Circulating HMGB1 is elevated in veterans with Gulf War Illness and triggers the persistent pro-inflammatory microglia phenotype in male C57Bl/6J mice

Carla Garza-Lombó,Christen L Mumaw,Chandrama Ahmed,Michelle L Block,Emily Quinn,Hendrik J Greve,Kimberly Sullivan,Morrent Thang,Evan J Messenger

doi:10.1038/s41398-021-01517-1

Abstract

Gulf War Illness (GWI) is a chronic, multi-symptom peripheral and CNS condition with persistent microglial dysregulation, but the mechanisms driving the continuous neuroimmune pathology are poorly understood. The alarmin HMGB1 is an autocrine and paracrine pro-inflammatory signal, but the role of circulating HMGB1 in persistent neuroinflammation and GWI remains largely unknown. Using the LPS model of the persistent microglial pro-inflammatory response, male C57Bl/6J mice injected with LPS (5 mg/kg IP) exhibited persistent changes in microglia morphology and elevated pro-inflammatory markers in the hippocampus, cortex, and midbrain 7 days after LPS injection, while the peripheral immune response had resolved. Ex vivo serum analysis revealed an augmented pro-inflammatory response to LPS when microglia cells were cultured with the 7-day LPS serum, indicating the presence of bioactive circulating factors that prime the microglial pro-inflammatory response. Elevated circulating HMGB1 levels were identified in the mouse serum 7 days after LPS administration and in the serum of veterans with GWI. Tail vein injection of rHMGB1 in male C57Bl/6 J mice elevated TNFα mRNA levels in the liver, hippocampus, and cortex, demonstrating HMGB1-induced peripheral and CNS effects. Microglia isolated at 7 days after LPS injection revealed a unique transcriptional profile of 17 genes when compared to the acute 3 H LPS response, 6 of which were also upregulated in the midbrain by rHMGB1, highlighting a distinct signature of the persistent pro-inflammatory microglia phenotype. These findings indicate that circulating HMGB1 is elevated in GWI, regulates the microglial neuroimmune response, and drives chronic neuroinflammation that persists long after the initial instigating peripheral stimulus.

Highlights

Gulf War Illness (GWI) is a chronic multi-symptom illness with chronic central nervous system (CNS) and peripheral symptoms and pathology [1,2,3,4,5]
These data indicate that persistent neuroinflammation occurs in brain regions that are affected in GWI 7 days after LPS-administration and that microglia themselves are a source of the persistent pro-inflammatory response, which continues long after the instigating stimulus
Peripheral immune dysfunction in GWI is hypothesized to contribute to persistent microglial neuroinflammation underlying the pathobiology years after the theater of war, but the mechanisms underlying how circulating factors can culminate in persistent neuroinflammation that continues long after the instigating stimulus has been largely unknown

Summary

Introduction

Gulf War Illness (GWI) is a chronic multi-symptom illness with chronic central nervous system (CNS) and peripheral symptoms and pathology [1,2,3,4,5]. A recent report documented the elevation of the [(11)C]PBR28 PET signal, in areas including precuneus, prefrontal, primary motor and somatosensory cortices, demonstrating a persistent response from microglia, the resident myeloid cells in the CNS parenchyma, in veterans with GWI, long after the theater of war [29]. The microglial and neuroimmune response to peripheral injury has been shown to augment ongoing neuropathology [32, 34,35,36], highlighting the potential importance of peripheral damage and immune dysfunction on CNS effects, as a possible source of pathology in GWI

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Conclusion