Technical note: Assessment of neutrophil endocytosis and proteolytic degradation and its relationship with phagocytosis and oxidative burst in dairy cows

Conventional assays of polymorphonuclear cell (PMN, neutrophil) function such as oxidative burst (OB) and phagocytosis (PC) are widely used to evaluate innate immunity in the transition period of dairy cows. Oxidative burst is commonly evaluated by measuring PMN median fluorescence intensity (MFI) involving the release of reactive oxygen species after in vitro stimulation. Phagocytosis can be measured by engulfment of fluorescent beads by PMN. DQ-ovalbumin (DQ-OVA) is a molecule suitable for the assessment of intracellular proteolytic degradation, so it might be informative about an additional pathway of pathogen handling by PMN. In this study, we evaluated the use of the DQ-OVA assay for the assessment of PMN function and the relationships among OB, PC, and DQ-OVA results in PMN isolated from blood of dairy cows between 5 and 21 d post partum. Results of the DQ-OVA validation assay were assessed with mixed linear regression models. Pearson correlation tests and kappa values for agreement were used to associate the MFI between each PMN function assay (OB, PC, and DQ-OVA). For the validation assay (9 cows in 3 replicates), PMN incubated with DQ-OVA were stimulated with IFN-γ or inhibited with cytochalasin D, and fluorescence was compared with untreated PMN. Stimulated and inhibited PMN had greater (970 ± 160 units) and lesser (593 ± 55 units) MFI relative to untreated PMN (791 ± 154 units), respectively, indicating that DQ-OVA fluorescence reflected enhanced or reduced endocytic and proteolytic function. To associate the MFI outcomes among OB, PC, and DQ-OVA, 153 samples from 40 cows were analyzed. Results showed significant, although weak association between DQ-OVA and PC MFI (Pearson r = 0.16). When values of MFI were categorized according to the first ("high" PMN functionality), second and third ("moderate" PMN functionality), or fourth ("low" PMN functionality) quartiles, agreement beyond chance (κ) was moderate: κ = 0.38 for DQ-OVA and OB, κ = 0.43 for DQ-OVA and PC, and κ = 0.43 for OB and PC. The DQ-OVA assay may complement traditional PMN functional assays because it provides additional information regarding the combination of endocytosis and proteolytic degradation, but it is not a substitute for assessment of OB or PC.