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Occludin and 18 distinct members of the claudin family are tetra-span transmembrane proteins that are localized in cell-specific tight junctions (TJs). A previous study showed that expression of chick occludin in Madin-Darby canine kidney (MDCK) cells raised transepithelial electrical resistance (TER) and, paradoxically, increased mannitol flux. In the present study, we employed epitope tagged canine occludin expression, under the control of the tetracycline repressible transactivator, to determine the extent to which the unexpected parallel increase in TER and mannitol flux was related to a structural mismatch between avian and canine occludins, which are only 50% identical. To determine whether the paradoxical changes in permeability was specific to occludin, we assessed the effect of over-expressing epitope tagged murine claudin-1. Our data revealed that over-expression of either of the epitope tagged mammalian tight junction proteins increased TER, mannitol and FITC-dextran flux. We observed a 2- and up to 5.6-fold over-expression of occludin-VSV-G and claudin-1-myc, respectively, with no change in ZO-1, endogenous occludin or claudin-1 expression. Confocal microscopy revealed that occludin-VSV-G, claudin-1-myc and ZO-1 co-localized at the TJ. In addition, claudin-1-myc formed aberrant strands along the lateral cell surface without an underlying ZO-1 scaffold. In fracture labeled replicas these strands consisted of claudin-1-myc with little accompanying occludin. These observations suggest that in epithelial cells claudin-1 can assemble into TJ strands without the participation of either ZO-1 or occludin. The proximity of the myc tag to the COOH-terminal YV sequence of claudin-1 appeared to interfere with its interaction with ZO-1, since over-expression of non-tagged claudin-1 increased TER but had a minimal effect on solute flux and no aberrant strands formed. From our data we conclude that differences in structure between avian and mammalian occludin do not account for the observed paradoxical increase in mannitol flux. Levels of ZO-1 remained unchanged despite substantial increases in induced TJ integral protein expression, suggesting that an imbalance between levels of ZO-1 and occludin or claudin-1 leads to altered regulation of pores through which non-charged solute flux occurs. We suggest that ion and solute flux are differentially regulated at the TJ.

1. Experiments were performed on exteriorized fetal lambs of between 69 days' gestation and term (147 days) in order to observe changes in lung volume and lung liquid secretion rate, and to delineate any alterations in solute permeability, ion transport and tight junction morphology in the maturing lung epithelium. Whilst it was technically possible to measure solute permeability as early as 69 days it was not feasible to apply the Ussing flux ratio technique before 84 days.

Occludin's role in mammalian tight junction activity was examined by 'labeling' the occludin pool with immunologically detectable chick occludin. This was accomplished by first transfecting MDCK cell with the Lac repressor gene. HygR clones were then transfected with chick occludin cDNA inserted into a Lac operator construct. The resulting HygR/NeoR clones were plated on porous inserts and allowed to form tight junctions. Once steady state transepithelial electrical resistance was achieved, isopropyl- beta-D-thiogalactoside was added to induce chick occludin expression. Confocal laser scanning microscopy of monolayers immunolabeled with Oc-2 monoclonal antibody revealed that chick occludin localized precisely to the preformed tight junctions. When sparse cultures were maintained in low Ca2+ medium, chick occludin and canine ZO-1 co-localized to punctate sites in the cytoplasm suggesting their association within the same vesicular structures. In low calcium medium both proteins also co-localized to contact sites between occasional cell pairs, where a prominent bar was formed at the plasma membrane. Chick occludin was detectable by western blot within two hours of adding isopropyl- beta-D-thiogalactoside to monolayers that had previously achieved steady state transepithelial electrical resistance; this coincided with focal immunofluorescence staining for chick occludin at the cell membrane of some cells. A gradual rise in transepithelial electrical resistance, above control steady state values, began five hours after addition of the inducing agent reaching new steady state values, which were 30-40% above baseline, 31 hours later. Upon removal of isopropyl- beta-D-thiogalactoside chick occludin expression declined slowly until it was no longer detected in western blots 72 hours later; transepithelial electrical resistance also returned to baseline values during this time. While densitometric analysis of western blots indicated that the presence of chick occludin had no detectable effect on E-cadherin or ZO-1 expression, the possibility cannot be excluded that ZO-1 might be a limiting factor in the expression of chick occludin at the cell surface. To test whether expression of chick occludin affected the process of tight junction assembly, monolayers in low Ca2+ medium were treated with isopropyl- beta-D-thiogalactoside for 24 or 48 hours, before Ca2+ was added to stimulate tight junction assembly. Chick occludin did not alter the rate at which transepithelial electrical resistance developed, however, steady state values were 30-40% above control monolayers not supplemented with the inducing agent. By freeze fracture analysis, the number of parallel tight junction strands shifted from a mode of three in controls to four strands in cells expressing chick occludin and the mean width of the tight junction network increased from 175 +/- 11 nm to 248 +/- 16 nm. Two days after plating confluent monolayers that were induced to express chick occludin, mannitol flux was reduced to a variable degree relative to control monolayers. With continued incubation with the inducing agent, mannitol flux increased on day 11 to 50%, and TER rose to 45% above controls. Both of these changes were reversible upon removal of isopropyl- beta-D-thiogalactoside. These data are consistent with the notion that occludin contributes to the electrical barrier function of the tight junction and possibly to the formation of aqueous pores within tight junction strands.

Lewis rats were given a single i.v. injection of soluble immune complexes containing human serum albumin (HSA) and rabbit anti-HSA antibodies, prepared in antigen excess. This resulted in localization of HSA and rabbit gamma globulin (RGG) in glomerular mesangial regions without producing definite histologic changes. 24 h after the injection of immune complexes, groups of these rats received lymph node cells or T-cell preparations from syngeneic donors sensitized to RGG, HSA, or ovalbumin; another group received no cells. All of these groups and a group of normal control rats were given injections of [3H]thymidine at 18, 27, and 44 h. The animals were killed 48 h after the time of cell transfer. In histologic sections, glomerular abnormalities were found only in some of the animals that had received immune complexes and lymph node cells or T-cell populations from donors sensitized to HSA or RGG; the lesions were characterized by focal and segmental increase in cells in mesangial regions. Autoradiographs revealed significantly greater numbers of labeled cells in mesangial regions and glomerular capillaries in the groups that had received immune complexes and cells from HSA- or RGG-sensitized donors than in any of the other groups. Electronmicroscopic studies suggested that the increase in cellularity in mesangial regions resulted from an influx of mononuclear phagocytes. The findings indicate that cell-mediated reactions can be initiated by the interaction between sensitized T lymphocytes and antigens present in immune complexes within mesangial regions.

Synthetic peptides that correspond to the COOH-terminal portion of C2b enhance vascular permeability in human and guinea pig skin. In human studies, 1 nmol of the most active peptide of 25-amino acid residues produced substantial local edema. A pentapeptide and a heptapeptide corresponding to the COOH-terminal sequence of C2b each induced contraction of estrous rat uterus in the micromole range; a peptide of 25 amino acids from this region induced a like contraction of rat uterus at a concentration 20-fold lower than the smaller peptides. The vascular permeability of guinea pig skin was enhanced by doses of these synthetic peptides in a similar fashion as that observed for the concentration of rat uterus. The induction of localized edema by intradermal injection in both the guinea pig and the human proceeds in the presence of antihistaminic drugs, suggesting that there is a histamine-independent component to the observed increase in vascular permeability. Cleavage of C2 with the enzymic subcomponent of C1, C1s, yields only C2a and C2b, and no small peptides, whereas cleavage of C2 with C1s and plasmin yields a set of small peptides. These plasmin-cleaved peptides are derived from the COOH terminus of C2b, and they induce the contraction of estrous rat uterus.

gamma Interferon (IFN-gamma) caused remarkable increases in class I (H-2Kk) and class II (I-Ak) antigens throughout the body by 6-9 d. Heart, kidney, and adrenals showed increases of 4-8 times their previous levels of class I antigen content, while the pancreas and small intestine increased 13-17-fold. Lesser increases were found in spleen, liver, and lung, which showed higher resting antigenic potency. Increases of class II antigenicity of 6-10-fold were found in heart, kidney, pancreas, lung, liver, adrenal, and small intestine, with lesser increases in thymus and spleen, and none in lymph node. Topographical analysis revealed that IFN-gamma induced class I and II antigens on most tissues in a highly selective fashion. For example, the renal proximal tubules expressed large amounts of both class I and II antigens, whereas the distal tubules and collecting ducts did not. In some epithelial cells class I and II determinants were induced only on the basal aspects of the cell membrane. IFN-gamma caused a remarkable increase in class II-positive dendritic cells in the liver, pancreas, salivary glands, and thyroid. Whether these cells were of local or systemic origin is uncertain, but the finding of a simultaneous depletion of dendritic cells from lymph nodes and spleen raises the possibility that they may have been derived, at least in part, from these sites. The dynamic and selective induction of class I and II antigen expression by IFN-gamma is likely to be important in regulation of the immune response in tissues.

A method has been developed for preparation of confluent monolayers of human monocytes from small volumes of blood and for maintenance of these pure monocyte cultures for up to 16 wk in vitro. These cells phagocytosed 5.7 mum diameter latex beads, rosetted with erythrocytes coated with IgG or with C3, killed Listeria monocytogenes, and synthesized both lysozyme and the second component of complement. Lysozyme was secreted at a rate of approximately 50,000 mol/min per cell for at least 12 wk in cultures. The maximal rate of C2 synthesis and secretion was considerably less; i.e., approximately 30 mol/min per cell between the 2nd and 12th wk in culture. Monocytes produced little C2 during the first 6 days in culture after which a marked increase in the rate of C2 production was noted. This increase was coincident with morphologic evidence of monocyte maturation.

Lewis rats were injected intravenously with rabbit anti-rat glomerular basement membrane (GBM) antisera in doses that were sufficient to cause glomerular fixation of rabbit gamma globulin (RGG) detectable by immunofluorescence, but which failed to induce histologically detectable lesions. 24 h later, groups of rats received lymph node cells or serum from syngeneic donors that had been immunized with either RGG or ovalbumin; they were injected with [3H]thymidine three times during the next 2 days, and sacrificed 48 or 96 h after transfer. Only the rats given anti-GBM antiserum plus lymph node cells from donors sensitized to RGG showed histological glomerular lesions, in the form of segmental hypercellularly and necrosis. Autoradiographs revealed the greatest number of labeled cells in glomeruli in the same group. In analogous experiments, it was shown that T-cell-enriched populations could induce hypercellular glomerular reactions. On the basis of electronmicroscopic and autoradiographic observations, it appears that the glomerular hypercellularity resulted from both infiltration of mononuclear cells and proliferation of endothelial cells. The findings indicate that interaction of specifically sensitized lymphocytes with glomerular-bound antigen can induce a cell-mediated (delayed-type) reaction in glomeruli.