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We have analyzed immunoglobulin light chain sequences from avian leukosis virus (ALV) induced bursal and metastatic tumors and from cell lines derived from these tumors. Sequence data presented demonstrate that ALV‐induced tumors and one cell line (DT40) derived therefrom continue to diversify their light chain genes outside of the bursal environment. Diversification within these tumor cells seems to occur by gene conversion events comparable with those observed in bursal B cells. Sequence analysis of spontaneously arising surface immunoglobulin negative subclones of the DT40 cell line revealed frameshifts within the rearranged light chain genes which most likely resulted from non‐functional recombination events. Superimposed gene conversion events can repair these frameshifts leading to re‐expression of surface immunoglobulin.

The chicken Ig lambda light chain locus is composed of a single V gene closely linked (1.8 kb) to a single J‐C unit in its natural configuration. In mice transgenic for this locus, the transgene becomes rearranged in B cells and to a much lesser extent in T cells. Modifications were introduced in the transgene in order to characterize elements which target the recombinase to the Ig loci. In the absence of either the promoter or the enhancer located 3′ of C lambda, rearrangement of the transgene is reduced 20‐ to 100‐fold. Moreover, rearrangement is increased 5‐fold when the DNA segment between V lambda and J lambda (‘Uo segment’), which is deleted during the joining process, is replaced by a neutral DNA segment of equal length. The Uo segment behaved as a strong transcriptional silencer when tested in a CAT assay in vitro. Control transgenic mice harbouring only the two 3 bp mutations that introduced restriction sites at both ends of the Uo segment to allow for its replacement were also analysed. Rearrangement was reduced 10‐ to 100‐fold in B cells from such transgenic lines. A model is proposed whereby the sites of these two mutations would function by counteracting transiently the repressing effect of the silencer, thus giving access of the chicken light chain locus to the recombinase.

The formation of B lymphoid restricted progenitors was followed during chicken embryonic development by monitoring the appearance of the various Ig gene rearrangements (DJH, VHDJH, V lambda J lambda), as a sensitivity that allows the detection of a single rearranged cell. By quantifying the DJH committed progenitor populations, we describe their evolution in different compartments at different developmental stages. The yolk sac is the first site where DJH‐positive cells are observed (at days 5–6 of development); via the general circulation, they then seed the various organs while undergoing VHDJH and V lambda J lambda rearrangements, which occur simultaneously but lag behind DJH by one to several days. These progenitor populations decline with time in most lymphoid sites and only expand in the bursa. RAG‐1 expression is observed in the bursa in the absence of ongoing rearrangement activity and thus appears to be an improper marker of rearrangement in the chicken. Commitment to the B cell lineage seems to result from an intrinsic cell program, but the survival and expansion of the committed B progenitors require the specific microenvironment of the bursa.