Analysis and application of the capability of immune system to improve antigen-specificity of antibodies.

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Recently, peripheral B cells have been shown to undergo secondary V(D)J rearrangement of Ig genes, but the physiological role of this event has not been fully elucidated. Here, we investigated whether rearrangement of L chain genes in the periphery is involved in the generation of high-affinity Abs. To test this possibility, we used a 17.2.25 rearranged VHDJH gene (VHT)-knockin mouse whose B cell diversity is limited due to the expression of the site-directed transgene. Immunization of the mouse with p-nitrophenylacetyl (pNP)-conjugated chicken γ-globulin (CGG) preferentially led to the production of anti-pNP IgG Abs comprised of non-VHT-encoded H chains and λ chains, which constituted a majority of high-affinity IgG to this hapten. We isolated three independent anti-pNP IgG1 mAbs (two bearing λ1 with high-affinity and one bearing κ with low-affinity), all of which used a common VHDJH containing an identical point mutation, thus suggesting that κ to λ1 replacement contributed to an increase of the Ab affinity. RAG-2 mRNA and the recombination signal sequence break reflecting the λ1 gene rearrangement were detected in the draining lymph node (LN) of immunized mice, but not of non-immunized animals. There was a close correlation between the levels of the λ gene rearrangement and λ^+ high-affinity anti-pNP IgG. Thus, our findings suggest that new rearrangement of λ genes in the periphery contributes to affinity maturation of anti-pNP IgG.