Single-Cell Analysis of Host Responses in Bovine Milk Somatic Cells (bMSCs) Following HPAIV bovine-H5N1 Influenza Infection.

The 2024 outbreak of highly pathogenic avian influenza virus (HPAIV) H5N1 in U.S. dairy cattle presented an unprecedented scenario where the virus infected bovine mammary glands and was detected in milk, raising serious concerns for public health and the dairy industry. Unlike previously described subclinical influenza A virus (IAV) infections in cattle, H5N1 infection induced severe clinical symptoms, including respiratory distress, mastitis, and abnormal milk production. To understand the host immune responses and changes particularly in the mammary gland, we performed scRNA-seq analysis on bovine milk somatic cells (bMSC) in-vitro infected with H5N1 isolate from dairy farm. We identified ten distinct cell clusters and observed a shift toward type-2 immune responses, characterized by T-cells expressing IL13 and GATA3, and three different subtypes of epithelial cells based on expression of genes associated with milk production. Our study revealed temporal dynamics in cytokine expression, with a rapid decline in luminal epithelial cells and an increase in macrophages and dendritic cells, suggesting a role in increased antigen presentation. These findings indicate that bovine H5N1 infection triggers complex immune responses involving both pro-inflammatory and regulatory pathways. This research fills a critical gap in understanding the immune responses of bovine mammary glands to H5N1 infection and highlights the need for further investigation into therapeutic strategies for managing such outbreaks. Bovine Milk Somatic Cells (bMSCs) were isolated from the raw milk of three multiparous Holstein-Friesian cows, followed by in-vitro infection (24 hrs) with bovine-H5N1 HPAIV (A/dairy cattle/Kansas/5/2024), and analyzed using scRNASeq.