BLACKSBURG, Va. – Newly published research identifies the immune cells present in the milk of healthy Holstein cows.

Researchers have long understood that milk contains various cell types including immune cells, Gabriela Pérez-Hernández, postdoctoral research associate at Virginia Polytechnic Institute and State University, said. However, technology stood in the way of characterizing the precise type of immune cells present. Now, researchers are using single-cell RNA sequencing to study fresh milk samples, providing a snapshot of the immune cells present by measuring the RNA molecules in individual cells.

“This technique is a powerful tool that enables us to analyze gene expression at the individual cell level,” she said. “So even across thousands of cells, we can tell what the main characteristics are of the cells and what cell types they are. We were finally able to get a more detailed picture of cell diversity in cow’s milk using this technique.”

In a study published in Animal Nutriomics, Pérez-Hernández and her colleagues sampled the milk of 25 multiparous healthy Holstein cows. The results demonstrated that the most common immune cells present in the samples were T cells, or immune cells that destroy pathogens and send signals to control the immune system’s response to threats, and granulocytes, a type of immune cell that releases small granules that release enzymes to fight pathogens.

Other immune cells identified included macrophages, immune cells that engulf and digest pathogens, and a small population of B cells, immune cells that create antibodies to attack pathogens. Interestingly, subpopulations of each cell type were discovered, meaning that several types of T cells, granulocytes, macrophages, and B cells were observed. Subpopulations of immune cells have unique characteristics that contribute different functions.

“Our research also has preliminary evidence of progenitor cells in milk,” Pérez-Hernández said. “These are cells that contribute to mammary gland regeneration and turnover. We know (mammary gland regeneration and turnover) happens during dry periods, but maybe this is happening during lactation periods, too.”

Knowing what immune cell types are present in milk is only the beginning of understanding their purpose. Pérez-Hernández said that the team is interested in exploring how the immune cells end up in milk, what role they play during lactation, and how they maintain mammary gland tissue.

While researchers and farmers have long been aware that colostrum contains a great number of immune cells, it’s unclear which types of immune cells are transferred to offspring. Pérez-Hernández said that single-cell RNA sequencing could be used in the future to study colostrum and characterize the immune cells transferred to calves.

“We know in general that some basic immunity is transferred to the offspring,” she added. “With these techniques, we can really learn more and maybe potentiate some cells that are going to be better for the calves when they are just born.”

Additionally, understanding the cellular landscape and how the mammary gland responds to infections and stressors will better equip scientists to study and possibly improve milk production. Pérez-Hernández and her colleagues believe that their research will lead to management strategies that optimize mammary gland health and milk production efficiency.

The research group is also interested in studying how different environmental or management factors, like heat stress and mastitis, affect milk’s immune profile.

“We see potential in exploring certain mammary gland immune characteristics, and we would like to know how these immune cell types are inherited across generations,” Pérez-Hernández said. “There are other researchers in Wisconsin and Florida that have shown that stressors like heat stress can have intergenerational effects, specifically for the udder. We are curious to see if the mammary immune traits are also passed down or affected due to different stress sources.”

Pérez-Hernández emphasized that while the research seems distanced from farm applications, fundamental and basic research plays a critical role in shaping the future of farm management strategies.

“Many of the practical solutions that are used for dairy farms today started with fundamental research studies,” she said. “At the end of the day, as a dairy or animal scientist, our work is always to support dairy farmers and animal welfare and improve production efficiency.”