Strategy to produce high amounts of a human κLC in mice. (A) The strategy consists of replacing the Jκ segments in a wild-type (WT) unrearranged mouse κ locus by a human VJ exon/neomycin cassette. The absence of the Jκ segments in the newly recombined κ locus blocks all possibilities of endogenous rearrangements. A Cre-mediated deletion of the neomycin resistance gene leads to the production of a chimeric human VJ/mouse κ constant LC. (B) Breeding with DH mice enables the production of only FLC by B cells and PCs. (C) Flow cytometric analysis of PCs isolated from spleen and stained with anti-mouse CD138 and anti-mouse κ antibodies. Note the increase of PCs in DH models compared with WT and that nearly all PCs are κ⁺/CD138⁺ in κF-DH mice. (D) Western blot analysis of the produced κLC in sera of κF-DH, DH, and WT mice with anti-mouse κ antibody (top) and anti-human Vκ4 V domain (bottom). The bands appear at the expected size of 25 kDa, and anti-human Vκ4 antibody reveals only the chimeric κLC of the κF-DH mice. (E) Serum and urine levels (in µg/mL) of κFLC in 2- and 5-month-old κF-DH mice compared with 5-month-old DH and WT mice. The κFLC levels increase in κF-DH mice with age. Serum results are expressed in log scale; means ± standard error of the mean.