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Polymerase μ is up‐regulated during the T cell‐dependent immune response and its deficiency alters developmental dynamics of spleen centroblasts

Mammalian DNA polymerase μ (Polμ), preferentially expressed in secondary lymphoid organs, is shown here to be up‐regulated in germinal centers after immunization. Alternative splicing appears to be part of Polμ regulation during an immune response. We generated Polμ‐deficient mice that are viable and show no anatomical malformation or serious alteration in lymphoid populations, with the exception of an underrepresentation of the B cell compartment. Young and aged homozygous Polμ–/– mice generated similar immune responses after immunization with the hapten (4‐hydroxy‐3‐nitrophenyl)acetyl (NP) coupled to chicken gammaglobulin (CGG), compared with their wild‐type littermates. Nonetheless, the kinetics of development of the centroblast population showed significant differences. Hypermutation analysis of the rearranged heavy chain intron region in centroblasts isolated from NP‐CGG‐immunized Polμ–/– mice showed a similar quantitative and qualitative somatic mutation spectrum, but a lower representation of heavily mutated clones. These results suggest that although it is not a critical partner, Polμ modulates the in vivo somatic hypermutation process.