Phagocytosis
2005; Cell Press; Volume: 22; Issue: 5 Linguagem: Inglês
10.1016/j.immuni.2005.05.002
ISSN1097-4180
AutoresLynda M. Stuart, R. Alan B. Ezekowitz,
Tópico(s)Immune responses and vaccinations
ResumoPhagocytosis requires receptor-mediated recognition of particles, usually in the guise of infectious agents and apoptotic cells. Phagosomes fuse with lysosomes to generate phagolysosomes, which play a key role in enzymatic digestion of the internalized contents into component parts. Recent findings indicate that a simple paradigm of a single cognate receptor interaction that guides the phagosome to phagolysosome formation belies the complexity of combinatorial receptor recognition and diversity of phagosome function. In fact, phagosomes are comprised of hundreds of proteins that play a key role in deciphering the contents of the phagosome and in defining host response. In this review we discuss how the challenge of recognizing diverse molecular patterns is met by combinatorial interactions between phagocytic receptors. Furthermore, these combinations are dynamic and both sculpt the balance between a proinflammatory or anti-inflammatory response and direct phagosome diversity. We also indicate an important role for genetically tractable model organisms in defining key components of this evolutionarily conserved process. Phagocytosis requires receptor-mediated recognition of particles, usually in the guise of infectious agents and apoptotic cells. Phagosomes fuse with lysosomes to generate phagolysosomes, which play a key role in enzymatic digestion of the internalized contents into component parts. Recent findings indicate that a simple paradigm of a single cognate receptor interaction that guides the phagosome to phagolysosome formation belies the complexity of combinatorial receptor recognition and diversity of phagosome function. In fact, phagosomes are comprised of hundreds of proteins that play a key role in deciphering the contents of the phagosome and in defining host response. In this review we discuss how the challenge of recognizing diverse molecular patterns is met by combinatorial interactions between phagocytic receptors. Furthermore, these combinations are dynamic and both sculpt the balance between a proinflammatory or anti-inflammatory response and direct phagosome diversity. We also indicate an important role for genetically tractable model organisms in defining key components of this evolutionarily conserved process. Phagocytosis was popularized at the end of the nineteenth century by the Russian embryologist Ilya Metchnikoff, who observed that amoeboid-like cells in transparent sea star larvae contained ingested cells. He hypothesized that these cells would be able to recognize and internalize foreign material. Metchnikoff proved this idea in a simple experiment in which he observed that these cells moved toward and engulfed a thorn that he had introduced into a larva. Based on these findings, he extrapolated that these so-called phagocytes were capable of ingestion and might play a key role in host defense and tissue homeostasis, which intricately linked them with inflammation in man (Metchnikoff, 1905Metchnikoff E. Immunity in the Infectious Diseases. Macmillan Press, New York1905Google Scholar). Remarkably, over the next century and a quarter, many of Metchnikoff’s ideas as to the origins of inflammation have been validated. What he could not have foreseen, however, is the enormous heterogeneity and complexity of phagocyte biology. In mammals professional phagocytes (i.e., macrophages, dendritic cells (DCs), and granulocytes) derive from a common myeloid progenitor cell. Specific combinations of inductive events instruct differentiation of these common progenitors to their mature progeny. It is pertinent to note that during early embryogenesis, coincident with the establishment of a circulation, myeloid cells constitutively populate newly formed organs (Gordon et al., 1986Gordon S. Crocker P.R. Morris L. Lee S.H. Perry V.H. Hume D.A. Localization and function of tissue macrophages.Ciba Found. Symp. 1986; 118: 54-67PubMed Google Scholar). These tissue macrophages have a distinct surface phenotype (CX3CR1hi, CCR2−,Gr1−) and appear to traffic constitutively from the blood to the tissues throughout adult life (Geissmann et al., 2003Geissmann F. Jung S. Littman D.R. Blood monocytes consist of two principal subsets with distinct migratory properties.Immunity. 2003; 19: 71-82Abstract Full Text Full Text PDF PubMed Scopus (2402) Google Scholar). In the mouse these cells are able to differentiate into DCs and, together with mast cells, NKT cells, B1 B cells, and γδT cells, form the sentinels at the potential portals of microbial entry. Diverse inflammatory signals rapidly mobilize polymorphonuclear leukocytes and a short-lived subset of inflammatory macrophages together with other plasma components, which all serve as key components of the innate immune response (Hoffmann et al., 1999Hoffmann J.A. Kafatos F.C. Janeway C.A. Ezekowitz R.A. Phylogenetic perspectives in innate immunity.Science. 1999; 284: 1313-1318Crossref PubMed Scopus (2069) Google Scholar). Phagocytes are required to continually sense and edit the extracellular environment. This constant surveillance requires a set of distinct cell surface receptors that have redundant and nonoverlapping repertoires for recognizing and responding to infectious and noninfectious injury. Janeway proposed that these canonical, germline-encoded, and invariant receptors directly recognize pathogens and popularized the concept of “pattern recognition receptors” (Janeway, 1989Janeway Jr., C.A. Approaching the asymptote? Evolution and revolution in immunology.Cold Spring Harb. Symp. Quant. Biol. 1989; 54: 1-13Crossref PubMed Google Scholar). In this review we will first focus on the complexity of pattern recognition receptors and subsequent phagosome diversity in mammalian phagocytes and then describe the potential role for model systems in deconstructing phagocytosis down to the essential nonredundant components. We will not discuss the regulation of the cytoskeleton in detail, as it is extensively covered in other specialist reviews (Aderem and Underhill, 1999Aderem A. Underhill D.M. Mechanisms of phagocytosis in macrophages.Annu. Rev. Immunol. 1999; 17: 593-623Crossref PubMed Scopus (1940) Google Scholar, Greenberg and Grinstein, 2002Greenberg S. Grinstein S. Phagocytosis and innate immunity.Curr. Opin. Immunol. 2002; 14: 136-145Crossref PubMed Scopus (413) Google Scholar, Underhill and Ozinsky, 2002Underhill D.M. Ozinsky A. Phagocytosis of microbes: complexity in action.Annu. Rev. Immunol. 2002; 20: 825-852Crossref PubMed Scopus (803) Google Scholar) but, rather, present a conceptual overview using certain selected examples to illustrate our points. The surface of the phagocyte is adorned with many receptors that are able to recognize and decode their cognate ligands expressed on the surface of infectious agents and apoptotic cells and trigger engulfment (Figure 1). These receptors either directly recognize the particle or recognize targets coated in opsonic molecules (see below). Although these ligands were originally referred to as pathogen-associated molecular patterns or PAMPs, this definition neither includes the recognition of commensal bacteria nor apoptotic and necrotic cells. For this reason we propose “molecular pattern” (MP) as a more inclusive term and will use it in this review. Particles are recognized by a combination of scavenger receptors, integrins (complement receptors), and lectins binding directly or via opsonins such as LBP, TSP, or collectins. Most of these receptors are able to recognize both pathogens and altered self-ligands such as apoptotic cells. In addition, phagocytes have specific receptors that discriminate pathogen-associated components causing inflammatory responses. As an example, recognition of gram-positive bacteria by TLR2 and TLR6 or gram-negative by TLR4 is shown. In contrast, anti-inflammatory signals are triggered after binding of apoptotic cells that expose phosphatidyl-serine (PS) on their cell surface. Ligation of receptors for apoptotic cells (especially the receptor for PS) causes liberation of TGFβ and other immunomodulatory cytokines. Hence, multiple receptors are involved in recognition with certain common receptors mediating internalization of both pathogens and self-ligands. Together these receptor complexes contribute to discriminating and initiating appropriate responses. Early experiments, constrained by the tools available at the time, were by necessity reductionist and aimed at identifying and defining the structure and function of individual phagocytic receptors. The macrophage mannose receptor (MMR) was one of the earliest phagocytic receptors cloned and represents a paradigm in this regard (Ezekowitz et al., 1990Ezekowitz R.A. Sastry K. Bailly P. Warner A. Molecular characterization of the human macrophage mannose receptor: demonstration of multiple carbohydrate recognition-like domains and phagocytosis of yeasts in Cos-1 cells.J. Exp. Med. 1990; 172: 1785-1794Crossref PubMed Scopus (397) Google Scholar, Taylor et al., 1990Taylor M.E. Conary J.T. Lennartz M.R. Stahl P.D. Drickamer K. Primary structure of the mannose receptor contains multiple motifs resembling carbohydrate-recognition domains.J. Biol. Chem. 1990; 265: 12156-12162Abstract Full Text PDF PubMed Google Scholar). MMR was first defined as an endocytic receptor that recognized mannosyl- and fucosyl-containing neoglycoproteins (Stahl et al., 1978Stahl P.D. Rodman J.S. Miller M.J. Schlesinger P.H. Evidence for receptor-mediated binding of glycoproteins, glycoconjugates, and lysosomal glycosidases by alveolar macrophages.Proc. Natl. Acad. Sci. USA. 1978; 75: 1399-1403Crossref PubMed Scopus (442) Google Scholar). The molecular characterization of this protein revealed that it was the first member of an ever-growing family of C type lectins that require calcium for ligand binding to the multiple carbohydrate binding domains. The mannose receptor contains a tandem array of eight prototypic lectin folds that consists of two antiparallel β strands and two α helices. Transient overexpression of the MMR in Cos7 cells revealed that MMR was able to recognize yeast, certain bacteria, and Pneumocystis carinii (Ezekowitz et al., 1990Ezekowitz R.A. Sastry K. Bailly P. Warner A. Molecular characterization of the human macrophage mannose receptor: demonstration of multiple carbohydrate recognition-like domains and phagocytosis of yeasts in Cos-1 cells.J. Exp. Med. 1990; 172: 1785-1794Crossref PubMed Scopus (397) Google Scholar, Ezekowitz et al., 1991Ezekowitz R.A. Williams D.J. Koziel H. Armstrong M.Y. Warner A. Richards F.F. Rose R.M. Uptake of Pneumocystis carinii mediated by the macrophage mannose receptor.Nature. 1991; 351: 155-158Crossref PubMed Scopus (345) Google Scholar). However, it was clear even at that time that preincubation of macrophages with the soluble yeast wall product mannan, a high-affinity MMR ligand, resulted in only partial inhibition of phagocytosis, thus providing circumstantial evidence for the existence of additional receptors that recognize yeast. The original reductionist view has been superseded as knowledge has advanced and many other receptors that are capable of recognizing similar MPs as those recognized by MMR have been characterized. These include other members of the family of C type lectin transmembrane proteins such as DC-SIGN, L-SIGN, DEC-205, Endo-180, Langerin, DCAL-1, BDCA-2 and Dectin-1. Each of these molecules has the potential to recognize subtle differences in the structure of displayed carbohydrate ligands that define their cognate MP. The array of these carbohydrate ligands, while generally conserved on the surface of micro organisms, have the potential to undergo subtle alterations that are predicted to redefine the binding affinity of any one lectin receptor for that particular infectious agent. In order to combat this potential evasive strategy adopted by infectious agents, phagocytes do not rely on any one receptor for recognition of a pathogen. An illustration of the cooperative action of two lectin receptors is best illustrated by experiments that demonstrate colocalization of MMR and DC-SIGN in a Candida albicans-containing phagosome (as discussed in the review Cambi et al., 2005Cambi A. Koopman M. Figdor C.G. How C-type lectins detect pathogens.Cell. Microbiol. 2005; 7: 481-488Crossref PubMed Scopus (288) Google Scholar). In this study the presence of only these two receptors was probed, and it is likely that other candidate molecules that all have the potential to recognize this ligand, including Toll-like receptor (TLR) 2, other multi-lectin receptors, and a class of receptors called scavenger receptors, might also be found within these phagosomes. Importantly, receptors not only trigger engulfment but also act to define the consequences of phagocytosis as either proinflammatory or anti-inflammatory. The TLRs play a key role in mediating sensing and signaling of downstream effectors in response to a number of well-defined ligands. TLR ligands are highly diverse but demonstrate a key common feature: these ligands are invariant and necessary components of pathogens that are absent from host cells and include bacterial derivatives (such as lipoteichoic acid [LTA], lipopolysaccharide [LPS], flagellin, peptidoglycan, and CpGDNA) and components associated with viral replication (ss and dsRNA). However, TLRs are not phagocytic receptors and for many TLRs, ligand engagement does not occur on the cell surface but after internalization into the endolysosomal or phagocytic compartment (Latz et al., 2004Latz E. Schoenemeyer A. Visintin A. Fitzgerald K.A. Monks B.G. Knetter C.F. Lien E. Nilsen N.J. Espevik T. Golenbock D.T. TLR9 signals after translocating from the ER to CpG DNA in the lysosome.Nat. Immunol. 2004; 5: 190-198Crossref PubMed Scopus (1074) Google Scholar, Underhill et al., 1999Underhill D.M. Ozinsky A. Hajjar A.M. Stevens A. Wilson C.B. Bassetti M. Aderem A. The Toll-like receptor 2 is recruited to macrophage phagosomes and discriminates between pathogens.Nature. 1999; 401: 811-815Crossref PubMed Scopus (1170) Google Scholar). However, a consensus is emerging that TLRs function not only in combinations with one another (reviewed in depth in Akira and Takeda, 2004Akira S. Takeda K. Toll-like receptor signalling.Nat. Rev. Immunol. 2004; 4: 499-511Crossref PubMed Scopus (6246) Google Scholar, Underhill and Ozinsky, 2002Underhill D.M. Ozinsky A. Phagocytosis of microbes: complexity in action.Annu. Rev. Immunol. 2002; 20: 825-852Crossref PubMed Scopus (803) Google Scholar) but also with a number of other pattern recognition and phagocytic receptors, thereby adding to the diversity of recognition. One such receptor is Dectin-1, a β-glucan receptor that recognizes the yeast cell wall product zymosan in the context of TLR2 and TLR6, as surmised from colocalization of these three molecules in the contact points around the zymosan particle (Gantner et al., 2003Gantner B.N. Simmons R.M. Canavera S.J. Akira S. Underhill D.M. Collaborative induction of inflammatory responses by dectin-1 and Toll-like receptor 2.J. Exp. Med. 2003; 197: 1107-1117Crossref PubMed Scopus (1269) Google Scholar). Recent work has begun to decipher the complex signaling cascades that allow Dectin-1 to trigger diverse responses to yeast (Rogers et al., 2005Rogers N.C. Slack E.C. Edwards A.D. Nolte M.A. Schulz O. Schweighoffer E. Williams D.L. Gordon S. Tybulewicz V.L. Brown G.D. Reis E.S.C. Syk-Dependent Cytokine Induction by Dectin-1 Reveals a Novel Pattern Recognition Pathway for C Type Lectins.Immunity. 2005; 22: 507-517Abstract Full Text Full Text PDF PubMed Scopus (676) Google Scholar). Upon ligation by zymosan, the protein tyrosine kinase Syk is recruited to the immunoreceptor tyrosine-based activation motif (ITAM) contained within the intracellular domain of Dectin-1 and stimulates production of IL10. This cytokine profile contrasts with that produced by DCs when costimulated via Dectin-1 and TLR2 to induce IL12 and TNF. These divergent outcomes are similar to those of Fc receptors (FcR) that also signal via immunoreceptor tyrosine-based motifs (see below). This interesting study provides important information concerning the molecular basis for the diverse responses possible after ligation of a single receptor and it is likely that other receptors that act to fine tune responses to pathogens will also have similar ligand-dependent intracellular signaling cascades. However, it is important to put the recognition of these particles in physiological context, and it should be noted that both Candida albicans and zymosan are excellent targets for three potential opsonins: complement, mannose binding lectin (MBL), and, in the primed host, antibody (see below). It is likely that these also contribute to the complexity of ligand recognition. To provide specific immunological meaning to phagocytosis is a particular challenge for multi-ligand receptors (including scavenger receptors) because of their broad specificity and capacity to bind a wide variety of pathogens. The ligands for these receptors are varied and include pathogen-derived LTA, LPS, and other lipopeptides. In addition, and a common emerging theme amongst multi-ligand receptors is their ability to recognize “modified self” such as β-amyloid, oxidized and acetylated lipid, and apoptotic cells. The immunological outcome of engagement of these receptors is diverse. It is not fully understood whether this depends on the receptor engaged, the cargo internalized, or whether phagocytic receptors subtly modify “hard-wired” signaling from other receptors, such as TLRs, that are also engaged during phagocytosis. CD36 is a prime example of such diversification of response from a single receptor; it is required for nonphlogistic recognition of apoptotic cells (Savill et al., 1992Savill J. Hogg N. Ren Y. Haslett C. Thrombospondin cooperates with CD36 and the vitronectin receptor in macrophage recognition of neutrophils undergoing apoptosis.J. Clin. Invest. 1992; 90: 1513-1522Crossref PubMed Scopus (659) Google Scholar) but has recently been shown to also contribute to LTA-dependent triggering of TLR2 (Hoebe et al., 2005Hoebe K. Georgel P. Rutschmann S. Du X. Mudd S. Crozat K. Sovath S. Shamel L. Hartung T. Zahringer U. Beutler B. CD36 is a sensor of diacylglycerides.Nature. 2005; 433: 523-527Crossref PubMed Scopus (679) Google Scholar). A similar paradoxical role is also true for scavenger receptor A (SRA), which is required for both response to pathogen invasion (Suzuki et al., 1997Suzuki H. Kurihara Y. Takeya M. Kamada N. Kataoka M. Jishage K. Ueda O. Sakaguchi H. Higashi T. Suzuki T. et al.A role for macrophage scavenger receptors in atherosclerosis and susceptibility to infection.Nature. 1997; 386: 292-296Crossref PubMed Scopus (969) Google Scholar, Thomas et al., 2000Thomas C.A. Li Y. Kodama T. Suzuki H. Silverstein S.C. El Khoury J. Protection from lethal gram-positive infection by macrophage scavenger receptor-dependent phagocytosis.J. Exp. Med. 2000; 191: 147-156Crossref PubMed Scopus (220) Google Scholar) and recognition of dying cells (Platt and Gordon, 1998Platt N. Gordon S. Scavenger receptors: diverse activities and promiscuous binding of polyanionic ligands.Chem. Biol. 1998; 5: R193-R203Abstract Full Text PDF PubMed Scopus (98) Google Scholar). How CD36, CD14, SRA, and other such molecules can be involved in both proinflammatory and anti-inflammatory recognition remains to be defined, but it’s likely that it is a result of distinct combinations of signaling molecules that associate with these receptors with different ligands (as discussed for Dectin-1 and Fc receptors) or due to events that occur after internalization such as signaling initiated from within the phagosome or the cytosol as discussed below. Opsonization coats the target and allows generic receptors to mediate engulfment, thereby increasing efficiency and diversifying the recognition repertoire of the phagocyte (Ezekowitz et al., 1984Ezekowitz R.A. Sim R.B. Hill M. Gordon S. Local opsonization by secreted macrophage complement components. Role of receptors for complement in uptake of zymosan.J. Exp. Med. 1984; 159: 244-260Crossref PubMed Scopus (141) Google Scholar). This is of particular importance for particles that are not immediate ligands for phagocytic receptors. In this regard the MBL is an important circulating opsonin that has a carboxy-terminal lectin domain and associates as multimers of trimers. This structure allows MBL to function as a canonical circulating pattern-recognition molecule, able to recognize a broad range of infectious agents ranging from bacteria, yeasts, parasites, and the envelope glycoproteins of certain viruses as well as apoptotic cells. The recent generation of MBL null animals has confirmed the in vivo importance of this molecule in recognition of S. aureus and apoptotic cells (Shi et al., 2004Shi L. Takahashi K. Dundee J. Shahroor-Karni S. Thiel S. Jensenius J.C. Gad F. Hamblin M.R. Sastry K.N. Ezekowitz R.A. Mannose-binding lectin-deficient mice are susceptible to infection with Staphylococcus aureus.J. Exp. Med. 2004; 199: 1379-1390Crossref PubMed Scopus (230) Google Scholar, Stuart et al., 2005Stuart L.M. Takahashi K. Shi L. Savill J. Ezekowitz R.A. Mannose-binding lectin-deficient mice display defective apoptotic cell clearance but no autoimmune phenotype.J. Immunol. 2005; 174: 3220-3226PubMed Google Scholar). MBL ligand interactions trigger activation of the complement cascade, and, thus, clearance of MBL ligand complexes by phagocytes may occur either via complement receptor CR3 or via so-called collectin receptors. The engagement of complement receptors triggers a distinct form of Rho-dependent phagocytosis, characterized by the “sinking” of the particle into the cell without triggering proinflammatory mediators (Aderem et al., 1985Aderem A.A. Wright S.D. Silverstein S.C. Cohn Z.A. Ligated complement receptors do not activate the arachidonic acid cascade in resident peritoneal macrophages.J. Exp. Med. 1985; 161: 617-622Crossref PubMed Scopus (81) Google Scholar). What is not clear is whether a critical density or distribution of the cleaved third complement component, C3bi (the ligand for CR3), is required to trigger a predominantly CR3-mediated effector function. It is likely that the same particle or pathogenic agent that fixes complement might also trigger an antibody response and thus become opsonized and engage one of the FcRs. The ligation of FcR induces Rac activation and pseudopodia formation (Caron and Hall, 1998Caron E. Hall A. Identification of two distinct mechanisms of phagocytosis controlled by different Rho GTPases.Science. 1998; 282: 1717-1721Crossref PubMed Scopus (768) Google Scholar). The diversity of Fc-mediated phagocytosis and signal transduction has been well defined and is the subject of several excellent reviews (Aderem and Underhill, 1999Aderem A. Underhill D.M. Mechanisms of phagocytosis in macrophages.Annu. Rev. Immunol. 1999; 17: 593-623Crossref PubMed Scopus (1940) Google Scholar, Ravetch and Bolland, 2001Ravetch J.V. Bolland S. IgG Fc receptors.Annu. Rev. Immunol. 2001; 19: 275-290Crossref PubMed Scopus (1303) Google Scholar). It is clear from these extensive studies that for certain classes of FcRs, proinflammatory signaling is initiated via an ITAM whereas others signal via an inhibitory motif (ITIM) to downregulate responses. Importantly, the regulation of this system provides a paradigm as to how receptors ligated by the same ligand (antibody) might evoke divergent responses. The ability of complement and antibody to define the response to phagocytosis is an important feature for many opsonins. This is exemplified by thrombospondin, which acts to bridge malaria-infected erythrocytes, apoptotic cells, and other ligands to its receptors, CD36, CD47, and integrin αvβ3 (CD51/CD61). This multifunctional molecule has numerous, distinct structural domains, providing it with a broad spectrum of biological activities including the ability to activate the important immunoregulatory cytokine, TGFβ (Crawford et al., 1998Crawford S.E. Stellmach V. Murphy-Ullrich J.E. Ribeiro S.M. Lawler J. Hynes R.O. Boivin G.P. Bouck N. Thrombospondin-1 is a major activator of TGF-beta1 in vivo.Cell. 1998; 93: 1159-1170Abstract Full Text Full Text PDF PubMed Scopus (941) Google Scholar). Similar functions have recently been ascribed to another family of opsonins, the lung collectins SPA and SPD. These molecules engage either the ITIM-containing molecule SIRPα via their globular heads to downregulate response or, via the collagenous “tail,” activate phagocytes through a CD91-calreticulin complex (Gardai et al., 2003Gardai S.J. Xiao Y.Q. Dickinson M. Nick J.A. Voelker D.R. Greene K.E. Henson P.M. By binding SIRPalpha or calreticulin/CD91, lung collectins act as dual function surveillance molecules to suppress or enhance inflammation.Cell. 2003; 115: 13-23Abstract Full Text Full Text PDF PubMed Scopus (543) Google Scholar). There is further complexity as numerous other molecules including the pentraxins, MFGE8, Gas6, matrix components like mindin, and coagulation factors such as protein S and fibrinogen mediate the humoral arm of phagocytosis (Anderson et al., 2003Anderson H.A. Maylock C.A. Williams J.A. Paweletz C.P. Shu H. Shacter E. Serum-derived protein S binds to phosphatidylserine and stimulates the phagocytosis of apoptotic cells.Nat. Immunol. 2003; 4: 87-91Crossref PubMed Scopus (303) Google Scholar, Hanayama et al., 2002Hanayama R. Tanaka M. Miwa K. Shinohara A. Iwamatsu A. Nagata S. Identification of a factor that links apoptotic cells to phagocytes.Nature. 2002; 417: 182-187Crossref PubMed Scopus (971) Google Scholar, He et al., 2004He Y.W. Li H. Zhang J. Hsu C.L. Lin E. Zhang N. Guo J. Forbush K.A. Bevan M.J. The extracellular matrix protein mindin is a pattern-recognition molecule for microbial pathogens.Nat. Immunol. 2004; 5: 88-97Crossref PubMed Scopus (131) Google Scholar). Interestingly, the phagocytes that utilize them actively secrete many opsonins. It is reasonable to suggest that proteomic analysis of serum is likely to reveal many more potential opsonins. The vast arrays of opsonins may not only facilitate engulfment but also define the responses after phagocytosis. However, several critical questions remain as to the mechanism of this “editing” of response. For instance, how does the phagocyte come to reconcile multiple ligands with similar methods of molecular recognition but apparently contrasting downstream consequence? In addition, what determines the destination of the phagocytosed cargo and how is this linked to the secretion profile that ensues? It seems unlikely that a particle engages only one receptor on the cell surface, and, normally, an array of receptors will interact with a specific pathogen. The involvement of a number of receptors is consistent either with sequential recognition or simultaneous recognition by a multimolecular complex. A precedent for cooperative, sequential recognition of a ligand has been established for LPS, the strongly proinflammatory component shed from the outer wall of gram-negative organisms. LPS is recognized by a low-specificity but high-affinity interaction with LPS binding protein (LBP) and CD14, which in turn act to deliver the ligand first to MD2 and then TLR4 to trigger signaling (Figure 2A ). The second (and not mutually exclusive) model is that a phagocytic synapse, broadly analogous to the T cell synapse (Figure 2B), might associate numerous molecules into a complex to mediate recognition. The T cell synapse is the point of contact of the T cell with an antigen-presenting cell (APC) and consists of low-specificity, high-affinity interactions between LFA-1 and ICAM-1 that mediate initial attachment, surrounded by a ring of lower affinity interactions between the TCR and its cognate MHC-peptide complex. Maturation of the synapse rearranges the position of these molecules and provides a scaffold for optimal TCR stimulation. It is conceivable that similar events occur during phagocytic recognition and would be important in coordinating intracellular signaling cascades (Figure 2B). In support of this, many molecules that cooperate in LPS signaling including CD11b/CD18, CD14, CD16, and CD36 exist in close proximity in the cell membrane, as indicated by fluorescence resonance energy transfer (FRET) between molecules upon ligation with LPS (Pfeiffer et al., 2001Pfeiffer A. Bottcher A. Orso E. Kapinsky M. Nagy P. Bodnar A. Spreitzer I. Liebisch G. Drobnik W. Gempel K. et al.Lipopolysaccharide and ceramide docking to CD14 provokes ligand-specific receptor clustering in rafts.Eur. J. Immunol. 2001; 31: 3153-3164Crossref PubMed Scopus (266) Google Scholar). Using similar techniques, it has also been shown that hsp70, hsp90, CXCR4, GDF5 and TLR4 localize to the site of CD14-LPS ligation within the lipid rafts (Triantafilou et al., 2002Triantafilou M. Miyake K. Golenbock D.T. Triantafilou K. Mediators of innate immune recognition of bacteria concentrate in lipid rafts and facilitate lipopolysaccharide-induced cell activation.J. Cell Sci. 2002; 115: 2603-2611Crossref PubMed Google Scholar), further emphasizing the potential complexity of the phagocytic interface. Together these data provide evidence for a model in which low-specificity receptors such as scavenger receptors and integrins “scan” the targets and mediate the initial interaction before more specific but lower affinity signaling receptors, such as TLRs, are recruited to the core of the phagocytic synapse. Importantly, although we have alluded to a complex of receptors, it is important to appreciate that it is likely to be a highly dynamic structure, undergoing constant remodeling during the process of internalization. (A) Sequential Recognition and engulfment of E. coli is initially mediated by high-affinity, low-specificity interaction wi
Referência(s)