A nonpeptide integrin antagonist can inhibit epithelial cell ingestion of Streptococcus pyogenes by blocking formation of integrin α5β1-fibronectin-M1 protein complexes

Streptococcus pyogenes can be efficiently internalized by a variety of human epithelial cells. β-lactam antibiotics, commonly used to treat S. pyogenes infections, do not readily permeate mammalian cells. There is growing evidence that the ability of streptococci to enter host cells contributes to the frequent failure of antibiotics to eradicate the organism from infected individuals. Recent studies have suggested that host cell entry requires the formation of a complex of a bacterial fibronectin (Fn) binding protein (e.g., M1 protein or protein F1/Sfbl), human Fn, and the epithelial cell Fn receptor, integrin α5β1. We report here that a low molecular weight, nonpeptide antagonist of integrin α5β1, SJ755, can inhibit internalization of streptococci by primary human tonsillar epithelial cells and immortalized human epithelial (A549) cells, thus increasing the extent of bacterial killing by antibiotics. SJ755 blocked Fn binding by human tonsillar epithelial and A549 cells, suggesting that integrin α5β1 is the major Fn receptor expressed by both cell types. SJ755 did not affect Fn binding by purified M1 protein or M1⁺ bacteria. Purified M1 protein failed to associate with integrin α5β1 unless the integrin had been prebound by Fn. Also, SJ755 blocked formation of α5β1-Fn-M1 complexes in vitro. These results support the previous proposal that Fn functions as a molecular bridge between M1 protein and integrin α5β1. Furthermore, these results suggest that integrin antagonists may enhance the efficacy of antibiotics in treatment of S. pyogenes infections.