Type b capsule inhibits ingestion of Haemophilus influenzae by murine macrophages: studies with isogenic encapsulated and unencapsulated strains.

Phagocytosis may be important in clearing Haemophilus influenzae from the bloodstream. To define the effect of type b capsule on phagocytosis, binding and ingestion by macrophages was measured for 5 isogenic sets of capsule-sufficient strains (clinical isolates and type b transformants of capsule-deficient mutants) and capsule-deficient mutants (strains lacking a 9-kb EcoRI fragment of chromosomal DNA associated with type b capsule expression). Capsule-sufficient strains were not bound in the absence of serum, whereas capsule-deficient strains were bound and ingested (1.8-5.1 organisms/macrophage; 59%-97% ingested). In the presence of nonimmune serum, capsule-sufficient strains were largely bound but not ingested (4.7-7.2 organisms/macrophage; 7%-21% ingested), whereas capsule-deficient strains were nearly all ingested (6.2-10.5 organisms/macrophage; 93%-97% ingested). Strains resisting ingestion caused persistent bacteremia 24 h after intravenous challenge in mice and were more likely than readily ingested strains to cause persistent bacteremia or death in infant rats. Thus, type b capsule inhibits ingestion by macrophages; resistance to ingestion may be an important virulence determinant of type b organisms.