Rab17 Mediates Differential Antigen Sorting Following Efferocytosis and Phagocytosis
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Citation: Cell Death and Disease (2016) 7, e2529; doi:10.1038/cddis.2016.431 OPEN Official journal of the Cell Death Differentiation Association www.nature.com/cddis Rab17 mediates differential antigen sorting following efferocytosis and phagocytosis Charles Yin1, Yohan Kim1, Dean Argintaru1 and Bryan Heit*,1 Macrophages engulf and destroy pathogens (phagocytosis) and apoptotic cells (efferocytosis), and can subsequently initiate adaptive immune responses by presenting antigens derived from engulfed materials. Both phagocytosis and efferocytosis share a common degradative pathway in which the target is engulfed into a membrane-bound vesicle, respectively, termed the phagosome and efferosome, where they are degraded by sequential fusion with endosomes and lysosomes. Despite this shared maturation pathway, macrophages are immunogenic following phagocytosis but not efferocytosis, indicating that differential processing or trafficking of antigens must occur. Mass spectrometry and immunofluorescence microscopy of efferosomes and phagosomes in macrophages demonstrated that efferosomes lacked the proteins required for antigen presentation and instead recruited the recycling regulator Rab17. As a result, degraded materials from efferosomes bypassed the MHC class II loading compartment via the recycling endosome – a process not observed in phagosomes. Combined, these results indicate that macrophages prevent presentation of apoptotic cell-derived antigens by preferentially trafficking efferocytosed, but not phagocytosed, materials away from the MHC class II loading compartment via the recycling endosome pathway. Cell Death and Disease (2016) 7, e2529; doi:10.1038/cddis.2016.431; published online 22 December 2016 Clearance of apoptotic cells in a timely and efficient manner is maturation of efferosomes appears to share much of the essential for preventing the induction of inflammation and the same cellular machinery.19 In both processes the target maintenance of homeostasis.1,2 The process of apoptotic cell microbe or apoptotic cell is internalized into a plasma clearance, termed efferocytosis, is performed by both profes- membrane-derived vacuole, respectively, termed the phago- sional phagocytes such as macrophages3,4 and dendritic some and efferosome, where these engulfed targets are cells,5,6 and by some non-phagocytic cell types such as degraded. The earliest maturation event following the inter- epithelial cells.7,8 Apoptotic cells that are not cleared undergo nalization of phagocytic and efferocytic cargos is the recruit- secondary necrosis, driving inflammation and autoimmunity ment of the small GTPase Rab5 to the nascent phagosome or through the release of self-antigens and proinflammatory efferosome, a process driven by the Rab5 GTPase-activating intracellular contents into the extracellular milieu.9 Efferocy- protein Gapex-5.20 Once activated, Rab5 mediates the fusion tosis is a particularly important physiological function of of early endosomes to the phagosome or efferosome via the macrophages, with defective macrophage efferocytosis Rab5 effectors EEA-1, Vps-34 and Mon1a/b.19,21 Within implicated in the development of a number autoimmune minutes of engulfment Rab5 is exchanged for Rab7, upon and inflammatory disorders including systemic lupus which Rab7 then mediates the fusion of late endosomes and erythematosus10–12 and atherosclerosis.6,13–16 Despite lysosomes with the phagosome or efferosome. While this recognition of the importance of efferocytosis in the main- latter portion of the efferocytosis pathway has not been fully tenance of homeostasis, the mechanisms regulating effer- characterized, it is believed to proceed in a manner identical to ocytosis remain poorly elucidated. It is thought that the that of phagocytosis. In phagocytosis, inactive GDP-bound molecular mechanisms of efferocytosis resembles those of Rab7 is exchanged for Rab5 by a complex of Mon1a/b and phagocytosis, with the important distinction that the ultimate Ccz-1, and is then activated by the exchange of GDP for GTP, – outcome of phagocytosis is the induction of inflammation and a process induced by either Ccz-1 or the HOPS complex.21 23 antigen presentation, whereas efferocytosis is immunologi- Active Rab7, through effectors such as RILP and ORPL1, cally silent.2,10,17 While a number of ligands, mediators and coordinates the movement of lysosomes along microtubules receptors that regulate efferocytosis have been identified and and their subsequent fusion with the phagosome, thereby characterized,10 little is known of the maturation process that delivering the vacuolar ATPase and catabolic enzymes that – degrades efferocytosed cells, or the processes that determine mediate the degradation of phagocytosed materials.24 26 the ultimate fate of degraded apoptotic cells. Ultimately, the phagolysosome containing this degraded In phagocytosis, the phagosome maturation process phagocytosed material matures into a tubular lysosome in responsible for the degradation of engulfed bacteria is well which the loading of MHC class II with phagocytosed antigens characterized (reviewed in Flannagan et al.18), and the occurs.27 1Department of Microbiology and Immunology and The Centre for Human Immunology, The University of Western Ontario, Schulich School of Medicine and Dentistry, London, ON, Canada N6A 5C1 *Corresponding author: B Heit, Department of Microbiology and Immunology, The University of Western Ontario, Schulich School of Medicine and Dentistry, London, ON, Canada N6A 5C1. Tel: +1 519 661 3407; Fax: +1 519 661 3499; E-mail: [email protected] Received 01.9.16; revised 17.11.16; accepted 18.11.16; Edited by G Raschella' Phagosome cargo sorting by Rab17 C Yin et al 2 The consequence of phagosome maturation is the induction 5 μm diameter synthetic phagocytic or efferocytic targets. of both inflammatory signaling and antigen presentation by the Both efferosomes and phagosomes sequentially recruited – phagocyte.28 30 In contrast, efferosome maturation is known Rab5 and Rab7, with efferosomes transitioning from a to be anti-inflammatory and non-immunogenic. Immediately Rab5-positive to Rab7-positive compartment with slightly following the efferocytic uptake of apoptotic cells, PPARγ and slower temporal dynamics than phagosomes (Figures 1c and the atypical chemokine receptor D6 initiate signaling, which d). To confirm that both phagosomes and efferosomes were suppresses early inflammatory processes such as the completing maturation, we immunostained human PBMC- oxidative burst.4,31–33 This suppression of inflammation is derived M0 macrophages for the lysosomal marker LAMP1 maintained over intermediary periods of time through tran- 40 min after engulfment of synthetic phagocytic or efferocytic scriptional processes initiated by these receptors. The targets (Figure 1e). Significant LAMP1 accumulation was upregulation of mIR-21 following efferocytosis inhibits inflam- observed on phagosomes and efferosomes, confirming that matory signaling by silencing PDCD4 and PI3K, resulting in vacuoles containing both types of cargo fuse with lysosomes. decreased TNFα expression through p38 MAPK-dependent Last, we assessed the possibility that macrophage polariza- signaling, while simultaneously enhancing IL-10 production tion may result in the selective uptake of phagocytic versus and further uptake of apoptotic cells through increased PI3K/ efferocytic targets (Figure 1f). M0 and M2 macrophages were AP-1 signaling.34,35 In addition to inducing mIR-21, continued highly efferocytic, while M1 macrophages and primary human signaling through PPARγ directly drives the expression of anti- dendritic cells were poorly efferocytic. Furthermore, no inflammatory cytokines including IL-10 and TGFβ.33,35 This selectivity was observed for phagocytic targets, indicating anti-inflammatory state can be maintained over prolonged that any capacity to differentially present efferosomal versus periods of time through the polarization of macrophages to an phagosomal antigens must occur following both target anti-inflammatory/pro-efferocytic M2 or M2c state.7,36 While engulfment and the canonical Rab5/Rab7-mediated matura- these processes contribute to creating an anti-inflammatory tion pathway. environment, it remains unclear how immunogenicity is avoided following efferocytosis. Tissue-resident macrophages Mass spectrometric identification of late regulators of constitutively express MHC class II and costimulatory mole- efferosome and phagosome maturation. Given that cules, as do in vitro unpolarized (M0) and M2-polarized efferosomes and phagosomes shared the same early macrophages.37–39 Indeed, M2 macrophages are fully com- maturation pathway, any selective processing of these targets petent to present antigens, and can induce naive CD4+ T cells likely occurs at a later time point. As such, we recovered to differentiate into Th17 effectors.40 Clearly, additional efferosomes and phagosomes using 3 μm diameter magnetic mechanisms must be in place to prevent the immunogenic bead mimics of efferocytic and phagocytic targets from M0- presentation of apoptotic cell-derived antigens. In this study, polarized human macrophages, 40 min after initiation of we tested the hypothesis that efferocytosed apoptotic cells efferocytosis or phagocytosis. Proteins from the recovered undergo a novel maturation process that bypasses the MHC efferosomes and phagosomes were resolved using SDS- class II loading compartment. Using mass spectrometry and PAGE, revealing a number of proteins selectively recruited to fluorescence