Extracellular vesicles from mesenchymal stromal cells primed with synthetic toll-like receptor 4 agonists treat hematopoietic acute radiation syndrome.

Whole-body exposure to ionizing radiation can lead to cellular DNA damage to bone marrow (BM), causing lethal hematopoietic acute radiation syndrome (H-ARS). Extracellular vesicles (EVs) from human BM-derived mesenchymal stromal cells were primed with CRX-527 (CRX), a synthetic TLR4 agonist, characterized and tested as a radiomitigator therapy. Using a xenogeneic H-ARS mouse model, a single in vivo treatment with CRX-EVs administered 4 or 24 hours after lethal irradiation significantly improved weight loss, clinical scores and prolonged survival compared to control treatments. Ex vivo generation of CRX-EV educated monocytes (CRX-EEMos) were also effective in a H-ARS model when administered 24 hours after lethal irradiation. CRX-EVs or CRX-EEMos significantly promoted hematopoiesis in BM and potentially the spleen, leading to restoration of peripheral complete blood counts. CRX-EEMos showed increased gene expression of IL-6 and IL-10: enriched for PD-L1 but low for CD16 in CD14-expressing monocytes. Antisense inhibition of Let-7 microRNAs in CRX-EEMos suppressed IL-10 gene expression and protein secretion, implicating a novel role for Let-7 in radioprotection. CRX-EVs can effectively treat H-ARS by increasing the secretion of anti-inflammatory molecules while stimulating monocytes to promote hematopoiesis in BM. The potential for large-scale production of CRX-EVs as an "off-the-shelf" treatment for H-ARS makes them a potential medical countermeasure for radiological and nuclear threats.