The roles of extracellular vesicles in the immune system

Abstract

The twenty-first century has witnessed major developments in the field of extracellular vesicle (EV) research, including significant steps towards defining standard criteria for the separation and detection of EVs. The recent recognition that EVs have the potential to function as biomarkers or as therapeutic tools has attracted even greater attention to their study. With this progress in mind, an updated comprehensive overview of the roles of EVs in the immune system is timely. This Review summarizes the roles of EVs in basic processes of innate and adaptive immunity, including inflammation, antigen presentation, and the development and activation of B cells and T cells. It also highlights key progress related to deciphering the roles of EVs in antimicrobial defence and in allergic, autoimmune and antitumour immune responses. It ends with a focus on the relevance of EVs to immunotherapy and vaccination, drawing attention to ongoing or recently completed clinical trials that aim to harness the therapeutic potential of EVs.

Fig. 1. Heterogeneity of extracellular vesicles.

Extracellular vesicles (EVs) are heterogeneous, phospholipid membrane-enclosed structures. Two main types of EV are distinguished based on their biogenesis, known as exosomes and ectosomes. Exosomes are small EVs of endosomal origin released by the exocytosis of multivesicular bodies (MVBs) and amphisomes. Amphisomes are formed by the fusion of autophagosomes and MVBs. By contrast, ectosomes are generated by plasma membrane budding and blebbing. Of note, some ectosomes may also carry endosomal cargo components. Ectosomes include small-sized EVs (such as small ectosomes and arrestin domain-containing protein 1-mediated microvesicles), medium-sized microvesicles and the larger-sized apoptotic bodies. Viruses can also bud from the plasma membrane or can be released from MVBs. En bloc-released virus clusters represent a novel type of large EV similar to the en bloc-released MVB-like EV clusters produced by tumour cells. Oncosomes are large EVs produced by tumour cells. Long protrusions of migrating cells give rise to EVs such as migrasomes, which detach from the end of the long retraction fibres of migrating cells. Secreted midbody remnants are released upon completion of cytokinesis by dividing cells. A special type of ectosome, known as ciliary ectosomes, are shed from the plasma membrane of cilia. Beaded apoptopodia release apoptotic vesicles during apoptosis. Neutrophils rolling on the vascular endothelium leave behind elongated neutrophil-derived structures (ENDs), which later round up. Cytoplasts are large remnants of neutrophils undergoing non-lytic NETosis (not shown). Follicular dendritic cells have long filiform processes from which a beading mechanism gives rise to iccosomes. In the immune synapse, T cell microvilli are fragmented by a similar beading process to give rise to EVs known as T cell microvilli particles (TMPs). Exophers are large vesicles hanging at the end of a stalk that contain damaged organelles and protein aggregates. Secretory autophagosomes are also released by cells. Of note, in the extracellular space, non-EV nanoparticles, such as exomeres, supermeres and T cell-derived supramolecular attack particles, are also present (not shown). These nanoparticles are distinguished from EVs by their smaller size and by the lack of a phospholipid bilayer membrane surrounding them. The biogenesis of non-EV nanoparticles remains to be explored.

Fig. 2. The role of extracellular vesicles in antigen presentation.

a | Extracellular vesicles (EVs) can present antigen on their surface MHC molecules directly to T cells. A more efficient form of semi-direct antigen presentation, known as cross-dressing, takes place when EVs attach to (or are possibly recycled to) the surface of dendritic cells (DCs), in which case the DC plasma membrane concentrates a large number of EV-associated peptide–MHC complexes for efficient immune synapse formation. Endocytic uptake of EVs by DCs leads to the intracellular processing of EV-associated antigens and peptides and their indirect presentation by the DC. b | Cross-presentation of MHC class I-restricted antigens to tumour-specific CD8⁺ T cells occurs when migratory DCs from the tumour microenvironment migrate to the draining lymph nodes and transmit tumour antigens to conventional DCs (cDCs) in the lymph nodes by synaptic vesicle transfer. c | Cross-presentation to CD8⁺ T cells can also be mediated by plasmacytoid DC-derived EVs and requires the uptake of EVs by cDC1 cells. It remains to be clarified if the cross-presentation by cDCs involves a process similar to cross-dressing or if it occurs through EV uptake and processing for indirect presentation. d | Platelet-derived EVs carry functional 20S proteasomes that can generate peptides from exogenously delivered proteins such as ovalbumin (OVA); these peptides are subsequently loaded onto the MHC class I molecules of the platelet-derived EVs and cross-presented to CD8⁺ T cells. Platelet-derived EVs can thus function as complex antigen-presenting units. Platelet-derived EVs also have co-stimulatory molecules (CD40, CD40L and OX40L) on their surface. TCR, T cell receptor.

Fig. 3. Immunoregulatory functions of extracellular vesicles.

Immunoregulatory molecules on the surface of extracellular vesicles (EVs), including the immune-checkpoint molecules programmed death ligand 1 (PDL1) and cytotoxic T lymphocyte antigen 4 (CTLA4) and the apoptosis-inducing ligands FASL and TNF-related apoptosis inducing ligand (TRAIL), interact with cognate ligands and receptors expressed by T cells and natural killer (NK) cells to inhibit their activity or induce apoptosis. The ectoenzymes CD39 and CD73 generate adenosine from ATP, which impairs cytotoxic T lymphocyte (CTL) responses and antigen presentation by dendritic cells (DCs). Regulatory T cell-derived EVs contain EV-associated microRNAs (miRNAs) that suppress CD4⁺ T cell responses (such as miR-155, Let7b and Let7d) or modulate cytokine production by DCs (such as miR-150-5p and miR-142-3p). The immunosuppressive cytokine transforming growth factor-β (TGFβ), which associates with betaglycan on the surface of EVs, activates regulatory T cells and myeloid derived suppressor cells (MDSCs) and downregulates expression of the activating receptor NKG2D on NK cells. EVs carrying MICA and MICB, which are ligands for NKG2D, can also lead to its downregulation on NK cells.

Fig. 4. Examples of antitumour effects of extracellular vesicles released by genetically engineered cells.

a | Chimeric antigen receptor (CAR) T cells release extracellular vesicles (EVs) carrying surface CARs. These EVs also contain perforin and granzyme B and can cause tumour cell death upon recognition of the CAR-specific tumour antigen. b | CAR T cells can be engineered to express the pattern recognition receptor agonist endogenous RN7SL1 RNA. RN7SL1-containing EVs derived from these CAR T cells are efficiently taken up by myeloid cells, in which RN7SL1 activates signalling through the pattern recognition receptors retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5). This inhibits the development of myeloid derived suppressor cells (MDSCs) and increases the co-stimulatory and antigen-presenting capacity of dendritic cells (DCs) in the tumour microenvironment to enhance antitumour immune responses. c | ‘Smart EVs’ were obtained by engineering the producing cells to release EVs with exofacial CD62L (also known as L-selectin), an adhesion molecule for leukocyte homing to lymph nodes, and OX40L, a co-stimulatory molecule that suppresses the differentiation and activity of regulatory T (T_reg) cells. These smart EVs home to lymph nodes upon subcutaneous injection into mice, where they interact with lymphatic endothelial cells. They also facilitate the activation of antitumour effector T cells and inhibit T_reg cells through OX40–OX40L interactions in the tumour-draining lymph nodes.