In this study, we demonstrate for the first timethat complement attack of target cells, in the presence of suitably high levels of serum, can induce the oligonucleosomal DNA fragmentation characteristic of apoptosis. This phenomenon requires membrane permeabilisation induced by formation of the complete membrane attack complex and relies on physiologically relevant levels of serum. TUNEL analysis detected complement mediated DNA fragmentation as early as 30 min after the addition of serum and electron microscopy confirmed that chromatin became condensed after complement attack. Various experiments implicate serum DNase I as the mediator of this DNA fragmentation. Intriguingly, membrane permeability induced by melittin gave rise to similar serum dependent DNA fragmentation. The implications of these results for the study of apoptosis in vitro and in vivo are discussed. Cell Death and Differentiation (2000) 7, 48 ± 58.
Experiments were carried out to determine the influence of antigen density on anti-complementary modulation, defined here as the conferring by anti-immunoglobulin (Ig) of resistance to lysis of guinea pig L2C leukaemic cells varied widely in their quantitative expression of surface membrane Ig as judged by the binding to cells of 125I-labelled Fab' fragments from anti-Ig, and a good correlation between the bulk antigen density and the percentage of cells lysed by anti-Ig plus C was obtained (P=0.02). In the presence of 10 mM sodium azide, which has been shown to diminish the modulation occurring during simultaneous incubations with anti-Ig and C, this correlation was even stronger (P less than 0.001). No zone could be defined in which the level of surface Ig expression was sufficient to serve complement lysis but too low for modulation. Furthermore, both the degrees and rates of modulation occurring on incubation with antibody at 37 degrees C, either before C addition or in the presence of lytic C, were similar for populations of high, low, or intermediate antigen density. Separation of cells by their size or density failed to yield populations differing in either their susceptibility to humoral killing through anti-Ig or their modulating capacity. IgG and IgM antibodies to the L2C cell surface Ig evoked similar levels of killing with syngeneic C, and when compared for their ability to promote anti-complementary modulation, no difference was revealed in either the rate or degree of modulation occurring during incubations at 37 degrees C with the two isotopes. The findings are discussed with particular reference to observations on the modulation of mouse thymus leukaemia (TL) antigens.
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