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(6) strongly supported this latter possibility. Thus EBV-positive Burkitt lymphoma (BL) cell lines displaying the latency I form of infection, in which only the nuclear antigen EBNAl is expressed, could readily be induced into apoptosis by signals such as serum withdrawal or treatment with calcium ionophore (5). In contrast BL lines that had progressed on serial passage to a latency III form of infection, with expression of the full spectrum of latent proteins, including nuclear antigens EBNAs 1, 2, 3A, 3B, 3C, and LP and the latent membrane proteins (LMPs) 1 and 2, showed enhanced cell survival under such conditions. Gene transfers into apoptosis-sensitive B-cell lines mapped this protective effect to LMP1 and indicated a likely mechanism, namely, LMPl-mediated up-regulation of the cellular protein Bcl-2 (7). Importantly Bcl-2, a membrane-associated protein with a distinctive pattern of cytoplasmic localization (8, 9), not only enhances cell survival in a variety of experimental situations (7, 9-15) but also appears to be involved in the physiological selection of B cells into memory (16 (Fig. 1).Transfections. Transient transfections into COS-1 cells using DEAE-dextran and stable transfections into the EBVpositive BL cell lines Wan-BL (23), Akata-BL (24), and Raji-BL (25) by using electroporation were performed as described (7,26). For the latter, selection was in hygromycin B at 300 ug/ml (pHEBO-based constructs) or G418 at 2.5 mg/ml ref. 27

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B cells undergo selection within germinal centers on the basis of their capacity to be activated by antigen held on follicular dendritic cells. Isolated germinal center B cells in culture kill themselves by apoptosis but this is prevented if their receptors for antigen are cross-linked. In this study it is confirmed that almost all germinal center B cells, unlike other B cells, do not express the 25-kDa protein encoded by the bcl-2 oncogene. Cross-linking the surface Ig of isolated germinal center cells causes them to express bcl-2 protein. Two other stimuli which inhibit the entry of germinal center cells to apoptosis result in the expression of bcl-2 protein. These stimuli are: (a) CD40 antibody and (b) recombinant 25-kDa fragment of the CD23 protein plus recombinant interleukin 1 alpha. Respectively, these induce germinal center cells to differentiate to resting B cells or plasmablasts. Dual-fluorescence studies on small lymphocytes confirm the presence of bcl-2 protein in mitochondria but show that this is also present in other extra-nuclear areas. Burkitt lymphoma cells have a phenotype which indicates that they are neoplastic cells of germinal center origin. The expression of bcl-2 protein by Burkitt lymphoma lines was also studied. Burkitt lines which retain the phenotype of fresh Burkitt lymphoma cells can be induced to enter apoptosis on culture with the Ca2+ ionophore ionomycin. These cells were found not to express bcl-2 protein. By contrast, Burkitt lines which have drifted towards a lymphoblastoid cell line phenotype and are resistant to the induction of apoptosis express high levels of the bcl-2 protein. The findings support the concept that the susceptibility of germinal center cells to entering apoptosis is associated with their lack of expression of bcl-2 protein. Aberrant expression of bcl-2 protein by some neoplastic germinal center cells may allow survival in situations where their normal counterparts die.