Circulating ECs display an apoptotic phenotype. Microphotograph of an apoptotic EC from a patient with SLE. Annexin V was visualized with FITC-conjugated Annexin V, CD146 was visualized by anti-CD146 conjugated to phycoerythrin, and nuclear material was visualized with Hoechst 33342. (A-C) Micrographs show the staining with each individual fluorochrome and the composite image (D). Panel C displays evidence of nuclear fragmentation, characteristic of apoptosis. Arrowhead denotes a prominent surface bleb containing nuclear material, characteristic of apoptosis. The other 2 cells in the figure represent cells that are CD146^– and, therefore, not of endothelial origin.