Autoantibodies in Mice Lacking Terminal Deoxynucleotidyl Transferase: Evidence for a Role of N Region Addition in the Polyreactivity and in the Affinities of Anti-DNA Antibodies

The generation of terminal deoxynucleotidyl transferase knockout mice (TdT⁰) has demonstrated that TdT is the only major activity involved in N region addition. This enzyme generates diversity by adding random nucleotides at the V-D-J junctions and by disrupting the formation of repetitive "homology-directed" junctions. Several studies have demonstrated that the Ig heavy chain third complementarity-determining region (H-CDR3) and the N regions play a critical role: 1) in distinguishing between poly reactive and monospecific combining sites in natural and Ag-induced Abs; and 2) in the specificity and polyreactivity of natural autoantibodies (autoAbs) and in particular of anti-DNA Abs. To examine the impact of the lack of TdT on the natural autoAb repertoire in adult mice, we have stimulated TdT⁰ and TdT⁺ littermates with LPS. Serum studies demonstrate that TdT is not critical for the generation of B cells expressing autoAbs including anti-DNA Abs and rheumatoid factors. However, the generation of a large collection of hybridomas indicates that the frequencies of these cells are reduced in TdT⁰ mice mainly due to a lower incidence of polyreactivity; also, the lack of N region diversity seems to negatively affect the affinity of anti-DNA Abs. The physiologic relevance of these data is discussed.