EVs

The history of research into extracellular vesicles (EVs) is an example of how a single term can hold back the development of an entire field of science. The commonly used term "debris" is a non-specific catch-all term for all undefined extracellular particles, and its negative connotation suggests that all such particles are cellular waste. For a long time, this assumption discouraged scientists from studying extracellular particles in depth, obscuring the discovery of both EVs and non-EV nanoparticles in this compartment. However, after several decades of sporadic observations of extracellular membrane-enclosed structures, the early 2000s saw a renewed focus of research on these EVs, leading to an exponential development of this field of research over the past two decades. The term "extracellular vesicles" was chosen in 2011 as a collective term for lipid double-membrane-enclosed cell-derived particles. EVs are released by all cellular organisms. For example, the release of outer membrane vesicles by Gram-negative bacteria and the release of cytoplasmic membrane vesicles by Gram-positive bacteria and archaea show that the production of EVs is characteristic of all three domains of life (archaea, bacteria and eukaryotes). The broad term "bacterial extracellular vesicles" is increasingly used to refer to all EVs released by bacteria (Buzas EI 2023).

Extracellular vesicles, or EVs for short, are small, heterogeneous phospholipid membrane particles that are released by cells into the extracellular environment. EVs contain proteins, RNA, DNA and are considered potential messengers and diagnostic markers, and depending on their biogenesis, two main types of EVs are distinguished, called exosomes and ectosomes. Exosomes are small EVs of endosomal origin that are released by exocytosis of multivesicular bodies (MVBs) and amphisomes. Amphisomes are formed by the fusion of autophagosomes and MVBs.

Ectosomes are formed by budding and vesicle formation of the plasma membrane. Ectosomes include small EVs (such as small ectosomes and arrestin domain-containing protein-1-mediated microvesicles), medium-sized microvesicles and the larger apoptotic bodies. Viruses can also egress from the plasma membrane or be released from MVBs. En bloc-released virus clusters represent a novel type of large EVs, similar to the en bloc-released MVB-like EV clusters produced by tumor cells. Oncosomes are large EVs produced by tumor cells. Small EVs, which occur in the largest quantities in biological fluids, have a diameter of about 50-150 nm. Medium EVs with a diameter of about 200-800 nm occur in smaller numbers than small EVs, and large EVs (diameter ≥ 1 μm; such as migrasomes, exophores, apoptotic corpuscles, large oncosomes and en bloc released MVB-like small EV clusters are the least abundant population of EVs.

EVs are of great importance for intercellular communication as they transport proteins, lipids and nucleic acids that influence various biological processes, including immune responses, disease progression and potential therapeutic strategies (Wang K et al. 2023; Jeppesen DK et al. 2023).

EVs can be divided into four main types:

• exosomes
• apoptotic corpuscles
• microvesicles
• oncosomes.

Exosomes are of endosomal origin and are released upon fusion of the limiting membrane of multivesicular bodies (MVBs) or amphisomes with the plasma membrane. Recent data suggest the involvement of additional endomembranes (such as the endoplasmic reticulum and the nuclear envelope) in the biogenesis of exosomes. The heterogeneity of EVs is a consequence of the diversity of types and functional states of the releasing cells as well as the different biogenetic pathways. EVs also include vesicles that are formed by different cell death mechanisms (such as apoptosis, necroptosis or pyroptosis) (Buzas EI 2023).

EVGs and immunity: EVs are thought to play an important role in T cell development, with the majority of EVs being released from the thymus by thymic epithelial cells. EVs transport tissue-specific antigens from thymic epithelial cells to conventional dendritic cells (cDC) in the thymus, where they serve for antigen presentation. In addition, thymic epithelial cell-derived EVs play a role in inducing the maturation of (CD4+ or CD8+) thymocytes by transporting proteins involved in their maturation and exit from the thymus. These proteins include: Sphingosine-1-phosphatase lyase 1 (SGPL1), Rho-GDP dissociation inhibitor 1 (GDIR1), Dedicator of cytokinesis protein 2 (DOCK2) and p21 protein-activated kinase 2 (PAK2).

EVs and B cells: In the case of B cell development, immature primary bone marrow B cells have been shown to release antibody-mediated binding of CD24+ plasma membrane-derived EVs. Since CD24 plays a role in B cell development and selection in the bone marrow, it was hypothesized that EVs may affect differentiating B cells. Animal experiments have shown that stimulation of the B-cell receptor (BCR) or CD24 on a mouse B-cell lymphoma cell line with anti-IgM or cross-linked primary and secondary antibodies, respectively, triggered the production of EVs that transported functional BCR and CD24 to recipient B cells. This transfer enabled recipient B cells to respond to new antigen stimuli.

EVs and antigen presentation: The demonstration of the antigen presentation ability of EVs was the first breakthrough discovery showing that EVs could play an important role in adaptive immunity. B cell-derived EVs can transport functional peptide-MHC complexes (pMHC) and present antigens directly to T cells(Buzas EI et al. 2023).

EVs, obesity and psoriasis: In obese patients, EV levels in peripheral blood are significantly higher than in healthy individuals, contributing to increased systemic inflammation. WAT-derived EVs mainly contain adipokines such as IL-6, MCP-1, adiponectin and resistin. Since these adipokines are highly involved in the inflammatory response associated with psoriatic lesions, it is hypothesized that adipose-derived EVs also play a role in the pathological development of psoriasis. Exosomes derived from mesenchymal stem cells of healthy WAT suppress local inflammation in psoriatic lesions and may improve clinical symptoms. This finding supports the assumption that WAT can interact with the skin via EVs. The differences in the composition of EVs released by WAT under different conditions may represent a key regulating factor for the pathological changes observed in psoriatic lesions (Iuliano M et al. 2024).

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