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Adjuvant intravesical Mycobacterium bovis BCG is the treatment of choice for recurrent superficial bladder cancer. Fibronectin (FN) was previously demonstrated to be necessary for the retention of BCG within the bladder and for the expression of antitumor activity. Recent studies have demonstrated that BCG attach and are ingested by bladder epithelial cells, suggesting the existence of a second bacterial attachment mechanism. We report the characterization of the molecules involved in BCG attachment and internalization by the human bladder transitional cell carcinoma cell line T-24. Pretreatment of T-24 cells with monoclonal antibodies to either alpha 5 or beta 1 integrin subunits significantly inhibited both BCG attachment and ingestion. Exogenous FN was observed to enhance both attachment and ingestion of BCG, and anti-FN was observed to inhibit both phenomena. Latex beads precoated with either FN or laminin (LN) but not BSA were ingested by T-24 cells, but only FN-coated beads inhibited BCG attachment and ingestion. Pretreatment of BCG with FN augmented both attachment and ingestion. The role of bacterial FN binding proteins was evaluated. A monoclonal antibody to a 55-kD FN-binding protein was observed to abrogate attachment and ingestion. These results demonstrate that attachment and ingestion of BCG are mediated in part by the alpha 5 beta 1 integrin receptor and are dependent on FN. These studies demonstrate a mechanism of entrance of mycobacteria into epithelial cells and suggest a second role for FN in the adjuvant antitumor effect of BCG.