Phagocytosis and Killing of Common Bacterial Pathogens of the Lung by Human Alveolar Macrophages

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Abstract
To investigate factors that determine susceptibility of the lungs to infection with common respiratory pathogens, we studied phagocytosis and killing of nontypable Haemophilus influenzae, H. influenzae type b, Streptococcus pneumoniae types III, VI, and XIV, an unencapsulated variant of S. pneumoniae type III, and Staphylococcus aureus Cowan I, by using human alveolar macrophages obtained by bronchoalveolar lavage of healthy nonsmokers. After opsonization with 10% pooled human serum, mean uptake (± standard deviation) of nontypable H. influenzae (67.5% ± 15.0%), unencapsulated S. pneumoniae type III (71.2% ± 4.8%) and S. aureus (79.1% ± 10.2%) was significantly greater (P < .01) than that of H. influenzae type b (40.1% ± 15.0%), and S. pneumoniae types III (4.4% ± 3.1%), VI (11.8% ± 9.6%), or XIV (8.7% ± 7.0%). Nontypable H influenzae was ingested after opsonization with much less pooled human serum than was H influenzae type b, and uptake of encapsulated S. pneumoniae was not enhanced by as much as 80% pooled human serum. Intracellular killing of unencapsulated S. pneumoniae type III and nontypable H influenzae was rapid and complete and corresponded to the degree of phagocytosis, but despite a high uptake, S. aureus were killed slowly and incompletely. The virulence of S. pneumoniae and H influenzae as lung pathogens is thus determined jointly by encapsulation and the inadequate opsonizing effect of normal human serum, where as that of S. aureus may be related to the organism's relative resistance to intracellular killing by alveolar macrophages.