Zebrafish B Cell Development without a Pre-B Cell Stage, Revealed by CD79 Fluorescence Reporter Transgenes

CD79a and CD79b proteins associate with Ig receptors as integral signaling components of the B cell Ag receptor complex. To study B cell development in zebrafish, we isolated orthologs of these genes and performed in situ hybridization, finding that their expression colocalized with IgH-μ in the kidney, which is the site of B cell development. CD79 transgenic lines were made by linking the promoter and upstream regulatory segments of CD79a and CD79b to enhanced GFP to identify B cells, as demonstrated by PCR analysis of IgH-μ expression in sorted cells. We crossed these CD79-GFP lines to a recombination activating gene (Rag)2:mCherry transgenic line to identify B cell development stages in kidney marrow. Initiation of CD79:GFP expression in Rag2:mCherry⁺ cells and the timing of Ig H and L chain expression revealed simultaneous expression of both IgH-μ- and IgL-κ-chains, without progressing through the stage of IgH-μ-chain alone. Rag2:mCherry⁺ cells without CD79:GFP showed the highest Rag1 and Rag2 mRNAs compared with CD79a and CD79b:GFP⁺ B cells, which showed strongly reduced Rag mRNAs. Thus, B cell development in zebrafish does not go through a Rag^hi CD79⁺IgH-μ⁺ pre-B cell stage, different from mammals. After the generation of CD79:GFP⁺ B cells, decreased CD79 expression occurred upon differentiation to Ig secretion, as detected by alteration from membrane to secreted IgH-μ exon usage, similar to in mammals. This confirmed a conserved role for CD79 in B cell development and differentiation, without the requirement of a pre-B cell stage in zebrafish.