Most, if not all, types of cells secrete extracellular vesicles (EVs) for intercellular communication (Théry et al. 2009). EVs influence recipient cells by delivering their cargoes under physiological and diverse pathological conditions (Yáñez-Mó et al. 2015). EVs are categorized into three main subtypes based on their biogenesis. Exosomes (40–160 nm diameter) originate from multivesicular endosomes (MVEs) (Kalluri and LeBleu 2020); microvesicles (ectosomes, 50–1000 nm) are generated by outward budding and fission from the cell membrane (Stahl and Raposo 2019); and apoptotic EVs (500–2000 nm) are released as fragments of cells undergoing apoptosis (Crescitelli et al. 2013). Based on their size, exosomes are classified as small EVs (sEVs), whereas microvesicles and apoptotic EVs are classified as large EVs (lEVs) (Jeppesen et al. 2019). EVs selectively transport their bioactive molecular cargoes (i.e., nucleic acids [DNAs and RNAs], proteins, metabolites, and lipids) to recipient cells (Valadi et al., 2007, Skog et al., 2008, Choi et al., 2013). The mechanisms of the transport, uptake, and intracellular signaling of EVs in recipient cells likely depend on their cell-specific properties and cargoes (Mathieu et al. 2019). EVs are physiological mediators of cell–cell communication (Yáñez-Mó et al., 2015, Maas et al., 2017). They also promote disease progression by facilitating the dissemination of pathological cargoes from dysfunctional cells (Carretero-González et al., 2018, Rajagopal and Harikumar, 2018, Cheng and Hill, 2022). The abilities of EVs to facilitate assessment of the molecular mechanisms of diseases and diagnose diverse pathological conditions are important in medical biotechnology and drug development (Rabinowits et al., 2009, Matsuzaka and Yashiro, 2022). In terms of cargoes, microRNAs (miRNAs) are small non-coding RNAs that mediate cell–cell crosstalk (Valadi et al., 2007, Kim et al., 2009). EV-mediated transfer of miRNAs induces the degradation and/or translational repression of target mRNAs by binding their 3′-untranslated regions (3′-UTRs) in recipient cells (Montecalvo et al., 2012, Alexander et al., 2015).

In the human placenta, trophoblast-derived EVs, including exosomes, are detectable as early as the fourth week of pregnancy (Dumont et al. 2017). Trophoblast exosomes are continuously synthesized and released into the maternal circulation throughout the gestational period (Sabapatha et al., 2006, Salomon et al., 2014, Dumont et al., 2017). By transporting immunomodulatory proteins [e.g., MHC class I-related chain, FASLG, and TNFSF10 (Hedlund et al., 2009, Atay et al., 2011, Stenqvist et al., 2013)] and being transferred to maternal cells, trophoblast exosomes promote feto-maternal immunotolerance (Tong and Chamley, 2015, Nair and Salomon, 2018). Furthermore, changes in trophoblast exosome levels in maternal blood reflect functional impairment of the placenta, leading to pregnancy complications (e.g., preeclampsia) (Salomon et al. 2017). Trophoblast exosomes carry miRNAs, including primate-specific chromosome 19 miRNA cluster (C19MC) miRNAs, which are almost exclusively expressed in the placenta (placenta-specific miRNAs) (Luo et al., 2009, Noguer-Dance et al., 2010, Donker et al., 2012). Exosomal placenta-specific miRNAs may regulate the migration of trophoblasts in the remodeling of the maternal uterine vasculature during pregnancy (Takahashi et al. 2017) and modulate the gene expression profiles of maternal immune cells (Kambe et al. 2014). By inducing autophagy, exosomal placenta-specific miRNAs confer resistance to viruses on recipient nonplacental cells (Delorme-Axford et al., 2013, Hamilton et al., 2021). Furthermore, exosomal placenta-specific miRNAs in maternal blood during pregnancy enable monitoring of the pathophysiological condition of the placenta (e.g., preeclampsia and gestational hypertension) (Hromadnikova et al. 2019). Therefore, trophoblast exosomes have potential not only as mediators of placenta–maternal cell communication but also as predictive/diagnostic biomarkers for complications of pregnancy.

Exomeres, a novel subpopulation of cargo carriers, were discovered using the asymmetric flow field-flow fractionation (AF4) method (Zhang et al. 2018). Unlike EVs, exomeres (< 50 nm) are non-vesicular extracellular nanoparticles (EPs) that lack a lipid bilayer membrane. Exomeres, which are secreted by some cancer cells and are found in blood, have physiological and pathological functions (Zhang et al., 2018, Zhang et al., 2019, Zhang et al., 2021). The molecular properties of exomeres are distinct from those of EVs. For example, exomeres from some cancer cells are selectively enriched for proteins involved in glycolysis and the mTORC1 metabolic pathway (Zhang et al. 2018). However, little is known of the characteristics of placental trophoblast-derived exomeres.

In this study, we characterized trophoblast-derived exomeres and investigated the cell–cell communication of placenta-specific miRNAs in those exomeres using an in vitro model system (BeWo trophoblasts as a source of exomeres and Jurkat T cells as recipient cells). We isolated exomeres from BeWo cells by the sequential ultracentrifugation method of Zhang et al. (Zhang et al. 2019). Surprisingly, placenta-specific miRNAs were significantly enriched in exomeres compared to exosomes. Moreover, transfer of BeWo exomere-derived placenta-specific miRNAs modulated the expression of the mRNAs targeted by those miRNAs in recipient Jurkat cells.