Fc-mediated nonspecific binding between fibronectin-binding protein I of Streptococcus pyogenes and human immunoglobulins

Fibronectin-binding protein I (SfbI) from Streptococcus pyogenes plays a ke y role in bacterial adhesion to, and invasion of, eukaryotic cells. In addi tion, SfbI exhibits: a considerable potential as mucosal adjuvant and can t rigger polyclonal activation of B cells, Here, we report that SfbI is also capable of binding human IgG in a nonimmune fashion. SfbI was reactive with IgG1, IgG2, IgG3, and IgG4 isotypes (type Ho IgG-binding profile). The aff inity constant (K-d) of the SfbI-IgG interaction was in the range of 1-2 x 10(-5) M, Further studies demonstrated that the SfbI binding was mediated b y the pc component of the IgG molecule. Experiments performed using purifie d recombinant proteins spanning different domains of SfbI showed that the I gG-binding activity was restricted to the fibronectin-binding domains, and in particular to the fibronectin-binding repeats, Finally, the presence of recombinant SfbI resulted in an impairment of both phagocytosis of IgG-coat ed RBCs and Ab-dependent cell cytotoxicity by macrophages, These results de monstrated for the first time that, in addition to its major role during th e colonization process, SfbI may also favor bacterial immune evasion after the onset of the infection by interfering with host clearance mechanisms.