Telomerase is up-regulated in human germinal center B cells in vivo and can be re-expressed in memory B cells activated in vitro.

The extensive proliferation that B lymphocytes undergo in germinal centers could compromise generation of long term B cell memory if there occurs shortening of the telomeres of germinal center B cells with cell division. Telomere length, which is thought to act as a "mitotic clock" for somatic cells that dictates cellular senescence, can be preserved by the enzyme telomerase. Human tonsil germinal center B cells consistently expressed 100- to 1000-fold higher levels of telomerase activity than naive or memory B cells, which had no or very low detectable activity, as analyzed by the telomere repeat amplification protocol assay. In vitro stimulation of human memory B cells through surface Ig or CD40 was capable of up-regulating telomerase. The findings suggest that longevity of B cell memory is maintained, despite multiple cell divisions in the generation of a memory B cell, by up-regulation of telomerase in germinal center and activated memory B cells.