The loss of opsonic activity of bovine milk whey following depletion of IgA

Assessment of the Opsonic Activity of Purified Bovine sIgA Following Intramammary Immunization of Cows with Staphylococcus aureus

The phagocytosis of Staphylococcus aureus by bovine polymorphonuclear neutrophils (PMN) requires the presence of antibodies. Among the major isotypes of bovine antibodies, IgG2 and IgM are considered opsonic for bovine PMN. However, the role of purified bovine secretory IgA (sIgA) as an opsonin has not been assessed. In the present study, IgG2 were obtained from serum and sIgA, IgG1, and IgM were purified from the colostrums of three cows intramammarily immunized with heat-killed Staphylococcus aureus. The Ig preparations were assayed for specific antibodies, and the opsonic capacity of every isotype was investigated. Despite the presence of antibodies, we observed no distinct chemiluminescence response of PMN stimulated with sIgA- or IgG1-opsonized S. aureus, whereas IgM or IgG2 bound to bacteria induced a marked chemiluminescence response. Moreover, the counting of internalized bacteria per PMN after phagocytosis revealed a low uptake of S. aureus opsonized with sIgA or IgG1, in contrast to IgM or IgG2, which triggered efficient ingestion of bacteria. Priming of neutrophils by TNF-alpha, IFN-gamma, or C5adesArg did not promote an oxidative burst or uptake of sIgA-opsonized S. aureus to a greater extent than with IgG1-opsonized bacteria. Furthermore, analysis of uningested bacteria by flow cytometry after incubation with PMN showed a preferential uptake of IgM-opsonized S. aureus by PMN and only few sIgA-positive stained bacteria were PMN-associated. These experiments indicate that sIgA, like IgG1 and unlike IgM or IgG2, could not be considered as a major opsonin for phagocytosis of S. aureus by bovine blood PMN.

Effect of whole Staphylococcus aureus and mode of immunization on bovine opsonizing antibodies to capsule

Exopolysaccharide capsule is a major virulence factor of Staphylococcus aureus because it inhibits neutrophil recognition of antibodies to highly antigenic S. aureus cell wall. To circumvent this inhibition, two modes of immunization were tested for ability to induce anticapsular opsonins. Cows were immunized at drying off and boosted on d 14 and 28 by injection of Smith diffuse S. aureus plus dextran sulfate in the area of the supramammary lymph node or intramammarily. In cows immunized in the area of the supramammary lymph node, IgG1 and IgG2 sera antibody titers to capsule increased and remained elevated to the end of the study, 120 d postcalving. The IgM titers increased during the dry period but declined to preimmunization levels at calving. Response of serum IgG1 and IgM to intramammary immunization was similar to that with supramammary lymph node immunization, but more delayed and lower in magnitude. Antibodies of all four isotypes, IgG1, IgG2, IgA, and IgM, increased in dry secretions following immunization via lymph node. In cows immunized in the lymph node, IgG1 antibodies remained elevated throughout the study, but IgG2 antibodies dropped to baseline 15 d postcalving. In cows immunized intramammarily, only IgA antibodies increased significantly in lacteal secretions and remained elevated throughout the study. Immunization of cows in the lymph node during the dry period enhanced the ability of dry secretions and colostrum to support phagocytosis.

Opsonization of Staphylococcus aureus by bovine immunoglobulin isotypes

The ability of specific bovine Ig isotypes to enhance phagocytosis of Staphylococcus aureus by polymorphonuclear neutrophils was studied. Polymorphonuclear neutrophils were isolated from the blood of 14 lactating Holstein cows. Antibodies against S. aureus M10 were produced by two Holstein cows immunized via intramuscular injections and injections in the area of the supramammary lymph node with M10 emulsified in dextran sulfate. The IgG1, IgG2, and IgM were prepared from immune sera. Fluorescein-labeled, formalin-killed S. aureus M10 were opsonized with the respective isotypes prior to incubation with polymorphonuclear neutrophils. Percentage of polymorphonuclear neutrophils phagocytosing averaged 37.4, 1.1, 15.9, and 9.4% for immune sera, IgG1, IgG2, and IgM, using a M10: polymorphonuclear neutrophils ratio of 10:1; and 77.1, 1.8, 32.1, and 57.9 using a 40:1 ratio. When IgG1 was incubated with either IgG2 or IgM, phagocytosis was reduced to 10.0 and 5.0%, respectively, using the 10:1 ratio and 24.2 and 44.7%, respectively, using the 40:1 ratio. Significant variation occurred among cows in the ability of polymorphonuclear neutrophils to undergo phagocytosis independent of isotype and S. aureus M10: polymorphonuclear neutrophil ratio. These data show that IgG2 and IgM are opsonic for bovine polymorphonuclear neutrophils and that IgG1 inhibits the activity of both. These results will be helpful to determine immunization protocols to solicit synthesis of bovine IgM and IgG2 specific for S. aureus.