Endocytosis, intracellular sorting, and processing of exosomes by dendritic cells

Abstract

IntroductionDendritic cells (DCs) are antigen (Ag)-presenting cells (APCs) that function as biosensors of the cellular microenvironment by detecting the presence of signals that determine T-cell tolerance or immunity. 1,2 To accomplish this task, DCs acquire extracellular Ags by receptor-mediated endocytosis, macropinocytosis, or phagocytosis [3][4][5] ; by incorporation of microvesicles shed from the surface of neighboring cells, 6,7 and by their recently described interaction with nanovesicles (Յ 100 nm) termed "exosomes." [8][9][10][11][12] Exosomes are formed by reverse budding of the membrane of late endosomes [13][14][15] or multivesicular bodies (MVBs) and are released to the extracellular space by fusion of MVB with the plasma membrane. [13][14][15] Originally described in neoplastic cell lines, 16 exosomes also are produced by leukocytes and epithelial cells. [17][18][19][20][21][22] Although the function of exosomes still is poorly understood, exosomes are a source of Ag for APCs and participate in Ag presentation to T lymphocytes. 11,12 High concentrations of exosomes expressing major histocompatibility complex (MHC) and costimulatory molecules activate T-cell clones and T-cell lines weakly 10,13 and fail to stimulate naive T cells. 9,11 This impaired naive T-cell stimulatory ability of exosomes has been attributed to their low T-cell receptor-cross-linking capacity (inadequate for naive T-cell activation) and their small size and membrane composition. 10 However, in the presence of DCs, exosomes increase their ability to stimulate T cells. 10,11,23,24 The mechanism of interaction of extracellular exosomes with DCs is unknown. Although there is evidence that exosomes may transfer functional MHC-I/peptide complexes to DCs, 24 it is unclear whether exosomes cluster or fuse with DCs or if they are internalized and processed, as occurs with vesicles derived from apoptotic cells. [2][3][4][5] Herein we demonstrate that exosomes are internalized efficiently by DCs. Targeting of exosomes to DCs depends on ligands on the exosome and DC surface and is independent of complement factors. Once internalized by DCs, exosomes are sorted into recycling endosomes and then through late endosomes/lysosomes. By this mechanism, DCs process and present peptides derived from the internalized exosomes to T cells. In vivo, blood-borne exosomes are captured by DCs and specialized phagocytes of the spleen and by hepatic Kupffer cells. In the steady state, uptake of circulating exosomes by splenic DCs does not induce DC maturation and does not prevent CD40-induced DC activation in vivo. Our results demonstrate that blood-borne allogeneic exosomes are efficiently targeted, internalized, and processed by splenic DCs in vivo, a phenomenon followed by presentation of exosome-derived allopeptides by CD8␣ ϩ DCs to CD4 ϩ T cells. Since allogeneic exosomes are a rich source of alloMHC and are targeted and processed in vivo by host DCs (without inducing their activation), intravenous administration of donor-derived exosomes may constitut...

Exosomes as intercellular signalosomes and pharmacological effectors

“…A diminution of exosome uptake by dendritic cells was observed in-vivo by inhibition of v3 integrins, CD11a and its ligands CD54, antibodies directed against the tetraspanins CD9 and CD81, or a soluble analogue of phosphatidylserine (PS) [71]. Thus exosomes adhesion to the recipient cell membrane are mediated through lipids (PS) and ligand-receptor interactions.…”

Circulating TNFR1 exosome-like vesicles partition with the LDL fraction of human plasma

“…We next assessed whether membrane proteins that are known to be present in exosomes, such as LAMP-1, LAMP-2, ICAM-1, and transferrin receptor (TfR), also co-segregate with the 48-kDa TNFR1 exosome-like vesicles in the LDL fraction of human plasma [13][14][15][16]. As shown in Figure 4A, LAMP-1, LAMP-2, ICAM-1 and TfR co-segregated with the HDL fraction and were not detected in the LDL fraction.…”

Exosomes in Viral Disease

“…First, exosomes can either be internalized or captured at the cell surface where they remain [4]. Both of these outcomes have been observed in dendritic cells [50]. Segura et al [51] observed CD8 + dendritic cells capture exosomes via dendritic cell (DC) lymphocyte function-associated antigen 1 (LFA-1) at the surface for presentation of exosome-borne major histocompatibility complex (MHC)-peptide complexes to CD4 + T cells.…”

“…This process was dependent on intracellular adhesion molecule 1 (ICAM-1) expression on exosomes and is termed Bcross-dressing^ [55]. Another method by which DCs could capture exosomal antigens is through internalization, processing, and repackaging of endocytosed exosomal antigens into the endosomal pathway for representation on DC MHC II to naïve CD4 + T cells [50]. This process was documented in elegant experiments by Morelli et al [50].…”

“…Another method by which DCs could capture exosomal antigens is through internalization, processing, and repackaging of endocytosed exosomal antigens into the endosomal pathway for representation on DC MHC II to naïve CD4 + T cells [50]. This process was documented in elegant experiments by Morelli et al [50]. Activation of CD8 + T cells is also possible but requires the presence of DCs capable of Bcross-presentation^, that is, internalization of exosomal antigens followed by processing and representation on MHC I.…”