Single-cell transcriptional atlas and cell compositions in OCS tissues
We conducted scRNA-seq to profile one ovarian carcinosarcoma (OCS) sample with the 10×Genomics Chromium platform. This 61-year-old female patient was diagnosed as OCS at the late tumor stage IIIC. The primary ovarian tumor was in large volume (Fig. 1A). The H&E and immunohistochemistry (IHC) staining was conducted to establish the diagnosis of OCS (Fig. 1B-1F). IHC staining for cytokeratin (CK) and E-cadherin showed diffuse strong staining of the epithelial component (Fig. 1C). The mesenchymal element stained for vimentin and desmin (Fig. 1D). This tumor presented high cellular proliferation markers Ki-67 and p16 labeling indexes (Fig. 1E). In addition, overexpression of p53 was found in the OCS tumor (Fig. 1F). Studies have demonstrated that Ki-67 and p53 overexpression affect OCS overall survival [8, 17]. Recent study has been reported that pembrolizumab could provide tumor control in a patient with metastatic OCS [10]. Here, we showed that this patient expressed focal PD1 expression (Fig. 1F), indicating that immunotherapy might be an effective treatment strategy for OCS.
To further explore the features of complex cellular components of OCS, we followed a customized workflow to isolate viable single cells from the primary surgical resections (Fig. 2A). Among the cells sequenced, 2,173 cells were retained after quality control filtering. By using a graph-based clustering method (see Materials and methods), cell clusters were annotated as epithelial cells, fibroblasts, ovarian cortex cells, endothelial cells, T cells, NK cells, NKT cells, or Myeloid cells (Fig. 2B) by established marker genes (Fig. 2C). All cell clusters identified from the OCS tumor could be visualized by a combined t-distributed stochastic neighbor embedding (t-SNE) plots (Fig. 2B). For example, endothelial cells were marked by VWF, CDH5 and ADGRL4 up-regulation, and epithelial cells were enriched with EPCAM, KRT7 and GPRC5A. Of note, the epithelial/cancer cells were the main cell type of this OCS sample and comprised 52% of the sorted cells. Fibroblasts expressed high levels of DCN, COL3A1 and LUM, and mast cells had a high level of CPA3, TPSB2 and CTSG expression. Myeloid cells were featured with HLA-DQA1, LYZ, IL1B overexpression, NK cells with GNLY, NKG7 and KLRD1 enrichment, NKT cells with FGFBP2 expression, and T cells with TRAC and CD3E overexpression. The ovarian cortex cells were enriched with TAGLN, RSG5 and THY1 expression (Fig. 2D).
Clustering-based copy-number variation resolves the malignant elements from non-cancer cells
Cancer cells are known to be associated with large-scale chromosomal alterations. Here, we applied copy-number variation (CNV) from the RNA expression data to classify epithelial cells as either cancer or non-cancer by compared with endothelial cells and macrophages (controls) [14, 18-20]. As shown in Fig. 3A and 3B, the epithelial cancer cells displayed much larger changes from the relative expression intensities across the genome. Of note, OCS contains both malignant epithelial and sarcomatous (mesenchymal) elements [3]. The sarcomatous component can be homologous (tissue physiologically native to the ovary) or heterologous (tissue foreign to the ovary). Homologous sarcomatous elements include fibrosarcoma and leiomyosarcoma, whereas the heterologous components usually contain chondrosarcoma, osteosarcoma, rhabdomyosarcoma or liposarcoma [21-23]. Thus, we also utilized CNV to verify fibroblasts and ovarian cortex cells as the malignant sarcomatous elements. As shown in Fig. 3C and 3D, we found that the fibroblasts displayed much larger chromosomal changes across the genome relative to endothelial cells and macrophages, but not the ovarian cortex cells. Furthermore, we validated the expression levels of epithelial cell markers EPCAM and KRT7 (Fig. 3E), and fibroblast markers DCN and COL3A (Fig. 3F) in human OCS tumor tissues using IHC staining. Taken together, malignant cell clustering reflected the epithelial cells and fibroblasts as the two malignant sources of OCS.
Transcriptional landscape intra-tumoral heterogeneity of OCS epithelial cells
We performed detailed clustering analysis of the 1,127 epithelial cells using Seurat and revealed the existence of six prominent cell subpopulations, namely C0 (RPL37+ / CDH6+), C1 (SLC2A1+ / ERO1A+), C2 (RPL13A+ / CXCR4+), C3 (S100A9+ / CEACAM6+ / LY6D+), C4 (XIST+ / CCNL1+ / WEE1+) and C5 (TOP2A+ / CENPF+ / MKI67+) (Fig. 4A, Fig. S1A). To learn more about the biology underlying these cell subgroups, we first employed the KEGG pathway analysis to identify distinct signaling pathways and interrogated for transcription factor consensus sites using single-cell regulatory network inference and clustering (SCENIC) method (Fig. 4B and 4C, Fig. S1B). These analyses identified that C3 was controlled by the transcription factors TP63, HES2 and BCL6, and enriched for antigen processing and presentation pathway. We also found this cluster cells expressed high levels of immune-related signaling pathways such as type I and II interferon-response (Fig. S1C), suggesting possible cell-cell communication between tumor cell and immune cells. C1 showed distinct signature of HIF-1 signaling pathway and expressed high level of FOSL2. C4 displayed increased levels of TAF1 and spliceosome. The C0 and C2 subpopulations indicated a link to ribosome. C5 was controlled by elevated MYBL1 as well as NFYB and CTCF, and enriched with cell cycle.
In addition, we applied metabolism-related pathway analysis for these cell clusters. As shown in Fig. 4D, increased lysine biosynthesis, lipoic acid metabolism, thiamine metabolism, oxidative phosphorylation, and drug metabolism-cytochrome P450 were observed in C0 cells. C1 was D-Glutamine and D-glutamate metabolism, lysine degradation, Vitamin B6 metabolism, nitrogen metabolism. C2 and C4 cells were in suppressive states with the lowest levels of most metabolic pathways. C3 was activated with caffeine metabolism, linoleic acid metabolism, steroid hormone biosynthesis, glycosphingolipid biosynthesis and glycosaminoglycan degradation, while C5 showed the characteristics of highly activated lysine biosynthesis, biotin metabolism, pyrimidine metabolism and cyanoamino acid metabolism. We next focused on the drug resistance analysis, and further characterized the potential drug resistant states in detail. Our data revealed that C0 as well as C1, C3 and C5 had the high levels of drug resistance scores (Fig. 4E). Interestingly, these clusters also harbored activated metabolism pathways, suggesting a potential association between metabolism and drug resistance of the tumor cells. Of note, C1 subpopulation displayed activated transcription factors and growth factor receptors by the expressing of NFKBIE and IGF1R (Fig. 4F and 4G, Fig. S1D). C5 subpopulation was positively correlated with DNA damage/repair and drug resistance, and marked by the expression of BRCA1 and TOP2A (Fig. 4F and 4G, Fig. S1D).
Distinct fibroblast subpopulations in human OCS tumor
The fibroblasts were clustered in two subpolulations (Fig. 5A). These two subclusters expressed high levels of canonical fibroblast markers such as DCN and COL3A1; however, each subcluster displayed distinct transcriptomic signatures (Fig. 5B). C0 fibroblasts accounted for the majority of the fibroblast populations (61.7%) expressed high levels of extracellular matrix (ECM) signatures, including COL6A2, COL6A1, COL3A1, COL6A3 and COL5A2. Interestingly, gene ontology (GO) analysis of this subcluster indicated significant enrichment for ECM organization and disassembly, as well as angiogenesis (Fig. 5C). C1 fibroblasts were characterized of signature genes such as SPINK6, RERGL, KERA, STEAP2, RAMP1 and TMEM176B. The GO terms enriched for this subcluster were associated with translational termination, translational elongation and SRP-dependent cotranslational protein targeting to membrane (Fig. 5C). Notably, we found that the C0 fibroblast signature was enriched with a set of metabolism-related pathways, such as linoleic acid metabolism, drug metabolism-cytochrome P450, oxidative phosphorylation and citrate cycle (TCA cycle) (Fig. 5D). However, most of these metabolic pathways were suppressed in C1 fibroblasts. The C1 signature was related to lysine biosynthesis (Fig. 5D). Next, we investigated whether these subtypes were correlated with known CAF types. However, we failed to group them into one certain CAF type (Fig. S2A). Interestingly, the C0 subtype was related to matrix CAFs (mCAFs), myofibroblastic CAFs (myCAFs), antigen-presenting CAFs (apCAFs) and inflammatory CAFs (iCAFs). As shown in Fig. 5E and 5F, the C0 fibroblasts expressed high levels of the type I interferon-response genes, such as IFITM3, IFI6, IFITM2, NAMPT, ADAR, LY6E, ISG15, and the type II interferon-response genes, such as STAT1, CDKN1A, HLA-A, HLA-B, HLA-C and HLA-E. This subcluster also expressed high levels of growth factors including PGF, VEGFB, IGF2, PTN, BMP2, BMP7, NRP1, GDF11, GPI and VEGFA (Fig. 5G), chemokines including CCL2, CXCL1, CCL28 and CXCL14 (Fig. S2B), and interleukins including IL11 and IL7 (Fig. S2C). These features of the C0 fibroblasts suggest an enrichment in the interplay between these fibroblasts and OCS cells.
Analysis of T cell and NK cell subpopulations defining the immune states in human OCS tumor
Tumor-infiltrating immune cells have been shown to play key roles in response to immunotherapy and tumor immune evasion [24]. As shown in the tSNE plots, the T and NK cells exhibited 9 distinct subclusters (Fig. 6A, Fig. S3A). According to their main marker genes, these subclusters were further grouped into seven subtypes: CD4+ T cells (CD3D+CD3E+CD4+CD8A-, subcluster 1 and 8); CD8+ T cells (CD3D+ CD3E+CD8A+CD4-, subcluster 4); gamma-delta (γδ) T cells (CD3D+ CD3E+TRDC+HOPX+CD8A-CD4-, subcluster 0 and 7); cycling immune cells (TOP2A+MKI67+, subcluster 6); NK cells (NKG7+GNLY+CD3D-CD3E-, subcluster 2); and NKT cells (NKG7+GNLY+CD3D+CD3E+, subcluster 5) (Fig. 6A and 6B, Fig. S3B). The subcluster 3 contained low quality cells, and were not further analyzed in this study. In addition, we found that NK cells, NKT cells and CD8+ T cells (subcluster 2, 5 and 4) expressed high levels of cytotoxic markers such as CST7, GZMA, GZMB, IFNG, NKG7 and PRF1 (Fig. 6C). Furthermore, the CD8+ T cells (subcluster 4) expressed a certain number of exhaustion markers, such as LAG3, PDCD1 and TIGIT, the γδ T cells (subcluster 7) expressed high level of exhaustion marker HAVCR2, and the CD4+ T cells (subcluster 8) expressed high level of TIGIT, suggesting that these cells became exhausted (Fig. 6D). Notably, the subcluster 8 CD4+ T cells were characterized by marker genes related to Tregs (Fig. 6E), and display immune checkpoint inhibition characteristics with high expression of CTLA4 and TIGIT (Fig. 6F, Fig. S3C). The γδ T cells play important roles in both innate and adaptive immune systems and certain subtype could contribute to IFNγ production [25, 26]. Similarly, hallmark pathways analysis in our study also revealed that the subcluster 7 of γδ T cells were enriched in IFNγ and IFNα response, suggesting that this subcluster might be IFNγ-producing γδ T cells (Fig. 6G).
OCS tumor contains distinct myeloid cell subtypes
Tumor-infiltrating myeloid cells, comprising of monocytes, macrophages, dendritic cells (DC), and neutrophils, have emerged as key regulators of cancer growth. We first dissected the gene signatures of 4 myeloid subtypes revealed in our study (Fig. S4A). Among these subclusters, monocytes (subcluster 1) were marked by APOE, C1QC and MAF, and macrophages (subcluster 2) were characterized by high expression levels of VCAN, S100A9 and CD14 (Fig. 7A-C). The DC cells (subcluster 0 and 3) accounted for the majority of the myeloid cells (55.7%) and were enriched with HLA-DRs and low expression of CD14 (Fig. S4A and S4B), further distinguished by specific expression of CD1A/FCER1A, LAMP3/CCR7, respectively (Fig. 7A-C). We found that the subcluster 0 of DC cells were enriched with oxidative phosphorylation, while the subcluster 3 of DC cells were marked by folate biosynthesis, histidine metabolism, D-Glutamine and D-glutamate metabolism, tryptophan metabolism, and ascorbate and aldarate metabolism (Fig. 7D). The macrophages were activated with a certain set of metabolic pathways including glycosaminoglycan biosynthesis, porphyrin and chlorophyll metabolism, sphingolipid metabolism, glycosaminoglycan degradation, riboflavin metabolism, glycosphingolipid biosynthesis, ether lipid metabolism, steroid biosynthesis and degradation of aromatic compounds (Fig. 7D). Furthermore, genesets analysis showed that the macrophages in OCS tumor were mainly M2-like macrophages with immunosuppressive properties (Fig. 7E and 7F). The CD1A+/FCER1A+ DC cells were enriched with genes related to cytolytic effector pathway, cell cycle, type II interferon response and immune checkpoint suppression (Fig. 7E and 7F), while the LAMP3+/CCR7+ DC cells were characterized by increased levels of genes related to cholinergic receptors, immune checkpoint activation, CD8+ T cell activation and glutamate receptors (Fig. 7E and 7G).
OCS malignant cells broadly interact with the immune and non-immune compartments
The complex cell communication networks were primarily mediated by interactions between ligands and receptors [27, 28]. In this study, we explore the ligands-receptors pairs using intercellular interaction analyses to reveal the central cellular components shaping the OCS tissue fate in single cell maps (Fig. S5A). As shown in Fig. 8A, the ANXA1-FPR1/FPR3 pair was enriched in the interactions between epithelial cells and myeloid cells, consistent with the finding that ANXA1 signals activate tumor-associated macrophage infiltration and tumor progression [29-31]. Moreover, we verified that ANXA1 expression was mainly localized in the malignant epithelial region of OCS tissues by IHC staining, in accordance with the scRNA-seq findings that ANXA1 was highly expressed in the epithelial cells (Fig. 8B and 8C). Our results indicate that the ANXA1-FPR1/FPR3 axis is enriched in the interplay between epithelial cells and myeloid cells. Furthermore, epithelial cells showed interactions with fibroblasts through IGF2-IDE axis (Fig. 8D, Fig. S5B). In addition, myeloid cells interacted with NK, NKT and T cells through CCL5-CCR1 pair (Fig. 8E, Fig. S5C).