Participation of human blood platelets in antimicrobial host defense

Background: Besides their classical functions in hemostasis, blood platelets actively contribute to inflammatory and defense reactions. As the first cellular elements that accumulate at any site of injury to the vascular wall, platelets start the innate host defense against any invading pathogens by means of internalization of bacteria and secretion of microbicidal peptides. Our own observations suggest that there are further platelet defense capabilities. Material and Methods: Gram-positive (Bacillus subtilis; B.s.), Gram-negative (Klebsiella pneumoniae; K.p.) and intracellular (Chlamydia pneumoniae; C.p.) bacteria were included in our study. Morphological aspects of bactericidal effects were investigated by transmission electron microscopy, and bacterial killing rates were determined by fluorescence double labeling of bacteria with LIVE/DEAD BacLight™. Bacteria were co-incubated with stimulated platelets or with stimulated, previously degranulated platelets, or were exposed to supernatants from stimulated platelets. Results: When co-incubated with stimulated platelets, bacteria from all three strains displayed swelling, loss of internal structures, and rupture of the cell wall. At binding sites of B.s., the platelet membrane often showed a ruffled appearance. Exposure to supernatants from stimulated platelets yielded moderate killing rates (36/52/68% for B.s./K.p./C.p.), whereas killing rates strongly increased if stimulated platelets were co-incubated instead (77/79/81% bei B.s./K.p./C.p.). Even degranulated platelets exerted bactericidal effects on B.s. (41%) if stimulated with thrombin. Conclusions: Besides the release of microbicidal peptides, human platelets also have a contact-dependent mechanism to kill invading bacteria. Since ruffled membranes are indicative for release of reactive oxygen species, platelets seem to contribute to antimicrobial host defense by a local release of free radicals.