The Etiology and Pathogenesis of Lupus
Autoimmune diseases such as systemic lupus erythematosis (SLE) are serious disorders in which the body makes antibodies against its own cellular components. SLE afflicts over 500,000 Americans and is characterized by the presence of anti-nuclear antibodies. Although the etiology of SLE remains poorly understood, clear evidence indicates a genetic basis for the disease. Our early work searched for genes involved in the disease by mating mice with difference auto-immune manifestations. When a non-autoimmune mouse strain (SWR) and mice from a strain prone to mild autoimmunity (NZB) were mated, animals that had severe lupus-like autoimmune disease arose. That study revealed that the “normal” strain contributed the structural gene for a critical pathogenic autoantibody produced by the F1 mice. We are now seeking to identify other loci that predispose to autoimmunity. To approach this question, molecular “knock-in” technology is being used to generate mice that produce a high frequency of cells making a specific autoantibody. The knock-in mice are then used in additional genetic studies designed to reveal new “autoimmune regulating” loci.
Figure 1. Breeding mice from different strains reveals the presence of several genes that control susceptibility to lupus. Identifying these genes should help to understand the pathogenesis of the disease and will also reveal fundamental mechanisms controlling the immune response.
One feature of pathogenic autoantibodies is that they typically have multiple somatic hypermutations. These mutations subtly alter the specificity and affinity of the antibodies. We are also interested in pinpointing when antibody producing B cells undergo somatic hypermutation. A surprising outcome of this analysis was the discovery that very early, immature B cells can generate somatic hypermutations. Just as surprising was the demonstration that these mutations can be generated in the absence of help from T cells. Previously, it was thought that B cells had to be activated by antigen and receive help from T cells before they would undergo somatic hypermutation.. These findings showed that somatic hypermutation is available to alter the repertoire of immature cells. Just what signals this process is unclear, as are any differences between the mechanisms of hypermutation in immature and mature B cells.
Figure 2. Mutations generated by hypermutation are localized in hotspots. To determine if a similar feature characterized the mutations found in the immature B cells, the nature and position of the mutations detected were localized. The immature B cells analyzed resulted in 61 mutant/404 total clones and the mature B cells in 51 mutant/323 total clones. The x axis indicates nucleotide position; y axis indicates number of mutations. The axis on the right represents the percent of mutations located at a particular nucleotide position among total mutations. The dominant individual hotspots are indicated with the position of the nucleotide.