Effect of protein A on the antistaphylococcal defence mechanisms of the murine lung

The importance of IgG attached to protein A in the initial reaction of inspired Staphylococcus aureus and alveolar macrophages was studied by infecting unimmunized mice with aerosols of S. aureus strains 566 and Wood 46 with high and low protein A content. At 2, 4, and 8 hr after infection, the presence of IgG attached to S. aureus and rates of staphylococcal ingestion and killing by macrophages were determined. IgG was detected by staining of sections of the right lung with fluorescein-labeled goat antibody to mouse IgG. For S. aureus strain 566, 25%--40% of the total number of bacteria, as determined in equivalently sized subjacent sections stained by the Brown and Brenn tissue gram stain, contained attached IgG. A few S. aureus strain Wood 46 were surrounded by dimly fluorescing complexes. Since rates of bacterial ingestion and killing were similar for both strains in this in vivo model of infection, IgG binding to protein A does not affect the bactericidal capacity of alveolar macrophages.