The tumor microenvironment (TME) plays an important role in the development and progression of hematological malignancies. In addition to several factors, such as growth factors, cytokines, extracellular matrix (ECM) molecules, etc., a growing body of evidence has indicated that extracellular vesicles (EVs) play a crucial role in the communication of tumor cells within the TME, thereby contributing to the pathogenesis of hematological malignancies.
Intercellular communication, an essential biological process of multicellular organisms, is mediated by three different mechanisms: (1) cytoplasmic bridges; (2) direct interactions between adjacent cells via membrane proteins; and (3) cellular secretary molecules 
. Recently, a fourth mechanism has been discovered, which includes the intercellular transfer of extracellular vesicles (EVs) 
. EVs are membrane-bound entities released by almost all types of cells into the extracellular environment 
. EVs are known to transport bioactive molecules, such as proteins, lipids, and nucleic acids in the form of DNA, RNA, miRNA (miR), etc., between cells 
. Detectable levels of EVs are found in nearly all biological fluids, such as blood, urine, synovial fluid, and saliva, and they are even found in the interstitial spaces between cells 
. Since they are protected from degradation by extracellular proteases and RNases, EVs can be stably stored for long-term use 
. Depending on their biogenesis, size, release mechanism, content, and function, EVs can be broadly classified into microvesicles, exosomes, and apoptotic bodies (Figure 1
Figure 1. EV biogenesis and uptake by target recipient cells. EVs mainly consist of MVs and exosomes. MVs are generated by plasma membrane outward budding, whereas exosomes are of endocytic origin. Both carry mRNAs, miRNAs, proteins, and other bioactive molecules. EVs are taken up by recipient cells, either by direct fusion with the plasma membrane or by the endocytic pathway. After EV uptake, the EVs’ cargo is released into the recipient cells, hence acting as intercellular communicators between cells.
. MVs are a type of EV generated by direct outward budding of plasma membrane from cells 
. The formation and release of MVs from cells typically require the interplay of cytoskeletal components, such as actin and microtubules; molecular motors, such as kinesin and myosin; and fusion machinery, such as SNAREs and tethering factors 
. MVs typically ranging from 100 nm to 1 µm in diameter 
. Because MVs are generated from plasma membrane by outward budding, they carry cytosolic and plasma membrane-associated proteins, e.g., proteins clustered at the plasma membrane, such as tetraspanins, which could serve as markers for MVs regardless of the originating cells 
. Other cytoskeletal proteins, such as heat shock proteins, integrins, and proteins containing post-translational modifications, such as glycosylation and phosphorylation, have also been shown to be present in MVs 
Exosomes are the other subtype of the EV of endocytic origin 
with a typical diameter of 30 to 150 nm 
. Specifically, exosomes are formed by inward budding of early endosomal membrane, which matures into multivesicular bodies (MVBs) 
. MVBs eventually fuse with the plasma membrane, releasing their content of exosomes into the extracellular space 
. The regulation of MVBs and the formation and subsequent release of exosomes are mediated by the endosomal sorting complexes required for transport (ESCRT) pathway 
. Since exosome generation is mediated by the ESCRT pathway, ESCRT and its accessory proteins (Alix, TSG101, HSC70, and HSP90β) are believed to be present in all exosomes regardless of the type of originating cells and hence serve as exosome markers 
. Other than the ESCRT pathway, exosome generation is also thought to be dependent on sphingomyelinase enzymes since, in some instances, cells with ESCRT deficiency also produce significant numbers of CD63+ exosomes 
. Both exosomes and MVs have been shown to participate actively in cell–cell communication, maintenance of cells, and tumor progression by transporting their cargo between cells 
. EVs are readily taken up by the recipient cells, either by direct fusion with the plasma membrane or by fusion with the endosomal membrane after endocytosis 
Apoptotic bodies, which are in general not considered to be a true form of EV, are larger, ranging from 50 nm to 5 µm in diameter 
, and are released from cells undergoing programmed cell death 
. These bodies are generated by the separation of plasma membrane from the cytoskeleton due to enhanced hydrostatic pressure during contraction of cells 
. In contrast with MVs and exosomes, apoptotic bodies contain cellular organelles, chromatin, and a few glycosylated proteins 
. Hence, higher levels of nuclear proteins (such as histones), mitochondrial proteins (such as HSP60), Golgi, and endoplasmic reticulum-associated proteins (such as GRP78) are expected to be observed in apoptotic bodies.
2. Tumor-Derived EVs
Tumor-derived EVs are distinguished from normal cell-secreted EVs due to the presence of unique tumor-specific ‘labels’ 
. Tumor-derived EVs have been shown to carry oncogenic proteins or nucleic acids (such as DNA, RNA, miRNAs, etc.) which facilitate tumor progression. Oncogenic bioactive molecules are enriched in tumor-derived EVs compared to normal cell-derived EVs 
. For example, chromosome segregation 1 like protein (CSE1L), a transmembrane protein, is enriched in tumor-derived EVs, not only triggering Ras-dependent EVs biogenesis but also promoting metastasis of B16F10 and melanoma cells 
. Adriamycin-resistant breast cancer cell-derived EVs were shown to carry transient receptor potential cation channel subfamily C member 5 (TrpC5) and to transfer of EV. TrpC5 confers endothelial cell resistance against chemotherapeutic regimens 
. On the other hand, the transfer of oncogenic nucleic acids such as miRNAs, specifically miR-221 from highly aggressive breast tumor cells to nonaggressive cancer cells, via EVs, contributing to the promotion of epithelial-to-mesenchymal transition (EMT) 
and leading to the induction of proliferation and metastasis while preventing drug-induced apoptosis of EVs’ fused recipient cells 
. Therefore, tumor cells more often induce oncogenic transformations into normal healthy cells via the transfer of oncogenic bioactive molecules through EVs 
3. Hematological Malignancies
Hematological malignancies are defined as tumors that commence in blood-forming tissues, such as bone marrow or cells of the immune system, resulting in leukemias, lymphomas, and myelomas 
. Hematological malignancies are considered to be among the leading causes of cancer-related deaths worldwide 
. In the United States itself, an estimated 184,710 new cases of hematological neoplasms were reported with 57,380 deaths in 2023, and the incidence increases with age 
. GLOBOCAN 2020 reported non-Hodgkin lymphoma to be the predominant hematological cancer worldwide with 544,352 new cases and 259,793 deaths, followed by leukemia, with 474,519 new cases and 311,594 deaths worldwide 
. The outbreak of COVID-19 further increased the death rate of patients suffering from various hematological malignancies 
EVs are known to play an important role in cell–cell communication via the transfer of bioactive cargo molecules. However, the role of EVs in the crosstalk of tumor cells with cells in the tumor microenvironment (TME) and other distant cells remains to be completely determined in the context of the pathogenesis of hematological malignancies.
4. EVs in Cell-Cell and Cell-Extracellular Matrix Communication in the TME
The TME is the environment surrounding the tumor cells in the body 
. It consists of immune cells, stromal cells, fibroblasts, extracellular matrix (ECM), and cells of the blood and lymphatic vessels 
. Tumor cells and their TME are in constant interaction, thereby regulating each other either positively or negatively 
. Dynamic interaction between cancer cells and TME components not only supports tumor growth and development 
but also promotes local invasion and metastatic dissemination of cancer 
. In hypoxic and acidic conditions, the TME often promotes angiogenesis, a process of restoring nutrient and oxygen supply, as well as removing metabolic waste 
. Additionally, the infiltration of various immune cells into the TME performs various pro- and anti-tumorigenic functions 
. EVs play an important role in intercellular communication via the transfer of bioactive cargo molecules between cells 
. Tumor-derived EVs also act as a communicating vehicle between cancer cells and cells in the TME and in some instances also with distant cells. On the other hand, cells in the TME often release EVs that interact with tumor cells, influencing tumor development and progression.
Effects of hematological malignancy-derived EVs on immune cells.
Immune cells play a major role in the elimination of tumors through diverse mechanisms, and the evasion of immune surveillance serves as an important step for developing tumor niches and successful establishment of tumors. Immune evasion, a strategy facilitated by tumor-derived EVs is utilized in different ways to target various immune cells (Figure 2
). Tumor-derived EVs, through their receptor-mediated uptake, can introduce several suppressive factors, e.g., miRNA, DNA, pro-apoptotic factors, metabolites, and various enzymes, into immune cells (Table 1
). They can also alter the activation of immune cells through inhibitory cell surface receptors 
Figure 2. Effect of EVs generated during hematological malignancies on different immune cells. EVs, derived from tumor cells not only (1) induce apoptosis of T-cells, but also reduce T-cell (2) migration and (3) proliferation. Moreover, tumor-derived EVs (4) decrease the cytotoxicity of NK-cells and (5) restrain the processing of antigen by APCs. On the other hand, tumor-derived EVs (6) induce immunosuppressive functions of MDSCs, as well as (7) promote MDSCs growth. Again, tumor-derived EVs (8) not only prevent the generation of ROS in macrophages but also (9) promote macrophages’ pro-inflammatory response. (10) The differentiation of monocytes into dendritic cells is often perturbed by the incorporation of tumor-derived EVs into monocytes. All of these processes contribute to the development and progression of the tumor.
In activated T-cells, tumor-derived EVs induce the down-regulation of CD3ζ and JAK3 expression via transcriptional regulation, thereby facilitating Fas/FasL-mediated apoptosis of CD8(+) T-cells 
. Chronic lymphocytic leukemia (CLL)-derived EVs were shown to down-regulate CD69 expression in T-cells via miR-363 transfer, thereby affecting effector T-cell migration 
. Tumor cells are often shown to evade the host immune system via the activation of the PD-L1/PD-1 pathway. PD-L1 is expressed on the surfaces of various tumor cells, whereas its receptor, PD-1, is present on T-cells. PD-L1 binding to PD-1 results in the apoptosis of T-cells, thereby evading host immune responses. The upregulation of PD-1 has been observed in various T-cell populations after exposure to diffuse large B cell lymphoma (DLBCL)-derived EVs 
. EVs released from B-cell lymphoma (BCL) under chemotherapy are enriched with CD39 and CD73, and they hydrolyze ATP, which is generated from chemotherapy-treated tumor cells and transformed into adenosine 
, which in turn affects the immune system by inhibiting T-cell activity and proliferation 
Table 1. The effect of EVs, derived from hematological malignancy on immune cell function.