Expression of a mouse-channel catfish chimeric IgM molecule in a mouse myeloma cell

Fusion genes encoding a murine V H domain and the constant region domains of the μ chain from the channel catfish, Ictalurus punctatus, were stably expressed in the λ light chain producing mouse myeloma cell line J558L. Although the pathways of pre-mRNA processing for expression of membrane (μm) and secreted (μs) forms of the μ chain differ between mammals and teleosts, mRNAs encoding both catfish μm and μs were correctly expressed in the mouse myeloma cells. The mouse-channel catfish chimeric μ chain polypeptide was able to associate covalently with the mouse λ light chain and assemble, intracellularly, into polymers of covalent structure ( μL) 2−8 which resembled those seen with native catfish IgM. In contrast to native catfish IgM, the mouse-catfish chimeric IgM showed the property of binding strongly to protein A of Staphylococcus aureus. The mouse-channel catfish chimeric IgM was core-glycosylated, but did not contain terminal sialic acid. Secretion rates for the chimeric IgM were low, and the possibility could not be excluded that extracellular chimeric IgM was released from dead or dying cells. The reason(s) for the intracellular retention of the chimeric IgM (probably in the endoplasmic reticulum) are not known, but those mechanisms involving retention via cysteine residues were excluded.