Immunoglobulin A from bronchopulmonary secretions blocks bactericidal and opsonizing effects of antibody to nontypable Haemophilus influenzae.

Patients with chronic bronchitis are colonized by and may develop acute bronchopulmonary infection due to nontypable Haemophilus influenzae (NTHI) despite the presence of bactericidal and opsonizing antibody to the infecting organism. To test the hypothesis that secretory immunoglobulin A (IgA) interferes with host defense mechanisms, we extracted secretory IgA from bronchopulmonary secretions of five patients with NTHI pneumonia. NTHI was incubated with IgA before or during incubation with each patient's own serum or normal human serum. IgA from four of these individuals blocked the bactericidal and opsonizing effects of normal human serum and/or their own serum against their own and/or other NTHI. IgA from bronchopulmonary secretions of patients not infected with NTHI or from the serum of a patient with an IgA myeloma had no such effect. Blocking appeared to result from a direct interaction between IgA and the bacteria. The presumed mechanism is an interaction with bacterial surface antigens, although it is not known whether this occurs at antigenic sites responsible for bactericidal and opsonizing activity or whether interaction with adjacent antigenic sites and subsequent steric interference is responsible. This blocking effect of IgA may be one mechanism that allows for the development of NTHI colonization or pneumonia in an individual who already has seemingly adequate antibody against the infecting organism.