Participation of complement in host defense against encapsulated Haemophilus influenzae types a, c, and d

Each of the serotypes of Haemophilus influenzae (types a to f) may colonize the respiratory tract of humans, but only type b strains commonly cause invasive systemic infections. To investigate the role of complement in immunity to encapsulated non-type b strains, rats were depleted of C3 with cobra venom factor and challenged with representative serotypes of H. influenzae (type a, b, c, or d) by different routes. After intravenous challenge, rats depleted of C3 had a greater incidence and magnitude of bacteremia with each of the serotypes when compared with normal controls. Intraperitoneal inoculation of type b organisms resulted in meningitis in normal and C3-depleted rats, but only C3-depleted, and not normal, rats developed meningitis after inoculation of serotype a, c, or d. In contrast to systemic inoculation, intranasal challenge with the different serotypes resulted in bloodstream invasion and meningitis only after challenge with type b organisms. These data suggest that complement plays a significant role in immunity to encapsulated, non-type b H. influenzae through its effect on bloodstream clearance.