Age- and tissue-specific differences in human germinal center B cell selection revealed by analysis of IgV_H gene hypermutation and lineage trees

The elderly produce increased levels of antibodies to autologous antigens and are less able to make high-affinity antibodies to foreign antigens. Ig gene hypermutation is integral to the affinity maturation process but previous studies of hypermutation with age have yielded conflicting results. The cells studied have represented post-germinal center (GC) populations and, therefore, the results may be complicated by possible differences in activation history. We studied Ig genes from GC B cells to elucidate which factors in the affinity maturation process change with age. Age-related changes in the pattern of hypermutation were seen, although the analysis of variable region heavy chain (V_H) genes and their lineage trees shows that an alteration in the mechanism of somatic hypermutation is unlikely. The changes are due to founder cell effects and/or the process of selection. Striking tissue-specific differences were seen. All measurements indicated that selection of Ig genes may decrease in Peyer's patch GC but increase in splenic GC with age. These tissue-specific differences highlight the importance of considering the activation and effector sites when studying immune senescence.