Mef2d regulates mutually exclusive expression of IgZ and IgM isotypes through epigenetic modulation in a zebrafish model

The discovery of IgZ, or its counterpart IgT, represents a novel immunoglobulin isotype (named ζ or τ) found in teleosts, introducing a new member to the existing Ig classes (µ, δ, γ, α, and ε) among vertebrates. The distinctive intrachromosomal organization of ighz and ighm loci implies the necessity of a distinct, mutually exclusive recombination process for the independent generation of IgZ and IgM isotypes. However, the molecular mechanisms governing this process remain elusive. In this study, we unveil the regulatory function of myocyte enhancer factor 2D (Mef2d) in the assembly of ighz genes through epigenetic modulation in a zebrafish model. Mechanistically, Mef2d selectively hinders the recombination of ighm locus in IgZ⁺ B cells by binding to the 3′Eµ site of the ighm locus and helping establish a repressive modification pattern of H3K4me0/H3K9me2/H3K27me2 in Dµ/Jµ regions through recruiting the co-repressive Sin3/Hdac1 complex with the assistance of cohesin complex and Setdb1/Ezh2 methyltransferases. Consequently, this renders the Dµ/Jµ regions inaccessible to Rag1/2, thus preventing ighm rearrangement. As a pivotal regulator for IgZ isotype production, Mef2d exhibits differential expression in committed IgZ⁺ B cells, a process regulated by the Il-7/Il-7r-mediated p38 Mapk signaling pathway. These results indicate the existence of a unique isotypic exclusion mechanism underlying recombination between ighz and ighm locus in teleosts. This mechanism highlights an unrecognized strategy for generating diverse isotypes in vertebrates, distinct from the well-established class switch recombination process. This study significantly contributes to our understanding of paradigms, diversifications, and the evolutionary history of vertebrate adaptive immunity.