Disentangling neuroimmune landscapes across peripheral activation paradigms resolves divergent glial state programs.

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 relative to a neurotoxic single high-dose challenge. 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, demonstrating a dissociation between gliosis marker intensity and inflammatory pathway activation. Whole-forebrain RNA-seq demonstrated selective enrichment of phagocytosis-related pathways under the repeated regimen in the absence of pro-inflammatory transcriptional amplification. Flow cytometry revealed an expansion of CD45