Platelets are known mainly for their clotting function, but because they are among the first cells to encounter blood-borne pathogens, they could potentially be important in defense. In support of that possibility, Weyrich and colleagues in PLoS Pathogens show that human platelets rapidly surround pathogenic Staphylococcus aureus and enforce their clustering. In addition to simply sequestering bacteria, platelets and their progenitors (megakaryocytes) also contain β-defensin 1, an important antimicrobial peptide more commonly expressed by epithelial cells. This peptide does not seem to be actively secreted by clustering platelets but instead is released after the membrane is made permeable by bacterial exotoxins. Platelet β-defensin 1 not only attacks S. aureus and impairs their growth directly but also induces the activation of neutrophils in the vicinity of platelet clusters. Platelets may therefore provide the very first line of defense against blood-borne bacteria, retarding their growth until more specialized cells of the immune system can join the fray.

PLoS Pathog. (10 November 2011) doi:10.1371/journal.ppat.1002355