IL-2 receptor alpha chain (CD25, TAC) expression defines a crucial stage in pre-B cell development

The analysis of the expression of the alpha chain of the IL-2 receptor (CD25, TAC) on the surface of B lineage cells in mouse bone marrow reveals that it is a useful marker to distinguish pre-B-I from pre-B-II cells. CD25 is not expressed on CD45R(B220)+ c-kit+ CD43+ TdT+ lambda 5+ c mu- sIg-IgH chain locus DJH-rearranged pre-B-I cells of mouse bone marrow. It is expressed on large cycling CD45R(B220)+ c-kit- CD43+ TdT- lambda 5+ c mu+ sIg- and on small resting CD45R(B220)+ c-kit- CD43- TdT- lambda 5- c mu- sIg- IgH chain locus VHDJH-rearranged pre-B-II cells. Therefore, the transition from pre-B-I to large pre-B-II cells is marked by the downregulation of c-kit and terminal deoxynucleotidyl transferase (TdT), and by the upregulation of CD25. SCID, RAG-2T, microMT and lambda 5T mutant mice do have normal, if not elevated numbers of pre-B-I cells but lack all CD25+ pre-B-II cells in their bone marrow. The expression of a transgenic H chain under control of the microH chain enhancer in RAG-2T bone marrow B lineage precursors allows the development of large and small CD25+ pre-B-II cells. The results suggest that the differentiation of pre-B-I to pre-B-II cells in mouse bone marrow requires the expression of microH chains and surrogate L chains in membranes, probably on the surface of precursor B cells.