Susceptibility Loci in C57BL/6 sle1, sle2 and sle3 Contain Genes that Alter Peripheral Selection of the CDR-H3 Sequences Enriched for Arginine

Systemic lupus erythematosus (SLE) is a multifactorial autoimmune disease characterized by deposition of ds- DNA binding autoantibodies in various body organs. These antibodies result from failure to control the composition of the B cell repertoire. Development of optimum B cell repertoire depends on the amino acid composition and the physicochemical characteristics at the center of the antigen binding site, the third complementarity determining region heavy chain (CDR-H3). Repertoire control involves positive selection for hydrophilic amino acids such as tyrosine and negative selection of hydrophobic and charged amino acids, specifically those containing arginine within the CDR-H3. Anti-dsDNA antibodies present in SLE patients exist in healthy individuals but at low levels, since dsDNA-specific B cells are deleted from the repertoire, but amplified in SLE patients. These antibodies contain arginine residues in CDR-H3, especially at positions 99-102, where they are positioned to bind negatively charged phosphate groups on the DNA backbone. Three genomic intervals, namely sle1 on chromosome 1, sle2 on chromosome 4, and sle3 on chromosome 7, were found to be associated with SLE susceptibility. We hypothesized that development of ds-DNA binding antibodies in SLE might result from failure to control CDR-H3 amino acid composition. We proposed that the SLE congenic loci might have unique effects in allowing survival/expansion of B cells expressing these auto-reactive antibodies. Our strategy was to change the composition of CDR-H3 by altering the germline composition of the DH gene segments. We created a ΔD-iD altered allele enriched for arginine while depleted of tyrosine at positions 99-102. We then monitored the influence of different SLE loci on the development and maintenance of B cells bearing CDR-H3 arginine. These findings support our hypothesis that peripheral B cell selection is altered by the presence of sle congenic alleles, allowing passage of B cells able to produce autoreactive antibodies binding ds-DNA. These findings may help in developing therapeutics to suppress autoimmunity in SLE.