Platelets Are Covercytes, Not Phagocytes.
Authors: White James G. 
Published: October 2019

Youssefian et al. (Blood2002; 99:4021) have recently reported that engulfment of Staphylococcus Aureus (S.A.) by human platelets is more like phagocytosis by polymorphonuclear leukocytes (PMN) than previously considered.1 Electron microsopy and ultrastructural immunocytochemistry revealed that the engulfment vacuoles containing bacteria were entirely made up of invaginated surface membrane. The surface-connected, open canalicular system (OCS), a tortuous system of interconnected channels continuous with the surface membrane, was not involved in uptake of bacteria. Many of our previous investigations have examined platelet-particle and platelet bacterial interaction, and disagree with the findings of Youssefian et al.1 The present study has used tannic acid as an electron dense tracer to evaluate the hypothesis indicating the OCS is not involved in bacterial uptake.1 Platelets in platelet-rich plasma (PRP) or in washed suspensions (WPS) and samples of PMN were incubated at 37°C with S.A. 502A or S.A. RN 450 for 15, 30, 60, and 120 minutes with or without added ADP.1 Fixation of samples was carried out in glutaraldehyde and osmic acid in the presence of tannic acid. The tannic acid selectively binds to the glycocalyx covering the exterior platelet surface and lining channels of the OCS, and converts osmium to osmium black visible as an electron dense stain. It also combines with fibrinogen/fibrin permitting visualization of α-granule secretion into the OCS. Interaction with bacteria during incubation is variable and infrequent, unless the sample is stirred or an aggregating agent like ADP is added.1 The bacterium attaches itself to the platelet surface, often at or near the opening of an OCS channel. The platelet cannot move the bacterium across the surface as it does small particles up to the size of α granules. Instead the platelet molds itself to the surface, just as it does to large latex spheres and flat surfaces. However, the latter surfaces cannot be internalized, and the platelet continues to spread causing evagination of the OCS to add to the total surface area covered. Bacteria can be internalized. Platelet spreading over the organism causes delilation of the openings of OCS channels. More canaliculi are recruited from the spiderweb of interconnected OCS channels to make a vacuole. Tannic acid staining shows the platelet engulfment vacuoles always contain osmium black around the bacterium, whereas PMN phagocytic vacuoles are never stained. α granules secreting fibrinogen/fibrin into OCS channels stained by tannic acid also discharge their contents into engulfment vacuoles containing bacteria. Direct communication between engulfment vacuoles and the platelet exterior through tannic acid stained OCS canaliculi were identified, while communicating OCS channels were out of the plane of section in other examples. Since the only way tannic acid could stain the engulfment vacuoles containing bacteria was by way of the OCS, it is clear that the OCS is involved in their interiorization. Thus the platelet is a covercyte, not a phagocyte, and its interaction with bacteria is very different from that of the PMN which does not have an OCS.