The Role of Opsonins in Recovery from Experimental Pneumococcal Pneumonia

The opsonization of type 1 and type 25 pneumococci in the rabbit lung early in the course of experimental pneumonia was studied. Bacteria washed from the lungs of animals with type 1 pneumonia were coated with IgG, IgA, and complement within 24 hr of infection; these bacteria were not phagocytized. Both type 1 and type 25 pneumococci, when incubated in concentrates of nonimmune bronchopulmonary washings (BPC), adsorbed IgG, IgA, and C3, but neither type of pneumococci was opsonized. Killing of pneumococci by surface phagocytosis, with or without the use of nonimmune BPC, was not demonstrated. When nonimmune serum was used, opsonization of type 25 pneumococci depended on the presence of heat-labile adsorbable factors, and when immune serum and immune BPC were used, opsonization was dependent on adsorbable factors. Depletion of serum complement with cobra venom factor before infection with type 25 pneumococci resulted in severe bacteremic pneumonia in the rabbit, but immunization before complement depletion allowed animals to clear type 25 pneumococci from the blood and lungs within 24 hr. Thus, opsonization is required for efficient clearance of pneumococci from the lung, and in the nonimmune animal, complement is also required for clearance.