Microglia-specific TLR4-KO Reduces Neuroinflammation in a Combined Model of Diesel Exhaust and Chronic Cerebral Hypoperfusion

Introduction: Air pollution derived particulate matter (PM) is recognized to have a neurotoxic effect on the brain. Our group has previously shown in murine models that PM has synergistic neurotoxicity in the setting of pre-existing neurological conditions. We have further shown that TLR4 (toll-like receptor 4) signaling mediates neurotoxicity in joint PM and chronic cerebral hypoperfusion (CCH) exposures in male mice. The objective of this study was to investigate inhibition of microglia-specific TLR4 neurotoxicity in the setting of PM and CCH in female mice.

Methods: We utilized a novel murine model of inducible monocyte/microglia-specific TLR4 knockout (i-mTLR4-ko). To model CCH, bilateral carotid artery stenosis (BCAS) was surgically induced. TLR4-intact (control) and i-mTLR4-ko female mice received 8 weeks of either inhaled filtered air (FA) or diesel exhaust particulate (DEP) (NIST SRM 2975) at 100μg/m3. There were 8 experimental groups (n=8/per group): (1) control/FA, (2) control/DEP, (3) control/FA+BCAS, (4) control/DEP+BCAS, (5) i-mTLR4-ko/FA, (6) i-mTLR4-ko/DEP, (7) i-mTLR4-ko/FA+BCAS, and (8) i-mTLR4-ko/DEP+BCAS. We assessed inflammation in the corpus callosum using Complement Component 5 (C5), and anaphylatoxin C5a, immunofluorescent markers of innate immune response activation.

Results: DEP exposure resulted in a trend towards an increase in neuroinflammation as quantified by C5 and C5a. The DEP effect on both C5 and C5a significantly worsened in the presence of CCH (+32%, p=0.02 and +151%, p=0.001, respectively). This effect was attenuated by i-mTLR4-ko in C5a (-71%, p=0.0003). A loss of significance was observed in C5 between the FA and DEP+BCAS groups in the i-mTLR4-ko animals.

Conclusion : Air pollution-induced neuroinflammation is attenuated by microglia-specific TLR4 knockout even in the setting of chronic cerebral hypoperfusion. The TLR4 pathway is a viable target for further clinical investigations as an important mediator of neuroinflammatory activation secondary to air pollution exposure.