The regulatory locus rλ1 affects the level of λl light chain synthesis in lipopolysaccharide‐activated lymphocytes but not the frequency of λl ‐positive B cell precursors

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Abstract
Several strains of mice, most notably the SJL strain, have a greatly reduced level of circulating λl immunoglobulins (rλ110 phenotype) compared with other mice. The locus responsible for this phenotype has been shown to be closely linked to the structural Cλl gene. Functionally this locus has been said to reduce the number of lymphocytes expressing surface λl molecules. In order to gain a better understanding of this phenomenon we compared the functional properties of activated B cells secreting λl immunoglobulins in the splenocytes of both BALB/c and SJL mice.Our results indicate that regulatory T cells, as well as regulatory ontogenetic processes, are not responsible for the rλ1lo phenotype. In addition, limiting dilution analyses revealed that the number of lipopolysaccharide‐sensitive precursors of λl‐secreting B cells was similar in the splenocytes of the two strains of mice tested. The quantity of λl molecules produced by a B cell clone, however, was found to be lower in SJL than in BALB/c mice. As the level of λl mRNA is greatly reduced in lipopoly‐saccharide blasts of SJL mice, as compared to the mRNA detected in BALB/c blasts, we conclude that the impairment responsible for the rλ110 phenotype is probably transcriptional. We tentatively propose that sequences 5′ to the Cλl region are defective in their capacity to enhance the λl transcripts in SJL mice.