IgM and IgD, the two forms of the B cell receptor on naïve mature B cells, are generated by alternative splicing of long primary RNA transcripts from the immunoglobulin heavy chain (Igh) locus. While immature B cells only express IgM, mature follicular or marginal zone B cells co-express IgM and IgD and self-reactive anergic B cells further down-regulate IgM and mainly express IgD. Despite great efforts, the factor regulating this alternative splicing has not been identified.
Through a flow cytometry screen in mice after ENU mutagenesis we identified zinc-finger protein ZFP318 as a critical regulator of IgD expression. ZFP318 is an evolutionary highly conserved zinc-finger protein that is predominantly expressed in mature B cells and the testis. Mice deficient for Zfp318 have increased expression of IgM and almost no IgD on all B cells. Despite this, B cell development appears to be normal in Zfp318 knock-out mice, with only a slight reduction in total B cells. This reduction becomes more pronounced in the competitive situation of 50:50 mixed bone marrow chimeras. Functional studies show little difference in germinal centre formation in vivo and in 24hour activation in vitro but proliferation after 72 hours was increased in all treatments that included anti-IgM. To identify differentially expressed genes and exons we have performed RNAseq of wild-type and Zfp318 knock-out follicular B cells. An initially analysis shows a near complete absence of Ighd exons. Our findings identify ZFP318 as a crucial factor regulating the expression of the two major antibody isotypes on the surface of most mature B cells.