The B29 and mb‐1 polypeptides are differentially expressed during human B cell differentiation

Abstract Surface immunoglobulin on mouse B cells is associated with a heterodimer comprising the products of the mb‐1 and B29 genes. Here we report that antibodies raised against a peptide sequence from the intracytoplasmic C terminus of the B29 murine gene product detect the 37‐kDa component of the human heterodimer, indicating that this component in man is also encoded by the B29 gene. The immunocytochemical reactivity of these anti‐B29 antibodies was compared with those of antibodies to the mb‐1 protein. Of 25 cases of acute lymphoblastic leukaemia (ALL), 24 were positive for mb‐1 whereas B29 was expressed in only 13 cases. Most of these B29‐positive ALL expressed immunoglobulin μ heavy chain in their cytoplasm (pre‐B ALL). In lymphoid tissue sections, anti‐B29‐labeled B cell follicles in a similar fashion to anti‐mb‐1, with the striking exception that plasma cells were unreactive for B29, but positive for mb‐1. These results suggest that the synthesis of B29 begins later in precursor B cells than that of mb‐1, and ceases before the terminal plasmacyte phase.