Abstract
Purpose :
Choroidal macrophages (cMΦ) are the predominant resident immune population in the choroid, and our goal is to characterize cMΦ heterogeneity and transcriptomic states in health and disease.
Methods :
We reanalyzed publicly available bulk and single-cell RNA-sequencing (scRNA-seq) data from the choroids of 150 non-diseased and AMD donors (scRNA-seq: 7 controls, 2 AMD; bulk RNA-seq: 115 controls, 26 AMD).
Results :
We identified 5 predominant myeloid populations including classical monocytes, nonclassical monocytes, conventional dendritic cells, monocyte-like macrophages, and cMΦ. cMΦ displayed a unique transcriptomic profile that was enriched for genes/pathways involved in phagocytosis, lipid metabolism, and immunosuppression. Of note, cMΦ contained uniquely high levels of TREM2 expression, a lipoprotein receptor that regulates the phagocytosis and removal of excess lipid. In macular AMD samples, cMΦ displayed a reduction in relative abundance and a transcriptomic shift that included upregulation of MHCII and NF-κB target genes and downregulation of TREM2 signaling modules. To expand our findings, we used bulk RNA-seq deconvolution and found a significant decrease in cMΦ in macular AMD samples (macular control=4.4%, macular AMD=1.6%, p=0.006). We confirmed that mouse cMΦ express TREM2 at baseline using publicly available scRNA-seq, immunofluorescence, western blot, and RT-qPCR. To test the hypothesis that TREM2 expression in cMΦ regulates vascular maintenance and lipid removal, we challenged TREM2 knock-out mice (TREM2KO) with 15wk of high-fat diet (HFD). Compared to WT mice on HFD, TREM2KO HFD mice showed an accelerated loss of visual function, assessed using optokinetic nystagmus (percent change from baseline at 10wk HFD: WT=-2.9% [p=0.24], TREM2KO=-20.9% [p=0.0004]). At 15wk of HFD, TREM2KO mice showed significantly decreased choroidal thickness on OCT (WT=49.5µm, TREM2KO=45.9 µm, p=0.009), significantly decreased choroidal vascular density in whole mount stains (vascular to avascular ratio: WT=1.6, TREM2KO=1.3 [p=0.04]) and significantly increased neutral lipid droplets (mean pixel intensity: WT=21.0, TREM2KO=25.9 [p=0.04]).
Conclusions :
Together, these findings expand our understanding of cMΦ heterogeneity, abundance, and expression profiles in health and AMD, and identify TREM2 as a novel regulator of choroidal homeostasis that is lost in disease.
This is a 2021 ARVO Annual Meeting abstract.