Dextran sulfate-induced degradation of spontaneously apoptotic B cells

Mature resting mouse splenic B cells undergo spontaneous apoptosis in vitro, unless rescued by specific agents including interleukin 4 and protein kinase C activators. Dextran sulfate, a B cell activator, has been reported to have such a protective effect on B cell apoptosis. This study was undertaken to elucidate the mechanism underlying the protective effect of dextran sulfate. The ratio of apoptotic B cells gradually increased to about one third with 24 h of incubation. Dextran sulfate dose-dependently reduced apoptotic cells, but it did not cause concomitant increase in viable cells. Both DNA levels and lactate dehydrogenase activities in the supernatants of dextran sulfate-treated cultures were significantly higher than those in the supernatants of untreated cultures. Concomitantly, DNA levels and lactate dehydrogenase activities in the cell pellets of dextran sulfate-treated cultures were lower than those in the cell pellets of untreated cultures. Addition of dextran sulfate to the culture of B cells 18 h after the start of incubation, when about one fifth of the B cells were dead, significantly reduced apoptotic cells during the next 6-h incubation. This decrease in the number of apoptotic cells was detectable as early as 1 h after addition of dextran sulfate and was prevented by Zn^{2 +}, Co ^{2 +}, Ni^{2 +}, the serine protease inhibitor 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF) or incubation on ice. These results indicated that dextran sulfate treatment did not prevent apoptosis but rather promoted degradation of apoptotic cells and suggest the involvement of DNase and protease in this process.