Probing fetal heart, liver, and thymus gene expression in congenital PRRSV infection: effect of fetal thymic viral load

Porcine reproductive and respiratory syndrome (PRRS) is economically devastating to pork producers worldwide. Our goal is to identify critical tissues and genes that forecast fetal resistance, resilience or susceptibility. We hypothesized that targeted gene expression may elucidate mechanisms involved in resistance and resilience, observed in fetuses which did not exhibit infection or maintained low viral load (VL), respectively. In this study, pregnant gilts ( N = 27) were infected with PRRSV at day 85 ± 0.9 of gestation. At 21 days post maternal inoculation, the gilts and fetuses were euthanized, and fetal tissues collected for investigation. A subset of fetuses was selected based on PRRS VL in placenta (PLC), fetal serum (SER), and fetal thymus (THY). We compared six distinct fetal groups: control (CTRL) from mock infected sows; and, from infected sows, uninfected (UNIF; 0.0 log VL in PLC, SER, THY); PLCO (VL only in PLC); PLCSER (VL in PLC, SER); THYL (VL in PLC, SER, low VL THY < 2.0 log VL), and THYM (VL in PLC, SER, medium VL THY > 2.5 log VL). RNA was extracted from fetal heart, liver, and thymus and differential gene expression (DEG) evaluated based on 168 gene targets and 12 groupings, hypothesized to be involved in fetal outcome (PRRS resistance, resilience or susceptibility), using NanoString transcriptomics. Our data revealed that anti-viral responses were only observed when the VL was > 2.5 log VL per μg in the thymus; almost no DEGs were found even with high fetal serum VL (PLCSER). In THYM vs CTRL fetal VL category, the heart had a relatively robust response with 34/168 DEG, liver followed with 12/168 DEG, whereas the thymus exhibited only 2/168 DEG. These results raise uncertainty regarding the primary role of the thymus in the fetus’ early anti-viral response. In contrast, the results indicate early responsiveness to the virus, as evidenced by differential gene expression in non-lymphoid tissues, as critical for fetal survival.