Localization of complement component 3 on Streptococcus pneumoniae: Anti-capsular antibody causes complement deposition on the pneumococcal capsule

We have previously shown that complement component 3 (C3) deposited onto encapsulated Streptococcus pneumoniae by anti-capsular antibody (Ab) is a more efficient opsonin in vitro and in vivo than C3 deposited by anti-cell wall Ab. In the present study, we explored the cellular location of C3b molecules that differ in opsonic efficiency by using avidin-ferritin to localize biotinylated Ab and C3 molecules on S. pneumoniae for electron microscopy. Anti-cell wall Ab and C3b molecules deposited by this Ab on unencapsulated S. pneumoniae were localized to S. pneumoniae cell walls. Anti-capsular Ab and C3b deposited by this Ab were seen in clusters on encapsulated S. pneumoniae at a distance from the cell wall. However, no avidin-ferritin staining of encapsulated S. pneumoniae was seen on incubation with biotinyl-anti-cell wall Ab, biotinylated C3 fixed by anti-cell wall Ab, or nonimmune serum containing biotinyl-C3. In each case, uptake of the biotinylated component was proven by radioactivity measurements, since biotinylated Ab and C3 were also radiolabeled with ¹²⁵I. When avidin-ferritin did not bind to biotinylated components, Ouchterlony analysis indicated that C3 was bound to cell wall components on the encapsulated organisms. Thus, we conclude that, for encapsulated S. pneumoniae, opsonically efficient C3b molecules, deposited by anti-capsular Ab, are located on the S. pneumoniae capsule, whereas the opsonically inefficient C3b molecules deposited by anti-cell wall Ab or nonimmune serum are located on the cell wall. A major reason for the increased virulence of encapsulated compared to unencapsulated S. pneumoniae is that, in the absence of anti-capsular Ab, the S. pneumoniae capsule interferes with the recognition of cell wall-bound C3b molecules by phagocytic cell receptors.