APOPTOSIS WITHOUT DECREASE OF CELL-DNA CONTENT

Apoptosis of human B cells and murine T and B cells was analyzed by DN A agarose gel electrophoresis, clamped homogeneous electric field, mea surement of cell DNA content by flow cytometry, transmission electron microscopy and by UV microscopy. Apoptosis was induced by etoposide (a n inhibitor of topoisomerase II), by the calcium ionophore ionomycin o r by cross-linking of membrane immunoglobulins (Ig) with anti-Ig-antib odies. Two types of apoptosis could be defined. Apoptosis resulting in small DNA fragments (180-200 base pairs and multiples thereof) was as sociated with a typical 'ladder' in agarose gel electrophoresis and a decrease in cell DNA content assessed by flow cytometry. Conversely ap optosis with large DNA fragments (100-150 kilobase pairs) was only dem onstrated by clamped homogeneous electric field but was not associated with decreased cell DNA content or the observation of DNA ladders. Nu clear condensation without fragmentation was more frequent when apopto sis generated large DNA fragments. The type of apoptosis appears to be an intrinsic property of each cell type.