Placental trophoblasts form the interface between the fetal and maternal environments and serve to limit the maternal–fetal spread of viruses. Here we show that cultured primary human placental trophoblasts are highly resistant to infection by a number of viruses and, importantly, confer this resistance to nonplacental recipient cells by exosome-mediated delivery of specific microRNAs (miRNAs). We show that miRNA members of the chromosome 19 miRNA cluster, which are almost exclusively expressed in the human placenta, are packaged within trophoblast-derived exosomes and attenuate viral replication in recipient cells by the induction of autophagy. Together, our findings identify an unprecedented paracrine and/or systemic function of placental trophoblasts that uses exosome-mediated transfer of a unique set of placental-specific effector miRNAs to directly communicate with placental or maternal target cells and regulate their immunity to viral infections.

Porcine reproductive and respiratory syndrome virus (PRRSV) infection of swine leads to a serious disease characterized by a delayed and defective adaptive immune response. It is hypothesized that a suboptimal innate immune response is responsible for the disease pathogenesis. In the study presented here we tested this hypothesis and identified several nonstructural proteins (NSPs) with innate immune evasion properties encoded by the PRRS viral genome. Four of the total ten PRRSV NSPs tested were found to have strong to moderate inhibitory effects on beta interferon (IFN-β) promoter activation. The strongest inhibitory effect was exhibited by NSP1 followed by, NSP2, NSP11, and NSP4. We focused on NSP1α and NSP1β (self-cleavage products of NSP1 during virus infection) and NSP11, three NSPs with strong inhibitory activity. All of three proteins, when expressed stably in cell lines, strongly inhibited double-stranded RNA (dsRNA) signaling pathways. NSP1β was found to inhibit both IFN regulatory factor 3 (IRF3)- and NF-κB-dependent gene induction by dsRNA and Sendai virus. Mechanistically, the dsRNA-induced phosphorylation and nuclear translocation of IRF3 were strongly inhibited by NSP1β. Moreover, when tested in a porcine myelomonocytic cell line, NSP1β inhibited Sendai virus-mediated activation of porcine IFN-β promoter activity. We propose that this NSP1β-mediated subversion of the host innate immune response plays an important role in PRRSV pathogenesis.