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Biomedical subjects

J Pieters

Publications and source records attributed to J Pieters.

At least 19 recordsLinked to original sources
PubMed2001

Evasion of host cell defense mechanisms by pathogenic bacteria.

From an immunological viewpoint, invasion of pathogenic bacteria into a susceptible host poses a potential life-threatening situation and thus has to be met with all weapons that are available. A crucial component of host defense mechanisms is the macrophage. The scavenger activity of this cell ensures the uptake and destruction of bacteria in phagolysosomes, on the one hand, and activation of the adaptive component of the immune system through presentation of bacterial antigens, on the other hand. From the bacterial perspective, entry into a phagolysosome is usually fatal and many pathogens have developed strategies that circumvent the destructive environment of this organelle. Such evasion strategies often exploit normal host cell function. Understanding these survival strategies will deepen our insight of the pathogenesis of infection as well as host cell biology.

Animals↗
PubMed2001

Entry and survival of pathogenic mycobacteria in macrophages.

Pathogenic mycobacteria, including Mycobacterium tuberculosis, are phagocytosed by macrophages but manage to survive within the mycobacterial phagosome. Recent work has shed some more light on the mechanisms of mycobacterial entry and survival inside macrophages. Two host cell components, the steroid cholesterol and a phagosomal coat protein termed TACO were found to play crucial roles in the establishment of an intracellular infection. This review describes how these findings may help to understand the circumvention of the normal trafficking routes inside host cells by mycobacteria.

Biological Transport↗
PubMed2001

Association of distinct tetraspanins with MHC class II molecules at different subcellular locations in human immature dendritic cells.

Dendritic cells have the capacity to trigger T cell responses in lymphoid organs against antigens captured in the periphery. T cell stimulation depends on the ability of MHC class II molecules to present peptides at the cell surface that are acquired in MHC class II compartments. The high capacity of dendritic cells to stimulate T lymphocytes is related to their ability to regulate the distribution of MHC class II molecules intracellularly. To analyze the molecular components involved in the generation of MHC class II-peptide complexes in human immature dendritic cells, mAb were raised against purified MHC class II compartments. One of the antigens turned out to be CD63, a member of the tetraspanin superfamily. CD63 localized exclusively intracellularly where it associated with peptide-loaded class II molecules. In contrast, the tetraspanins CD9, CD53 and CD81 associated with class II molecules at the plasma membrane. Selective association of distinct tetraspanins may be involved in the regulation of MHC class II distribution in human dendritic cells.

Animals↗
PubMed2000

Essential role for cholesterol in entry of mycobacteria into macrophages.

Mycobacteria are intracellular pathogens that can invade and survive within host macrophages, thereby creating a major health problem worldwide. The molecular mechanisms involved in mycobacterial entry are still poorly characterized. Here we report that cholesterol is essential for uptake of mycobacteria by macrophages. Cholesterol accumulated at the site of mycobacterial entry, and depleting plasma membrane cholesterol specifically inhibited mycobacterial uptake. Cholesterol also mediated the phagosomal association of TACO, a coat protein that prevents degradation of mycobacteria in lysosomes. Thus, by entering host cells at cholesterol-rich domains of the plasma membrane, mycobacteria may ensure their subsequent intracellular survival in TACO-coated phagosomes.

Animals↗
PubMed1999

A secreted form of the major histocompatibility complex class II-associated invariant chain inhibiting T cell activation.

Major histocompatibility complex (MHC) class II molecules function at the cell surface to present antigenic peptides to T helper cells. Intracellularly, MHC class II molecules are associated with the invariant chain (Ii). Ii can modulate MHC class II-dependent T cell activation through (i) assistance in the export of MHC class II molecules from the endoplasmic reticulum, (ii) providing a targeting signal for endosomal/lysosomal compartments, and (iii) preventing peptides from associating prematurely with MHC class II molecules. Here we describe the generation and subsequent secretion of a lumenal form of Ii, IiP25. IiP25 lacked the targeting sequences for transport to MHC class II compartments but contained part of the CLIP region that is known to compete with antigenic peptides for binding to MHC class II molecules. When added to an antigenic peptide presentation model system, IiP25 inhibited T cell activation by competing for the CLIP binding site at the plasma membrane. Secretion of a lumenal Ii fragment may represent an additional mechanism to modulate T cell activation by MHC class II molecules.

Binding Sites↗
PubMed1999

A coat protein on phagosomes involved in the intracellular survival of mycobacteria.

Mycobacteria are intracellular pathogens that can survive within macrophage phagosomes, thereby evading host defense strategies by largely unknown mechanisms. We have identified a WD repeat host protein that was recruited to and actively retained on phagosomes by living, but not dead, mycobacteria. This protein, termed TACO, represents a component of the phagosome coat that is normally released prior to phagosome fusion with or maturation into lysosomes. In macrophages lacking TACO, mycobacteria were readily transported to lysosomes followed by their degradation. Expression of TACO in nonmacrophages prevented lysosomal delivery of mycobacteria and prolonged their intracellular survival. Active retention of TACO on phagosomes by living mycobacteria thus represents a mechanism preventing cargo delivery to lysosomes, allowing mycobacteria to survive within macrophages.

Animals↗
PubMed1998

Subcellular fractionation by organelle electrophoresis: separation of phagosomes containing heat-killed yeast particles.

Uptake of foreign material and its subsequent lysosomal degradation is an important function of macrophages. The mechanisms involved in the binding, uptake and delivery of such material to lysosomal organelles, are, however, poorly understood. Here we describe a method using organelle electrophoresis to study the uptake and trafficking of heat-killed yeast particles within murine macrophages. Such yeast particles, which were fluorescently labeled, could be readily detected in intact cells as well as in subcellular fractions. Organelle electrophoresis of a homogenate from macrophages that had internalized yeast particles resulted in the separation of yeast-containing organelles from most other subcellular membranes. In addition, this method was used to follow the kinetics of yeast particle transport within macrophages could be followed readily. Organelle electrophoresis may be a valuable tool for the analysis of phagocytosis.

Animals↗
PubMed1998

MHC class II and invariant chain biosynthesis and transport during maturation of human precursor dendritic cells.

Dendritic cells (DC) are highly potent activators of the immune response. The precise mechanisms that give rise to the DC phenotype are not known. To investigate the mechanisms that contribute to the generation of the DC phenotype, precursor DC were freshly isolated from human blood and allowed to mature in vitro. These matured DC showed the phenotypical and functional characteristics of DC. Analysis of the MHC class II and invariant chain (li) biosynthesis revealed that upon maturation, class II synthesis was induced whereas li synthesis was significantly up-regulated. In mature DC, despite the presence of large amounts of li, export of MHC class II molecules from the endoplasmic reticulum was incomplete, up to 4 h after biosynthesis. Thus, MHC class II-li synthesis and transport in DC is highly regulated during maturation of DC. Analysis of the regulatory mechanisms may contribute to a better understanding of antigen-presenting capacities during the differentiation of DC.

Antigen Presentation↗
PubMed1997

Distinct intracellular compartments involved in invariant chain degradation and antigenic peptide loading of major histocompatibility complex (MHC) class II molecules.

Major histocompatibility complex (MHC) class II molecules are transported to intracellular MHC class II compartments via a transient association with the invariant chain (Ii). After removal of the invariant chain, peptides can be loaded onto class II molecules, a process catalyzed by human leukocyte antigen-DM (HLA-DM) molecules. Here we show that MHC class II compartments consist of two physically and functionally distinct organelles. Newly synthesized MHC class II/Ii complexes were targeted to endocytic organelles lacking HLA-DM molecules, where Ii degradation occurred. From these organelles, class II molecules were transported to a distinct organelle containing HLA-DM, in which peptides were loaded onto class II molecules. This latter organelle was not directly accessible via fluid phase endocytosis, suggesting that it is not part of the endosomal pathway. Uptake via antigen-specific membrane immunoglobulin resulted however in small amounts of antigen in the HLA-DM positive organelles. From this peptide-loading compartment, class II-peptide complexes were transported to the plasma membrane, in part after transit through endocytic organelles. The existence of two separate compartments, one involved in Ii removal and the other functioning in HLA-DM-dependent peptide loading of class II molecules, may contribute to the efficiency of antigen presentation by the selective recruitment of peptide-receptive MHC class II molecules and HLA-DM to the same subcellular location.

Antibodies, Bacterial↗
PubMed1997

Inflammatory stimuli induce accumulation of MHC class II complexes on dendritic cells.

Dendritic cells have the remarkable property of presenting any incoming antigen. To do so they must not only capture antigens with high efficiency and broad specificity, but must also maximize their capacity to load class II molecules of the major histocompatibility complex (MHC) with antigenic peptides in order to present a large array of epitopes from different proteins, each at a sufficient copy number. Here we show that formation of peptide-MHC class II complexes is boosted by inflammatory stimuli that induce maturation of dendritic cells. In immature dendritic cells, class II molecules are rapidly internalized and recycled, turning over with a half-life of about 10 hours. Inflammatory stimuli induce a rapid and transient boost of class II synthesis, while the half-life of class II molecules increases to over 100 hours. These coordinated changes result in the rapid accumulation of a large number of long-lived peptide-loaded MHC class II molecules capable of stimulating T cells even after several days. The capacity of dendritic cells to load many antigenic peptides over a short period of initial exposure to inflammatory stimuli could favour presentation of infectious antigens.

Antigen Presentation↗
PubMed1997

The mannose receptor functions as a high capacity and broad specificity antigen receptor in human dendritic cells.

Dendritic cells, in contrast to B lymphocytes, must be able to efficiently internalize a diverse array of antigens for processing and loading onto major histocompatibility complex (MHC) class II molecules. Here we characterize the mannose receptor pathway in dendritic cells and show that mannose receptor-mediated uptake of antigens results in a approximately 100-fold more efficient presentation to T cells, as compared to antigens internalized via fluid phase. Immunocytochemistry as well as subcellular fractionation revealed the localization of the mannose receptor and MHC class II molecules in distinct subcellular compartments. The mannose receptor thus functions in rapid internalization and concentration of a variety of glycosylated antigens that become available for processing and presentation. This may contribute to the unique capacity of dendritic cells to generate primary T cell responses against infectious agents.

Antigen-Presenting Cells↗
PubMed1997

Analysis of subcellular organelles involved in major histocompatibility complex (MHC) class II-restricted antigen presentation by electrophoresis.

Presentation of material derived from pathogenic organisms to the immune system requires uptake of antigens into antigen presenting cells, processing into peptide fragments and loading of the resulting fragments onto major histocompatibility complex (MHC) class II molecules. MHC class II-restricted antigen presentation involves both the biosynthetic as well as the endocytic pathway of antigen-presenting cells. In recent years, the general mechanisms that govern these processes have been delineated, and specialized organelles have been characterized in which processing and loading of antigens takes place. Here, we review the work that has led to the characterization of these MHC class II compartments, and describe the use of organelle electrophoresis and two-dimensional gel electrophoresis to analyze the molecular composition of the different subcellular organelles involved in MHC class II-restricted antigen presentation as well as in antigen uptake.

Antigen Presentation↗
PubMed1997

Mannose receptor mediated antigen uptake and presentation in human dendritic cells.

In an immature state, dendritic cells (DC) can capture antigen via at least two mechanisms. First, DC use macropinocytosis for continuous uptake of large amounts of soluble antigens. Second, they express high levels of mannose receptor that can mediate internalization of glycosylated ligands. We found that dendritic cells can present mannosylated antigen 100-1000 fold more efficiently than non-mannosylated antigen. Immunocytochemistry as well as subcellular fractionation demonstrated that the mannose receptor and MHC class II molecules were located in distinct subcellular compartments. These results demonstrate that the mannose receptor endows DC with a high capacity to present glycosylated antigens at very low concentrations.

Antigen Presentation↗
PubMed1997

MHC class II restricted antigen presentation.

Presentation of antigenic peptides by MHC class II molecules to CD4(+) T cells requires many events in both the biosynthetic and endocytic pathways that must all occur in a controlled and coordinated fashion. In recent years the roles of two important chaperones, the invariant chain and the HLA-DM dimer, in promoting the acquisition of peptides by MHC class II molecules have largely been elucidated. The different compartments within the endosomal/lysosomal pathway that are involved in peptide loading are now being characterized. In addition to the specialized MHC class II compartments that exist in antigen-presenting cells, other intracellular compartments may also be involved in peptide loading. The precise mechanisms and intracellular sites of MHC class II peptide loading appear to dictate the nature of the T-cell epitopes presented by the antigen-presenting cell.

Animals↗
PubMed1997

Isolation and characterization of the mycobacterial phagosome: segregation from the endosomal/lysosomal pathway.

Mycobacteria have the ability to persist within host phagocytes, and their success as intracellular pathogens is thought to be related to the ability to modify their intracellular environment. After entry into phagocytes, mycobacteria-containing phagosomes acquire markers for the endosomal pathway, but do not fuse with lysosomes. The molecular machinery that is involved in the entry and survival of mycobacteria in host cells is poorly characterized. Here we describe the use of organelle electrophoresis to study the uptake of Mycobacterium bovis bacille Calmette Guerin (BCG) into murine macrophages. We demonstrate that live, but not dead, mycobacteria occupy a phagosome that can be physically separated from endosomal/lysosomal compartments. Biochemical analysis of purified mycobacterial phagosomes revealed the absence of endosomal/lysosomal markers LAMP-1 and beta-hexosaminidase. Combining subcellular fractionation with two-dimensional gel electrophoresis, we found that a set of host proteins was present in phagosomes that were absent from endosomal/lysosomal compartments. The residence of mycobacteria in compartments outside the endosomal/lysosomal system may explain their persistence inside host cells and their sequestration from immune recognition. Furthermore, the approach described here may contribute to an improved understanding of the molecular mechanisms that determine the intracellular fate of mycobacteria during infection.

Animals↗
PubMed1997

MHC class II compartments: specialized organelles of the endocytic pathway in antigen presenting cells.

Mounting an immune response against foreign, extracellular material requires that this material be internalized by antigen presenting cells, processed to peptide fragments and then displayed on the cell surface for recognition by the T cell receptor on T helper cells. Such peptides, derived from internalized antigens are generally presented to T cells in association with Major Histocompatibility Complex (MHC) class II molecules. Recent work has identified subcompartments of the endosomal/lysosomal system that appear to be important sites for the generation of peptide-MHC class II complexes. These so-called MHC class II compartments receive antigenic fragments from endosomes and lysosomes and load them onto class II molecules. From these compartments, peptide-loaded class II molecules are transported to the plasma membrane, where they can trigger T cells bearing appropriate receptors. This review summarizes recent work characterizing MHC class II compartments as specialized organelles of the endosomal/lysosomal pathway. Interestingly, MHC class II compartments share many features common to organelles present in a wide variety of specialized cells with quite different functions. Modifying the endocytic pathway to serve the needs of specialized cells may be a common mechanism which allows very different cell types to carry out their disparate functions.

Animals↗
PubMed1994

Isolation and characterization of the intracellular MHC class II compartment.

An intracellular compartment has been isolated to which MHC class II molecules are transported on their way to the plasma membrane. They arrive with an associated invariant chain which is then proteolytically processed while MHC class II molecules acquire antigenic peptide. These loaded class II molecules then leave the compartment devoid of invariant chain and bound for the plasma membrane. This compartment represents a new stage in the endocytic/lysosomal pathway.

Antigens, Differentiation, B-Lymphocyte↗