Background: During B-cell development in the mouse, the V_H, D_H and J_H elements of the immunoglobulin heavy chain (IgH) locus are rearranged, firstly by D_H–J_H joining, and then by V_H–D_HJ_H joining. In-frame (‘productive') V_HD_HJ_H joints and D_HJ_H joints in reading frame 2 (one of the three possible D_H reading frames) allow the expression of μ and truncated μ chains (Dμ proteins), respectively. The expression of such molecules from one of the two IgH loci of a cell is thought to interfere with V_H–D_HJ_H recombination on the other IgH locus, and to guide the cells through further development.

Results We have developed a gene amplification assay that permits the examination of rearranged immunoglobulin genes in single cells. Using this assay, we monitored cells bearing D_HJ_H and/or V_HD_HJ_H joints at early stages of development: in CD43⁺ B-cell progenitors, subdivided into fractions A, B, C and C′ by flow cytometry, and in CD43⁻ pre-B cells (fraction D). Fraction C was enriched for cells with two non-productive V_HD_HJ_H joints. Cells containing both a D_HJ_H joint in D_H reading frame 2 and a V_HD_HJ_H joint were not seen in any fraction. All fraction D cells harbored an in-frame V_HD_HJ_H joint. Cells with two productive V_HD_HJ_H joints appear to be selected against throughout development.

Conclusions Cells expressing Dμ proteins appear to be arrested in development as a result of inhibited V_H–D_HJ_H joining. Expression of the μ chain is required for maturation into CD43⁻ pre-B cells; accordingly, cells carrying two non-productive V_HD_HJ_H joints accumulate in the CD43⁺ compartment. Such a developmental arrest may also affect cells that express self-reactive V_HD_HJ_H antibody domains. Our results indicate further that allelic exclusion at the IgH locus is already established at the pre-B cell stage.