Exosomes in meconium mediate horizontal transmission and immune evasion of reticuloendotheliosis virus

Reticuloendotheliosis virus (REV) is an immunosuppressive virus that can be vertically transmitted through chicken embryos. Infected chicks continuously shed the virus in their meconium and feces, facilitating horizontal transmission. Even in flocks with positive maternal antibodies, horizontal transmission can still occur. It is hypothesized that REV-associated exosomes present in the meconium of chicks mediate this horizontal transmission and immune evasion. In this study, exosomes were extracted from the meconium of chicks that tested positive for REV after the virus was experimentally introduced to chicken embryos. Through techniques including electron microscopy, genomic sequencing, and proteomic analysis, it was confirmed that these exosomes contain the complete REV genome and three major viral proteins. DF-1 cells were inoculated with REV-positive meconium exosomes, either directly or after neutralization with specific REV antibodies. In both cases, REV infection was detected in the cells via indirect immunofluorescence assay (IFA), confirming that REV-positive meconium exosomes can evade antibody neutralization and establish infection in vitro. chicks with positive maternal antibodies against REV and antibody-negative chicks were inoculated with REV. Parameters including body weight, organ somatic index, and viral replication were monitored at different time points. The results demonstrated that inoculation with REV caused severe growth retardation and hepatosplenomegaly in antibody-negative chicks. In contrast, the presence of maternal antibodies significantly attenuated the pathogenicity of the virus through neutralizing activity. However, when REV-positive meconium exosomes were inoculated to both maternal antibody-positive and antibody-negative chicks using the same protocol, no significant differences were observed in the relevant pathogenicity indicators between the two groups. This demonstrates that maternal antibodies are unable to effectively inhibit the pathogenic effects mediated by REV-positive meconium exosomes. This study characterized REV-positive meconium exosomes and demonstrated their ability to mediate effective infection both in vivo and in vitro, unaffected by antibodies. It is hypothesized that meconium exosomes represent one of the pathways through which REV achieves early horizontal transmission and immune evasion.