How Microglia React to Inflammation Depends Heavily on Timing and Trigger

Microglia, the brain's resident immune cells, are central players in neuroinflammation and neurodegeneration. Understanding how they activate in response to specific inflammatory triggers—and how those responses evolve over time—is critical for modeling diseases like Alzheimer's and developing targeted therapies. This study provides one of the most temporally and mechanistically detailed characterizations of human microglial activation to date.

The researchers used human iPSC-derived microglia (hiMG) and exposed them to three conditions: lipopolysaccharide (LPS), interferon gamma (IFNγ), and a combination of both. LPS mimics bacterial infection and is commonly used in inflammation research but has debated physiological relevance in CNS contexts. IFNγ is a key pro-inflammatory cytokine more directly relevant to CNS immune priming. By examining responses at multiple time points—from 1 hour to 96 hours—the team captured early, mid, and late activation dynamics using bulk RNA sequencing, multiplex cytokine assays, morphological imaging, western blotting, and targeted mass spectrometry of secreted proteins.

Transcriptomic analysis at 24 hours revealed massive gene expression remodeling: LPS altered over 7,000 genes and LPS/IFNγ co-stimulation affected more than 8,500. IFNγ alone produced a much more modest transcriptomic footprint. Several changes overlapped with DAM (disease-associated microglia) gene signatures relevant to neurodegeneration: upregulation of S100A9, CD44, ACSL1, and HIF1A, alongside downregulation of TREM2, GPNMB, FABP3, LGMN, and LPL. Cytokine changes were detectable as early as 1 hour post-treatment, primarily driven by LPS, and followed distinct early (≤2 h), mid (4–12 h), and late (24–96 h) temporal patterns. IFNγ contributed to sustained inflammatory signaling when combined with LPS, even when its standalone effects were limited.

Morphological analysis confirmed structural remodeling consistent with activation under LPS and LPS/IFNγ conditions. Targeted mass spectrometry quantified five secreted proteins—ApoE, CD44, FUCA1, Galectin-3, and Osteopontin—all relevant to microglial activation. Most showed time-dependent increases greatest under LPS and LPS/IFNγ conditions, but Osteopontin secretion was notably highest with IFNγ alone, pointing to stimulus-specific secretory programs that cannot be inferred from transcriptomics alone.

These findings carry important implications for neurodegeneration research. The partial overlap of LPS- and IFNγ-induced gene expression with DAM signatures suggests these stimuli can model aspects of pathological microglial states seen in Alzheimer's and other neurodegenerative diseases. However, no single stimulus fully recapitulates the DAM phenotype, underscoring the need for context-appropriate experimental models. The temporal granularity of this dataset also reveals that single-timepoint studies risk missing key activation dynamics—an important methodological caution for the field.