The role of programmed cell death in the regulation of bone marrow and germinal center B cells in a bcl-2 transgenic animal

Abstract

Antibody-mediated autoimmunity is thought to result from the dysregulation of B cells, rather than inheritance of specific autoantibody-encoding genes. Thus, normal individuals are constantly generating autoreactivity and then downregulating these anti-self B cells. Apoptosis, or programmed cell death, is one mechanism by which the immune system removes potentially pathogenic cells. The following experiments use a transgenic mouse expressing the anti-apoptotic bcl-2 gene in B cells to explore the role of programmed cell death in this compartment.;Deletion of autoreactive B cells was studied in double transgenic mice expressing both bcl-2 and the R4A anti-DNA antibody heavy chain gene. High affinity anti-DNA B cells were isolated from spleens of R4A/bcl-2 double transgenic animals, suggesting that bcl-2 can promote the survival of anti-DNA B cells that are normally destined for elimination.;Autoreactive B cell clones may also be activated by an ongoing immune response. This was studied using a model in which anti-DNA B cells are generated during the antibody response against phosphorylcholine (PC). We have shown that crossreactive anti-DNA, anti-PC clones are rescued in the bcl-2 transgenic mouse during the primary anti-PC response. These B cells produced unmutated antibodies. We believe that they have been rescued from immunologic tolerance by a combination of the bcl-2 transgene and an anti-PC response. Serum anti-DNA antibodies were not increased, suggesting that the crossreactive B cells were not able to fully escape tolerance.;Germinal centers are zones of antigen-activated B cells where apoptosis is thought to mediate antigen selection. While no evidence for altered antigen selection was observed in bcl-2 transgenic mice immunized with PC, somatic mutations in germinal center antibody clones from these mice were not clustered at hotspot motifs, suggesting that the targeting of mutations to hotspots may be uncoupled from somatic hypermutation.;These studies show that B cell regulation by bcl-2 occurs at multiple levels. Both central and peripheral autoreactive B cells have prolonged survival, but secretion of autoantibodies into the serum only occurs in the double transgenic R4A/bcl-2 model. In germinal centers, the bcl-2 transgene led to an altered pattern of somatic hypermutation, but did not affect antigen selection.