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.
