[Analysis of expression of the molecules Igalpha and Igbeta in human pro-B leukemic lines]

Ig alpha and Ig beta are two glycosylated transmembrane proteins of the Ig superfamily that are encoded by the B cell-specific genes mb-1 and B29, respectively. Ig alpha/Ig beta heterodimers may associate with the mu/surrogate light chains (psi LC) complex and with membrane Immunoglobulins on the surface of pre-B and B cells, respectively. They play a crucial role in the signal transduction that follows pre-B and B cell receptor cross-linking. Previous works have shown that mb-1 and B29 transcripts are expressed in normal mouse and human pro-B cells. However, little is known about the expression of Ig alpha and Ig beta molecules in pro-B cells. To address this issue we first analysed the expression of the Ig alpha and Ig beta molecules in the RS4; 11 and Nalm16 human pro-B cell lines using specific monoclonal antibodies. We found that both cell lines expressed Ig alpha and Ig beta but this expression was limited to the cytoplasm compartment. Three forms (44, 40 and 36 kDa) of the Ig alpha molecule and a single form (36 kDa) of the Ig beta molecule were detected in these lines. The heterogeneity of the Ig alpha molecule was partly related to the presence of a truncated Ig alpha protein which is likely the product of a short mb-1 transcript expressed in these cell lines. This short transcript is generated by alternative splicing of the mb1 mRNA with loss of exon 2. Ig alpha heterogeneity was also related to the expression of different glycosylated forms of the Ig alpha molecule. Only a minor fraction of the Ig alpha and Ig beta molecules associate with each other to form Ig alpha/Ig beta heterodimers and Ig beta homodimers. In these pro-B cell lines Ig alpha and Ig beta were found to associate with the lyn tyrosine kinase, suggesting that they may play some functional role at this B cell differentiation stage. Transfection of muHC gene in the Nalm16 cells results in the assembly of the pre-B receptor and in its expression on the cell surface. The level of surface expression of the pre-B receptor was found to correlate with the level of muHC and psi LC synthesis and with the level of association of the different components of the pre-B receptor with each other. Analysis of the 697 and Nalm6 pre-B cells and of the Ramos B cells indicated that heterogeneity of Ig alpha and Ig beta increases as a function of B cell differentiation.