# CD4+ T Cell Activation and Peripheral Immune Cell Influx into the Brain Following Wildfire Smoke Exposure is Modulated by a Saturated Fat Diet

## TL;DR

Wildfire smoke exposure triggers immune cell movement into the brain, and a high-fat diet affects this process and brain inflammation.

## Contribution

The study reveals how diet modulates immune cell infiltration and neuroinflammation after wildfire smoke exposure.

## Key findings

- CD4+ T cells infiltrate the brain in a time- and diet-dependent manner following woodsmoke exposure.
- Inflammatory markers like LFA-1, VCAM-1, and ICAM-1 peak one day after the last smoke exposure.
- A high saturated fat diet alters brain metabolism, increasing susceptibility to inflammation.

## Abstract

Over the past 40 years, wildfires across the United States have steadily increased, in terms of acreage burned. Smoke from wildfires can exacerbate several cardiovascular and respiratory diseases, with new studies highlighting potential sustained neurological outcomes. The purpose of this study was to delineate the role of peripheral immune populations in woodsmoke-induced neuroinflammation, the timeline of this response, and the impact of diet on each of these factors. Based on previous research on other pollutants and diet introduction, we included a highly saturated fatty acid (coconut oil) to represent the effects of a diet intervention that may negatively influence the severity and time course of peripheral immune infiltration. 8-week-old female C57BL/6 mice were exposed to either a sham filtered air or a woodsmoke exposure of a 0.5 mg/m3 concentration every other day for 14 days, 4 hr/day. Three time points were analyzed: 1-, 14-, and 28-days post exposure to woodsmoke to determine neuroinflammation at its peak, and when it begins to resolve. At each time point, brains were queried by high dimensional flow cytometry analysis. Markers for peripheral immune infiltration included T cells (CD3, CD4), lymphocyte trafficking (LFA-1), resident immune cells (CD11b, ACSA-2) and other inflammation-dependent markers. Endothelial cell expression (CD31) and subsequent adhesion molecules (VCAM-1, ICAM-1) were also analyzed. Results demonstrate a time and diet-dependent influx of CD4 T cells (CD3+, RORg−, FOXP3−, CD44+) potentially to resolve neuroinflammation from woodsmoke inhalation. We also see a pattern of inflammation across several trafficking and adhesion markers, including LFA-1, VCAM-1 and ICAM-1, that peaks at 1 day after the last exposure (14 days after the initial insult). Conferred with these results are metabolomic changes in the brain including upregulated taurine and glycerolipid metabolism that is mainly tied to increased susceptibility to inflammatory phenotypes in the brain. Here, we demonstrate that woodsmoke inhalation can initiate the recruitment of peripheral immune populations and activation of endothelial cell markers within the brain. The presence of these cells implicates a larger, more systemic impact on the central nervous system following woodsmoke exposure.