Abstract
Channel catfish, Ictalurus punctatus, express two different IgD+ B cell populations. Catfish IgM+/IgD+ B cells are small and agranular, while IgM-/IgD+ B cells are larger and exhibit plasmablast morphology. The use of cell sorting, flow cytometry and RT-PCR demonstrated that IgD+ B cell expression varies between individual catfish. For example, some catfish have little to no IgD+ B cells in their peripheral blood leukocytes (PBL), while in others the IgM-/IgD+ B cell population can represent as much as 72%. This contrasts to the situation in mammals where IgM-/IgD+ B cells are rare. Also, transfection studies show IgD functions as a typical BCR since Igδ chains associate with accessory molecules CD79a and CD79b. Furthermore, IgM-/IgD+ B cells preferentially express IgL σ chains, while IgM+/IgD+ B cells can express any of the four catfish IgL isotypes. As assessed by 5'-RACE, all membrane IgD transcripts from sorted IgM-/IgD+ B cells contained viable VH rearrangements with no bias in family member usage. However, all secreted IgD transcripts were V-less and began with a leader sequence spliced to the Igδ CH1 domain and it is likely that these transcripts encode for the secreted form of IgD previously found in catfish serum. Importantly, the Igδ leader mediated IgD protein secretion when transfected into catfish B cells. This characterization of distinct IgM+/IgD+ and IgM-/IgD+ B cells in teleosts provides an important first step in understanding the evolution of IgD function.
