Abstract Body: The acute phase of ischemic stroke triggers a complex cascade of cellular responses in the brain and the immune system. Previous studies show that ischemic stroke induces microglial lipid accumulation from 3d to 7d after stroke, linked to pro-inflammatory activation. However, little is known about cellular lipid alterations in peripheral immunity acutely after stroke. We hypothesized that acute ischemic stroke would increase neutral lipid content (lipid droplets) in peripheral myeloid cells and promote immunosuppression phenotypes.

To assess neutral lipid levels during the first week of stroke, we used the BODIPY493/503 dye probe and performed flow cytometry on human PBMCs of healthy volunteers (N=31), and ischemic stroke patients at 3d (N=43) and 7d (N=52). In time course experiments on C57BL/6 mice (1d, 3d and 7d post-MCAO), the same technique was performed using Staph A pHrodo bioparticles, BODIPY, LipiM and Bodipy-cholesterol probes to explore changes in neutral lipid level, lipid uptake, and phagocytic activity of leukocytes. We collected white adipose tissue (WAT)-conditioned PBS from sham and 1d post-MCAO mice to stimulate naive spleen cells and examine whether factors released from WAT alter lipid content and phagocytosis.

In human PBMCs, we found a significant increase in neutral lipids across innate and adaptive cells for up to 7d post-stroke (p<0.01). Consistently, we observed similar results in mice after MCAO (p<0.01). Peripheral monocytes exhibited higher uptake C12 fatty acids and cholesterol and had persistently decreased phagocytic ability up to day 7 post-MCAO (p < 0.05). The impairment in bacterial engulfment was more severe in BODIPY-Hi versus BODIPY-Lo monocytes at both day 1 and day 3 after MCAO, suggesting that higher lipid loads are associated with greater impairment in bacterial phagocytosis (p<0.05). Secretory factors from WAT juice profoundly affected splenic cells of naive mice. We observed an increasing trend in monocyte lipid levels (p = 0.07) after stimulation with stroke (versus sham) WAT-derived supernatant. Immune functions were also impacted, as evidenced by reduced CD68 and MHCII expression, and decreased phagocytosis in dendritic cells.

To conclude, our study provides evidence for lipid accumulation in circulating leukocytes after stroke in humans and mice, which was shown to impair phagocytic engulfment of bacteria. Our findings suggest a novel mechanism of post-stroke immune suppression implicating the brain-fat axis.