Disentangling neuroimmune landscapes during divergent peripheral activation states reveals distinct glial signatures

Lipopolysaccharide (LPS), a gram-negative bacterial cell-wall component, is a well-characterized immunostimulant acting via Toll-like receptor 4 (TLR4) and is widely used to model systemic inflammation–to–brain immune signaling. A single intraperitoneal high dose evokes a robust peripheral inflammatory state that is rapidly relayed to the central nervous system (CNS), resulting in profound neuroinflammation. By contrast, repeated low-dose LPS engages innate immune memory and has been associated with neuroprotective effects. Here we sought to comprehensively characterize the CNS-specific effects of the repeated LPS regimen in contrast to the effects of a neurotoxic single high-dose LPS. High-dose LPS induced robust forebrain inflammatory cytokine expression, disrupted homeostatic microglial and astrocytic marker programs, and produced transcriptomic signatures enriched for NF-κB signaling and apoptosis. In striking contrast, repeated low-dose LPS preserved homeostatic glial marker expression while increasing IBA1/F4/80-positive microglial signal across forebrain regions without a parallel increase in inflammatory cytokine transcripts. Whole-forebrain RNA-seq demonstrated selective enrichment of phagocytosis-related pathways under the repeated regimen, distinguishing it from the high-dose condition. Flow cytometry revealed an expansion of CD45high CD11b+ myeloid cells expressing the phagocytic marker CD206 following repeated low-dose LPS. Cell-type–resolved transcriptional profiling showed that this CD45high CD11b+ subset preferentially upregulated phagocytic programs while lacking prominent pro-inflammatory and apoptotic pathway activation under the repeated low-dose regimen. In parallel, astrocytes maintained homeostatic gene expression without enrichment of neurotoxic inflammatory signatures. Together, these findings delineate how distinct systemic LPS dosing paradigms differentially shape glial transcriptional and phenotypic responses in the CNS.