YY1 Controls Em-3’RR DNA Loop Formation and Immunoglobulin Heavy Chain Class Switch Recombination.. Mus musculus

Activation of splenic B cells induces formation of a 220kb DNA loop between Em and 3’RR enhancers in the immunoglobulin heavy chain locus (IgH). This DNA loop has been proposed to be necessary for the crucial immune diversification mechanism of IgH class switch recombination, but the factors that control its formation are unknown. We show that conditional deletion of transcription factor YY1 in primary splenic B cells results in a dramatic drop in formation of this DNA loop, as well as immunoglobulin class switch recombination. Reconstitution of YY1-deleted splenic B cells with various YY1 mutants showed that the C-terminal half of YY1 lacking the transactivation domain restored both Em-3’RR DNA loop formation as well as class switch recombination. RNA transcript analyses of YY1 conditional deleted splenic B cells suggest that YY1 does not regulate genes needed for DNA looping or CSR. Our results argue for a direct physical mechanism of YY1 mediating long-distance DNA loops and provide strong evidence of the importance of this DNA loop for class switching. Our results provide foundational mechanistic insight into a crucial immune function. Overall design: Follicular B cells were isolated from the spleens of three C57Bl/6 yy1 fl/fl mice. For each spleen, half the cells received mock treatment and half received TATCRE. The 6 samples were then grown in RPMI medium along with LPS, Il4, OPI, and 20% FBS for 72 hours. The 6 groups of cells were lysed and RNA was isolated for library preparation. Expression differences between Mock and TATCRE treated cells were determined to understand the role of yy1 in B cell class switching.