TRANSCRIPTION AND PROTEIN EXPRESSION OF MB-1 AND B29 GENES IN HUMAN HEMATOPOIETIC MALIGNANCIES AND CELL-LINES

The transmembrane forms of all immunoglobulin (lg) classes are associa ted with two glycoproteins, mb-1 and B29, that are crucial for signal transduction following antigen binding to the lg molecule. We have inv estigated the transcription and protein expression of mb-1 and B29 gen es during B-cell development. Sixty human continuous cell lines (35 B- lineage, 11 T-lineage, 11 myeloid-lineage and three non-hematopoietic) and 75 hematopoietic malignancies (55 B-lineage, 12 T-lineage and eig ht myeloid-lineage), were tested for RNA expression by Northern blotti ng experiments with the mb-1 pRA3 cDNA probe, and a newly isolated B29 cDNA probe. Protein expression was analyzed by immunofluorescence mic roscopy of cytocentrifuge preparations, which were labeled with the an ti-mb-1 HM57 monoclonal antibody (mAb) and an anti-B29 polyclonal anti serum, directed against intracellular epitopes of these polypeptides. Except for two early precursor B-cell lines, mb-1 and B29 transcripts and proteins were detected in all B-cell lines and B-cell malignancies , i.e. from immature to more mature B cells, irrespective of their lg class expression. Transcription of mb-1 genes seems to be down-regulat ed at the plasma cell stage, because no mb-1 transcripts and mb-1 prot eins could be detected in the four plasma cell lines and two plasma ce ll leukemias tested. B29 transcripts were detectable in these cell sam ples, but low levels of B29 proteins were only detected in one plasma cell line. The HM57 mAb gave strong labeling on fresh cytocentrifuge p reparations of all B-cell samples, and this mb-1 protein expression ap peared to be B-cell specific. We therefore conclude that the HM57 mAb is well suited for the detection of the mb-1 molecule as a pan-B-cell marker for the diagnosis of immature and mature B-cell malignancies. T he expression pattern of the mb-1 protein is comparable to that of the CD19 and CD22 antigens, but has the advantage of being B-lineage spec ific. Although B29 protein expression was restricted to B-lineage cell s, the anti-B29 antiserum is less suitable for diagnosis of B-cell mal ignancies, because of the variable and generally weak signals on cytoc entrifuge preparations.