Bovine neutrophils ingest but do not kill Haemophilus somnus in vitro

Phagocytosis of Haemophilus somnus by bovine blood neutrophils required opsonization of the bacteria with antibodies against H. somnus. Few bacteria were phagocytosed in the absence of serum; in addition, absorption of immune serum with Formalin-killed H. somnus significantly reduced ingestion of H. somnus. Heat inactivation of antiserum (56 degrees C for 30 min) to eliminate complement activity had little effect on its ability to opsonize H. somnus for uptake by bovine neutrophils. Antiserum from an H. somnus-immunized calf and autologous sera from adult cattle supported equivalent phagocytosis of H. somnus by bovine neutrophils, suggesting that normal, healthy cattle may contain sufficient antibodies in their sera to facilitate phagocytosis of H. somnus. Although bovine neutrophils readily ingested H. somnus, they were unable to kill the bacterium in vitro. These same neutrophils readily killed opsonized Escherichia coli and Staphylococcus epidermidis, suggesting that H. somnus is able to survive and perhaps multiply within bovine neutrophils. Because bovine neutrophils stimulated with opsonized H. somnus demonstrated a reduced oxidative burst (as measured by chemiluminescence) compared with neutrophils stimulated by opsonized E. coli, we suggest that reduced production of reactive oxygen intermediates during the phagocytosis of H. somnus may account, in part, for the enhanced survival of H. somnus in bovine neutrophils.