Increased expression levels of ACE2 after SARS-CoV-2 infection
ACE2, the molecular pathway through which SARS-CoV-2 enters host cells (Fig. 1a), was significantly higher (Log2FC 0.22; p value 0.01) expressed in blood samples of COVID-19 patients as determined by bioinformatics analyses of transcriptomic data obtained from 24 healthy controls and 62 COVID-19 patients (COVID19db ID: COVID000010) (Fig. 1b). The increased ACE2 mRNA levels in COVID-19 patients were comparable to that of A549 lung carcinoma epithelial cells after ACE2 transfection and/or SARS-CoV-2 infection (Fig. 1c). Significantly increased ACE2 expression levels were also found upon infection with other respiratory viruses, e.g., HPIV3 infection of A549 cells (Fig. 1d) and IAVdNS1 infection of normal human bronchial epithelial (NHBE) cells (Fig. 1e).
Identification and function of ACE2-regulated genes in MCF-7 BC cells
RNA-seq analysis of ACE2high and ACE2low MCF-7 cells revealed a total of 2801 differentially expressed genes (DEGs) (padj < 0.05) with 1445 significantly upregulated and 1356 significantly downregulated genes (Fig. 2a). Gene ontology (GO) enrichment analysis was performed using the DEGs to assess the functional categories of biological process (BP), molecular function (MF) and cellular component (CC). The top 20 significantly enriched GO terms of the upregulated genes in ACE2high and ACE2low MCF-7 cells include the categories defense response to other organism' (ontology: BP; gene ratio 108/1174; p-value 2.57E-37), 'cytokine activity' (ontology: MF; gene ratio 46/1178; p-value 1.84E-16) and 'MHC protein complex' (ontology: CC; gene ratio 16/1214; p-value 5.09E-16) (Fig. 2b). The top 20 enriched GO terms of downregulated genes in ACE2low MCF-7 cells contain 'sister chromatid segregation' (ontology: BP; gene ratio 60/1176; p-value 7.62E-16), 'structural constituent of ribosome' (ontology: MF; gene ratio 41/1177; p-value 1.37E-11) and 'chromosomal region' (ontology: CC; gene ratio 80/1236; p-value 2.95E-19) (Supplementary Fig. 1). The top 10 upregulated genes by ACE2 were IFI6, IFIT1, IFIT2, IFIT3, OAS2, OASL, HLA-B, OAS1, DDX60 and CMPK2, the top 10 downregulated genes were SCD, ABCG1, SREBF1, FGFR4, PHGDH, FBXO27, PREX1, CRAT, AIF1L and PXMP4. Disease annotation of the top upregulated genes demonstrated a link to viral infections (Supplementary Table 2), most commonly to influenza (disease id: C0021400) (Supplementary Fig. 2A), while the downregulated genes were annotated to BC (malignant tumor of breast (disease id: C0006142) and breast carcinoma (disease id: C0678222) (Supplementary Fig. 2B). The ACE2-mediated differential expression profiles were independently confirmed for selected DEGs ACEhigh and ACElow cell systems by qPCR using DEG-specific primers (data not shown).
Correlation of the GO terms and DEGs between ACE2high MCF-7 cells and SARS-CoV-2-infected PBMNCs
Comparison of the GO terms of the significantly upregulated genes in ACE2high vs. ACE2low MCF-7 cells with those in whole blood obtained from 62 COVID-19 patients and 24 healthy volunteers (COVID19db ID: COVID000010) demonstrated that 9/10 selected upregulated genes in ACE2high MCF-7 cells were expressed at higher levels in blood samples of COVID-19 patients as visualized by a volcano plot (Fig. 2c). Additionally, 8/10 GO terms were commonly enriched in both ACE2high MCF-7 cells and blood samples of COVID-19 patients, as indicated by green circles. These enriched GO terms included 'response to virus', 'nucleosome', and 'type I interferon signaling pathway' (Fig. 2d). As expected, the GO terms ‘neutrophil degranulation and neutrophil activation’ were only found in blood samples of COVID-19 patients, but not in ACE2high MCF-7 cells. The analysis further focused on the significantly upregulated genes within the top five GO terms, namely 'defense response to other organism', 'response to type I IFN', 'defense response to virus', 'response to virus', and 'type I IFN signaling pathway'. Notably, 14 common genes upregulated in ACE2high MCF-7 cells within these top five GO enrichments (Supplementary Fig. 3) were also enhanced in blood samples from COVID-19 patients (COVID19db). These genes include OAS1 (Log2FC 1.78; p value 5.47E-09), OAS2 (Log2FC 1.34; p value 0), OAS3 (Log2FC 1.67; p value 0), OASL (Log2FC 1.87; p value 2.91E-10), STAT1 (Log2FC 0.93; p value 9.34E-08), IFITM3 (Log2FC 1.61; p value 5.94E-09), IRF1 (Log2FC 0.48; p value 3.38E-06), IRF2 (Log2FC 0.19; p value 0.01), IRF7 (Log2FC 1.32; p value 6.86E-07), IRF9 (Log2FC 0.54; p value 0.001), BST2 (Log2FC 0.68; p value 0), IFITM1 (Log2FC 1.19; p value 2.26E-08), IFITM2 (Log2FC 0.67; p value 4.91E-06) and NLRC5 (Log2FC 0.36; p value 0.002). All 14 genes upregulated in genes of ACE2high MCF-7 cells were statistically higher (p < 0.05) in COVID-19 blood samples compared to that of healthy controls (Fig. 2e). Additionally, the top 10 up- and downregulated genes of ACEhigh MCF-7 cells were compared to those of SARS-CoV-2-infected Calu3, A549 and NHBE cells (Supplementary Fig. 4A) as well as to infection with other respiratory viruses, such as IAV, IAVdNS1, HPIV3, and RSV (Supplementary Fig. 4B). Interestingly, except for IFIT2 in SARS-CoV-2-infected NHBE cells, the expression of the top 10 genes in ACE2high MCF-7 cells exhibited a similar increased trend upon viral infections. Among the top 10 downregulated genes, SREBF1, FGFR4, CRAT and PXMP4 showed a similar decrease following different viral infections. Hence, the global transcriptomic profile and functional annotations of ACE2high MCF-7 cells were mainly comparable to those of SARS-CoV-2-infected cells and COVID-19 patients.
Upregulation of HLA class I surface expression after ACE2 overexpression and SARS-CoV-2 infection
Since the effect of SARS-CoV-2 infection-relevant molecules on the expression of immune modulatory molecules has not yet been analyzed in detail, the consequence of ACE2 overexpression in MCF-7, RKO, A549 and EA.Hy926 cells on the expression of HLA class I antigens was determined by flow cytometry. As shown in Fig. 3a, an up-regulation of HLA class I surface expression was detected upon ACE2 overexpression in all cell lines mentioned. This finding is consistent with high levels of HLA-B (Log2FC 0.2; p value 0.02) and -C (Log2FC 0.59; p value 1.17E-06) expression in blood samples of COVID-19 patients compared to healthy controls (Fig. 3b).
Furthermore, RNA-seq results from lung biopsies of COVID-19 patients (GEO: GSE1488290) displayed a similar correlation with higher mRNA levels of HLA class I antigens (Supplementary Fig. 5). SARS-CoV-2 infection of ACE2high A549 cells (Fig. 3c) and infection with other respiratory viruses, such as IAV, IAVdNS1, HPIV3 and RSV, upregulated HLA class I antigens (Supplementary Fig. 6) when compared to the uninfected controls. HLA-B, but not HLA-A and HLA-C antigens were enhanced in SARS-CoV-2-infected Calu3 cells (Fig. 3d).
Association of the ACE2-mediated upregulation of HLA class I surface antigens with increased APM and IFN signaling component expression
In order to determine whether the ACE2-mediated increase of HLA class I surface expression was due to an enhanced expression of HLA class I APM components, the human ACE2high and ACE2low model systems were analyzed for the mRNA and protein expression of the major HLA class I APM molecules, such as the transporter associated with antigen processing (TAP)1, TAP2, TAPBP, β2-microglobulin (B2M), the IFN-γ inducible proteasome subunits, the low molecular weight proteins PSMB8, PSMB9 and PSMB10 as well as the chaperones calreticulin (CALR) and calnexin (CANX). Except for calnexin, calreticulin and tpn, an ACE2-mediated upregulation of the mRNA expression of the other HLA class I APM components analyzed was detected (Fig. 4a).
Despite SARS-CoV-2 infection has been reported to influence cytokine signaling, including the IFN signaling pathway [48], and IFN-γ has been shown to increase ACE2 surface expression [49], a possible link between ACE2 overexpression in tumors and IFN signaling has not yet been analyzed. Expression analyses of various IFN type I and II signaling components revealed a strong upregulation of the mRNA expression of IRF1, IRF9, JAK2, STAT1, STING and TYK2 in ACE2high transfectants compared to the ACE2low mock controls (Fig. 4b). These data were confirmed by Western blot analyses as representatively demonstrated by an increased TAP1 and IRF1 protein expression in ACE2high vs. ACE2low cells (Fig. 4c).
The link between ACE2 expression and immune response relevant profiles was also examined in the context of cancer through in silico analyses of cancer genome databases. As shown in Fig. 4d and Supplementary Table 3, ACE2 showed a positive correlation with the expression of components of the HLA class I APM as well as the IFN type I and II pathways in pan-cancer and breast cancer samples.
The upregulation of HLA class I by ACE2 influenced the NK cell-mediated recognition of ACE2high MCF-7 cells with a decreased cytotoxicity of NK cells compared to ACE2low MCF-7 cells as determined by a CD107a degranulation assay (Fig. 4e).
Correlation of ACE2 expression with the expression of the immune checkpoint molecule PD-L1 (CD274)
It was postulated that the ACE2-mediated upregulation of immune modulatory molecules might be associated with an increased response to immunotherapy [50], such as e.g. the treatment of SARS-CoV-2-infected patients with ICPi might enhance anti-viral T cell responses by affecting PD-L1 expression [51]. Indeed, ACE2high MCF-7 cells expressed higher levels of PD-L1 than the ACE2low control cells (Fig. 5a), which was accompanied by an increased expression of different IFN-γ signaling pathway components in ACE2high EA.Hy926, A549, RKO and MCF-7 cells and is in line with the IFN-γ-mediated upregulation of PD-L1. Comparable results were retrieved from in silico data of blood samples (Log2FC 1.19; p value 4.43E-06) (Fig. 5b) and lung biopsies from COVID-19 patients compared to their healthy counterparts (Supplementary Fig. 5) as well as in both SARS-CoV-2-infected (Fig. 5c) and other respiratory virus-infected cells (Fig. 5d).
ACE2high, not ACE2low MCF-7 cells, increased immune cell migration and apoptosis upon nivolumab treatment
In the next step, immune cell migration, cancer cell proliferation and apoptosis was investigated over a period of three days upon co-culturing ACE2low/high MCF-7 cells with immune cells in the presence and absence of the anti-PD1 monoclonal antibody nivolumab. In contrast to the ACE2low controls, the PD-1 inhibitor induced significant immune cell infiltration towards ACE2high MCF-7 cells (Fig. 6a), which significantly increased over time. The number of immune cells migrating towards cancer cells was minimal in ACE2mock/low MCF-7 and was unaffected by nivolumab treatment (Fig. 6). The induced migration of immune cells in the presence of nivolumab was correlated with a trend of increased apoptosis of ACE2high cells (Fig. 6b), but not of ACE2low MCF-7 cells (Fig. 6d), while nivolumab did neither influence the proliferation of ACE2high nor of ACE2low MCF7 cells (data not shown). The higher expression of HLA class I on ACE2high MCF-7 cells might be responsible for the increased migration and apoptosis, potentially leading to the activation of T cells. At the same time, the inhibitory effect of the increased PD-L1 expression in ACE2high cells would be inhibited by nivolumab (Fig. 7a).
Association of altered cytokine release with ACE2 and PD-L1 expression upon nivolumab treatment
Despite a cytokine storm has been widely reported caused by in viral respiratory infections of influenza viruses and SARS-CoV-2 [52, 53], it has not been directly linked to specific immune cell subpopulations. To investigate the impact of nivolumab on cytokine release in cell supernatants during the co-culture of PBMNCs with ACE2high and ACE2low MCF-7 cells, a human FirePlex®-96 key cytokine immunoassay panel was used, which consists of 17 cytokines (CSF2, IL1B, IL2, IL4, IL5, IL6, CXCL8, IL9, IL10, IL12A, IL13, IL17A, IFNG, CCL2, CCL3, CCL4, and TNF). As shown in Fig. 7b, treatment of ACE2high and ACE2low MCF-7 cells with nivolumab decreased the release of the innate immunity-related cytokine CCL2 and an increased in the secretion of the adaptive immunity-related cytokine IL-2. Furthermore, the secretion of the anti-inflammatory cytokine IL-10 known to inhibit MHC class I expression was significantly reduced following nivolumab treatment (Fig. 7b), while the other cytokines analyzed did not show statistically significant changes (Supplementary Table 4A).
Altered cytokine expression profile in peripheral blood cells and lung epithelium upon SARS-CoV-2 infection
To assess the expression status of the previously mentioned differentially expression cytokines, single cell (sc) RNA-seq data from PBMNCs obtained from a cohort of 425,398 single cells from COVID-19 patients [54] were examined using CZ CELLxGENE Discover. Using the uniform manifold approximation and projection (UMAP), the expression of the cytokines IFNG, IL6, IL2, IL10, and CCL2 was determined across various immune cell subtypes, including plasmablasts, B cells, CD4+, CD8+, and γδ T cells, NK cells, conventional and plasmacytoid dendritic cells, classical and non-classical monocytes and hematopoietic progenitor cells (Fig. 7c). The UMAP revealed that IFNG mRNA levels were notably high in CD4+, CD8+ and γδ T cells as well as NK cells (Fig. 7d), while IL2 expression was found to be in CD4+ T cells (Fig. 7e). In addition, CCL2 expression exhibited higher levels in classical monocytes and dendritic cells (Fig. 7f) as well as an upregulation of IL10 mRNA in monocytes and CD4+ T cells of COVID-19 patients (Fig. 7g). Since the lung epithelium is a major target of the cytokine storm [44], a separate in silico analysis was performed on lung biopsies from severe COVID-19 patients (GEO: GSE147507) to assess the expression of these cytokines. The analysis revealed low IL2, but high CCL2 and IL10 mRNA expression (Supplementary Table 4B).