<p>The mechanism of non-/hypo-response to hepatitis B vaccine among infants born to Hepatitis B surface Antigen (HBsAg) positive mothers has not been fully clarified. Intrauterine exposure to HBsAg may alter the innate and adaptive immunity of the offspring, further complicating vaccine response. One potential factor is myeloid differentiation factor 88 (MyD88), which is a key adaptor protein linking innate and adaptive immunity and may be regulated by microRNAs (miRNAs). The study aimed to determine the impacts of intrauterine exposure to HBsAg on the immune response to hepatitis B vaccine in the offspring and the potential mechanisms. In the cohort of infants born to HBsAg positive mothers, three pairs of hepatitis B vaccine non-responsive/responsive infants were matched. And miRNA sequencing and bioinformatics were performed to identify miRNAs that are involved in regulating MyD88. Moreover, to mimic human intrauterine exposure to HBsAg, female Hepatitis B Virus-Transgenic (HBV-Tg) mice were mated with male C57BL/6J mice. After birth, pups including HBV-Tg⁺ and HBV-Tg⁻ pups were administered 1 µg hepatitis B vaccine according to the 0-2-4week vaccination schedule. Among infants born to HBsAg positive mothers, microRNA-155-5p (miR-155-5p) expression was higher in non-responders than responders. miRNA sequencing and bioinformatics suggested miR-155-5p may regulate MyD88 expression in these neonates. In the animal experiment, pups with intrauterine exposure to HBsAg produced less anti-HBs after vaccination. Flow cytometry showed that both HBV-Tg⁻ pups and HBV-Tg⁺ pups had less generation of CD4⁺ T cells, B cells, dendritic cells (DCs), and Germinal Center B (GC B) cells in comparison with wild type (WT) mice pups. Cytokine analysis demonstrated that Interleukin-6 (IL-6) and Interleukin-12 (IL-12) were expressed at lower levels in the pups with intrauterine exposure to HBsAg. Further investigation of MyD88 signaling pathway and miR-155-5p pointed that in DCs of pups with intrauterine exposure to HBsAg, there were lower expression of MyD88 and Nuclear Factor-kappa B (NF-κB), while miR-155-5p was elevated. There is a potential link between intrauterine HBsAg exposure, compromised DC generation and activation, and subsequent suboptimal vaccine response. The current data implied that intrauterine HBsAg exposure may have an inhibitory effect on the MyD88 signaling pathway in DCs, with miR-155-5p potentially negatively regulating MyD88.</p>