Background/Purpose: While NPSLE are among the least understood complications, increasing evidence points to microglia, a brain-resident innate immune cell population, as a driver of disease. We showed that microglial expression of genes linked to the disease-associated microglia (DAM) subset in neurodegenerative disease correlates with the severity of behavioral deficits in a NPSLE model prior to overt systemic disease. Prior data show NPSLE-like disease persists in bone marrow (BM) chimeric MRL/lpr mice (MRL BM::MRL/lpr) despite mitigation of systemic disease, suggesting a brain-intrinsic mechanism. However, these findings do not negate the contribution of circulating mediators of SLE-like disease at subclinical levels (antibodies, proteins, immune cells) to brain dysfunction as the reciprocal chimera (MRL/lpr BM::MRL) was not evaluated. Further, radiation impacts microglial function, and recipient mice from this study were not head-shielded.

Methods: Young female CD45.1 (Jackson 033076) and SLE-prone B6.Sle1Sle2Sle3 (TC; CD45.2; Jackson 007228) mice were used to generate head-shielded reciprocal BM chimeric mice with busulfan treatment to clear remaining BM (CD45.1 BM::CD45.1; TC BM::CD45.1; CD45.1 BM::TC; TC BM::TC). Mice underwent behavioral tasks 10 weeks post-transfer. Perfused brains were extracted following intravenous CD45 labeling to exclude remaining circulating immune cells, dural meninges were removed and live CD45+ cells were FACSorted from pooled cell suspensions (n=3/group to account for biological variability) for cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq; 10X Genomics 3’ v3.1). Data were analyzed in R using the Seurat package. Post-filtering, ~10K cells/sample were maintained.

Results: Transfer of CD45.1 BM into TC hosts reduces SLE-like disease, while reconstitution of CD45.1 hosts with TC BM induces SLE-like disease. As in NPSLE patients, TC mice exhibit heightened anxiety and coordination defects. Anxiety persists in chimeric mice of TC, but not CD45.1, host origin, suggesting brain-intrinsic defects in TC mice are required for this behavior. However, impaired coordination in TC mice is recapitulated by both brain-intrinsic and hematopoietic-derived defects in TC mice. CITE-seq data of CD45.1 BM::TC chimeras reveal a persistent increase in DAM, macrophages, and T cells of TC host origin despite reconstitution with CD45.1 BM, suggesting these niches are not turning over but are locally expanding and contributing to anxiety and coordination defects. In contrast, TC BM::CD45.1 chimeras show partial replacement of CD45.1 brain-resident macrophages and T cells by TC donor cells without expansion, suggesting that aberrantly activated TC-derived macrophages and T cells that initiate localized inflammation, or circulating factors derived from systemic inflammation, contribute to coordination defects.

Conclusion: NPSLE manifestations diverge in mechanistic origin within the same organism; namely, anxiety is associated with local expansion of TC-derived DAM, macrophages, and T cells, while coordination defects may arise from either the increased activity of TC-derived macrophages and T cells or from a peripheral circulating mediator.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/simultaneous-contribution-of-brain-intrinsic-and-peripheral-disease-mechanisms-to-neuropsychiatric-symptoms-of-systemic-lupus-erythematosus-npsle/