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CELL SCIENCE AT A GLANCE |
Depts of Medicine and Pharmacology, Columbia University, 630 West 168th Street, New York, NY 10032, USA
(e-mail: greenberg{at}cuccfa.ccc.columbia.edu)
Phagocytosis is a phylogenetically ancient process triggered by particulate ligands, such as bacteria, fungi and environmental particles. The spectrum of phagocytically competent cell types and receptors is diverse, but many of the basic processes of cytoskeletal rearrangement and membrane trafficking that accompany phagocytosis are conserved. Insights from studies of lower eukaryotes, such as Dictyostelium, Caenorhabditis elegans and Drosophila have contributed greatly to our understanding of the phagocytic pathway. Here, I focus on only a few examples of phagocytosis and bacterial invasion in mammalian cells, in which specific receptors have been identified and signal transduction pathways have been mapped. Owing to space constraints, the roles of lipid microdomains in phagocytosis and clearance of apoptotic bodies are not covered. The examples provided below emphasize the ability of the host cell to utilize different surface receptors to engage the phagocytic machinery.
Phagocytosis mediated by receptors for the Fc portion of IgG (Fc
Rs)
Receptors for the Fc portion of immunoglobulins are expressed in myeloid cells obtained from organisms ranging from lower vertebrates to humans. There are two fundamental types of FcR in phagocytic leukocytes: those that either contain or are associated with immunoreceptor tyrosine-based activation motifs (ITAMs); and those that contain immunoreceptor tyrosine-based inhibition motifs (ITIMs). The former, but not the latter, trigger phagocytosis. Signalling is initiated by ligand binding and clustering of cell surface FcRs. Members of the Src family of tyrosine kinases associate with FcRs and are probably responsible for the early phase of phosphorylation of tyrosine residues within FcR-associated ITAMs. This serves to recruit Syk, an SH2-domain-containing tyrosine kinase, which becomes activated to phosphorylate multiple substrates, including neighboring ITAMs. Syk is recruited from a cytosolic as well as a plasma-membrane-associated pool. It is also possible that a small number of preformed Syk-ITAM complexes exist in resting cells. Following receptor clustering, the local concentration of Syk increases relative to the local concentrations of protein tyrosine phosphatases (PTPases), thus favoring the localized accumulation of phosphorylated substrates.
Events downstream of Syk activation are less well understood; however, it is clear that programs of both actin assembly and membrane trafficking are needed to trigger cytoskeletal alterations, pseudopod extension and phagosomal closure. It is apparent that Arp2/3, members of the Rho family and ARF6 are essential for FcR-directed actin assembly, whereas phosphoinositide 3-kinase (PI 3-kinase), members of the Rab family and multiple SNARE proteins are required for pseudopod extension. In addition, unconventional myosins are likely to play keys roles in pseudopod extension and phagosomal closure.
Phagocytosis mediated by the leukocyte integrin complement receptor 3
Unlike most phagocytosis-promoting receptors, complement receptor 3 (CR3; also known as CD11b/CD18 or 
Mß2), a ß2 integrin, requires an activation step (‘inside-out’ signalling) for phagocytic competence. CR3 activation encompasses enhanced receptor affinity and avidity (i.e., clustering of CR3 to augment potentially low-affinity receptor-ligand interactions and, perhaps, to engage the cytoskeleton). Like FcR-mediated phagocytosis, CR3-mediated phagocytosis requires the participation of Rho family members, particularly RhoA, in an Arp2/3-dependent ingestion process. Evidence suggests that CR3-mediated phagocytosis triggers the formation of morphologically distinct phagosomes in which ‘sinking in’ of the phagocytic targets into the host cell plays a prominent role. The integrity of microtubules is essential for CR3-mediated phagocytosis; the reasons for this are unclear.
Phagocytosis triggered by Salmonella and Shigella
These pathogens trigger phagocytosis in an array of host cells, such as epithelia, via a type III secretion system. By injecting cytoskeleton-altering proteins and activators of Rho family GTPases into the host cell cytoplasm, these pathogens induce the formation of cell extensions that resemble macropinosomes (a ‘trigger mechanism’) rather than pseudopods characteristic of FcR-mediated phagocytosis (a ‘zipper mechanism’).
Phagocytosis of Listeria monocytogenes
This Gram-positive bacterium engages receptors that otherwise function in cell-cell adhesion (E-cadherin) or growth and differentiation (Met) by the bacterial adhesins InlA and InlB, respectively. Like many other examples of phagocytosis, ingestion of Listeria monocytogenes is PI 3-kinase dependent. The exact role of PI 3-kinase in the ingestion of Listeria is unclear.
Phagocytosis of unencapsulated Neisseria
Adhesion to host cells is by a bacterial surface appendage (the type IV pilus) and outer membrane opacity (Opa) proteins. Ingestion is accomplished by multiple signal transducing events, including activation of Src family tyrosine kinases, ligation of host cell integrins by vitronectin-associated bacteria and generation of ceramide by sphingomyelinase.
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