Correspondence Lethal Synergism between Influenza Virus and Streptococcus

pneumoniae To the Editor—Occasionally, scientific discoveries sink into oblivion, and later investigators find difficulties in explaining forgotten connections. McCullers and Rehg [1] discuss the role that platelet-ac-tivating factor receptor (PAFr) plays in the lethal synergism between influenza virus and Streptococcus pneumoniae. PAFr is an important factor in the attachment and invasion of cells by virulent pneumococcal strains. Different effects—that is, acid, in-flammatory cytokines, and viral neura-minidase (NA)—are enhanced by the up-regulation of PAFr. Moreover, the authors discuss the cleavage of terminal sialic acid moieties by influenza NA–exposing cryp-tic receptors. In all of those ways, pneu-mococcus adheres and invades. NA is, in-deed, a key influenza-virus pathogenic factor, as the new NA inhibitors show. However, pneumococci also produce NA. Thirty years ago, several reports were published that describe the pathogenic role that NA plays in pneumococcal in-fections [2–5]. The NAs of influenza virus and of pneumococcus are likely to be syn-ergistic, although pneumococcal NA is more powerful because of its stronger ac-tivity and broader substrate spectrum [6]