IgD+ cell populations in channel catfish, Ictalurus punctatus

Channel catfish, Ictalurus punctatus, express two Ig isotypes, IgM and IgD. While catfish IgM has been extensively studied at the functional and structural levels, much less is known about IgD. Catfish mIgD cDNA encodes for a chimeric molecule consisting of a rearranged VDJ region spliced to the Cmicro1 domain, followed by seven Cdelta domains, a transmembrane region followed by a short cytoplasmic tail. Here, through the use of anti-IgM and anti-IgD monoclonal antibodies three different B cell populations were identified that differentially express IgM and IgD; IgM+/IgD -, IgM+/IgD+ and IgM-/IgD +. Catfish IgM+/IgD- and IgM +/IgD+ B cells are small and agranular. In contrast, IgM-/IgD+ B cells are larger and exhibit a plasmablast morphology. The use of cell sorting, flow cytometry and RT-PCR demonstrates that IgD+ B cell expression varies between individuals. For example, some catfish have <5% IgD+ B cells in their PBL, while in others the IgM-/IgD+ B cell population can represent as much as 72% of the total PBL. Furthermore, IgD expressed by IgM-/IgD+ B cells preferentially associates with IgL sigma, while IgM+/IgD+ B cells can express any of the four catfish IgL isotypes. Also, (1) transfection studies show IgD functions as a typical B-cell receptor, since Igdelta chains associate with CD79a and CD79b molecules; and (2) all membrane IgD transcripts from sorted IgM-/IgD+ B cells contain viable VDJ rearrangements with no bias in VH family member usage. Interestingly, all secreted IgD transcripts from IgM+/IgD+ and IgM-/IgD+ B cells were V-less and began with a leader spliced to Cdelta1. Importantly, this leader mediated IgD protein secretion when transfected into catfish B cells. Finally in some catfish, a third IgD+ population consisting of granulocyte-like cells of unknown origin was identified. These granulocytes do not express message for IgD or IgM and are armed with exogenous IgD from serum that is placed on the cell surface by a putative IgD- binding FcdeltaR. Additionally, it appears that upon IgD cross-linking these granulocytes undergo degranulation. Together, these findings imply that catfish IgM-/IgD + B cells likely expand in response to certain pathogens and that secreted IgD, which has previously been shown to be expressed in serum functions through its Fc-region. Thus catfish secreted IgD, as has been suggested for human IgD, may function as a pattern recognition molecule.