Abstract
For decades, it has remained unknown whether artiodactyls, such as cattle, pigs, and sheep, express immunoglobulin D (IgD), although the δ gene was identified in these species nearly 10 years ago. By developing a mouse anti-bovine IgD heavy chain monoclonal antibody (13C2), we show that secreted bovine IgD was present mainly as a monomer in serum and was heavily glycosylated by N-linked saccharides. Nonetheless, IgD was detectable in some but not all of the Holstein cattle examined. Membrane-bound IgD was detected in the spleen by western blotting. Flow cytometric analysis demonstrated that IgD-positive B cells constituted a much lower percentage of B cells in the bovine spleen (∼6.8% of total B cells), jejunal Peyer's patches (∼0.8%), and peripheral blood leukocytes (∼1.2%) than in humans and mice. Furthermore, IgD-positive B cells were almost undetectable in bovine bone marrow and ileal Peyer's patches. We also demonstrated that the bovine δ gene can be expressed via class switch recombination. Accordingly, bovine δ germline transcription, which involves an Iδ exon and is highly homologous to Iμ, was confirmed. However, we could not identify an Iδ promoter, despite bovine Eμ demonstrating both enhancer and promoter activity. This study has answered a long-standing question in cattle B cell biology and significantly contributes to our understanding of B cell development in this species.
Highlights
Immunoglobulin D (IgD) was initially discovered by Rowe and Fahey in the serum of a myeloma patient in 1965 [1]
We performed a rapid amplification of cDNA ends (RACE) PCR using primers derived from the bovine dCH3 exon and spleen total RNA as a template
We revealed that the bovine d gene can be expressed via a class switch recombination (CSR) process
Summary
Immunoglobulin D (IgD) was initially discovered by Rowe and Fahey in the serum of a myeloma patient in 1965 [1]. Since its discovery in humans and mice, IgD has long been thought to exist only in primates and rodents and to represent a recently developed Ig class. This belief was not challenged until a milestone paper published by Wilson and colleagues in 1997 that reported the presence of the d gene in a teleost, the channel catfish (Ictalurus punctatus) [2]. IgD is believed to be as ancient as IgM in terms of its evolution [3], despite its absence in a number of species, such as birds [7] and select mammalian species, including rabbits and opossums [8,9]
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