Vitronectin mediates internalization of Neisseria gonorrhoeae by Chinese hamster ovary cells

Gonococci producing a distinct opacity protein (OpaA in strain MS11) adhere to and are efficiently internalized by cultured epithelial cells such as the Chang conjunctiva cell line. Both adherence and uptake require interactions between OpaA and heparan sulfate proteoglycans on the mammalian cell surface. Chinese hamster ovary (CHO) cells also support adherence of gonococci through interactions of OpaA with cell surface heparan sulfate proteoglycans. However, despite this similarity in the requirements for adherence, CHO cells are not capable of internalizing gonococci. In this report, we characterized this apparent deficiency and identified a factor in fetal calf serum (FCS) which is capable of mediating uptake of gonococci by CHO cells. In the absence of FCS, OpaA+ gonococci adhered to but were not internalized by CHO cells, whereas in the presence of up to 15% FCS, the bacteria were efficiently internalized by the cells. Preincubation of bacteria, but not cells, with FCS also stimulated internalization, suggesting that a factor present in FCS was binding to the surface of gonococci and subsequently stimulating entry. Using a combination of chromatographic purification procedures, we identified the adhesive glycoprotein vitronectin as the serum factor which mediates the internalization of gonococci by CHO cells. Vitronectin-depleted serum did not support gonococcal entry, and this deficiency was restored by the addition of purified vitronectin. Further experiments using a set of gonococcal recombinants, each expressing a single member of the family of Opa outer membrane proteins, demonstrated that vitronectin bound to the surface of OpaA-producing gonococci only and that the vitronectin-mediated uptake by the CHO cells was limited to this bacterial phenotype. To our knowledge, our data are the first example that vitronectin can serve as a molecule that drives bacterial entry into epithelial cells.