Our objective was to characterize microglial quantity and morphology after a recovery interval following diesel exhaust particulate (DEP) exposure.

Air pollution exposure has been strongly linked with dementia and cerebrovascular diseases. So far, no studies have modeled improvements in air quality by examining the pollution-exposed brain after a recovery period with filtered air. Our group has previously established through immunofluorescence staining in this cohort that microglial activation persists following an 8-week recovery period.

8-week-old male and female C57BL/6 mice were given inhaled Filtered Air (FA) and DEP (NIST SRM 2975) at 100μg/m³. There were three arms: 1) 8 weeks of FA or DEP exposure (n=16/group); 2) 8 weeks of FA or DEP exposure preceding an 8-week recovery period with FA (n=16/group); and 3) 16 continuous weeks of FA or DEP exposure (n=16/group). Mice were humanely euthanized and corpus callosum sectioned at 5 um. Microglial morphology was visualized using non-fluorescent staining for ionized calcium-binding adapter molecule 1 (Iba-1). Visible Iba-1 positive microglia were counted and cell area assessed using thresholding in ImageJ software.

The 8-week DEP group demonstrated no difference in visible Iba-1 positive microglia cell count or cell area in comparison to the 8-week FA group. The 8-week DEP recovery group showed an increase in both microglia cell count (+44%, p<0.0001) and cell area (+7% p<0.01) in comparison to the 8-week FA recovery group. The 16-week DEP group demonstrated an increase in microglia count (+34%, p=0.0001) but no difference in cell area in comparison to the 16-week FA group.

There is a robust effect on microglial recruitment and morphology after an 8-week DEP exposure followed by an 8-week recovery period. Our findings suggest that air pollution produces a sustained microglial immune response that persists even after air quality improvements.