**1. Introduction**

Cellular communication is essential for all life forms to observe, comprehend and affect their surroundings [1–6]. One pathway that cells employ for the transfer of information is the use of extracellular vesicles (EVs) – lipid-bilayered secreted vesicles that carry lipids, nucleic acids and proteins that can cause physiological changes in other cells. The use of EVs for cellular communications is a highly conserved process

of life. The EV secretion was observed in all types of cells and organisms studied up to date, including plants [7–9], prokaryotes [3, 10, 11] and protozoans [12–21]. Moreover, evidence suggests that EVs can affect cells of different species, even across different kingdoms [10, 11, 13, 16, 20]. Cross-kingdom EV interactions were shown to take part in the pathogenesis of some parasitic diseases such as those caused by protozoan parasites [22, 23].

Protozoan parasites, also known as first animals, are single-celled organisms that display diversity among unicellular eukaryotic organisms with a complex life cycle on the host system [20]. They have developed many strategies not only to provide their survival and reproduction, but also to enable the invasion into the hosts by means of immune strategies including change in host antigens, development of self-tolerance, immune inactivation, immunosuppression and intervention of molecule-mimetic mechanisms between parasites and host antigens [16, 24, 25]. Recent studies propose that the parasites actually utilize the extracellular vesicles as one infection strategy [18, 20, 21, 26–31], where the questions are arisen on how EVs modulate the host immune system and ultimately cause the infection. Based on the cell of origin, the release mechanisms of EVs from different protozoan parasites, including Apicomplexa and Kinetoplastids such as *Leishmania* species (spp.) [22, 23, 26, 32–35], *Plasmodium spp.* [31, 36–41], *Toxoplasma spp.* [36, 42, 43] and *Trypanosoma spp.* [44–49] were described, where the parasitic infections were studied in detail for leishmaniasis, malaria, toxoplasmosis and Chagas disease independently.

Among the many species and subspecies of protozoa, *Leishmania* are digenetic intracellular protozoan parasite that cause leishmaniasis through the localization either in mononuclear phagocytes of vertebrates as amastigote form or in the sandfly vector as promastigote form. There are three main forms of leishmaniasis, including a localized form- cutaneous leishmaniasis (CL) or mucocutaneous leishmaniasis (MCL), and a life-threatening form – visceral leishmaniasis (VL) (also known as "Kala-azar") [50].

The EVs released from parasites or infected cells play a significant role in hostpathogen communications and thus contribute to pathogenesis [12, 13, 15, 16, 18–21, 51]. Studies indicated that *Leishmania* exosomes can modulate the host immune system through monocyte cytokine production occurring in response to *Leishmania* infection, which in return further exacerbates *Leishmania* infection [14, 21–23, 26, 32–35, 52–54]. Likewise, Evs' role in the occurrence of infection was also confirmed later for more protozoan family members such as *Plasmodium spp.* [31, 36–41], *Toxoplasma spp.* [36, 42, 43] and *Trypanosoma spp.* [44–49], which further directed the attention of researchers on protozoan EVs and their mechanism of action.

This chapter largely focuses on the role of EVs in *Leishmania*-host interaction, immunomodulation of the host immune system by *Leishmania* EVs, manipulation of the cellular microenvironment in favor of *Leishmania* species. In addition, the role of EVs in the pathogenesis of other protozoan parasites including *Plasmodium spp., Toxoplasma spp.* and *Trypanosoma spp.* are discussed and compared at the biological level to get a better insight on strategies in immunomodulation mechanisms. At the end of the chapter, novel and potential immunotherapeutic approaches utilizing the strategies of protozoan EVs are briefly discussed.
