A novel population of extracellular vesicles smaller than exosomes promotes cell proliferation

Background: Extracellular vesicles (EVs) play important roles in intercellular communication by delivering RNA, lipid, and proteins to neighboring or distant cells. Identification and classification of EVs secreted from diverse cell types are essential for understanding their signaling properties.

Methods: In this study, EVs from the culture media were isolated by ultracentrifugation and analyzed by electron microscopy (EM) and nanoparticle tracking analyses. Conditioned media (CM) from HEK293 cells culture grown either in serum-free (SF) or 10% fetal bovine serum (FBS) containing media were centrifuged at 100,000×g to separate the SN_Δ supernatant and the P100 pellet in which exosomes are enriched. Then, the SN_Δ fraction was centrifuged at 200,000×g to yield the P200 pellet fraction containing novel EVs smaller than exosomes. The exosomal markers in the EV subgroups were examined by western blotting and immune-EM, and the functional analyses of EVs were conducted on HEK293 and THP-1 cell culture.

Results: We identified a new group of EVs in the P200 fraction that was smaller than exosomes in size. Typical exosome markers such as Hsp70, TSG101, and CD63 were found in both P100 exosomes and the P200 vesicles, but CD81 was highly enriched in exosomes but not in the P200 vesicles. Furthermore, chemicals that inhibit the major exosome production pathway did not decrease the level of P200 vesicles. Therefore, these small EVs indeed belong to a distinguished group of EVs. Exosomes and the P200 vesicles were found in CM of human cell lines as well as FBS. Addition of the exosomes and the P200 vesicles to human cell cultures enhanced exosome production and cell proliferation, respectively.

Conclusions: Our study identifies a novel population of EVs present in the P200 fraction. This EV population is distinguished from exosomes in size, protein contents, and biogenesis pathway. Furthermore, exosomes promote their own production whereas the P200 vesicles support cell proliferation. In sum, we report a new group of EVs that are distinct physically, biologically and functionally from exosomes.