Fibrinogen-mediated adherence of group A Streptococcus to influenza A virus-infected cell cultures

A quantitative radioassay was used to study the adherence of group A Streptococcus to Madin-Darby canine kidney cells infected with influenza A virus (strains FM1, Jap 305, and NWS) and reacted with fibrinogen. Treatment of virus-infected cell cultures with human fibrinogen significantly enhanced streptococcal adherence (P less than 0.0005) compared with adherence to untreated, virus-infected cells and uninfected control cells. Enhanced adherence was not seen with NWS virus-infected cell cultures or with virus-infected cells treated with human fibronectin, canine fibrinogen, or porcine fibrinogen. Human fibrinogen was shown to bind directly to surface membranes of virus-infected cells. Virus-infected cell cultures were incubated in the presence of tunicamycin, an antibiotic that inhibits glycosylation of virus-specific surface membrane glycoproteins. We found that with increasing antibiotic concentration there was a progressive decrease in fibrinogen-mediated streptococcal adherence. Adherence of 3H-labeled streptococci to fibrinogen-treated, virus-infected cell cultures showed saturation kinetics and could be blocked with monospecific antibodies against fibrinogen. These results suggest that human fibrinogen binds to a glycoprotein moiety on the surface of influenza A virus-infected cells, and that once bound the fibrinogen molecule acts as an "acquired" receptor for the attachment of group A Streptococcus. We postulate that this mechanism, it if occurs in vivo, might help explain the observed association between influenza A virus infection and subsequent bacterial superinfection with group A Streptococcus.