Pulmonary opsonins in Klebsiella pneumoniae pneumonia in rats.

Klebsiella pneumoniae was inoculated intrabronchially into rats, and bronchoalveolar lavage fluid and sera were obtained during the ensuring pneumonia. Klebsiellae recovered by lavage were not maximally coated with C3, as judged by studies with fluorescent antibody, whereas the organisms could be coated fully with C3 by a brief incubation in rat serum. The levels of C1 and C3 in lavage fluid obtained during infection were only a small fraction of the levels in the serum, and klebsiellae were not opsonized during incubation with concentrated lavage fluid. Systemic decomplementation did not affect the severity of K. pneumoniae pneumonia, as judged by the measurement of lung weight or by the numbers of klebsiellae in the lungs, but decomplemented rats had a larger number of klebsiellae in the blood at 24 h of infection than did controls. There were fewer klebsiellae in the lungs 4 h after inoculation of preopsonized organisms than after inoculation of organisms which were incubated in control (heat-inactivated) sera. These studies indicate that the concentration of complement and heat-labile opsonins within the alveoli is lower than that in the systemic circulation and is not adequate for effective opsonization of klebsiellae.