血管内皮細胞に於けるACE2の発現は仮説検証を支持することが証明されておらず、SARS-CoV-2によるECの直接感染を支持する十分な証拠もない。
However, expression of ACE2 in EC has not been convincingly demonstrated to support this assumption, nor has there been sufficient evidence to support a direct infection of EC by SARS-CoV-2.
Ian R. McCracken, et al.
https://www.ahajournals.org/doi/pdf/10.1161/CIRCULATIONAHA.120.052824
Publicly available single-cell RNA-sequencing (scRNAseq) of human organ donor hearts2 showed that while ACE2 sequence reads are abundant in pericytes (PC), they are rare in EC (Figure D). Out of 100,579 EC, only 468 (0,47%) were ACE2+, and in the majority (424) only a single ACE2 transcript was detected. This could reflect true low and rare endothelial ACE2 expression, but also contamination from adherent PC fragments, a common confounder in vascular scRNAseq data. If such fragments contributed the ACE2 transcripts observed in certain EC, we would expect to detect other pericyte transcripts in the same cells. Indeed, among the top-50 gene transcripts enriched in ACE2+ vs. ACE2- EC, we noticed several known pericyte markers, including PDGFRB, ABCC9, KCNJ8and RGS5 (Figure E).
Comparison of transcript abundance across the three major vascular and mesothelial cells showed that the top-50 gene transcripts were expressed at the highest levels in PC (Figure E). This suggests that the rare occurrence of ACE2 transcripts in human heart EC is likely caused by pericyte contamination. Similar conclusions have previously been reached in mouse tissues
Analysis of the chromatin landscape at the ACE2 gene locus in human umbilical vein EC (HUVEC) using data from ENCODE further supports this concept. The histone modification mark H3K27me3, which indicates repressed chromatin, was enriched at the ACE2 transcription start site (TSS); conversely, promoter, enhancer and gene body activation marks (H3K27ac, H3K4me1, H3K4me2, H3K4me3, H3K36me3), RNA polymerase-II and DNase I hypersensitivity were absent or low, suggesting that ACE2 is inactive in EC. In marked contrast, the adjacent gene BMX, an endothelial-restricted non-receptor tyrosine kinase displays an epigenetic profile consistent with active endothelial expression (Figure F).
Thus, transcriptomic and epigenomic data indicate that ACE2 is not expressed in human EC. Other cell surface molecules have been suggested as possible receptors for the virus, but their role in supporting SARS-CoV-2 cell infection remains to be demonstrated. We therefore tested directly whether EC could be capable of supporting coronavirus replication in vitro.
Productive levels of replication in primary human cardiac and pulmonary EC were observed for the human coronavirus 229E GFP reporter virus4, which utilises CD13 as its receptor, demonstrating directly that human EC can support coronavirus replication in principle (Figure G).
However, when cells were exposed to SARS-CoV-2, replication levels were extremely low for EC, even following exposure to very high concentrations of virus compared to more permissive VeroE6 cells (Figure H).
The observed low levels of SARS-CoV-2 replication in EC are likely explained by viral entry via a non-ACE2 dependent route, due to exposure to extremely high concentrations of virus in these experiments (MOI 10 and 100).
These data indicate that direct endothelial infection by SARS-Cov-2 is not likely to occur.
The endothelial damage reported in severely ill COVID19 patients is more likely secondary to infection of neighbouring cells and/or other mechanisms, including immune cells, platelets and complement activation, and circulating proinflammatory cytokines. Our hypothesis is corroborated by recent evidence that plasma from critically ill and convalescent patients with COVID-19 causes endothelial cell cytotoxicity5. These finding have implications for therapeutic approaches to tackle vascular damage in severe COVID19 disease.
