The histone methyltransferase Suv39h1 increases class switch recombination specifically to IgA

Ab class (isotype) switching allows the humoral immune system to adaptively respond to different infectious organisms. Isotype switching occurs by intrachromosomal DNA recombination between switch (S) region sequences associated with C, region genes. 14 Although isotype-specific transcription of unrearranged (germline) C-H genes is required for switching, recent results suggest that 14 isotype specificity is also determined by the sequences of downstream (acceptor) S regions. In the current study, we identify the histone methyltransferase Suv39h1 as a novel S alpha-specific factor that specifically increases IgA switching (S mu-S alpha recombination) 14 in a transiently transfected plasmid S substrate, and demonstrate that this effect requires the histone methyltransferase activity 14 of Suv39h1. Additionally, B cells from Suv39h1-deficient mice have an isotype-specific reduction in IgA switching with no effect on the level of germline I alpha-C alpha transcripts. Taken together, our results suggest that Suv39h1 activity inhibits the activity of a sequence-specific DNA-binding protein that represses switch recombination to IgA.