α1-Acid glycoprotein modulates phagocytosis and killing of Escherichia coli by bovine polymorphonuclear leucocytes and monocytes

α1-Acid glycoprotein (AGP) is an acute phase protein that modulates innate immunity and increases in response to infection or injury. The effects of native (phosphorylated) and partially dephosphorylated AGP on the antimicrobial activities of bovine polymorphonuclear leucocytes (PMNs) and monocytes were evaluated. Native AGP inhibited phagocytosis and killing of Escherichia coli by PMNs and monocytes. Engulfment and killing of E. coli were reduced at the acute phase concentration of AGP (0.9 mg/mL) compared with a physiological concentration (0.3 mg/mL). The ability of AGP to inhibit phagocytosis by monocytes and the killing of E. coli by PMNs was reduced following dephosphorylation. The findings indicate that the functions of PMNs and monocytes are differentially regulated by varying concentrations of AGP and its phosphorylation state.