Friend or Foe? Turning a Host Defense Protein Into a Pathogenʹs Accomplice

Cystatins are cysteine protease inhibitors that are at the front-line of defense against pathogens that secrete proteases as virulence factors. In this issue, reveal how the bacterial protease IdeS from Streptococcus pyogenes hijacks normal cystatin C function to convert it into a cofactor that enhances proteolytic destruction of host-defense antibodies.