Dr. Kallas asks for interpretation of the finding that monocytes of the septic patients are positive in terms of binding of Annexin V. Monocytes ingest apoptotic bodies and remnants of the cells that undergo apoptosis. During phagocytosis the plasma membrane of the monocyte fuses with the plasma membrane of the apoptotic body and therefore phosphatidylserine of the apoptotic body becomes exposed on the outer leaflet of the phagocyte. The phagocytes have also other attributes apoptotic cells - e.g. because they ingest chromatin fragments of apoptotic cells they are positive in the TUNEL assay, demonstrating numerous DNA strand breaks. The breaks, however, are in the cytoplasm which can be detected by laser scanning cytometry or image analysis but not by flow cytometry (see Bedner et al., Cytometry, 35: 181-195,1999). Because not only "professional" macrophages are able to phagocytize apoptotic bodies, but the neighboring epithelial cells or fibroblasts often do the same, one has to be careful not to include these false positive "apoptotic" cells while estimating apoptotic index. Zbigniew Darzynkiewicz Brander Cancer Research Institute New York Medical College 19 Bradhurst Ave. Hawthorne, NY 10532 tel: 914-347-2801 fax: 914-347-2804 http://www.geocities.com/z_darzynkiewicz
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