throughout the dendritic cell cytosol. Other small extracellular molecules may thus attain a similar intracellular distribution, perhaps activating endogenous proteases that contribute to initiation of apoptosis.

Enhancement of endocytosis due to aminophospholipid transport across the plasma membrane of living cells

Formation of intracellular vesicles is initiated by membrane budding. Here we test the hypothesis that the plasma membrane surface area asymmetry could be a driving force for vesicle formation during endocytosis. The inner layer phospholipid number was therefore increased by adding exogenous aminophospholipids to living cells, which were then translocated from the outer to the inner layer of the membrane by the ubiquitous flippase. Addition of either phosphatidylserine or phosphatidylethanolamine led to an enhancement of endocytosis, showing that the observed acceleration does not depend on the lipid polar head group. Conversely, a closely related aminophospholipid that is not recognized by the flippase, lyso-alpha-phosphatidylserine, inhibited endocytosis, and similar results were obtained with a cholesterol derivative that also remains in the plasma membrane outer layer. Thus an increase of lipid concentration in the inner layer enhanced internalization, whereas an increase of the lipid concentration in the outer layer inhibited internalization. These experiments suggest that transient asymmetries in lipid concentration might contribute to the formation of endocytic vesicles.

Apoptosis of epithelial cells and macrophages due to infection with the obligate intracellular pathogen Chlamydia psittaci

We have characterized the cytotoxic activity of the obligate intracellular bacterium Chlamydia psittaci, which resides within a membrane-bound vacuole during the 2-day infection cycle. We have established that infected epithelial cells and macrophages die through apoptosis, which is measurable within 1 day of infection and requires productive infection by the bacteria. Inhibition of host cell protein synthesis has no effect on cell death, but blocking bacterial entry or bacterial protein synthesis prevents apoptosis, implying that bacterial growth is required for death of the host cell. Apoptosis was confirmed through the use of electron microscopy, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, gel agarose electrophoresis of fragmented DNA, and propidium-iodide labeling of host cell nuclei. Although infected cells died preferentially, both infected and uninfected cells became apoptotic, suggesting that the infected cells may secrete proapoptotic factors. Inhibition of either of two proapoptotic enzymes, caspase-1 or caspase-3, did not significantly affect Chlamydia-induced apoptosis. These results suggest that, as in the case of apoptosis due to Bax expression or oncogene dysregulation, which initiate the apoptotic program within the cell interior, the Chlamydia infection may trigger an apoptotic pathway that is independent of known caspases. As apoptotic cells secrete proinflammatory cytokines, Chlamydia-induced apoptosis may contribute to the inflammatory response of the host.

Functional gene transfer from intracellular bacteria to mammalian cells

We provide evidence of direct transfer of functional DNA from bacteria to mammalian cells. An Escherichia coli K12 diaminopimelate auxotroph made invasive by cloning the invasin gene from Yersinia pseudotuberculosis transfers DNA after simple co-incubation, into a variety of mammalian cell lines. Transfer efficiency was enhanced in some cells by coexpression of the gene for listeriolysin from Listeria monocytogenes. Expression of the acquired genes occurs in both dividing and quiescent cells. The only requirement for bacteria to transfer genetic material into nonprofessional phagocytic cells and macrophages is the ability to invade the host cell.

Enhancement of ATP levels and glucose metabolism during an infection by Chlamydia. NMR studies of living cells

The Chlamydia species are obligate intracellular bacteria that proliferate only within the infected cell. Since the extracellular bacteria are metabolically inert and there are no cell-free systems for characterizing Chlamydia metabolism, we studied metabolic changes related to ATP synthesis and glycolysis in HeLa cells infected with Chlamydia psittaci during the course of the 2-day infection cycle using noninvasive 31P and 13C NMR methods. We find that the infection stimulates ATP synthesis in the infected cell, with a peak of ATP levels occurring midway through the infection cycle, when most of the metabolically active bacteria are proliferating. The infection also stimulates synthesis of glutamate with a similar time course as for ATP. The stimulation is apparently due to an enhancement in glucose consumption by the infected cell, which also results in an increased rate of lactate production and glutamate synthesis as well as higher glycogen accumulation during the infection. Concurrently, infection leads to an increase in the expression of the glucose transporter, GLUT-1, on HeLa cells, which may account for the enhanced glucose consumption. The chlamydiae are thus able to stimulate glucose transport in the host cell sufficiently to compensate for the extra energy load on the cell represented by the infection.

Internalization of Chlamydia by dendritic cells and stimulation of Chlamydia-specific T cells

Chlamydia species are the causative agents of trachoma, various forms of pneumonia, and the most common sexually transmitted diseases. Although the infection cycle has been extensively characterized in epithelial cells, where the Chlamydia entry-vacuoles avoid fusion with host-cell lysosomes, the cellular immune response has received less attention. Moreover, despite the abundant presence of dendritic cells (DC) in the sites of infection, the interaction between Chlamydia and DC has never been studied. We observe that DC kill Chlamydia trachomatis and Chlamydia psittaci. The chlamydiae are internalized by the DC in a nonspecific manner through macropinocytosis, and the macropinosomes fuse subsequently with DC lysosomes expressing MHC class II molecules. The interaction induces maturation of the DC, since presentation of an exogenous Ag is severely inhibited after a 1-day incubation, although chlamydial Ags are still presented and recognized by Chlamydia-specific CD4+ T cells. Thus, DC most likely play a role in initiating the T cell response in vivo and could potentially be used in adoptive transfer therapies to vaccinate against Chlamydia.

Distribution of endosomal, lysosomal, and major histocompatability complex markers in a monocytic cell line infected with Chlamydia psittaci

The intracellular fate of Chlamydia psittaci during infection of a monocytic cell line, THP1, was characterized. Cytochalasin D inhibited phagocytosis of latex beads but had no effect on infection by C. psittaci, and vacuoles expressed the transferrin receptor, suggesting accessibility to the endocytic pathway. Early Chlamydia-containing vacuoles expressed major histocompatibility complex (MHC) class I molecules, and most vacuoles fused with host cell lysosomes, since they expressed LAMP-1 and had acidic pHs. In cells prestimulated with gamma interferon, vacuoles also expressed MHC class II molecules, suggesting that the monocytes might effectively process Chlamydia-derived antigens for presentation by MHC class I and class II molecules.

Determinant selection for T-cell tolerance in HEL-transgenic mice: dissociation between immunogenicity and tolerogenicity

The induction of T-cell tolerance to self-antigens has been extensively characterized for immunodominant (ID) regions. However, tolerance toward other minor self-determinants has received less attention. In the H-2(d) haplotype, HEL contains a single ID determinant (region 102-120) presented by I-E(d) MHC class II molecules. The present study evaluates the role of subdominant and cryptic HEL regions in maintaining tolerance. We have generated a mutated HEL antigen, HEL mu, whose ID region does not bind to I-E(d). Lymph node cells from HEL-immunized mice proliferated strongly to HEL mu in vitro. Two new stimulatory regions common to HEL and HEL mu were uncovered. They are produced during antigen processing and prime specific T lymphocytes. HEL-Tg mice were tolerant to these determinants, thus confirming their in vivo presentation. These HEL regions were as tolerogenic as the HEL ID determinant, despite their poor immunogenicity. These results demonstrate that there is not always a correlation between tolerogenicity and immunogenicity, a finding that may be critical for understanding T-cell tolerance.

Heparin-mediated inhibition of Chlamydia psittaci adherence to HeLa cells

The adherence of human strains of Chlamydia trachomatis has been recently shown to be inhibitable by heparin and heparitinase, leading to the proposal that Chlamydia binding to host cells may be mediated by a glycosaminoglycan (GAG)-dependent mechanism. We here describe the adherence of the guinea-pig pathogen, Chlamydia psittaci GPIC, to HeLa cells, which was measured by cytofluorometry with chlamydiae whose DNA was fluorescently labelled. Adherence could be inhibited by heat or trypsin pretreatment of the bacteria, and binding was much faster at 37 degrees C (reaching a plateau within 1 h) than 4 degrees C. Little binding remained when host cells were pre-fixed with paraformaldehyde, suggesting that host cell receptor mobility may be required for effective adherence. Visualization by confocal microscopy confirmed that the bacteria were at or near the host cell surface during the entire time-course of these experiments. Adherence increased as a function of pH between pH 6 and pH 8.0-8.5. Both adherence and infection of HeLa cells could be inhibited with heparin when the adherence step was performed at 4 degrees C, but only infection was inhibited when the adherence step was performed at 37 degrees C, even though heparitinase could block adherence at either 4 degrees C or 37 degrees C. Even at 4 degrees C, heparin-mediated inhibition was significantly lower at pH 8 than pH 7.4, suggesting that GAG-independent mechanisms may play a role in the higher adherence observed at basic pH. These results therefore demonstrate that a GAG-dependent adherence step may be operative in C. psittaci, and raise the possibility that other adherence mechanisms may also contribute to binding by this chlamydial strain. Furthermore, they suggest that there may not be a strict correlation between C. psittaci adherence and the ability to cause productive infections.

High incidence of Coxiella burnetii markers in a rural population in France

Since Coxiella burnetii, the causative agent of Q fever, is often transmitted from goats and sheep to humans through aerosols, we examined the sera from 168 persons involved in goat breeding in the Centre region of France and 40 members of veterinary and medical staff from the same region for the presence of antibodies against C. burnetii. An immunofluorescence assay was used to detect the presence of antibodies of the IgG isotope against epitopes from phase II of C. burnetii, which are the first antibodies to appear in infected people, and from phase I, which reflect more chronic stages of the infection. Our serological survey showed that most of the tested sera were positive for C. burnetii markers, indicating at least an encounter with the bacterium. In the overall population of 208 subjects, 71% of the sera had antibodies against phase II epitopes (titres > or = 1:40). Among the goat farmers and their immediate families, 78% had antibodies against phase II and 33% against phase I (titres > or = 1:40). Considering only high titres (> or = 1:320), though, only 37% of the farmers had antibodies against phase II and 15% against phase I. Only 3 out of 12 veterinarians working in the field had high titres of antibodies against phase II and phase I, while none of 28 members of veterinary and medical laboratories had significant levels of antibodies. These results emphasize the need for closer surveillance of populations at risk for Q fever, to prevent the infection by C. burnetii from reaching chronic stages of the disease.

Interleukin-10 inhibits tumor metastasis through an NK cell-dependent mechanism

Interleukin-10 (IL-10) is a recently described pleiotropic cytokine secreted mainly by type 2 helper T cells. Previous studies have shown that IL-10 suppresses cytokine expression by natural killer (NK) and type 1 T cells, thus down-regulating cell-mediated immunity and stimulating humoral responses. We here report that injected IL-10 protein is an efficient inhibitor of tumor metastasis in experimental (B16-F10) and spontaneous (M27 and Lox human melanoma) metastasis models in vivo at doses that do not have toxic effects on normal or cancer cells. Histological characterization after IL-10 treatment confirmed the absence of CD8+ and CD4+ T cells and macrophages at the sites of tumor growth, but abundant NK cells were localized at these sites. This unexpected finding was confirmed by showing that IL-10 inhibits most B16-F10 and Lox metastases in mice deficient in T or B cells (SCID and nu/nu mice), but not in those deficient in NK cells (beige mice or NK cell-depleted mice). However, IL-10 downregulation of pro-inflammatory cytokine production and/or recruitment of additional effector cells may also be involved in the anti-tumor effect at higher local concentrations of IL-10, since transfected B16 tumor cells expressing high amounts of IL-10 were rejected by normal, nu/nu, or SCID mice at the primary tumor stage, and there was still a 33% inhibition of tumor metastasis in beige mice.

Immunology and the confocal microscope

The techniques of classical epifluorescence microscopy are already widely used by the immunological community to detect antigens at the cellular level. Coupled with the use of specific inhibitors that affect diverse intracellular events, these techniques have provided valuable information on the mechanisms involved in antigen presentation. The same biological samples can now be examined by confocal microscopy, which has a higher resolution than conventional microscopy and allows one to analyse quantitatively single cross-sections of the sample. The confocal microscope is therefore especially well-suited for studies on intracellular membrane traffic, cell-to-cell interactions, and the distribution of particular antigens and their co-localization with other intracellular markers. This review describes the technique of confocal microscopy and the goals of sample preparation, along with several detailed protocols for fixing and permeabilizing cells and mounting them on microscope slides. Representative examples are cited from studies on the endocytosis of surface receptors, the distribution of adhesion and major histocompatibility complex (MHC) molecules, and the interaction of an intracellular parasite with MHC molecules of the host cell.

Presentation of antigens derived from microorganisms residing in host-cell vacuoles

Antigens presented by major histocompatibility complex molecules have been classified into those presented by 'endogenous' and 'exogenous' pathways. Some microorganisms reside within host-cell vacuoles that appear to avoid both pathways. Novel presentation mechanisms are being unraveled for these microorganisms, and their antigens, rather than being just peptides, can also consist of lipids or DNA fragments.

pH and calcium dependence of hemolysis due to Rickettsia prowazekii: comparison with phospholipase activity

Rickettsia prowazekii invades nucleated cells through phagocytosis and subsequently proliferates in the cytoplasm of the host cell. Hemolysis and a phospholipase A2 (PLA2) activity at neutral pHs have previously been reported; even though the phagosomal environment is most likely acidic. We here show that R. prowazekii and R. typhi also lyse erythrocytes at mildly acidic pHs, compatible with an early phagosomal compartment. For R. prowazekii, hemolysis at an acidic pH but not a neutral pH is enhanced by Ca2+, raising the possibility that more than one membranolytic factor may be produced by the rickettsiae. The rickettsiae alone display PLA2 activity, implying that the enzyme is of bacterial rather than erythrocyte or host cell origin. Moreover, the PLA2 activity requires divalent cations (Ca2+ or Mg2+), and, as with many extracellular PLA2s from other species, it has a preference for acidic over neutral phospholipids. The pH dependence of PLA2 is similar to that of the hemolysis without Ca2+, but in the presence of the hemolysis buffers (which contain Mg2+), there is no calcium-induced enhancement at acidic pHs. Thus, these rickettsiae are endowed with a membranolytic activity that could contribute to the escape of the bacteria from early phagosomal compartments, and it is likely that multiple toxins may be used for membrane lysis.

Role of the CDR1 region of the TCR beta chain in the binding to purified MHC-peptide complex

Single alanine substitutions were introduced into the CDR1 region of the beta chain of a Kd-restricted TCR. Mutants and wild-type TCR were attached to the zeta chain of the CD3 complex and expressed at the surface of a rat basophil cell line. Transfectants were tested for the binding of purified soluble Kd-peptide complexes. With this experimental system, accessory molecules are unlikely to play a major role and the contribution of each residue to the interaction can be addressed. Results show that all positions in the CDR1 region are involved in the binding to the Kd-peptide complex but at varying degrees. These effects are discussed in relation to a molecular model of the TCR. Comparison of these results with previous data obtained in a T cell hybridoma system suggests the existence of a threshold in the TCR affinity necessary for mature T cell activation.

Ly-49-independent inhibition of natural killer cell-mediated cytotoxicity by a soluble major histocompatibility complex class I molecule

Natural killer (NK) cells lyse their targets in a non-major histocompatibility complex (MHC)-restricted manner, and the cytotoxicity can be inhibited by a number of MHC class I allele products, suggesting that NK cells may have a "positive receptor" that recognizes the target and a "negative receptor" that receives an inhibitory signal from class I. Since negative receptors could also exert their effect by masking a positive ligand, we have determined whether there may be a direct interaction between class I and an NK surface receptor by measuring cytotoxicity in the presence of a soluble class I molecule, Kd. Soluble Kd at micromolar concentrations could efficiently block NK cell cytotoxicity, suggesting that class I has a direct effect on cytotoxicity, rather than masking another target cell ligand. Inhibition required that Kd be at least divalent, probably because of its higher affinity or its ability to cross-link the NK surface receptor. In addition, the effect was independent of the peptide used to load Kd, and there was inhibition of cytotoxicity of NK cells derived from either H-2d or H-2b mice. Finally, depletion of NK cells expressing Ly-49 had no effect on the specific inhibition by Kd, raising the possibility that NK cells are endowed with additional negative receptors besides Ly-49. Taken together, these results suggest that there may be a family of NK receptors recognizing different class I alleles, which can receive negative signals by directly binding to class I on the target cell surface.

Is antigen processing guided by major histocompatibility complex molecules?

Major histocompatibility complex class I (MHC-1) molecules bind peptide fragments derived from cytosolic antigens, and class II (MHC-2) molecules bind fragments of proteins that enter the endocytic pathway. How peptides of the right affinity and size are generated in vivo is still the focus of intense research. Current data are consistent with the view that precursor peptides of varying length are produced in the cytosol and transported into the endoplasmic reticulum lumen where nascent MHC-1 could sample the peptides for their affinity. High-affinity peptides would form stable complexes with MHC-1, which are resistant to proteolysis by luminal enzymes; peptides unable to bind to MHC-1 presumably undergo proteolysis in the lumen. On the other hand, multiple mechanisms are probably used to load MHC-2. Some proteins denatured in the acidic and reducing environment of the endosomes most likely bind to MHC-2 through the antigen's immunodominant region, and the exposed portions of the antigen are degraded by endosomal proteases. Other antigens must first be proteolysed into peptide fragments, which compete among themselves for binding to MHC-2, whereas heat shock proteins could also contribute peptides for MHC-2 loading. Because of their respective loading modes, there is a partial correlation between the MHC-2 binding affinity of the protein fragments and their in vivo immunodominance, which may not necessarily be the case for MHC-1.

Major contribution of the beta chain to the antigen specificity of a T cell receptor

T cell receptors (TCR) recognize peptides presented by major histocompatibility complex (MHC) molecules on the surface of cells. Sequence homology between the variable regions of the T cell receptor and of antibodies suggests that similarly-folded domains participate in ligand binding in both cases. However, most current models assume that both TCR chains (alpha and beta) are required for specific binding, whereas the heavy chain alone can confer specificity on many antibodies. We have therefore constructed chimeric molecules with alpha and beta from two different TCR, one restricted by the class II MHC protein, Ek, and the other by the class I MHC, Kd. The beta chain alone was sufficient for specific recognition of a peptide, Cw3, bound to Kd, but the alpha chain contributed to the overall avidity. These results suggest that other TCR may recognize their ligands primarily through the beta chain.

Picosecond fluorescence spectroscopy of a single-chain class I major histocompatibility complex encoded protein in its peptide loaded and unloaded states

The tryptophan fluorescence properties of two different peptide complexes of the single-chain H-2Kd (SC-Kd) were studied by means of the single-photon counting technique. The latter enables time-resolved measurements of fluorescence intensity and anisotropy decay parameters relevant to structural and dynamic properties of proteins. While the isolated SC-Kd molecules in their 'original' purified form represent the unloaded state, i.e., containing endogenous low-affinity peptides, the loaded SC-Kd protein is obtained by introducing well-defined high-affinity peptides that replace the low-affinity ones. These two SC-Kd forms were found to exhibit different time-resolved tryptophan emission patterns; the unloaded complexes show a slightly faster fluorescence intensity decay rate than the loaded one. Three well-resolved time domains were distinguished in the anisotropy decay course of both forms: a short one in the picosecond range, an intermediate one of several nanoseconds, and a long one spanning several dozens to hundreds of nanoseconds. They are assigned to superposition contributions of (short- and long-distance) non-radiative energy transfer processes, to motions of the tryptophans, and to rotation of the whole protein globule. In the loaded SC-Kds, the first two processes were found to be attenuated. It is therefore suggested that upon binding of high-affinity peptides, the SC-Kd structure becomes more compact and certain tryptophans become less accessible to quenchers. The faster anisotropy decay observed in the unloaded form reflects both an enhancement in the energy-transfer between the tryptophans and an acceleration of their motions.(ABSTRACT TRUNCATED AT 250 WORDS)

Role for MHC class I molecules in selecting and protecting high affinity peptides in the presence of proteases

Ag fragments derived from the cytosol are transported into the endoplasmic reticulum (ER) lumen, where they bind to nascent MHC class I molecules. However, it is not known whether only high affinity peptides enter the ER, or whether ER proteases must trim longer precursor peptides down to optimal size. To evaluate the feasibility of proteolytic fine trimming in vitro, soluble Kd and Kb were preincubated with peptides that bind to Kd or Kb and the mixture was exposed to three different proteases. Class I protected allele-specific peptides against proteolysis, whereas the other peptides were degraded to the amino acid level. When a Kd/S11E (SYIPSAEYILE) complex was immunoprecipitated after incubation with carboxypeptidase, both S11E and the optimal sized S9I (SYIPSAEYI) were found to be specifically bound to Kd. However, only S91 was recovered if S11E, Kd and carboxypeptidase were mixed at the same time, and there was no fine-trimming of bound S11E if high protease concentrations and short proteolysis times were used, which suggests that trimming occurs only in the unbound state. The amount of peptide that immunoprecipitated with Kd increased after carboxypeptidase treatment of Kd/S11E, implying that the peptide affinity had increased. Kd also protected S9I against proteolysis by a lysed microsome preparation, demonstrating that class I could also protect high affinity peptides in vivo. These results suggest that class I participates in the selection of high affinity peptides in the ER, by sampling transported unbound peptides are degraded by ER proteases or efflux back to the cytosol.

Role for MHC class I molecules in selecting and protecting high affinity peptides in the presence of proteases

Ag fragments derived from the cytosol are transported into the endoplasmic reticulum (ER) lumen, where they bind to nascent MHC class I molecules. However, it is not known whether only high affinity peptides enter the ER, or whether ER proteases must trim longer precursor peptides down to optimal size. To evaluate the feasibility of proteolytic fine trimming in vitro, soluble Kd and Kb were preincubated with peptides that bind to Kd or Kb and the mixture was exposed to three different proteases. Class I protected allele-specific peptides against proteolysis, whereas the other peptides were degraded to the amino acid level. When a Kd/S11E (SYIPSAEYILE) complex was immunoprecipitated after incubation with carboxypeptidase, both S11E and the optimal sized S9I (SYIPSAEYI) were found to be specifically bound to Kd. However, only S91 was recovered if S11E, Kd and carboxypeptidase were mixed at the same time, and there was no fine-trimming of bound S11E if high protease concentrations and short proteolysis times were used, which suggests that trimming occurs only in the unbound state. The amount of peptide that immunoprecipitated with Kd increased after carboxypeptidase treatment of Kd/S11E, implying that the peptide affinity had increased. Kd also protected S9I against proteolysis by a lysed microsome preparation, demonstrating that class I could also protect high affinity peptides in vivo. These results suggest that class I participates in the selection of high affinity peptides in the ER, by sampling transported unbound peptides are degraded by ER proteases or efflux back to the cytosol.

Separate metabolic pathways leading to DNA fragmentation and apoptotic chromatin condensation

Apoptosis is the predominant form of cell death observed in a variety of physiological and pathological conditions such as cancer involution, insect metamorphosis, the development of the immune and nervous systems, and embryogenesis. The typical nuclear changes taking place in apoptotic cells include extensive condensation of chromatin and internucleosomal DNA fragmentation into units of 200 base pairs. However, the mechanisms responsible for both chromatin condensation and DNA fragmentation have yet to be elucidated. In this study, micrococcal nuclease and the divalent cations, Ca2+ and Mg2+, were applied to isolated nuclei in an attempt to reconstitute in vitro the digestion of genomic DNA associated with apoptosis. Micrococcal nuclease was found to induce a typical pattern of DNA fragmentation, but did not give rise to chromatin condensation, whereas Ca2+/Mg2+ induced both chromatin condensation and DNA fragmentation in isolated mouse liver nuclei. When the endonuclease inhibitor ZnCl2 was used, the DNA fragmentation induced by Ca2+/Mg2+ in nuclei could be completely inhibited, but chromatin condensation still occurred. For comparison, intact liver cells were treated with valinomycin, a potassium ionophore, which gave rise to an atypical cell death, with chromatin condensation appearing without DNA fragmentation. Our results suggest that endonuclease activation in apoptosis is neither necessary nor sufficient to induce chromatin condensation, and that DNA fragmentation and chromatin condensation may be triggered through separate pathways during apoptosis.

Cytolysis mediated by ionophores and pore-forming agents: role of intracellular calcium in apoptosis

Apoptosis is a term used to describe certain forms of physiological cell death that occur during embryogenesis, differentiation, and normal cell turnover. Previous reports concerning the effects of calcium ionophores on rodent thymocytes and the pore-forming proteins perforin and staphylococcal alpha-toxin on murine tumor cells led to the suggestion that simply raising intracellular calcium causes apoptotic cell death. This hypothesis was tested using two ionophores, A23187 and valinomycin, and two pore-forming agents, melittin and staphylococcal alpha-toxin, on four murine tumor cell lines. Although treatment with these agents could raise intracellular calcium, and in some instances cause DNA fragmentation, only valinomycin caused apoptosis. In contrast to previous reports, our results suggest that raising intracellular calcium and inducing internucleosomal DNA fragmentation is not sufficient to elicit apoptotic cell death in all cell types.

Major histocompatibility complex class I molecules and resistance against intracellular pathogens

The immune system employs a temporal hierarchy of effector mechanisms to combat infections by intracellular pathogens. The nonspecific response is independent of MHC and can be activated rapidly, while the specific response is slower, more specific, and requires major histocompatibility complex (MHC) molecules. MHC-dependent responses have been characterized extensively in vitro for antigens presented by polymorphic MHC class Ia and class II proteins and recognized by T lymphocytes carrying alpha/beta T-cell receptors (TcR). Growing indirect evidence has implicated monomorphic MHC class Ib proteins and gamma/delta T lymphocytes in defense against bacterial infections, but the biochemical and immunological behavior of class Ib proteins and gamma/delta TcR has not been well characterized, and most hypotheses involving these proteins have relied on data obtained with polymorphic MHC proteins and alpha/beta TcR. An overview of studies describing bacterial infections in vivo suggests that, in many cases, MHC class I-dependent effector cells may not be indispensable for effective immune responses, exerting instead a modulatory effect during the course of infection. Furthermore, many class Ib proteins have probably specialized to present stress antigens and conserved microbial antigens, which may be recognized by gamma/delta T cells through an interaction that is qualitatively very different from alpha/beta TcR binding to class I and class II proteins.

Dissociation of the peptide/MHC class I complex: pH dependence and effect of endogenous peptides on the activation energy

Dansylated peptides were used to characterize the dissociation of peptides from a recombinant class I major histocompatibility complex protein. Dissociation of endogenous, low-affinity peptides from the class I molecule Kd had an activation energy of 6.78 +/- 0.64 kcal/mol in the 14 to 26 degrees C temperature range, but there was a break in the Arrhenius plot between 12 and 14 degrees C. Dissociation of a dansylated, high-affinity peptide had an activation energy of 20.24 +/- 1.69 kcal/mol, and there was similarly a break in the plot. Both direct interactions between peptide and the class I heavy chain and indirect effects of the peptide affinity on the extent of light chain association with heavy chain may contribute to the difference in activation energies, while the break in the Arrhenius plots implies a temperature-dependent conformational change. Dissociation was also slowest at neutral pH, but the peptide/class I complex dissociated rapidly at pH greater than 9 and less than 5, suggesting that endocytosed class I proteins would most likely lose their bound peptides at the acidic pH of endosomes.

Dissociation of the peptide-MHC class I complex limits the binding rate of exogenous peptide

Soluble, single-chain molecules for two MHC class I alleles, H-2Kd and H-2Kb, were used to analyze the kinetics of antigenic peptide binding to MHC. After MHC preloading with radiolabeled or fluorescent peptides, the observed rate of MHC-peptide complex dissociation increased after addition of an excess of unlabeled competitor peptide. Although exogenous peptides conforming to the allele-specific motif were required for the enhanced complex dissociation to occur, the dissociation rate of the complex was independent of exogenous peptide concentration. Similarly, the association rate of exogenous peptides was independent of concentration, reflecting the presence of low affinity peptides in the binding sites of the recombinant MHC proteins; the sequences of these endogenous peptides conform to the consensus motif for the MHC allele studied. Finally, the association rate of exogenous peptide decreased when MHC molecules were preloaded with high affinity peptides, and the binding of labeled high affinity peptide to isolated recombinant MHC was faster than the subsequent dissociation observed in the presence of competitor peptide. Taken together, these results imply that the rate of exogenous peptide binding is limited by the dissociation rate of the previously bound peptides.

Dissociation of the peptide-MHC class I complex limits the binding rate of exogenous peptide

Soluble, single-chain molecules for two MHC class I alleles, H-2Kd and H-2Kb, were used to analyze the kinetics of antigenic peptide binding to MHC. After MHC preloading with radiolabeled or fluorescent peptides, the observed rate of MHC-peptide complex dissociation increased after addition of an excess of unlabeled competitor peptide. Although exogenous peptides conforming to the allele-specific motif were required for the enhanced complex dissociation to occur, the dissociation rate of the complex was independent of exogenous peptide concentration. Similarly, the association rate of exogenous peptides was independent of concentration, reflecting the presence of low affinity peptides in the binding sites of the recombinant MHC proteins; the sequences of these endogenous peptides conform to the consensus motif for the MHC allele studied. Finally, the association rate of exogenous peptide decreased when MHC molecules were preloaded with high affinity peptides, and the binding of labeled high affinity peptide to isolated recombinant MHC was faster than the subsequent dissociation observed in the presence of competitor peptide. Taken together, these results imply that the rate of exogenous peptide binding is limited by the dissociation rate of the previously bound peptides.

Distribution of perforin-containing cells in normal and pregnant mice

Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells elaborate a cytolytic protein named perforin or cytolysin. It was widely held that, in vivo, high quantities of perforin are not present in resting lymphocytes and are usually produced only by activated lymphocytes found under pathological conditions. Until now, only one tissue was known to synthesize abundant quantities of perforin under nonpathological conditions, the uterus during pregnancy. To investigate the possibility that perforin might also be synthesized by other tissues, several tissues besides the uterus from pregnant and normal mice were tested by immunofluorescence and immunoperoxidase for the presence of perforin. The tissues studied were the ears, brain, nasal epithelium, tongue, salivary gland, larynx, thymus, stomach, liver, spleen, small intestine, and lymph nodes; two cell populations with different sizes and levels of perforin expression were found. Large cells, displaying the NK cell phenotype and expressing high levels of perforin, were detected not only in the uterus but also in the salivary gland and lungs of pregnant mice. Small cells, expressing low levels of perforin, were detected mainly in the stomach and small intestine, and they were expressed in both pregnant and normal mice. Taken together, these results imply that perforin-containing cells exist in vivo under nonpathological conditions, and that the immune system is endowed with heretofore unknown mechanisms for stimulating the activation of NK cells in a limited number of tissues during pregnancy.

A soluble, single-chain Kd molecule produced by yeast selects a peptide repertoire indistinguishable from that of cell-surface-associated Kd

Peptide binding to a soluble, single-chain Kd protein produced by the yeast strain Kluyveromyces lactis, and to Kd molecules on Kd-expressing cells (P815) was studied using radiolabeled Kd-restricted peptides. The stability of the peptide-Kd complexes formed was monitored in the absence and presence of unlabeled competitor peptides. Radioiodination of the Tyr anchor residue in position 2 of the peptide interferes with binding. A Kd-biased peptide library and a modified antigenic peptide in which a second Tyr was added in positions 6 and 8, respectively, were therefore used to assay binding. Recombinant and cell-associated Kd molecules are very similar in the following respects: the ease with which the proteins can be loaded with labeled peptide; the spectrum of peptides selected from a peptide library; the stability of the labeled peptide-Kd complex formed; and the ability to partially dissociate the class I-peptide complex with exogenous, unlabeled peptides. These results imply that measurements of peptide binding to soluble Kd molecules are a reliable indicator of the peptide-binding properties of Kd proteins on living cells. The large quantities of soluble recombinant Kd protein currently available represent an invaluable tool not only for dissecting the molecular mechanisms of antigen presentation but also for vaccinations and the design of T cell-specific toxins.

Real-time measurement of antigenic peptide binding to empty and preloaded single-chain major histocompatibility complex class I molecules

Cytotoxic T lymphocytes (CTL) recognize peptides in association with major histocompatibility complex (MHC) class I proteins, but how peptides bind to class I is not well understood. We used a fluorescence technique to measure antigenic peptide binding to a soluble, single-chain Kd (SC-Kd) molecule in which the Kd heavy chain was connected by a 15-residue link to beta 2-microglobulin. Peptides were covalently labeled at their N terminus with dansyl, and binding of dansylated Kd-restricted peptides to SC-Kd resulted in significant fluorescence enhancement, which could be inhibited by unmodified Kd-restricted peptides. Real-time binding of a dansylated peptide could be followed by monitoring the fluorescence at 530 nm. The dansylated Plasmodium berghei circumsporozoite (PbCS) 263-260 peptide bound to "empty" SC-Kd with an association rate constant of 1140 M-1s-1, and the subsequent spontaneous dissociation of the SC-Kd-peptide complex was slow. The dissociation increased dramatically after addition of excess unlabeled PbCS 253-260 peptide, but with a slower association constant for unlabeled peptide, 77 M-1s-1. Thus, the Kd-peptide complex on the surface of antigen-presenting cells should be stable, but high concentrations of peptides in the endoplasmic reticulum (ER) lumen would allow for peptide exchange on Kd before export to the surface. The apparent activation energy for PbCS 253-260 peptide binding to SC-Kd was 6.78 +/- 0.64 kcal/mole, similar to values previously reported for antigen-antibody interactions.