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Chapter 13 : Interactions of Streptococcus pneumoniae with the Proteins of the Complement Pathways
Thirty-three years later, the heat-stable, specific components of the opsonic interaction (antibodies) were distinguished from nonspecific, heat-labile serum proteins, now named complement. In the presence of antibody alone, the rate of neutrophil phagocytosis of serotype 3 Streptococcus pneumoniae was accelerated more than sevenfold by the addition of active complement proteins in fresh human serum. Mannose binding lectin (MBL), a member of the collectin family, exhibits the collagen-like domain and the carbohydrate recognition domain of the collectin family. The most compelling evidence for the role of innate immunity in defense against pneumococcal infection focuses on C3. Recent genetic investigations have identified a number of pneumococcal proteins that affect the activation, deposition, or cleavage of C3 so as to interfere with C3-mediated opsonization and phagocytosis. Deleting the choline-binding region of the family 2 PspA generated a mutant that failed to bind human lactoferrin but retained most of its complement deposition. Virulence was moderately attenuated. The pspC gene is present in 75% or more of pneumococcal strains, but the encoded proteins are highly polymorphic and are divided into 11 groups. Studies of type 3 pneumococci, highly resistant to phagocytosis, also pointed to absorption of factor H from plasma and led to the isolation of Hic. Pneumococcal proteins that interact with components of the classical or alternative complement pathways are predominantly surface expressed (except for Pla), related if not completely conserved (e.g., CbpA, PspA, PspC, and Hic), and potently immunogenic.