UC Irvine

Alzheimer’s Disease (AD) is the most common cause of dementia impacting almost 7 million people in the United States with cases projected to increase over 13 million by 2050. Numerous large genome-wide association studies (GWAS) have repeatedly identified AD risk genes associated with immunity and specifically expressed in myeloid cells, implicating neuroimmune processes and neuroinflammation as a major factor of AD etiology. Microbial infection has been broadly explored as a mechanism through which both genetic and environmental contributions interact together to influence AD risk and neuropathology. Pathogenic microbes that invade the CNS can cause significant neuroinflammation through CNS-resident immune cells and infiltration of peripheral immune cells such as monocyte/macrophages and T cells. Recent epidemiological studies using human datasets have demonstrated several viral pathogens tightly associated with different neurological diseases, while the strongest association was between AD and viral encephalitis. While the field’s current understanding of the contributions of viruses and their resulting inflammation on AD remains nuanced and complex, ongoing investigations are precisely identifying direct and indirect viral mechanisms leading to CNS damage and associated cognitive impairments like dementia. This is further highlighted by the increasing frequency of emerging viral pathogens like SARS-CoV-2 in which the long-term neurologic sequalae are unknown. To address this, several experiments were carried out to better understand the role of brain-resident microglia and infiltrating peripheral immune cells in mediating neurological damage following infection with a neurotropic strain of murine coronavirus (JHMV). Following intracranial inoculation and the development of immune-mediated demyelination, JHMV-infected mice were treated with the small molecular inhibitor of colony stimulating factor 1 receptor (CSF1R), PLX5622, to deplete microglia. Treatment with PLX5622 did not affect viral replication within the CNS yet the severity of demyelination was increased and remyelination impaired compared to control mice. Gene expression analysis revealed that targeting microglia resulted in altered expression of genes associated with immune cell activation and phagocytosis of myelin debris. These findings indicate that microglia are not critical in viral surveillance in persistently JHMV-infected mice yet restrict white matter damage and remyelination, in part, by influencing phagocytosis of myelin debris.Furthermore, utilizing a tetracycline-inducible CXCL1 mouse line, we sustained neutrophil infiltration in the CNS of JHMV-infected mice to determine their contributions to increased demyelinating damage. Single-cell RNA sequencing revealed upregulation of neutrophil transcriptional programs associated with neutrophil activation, cytotoxicity, and degranulation. Flow cytometry confirmed increased numbers of CD11b⁺ and CD63⁺ neutrophils in DTg spinal cords, consistent with enhanced degranulation. Treatment with Nexinhib20 (N20), a small-molecule degranulation inhibitor reduced spinal cord demyelination in DTg mice without substantially altering neutrophil infiltration or surface expression of degranulation markers, indicating that limiting granule release—rather than immune cell recruitment—is sufficient to attenuate tissue damage. Lastly, spatial transcriptomic analysis, via the CosMx platform, revealed that N20 treatment reprogrammed the lesion microenvironment, downregulating inflammatory mediators Ccl2, Cxcl9, B2m, Ptprc, Srgn, and upregulating a disease-associated myeloid (DAM) signature, ie. Apoe, Trem2, Lpl, Gpnmb, Ctsd, and Spp1, consistent with enhanced debris removal and tissue repair programs. Together, these findings signal neutrophil degranulation as a contributor to white matter pathology during chronic viral-induced demyelination and suggest that targeting granule exocytosis represents a viable strategy to limit immune-mediated CNS injury.To understand whether systemic SARS-CoV-2 could induce effects on AD neuropathology, aged 5xFAD and wildtype (WT) mice were infected with a mouse-adapted SARS-CoV-2 (MA10) and extensive characterization of molecular and cellular changes within the brains was performed. We found that while MA10 infection induced acute viral pneumonia, viral RNA was not detectable in the brains of infected mice nor induced glial activation and neuroinflammation. Furthermore, MA10 infection did not affect Ab plaque volume or numbers in 5xFAD mice compared to uninfected mice. However, spatial transcriptomics did reveal altered expression of genes associated with homeostatic function in neurons, glia, and vascular endothelial cells. Although MA10 infection did not affect AD neuropathology, systemic infection did result in numerous down-stream effects on gene expression associated with resident CNS cell functions that may impact neurologic disease.Finally, to investigate how viral encephalitis may impacted AD neuropathology, we infected aged 5xFAD and WT mice with JHMV. We observed that JHMV-induced encephalitis at 12 days post-infection resulted in minimal changes to overall Aβ protein, despite increased CD4+ and CD8+ T cell infiltration and Lgals3/MAC2-expressing macrophages surrounding more compact Aβ plaques in the brain. Spatial transcriptomic imaging and pathway analysis of differentially expressed genes (DEGs) within myeloid cells demonstrate down-regulated disease-associated (DAM) pathways involving Aβ clearance, response to lipids, and macrophage activation within infected 5xFAD brains. These findings reveal that viral encephalitis and its resulting inflammatory mechanisms impacts the expression of key genes within myeloid cells responding to Aβ plaque pathology, which has implications to the role of viral encephalitis as a risk factor for Alzheimer’s disease.