The IgA/IgM receptor expressed on a murine B cell lymphoma is poly-Ig receptor

T560, a mouse B lymphoma that originated in gut-associated lymphoid tissue, expresses receptors that bind dimeric IgA and IgM in a mutually inhibitory manner but have little affinity for monomeric IgA, Evidence presented in t his paper indicates that the receptor is poly-Ig receptor (pIgR) known in h umans and domestic cattle to hind both IgA and IgM. The evidence includes t he demonstration that binding of IgM is J chain dependent, and that pig-pre cipitated receptor has an appropriate M-r of 116-120 kDa and can be detecte d on immunoblots with specific rabbit anti-mouse pIgR, Overlapping RT-PCR p erformed using template mRNA from T560 cells and oligonucleotide primer pai rs designed from the published sequence of mouse liver pIgR indicate that T 560 cells express mRNA virtually identical with that of the epithelial cell pIgR throughout its external, transmembrane, and intracytoplasmic coding r egions. Studies using mutant IgAs suggest that the C alpha2 domain of dimer ic IgA is not involved in high-affinity binding to the T560 pIgR, Inasmuch as this mouse B cell pIgR binds IgM better than IgA, it is similar to human pIgR and differs from rat, mouse, and rabbit epithelial cell pIgRs that bi nd IgA but not IgM. Possible explanations for this difference are discussed . All clones of T560 contain some cells that spontaneously secrete both IgG 2a and IgA, but all of the IgA recoverable from the medium and from cell ly sates is monomeric; it cannot be converted to secretory IgA by T560 cells.