The ability of choline-binding protein A (CbpA) of Streptococcus pneumoniae to bind the third component of complement (C3) suggests possible interactions with opsonic C3 in the bloodstream or with C3 secreted by epithelial cells. The latter possibility was investigated by measuring C3 in supernatants of resting and cytokine-activated monolayers of type II pulmonary epithelial cells (A549 cells). Expression of C3 on the epithelial cell surface was confirmed by immunofluorescence. Epithelially produced C3 bound to CbpA, as determined by Western blot test. cbpa- mutants and lysates therefrom failed to bind C3, were completely deficient in adhesion to a matrix in which C3 was the sole substrate, and demonstrated a moderate yet significant decrease in adhesion to type II pulmonary epithelial cells. These results confirm the interaction of the pneumococcal protein CbpA and its substrate, C3, in 2 in vitro models of adhesion.
Bibliographical noteFunding Information:
Received 3 April 2000; electronically published 19 July 2000. Financial support: National Institutes of Health grant AI24162 (to M.K.H.). a Present affiliation: Yale Child Health Research Center, Yale University School of Medicine, New Haven, Connecticut. Reprints or correspondence: Dr. Margaret K. Hostetter, Yale Child Health Research Center, Yale University School of Medicine, 464 Congress Ave., New Haven, CT 06520 (email@example.com).