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Modeling Multiple Sclerosis Medication in Mice: Regulatory B Cells Escape Long-Term αCD20 B Cell-Depletion, Preserving Health in the Face of Autoimmunity

Neu, SD;

αCD20 B cell-depleting antibody (αCD20) therapy has become a first-line treatment for multiple sclerosis (MS), and while the mechanism for this efficacy is incompletely understood, clinical success has led to the development of multiple iterations
of αCD20 drugs. αCD20 therapies deplete B cells by targeting the cell surface protein, CD20, which is expressed by nearly all B cell subsets. As part of the adaptive immune response, B cells are best known for their role in immune protection by antibody
production. However, mounting evidence shows that B cells are also potentially antiinflammatory mediators in MS. Regulatory B cells (Breg) as a group are heterogeneous, but two subsets have garnered much attention for their efficacy in the animal model of MS
known as experimental autoimmune encephalomyelitis (EAE). During EAE, induction of encephalitogenic T cells leads to ascending paralysis in mice that resembles the locomotive disability experienced by MS patients.  First characterized by the Dittel Lab, B cells with IgDlo expression (BDL) can be found in the spleen and have a phenotype distinct from other B cells subsets. When BDL
are absent, mice do not recover from EAE. This BDL-mediated recovery appears to be linked to CD4+ FoxP3+ regulatory T cells (Treg). Treg are crucial for mitigating inflammation and limiting the development of autoimmunity. The other Breg population of
interest is gut-derived IgA antibody-secreting cells (IgA-ASC). IgA is a type of immunoglobulin that is secreted at mucosal barriers like the intestinal tract. Rather than eliminating gut microbes, a unique role of IgA is to facilitate homeostasis within the gut
microbiome: the compendium of microorganisms that continuously live within our intestines. Unexpectedly, IgA-ASC have been found to migrate from the gut to the brain 2 during EAE/MS and secrete the anti-inflammatory cytokine IL-10 to dampen inflammation
therein.  Preliminary evidence in mice indicated that both BDL cells and IgA-ASC are refractory to αCD20-mediated depletion during short-term timeframes. However, as MS is a life-long disease, αCD20 therapy requires continuous dosing over years to deplete B
cells as they are regenerated from the bone marrow. While αCD20 treatment is widely considered safe, the long-term and off-target effects of extended αCD20 usage have not been well investigated. Further, it remains to be seen how prolonged B cell-depletion
impacts Breg populations and their roles in the spleen and intestines.  In this study, we developed an experimental dosing strategy to deplete B cells long-term using αCD20 therapies in humanized-CD20-transgenic mice. By comparing short-term and continuous treatment as well as weak and strong B cell-depletion, we investigated the impact of αCD20 therapy on B cells at a high-resolution subset level in multiple B cell-containing organs including the blood, spleen, bone marrow, and small and  large intestines. While conventional B cells are largely depleted in tissues examined, our results demonstrate that both BDL cells and IgA-ASC are resistant to “weak” αCD20-
mediated depletion, even after several treatments. However, long-term use of “strong” B cell-depleting αCD20 therapy decimates splenic BDL cells and skews intestinal IgA-ASC subsets. Surprisingly, these changes in BDL and IgA-ASC availability appear to minimally
effect the disease course of EAE or the production of IgA in the intestines. Taken together, our findings affirm the overall safety profile of αCD20 therapy, and its long-term administration as used for MS.