The role of mammary gland macrophages in chronic bovine brucellosis

Abstract

Brucella is a facultative intracellular pathogen which causes chronic infections in many mammalian species by parasitizing monocytes and macrophages. Chronic bovine brucellosis is characterized by persistent infections in the mammary gland. Therefore, we studied the in vitro interaction of B. abortus and mammary gland macrophages collected from infected and non-infected cows. Bovine macrophages from involuted mammary glands were collected and challenged with aggregated gamma globulin, opsonified zymosan, non-opsonified zymosan, B. abortus, and opsonified B. abortus in a chemiluminescence assay. A measurable oxidative burst occurred when macrophages were challenged with aggregated immunoglobulin, opsonified zymosan, and non-opsonified zymosan. Opsonification of B. abortus with serum or lacteal antibody was required for stimulation of macrophage oxidative burst activity. Addition of superoxide dismutase resulted in a concentration dependent decrease in macrophage chemiluminescence. Mammary gland macrophages collected from four B. abortus unexposed cows supported intracellular growth of B. abortus strain 2308, but not that of B. abortus strain 45/20 in an in vitro assay. Opsonification of both strains was required for phagocytosis and generation of a measurable oxidative burst. Electron microscopy of mammary gland macrophages challenged in vitro with live opsonified B. abortus strain 2308 demonstrated Brucella within phagosomes and phagolysosomes of macrophages shortly after phagocytosis. Lacteal secretions from five B. abortus infected cows displayed greater opsonic activity for Brucella than did lacteal secretions from five unexposed cows, when measured in a macrophage chemiluminescence assay. Both specific and non-specific opsonic activity were present in lacteal secretions. Mammary gland macrophages collected from B. abortus infected and B. abortus challenge resistant cows were compared in several in vitro functional assays. Macrophages from challenge resistant cows produced significantly higher levels of chemiluminescence than did macrophages from B. abortus infected cows when challenged with aggregated gamma globulin, opsonified zymosan, and opsonified Brucella. Macrophages from four of five challenge resistant cows demonstrated greater bacteriostatic activity against intracellular Brucella than did macrophages from five of five B. abortus infected cows. Lysosomal enzymatic activity and Fc receptor expression were not significantly different for macrophages from the two groups of cows.