Somatic mutation of the CD95 gene in human B cells as a side-effect of thegerminal center reaction

Somatic hypermutation specifically modifies rearranged immunoglobulin (Ig) genes in germinal center (GC) B cells. However, the bcl-6 gene can also acq uire somatic mutations during the GC reaction, indicating that certain non- Ig genes can be targeted by the somatic hypermutation machinery. The CD95 g ene, implicated in negative selection of B lymphocytes in GCs, is specifica lly expressed by GC B cells and was recently identified as a tumor suppress or gene being frequently mutated in (post) GC B cell lymphomas. In this stu dy, the 5' region (5'R) and/or the last exon coding for the death domain (D D) of the CD95 gene were investigated in naive, GC, and memory B cells from seven healthy donors. About 15% of GC and memory, but not naive, B cells c arried mutations within the 5'R (mutation frequency 2.5 x 10(-4) per base-p air). Mutations within the DD were very rare but could be efficiently selec ted by inducing CD95-mediated apoptosis: in 22 apoptosis-resistant cells, 1 2 DD mutations were found. These results indicate that human B cells can ac quire somatic mutations of the CD95 gene during the GC reaction, which pote ntially confers apoptosis resistance and may counteract negative selection through the CD95 pathway.