Immunological responses of Holstein-Friesian cattle to Staphylococcus aureus in vitro
The aim of this study was to develop and evaluate an in vitro proliferative response of peripheral blood mononuclear cells (PBM) isolated from Holstein-Friesian cattle, induced by formalin-killed Staphylococcus aureus, a major mastitis pathogen, as a potential indicator of resistance or susceptibility to mastitis. Different strains of S. aureus, isolated from cows with subclinical mastitis, and identified by restriction enzyme fragmentation pattern analysis, were shown to induce different levels of proliferation of PBM in vitro. One particular strain (strain A), induced the strongest proliferation response compared to the other strains tested and differences in the magnitude of the proliferative response, induced by S. aureus strain A, was noted among cows. Phenotypic analysis of PBM of cows, by flow cytometry, showed that normal proportions of cell sub-populations were present at the start of culture and that the proliferating cell population consisted mainly of T cells expressing CD4, CD8 or markers. Proliferation of PBM of cows, induced by Staphylococcal Enterotoxin B (SEB), showed that the peak day of proliferation occurred 72 to 96 hours earlier in the presence of SEB compared to in the presence of S. aureus strain A antigen. A statistically significant difference in the proliferative response induced by S. aureus strain A was seen in two progeny groups sired by two different commercial bulls, suggesting that genetic control of this response may be important in protection of the mammary glands against infection. The proliferative assay was, therefore, subsequently performed in bulls.