Systemic lupus erythematosus (SLE) is a chronic autoimmune disease that affects an estimate of 1.5 million people in the US. It currently has no cure and can cause a variety of symptoms at multiple locations, including anemia, joint pain, skin lesion, kidney damage, and neurological disorders. SLE is caused by autoreactive B cells that produce auto-antibodies, antibodies that attack the body’s own healthy tissues. Each B cell produces a unique antibody by virtue of their ability to undergo somatic recombination. Thus, B cells that produce different antibodies are called “clones.” Many B cell clones exist in the body, and every clone produces a single type of antibody against a single antigen, a molecule that the antibody can detect as foreign. Upon binding to its cognate antigen, an antibody tags it for destruction by other immune cells and triggers inflammation. Although autoreactive B cells are usually eliminated during early developmental stages, they occasionally escape this process and cause a variety of autoimmune diseases, including SLE. The origin of these autoreactive B cells is still largely unknown, and a better understanding of this problem is crucial for finding a cure for SLE.

The diversity of SLE symptoms mirrors the diversity of autoantibodies. At least 180 types of autoantibodies have been discovered in SLE patients, targeting a wide range of antigens including nucleic acids, cell membrane components, and secreted proteins [1]. These auto-antibodies, however, usually do not arise all at once during disease progression. Anti-DNA autoantibodies, a common molecular marker for SLE, may appear in the bloodstream as early as a few years before any noticeable symptoms [2]. At the onset of disease, B cells producing auto-antibodies against other antigen targets rapidly expand and cause widespread inflammation, a phenomenon called epitope spreading. Understanding the triggers of epitope spreading is crucial for developing more effective therapeutic and preventive measures against SLE.
 

Recently, a team of scientists led by Michael Carroll at Harvard Medical School offered new insights into the mechanism of epitope spreading in their paper published in Cell [3]. Using a novel mouse model, they discovered that introduction of a single autoreactive B cell clone drives the production of other B cell clones against different auto-antigens. In their study, they used a method called “mixed chimera” to generate a mouse strain that contains both wild-type B cells (WT) and an autoreactive B cell clone (named 564Igi) against a known DNA antigen. To do so, they ablated the immune system of wild-type mice with irradiation, reconstituted their hematopoietic system with marrow from both WT and 564Igi mice, and allowed the immune system to regenerate with the transferred bone marrow. The 564Igi B cells expressed a fluorescent protein, which allowed the authors to track their location and proportion.

Surprisingly, although the chimeric mice showed an increase in total autoreactive antibodies compared to fully wild-type mice, the majority of these antibodies were produced by WT B cells. In support of this finding, the authors observed that 6-8 weeks after bone marrow transfer, almost all B cells were wild-type in germinal centers (GCs), tissue sites where B cells aggregate, mature, and produce antibodies. Analysis of autoantibodies in GCs by sequencing and antigen binding assay revealed a diverse range of targets, similar
to the phenomenon observed in epitope spreading. As a control, the authors also generated a chimeric mouse strain containing WT B cells and a B cell clone expressing an inactive antibody. These mice displayed minimal amounts of auto-antibodies, indicating the chimera experiment itself did not have an effect on autoimmunity. These results suggested that instead of directly contributing to auto-immunity, 564Igi B cells triggered generation of other autoreactive B cells clones.

The authors then investigated whether the 564 Igi B cells were also required for the proliferation and survival of other autoreactive B cells once they were produced. To do so, they generated another batch of chimeric mice similar to the previous ones, except the 564Igi B cells in these mice also expressed a receptor that can bind to tamoxifen. Tamoxifen administration would selectively kill these cells, leaving other cells, including WT B cells, unharmed. Tamoxifen administration effectively eliminated 80-90% of 564Igi B cells, while the total B cell frequency in autoreactive GCs were unaffected. These results suggest that once other autoreactive B cells are induced by one autoreactive strain, the original strain is no required for their proliferation and survival.

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Despite the diversity of antigens targeted by WT autoreactive B cells, sequencing analysis showed that the antibodies produced by these cells converged on specific sequences even in GCs from different mice. To further study whether the convergence of antibodies is caused by a convergence of B cell clones, the authors constructed another 564Igi-WT chimera mouse model containing 564Igi B cells and WT Confetti B cells. Every Confetti B cell clone expresses one of ten fluorescence markers with different colors, which serves as a marker for their antibody identity. Four weeks following bone marrow transfer, the authors observed a decrease in color diversity in autoreactive GC and dominance of a few B cell clones. In comparison, the Confetti-only control mice without 564Igi B cells showed constant B cell diversity. These results suggest that once autoimmunity is established by 564Igi B cells, WT B cell clones in GCs compete against each other, resulting in the survival of a few clones that are the most autoreactive.

This study conducted by Carroll and colleagues was the first to demonstrate that a specific autoreactive B cell clone can cause expansion of other autoreactive clones. Although epitope spreading had long been observed, it was unclear whether the early autoreactive B cell clone is causally linked to the subsequent expansion of other autoreactive clones. Establishment of this link will potentially open doors for more effective treatment against autoimmune diseases such as SLE. A major hurdle in treating SLE is the heterogeneity of this disease: patients harbor a wide spectrum of autoantibodies against diverse targets, making a one-for-all therapy unlikely [4]. Now that clinicians know that a single autoreactive B cell clone can lead to full-blown disease progression, researchers may develop new strategies to combat SLE by identifying and eliminating this clone at early stages, even before the induction of other autoreactive B cells and presentation of symptoms.