Phagocytes have a critical function in remodelling tissues during embryogenesis and thereafter are central effectors of immune defence1,2. During phagocytosis, particles are internalized into ‘phagosomes’, organelles from which immune processes such as microbial destruction and antigen presentation are initiated3. Certain pathogens have evolved mechanisms to evade the immune system and persist undetected within phagocytes, and it is therefore evident that a detailed knowledge of this process is essential to an understanding of many aspects of innate and adaptive immunity. However, despite the crucial role of phagosomes in immunity, their components and organization are not fully defined. Here we present a systems biology analysis of phagosomes isolated from cells derived from the genetically tractable model organism Drosophila melanogaster and address the complex dynamic interactions between proteins within this organelle and their involvement in particle engulfment. Proteomic analysis identified 617 proteins potentially associated with Drosophila phagosomes; these were organized by protein–protein interactions to generate the ‘phagosome interactome’, a detailed protein–protein interaction network of this subcellular compartment. These networks predicted both the architecture of the phagosome and putative biomodules. The contribution of each protein and complex to bacterial internalization was tested by RNA-mediated interference and identified known components of the phagocytic machinery. In addition, the prediction and validation of regulators of phagocytosis such as the ‘exocyst’4, a macromolecular complex required for exocytosis but not previously implicated in phagocytosis, validates this strategy. In generating this ‘systems-based model’, we show the power of applying this approach to the study of complex cellular processes and organelles and expect that this detailed model of the phagosome will provide a new framework for studying host–pathogen interactions and innate immunity.
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We thank R. Kearney and J. Bergeron from the Montreal Proteomics Network, and Genome-Quebec-Canada for their support. J.S.B., M.D. and R.A.B.E. thank their laboratories for their support. The work was supported by a Wellcome Trust Clinician Scientist Award to L.M.S., grants from the Whitaker Foundation and NIH/NIGMS to J.S.B, grants from the Canadian Institute for Health Research and Genome-Canada-Québec to M.D., and NIH grants to R.A.B.E. The work was conceived through discussions between the Laboratory of Developmental Immunology, Massachusetts General Hospital/Harvard Medical School and the Département de pathologie et biologie cellulaire, Université de Montréal. The bioinformatics and RNAi screens were performed in the Laboratory of Developmental Immunology, Massachusetts General Hospital/Harvard Medical School; the protein–protein networks were generated in the Department of Biomedical Engineering and High-Throughput Biology Center, Johns Hopkins University; the proteomics, the annotation of the components and the phagosome isolation were performed in the Département de pathologie et biologie cellulaire, Université de Montréal. Author Contributions L.M.S. and J.B. contributed equally to this work. J.S.B., M.D. and R.A.B.E. contributed equally to this work. The manuscript was written by L.M.S. and the website linked to this paper was designed by G.M.C.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
This file contains Supplementary Methods, Supplementary Data, Supplementary Figures 1-5 and Supplementary Tables 2- 5 and Supplementary Table 9. (PDF 5985 kb)
This file contains full list of proteins enriched in phagosome preparations isolated from Drosophila S2 cells . (XLS 1377 kb)
This file contains results of RNAi screen for phagocytosis of E.coli and S.aureus. (XLS 314 kb)
This file contains results of RNAi screen of the secondary components for phagocytosis of E.coli and S.aureus. (XLS 36 kb)
This file contains plate and well i.d.s for MRC RNAi library primer. (XLS 129 kb)
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