Investigating the role of CCL11 in shaping microglial age-and long-COVID-related inflammatory activation

Microglia are the innate immune cells of the brain and are at the centre of the neuroinflammatory cascades that are triggered during ageing and in various neuropsychiatric diseases ([1]). In recent years, a specific, systemically circulating cytokine, CCL11, has been identified that is elevated in both blood and cerebrospinal fluid during ageing in mice and humans ([2]). It is one of the main inflammatory cytokines that is elevated in blood and CSF during acute COVID-19 infection and in patients with neuropsychiatric long COVID syndrome (LC) up to several weeks after infection ([3]). However, a systematic and unbiased analysis of the effects of CCL11 on the molecular architecture of the CNS and specifically on microglial phenotypes and responses has not yet been performed. In this study, we used mouse models injected with CCL11 at different doses and time periods to investigate the effects of short- and long-term exposure of the brain to CCL11. In addition, we also used primary microglial cell cultures to study the effects of CCL11 on microglial activation without the complex cellular microenvironment of the CNS. Both experimental approaches were accompanied by several proteomics and RNA sequencing experiments to investigate the effects of CCL11 on microglia across the lifespan in a systemic manner. In particular, we found that systemic administration of CCL11 over sev-eral weeks leads to a profound remodelling of the CNS proteome in different brain regions, with several age-related molecular neuroinflammatory signalling pathways induced by CCL11. Analysis of the microglia phenotype by MGenrichment showed the induction of inflammatory microglia phenotypes as observed in neurodegenerative diseases. In addition, this was accompanied by morphological changes towards a more activated microglia phenotype and the induction of a DNA damage response. RNA sequencing of CCL11-stimulated cultured primary mouse microglia also revealed the induction of a microglia phenotype transcriptionally similar to the inflammatory and disease-associated microglia observed in Alzheimer's disease mouse models. Therefore, we used global CCL11 knockout mice to study the changes in microglia during aging in the absence of CCL11 (homozygous knockout) or with reduced CCL11 levels (heterozygous knockout). Interestingly, we observed different responses, with a partial rescue of specific age-related molecular subdomains in isolated microglia from old knockout mice on the one hand and even more strongly regulated age-associated signalling pathways and proteome clusters on the other. In summary, CCL11 plasma levels increase during aging and are more elevated in patients with a higher symptomatic PASC score, and several weeks of CCL11 treatment leads to morphological changes of microglia in different brain regions. Furthermore, CCL11 stimulation of pMG and mouse brains leads to tran-scriptomic and proteomic changes associated with inflammation, neurodegenerative diseases and aging, and finally, systemic knockout of CCL11 can partially abrogate specific age-related microglial proteomic changes. Taken together, our data suggest that CCL11 may be important in triggering age- and LC-associated microglial dysfunction, which may subsequently contribute to age-associated neuropsychiatric diseases as well as long-term neuropsychiatric COVID.

Publication History

Article published online:
30 April 2025

• References

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