Compartment-specific immune responses to BoAHV-1 transplacental infection in late gestation

Varicellovirus bovinealpha1 (BoAHV-1) is an important cause of reproductive failure in cattle and can induce abortion following transplacental infection during late gestation. However, immune-associated responses at the conceptus (placenta and fetus) remain incompletely understood. This study aimed to characterize maternal and fetal immune-associated responses to BoAHV-1 vertical infection by evaluating the expression of endosomal Toll-like receptors (TLR3, TLR7 and TLR9), cytokines, and the antimicrobial peptide BMAP28, together with the distribution of immune-cell markers in placental and fetal tissues. Pregnant dams (infected, n = 7; controls, n = 4) were experimentally infected with BoAHV-1 during late gestation and euthanized at 15 days post-infection (dpi). Peripheral blood leukocytes (PBLs) were collected at 3 and 10 dpi, and placental and fetal tissue samples were collected during necropsy. Relative mRNA expression of innate immune genes was assessed by RT-qPCR, while immune-cell markers were characterized by immunohistochemistry. Maternal tissues, including PBLs and placental caruncles, showed reduced expression of TLR9 and TNF-α. Fetal lungs showed increased expression of endosomal TLRs, TNF-α, IL-12 and BMAP28, accompanied by increase of CD3⁺ cells and iNOS-associated cells. The fetal spleen showed specific changes in gene expression, whereas the fetal liver presented a distinct transcriptional pattern with increased TLR expression and reduced cytokine transcript levels. Overall, BoAHV-1 transplacental late infection was associated with distinct patterns of immune-related gene expression and immune-cell marker distribution across maternal, placental and fetal tissues, providing an integrated view of immune-associated responses at day 15 post-infection.