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Why human platelets fail to kill bacteria

Recent investigations have suggested that bacteria are taken up by platelets in the same manner as by polymorphonuclear (PMN) leukocytes and monocytes. The phagocytic vacuole containing the organism is completely separated from the cell exterior and the cytoplasm in order to form a killing chamber. Yet, earlier studies demonstrated that platelets do not kill bacteria. The present work has provided a basis for the lack of platelet bactericidal activity. Platelets incubated with Staphylococcus aureus 502A or RN 450 for intervals up to 2 h can take up organisms into sequestration vacuoles that resemble the phagosomes formed by PMN and monocytes. However, staining with tannic acid which forms an electron dense stain with osmic acid reveals that the phagosomes in PMN and monocytes are completely separated from the cell exterior and the cytoplasm. As a result, they are true killing chambers. The engulfment vacuoles containing bacteria in platelets on the other hand are almost never sealed from the cell exterior. Osmium black reaction product stains the inside of the engulfment vacuole and channels of the open canalicular system connecting it to the cell exterior. Thus platelets do not form the killing chamber phagosomes observed in leukocytes and, as a result, cannot kill bacteria.