Abstract
Human alveolar macrophages (AMs) phagocytose Pneumocystis (Pc) organisms predominantly through mannose receptors, although the molecular mechanism mediating this opsonin-independent process is not known. In this study, using AMs from healthy individuals, Pc phagocytosis was associated with focal F-actin polymerization and Cdc42, Rac1, and Rho activation in a time-dependent manner. Phagocytosis was primarily dependent on Cdc42 and RhoB activation (as determined by AM transfection with Cdc42 and RhoB dominant-negative alleles) and mediated predominantly through mannose receptors (as determined by siRNA gene silencing of AM mannose receptors). Pc also promoted PAK-1 phosphorylation, which was also dependent on RhoGTPase activation. HIV infection of AMs (as a model for reduced mannose receptor expression and function) was associated with impaired F-actin polymerization, reduced Cdc42 and Rho activation, and markedly reduced PAK-1 phosphorylation in response to Pc organisms. In healthy AMs, Pc phagocytosis was partially dependent on PAK activation, but dependent on the Rho effector molecule ROCK. These data provide a molecular mechanism for AM mannose receptor-mediated phagocytosis of unopsonized Pc organisms that appears distinct from opsonin-dependent phagocytic receptors. Reduced AM mannose receptor-mediated Cdc42 and Rho activation in the context of HIV infection may represent a mechanism that contributes to the pathogenesis of opportunistic pneumonia.
Publication types
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Research Support, N.I.H., Extramural
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Research Support, U.S. Gov't, P.H.S.
MeSH terms
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Actins / metabolism
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Adaptor Proteins, Signal Transducing
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Blotting, Western
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Bronchoalveolar Lavage Fluid / cytology
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Bronchoscopy
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Carrier Proteins / genetics
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Carrier Proteins / metabolism*
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Cells, Cultured
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Cytoskeletal Proteins
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Enzyme Activation
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Enzyme-Linked Immunosorbent Assay
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Fluorescein-5-isothiocyanate
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Fluorescent Dyes
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Gene Silencing
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HIV Infections / complications
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HIV Infections / immunology*
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HIV Infections / virology
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HIV-1 / isolation & purification
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Humans
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Immunity, Innate
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Lectins, C-Type / metabolism*
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Macrophage Activation
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Macrophages, Alveolar / immunology*
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Macrophages, Alveolar / microbiology
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Mannose Receptor
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Mannose-Binding Lectins / metabolism*
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Microscopy, Confocal
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Phagocytosis / drug effects
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Phagocytosis / physiology
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Pneumocystis / physiology
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Precipitin Tests
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Protein Serine-Threonine Kinases / metabolism
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RNA, Small Interfering / metabolism
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Receptors, Cell Surface / metabolism*
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Tetradecanoylphorbol Acetate / pharmacology
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Transfection
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Zymosan / pharmacology
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cdc42 GTP-Binding Protein / genetics
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cdc42 GTP-Binding Protein / metabolism*
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p21-Activated Kinases
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rho GTP-Binding Proteins
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rhoB GTP-Binding Protein / genetics
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rhoB GTP-Binding Protein / metabolism*
Substances
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Actins
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Adaptor Proteins, Signal Transducing
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Carrier Proteins
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Cytoskeletal Proteins
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Fluorescent Dyes
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Lectins, C-Type
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Mannose Receptor
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Mannose-Binding Lectins
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RNA, Small Interfering
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Receptors, Cell Surface
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CDC42EP1 protein, human
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Zymosan
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PAK1 protein, human
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Protein Serine-Threonine Kinases
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p21-Activated Kinases
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cdc42 GTP-Binding Protein
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rho GTP-Binding Proteins
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rhoB GTP-Binding Protein
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Fluorescein-5-isothiocyanate
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Tetradecanoylphorbol Acetate