DNA polymerase η is a limiting factor for A:T mutations in Ig genes and contributes to antibody affinity maturation

DNA polymerase η (POLH) is required for the generation of A:T mutations during the somatic hypermutation of Ig genes in germinal center B cells. It remains unclear, however, whether POLH is a limiting factor for A:T mutations and how the absence of POLH might affect antibody affinity maturation. We found that the heterozygous Polh^+/- mice exhibited a significant reduction in the frequency of A:T mutations in Ig genes, with each type of base substitutions at a level intermediate between the Polh^+/- and Polh^-/- mice. These observations suggest that Polh is haplo-insufficient for the induction of A:T mutations in Ig genes. Intriguingly, there was also a reduction of C to T and G to A transitions in Polh^+/- mice as compared with WT mice. Polh^/- produced decreased serum titers of high-affinity antibodies against a T-dependent antigen, which was associated with a significant reduction in the number of plasma cells secreting high-affinity antibodies. Analysis of the V region revealed that aa substitutions caused by A:T mutations were greatly reduced mice. Polh^/- mice. These results demonstrate that POLH is a limiting factor for A:T mutations and contributes to the efficient diversification of Ig genes and affinity maturation of antibodies.