In addition to adjuvants, EVs can also deliver immunomodulators. Small RNAs (smRNA), such as miRNA and small interfering RNA (siRNA), hold promise for cancer immunotherapy due to their targeted gene silencing via RNA interference. But its effectiveness can be limited by tumor heterogeneity, immune suppression, and potential off-target effects. EVs as nanocarriers can improve cytosolic delivery of smRNAs and protect them from degradation because of their liquid bilayer. Currently, 3 approaches have been developed for delivering smRNAs via EVs. (a) Exogeneous smRNAs can be loaded into EVs via mechanical methods. (b) Genetically modified donor cells can be engineered to express endogenous smRNAs, which are then sorted into EVs. (c) EVs with cancer immunotherapeutic potential can be directly harvested from stem cell culture supernatant. While optimizing loading efficiency remains an area of ongoing research, studies already demonstrated the effectiveness of smRNA-loaded EVs in cancer immunotherapy (
Figure E). A study explored miR-130-loaded EVs derived from 4T1 cells to reprogram M2 macrophages to the antitumor M1 phenotype. These EVs up-regulated MA markers [CD86, Irf5, Nos2, tumor necrosis factor-α (TNF-α), and IL-1β] and down-regulated M2 markers (CD206, Ym1, Arg, TGF-β, and IL-10). Consequently, the reprogrammed macrophages inhibited the migration and invasion of 4T1 cells [
112]. Another approach involves miR-155-loaded EVs derived from CT26 cells to enhance DC maturation and antibody presentation. miR-155 delivery significantly up-regulated MHC-II, CD86, CD40, and CD83 on DC membranes, indicating successful DC maturation. In addition, miR-155 increased the expression levels of IL12p70 and interferon-γ (IFN-γ), further promoting antitumor immunity [
113]. A recent study explored a new approach using M1 macrophage-derived EVs loaded with anti-PD-L1 siRNA. To enhance delivery, these EVs were decorated with vesicular stomatitis virus glycoprotein, a pH-responsive viral fusogen. This approach effectively silenced PD-L1 expression, leading to increased CTL population and repolarization of M2 macrophages to the M1 phenotype, demonstrating the potential to overcome immune checkpoint suppression [
114]. siRNA-loaded EVs can also modulate immune responses. L929 EVs carrying anti-TGF-β1 siRNA inhibited tumor development and induced apoptosis [
115]. In another study, EVs containing anti-VEGFR (vascular endothelial growth factor receptor) siRNA showed higher antitumor effect and lower toxicity compared to free drug, apatinib, for lung metastatic osteosarcoma (25% survival at day 80 versus 0% at day 58) [
116]. In addition to the exogeneous loading of smRNAs into EVs, engineered donor cells can directly secrete EVs containing endogenous smRNAs. In brief, donor cells undergo genetic modification, enabling specific smRNAs to be overexpressed and encapsulated into EVs through intracellular cargo sorting. For example, human miR-371b-5p sequence was inserted into the LAMP2A gene, enabling HEK293T cells to continuously generate EVs carrying endogenous miR-371b-5p [
117]. In animal models of osteosarcoma, these engineered EVs inhibited tumor growth and extended overall survival. Furthermore, EVs derived from stem cells naturally containing certain smRNAs possess cancer immunotherapeutic effects. A study found that mesenchymal stem cell-derived EVs containing miR-133b can suppress glioma progression via Wnt/β-catenin signaling pathway [
118]. Mesenchymal stem cell-derived EVs containing miR-3940-5p inhibits colorectal cancer metastasis by targeting integrin α6 [
119]. Bone mesenchymal stem cell-derived EVs containing miR-512-5p can inhibit glioblastoma progression by targeting JAG1 [
120]. miR-199a-expressing EVs from adipose tissue-derived mesenchymal stem cells [
121], miR-30b-5p-expressing EVs from bone mesenchymal stem cells [
122], miR-320a-expressing EVs from human umbilical cord mesenchymal stem cells [
123], and many other examples indicate that these EVs from stem cells can be used for cancer immunotherapy. Despite the promise of stem cell-derived EVs in cancer immunotherapy, their heterogeneity and substantial cargo variations raise concerns about therapeutic efficacy and reproducibility. The same concern also exists with genetically modified donor cell-derived EVs that carry certain smRNAs.