Therapeutic B-cell depletion reverses progression of Alzheimer’s disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized by impaired clearance of toxic aggregates such as amyloid-β (Aβ) peptides derived from amyloid precursor protein (APP). Microglia and astrocytes normally help aggregate and clear Aβ, with TGFβ signaling supporting microglial homeostasis, but chronic inflammation and dysregulated Aβ production promote disease-associated microglia (DAM) that exacerbate neuroinflammation and reduce Aβ phagocytosis. Aging, with its associated increase in systemic inflammation, further influences AD progression via peripheral immune activation. While roles for T cells in AD have been reported as both beneficial and harmful, the contribution of B cells has remained unclear and underexplored beyond their production of immunoglobulins. The authors hypothesize that B cells contribute to AD pathogenesis beyond antibody production, potentially by infiltrating the brain and modulating microglial states, and test whether genetic loss or therapeutic depletion of B cells can ameliorate AD pathology and behavioral deficits in multiple transgenic mouse models.

Prior work has established that microglia, aided by astrocytes and TGFβ signaling, participate in Aβ plaque aggregation and clearance, but chronic inflammation and Aβ dysregulation promote DAM, which secrete pro-inflammatory cytokines and show reduced phagocytosis of Aβ. Aging increases systemic inflammation and is associated with heightened AD risk. The adaptive immune system’s role in AD is complex, with evidence for both protective and pathogenic T cell effects. The role of B cells in AD has been largely unexplored; earlier observations included microglia-induced B cell activation and correlations of B cell changes with cognitive decline, but mechanistic insight was lacking. The current study builds on these observations to directly test B-cell involvement in AD progression across established transgenic mouse models.

Models and cohorts: Three AD mouse models were used: 3xTgAD (female, 60–70 weeks old, for pronounced phenotype), APP/PS1, and 5xFAD (female, 35–47 or ~40–47 weeks for aggressive pathology). B-cell-deficient lines were generated by crossing AD models with JHT mice (immunoglobulin heavy chain locus deletion arresting B cell development at pro-B stage), yielding 3xTgAD-BKO and APP/PS1-BKO mice. Therapeutic B-cell depletion was performed using intraperitoneal anti-CD20/B220 antibody administered for 2 months; controls received isotype IgG. Assessments: Flow cytometry quantified B cell subsets (B1a, B1b, B2), activation markers (4-1BBL), and intracellular cytokines (IFNγ, IL-6, IL-10, TGFβ) in blood, spleen, and cervical lymph nodes. Brain microglia (CD11b+CD45+) were analyzed ex vivo for cytokines (IL1β, TGFβ, IFNγ, IL-10) by flow cytometry after brief stimulation in the presence of monensin. Immunofluorescence on perfused brain sections quantified Aβ plaques (anti-Aβ), Iba1+ microglia, B220+ B cells, and brain IgG. Aβ1–40 and Aβ1–42 levels in cortical extracts (TBS- and formic acid-soluble fractions) were measured by ELISA. Behavioral analyses included Morris water maze (MWM) for spatial learning and memory (training over multiple days with probe trial) and open field activity (OFA) for locomotion/exploration. Gene expression profiling: microarray analysis of hippocampus and whole brain (without hippocampus) compared AD vs B-cell-deficient AD mice to assess TGFβ isoforms and DAM-associated genes. Statistics: unpaired t tests, one-way or two-way ANOVA, and Kruskal–Wallis tests as appropriate; data reported as mean ± SEM with significance thresholds at p < 0.05, 0.01, 0.001.

- Circulating and lymphoid B cells are increased and activated in AD mice. In 3xTgAD mice, B1a (CD5+CD19+) and B1b (CD5−CD19+) cells were increased while B2 cells were decreased in cervical lymph nodes relative to WT controls. Peripheral B cells, particularly B1a, upregulated IFNγ, IL-6, IL-10, TGFβ, and 4-1BBL, indicating an activated, age-like pathogenic phenotype. - Genetic loss of B cells ameliorates AD phenotypes. In 3xTgAD-BKO and APP/PS1-BKO mice, despite continued expression of AD transgenes, B-cell deficiency reduced Aβ plaque burden in the hippocampus, lowered large-sized Iba1+ microglia, and improved behavioral performance (spatial learning and memory in MWM; exploratory/locomotor activity where tested). Soluble Aβ1–42 and Aβ1–40 elevations seen in AD mice were reduced in BKO mice. - Microglial homeostatic/anti-inflammatory states are restored by removing B cells. AD mice exhibited decreased TGFβ+ and IFNγ+ microglia; B-cell deficiency or transient anti-CD20/B220-mediated depletion restored TGFβ+ microglia to near WT levels and increased IL-10+ microglia (notably in 5xFAD after depletion). Transcriptome analyses indicated alterations in TGFβ1 expression and DAM-related gene signatures with B-cell deficiency. - Therapeutic B-cell depletion after disease onset improves pathology and behavior. Two-month anti-CD20/B220 treatment in 3xTgAD and 5xFAD cohorts efficiently depleted circulating B cells, reduced hippocampal Aβ plaque burden, decreased large Iba1+ microglia, increased frequencies of TGFβ+ and IL-10+ microglia, and improved locomotor behavior in OFA. - B cells infiltrate AD brain and are associated with IgG deposits around plaques. B220+ B cells were increased in brain parenchyma of 5xFAD mice and reduced after B-cell depletion. Brain IgG levels were elevated in 5xFAD and were reduced by B-cell depletion, while serum Aβ-specific antibody levels were not altered by the depletion regimen, suggesting central rather than peripheral antibody changes are relevant. Overall, across three AD models, the presence of mature B cells is required for full AD progression; their removal reverses pathology and behavioral deficits and shifts microglia toward a TGFβ-associated, less DAM-like phenotype.

The study provides evidence that B cells have a pathogenic role in murine AD progression beyond antibody production. AD is associated with activated B cells in the periphery and their infiltration into the brain, with immunoglobulin deposition around Aβ plaques. Genetic ablation or therapeutic depletion of B cells reduced Aβ plaque burden, decreased activated microglia, restored TGFβ+ and IL-10+ microglial populations, and improved behavior in multiple models. The findings suggest that brain IgG and/or B-cell-derived cytokines may exacerbate neuroinflammation, potentially via Fc receptor and complement pathways, leading to suppression of TGFβ1+ homeostatic microglia and promotion of DAM. Conversely, removing B cells increases TGFβ+ microglia and expression of receptors such as Trem2, Clec7a, and Tyro3 associated with debris clearance, supporting improved Aβ handling and behavioral outcomes. While human data are limited, observed correlations of altered B cell subsets with AD severity imply potential translational relevance, supporting the concept that targeting B cells could modulate disease course.

Across three transgenic AD mouse models, both genetic loss and therapeutic depletion of B cells reverse or retard AD progression, reducing Aβ pathology, normalizing microglial phenotypes toward TGFβ-associated states, and improving behavioral deficits. These results identify B cells as contributors to AD pathogenesis and suggest B-cell–targeted therapies (for example, anti-CD20-mediated depletion) as potential strategies to treat AD, even after disease onset. Future work should elucidate the precise mechanisms by which B cells and brain IgG influence microglia and neuroinflammation and assess safety, timing, and efficacy of B-cell–modulating therapies in clinical settings.

Mechanistic details of how B cells and brain IgG alter microglial states in AD are not fully delineated in this study. Findings are based on mouse models; applicability to human AD, while suggested, remains to be established.