Variability in the developing antibody repertoire is focused on the third complementarity determining region of the H chain (CDR-H3), which lies at the center of the antigen binding site where it often plays a decisive role in antigen binding. The power of VDJ recombination and N nucleotide addition has led to the common conception that the sequence of CDR-H3 is unrestricted in its variability and random in its composition. Under this view, the immune response is solely controlled by somatic positive and negative clonal selection mechanisms that act on individual B cells to promote production of protective antibodies and prevent the production of self-reactive antibodies. This concept of a repertoire of random antigen binding sites is inconsistent with the observation that diversity (DH) gene segment sequence content by reading frame (RF) is evolutionarily conserved, creating biases in the prevalence and distribution of individual amino acids in CDR-H3. For example, arginine, which is often found in the CDR-H3 of dsDNA binding autoantibodies, is under-represented in the commonly used DH RFs rearranged by deletion, but is a frequent component of rarely used inverted RF1 (iRF1), which is rearranged by inversion. To determine the effect of altering this germline bias in DH gene segment sequence on autoantibody production, we generated mice that by genetic manipulation are forced to utilize an iRF1 sequence encoding two arginines. Over a one year period we collected serial serum samples from these unimmunized, specific pathogen-free mice and found that more than one-fifth of them contained elevated levels of dsDNA-binding IgG, but not IgM; whereas mice with a wild type DH sequence did not. Thus, germline bias against the use of arginine enriched DH sequence helps to reduce the likelihood of producing self-reactive antibodies.
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