Using High-Resolution Differential Cell Counts (HRDCCs) in Bovine Milk and Blood to Monitor the Immune Status over the Entire Lactation Period

SIMPLE SUMMARY: Since leukocytes enter the mammary gland as part of the immune defense, they can be analyzed in raw milk samples and used for diagnostics. In our study, we differentiated and measured several subtypes of immune cells in blood and milk samples of eight dairy cows over their whole lactation to monitor their immune status. In addition to these so-called high-resolution differential cell counts (HRDCCs), we determined routine laboratory biomarkers. We found considerable differences between the documented cell count progressions in the blood and milk. Some cell populations remained mostly stable, while others showed a certain dynamic during the course of lactation. Moreover, two types of lymphocytes were also noticeable due to changes in the case of inflammation. These findings suggest that HRDCCs can be a promising tool for more efficient milk diagnostics, ensuring milk quality, and supporting cattle health and wellbeing. ABSTRACT: Differential cell counts in milk offer a deeper insight into the immunology of the mammary gland and might even provide information about the systemic health status of a dairy cow. Consequently, their potential as a diagnostic method to identify biomarkers has been a subject of research for quite some time. The objective of our study was to closely monitor the immune status of eight healthy dairy cows throughout their whole lactation. For this, high-resolution differential cell counts in milk and blood were determined by means of flow cytometry, which included 10 subpopulations of the 3 main populations of immune cells and their viability. Milk and blood samples were taken twice a week in the first 100 days after calving and once a week during the remaining lactation period: in total, 55 (52–57) blood and 55 (52–57) milk samples per animal. In addition, six well-established routine laboratory biomarkers, i.e., haptoglobin, calcium, and different metabolic parameters (non-esterified fatty acids, β-hydroxybutyric acid, bilirubin, and glutamate dehydrogenase), were analyzed in all blood samples. Furthermore, a standard differential blood cell count was performed on all blood samples. We found substantial differences between cell count progressions in the blood and milk. The distribution of cell populations in the blood remained mostly stable throughout the lactation, albeit at different individual levels. Several cell populations in the milk showed a noticeable dynamic over time, which caused a separation of different lactation stages in clustering analyses. Gamma delta T cells and CD4(+) T cells in the milk stood out as they showed characteristic fluctuations during the course of lactation as well as minor changes in the case of inflammation. The determination of a differential cell count has the potential to be a sensitive diagnostic and prognostic tool in bovine milk. Further studies need to show to what extent the method is suitable for routine diagnostics and how to deal with animal-specific differences.