Neutrophil Kinetics in Endotoxin-Induced Mastitis

Abstract

SUMMARY Milk, blood, and bone marrow samples were obtained from cows inoculated with Escherichia coli endotoxin into a lactating mammary gland to understand changes in neutrophil numbers in blood and bone marrow during acute mastitis. Neutrophil diapedesis into the udder and milk (within 4–6 hours) initially caused neutropenia or neutrophilia followed by neutropenia, and depletion of bone marrow reserve of neutrophils. Mobilization of neutrophils from the bone marrow subsequently tended to increase the blood neutrophil counts even though more neutrophils migrated into the milk. Immature neutrophils, from band cells to myelocytes (left shift), also appeared in blood during this period in some experiments. Results in 2 experiments, in which 2 mammary glands of the same cow were inoculated 24 hours apart with identical doses of endotoxin, indicated that neutrophil mobilization into the glands given the 2nd injection was less than in glands given the 1st injection. Furthermore, the added burden of neutrophil diapedesis into another gland did not seem to alter the trend of neutrophil counts in peripheral blood, although it caused a greater and longer reduction in the bone marrow reserve of neutrophils in comparison with single-injection experiments. Subsidence of acute mastitis and recovery were associated with lessened neutrophil migration into the milk, disappearance of left shift when seen, and replenishment of blood and bone marrow neutrophil pools due to compensatory stimulation of granulopoiesis. Blood neutrophil counts underwent a secondary decrease between 24 and 72 hours, perhaps due to intravascular distribution of cells to restore the marginal pool, and then increased to base-line or above base-line value. The granulopoietic activity subsided after 72 to 120 hours; however, the various granulocytic pools were generally at higher than base-line value at the termination of the experiments (168th hour).