Functional characterization and epigenetic regulation of Tcf3a and Tcf3b during IgM B cell activation in large yellow croaker (Larimichthys crocea).

The transcription factor Tcf3 is a key regulator during mammalian B cell development and activation, which is highly expressed in precursor B cells, downregulated in mature B cells, and re-expressed upon B cell activation. Despite the extensive studies in mammals, its role in teleost B cells remains poorly characterized. Here, using the economically important marine teleost large yellow croaker (Larimichthys crocea) as a model, we identified two Tcf3 homologs (LcTcf3a and LcTcf3b) and found that both genes were upregulated during B cell activation. By cloning the predicted promoter region and serial truncation, we found that the core promoter regions of LcTcf3a and LcTcf3b were located about 600 bp around the transcription start sites. Interestingly, while both of the two gene promoters contain CpG islands, bisulfite sequencing showed constitutive hypomethylation both before and after B cell activation. However, in vitro methylation suppressed the promoter activities of LcTcf3a and LcTcf3b, indicating their DNA methylation-dependent regulation. Together, these results suggested a "poised" state of LcTcf3a and LcTcf3b in resting B cells, where LcTcf3a and LcTcf3b were repressed but preparing for fast upregulation during immune response. Our findings proposed that LcTcf3a and LcTcf3b likely contribute to B cell activation in a teleost model and their promoter activities may be regulated by DNA methylation, thus providing insights into the understanding of the teleost B cell immunity.