Recombinant thyroid peroxidase-specific autoantibodies. I. How diverse is the pool of heavy and light chains in immunoglobulin gene libraries constructed from thyroid tissue-infiltrating plasma cells.

Thyroid peroxidase (TPO) autoantibodies are a distinguishing feature of autoimmune thyroid disease. We have previously constructed immunoglobulin G heavy (H) and light (L) chain cDNA libraries from intrathyroidal B-cells. TPO-selected autoantibodies expressed by combined H and L chain libraries (combinatorial libraries) recognized a limited number of epitopes on TPO and used only a few of the many H and L chain variable region genes present in the genome (germline genes). One possible explanation for this restriction is a lack of diversity in the parental H and L chain gene libraries used to construct the combinatorial library. To address this issue, we determined the nucleotide sequences of randomly selected H and kappa L chain variable region genes from a pair of H and L chain libraries. The 12 H chain gene sequences analyzed were highly diverse, and none resembled the genes of TPO-selected autoantibodies. The sequences of 14 randomly selected kappa L chain genes were less diverse; 12 of 14 were closely related to the same germline gene (KL012) used by TPO-specific autoantibodies. However, we observed previously that only about 1 in 500 of the L chains in this library can pair with an H chain and bind TPO. We now find that, with 1 exception, the randomly selected KL012-like genes in the L chain library differ significantly from the antigen-specific KL012-like genes, particularly in the antigen-binding regions. In summary, the present data indicate that 1) the restricted number of H chain genes used by TPO-specific autoantibodies cannot be ascribed to limited H chain gene diversity in the parent library; and 2) L chains from combinatorial libraries (even when related to the same germline gene) cannot simply be regarded as plastic, or promiscuous, partners for high affinity antigen binding by a particular H chain.