CD79 alpha/CD79 beta heterodimers are expressed on pro-B cell surfaces without associated mu heavy chain.

During B cell development, the surface expression of CD79a/CD79b heterodimers had been thought to begin in the pre-B cell stage where the heterodimers constitute pre-B cell receptors together with m heavy and surrogate light chains. Thereafter, in mature B cells, CD79a/CD79b associates with surface Ig to form B cell antigen receptors. In this study, we revealed by using newly established mAb that CD79b was expressed on the surface of pro-B cells which had not undergone the productive Ig gene rearrangement. Biochemical analysis showed that CD79b on pro-B cells existed either as monomers or as disulfide-linked heterodimers with CD79a, noncovalently associated with four unidentified membrane molecules. Our finding that CD79b is expressed on earlier B-lineage cells than previously expected coincides with the recent study in which CD79b-deficient mice exhibit a blockade of B cell differentiation at the pro-B cell stage. Thus, it is speculated that the CD79b-containing complexes on pro-B cell surfaces may function to induce early B cell differentiation.