Reprogramming IgH isotype-switched B cells to functional-grade induced pluripotent stem cells.

Induced pluripotent stem cells (iPSCs) can be formed from somatic cells by a defined set of genetic factors; however, aberrant epigenetic silencing of the imprinted Dlk1-Dio3 gene cluster often hinders their developmental potency and ability to contribute to high-grade chimerism in mice. Here, we describe an approach that allows splenic B cells activated to undergo Ig heavy-chain (IgH) class-switch recombination (CSR) to be reprogrammed into iPSCs that contribute to high-grade chimerism in mice. Treatment of naïve splenic B cells in culture with anti-CD40 plus IL-4 induces IgH CSR from IgM to IgG1 and IgE. CSR leads to irreversible IgH locus deletions wherein the IgM-producing Cμ exons are permanently excised from the B-cell genome. We find that anti-CD40 plus IL-4-activated B cells produce iPSCs that are uniformly hypermethylated in the imprinted Dlk1-Dio3 gene cluster and fail to produce chimerism in mice. However, treatment of activated B cells with the methyltransferase inhibitor 5-aza-2'-deoxycytidine before and at early stages of reprogramming attenuates hypermethylation of the Dlk1-Dio3 locus in resultant iPSCs and enables them to form high-grade chimerism in mice. These conditions allowed us to produce chimeric mice in which all mature B cells were derived entirely from IgG1-expressing B-cell-derived iPSCs. We conclude that culture conditions of activated B cells before and at early stages of reprogramming influence the developmental potency of resultant iPSCs.