Perspective Chapter: Exosome-Mediated Pathogen Transmission

Abstract

Exosomes are membrane-bound vesicles. They are considered as waste-management system of cells, crucial for intercellular communication of information and have emerged to be mediators of pathogen transmission. Pathogen derived exosomes advance infections by suppression of host immune response, transmission of pathogen-related molecules and immune evasion. The ability of exosomes derived from the virus infected cells to modulate the host immune response and/or further viral replication in the host has been reported in several viruses infecting human and animals. Apart from the virus infected cells, parasites have also known to release exosomes, parasite derived exosomes help in the attachment of parasite to the host and facilitate evasion of host immune responses. Tick-derived exosomes aid transmission of vector-borne pathogens. Similar to certain viral and parasitic infections, exosomes derived from bacteria infected cells could also play a key role in dissemination of the infection. An understanding of the exosome mediated pathogen transmission, its pathway and host-pathogen interactions could pave way to discovery of novel therapeutic targets.

1. Introduction

Exosomes are small extracellular vesicles ranging from 50 to 100 nm, that were first described in the late 1980s as “garbage bags” for cells to dispose the unwanted material and cellular waste from the cytosol. However, it has ever since become clear that they play a much broader role in intercellular communication by transferring bioactive molecules between cells [1]. Exosomes are composed of diverse bioactive molecules, such as proteins, lipids, and nucleic acids, such as DNA and RNA. These molecules can be taken up by other cells and influence cellular behavior, making exosomes a potentially important mode of intercellular communication. The two mechanisms of exosome biogenesis are the ESCRT-dependent and ESCRT-independent pathways [2]. The ESCRT-dependent pathway utilizes the endosomal sorting complexes required for transport (ESCRT) machinery which consists of several protein complexes (ESCRT-0, -I, -II, and -III) that recognize and cluster cargo molecules on the endosomal membrane and facilitate the budding of intraluminal vesicles (ILVs) within the lumen of late endosomes or multivesicular bodies (MVBs). After the formation of MVBs containing (ILVs), the MVBs either fuse with lysosomes for degradation or fuse with the plasma membrane for exosome release. The ESCRT-independent pathway, on the other hand, does not require the ESCRT machinery for cargo sorting and ILV formation. Instead, it involves the direct budding of the plasma membrane to form exosomes. This process is thought to be mediated by lipid rafts and tetraspanin-enriched microdomains on the plasma membrane, which recruit specific cargo molecules and drive the formation of small membrane vesicles [3, 4]. The resulting vesicles are then released into the extracellular space as exosomes. Exosomes are known to play a crucial role in infections as carriers of pathogen-related molecules. Microorganisms such as bacteria, Protozoa, and fungi have been found to secrete various types of microvesicles, including exosomes, which are used by pathogens to spread infection and evade the host immune system. In addition to these microorganisms, viruses, have been shown to stimulate the production of exosomes in host cells, which in turn can regulate the host immune response [5]. Exosomes can directly transmit substances of pathogen origin and also indirectly influence the progression of infection by modulating processes such as immune evasion and apoptosis (Figure 1). Thus, the study of microvesicles and their role in host-pathogen interactions is an important area of research that could lead to the development of new therapeutics for infectious diseases.

Figure 1.

Exosomes mediate further infection. Exosomes mediate further infection through transferring pathogen-related molecules (pathogenic genes and proteins) or even the entire pathogens. Therefore, exosomes can be either directly infectious, alter nuclear gene expression, or mediate toxic reactions.

2. Exosome-mediated parasite transmission

Exosome research in parasite infections is particularly intriguing because it suggests that the communication between the host and the parasite via exosomes may play a key role in pathogenesis. Exosomal vesicles are an important component of microbial communication and can facilitate the exchange of genetic material, which can have significant implications for microbial evolution and adaptation [6].

2.1 Haemoprotozoan parasites

Studies have shown that promastigote and amastigote forms of Leishmania donovani and Leishmania major can release exosomes, which are detected in host cells and selectively induce IL-8 secretion from macrophages [7, 8]. The chemokinetic recruitment of neutrophils helps Leishmania invade cells and gain access to macrophages upon phagocytosis of the infected neutrophils. This process is thought to occur through the release of chemoattractants by the infected macrophages, which then recruit neutrophils to the site of infection [9, 10]. This suggests that exosomes released by Leishmania species may play a role in modulating the host immune response and contributing to the pathogenesis of leishmaniasis. Research has demonstrated that Leishmania exosomes can induce the release of the immunosuppressive cytokine IL-10 while inhibiting the inflammatory cytokine tumor necrosis factor (TNF) in human monocyte-derived dendritic cells (DCs) in response to interferon gamma (IFNg). Dendritic cells are a type of immune cell that play a crucial role in initiating and regulating immune responses. The inhibition of TNF and induction of IL-10 by Leishmania exosomes can have important implications for the ability of the immune system to effectively respond to and clear Leishmania infections. TNF is a pro-inflammatory cytokine that helps to recruit immune cells to sites of infection and activate their antimicrobial functions, while IL-10 is an immunosuppressive cytokine that can dampen immune responses and promote the persistence of pathogens [9]. Several protozoan parasites, including Leishmania species and T. cruzi, have been shown to release exosomes and/or microvesicles [11, 12, 13]. Leishmania species are the parasites that cause human leishmaniasis, while T. cruzi causes Chagas disease. Similarly, studies have shown that T. cruzi can release exosomes that contain parasite-derived molecules, such as proteins and nucleic acids, which can modulate the host immune response and aid in parasite survival and proliferation. T. cruzi-derived exosomes have also been shown to induce pro-inflammatory cytokine production and apoptosis in host cells [14]. T. cruzi-derived exosomes have also been shown to contain immunomodulatory molecules, including miRNAs, which can regulate theexpression of host immune genes and contribute to the pathogenesis of Chagas disease. T. cruzi induces the release of exosomes from infected host cells, which expresses TGF-β, which has proven to facilitate parasite invasion and maturation in host cells [15]. The exosomes are known to protect extracellular life cycle stages of T. cruzi, such as epimastigotes from the vector and trypomastigotes from ruptured cells, from complement-mediated attack, facilitating parasite invasion of host cells [16]. The secretion of exosomes by Leishmania spp. and T. cruzi induce the release of exosomes from the cells that they infect [7]. Extracellular vesicles (EVs) have been shown to play a role in intercellular communication between parasites. Recent studies have shown that microvesicles play a crucial role in the transmission of malaria caused by the Plasmodium falciparum parasite (DEBS). These microvesicles are small membrane-bound particles that are released by infected red blood cells and can interact with uninfected cells in the vicinity. It has been found that these microvesicles contain specific molecules that can influence the behavior of the parasite. In particular, they can increase the commitment of asexual parasites to differentiate into sexual stages, known as gametocytes. This is important for the transmission of the parasite, as only the sexual stages can be transmitted to mosquitoes and therefore continue the life cycle of the parasite. By increasing the production of gametocytes, the microvesicles can effectively enhance the transmission potential of the parasite, making it more likely to be passed on to mosquitoes and therefore increase the spread of malaria [17, 18].

2.1.1 Plasmodium falciparum

Malaria parasite Plasmodium falciparum has been found to use exosomes for communication between infected red blood cells. This communication between infected and uninfected red blood cells via exosomes is thought to play a key role in the pathogenesis of malaria [7]. Exosomal vesicles released from P. falciparum infected erythrocytes have been shown to help in parasite survival, transmission, density sensing and differentiation of gametocytes [19, 20, 21]. Plasmodium falciparum-infected RBCs (iRBCs) can communicate with each other via different mechanisms, including the exchange of genetic material through a process called cell-cell transfer or tunneling nanotubes (TNTs). This communication can result in the increased production of gametocytes, which are the parasite’s sexual forms that can be transmitted to mosquito vectors and infect other hosts. In addition to TNTs, iRBCs can also release exosome-like vesicles that contain different types of cargo, including proteins, lipids, and nucleic acids. These vesicles can be taken up by other iRBCs or host cells, modulating their functions and promoting parasite survival in different environments, such as drug pressure or immune attack. One of the proteins that play a critical role in exosome-like vesicle production in P. falciparum is PfPTP2. This protein is a phosphatase that regulates different signaling pathways in the parasite, including those involved in vesicle biogenesis and secretion. Disrupting PfPTP2 function can reduce exosome-like vesicle production and affect parasite survival and virulence.

2.2 Protozoan parasites

2.2.1 Trichomonas vaginalis

Trichomonas vaginalis, a parasitic protozoan that is responsible for the common sexually transmitted infection trichomoniasis, has been shown to release functional exosomes that play a role in both parasite-to-parasite and parasite-to-host communication. One study published in 2013 showed that T. vaginalis exosomes contained virulence products that could specifically downregulate the secretion of the pro-inflammatory cytokine IL-8 by ectocervical cells [22]. This downregulation of IL-8 secretion could potentially limit neutrophil migration, which in turn could prevent pathogen clearance and facilitate the establishment of infection. Furthermore, T. vaginalis exosomes have been shown to contain a range of other bioactive molecules, including proteins, lipids, and nucleic acids, that are capable of modulating host cell behavior. They are known to induce cell death in host immune cells, impair host cell signaling pathways, and modulate host cell gene expression. The detection of exosomes secreted by T. vaginalis suggests a potential role for these extracellular vesicles in the pathogenesis of trichomoniasis. Furthermore, the detection of specific parasite proteins in T. vaginalis exosomes suggests that these vesicles may also play a role in the parasite’s adherence to host epithelial cells, which is a critical step in the infection process.

2.2.2 Toxoplasma gondii

Toxoplasmosis is known to be caused by Toxoplasma gondii. The human foreskin fibroblasts infected with T. gondii release a type of exosome-like vesicle that contains abundant miRNAs and shows a significant increase in mRNAs compared to uninfected fibroblasts. The mRNAs that are most enriched in these vesicles include thymosin beta 4, eukaryotic elongation factor-1α (EF-1α), Rab-13, and LLP homolog. These mRNAs have been previously associated with neurologic activitysuggesting that T. gondii exosomes may play a role in mediating neurologic effects in toxoplasmosis, a parasitic disease caused by T. gondii [23].

2.3 Helminths

Various helminths, including trematodes like Fasciola hepatica and Echinostoma caproni, secrete exosomes and other extracellular vesicles (EVs) that can be internalized by host cells. Electron microscopy images have been used to study the morphology and distribution of EVs released by these helminths, including those that can be detected on the tegumental surface. The tegument is the outermost layer of the parasite, and it plays a critical role in the host-parasite interaction. By releasing EVs that can interact with the tegument, these helminths may be able to modulate the host immune response and evade host defenses [24]. Exosomes released by Heligmosomoides polygyrus (H. polygyrus), a parasitic helminth, can block the activation of type 2 innate lymphoid cells (ILC2s), which are immune cells that play a critical role in the host response to helminth infections. This blockade of ILC2 activation is thought to contribute to the ability of H. polygyrus to establish chronic infections in its host [25]. Furthermore, H. polygyrus-derived exosomes have downstream effects on eosinophilic recruitment. Eosinophils are immune cells that play a role in the host response to helminth infections, and studies have shown that H. polygyrus-derived exosomes can induce the recruitment of eosinophils to sites of infection. This recruitment is thought to be mediated by the activation of IL-5, a cytokine that plays a role in the production and recruitment of eosinophils. Analyses of the secretion products of the tapeworm E. granulosus have revealed the presence of exosome-associated proteins, including CD63-like tetraspanins. CD63 is a transmembrane protein that is commonly used as a marker of exosomes, and tetraspanins are a family of proteins that are associated with the membrane of exosomes and play a role in their biogenesis and function. The presence of CD63-like tetraspanins in the secretion products of E. granulosus suggests that the parasite is capable of releasing exosomes, which could play a role in the pathogenesis of echinococcosis [26]. Exosomes released by parasites such as Heligmosomoides polygyrus, Schistosoma mansoni, and Schistosoma japonicum have been shown to contain immunomodulatory molecules, including proteins and miRNAs, which can modulate the host immune response and aid in parasite survival and proliferation. Exosome-like extracellular vesicles (EVs) was isolated from excretory-secretory (ES) products of fourth stage larvae (Tci-L4ES) of Telodorsagia circumcincta, a parasitic nematode that affects sheep. Proteomic characterization of these EVs and identified several proteins involved in various functions such as structure and metabolism of the parasite. Importantly, it was found that some of these proteins can be bound by two types of antibodies, IgA and IgG, in T. circumcincta-infected sheep suggesting that these proteins may have potential as vaccine targets for the development and production of a vaccine against T. circumcincta infection [27]. Proteomic analysis could identify proteins carried by extracellular vesicles (EVs) released by tapeworms. Parasite-derived proteins such as antigen-5, severin/gelsolin/villin lipid transport protein, alpha-mannosidase, and malate-dehydrogenase, as well as host-origin proteins such as carbonic anhydrase, fructose-bisphosphate aldolase, peroxiredoxin, hemoglobin alpha and beta, pyruvate kinase, serum albumin, and triose phosphate isomerase were identified in the EVs. The study also revealed that the EVs carried virulence factors, including highly immunogenic and tolerogenic antigens and peptidases, that were associated with cyst survival. This finding suggests that EVs may play a crucial role in tapeworm infection [28].

2.3.1 Filarial parasites

Lymphatic filariasis is a parasitic disease caused by filarial worms, including Brugia malayi, Wuchereria bancrofti, and Brugia timori, which are transmitted through the bites of infected mosquitoes. Studies have suggested that extracellular vesicles (EVs), including exosomes, released by these worms may play a role in the pathogenesis of the disease [29]. Exosome-like vesicles secreted by the infective L3 stage of B. malayi are designated a set of proteins, including actin, EF-1α, EF-2, Rab-1, and HSP70, as exosome markers based on their presence in the vesicles. These proteins are known to be involved in various cellular processes, such as cytoskeletal organization, protein synthesis, and vesicle trafficking, suggesting that the EVs may play a role in modulating the host immune response and promoting parasite survival.

3. Exosome-mediated pathogen transmission by arthropods

Arthropods, such as ticks and mosquitoes, have been shown to release extracellular vesicles (EVs) in their saliva during feeding. EVs are double-layer vesicles that are secreted by all cells and play a critical role in cell-to-cell communication. These vesicles contain various molecules, including proteins, lipids, and nucleic acids, that can be transferred to other cells to influence their behavior. In the context of pathogen transmission, infected cells can secrete EVs that carry infectious cargo, such as viral RNA, which can enhance pathogen transmission and replication. This has been demonstrated in the case of Zika virus, where infected mosquito saliva was found to contain EVs that carry viral RNA and can promote infection in recipient cells. Ticks are ectoparasites that feed on the blood of their hosts, and their saliva contains a complex mixture of proteins, lipids, and other molecules that help them to obtain a blood meal and evade the host immune response. It is likely that EVs are also present in tick saliva and play a role in modulating the host immune response. The argasid tick Ornithodoros moubata secretes immunomodulatory proteins in the saliva. Proteomic analysis of tick saliva has revealed several exosome-associated proteins, such as aldolase and enolase, as well as lipocalins that have anti-inflammatory properties. These lipocalins can scavenge leukotrienes, which are inflammatory mediators, and adenosine nucleotides, which can modulate the immune response. It is reported that exosomes are critical for the transmission life cycle of Langat virus (LGTV), a tick-borne virus closely related to tick-borne encephalitis virus (TBEV), which is a causative agent of a neurological tick-borne disease [30]. A study demonstrated that LGTV can infect tick cells and replicate within them. The virus is then secreted into the extracellular space via. Exosomes, which are taken up by neighboring cells, including both tick and mammalian cells. The exosomes containing LGTV were found to be infectious and could transfer the virus from infected to uninfected cells, indicating that exosomes play a crucial role in LGTV transmission. Furthermore, research shows that the exosomal cargo of LGTV-infected tick cells contained viral RNA and proteins, which could induce an antiviral response in uninfected cells, potentially limiting viral spread. It is also suggested that exosomes derived from neuronal cells are likely able to mediate transmission of tick-borne flavivirus RNA and proteins from one neuronal cell to the other in the CNS. These findings suggest that exosomes play a complex role in the transmission and pathogenesis of LGTV, and potentially other related tick-borne viruses such as TBEV. RNAi-mediated silencing of synaptobrevin expression in A. americanum adult ticks resulted in a significant decrease in feeding success. Specifically, the silenced ticks exhibited increased mortality, premature detachment from the host, and lower engorgement weights compared to control ticks. These findings suggest that synaptobrevin is critical for successful tick feeding and survival [31]. Arthropods such as mosquitoes are known to be important vectors for the transmission of flaviviruses, including DENV. Recent studies have shown that arthropod-derived EVs can contain viral RNA, including full-length viral genomes, and can transfer this RNA to neighboring cells or even to other hosts, potentially leading to the spread of infection. In addition to DENV, other flaviviruses such as Zika virus, Japanese encephalitis virus, and West Nile virus have also been shown to be transmitted by arthropod vectors and may potentially be contained within EVs. The mechanisms by which flaviviruses are packaged into arthropod-derived EVs and how they are transmitted to new hosts are not yet fully understood, and further research is needed to elucidate these processes. However, the discovery of viral RNA in arthropod EVs suggests that these structures may play an important role in the transmission and dissemination of flaviviruses.

4. Exosome mediated fungal transmission

In addition to the parasites, other eukaryotes such as pathogenic fungi also release extracellular vesicles (EVs) that play important role in mediating the pathogenesis. Exosomes can play a role in the proliferation of fungal infections by several mechanisms. Firstly, fungal exosomes can carry virulence factors and antigens that can directly contribute to the pathogenesis of the infection. For example, fungal exosomes have been shown to contain proteins and lipids that promote the adhesion and invasion of host cells, as well as molecules that suppress the immune response and promote the survival of the pathogen within the host. Secondly, exosomes secreted by infected host cells can also indirectly promote the proliferation of fungal infections by modulating immune responses. For instance, exosomes released by infected immune cells can contain cytokines and other immune modulators that suppress the activity of immune cells, such as macrophages and neutrophils, which are crucial for controlling fungal infections. This, in turn, can facilitate the proliferation of the fungus within the host. Moreover, recent studies suggest that exosomes may play a role in the horizontal transfer of antifungal resistance among fungal populations. Fungal exosomes can carry genetic material, such as RNA and DNA, which can be transferred to other fungi, leading to the acquisition of antifungal resistance. Exosomes can proliferate fungal infections by carrying virulence factors, modulating immune responses. For example, the pathogenic fungus Paracoccidioides brasiliensis releases highly immunogenic EVs that contain the carbohydrate galactose-/-1,3-galactose (/-Gal), which is not found in human cells. These/-Gal-enriched EVs may generate a robust immune response in the host, but they may also be beneficial to the pathogen by binding to host lectins and potentially stimulating a suppressive type 2 response. Other opportunistic fungi, including Cryptococcus neoformans, Candida albicans, and Histoplasma capsulatum, also release EVs that contain virulence-associated factors such as polysaccharides and lipids [32]. For example, C. neoformans EVs are enriched in virulent capsular components such as glucosylceramide and glucuronoxylomannan (GXM), and a recent study has shown that phospholipid translocases (flippases) are important for C. neoformans exosome packaging and transport [33]. Interestingly, fungus-released EVs can also induce antimicrobial activity by host cells. C. neoformans EVs are taken up by macrophages and stimulate the production of TNF, IL-10, TGF-b, and nitric oxide [34]. EVs released by Malassezia sympodialis, a component of human flora, can generate IL-4 and TNF secretion from peripheral blood mononuclear cells, enhancing an inflammatory response in cases of atopic dermatitis [35].

5. Exosome mediated bacteria transmission

Extracellular vesicles (EVs) have been identified as a mechanism for dissemination of bacterial components. Gram-negative bacteria such as Escherichia coli and Salmonella have been shown to produce outer membrane vesicles (OMVs) that contain lipopolysaccharide (LPS), a potent endotoxin that can trigger an inflammatory response in host cells. OMVs have also been shown to carry other virulence factors, such as adhesins and toxins, and can promote bacterial survival and dissemination within the host. Similarly, Gram-positive bacteria such as Staphylococcus aureus and Streptococcus pyogenes have been shown to release membrane vesicles that carry lipoteichoic acid (LTA), another pathogen-associated molecular pattern (PAMP) that can activate host immune responses. In addition to OMVs and membrane vesicles, bacteria can also release exosomes, which are thought to originate from the bacterial cytoplasmic membrane and can carry a range of bacterial components, including nucleic acids, proteins, and lipids. Extracellular vesicles (EVs) are a newly described mechanism for bacterial dissemination and can contribute to the pathogenesis of bacterial infections.

5.1 Mycobacterium spp.

In the case of bacterial infections, much of our understanding of exosome production and function comes from studies on Mycobacteria. Additionally, there is increasing evidence that other bacterial species also produce exosomes that contribute to disease pathogenesis. Mycobacterial exosomes have been shown to carry bacterial components that modulate host immune responses and promote bacterial survival, as well as contributing to the dissemination of mycobacteria to other cells and tissues in the host. Mycobacterium avium-infected macrophages release vesicles that can stimulate a pro-inflammatory response in neighboring macrophages that are not infected [36]. Mycobacterium tuberculosis PAMPs can be transported from the phagosome to the MVB during macrophage infection, and these PAMPs are also found in extracellular vesicles released by infected macrophages [37]. These vesicles have been shown to have markers of a late endosomal/lysosomal compartment and are released through calcium-dependent exocytosis, suggesting that they are exosomes [38]. The content of these exosomes can be detected inside neighboring uninfected cells, suggesting a potential role in intercellular communication during infection. The release of pro-inflammatory exosomes has also been observed in macrophages infected with Mycobacterium tuberculosis or Mycobacterium bovis BCG. These exosomes carry mycobacterial components that can stimulate an immune response and contribute to disease pathogenesis. The pro-inflammatory response is thought to be mediated by the activation of pattern recognition receptors (PRRs) on the surface of the macrophages, which recognize the mycobacterial components carried by the exosomes. This activation leads to the production of pro-inflammatory cytokines and chemokines that recruit and activate other immune cells to the site of infection. While mycobacterial exosomes have been shown to stimulate pro-inflammatory responses in macrophages, it is also possible that the mycobacterial components present on or in the exosomes could function to suppress the immune response. Mycobacterial exosomes can carry immunosuppressive components, such as mycobacterial lipids, that can downregulate the immune response and promote bacterial survival within host cells. In addition to carrying immunosuppressive components, mycobacterial exosomes may also promote bacterial persistence by facilitating intercellular communication and promoting the formation of bacterial aggregates within host cells. This can protect the bacteria from immune surveillance and promote their survival within the host. Exosomes have been shown to play a role in anthrax infection by serving as carriers of anthrax toxin components [39]. Tissue factor, is a blood coagulation protein that is also involved in a variety of cellular processes such as cell proliferation, migration, and apoptosis. It has been found on the surface of various cell types, including endothelial cells and macrophages.

5.2 Helicobacter pylori

miRNA expression in exosomes plays a role in the regulation of inflammation in macrophages and can affect the infectivity and pathogenicity of Helicobacter pylori. Specifically, miR-155 expression in exosomes derived from H. pylori-infected macrophages was found to increase significantly and could be delivered to surrounding macrophages to induce a stronger inflammatory response. Moreover, miR-155 loaded in exosomes derived from H. pylori-infected macrophages was found to promote the production of cytokines such as TNF-α, IL-6, and IL-23 to regulate inflammatory responses, thereby enhancing the expressions of cellular signal transduction proteins such as CD40, CD63, CD81, and MHC-I for immune-regulation responses. However, overactive macrophages can produce a multitude of proinflammatory cytokines and chemokines, leading to inflammation-related diseases or autoimmune diseases. During H. pylori infection, exosomes may act as vectors to carry virulence factors or proteins of H. pylori to host cells and target organs, thus playing a role in the pathogenicity of H. pylori [18].

5.3 Bacteroides fragilis

Bacteroides fragilis, a representative strain of Bacteroides spp., has been found to enhance immune function. This is achieved through the transfer of bacterial lipopolysaccharide to intestinal dendritic cells via exosomes. This process promotes the secretion of IL-10 and IL-6 by dendritic cells and the differentiation of T lymphocytes, which in turn intensifies the immune reactions of the host. Exosomes, which are closely associated with bacterial infection, are believed to act as signal transduction messengers [40].

5.4 Other bacteria

It is shown that “microparticles” released from Chlamydia pneumoniae-infected cells contain tissue factor, and that these microparticles can activate NF-κB, a transcription factor involved in the regulation of TF expression in endothelial cells suggesting that Chlamydia pneumoniae may use exosomes or exosome-like vesicles as a mechanism for spreading the infection and modulating host cell responses [37]. Other bacterial species, such as Pseudomonas aeruginosa, Burkholderia cenocepacia, and Staphylococcus aureus, also produce exosomes that carry virulence factors and other bacterial components, which can modulate host immune responses and promote bacterial survival and dissemination. Chlamydia trachomatis is an intracellular bacterial pathogen that causes a variety of diseases in humans, including sexually transmitted infections and ocular infections. To establish and maintain infection, C. trachomatis has evolved several mechanisms to interact with host cells and manipulate host cellular processes. One such mechanism is the release of host cell vesicles that contain bacterial effector proteins. These vesicles can be internalized by neighboring cells, allowing C. trachomatis to spread and establish new infection foci. Several cytotoxic and secreted proteins have been identified in these host vesicles, and they are believed to play a role in the delivery of virulence factors. One such protein is CT166, a cytotoxic protein that has been shown to induce cell death in host cells. Another is CT694, a secreted protein that has been shown to interact with host proteins involved in cell signaling pathways. These proteins, along with others found in host vesicles, likely play a critical role in C. trachomatis pathogenesis by facilitating the delivery of virulence factors and manipulating host cellular processes to the bacterium’s advantage. Exosomes have been found to play a role in the pathogenicity of Staphylococcus aureus, specifically through the actions of the pore-forming α-toxin [41]. This toxin targets human non-virally transformed keratinocytes (HaCaT cells) and can be endocytosed by the cells to prevent cell lysis. The toxin-containing vesicles are then transported to late endosomes and incorporated into exosomes, which are secreted by the cells [42]. Interestingly, these exosomes contain both mono- and multi-meric toxins, which can be activated after being taken up by naive cells. This mechanism allows the bacteria to spread its virulence factors and evade the immune system, ultimately leading to the development of infections.

6. Exosome-mediated viral transmission

Exosomes have been shown to play a role in a range of viral infections, including HIV, Hepatitis B and C, Influenza, and Zika virus, among others. Exosomes can contribute to viral pathogenesis by promoting viral replication and spread, inducing apoptosis in infected cells, and modulating the immune response to favor viral persistence. Additionally, exosomes can serve as vehicles for the transfer of viral components, including nucleic acids and proteins, between cells, facilitating viral spread and potentially contributing to the development of chronic infections. Viruses can hijack the host cell’s exosomal pathway to promote the transfer of viral components, including nucleic acids such as viral RNA or DNA, between cells. Exosomes containing viral genomes can be taken up by susceptible cells, potentially leading to the establishment of a productive viral infection. When a cell is infected with a virus, it may secrete exosomes that contain viral components. These exosomes can then be taken up by other cells, potentially leading to the spread of the virus. Exosomes derived from viral-infected cells can contain a range of viral components, including viral proteins, nucleic acids (such as RNA or DNA), and even intact viruses themselves. These exosomes can therefore serve as a means of exporting viral components from the infected cell, potentially contributing to viral pathogenesis [43, 44]. The viral components contained within exosomes derived from viral-infected cells can contribute to the pathophysiological effects on recipient cells. These effects can be mediated by a variety of mechanisms, including the activation of cellular signaling pathways, the induction of inflammation, and the suppression of antiviral responses.

6.1 Human immunodeficiency virus (HIV)

Exosomes derived from HIV-infected cells have been shown to contain viral proteins that can induce apoptosis (programmed cell death) in recipient cells. Similarly, exosomes derived from cells infected with the Respiratory Syncytial Virus (RSV) have been shown to contain viral proteins that can trigger an inflammatory response in recipient cells. HIV-1 is known to exploit exosomes to facilitate viral spread and evade host immune responses. The transfer of HIV-1 coreceptors CCR5 and CXCR4 within exosomes from infected to uninfected cells is one mechanism by which the virus can enhance its infectivity and spread to new cells. Exosomes from HIV-1-infected cells can transfer viral proteins and RNA to uninfected cells, leading to the activation of host immune responses and the promotion of viral replication and dissemination [45, 46]. In addition to promoting viral spread, exosomes can also serve as a mechanism for the virus to evade host immune surveillance. HIV-1 has been shown to use exosomes to downregulate host immune responses by transferring viral proteins such as Nef and Vpu to immune cells, leading to the degradation of host immune factors such as CD4 and MHC class I molecules [47].

6.2 Hepatitis A virus (HAV)

Exosomes can acquire Hepatitis A Virus (HAV) components after HAV-infected plasmacytoid dendritic cells. These exosomes can protect HAV from neutralization by HAV antibodies and assist in the transmission of HAV among liver cells. Additionally, these HAV-carrying exosomes can also directly invade and infect uninfected cells with modest pathogenicity. In the case of HAV, infected plasmacytoid dendritic cells can release exosomes containing HAV components, which can then be taken up by uninfected liver cells. These exosomes can protect HAV from neutralization by HAV antibodies, allowing the virus to more easily infect liver cells and spread throughout the liver [48].

6.3 Hepatitis C virus (HCV)

In the case of HCV, studies have shown that the virus can incorporate into exosomes either as whole virions or as nucleocapsids, envelope proteins, and replication-competent viral RNA. The mechanism by which HCV incorporates into exosomes and how this process is regulated is not yet fully understood. However, it is believed that the incorporation of HCV into exosomes may help the virus to evade the immune system and spread throughout the body and play a role in the pathogenesis of HCV infection [49]. Hepatitis C virus (HCV) is a small enveloped virus with a positive-sense single-stranded RNA genome, belonging to the Flaviviridae family. Recent research has shown that the assembly and release of HCV virions in hepatocytes are closely correlated with the exosome secretory pathway. This pathway can incorporate either the whole virions or only nucleocapsids, envelope proteins, and replication-competent viral RNA into exosomes. In addition to classical transmission by free viral particles, HCV can also be transferred by exosomes to naive human hepatoma Huh7.5.1 cells, resulting in productive infection with efficiency like that of free infectious particles. Exosomes derived from HCV-infected Huh7.5 cells or individuals both contain miR-122, which promotes HCV replication and transfer. Exosomes can transmit HCV to naive cells and modestly protect antibodies from being neutralized by HCV. This suggests that HCV may use transmission via exosomes as an immune evasion mechanism, allowing it to resist neutralization by anti-HCV antibodies.

6.4 Epstein-Barr virus (EBV)

Epstein-Barr virus (EBV), exosomes are known to play a role in the maintenance of latent infection. EBV is a virus that can cause infectious mononucleosis and is associated with several types of cancer. When EBV infects a cell, it can enter a latent phase in which it remains in the host cell without causing any symptoms. During this phase, the virus can be reactivated and start replicating, leading to the production of new viral particles and the spread of infection. EBV can exploit exosomes to deliver its genetic material, including proteins, RNA, and miRNA, to target cells. This allows the virus to maintain its latent infection in the host by regulating the expression of viral and host genes [50]. Apart from Burkitt lymphoma and nasopharyngeal carcinoma, EBV has also been linked to other malignancies, including Hodgkin’s lymphoma, gastric cancer, and certain types of lymphomas and leukemias. Exosomes released by EBV-infected cells can play a role in the pathogenesis of these diseases by transferring viral proteins, RNA, and miRNA to surrounding cells and tissues. This can lead to the activation of signaling pathways that promote tumor growth and metastasis, as well as the suppression of host immune responses against the virus and cancer cells. Therefore, understanding the role of exosomes in EBV-associated malignancies may provide new insights into the mechanisms of tumor progression and immune evasion, as well as potential targets for therapeutic intervention [51].

6.5 Herpes simplex virus (HSV)

Exosomes derived from Herpes Simplex Virus (HSV) infected cells have been shown to contain viral proteins, RNA, and miRNAs that can be transmitted to uninfected cells and modulate their gene expression to promote viral replication and transmission [52]. The presence of these viral components in exosomes suggests that they may play a role in HPV-mediated immune evasion and tumor progression. Furthermore, the ability of exosomes to transfer their contents to neighboring cells may contribute to the spread of HPV infection. Dias et al. [53]. found that the prion protein (PRNP) plays a role in directing multivesicular bodies (MVBs) containing intraluminal vesicles (ILVs) toward the plasma membrane for the release of exosomes. Specifically, PRNP was shown to interact with components of the endosomal sorting complex required for transport (ESCRT) machinery, which is involved in the formation of ILVs within MVBs. This interaction was found to promote the association of MVBs with the plasma membrane and the subsequent release of exosomes. These findings suggest that PRNP may play a key role in regulating the secretion of exosomes in various physiological and pathological contexts.

6.6 Porcine reproductive and respiratory syndrome virus (PRRSV)

Exosomes derived from Porcine Reproductive and Respiratory Syndrome Virus (PRRSV)-infected cells can contain viral RNAs and transfer productive infections to naive cells, even in the presence of PRRSV-specific neutralizing antibodies (NAbs). PRRSV is a highly contagious virus that causes significant economic losses to the swine industry worldwide. The virus is known to replicate in the respiratory tract and can cause respiratory distress in infected pigs, as well as reproductive failure in pregnant sows. Recent studies have shown that exosomes derived from PRRSV-infected cells can contain viral RNAs, proteins, and even infectious virions. These exosomes can then be taken up by naive cells, which can lead to the establishment of a productive infection. It has been shown that PRRSV-specific NAbs are not effective in neutralizing the virus when it is packaged within exosomes. This suggests that exosomes may provide a mechanism for PRRSV to evade the host immune response and spread the infection to other cells [54].

6.7 West Nile virus (WNV)

It has been demonstrated that exosomes containing mosquito-borne West Nile Virus (WNV) can facilitate the transmission of viral RNA and proteins from one neuronal cell to others, suggesting a potential role for exosomes in WNV neuropathogenesis. West Nile Virus is a neurotropic virus that can cause severe neurological disease in humans and animals. The virus is thought to replicate in neurons and can spread from cell to cell within the nervous system. Recent studies have shown that exosomes derived from WNV-infected cells can contain viral RNA and proteins, which can be transferred to neighboring neuronal cells. This suggests that exosomes may play a role in the spread of WNV within the nervous system [44]. Furthermore, it has been suggested that exosomes may also be involved in the development of WNV neuropathogenesis, as the transfer of viral RNA and proteins to neighboring cells may alter the function of the recipient cells and contribute to disease progression.

7. Conclusion

In conclusion, exosomes play a significant role in mediating pathogen transmission between cells. Through their ability to transfer various types of bioactive molecules, including nucleic acids, proteins, and lipids, exosomes can facilitate the transfer of infectious agents, including bacteria, viruses, and parasites. Exosomes have been shown to act as vectors for the spread of several human pathogens, including HIV, HCV, and prion proteins. In addition, exosomes released from infected cells can promote the spread of infection by suppressing the host immune response and facilitating pathogen replication. However, the mechanisms by which exosomes mediate pathogen transmission are still not fully understood, and further research is needed to better characterize the specific roles of exosomes in the pathogenesis of different infectious diseases. Additionally, the potential use of exosomes as diagnostic markers or therapeutic targets for infectious diseases warrants further investigation. Despite the remaining uncertainties, the emerging evidence suggests that exosome-mediated pathogen transmission is a crucial aspect of infectious disease biology and has significant implications for the development of new diagnostic and therapeutic approaches.