Introduction: Air pollution is strongly associated with increased risk of cerebrovascular disease and neurodegeneration. However, few studies have modeled the effects of air quality improvement by examining neurotoxicity after a recovery period with clean air following termination of air pollution exposure. Our group has previously established in the same cohort that microglial activation persists following an 8-week recovery period. The objective of this study was to characterize the pro-inflammatory vs. anti-inflammatory phenotypes of the activated microglia.
Methods: Male and female 8-week-old C57BL/6 mice were exposed to inhaled Filtered Air (FA) or Diesel Exhaust Particulate (DEP) (NIST SRM 2975) at 100μg/m3 in the following three arms: 1) 8 weeks of inhaled FA or DEP (n=16/group); 2) 8 weeks of FA or DEP followed by 8 weeks of recovery with FA (n=16/group); and 3) 16 weeks of FA or DEP (n=16/group). Microglial phenotype was characterized using immunohistochemical double staining. Co-localization of ionized calcium-binding protein-1 (Iba-1) with inducible nitric oxide synthase (iNOS) was used to identify pro-inflammatory microglia, and co-localization of Iba-1 with arginase-1 (Arg) identified anti-inflammatory microglia.
Results: Microglia (Iba-1) co-expressing iNOS increased in DEP groups in the 8-week and 16-week exposure arm (+33%, p=0.009 and +23%, p=0.03, respectively). There was no increase in the 8-week recovery arm. On its own, iNOS-positive cell count was elevated at all time points (p=0.004, p=0.01, p=0.0001). However, there was no significant difference in Arg-positive cell count and Iba-1/Arg double-positive cells between Filter and DEP in any arm.
Conclusion : Air pollution exposure results in a predominantly pro-inflammatory microglia phenotype. However, this phenotype is not sustained during the recovery phase, even when microglial activation persists. Changes in microglial phenotypes and activation patterns can contribute to the understanding of the neurotoxic effects of air pollution and the brain's potential to recover from subacute exposures.
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