Early Intracellular Events During Internalization of Listeria monocytogenes by J774 CellsPaul Websteraa House Ear Institute, Los Angeles, California Correspondence to: Paul Webster, House Ear Institute, 2100 West Third Street, Los Angeles, CA 90057. E-mail: pwebster@hei.org The gram-positive bacillus Listeria monocytogenes gains entry into host cells through a phagosome membrane that forms around entering bacteria. During the early stages of internalization the invading bacteria appear to modify the protein composition of the forming phagosome membrane in J774 cells. MHC class II molecules on the cell surface and exposed surface molecules available for biotinylation are excluded from the bacteriahost cell membrane interface and from the forming phagosome. This exclusion of MHC class II molecules from the early phagosome may partially help to explain previous reports suggesting that L. monocytogenes is able to interfere with antigen presentation. Inside the host cell, MHC class II molecules are delivered to the phagosome membrane. This is followed by delivery of LAMP 1, a marker of late endocytic compartments, and fusion with low-pH compartments. The bacteria then escape into the cell cytoplasm, possibly assisted by rapid delivery of this low-pH environment. (J Histochem Cytochem 50:503517, 2002) Key Words: electron microscopy, phagocytosis, macrophages, MHC class II, immunocytochemistry, lysosomes
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