Apoptosis is a programmed, physiological mode of cell death that plays an important role in tissue homeostasis. Understanding of the basic mechanisms that underlie apoptosis will point to potentially new targets of therapeutic treatment of diseases that show an imbalance between cell proliferation and cell loss. In order to conduct such research, techniques and tools to reliably identify and enumerate death by apoptosis are essential. This review focuses on a novel technique to detect apoptosis by targeting for the loss of phospholipid asymmetry of the plasma membrane. It was recently shown that loss of plasma membrane asymmetry is an early event in apoptosis, independent of the cell type, resulting in the exposure of phosphatidylserine (PS) residues at the outer plasma membrane leaflet. Annexin V was shown to interact strongly and specifically with PS and can be used to detect apoptosis by targeting for the loss of plasma membrane asymmetry. Labeled annexin V can be applied both in flow cytometry and in light microscopy in both vital and fixed material by using appropriate protocols. The annexin V method is an extension to the current available methods. This review describes the basic mechanisms underlying the loss of membrane asymmetry during apoptosis and discusses the novel annexin V‐binding assay. Cytometry 31:1–9, 1998. © 1998 Wiley‐Liss, Inc.

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SUMMARYTwo MoAbs, independently raised against ovarian carcinoma cells and referred to as 0V-TL3 and 0V-TL16, display an identical reaction pattern with a membrane-associated protein in both normal and malignant ovarian cells. Also, a similar binding affinity constant and a similar number of binding sites per cell indicate that both MoAbs bind to the same antigen. Competition assays reveal that OV-TLI6 is able to compete with 0V-TL3 for binding to OVCAR-3 cells. Epitope mapping using a filamentous phage hexapeptide epitope library showed that both MoAbs are able to select identical phages, suggesting that their epitopes are identical or at least overlapping. However, purified polyclonal and monoclonal anli-idiotypic antibodies directed against 0V-TL3 failed lo recognize the 0V-TL16 idiotype, indicating that the .structure of the antigen-binding regions of both antibodies is distinct. This was corroborated by molecular cloning and sequencing of the variable heavy (VH) and light (VJ chain immunogiobulin regions of both MoAbs. The Vr egions of both antibodies were found to be distinct, whereas the VL regions are almost identical. Computer modelling of the idiotypes suggests that the complementarity determining regions (CDR). with the exception of VHCDR3, have (almost) identical spatial configurations. Our data indicate that, although structurally different in their VH regions, OV-TL3 and OV-TL16 are able to bind to identical epitopic regions on the antigen, because differences in primary structure do not exclude the formation of sufficient and similar spatial structures for the interaction with an epitope.Keywords immunogiobulin variable domains ovarian carcinoma phage display epitope library antibody modelling INTRODUCTIONFor the management of cancer, MoAbs raised against tumourspecific determinants have proven to be of great help. For ovarian carcinomas MoAbs have been generated, among which are 0V-TL3 and 0V-TL16, that have been raised by different immunization procedures. Both react with a membrane antigen (OA3) [1] expressed in about 90% of the ovarian tumours and only weakly expressed in normal tissue [2]. 0V-TL3, raised after immunization with an extract of an endomctrioid cancer biopsy, has a reactivity pattem highly restricted to ovarian carcinomas [3,4], and was recently shown to react with a surface Correspondence: G.J