Two Maturation-associated Mouse Erythrocyte Receptors of Human B Cells. II. Isolation and Partial Characterization of a B-cell Lectin with Specificity of R1

Trypsin treatment of chronic lymphocytic leukaemia (CLL) cells which have the capacity to rosette with mouse erythrocytes (M), the BM+ subtype, inhibits their capacity to rosette and releases a substance into the supernatant which agglutinates mouse and rat erythrocytes but not erythrocytes of five other species tested. This substance has been named immature B-cell lectin (IBL). The specificity of IBL was further demonstrated by fluorescence labelling, absorption and latex rosetting. IBL does not bind to pronase-treated M (pro M), indicating that it has the specificity of R1 as distinct from R2 which binds to a pronase-resistant ligand on M. Other evidence that IBL is associated with B-cell membrane receptors for mouse erythrocytes is as follows: (1) The amount of IBL released into the supernatant correlated with the trypsin sensitivity of M rosetting with different clones of BM+ CLL cells. (2) Only small amounts of IBL were released from non-rosetting cells (T cells and mature B cells). (3) Binding properties of IBL were inhibited by extract of M but not extract from ox erythrocytes. (4) High-titre solutions of IBL conferred the capacity to form M rosettes on certain types of non-rosetting B cells. IBL has a dual binding specificity. Its binding to M is inhibited by fetuin and mannan, while its binding to B cells is not inhibited by these substances. The relationship of IBL to other membrane lectins including fibronectin is discussed. Preliminary characterization indicates a high-molecular-weight (at least 300,000 daltons) glycoprotein which has a pronounced tendency to aggregate in solution. The relationship of IBL to stages of human B-cell maturation is discussed.