Rapid and complete fusion of macrophage lysosomes with phagosomes containing Salmonella typhimurium.

The virulence of Salmonella typhimurium for mice results, in part, from its ability to survive after phagocytosis by macrophages. Although it is generally agreed that intracellular bacteria persist in membrane-bound phagosomes, there remains some question as to whether these phagosomes fuse with macrophage lysosomes. This report describes the maturation of phagosomes containing S. typhimurium inside mouse bone marrow-derived macrophages. Macrophages were infected briefly and incubated for various intervals; then they were examined by fluorescence microscopy for colocalization of bacteria with lysosomal markers. These markers included LAMP-1, cathepsin L, and fluorescent proteins or dextrans preloaded into lysosomes by endocytosis. By all measures, phagosomes containing S. typhimurium merged completely with the lysosomal compartment within 20 min of phagocytosis. The rate of phagosome-lysosome fusion was similar to the rate for phagocytosed latex beads. Phagolysosomes remained accessible to fluid-phase probes and contained lysosomal markers for many hours. Moreover, a large percentage of the wild-type bacteria that were viable 20 min after infection survived longer incubations inside macrophages, indicating that the survivors were not a minor subpopulation that avoided phagosome-lysosome fusion. Therefore, we conclude that S. typhimurium survives within the lysosomal compartments of macrophages.

Spacious phagosome formation within mouse macrophages correlates with Salmonella serotype pathogenicity and host susceptibility.

Light microscopic studies indicated a correlation between the virulence for mice of different Salmonella serotypes and the ability to form or maintain spacious phagosomes (SP) within mouse macrophages. Although Salmonella typhimurium induced membrane ruffling, macropinocytosis, and SP formation in macrophages from BALB/c mice, serotypes which are nonpathogenic for mice produced markedly fewer SP. SP formation correlated with both serotype survival within mouse macrophages and reported lethality for mice. Time-lapse video microscopy demonstrated that the human pathogen S. typhi induced generalized macropinocytosis and SP formation in human monocyte-derived macrophages, indicating a similar morphology for the initial phases of this host-pathogen interaction. In contrast to bone marrow-derived macrophages from BALB/c mice, macrophages from S. typhimurium-resistant outbred (CD-1) and inbred (CBA/HN) mice did not initiate generalized macropinocytosis after bacterial infection and formed markedly fewer SP. These deficiencies were not due to the Ihy resistance genotype of these mice, as macrophages from mice that were congenic except for the Ihy locus demonstrated equal SP formation in response to S. typhimurium. The observation that S. typhimurium-resistant CD-1 and CBA/HN mice are deficient in the ability to form and/or maintain SP indicates that a variable host component is important for SP formation and suggests that the ability to induce or form SP affects susceptibility to S. typhimurium. When serotypes nonpathogenic for mice were used to infect BALB/c macrophages, or when CD-1 or CBA/HN mouse macrophages were infected by S. typhimurium, some of the SP that formed shrank within seconds. This rapid shrinkage suggests that SP maintenance is also important for S. typhimurium survival within macrophages. These studies indicate that both host and bacterial factors contribute to SP formation and maintenance, which correlate with Salmonella intracellular survival and the ability to cause lethal enteric (typhoid) fever.

Modern war surgery: the experience of Bosnia. 1: Deployment.

In the first of two papers on the experience of a Canadian military surgical team in the former Yugoslavia, the authors describe the deployment of the field surgical hospital, the medical structure that supported the Canadian battle group. The hospital was made up of tent sections erected within an unfinished concrete factory building. The hospital comprised a treatment area for sick parades and reception, a pharmacy, a resuscitation area for nonambulatory casualties, a laboratory, an x-ray section, an operating room and sterilization section and a ward. The hospital could be mobilized if necessary. The setup proved to be functional for the treatment of injured soldiers. Although long delays were expected because of difficulties in transporting the injured, the patients reached the hospital in a reasonable time after injury and could be treated satisfactorily. During the period of its deployment, this hospital was used more than any other Canadian hospital in the United Nations mission. This experience allowed the authors to identify deficiencies and to correct them quickly.

Molecular size-fractionation during endocytosis in macrophages.

The sorting of macromolecules within and between membranous organelles is often directed by information contained in protein primary or secondary structure. We show here that absent such structural information, macromolecules internalized by endocytosis in macrophages can be sorted by size. After endocytosis, small solute probes of fluid-phase pinocytosis were recycled to the extracellular medium more efficiently than large solutes. Using macropinosomes pulse labeled with fluorescent dextrans, we examined the ability of organelles to exchange solute contents. Dextran exchange was optimal between organelles of similar age, and small dextrans exchanged more efficiently than large dextrans. Efferent solute movement, from lysosomes or phagolysosomes toward the plasma membrane, occurred through the same endocytic vesicles as afferent movement, toward lysosomes and this movement was solute size dependent. Remarkably, uniform mixtures of different-sized dextrans delivered into lysosomes separated into distinct organelles containing only one dextran or the other. Thus, the dynamics of endosomes and lysosomes were sufficient to segregate macromolecules by size. This intracellular size fractionation could explain how, during antigen presentation, peptides generated by lysosomal proteases recycle selectively from lysosomes to endosomes for association with class II MHC molecules.

Quantitative measurement of intraorganelle pH in the endosomal-lysosomal pathway in neurons by using ratiometric imaging with pyranine.

Organelle acidification is an essential element of the endosomal-lysosomal pathway, but our understanding of the mechanisms underlying progression through this pathway has been hindered by the absence of adequate methods for quantifying intraorganelle pH. To address this problem in neurons, we developed a direct quantitative method for accurately determining the pH of endocytic organelles in live cells. In this report, we demonstrate that the ratiometric fluorescent pH indicator 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) is the most advantageous available probe for such pH measurements. To measure intraorganelle pH, cells were labeled by endocytic uptake of HPTS, the ratio of fluorescence emission intensities at excitation wavelengths of 450 nm and 405 nm (F450/405) was calculated for each organelle, and ratios were converted to pH values by using standard curves for F450/405 vs. pH. Proper calibration is critical for accurate measurement of pH values: standard curves generated in vitro yielded artifactually low organelle pH values. Calibration was unaffected by the use of culture medium buffered with various buffers or different cell types. By using this technique, we show that both acidic and neutral endocytically derived organelles exist in the axons of sympathetic neurons in different steady-state proportions than in the cell body. Furthermore, we demonstrate that these axonal organelles have a bimodal pH distribution, indicating a rapid acidification step in their maturation that reduces the average pH of a fraction of the organelles by 2 pH units while leaving few organelles of intermediate pH at steady state. Finally, we demonstrate a spatial gradient or organelle pH along axons, with the relative frequency of acidic organelles increasing with proximity to the cell body.