Regulation of B Cell Apoptosis

Abstract

Only 1% of thymocytes have hypodiploid nuclei, an index of apoptosis. Yet there is evidence that 99% of thymocytes die by apoptosis, and are destroyed so quickly by phagocytes they are hardly apparent histologically. So when we see 1% of spleen B (or T) cells have hypodiploid nuclei, it tells us little about the apoptotic turnover of these cells. Despite evidence that germinal center B cells enter apoptosis when activated through their antigen receptor in the absence of costimulation¹, and that terminal differentiation also leads to apoptosis, life span estimates for the majority of mature B cells run to several months^2,3. So we were surprised to find that small dense B cells from the spleens of specific pathogen-free mice spontaneously undergo apoptosis in vitro ⁴ so fast that about 75% of small dense B cells have hypodiploid nuclei by 64h. The rate is linear with no lag period to indicate an induction process. Varying fetal calf serum concentration from 1 to 10%, varying the B cell (or T cell) concentration from 0.1 to 10 × 10⁶/ml (to rule out autocrine mechanisms), and purifying the cells (adherence to BSA-coated plastic, exposure to anti-thy 1.2 plus complement, centrifugation through Percoll) had no major effect on the spontaneous apoptosis rate⁵(and our unpublished data). Neither did inclusion of 10% normal mouse serum, or coculture with other spleen cell types⁵. Cycloheximide (CHX) and other protein synthesis inhibitors accelerated apoptosis in B and T cells^4,5 while blocking apoptosis in thymocytes^5,6. We appeared to be observing an apoptosis program “hard-wired” into the B cell, very different from that of the thymocyte, requiring no stimulation and no “death protein” synthesis.