The Ig light chain restricted B6.κ^-λ^SEG mouse strain suggests that the IGL locus genomic organization is subject to constant evolution

Abstract.

In laboratory mice, the different Ig λ light chain subtypes (λ1, λ2, λx and λ3) are expressed on 60, 16, 16 and 8%, respectively, of the λ-positive peripheral B cells. Eighteen years ago, our laboratory characterized a λ1^– wild mouse strain: SPE (Mus spretus). In this report, we describe the characterization of another wild-derived Mus spretus inbred strain, SEG, that presents the same characteristic, namely the absence of λ1 expression. An almost congenic strain, B6.λ^SEG, was detected in a series of recombinant congenic strains carrying 2% of SEG/Pas genome in a C57BL/6J background. This B6.λ^SEG strain was crossed to Igh^a Cκ^– mice in order to derive two different additional congenic strains: B6.κ^–λ^SEG Igh^a and B6.κ^–λ^SEG Igh^b. In this paper, we characterize the genomic organization and the expression of the SEG IGL locus. Altogether, our data show that the SEG IGL locus is constituted by a single functional IGLJ2SEG-IGLC2SEG, two pseudo IGLJ4SEG1/2-IGLC4SEG1/2 gene clusters and two V gene segments: IGLV2SEG and IGLVXSEG. In particular, we show the absence of IGLV1 and IGLVSD26 gene segments. IGLVSD26 was reported to be present in some Mus m. musculus mice and absent in BALB/c. Here, we confirm its presence not only in other Mus m. musculus mice but also in Mus spretus mice. Consequently, we propose that IGLVSD26-related gene segments define a new family that we name V λ 4. The study of the organization of different IGL loci, in addition to the V λ 4 ⁺ reported here, could elucidate questions concerning the evolution of the λ locus.