Intracellular traffic to compartments for MHC class II peptide loading: signals for endosomal and polarized sorting

In this review we focus on the traffic of MHC class II and endocytosed antigens to intracellular compartments where antigenic peptides are loaded. We also discuss briefly the nature of the peptide loading compartment and the sorting signals known to direct antigen receptors and MHC class II and associated molecules to this location. MHC class II molecules are expressed on a variety of polarized epithelial and endothelial cells, and polarized cells are thus potentially important for antigen presentation. Here we review some cell biological aspects of polarized sorting of MHC class II and the associated invariant chain and the signals that are involved in the sorting process to the basolateral domain. The molecules involved in sorting and loading of peptide may modulate antigen presentation, and in particular we discuss how invariant chain may change the cellular phenotype and the kinetics of the endosomal pathway.

Exon 6 is essential for invariant chain trimerization and induction of large endosomal structures

Invariant chain (Ii) is a transmembrane type II protein that forms a complex with the major histocompatibility complex (MHC) class II molecules in the endoplasmic reticulum (ER). The membrane proximal luminal region of Ii is responsible for the non-covalent association with MHC class II molecules. Chemical cross-linking in COS cells was used to study the effect of luminal and cytoplasmic deletions on trimerization of Ii. We demonstrate that trimerization of Ii is independent of the cytosolic tail of Ii, whereas residues 162-191 (the sequence encoded by exon 6) in the luminal part of Ii are essential for trimer formation. Immunofluorescence studies of the transfected luminal deletion constructs show that the amino acids encoded by exon 6 of Ii are also essential for the induction of large endosomal vesicles. The data suggest that Ii must be in a trimeric form to modify the endosomal pathway.

A flow cytometric and immunofluorescence microscopic study of tumor necrosis factor production and localization in human monocytes

The production and localization of tumor necrosis factor (TNF) in human monocytes were investigated by using monoclonal and polyclonal antibodies against recombinant human TNF together with flow cytometry and immunofluorescence microscopy. Lipopolysaccharide (LPS) induced a rapid and transient accumulation of TNF in perinuclear vesicles which was detected 20 min after the addition of LPS. The fluorescence intensity of the vesicles peaked at 40 min of LPS exposure, concomitantly with the release of TNF into the medium. Thus, our results indicate that the secretion of TNF is typical for secretory proteins as it involves passage through the secretory apparatus. Additional studies demonstrated that plasma membrane-associated TNF could not be detected in live monocytes not exposed to LPS. However, after 90 min with LPS, a small population of monocytes expressed membrane-associated TNF, and by 24 hr approximately 50% of the monocytes displayed TNF on the plasma membrane. Furthermore, our results indicate that plasma membrane-associated TNF does not represent released TNF bound back to its own receptor. Thus, our findings support the view that TNF exists as a surface trans-membrane protein in LPS-stimulated monocytes.

Sorting of MHC class II molecules and the associated invariant chain (Ii) in polarized MDCK cells

Epithelial cells have been found to express MHC class II molecules in vivo and are able to perform class II-restricted antigen presentation. The precise intracellular localization of these molecules in epithelial cells has been a matter of debate. We have analyzed the polarized targeting of human MHC class II molecules and the associated invariant chain (Ii) in stably transfected MDCK cells. The class II molecules are located at the basolateral surface and in intracellular vesicles, both when expressed alone or together with Ii. Ii is located in basolateral endosomes and can internalize through the basolateral plasma membrane domain. We show that the cytoplasmic tail of Ii contains information for basolateral targeting as it is sufficient to redirect the apical protein neuraminidase (NA) to the basolateral surface. We find that the two leucine-based motifs (LI and ML) in the cytoplasmic tail of Ii are individually sufficient for endosomal sorting and basolateral targeting of Ii in MDCK cells. In addition, basolateral sorting information is located within the 10 membrane-proximal residues of the Ii cytoplasmic tail. As several different signals mediate basolateral sorting of the class II/Ii complex, a polarized distribution of these molecules may be an essential feature of antigen presentation in epithelial cells.

Cell cycle-specific glucocorticoid growth regulation of a human cell line (NHIK 3025)

It has been reported that the human cell line NHIK 3025 has a specific cytoplasmic glucocorticoid receptor. When these cells were exposed to glucocorticoids, the cell cycle time was prolonged. Cells, synchronized by mitotic selection, were subjected to the synthetic glucocorticoid dexamethasone throughout the cell cycle. Only cells exposed in the first half of G1 phase had a lengthened cell cycle time. Most of the prolongation was also located within the G1 phase. The dexamethasone growth inhibition was reversible and could be detected only in the cell cycle where the cells were exposed to the steroid. DNA-histograms of asynchronous cells were recorded by flow cytometry at various times after steroid exposure. These histograms also showed G1 phase sensitivity and G1 phase prolongation after exposure to dexamethasone. Our results thus indicate that these cells have a dexamethasone-sensitive restriction point in mid-G1 phase of the cell cycle.

The cytoplasmic tail of CD1d contains two overlapping basolateral sorting signals

CD1d is a member of the CD1 polypeptide family that represents a new arm of host defense against invading pathogens. In our previous work (Rodionov, D. G., Nordeng, T. W., Pedersen, K., Balk, S. P., and Bakke, O. (1999) J. Immunol. 162, 1488-1495) we have shown that CD1d contained a classic tyrosine-based internalization signal (YQGV) in its short cytoplasmic tail. CD1d is expressed in polarized epithelial cells, and we found that the cytoplasmic tail of CD1d also contained information for basolateral sorting. Interestingly, a mutation of the critical tyrosine residue of the endosomal sorting signal did not result in the loss of basolateral targeting of the mutant CD1d. To search for a basolateral sorting signal we have constructed a full set of alanine mutants, but no single alanine substitution inactivated the signal. However, deletions or mutations of either the C-terminal valine/leucine pair or the critical tyrosine residue from the internalization signal and either residue from the C-terminal valine/leucine pair inactivated basolateral sorting. Our data thus suggest that the cytoplasmic tail contains two overlapping basolateral signals, one tyrosine- and the other leucine-based, each being sufficient to direct CD1d to the basolateral membrane of polarized Madin-Darby canine kidney cells.

Major histocompatibility complex class II-dependent antigen presentation by human intestinal endothelial cells

BACKGROUND & AIMS

In the normal gut, human intestinal microvascular endothelial cells (HIMECs) express major histocompatibility complex (MHC) class II molecules. Enhanced expression is found in chronic inflammation. We examined the cytokine regulation of MHC class II molecules and the associated invariant chain (Ii) in HIMECs and investigated whether such cells can process and present a complex protein antigen to T cells.

METHODS

Enzyme-linked immunosorbent assay, flow cytometry, immunoelectron microscopy, as well as T-cell activation assay with HIMECs and HLA-DR-restricted T-cell clones were employed.

RESULTS

In unstimulated HIMEC monolayers, HLA-DR, -DP, and -DQ and Ii were undetectable at the protein level, but interferon gamma (IFN-gamma) (100 U/mL) induced expression that peaked for DR after 2-3 days, for DP after 4-6 days, for DQ after 10-12 days, and for Ii after 2-3 days. Tumor necrosis factor alpha had no effect alone but enhanced class II expression in combination with IFN-gamma, most notably for DQ and DP. HLA-DR3-restricted and Mycobacterium tuberculosis heat shock 65-kilodalton-specific T-cell clones were activated to produce IFN-gamma in response to relevant antigen presented by IFN-gamma-treated HIMECs. This response was inhibited by blocking monoclonal antibody to HLA-DR and by chloroquine when compared to professional antigen-presenting cells, HIMECs activated T-cell clones quite efficiently.

CONCLUSIONS

These data suggest that microvascular endothelial cells can present complex protein antigens in the human gut.

T-lymphocytes and the subpopulations of T-helper and T-suppressor cells measured by monoclonal antibodies (T11, T4, and T8) in relation to surgery under epidural and general anaesthesia

Monoclonal antibodies reacting with specific determinants on lymphocytes and their subpopulations were used to study the influence of surgery under general or epidural anaesthesia on the number of T-cell (T11), T-suppressor cells (T8), and T-helper cells (T4). The T-cell fraction decreased significantly during surgery, but was only moderately reduced on the first postoperative day. The change in the T-cell fraction was the same under both types of anaesthesia. The helper and suppressor cell fractions were not significantly altered at any time during the study, and again there was no difference between the two anaesthetic regimens. The ratio of helper cells to suppressor cells (the T4/T8 ratio), which is widely used as an indicator of immunosuppression, showed no significant variations during the study, even though there were individual variations. Changes in the T4/T8 ratio were not correlated to the amount of blood transfusions, nor was there any correlation between the T4/T8 ratio and the thymidine uptake in mitogen-(PHA)-stimulated lymphocytes. T-lymphocytes are depressed during and after surgery under general anaesthesia. From the present study, this does not seem to be due to changes in the balance between helper- and suppressor T-cells.

The leucine-based motif DDQxxLI is recognized both for internalization and basolateral sorting of invariant chain in MDCK cells

The major histocompatibility complex (MHC) class II-associated invariant chain (Ii) contains signals for transport to endocytic compartments where the class II molecules bind antigenic peptides for presentation to CD4+ T cells. Two leucine-based signals in the Ii cytoplasmic tail can be independently recognized for endosomal sorting of Ii, and we have recently shown that each signal is sufficient for basolateral sorting and internalization of Ii in polarized Madine Darby Canine Kidney (MDCK) II cells. The recognition motif for endosomal sorting is complex and consists of two critical leucine-like residues as well as surrounding amino acids. Here, we have analyzed the importance of residues surrounding the membrane-distal leucine-based signal in basolateral sorting and internalization of Ii in MDCK II cells. We find that the DDQxxLI motif is involved in both sorting events indicating the presence of similar signal recognition components both at the TGN and at the plasma membrane. The identical motif is required for endosomal localization and internalization of Ii also in simian COS cells and the human HeLa and M1 cells.

A new triple-stranded alpha-helical bundle in solution: the assembling of the cytosolic tail of MHC-associated invariant chain

BACKGROUND

The invariant chain (li) is a transmembrane protein that associates with the major histocompatibility complex class II (MHC II) molecules in the endoplasmic reticulum. The cytosolic tail of li contains two leucine-based sorting motifs and is involved in sorting the MHC II molecules to the endosomal pathway where the peptide antigen is bound. This region of li also contributes to phenotypical changes in cells, such as the formation of large endocytic structures.

RESULTS

We report here the three-dimensional structure of a 27 amino acid peptide corresponding to the cytosolic tail of li. The structure was determined by nuclear magnetic resonance (NMR) spectroscopy using a computational strategy. At high concentration, this structure reveals a new triple-stranded alpha-helical bundle in which the helices, two parallel and one antiparallel, are almost coplanar. Trimerization is mediated by electrostatic interactions intercalated by three hydrophobic layers.

CONCLUSIONS

The new trimer fold, the first to be identified by NMR data alone, can be used to improve understanding of protein-protein interactions and to model multiple-helical transmembrane proteins and receptors. We suggest that interactions of the li cytosolic tails may form part of a mechanism that could cause the endosomal retention and enlarged endosomes induced by li.

Targeting major histocompatibility complex class II molecules to the cell surface by invariant chain allows antigen presentation upon recycling

We studied the functional consequences of targeting class II molecules to either the cell surface or to endocytic structures by expressing HLA-DR1 in human kidney cells in the presence or absence of different forms of the invariant chain (Ii). Transfectants expressing class II molecules in the absence of Ii present influenza virus efficiently and co-expression of full length Ii does not further increase antigen presentation. Chimeric Ii containing the cytoplasmic domain of the transferrin receptor (Tfr-Ii) delivers class II molecules associated with Tfr-Ii to endosomal compartments, but this does not result in efficient antigen presentation. When class II molecules are targeted to the cell surface by Ii lacking either 15 (delta 15Ii) or 23 (delta 23Ii) amino acids from the cytoplasmic domain, a fraction of free class II molecules is also observed. Whereas delta 15Ii did not affect antigen presentation by class II molecules, delta 23Ii inhibited, but did not abrogate, the response. We show that class II molecules expressed in the presence of delta 23Ii can be internalized, followed by degradation of delta 23Ii and return of free class II alpha beta heterodimers to the cell surface. A fraction of the resulting free class II molecules is sodium dodecyl sulfate stable, indicating that internalization and reappearance of class II molecules at the cell surface can be an alternative route for antigen presentation. In all transfectants, class II molecules were found in endocytic compartments that labeled for CD63 and resembled the multilaminar MIIC compartments found in B cell lines. Ii is not required for endosomal targeting of class II molecules. The number of class II molecules observed in the multilaminar compartments correlates with the efficiency of antigen presentation.

Concentration-dependent effects of potassium dichromate on the cell cycle

Hexavalent chromium is found to be a strong mutagen, and it also is a potential carcinogen in man. DNA flow cytometry, growth measurements, and determinations of mitotic index show that 1-2 microM K2Cr2O7 produces a prolongation of the G2 phase of the cell cycle in NHIK 3025 cells. By increasing the chromate concentrations (greater than 2 microM K2Cr2O7) the cells are also arrested in G2 phase. We have found, using synchronized cells and measuring cell cycle time, that the most chromate-sensitive part of the cell cycle is S phase. This phase is also somewhat prolonged, and the cells became arrested in early S phase at high toxic K2Cr2O7 concentrations (8 microM). Our results thus indicate that K2Cr2O7 has an effect within S phase--maybe on DNA/RNA synthesis--and also interferes with processes necessary for progression through the G2 phase.

Estimation of offspring production from a limited number of stage-structured censuses

We propose a procedure for maximum likelihood estimation of the number of animals or offspring in a closed population where the individuals counted go through stages or age-groups. Application of the procedure requires knowledge of the distributions of the stage durations. A procedure for maximum likelihood estimation of those based on marked animals is also given. The procedures are illustrated by applying them to gray seal (Halichoerus grypus) data from Froan Nature Reserve, Central Norway, from the breeding seasons 1990-1999.

Cell cycle characteristics of synchronized and asynchronous populations of human cells and effect of cooling of selected mitotic cells

The method of synchronizing cells by means of mitotic selection has been adapted to the human line NHIK 3025. Increase in cell number as a function of time in asynchronous and synchronous populations was studied as well as mitotic index as a function of time after selection of synchronized populations. Phase durations of the cell cycle of synchronous populations were determined by 3H-thymidine incorporation and scintillation counting. The relative phase durations of exponentially growing asynchronous populations were determined by mathematical analysis of DNA-histograms recorded by flow cytofluorimetry. Both the generation time and the various phase durations of the cell cycle were found to be the same in asynchronous and synchronous populations. It was found that NHIK 3025 cells are damaged by cooling to 4 and 0 degrees C so that cooling of selected cells in order to increase the yield would reduce the quality of the synchronized populations.

Soluble cytostatic factor(s) released from human monocytes. II. Effects on target cell kinetics

Human monocytes release a stable cytostatic factor during in vitro culture after stimulation with lymphokine and endotoxin. The cell cycle time of synchronized NHIK 3025 cells increased from 20.3 to 23.2 h in the first cell cycle when the target cells were exposed to the factor during the whole cell cycle. In exponentially growing NHIK 3025 cell cultures the cell doubling time increased from 18.9 h to 23.1 h under continuous factor exposure for 70 h. These cells regained normal cell division rate when fresh culture medium replaced the cytostatic factor. Continuous exposure to the cytostatic factor for 96 h increased the cell doubling time of asynchronous K-562 cells from 19.7 to 31.8 h. The target cell DNA synthesis, evaluated by thymidine incorporation, was markedly depressed after culture with the factor for 24 h, and this depression was detectable already within 4 h of culture. The factor showed no cytolytic effect on the two cell lines tested. The cytostatic factor influenced the cell cycle distribution of both target cells tested, since cell cycle analysis by DNA flow cytometry demonstrated a reversible inhibition of factor-exposed NHIK 3025 cells in G1- and early S-phase, whereas K-562 cells accumulated in G1-phase.

The MHC class II-associated chicken invariant chain shares functional properties with its mammalian homologs

The nucleotide sequence of chicken invariant chain (Ii) was determined, and the amino acid sequence similarity with human Ii is 61%. Certain regions important for the biological function of human Ii are highly conserved between chicken and mammals. The cytoplasmic tail of chicken Ii fused to the plasma membrane reporter molecule neuraminidase relocated the protein to endosomes. Moreover, like the mammalian orthologs, the cytoplasmic tail was found to contain two independent leucine-based endosomal sorting signals. Chicken Ii was found to interact with human Ii and crosslinking studies also indicate that chicken Ii assembles as a trimer. The chicken Ii can furthermore bind the human MHC class II (HLA-DR1). Many of the functional properties between the chicken Ii and its mammalian orthologs are thus maintained in spite of their sequence differences.

Characterization and sequence-specific binding to mouse mammary tumor virus DNA of purified activated human glucocorticoid receptor

Activated glucocorticoid receptor (GR) from the human cell line HeLa S3 was purified by differential chromatography on DNA-cellulose followed by DEAE-Sepharose chromatography to 50-60% homogeneity according to sodium dodecyl sulfate gel electrophoresis and densitometric scanning of silver-stained gels. These gels routinely demonstrated a main band of Mr 94,000 (94K band) and two minor bands of Mr 79,000 (79K band) and 39,000 (39K band), respectively. Photoaffinity labeling indicated that the hormone was bound to the 94K and 79K components. In some preparations, a 72K band was observed. Further characterization of the purified receptor by gel permeation chromatography on Sephadex G-200 revealed a receptor complex with a Stokes radius of 5.8 nm. The sedimentation coefficient of the purified receptor was 4.4 Sw. In analogy to the rat hepatic GR, limited proteolysis of the purified GR with trypsin or alpha-chymotrypsin led to degradation of the 94K and 79K components and appearance of 28K and 39K fragments, respectively. In addition, no difference in the protease digestion pattern using Staphylococcus aureus V8 protease was observed. Immunoblotting using a monoclonal antibody raised against the 94K GR from rat liver demonstrated cross-reactivity with the human 94K and 79K proteins from HeLa S3 cells, indicating similar antigenic characteristics between rat and human GR. In our study, five out of nine tested monoclonal antibodies against the rat liver GR cross-reacted with human GR. DNase I and exonuclease III protection experiments demonstrated binding of the purified human GR to specific GR binding regions in mouse mammary tumor virus DNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of potassium dichromate on the cell cycle of an established human cell line (NHIK 3025)

The effect of K2Cr2O7 exposure on the cell cycle phases of the human cell line NHIK 3025 has been studied. Inhibition of cell proliferation was found to depend on the concentration and length of exposure. An effect on cell proliferation was observed 6-9 h after addition of K2Cr2O7, whereas cell death was not observed until 3-4 days later. The cells were most sensitive in G2 phase. After exposure to 8 mumol/l K2Cr2O7 the greatest prolongation of the cell cycle was in G2 + M phase, but an increase of S phase was also observed. No prolongation of G1 phase could be measured. Our results thus indicate that K2Cr2O7 delays progression through the cell cycle.

Selection of phage displayed peptides from a random 10-mer library recognising a peptide target

BACKGROUND

Peptide display libraries are powerful tools in the search for detailed information about protein-protein interactions. Usual targets for isolation of phage displayed peptide ligands include antibodies, various receptors, other full size proteins or larger fragments thereof. Smaller protein fragments such as synthetic peptides have not been reported as targets for screening of peptide display libraries.

OBJECTIVES

To investigate whether a protein target used for screening of a peptide display library could be scaled down to peptide size. As the peptide target we wanted to use a sequence derived from the cytosolic tail of MHC class II associated invariant chain containing a leucine class endosomal sorting signal, known to be recognised as an autonomous functional unit during targeting of class II complexes to antigen processing compartments.

STUDY DESIGN

A screening procedure where a synthetic 15-mer invariant chain peptide was coupled to a methacrylate matrix of high binding capacity was developed, and three rounds of selection were performed from a random 10-mer fUSE5 display library.

RESULTS

The peptide display library was successfully enriched for phage clones with affinity for the invariant chain peptide. Furthermore, the binding phage clones were able to distinguish between a functional and a mutated form of the target. These clones therefore displayed possible peptide mimetics of signal recognition sites in the cellular sorting machinery.

CONCLUSION

The size of a protein target may be scaled down to peptide size and be recognised by a 10-mer peptide displayed on filamentous phage. This approach may particularly be useful when the peptide target contains a functional unit for recognition.

Overexpression of proteins containing tyrosine- or leucine-based sorting signals affects transferrin receptor trafficking

Targeting of many transmembrane proteins to post-Golgi compartments is dependent on cytoplasmically exposed sorting signals. The most widely used signals conform to the tyrosine- or the leucine-based motifs. Both types of signals have been implicated in protein localization to the same intracellular compartments, but previous results from both cell-free experiments and studies of transfected cell lines have indicated that the two types of signals interact with separate components of the sorting machinery. We have overexpressed several transmembrane proteins in stably transfected Madin-Darby canine kidney cells using an inducible promoter system. Overexpression of proteins containing tyrosine- or leucine-based sorting signals resulted in reduced internalization of the transferrin receptor, whereas recycling and polarized distribution was not influenced. Our results indicate that proteins with tyrosine- and leucine-based sorting signals can be transported along common saturable pathways.

A change in the oxygen effect throughout the cell-cycle of human cells of the line NHIK 3025 cultivated in vitro

NHIK 3025 cells were synchronized by repeated mitotic selection. The S-phase was determined by 3H-thymidine incorporation and scintillation counting. By comparing the age-response surves of aerobic cells irradiated with 500 rad with those of extremely hypoxic (less than4 p.p.m. O2) cells irradiatedwith 1500 rad, it was found that the sensitizing effect of oxygen was not constant throuhgout the cycle. It was significantly higher in S, G2 and mitosis than in G1. No significant sensitizing effect of 120 p.p.m. O2 (compared with less than4 p.p.m.O2) was found on cells in G1 when the cells were irradiated with 1500 rad. In S, G2 and mitosis, however, the sensitizing effect of oxygen at 120 p.p.m. is considered to be significant. Experiments performed with cells irradiated with 2000 rad incontact with either less than4 p.p.m. O2 or 80 p.p.m. O2 showed the same trend, little sensitizing effect in G1 and higher in S, G2 andmitosis. Dose-response curves for cells in mid-G1 and mid-S under aerobic and extremely hypoxic conditions were well fitted by the formula S=exp (-alphaD-betaD2). From the dose-response curves it was conculded that the change in the sensitizing effect of oxygen throughout the cell-cycle only appeared for low doses (in the dose region where alpha dominates). The sensitizing effect of oxygen on cells in mid-G1 was found to be increasing with increasing dose.

The bio-logical role of invariant chain (Ii) in MHC class II antigen presentation

Foreign antigens are internalized by antigen presenting cells by endocytosis and processed to peptides. To enable presentation of antigenic peptides by MHC class II molecules, these molecules have to be sorted to endosomal compartments where they can meet and bind the peptides. Invariant chain is complexed with MHC class II molecules and contains sorting signals responsible for MHC class II accumulation in endosomes. Invariant chain also has several other features contributing to the immune system's specific combat against invaders.

Effect of oral high-dose progestins on the disposition of antipyrine, digitoxin, and warfarin in patients with advanced breast cancer

The influence of two progestins, medroxyprogesterone acetate (MPA) and megestrol acetate (MA), given orally in high doses, on the pharmacokinetics of antipyrine, digitoxin, and warfarin were studied in patients with advanced breast cancer. Antipyrine and warfarin were given as a single test dose before and after 5 weeks of progestin treatment. The pharmacokinetics of digitoxin was investigated at steady state in patients receiving this drug therapeutically before and during treatment with progestins. Small changes in clearance rates for antipyrine, warfarin, and digitoxin were found. A minor decrease observed in warfarin clearance however may be of clinical importance. Half-lives decreased by 13% for antipyrine and increased by 71% for warfarin. High-dose progestins given orally do not seem to have a major influence on drug metabolism, probably reflecting a minor effect on drug and steroid-metabolizing microsomal mono-oxygenases in the liver.