Background

Breast cancer has become the most commonly diagnosed cancer in women [1]. In the absence of distant metastases, breast cancer is a potentially curable disease [2]. Breast conserving surgery (BCS) has been favored more recently due to its highest success rate in women with early-stage breast cancer, but it is not recommended for women at high risk of local recurrence [3]. A negative margin reduces the risk of local recurrence in invasive breast cancer based on a study-level meta-analysis [4]. For esthetic reasons, the goal of BCS is to remove a sufficient margin of normal tissue around the tumor without affecting the breast appearance. A 5-mm-wide tissue zone surrounding the tumor burden and adjoining the normal is a traditionally and widely used gross surgical margin showing low recurrence [5]. However, in actual practice, a surgical margin ranges from no tumor on ink to 10 mm or more depending on the age, cancer type, therapies employed and ethnicity [6]. Consequently, for an accurate intraoperative surgical margin assessment of BCS, it is necessary to clarify the cellular, structural and molecular information of the whole specimen surface as it relates to the tumor burden zone.

Tumor microenvironment (TME) plays a decisive role in tumor malignant biological behaviors and response to therapies through its constant interaction with tumor cells [1: Table S2. RT-qPCR data were calculated by the ΔΔCt of genes of interest relative to the housekee** gene GAPDH.

Cell proliferation assay

Cell proliferation was performed using Cell Counting Kit-8 (TransGene, FC101-01). Cells were plated in 96-well plates and cultured in an incubator. At indicated time point after seeding, CCK-8 solution was added to the wells and incubated for 3 h; then, the absorbance was measured at 450 nm (BioTek, Epoch2 Instruments).

Colony formation assay

Cells were seeded in 6-well plates at a density of 1000 cells/3 ml per well. The medium was changed every 4 days. On the 6th day (MCF7 cells), 8th day (MDA-MB-231) and 20th day (SKBR3 cells), cell clones were photographed using OLYMPUS inverted fluorescence microscope.

Wound-healing assay

A thin ruler was used to scrape a straight line in the confluent cell monolayer in 6-well plate to create a wound. Then detached cells were washed away with PBS and the cells were incubated in media with 1% FBS to allow cells to grow and close the wound. The scratches were photographed at 0 h,12 h, 24 h, 48 h, 72 h and 144 h using OLYMPUS inverted fluorescence microscope. The scratch area was calculated by the software OLYMPUS cellSens Entry, and the average value of the area was calculated by multiple scratches. The change rate of area represented the speed of cell migration.

Tumor-bearing mice

Female BALB/cNj-Foxn1nu/Gpt mice were purchased from corporation (Gempharmatech Co., Ltd, China). Mice were housed in specific pathogen-free facilities with ad libitum access to water and food under a 12-h dark and 12-h light cycle in animal housing with constant humidity (40–60%) and temperature (23 ± 1 °C). 5 × 106 cells were collected and injected into subcutaneous location of both groin in nude mice. Cells expressing BMPR1B gene were injected on the left side, and control cells were injected on the right side. All programs have been approved by the Research Ethics Committee of Northwest University.

Statistical analysis

The log-rank test, Pearson’s correlation and two-sided paired Student’s t-test were used in the study. The method for score of gene signatures is one-way ANOVA. Unless otherwise specified, the quantitative data were presented as mean (± standard deviation) from three independent experiments. Significant differences were considered as follows: ns, no significance, p >  = 0.05; *p < 0.05; **p < 0.01.

Results

scRNA-seq reveals cell subpopulations of different zones of breast tumor tissues

We collected nine sets of fresh samples from the tumor zone, the interface zone and the normal zone of 3 patients with untreated and invasive breast cancer who underwent total mastectomy. After resection, samples were rapidly digested to single-cell suspension for sequence, analysis and functional study (Fig. 1a). A total of 88,548 cells were obtained from clean data. Among them, 28,718 cells originated from the tumor zone, 29,192 cells from the interface zone and 30,638 cells from the normal zone (Additional file 1: Table S3). Based on marker genes and artificial correction, all identified clusters could be assigned to known cell types: Fibroblasts, B cells, T cells, epithelial cells, stem cells, macrophages and endothelial cells, and different cell types had different transcriptional activities (Fig. 1b).

Fig. 1
figure 1

Overview of the 88,548 single cells from breast tissues of breast cancer patients. a Design and workflow of this study. b tSNE plots of the 88,548 cells profiled (top to bottom): the annotated cell type, different patients of origin (patient 1, patient 2 and patient 3), different sample types of origin (normal, interface or tumor zone samples) and the number of transcripts. K, Thousand. c Density dot plot showing the correlation of genes in bulk sequencing and single-cell sequencing. d Subclusters of stem cells, macrophages, T cells, B cells, endothelial cells, fibroblasts and epithelial cells (left to right): tSNE plots of marker genes for cell types, the fraction of cells deriving from each of 3 patients, the fraction of cells deriving from the normal, interface and tumor zone samples and the number of cells in each subcluster

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Meanwhile, the correlation analysis of single-cell RNA sequencing and matched bulk-seq showed that they had a good correlation (r = 0.86; Fig. 1c), indicating that gene expression was not significantly affected by cell dissociation. Overall, the data revealed the presence of a complex cellular heterogeneity containing a total of 87 different cell subclusters (Fig. 1d), and many subclusters had strong zone specificity, which implied that the interface zone has a unique microenvironment. Here, we mainly explored the zone-specific cell subtypes and functions of specific genes in epithelial cells.

Comparison of characteristics of T cell subtypes in different zones

T cells play crucial roles in the process of tumor inflammatory response. Here, we detected 12,941 T cells that were reclustered into 11 separate subsets (Fig. 2a), respectively, corresponding to natural killer cells (NK cells, cluster 9, KLRF1 +), CD8 + T cells (clusters 0, 4, 5 and 7, CD8A +), CD4 + T cells (clusters 1, 2 and 8, CD4 + /FOXP3-) and regulatory T cells (Tregs, clusters 3 and 6, CD4/IL2RA/FOXP3 +) (Fig. 2b). In addition, one cluster did not correspond to known cells (cluster 10, TOP2A +), and ontology analysis showed cell cycle pathway had strong enrichments in this cluster (Additional file 1: Figure S2a), suggesting cluster 10 had an active biological process.

Fig. 2
figure 2

T cell clusters. a tSNE plots of 12,941 T cells, color by relevant clusters (top) or sample type (bottom). b Color coding for expression (gray to blue) of marker genes of natural killer cells, CD8 + T cells, CD4 + T cells, regulatory T cells and TOP2A + cells. c Differences in pathway activity of each cell scored by GSVA between T cells isolated from normal, interface and tumor zone zones. d As in c, but for CD4 + T cells (clusters 1, 2 and 8), CD8 + T cells (clusters 0, 4, 5 and 7) and regulatory T cells (clusters 3 and 6). e Violin plots displaying the expression of selected genes encoding granzyme (GZMA, GZMM and GZMK) and immune checkpoint (CTLA4 and TIGIT) in T cell clusters. f Overall survival curves of breast cancer patients stratified by the average expression of top 10 marker genes of regulatory T cell clusters. Binary: high versus low, n = 535 or 535 patients, respectively. Expression threshold set to median to split the high-expression and low-expression cohorts. g Heatmap of single-cell regulon scores inferred by SCENIC. Number in brackets is the number of the target genes. g, genes

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T cell types of NK cells, CD8 + cells and CD4 + cells were detected in all three zones, with the exception of CD8 + cluster 7 and CD4 + cluster 8 mainly presenting in tumor zone. For Tregs, cluster 3 apparently enriched in tumor zone, and cluster 6 enriched in both tumor and interface zone (Fig. 2a and b). The enrichment of specific T cell clusters in tumor and interface zones suggested a transcriptome difference between T cell types that were influenced by microenvironment.

Subsequently, we compared levels of the pathway gene signatures among cells from the three zones (Fig. 2c). Analysis showed that overall pathway signals of CD4 + and CD8 + T cell were weak in the interface zone, which indicated that these T cells have weak bioprocess and functions. However, angiogenesis signal in the interface-derived Tregs was stronger than that in the normal zone, which might contribute to the induction and/or maintenance of Tregs in the interface zone [20]. Besides, some pathways differed between subclusters; for instance, CD8 + cluster 7 and CD4 + cluster 8 showed strong IFN-γ and IFN-α responses, enhanced complement activities, high allograft rejection activities (Fig. 2d) and granzyme expressions of GZMA, GZMM and GZMK (Fig. 2e), indicating these two subclusters of T cells showed some certain anti-tumor activities. For Tregs, compared with cluster 6, cluster 3 had stronger inflammatory response states, such as IFN response and complement reaction (Fig. 2d), although both of them expressed immunosuppressive molecules TIGIT and CTLA4 (Fig. 2e). However, survival analysis of marker genes showed that cluster 6 positively contributed to the prognosis of patients based on The Cancer Genome Atlas (TCGA) datasets, while cluster 3 had no relationship with patients’ prognosis (Fig. 2f), which indicated tregs had complex functions that need to be further explored.

In order to assess which transcription factors underlay differences in expression patterns of pathways among clusters, we performed single-cell regulatory network inference and clustering (SCENIC). SCENIC scanned transcription factor binding sites near differentially expressed gene positions and analyzed co-expression of these genes and transcription factors. There were significant differences in the expression of transcription factors (Fig. 2g). We noticed high expression levels of transcription factors STAT1, STAT2 and IRF7 in tumor-derived cluster 3, cluster 7 and cluster 8 (Fig. 2g). These factors are required for IFN production [21,22,23], which suggested that they played important roles in inflammatory response. Upregulation of transcription factor BATF controlled the activation program of clusters 3 and 6 in tumor and interface zones [24].

Recently, immune-hot and immune-cold regions are considered additional elements of heterogeneity of TME [25, 26]. Here we found clusters 0, 1, 4, 5, 7 and 8 can express granzyme (Fig. 2e), which suggests that these clusters have immune activity, and thus, they belong to the immune-hot clusters. However, clusters 2, 3 and 6 had no obvious immune activity based on their low granzyme expression level, and they belong to the immune-cold clusters. Notably, we noticed that total cell numbers of immune-hot clusters were higher in the interface or the tumor zone than that in the normal zone in all three patients (Additional file 1: Table S4), indicating the interface zone share some characteristics with the tumor zone at the immune level. Altogether, the interface-derived T cells were different from those in normal and tumor zone.

Macrophages show distinct phenotypes in interface zone

Tumor-associated myeloid cells can differentiate into different cellular subsets based on their microenvironment, and every subset has unique characteristics and plays different roles [27]. A total of 2719 myeloid cells were obtained and reclustered into 11 subclusters (Fig. 3a). Clusters 3 and 5, mainly enriched in the tumor and interface zones, were characterized by genes S100A4, AP1S2 and S100A6 that defined monocytes [28, 29]. Cluster 7, across the three zones, was identified as dendritic cells (DCs) that selectively expressed DC markers IRF8, BIRC3, BASP1 and NAPSB [30, 31]. The remaining clusters corresponded to macrophages that expressed C1QA, C1QB, C1QC and APOE enabling the distinction from monocytes (Fig. 3b) [32, 33]. In addition, C1QA, C1QB and C1QC genes could well distinguish macrophages from monocytes and DCs, and commonly used markers MSR1, CD14 and CD163 could not achieve this discrimination (Fig. 3c).

Fig. 3
figure 3

Myeloid cell and B cell clusters. a tSNE plots of 2,719 myeloid cells, color by relevant clusters (left) or sample type (right). b Bubble plots displaying expressions of specific genes in DCs (cluster 7), monocytes (clusters 3 and 5) and macrophages (clusters 0, 1, 2, 4, 6, 8, 9 and 10). c Violin plots displaying expressions of the selected marker genes in macrophage clusters. d Differences in pathway activity of each cell scored by GSVA. e–f Pseudotime trajectory of macrophage inferred by Monocle and showed by subcluster (e) and state (f). g Heatmap of single-cell regulon scores inferred by SCENIC. Number in brackets is the number of the target genes. g, genes. h tSNE plots of 1,485 B cells and myeloid cells, color by relevant clusters (left) or sample types (right). i Color coding for expression (gray to blue) of marker genes of plasma B cells, follicular B cells, IL7R + B cells and mast cells. j Pseudotime trajectory of B cell and mast cell state inferred by Monocle and characterized by state (left) and cell types (right). k Differences in pathway activity of each cell scored by GSVA. l Bubble plots displaying expressions of selected genes in subclusters

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According to the analysis of pathway gene signatures, macrophage subtypes had different pathway activities (Fig. 3d). Decreased signals of inflammatory response, IFN response, reactive oxygen species and allograft rejection corresponded to M2-like macrophage phenotypes in cluster 9 that expressed scavenger receptors (SCARA3) (Fig. 3d, Additional file 1: Figure S2b), which indicated cluster 9, across the three zones, had phagocytic function in the body [34]. For tumor-derived cluster 6, a high expression level of VEGFA and a strong hypoxia signal supported that cluster 6 played an important role in promoting angiogenesis (Fig. 3d, Additional file 1: Figure S2b).

Clusters 0, 2 and 10 were mainly enriched in the tumor and interface zones and displayed a specificity of zone distribution of macrophages with strong IFN response and reactive oxygen species (Fig. 3d). Despite all this, the poor separation of these clusters indicated they did not represent separate entities but diverse cell states, which was consistent with a spectrum of macrophage activation states [35]. In order to determine the differentiation state of macrophages, we performed a pseudotime analysis by Monocle and found that macrophage subclusters did not have a specific polarization state, except for M2-like cluster 9 (Fig. 3e and f). Therefore, clusters 0, 2 and 10 did not belong to polarized macrophages. SCENIC analysis showed PPARG transcription factor as classical M2 marker was upregulated in cluster 0 (Fig. 3g) [36], indicating cluster 0 might tend to polarize to M2 macrophages. Transcription factors IRF7, IRF1, STAT2 and STAT1, which boosted inflammatory response, were upregulated in cluster 2, indicating cluster 2 might tend to polarize to M1 macrophages. Moreover, cluster 10 with high expression level of EZH2 had a function in tumor promotion [37]. Together, results displayed macrophages with various states in interface zone, and cluster 0 and cluster 10 may provide an environment for tumor occurrence in the interface zone.

Follicular B cells and mast cells in interface and tumor zones possess specific characteristics

We detected 1,363 B cells and 122 mast cells that clustered near B cells. Given that TME could affect tumor infiltrating immune cells [16, 38], we explored the differences of B cells in three zones. Reclustering revealed 11 distinct subclusters (Fig. 3h), of which nine were enriched in tumor and interface zones and designated as plasma B cells (clusters 2 and 10, IGHG1 +), follicular B cells (clusters 0, 3, 6, 8 and 9, MS4A1 +), immature B cells (cluster 7, IL7R +) and mast cells (cluster 5, TPSAB1 +) (Fig. 3i). The remaining two clusters were derived from all three zones and designated as plasma B cells with high level of IGHA1 (clusters 1 and 4). Further, pseudotime analysis revealed the differentiation trajectory of B cells was divided into three branches corresponding to the main cell types follicular B cells, plasma B cells and mast cells (Fig. 3j). Wherein, immature B cells distributed along axes of the trajectory, indicating it was under a state of differentiation from follicular B cells to plasma B cells. Mast cells were independently located at one end of the differentiation trajectory because they did not belong to B cell lineage. Other clusters were well distributed at the corresponding location of cell types in the differentiation trajectory, indicating the differentiation process of follicular B cells to plasma B cells.

Pathway analysis showed no significant difference in plasma B cells among the three regions. However, interface-derived and tumor-derived follicular B cells expressed strong interferon response signal (Fig. 3k, Additional file 1: Figure S2c) and high levels of HLA class II molecules (Fig. 3l), which can enhance T cell-independent antibody response [39] and antigen presentation function. Additionally, interface-derived and tumor-derived mast cells had high expression levels of VEGFA and VEGFB genes (Additional file 1: Figure S2d), suggesting potential functions of promoting angiogenesis.

Stem cells harbor different differentiation states in interface zone

A total of 20,810 stem cells were obtained and classified into 13 subclusters, among which clusters 1 and 6 were enriched in the interface and tumor zones, while cluster 4 was mainly derived from the interface zone, and the remaining clusters distributed in all three zones (Fig. 4a).

Fig. 4
figure 4

Stem cell clusters. a tSNE plots of 20,810 stem cells, color by relevant clusters (left) or sample type (right). b Pseudotime trajectory of stem cells state inferred by Monocle and characterized by subcluster (top to bottom): pseudotime characterized by a gradient from dark to light blue, pseudotime trajectory of MSCs, PSCs and progenitor cells. MSCs, Mesenchymal stem cells; PSCs, pluripotent stem cells. c Bubble plots displaying expressions of specific genes in MSCs (clusters 1 and 6), PSCs (clusters 3 and 5) and progenitor cells (clusters 0, 2, 4, 7, 8, 9, 10, 11 and 12). d Boxplots showing signature score of marker genes. The signature score is calculated by the average expression of log2(TPM + 1) of each gene in top 10 marker genes for MSCs (cluster 1 or 6) from breast cancer tissues (TCGA datasets, n = 1085) and normal breast tissues (TCGA and GTEx datasets, n = 291), **p < 0.01. e Heatmap of score differences of selected pathway activity of each cell by GSVA

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Pseudotime analysis yielded a connected trajectory with three main branches (Fig. 4b). Clusters 1 and 6 were markedly enriched at the start branch of pseudotime, suggesting they represented mesenchymal stem cells (MSCs, Fig. 4b), which was further confirmed by expression of FN1, a marker gene of MSCs (Fig. 4c) [40]. Besides, expressions of maker genes that separated clusters 1 and 6 from other clusters were confirmed in TCGA data (Fig. 4d). Interestingly, clusters 1 and 6 displayed high expressions of collagen type IA1, A2 and type IIIA1 (Fig. 4c), which have been previously reported to promote tumor growth and invasion. Clusters 3 and 5 were enriched at three zones that were merged into the first branch point (Fig. 4b), revealing the multilineage potential of these two clusters; thus, they were identified as pluripotent stem cells (PSCs). The remaining clusters were enriched at one end branch of the trajectory (Fig. 4b) and expressed some epithelial marker genes (Fig. 4c), so they were defined as epithelial progenitor cells.

Analysis of pathway gene signatures showed that each subtype had specific pathway activities (Fig. 4e). Wherein, MSCs (cluster1 and 6) had a strong IFN response signal, indicating the immunosuppressive capacity [41]. Notch signaling and myc targets had different expression levels between clusters 3 and 5, both of which belong to PSCs, suggesting PSCs possessed different differentiation and proliferation capabilities [42, 43]. Notably, interface-derived cluster 4 showed a weak DNA repair ability, which might promote breast carcinogenesis indicating a special interface microenvironment [44].

Special endothelial cell subtypes in the interface and tumor zones

Endothelial cells located between tissue and blood play a role in presenting tumor microenvironment signals. Here, we obtained 1258 endothelial cells and reclustered them into 10 clusters, and more interface-derived cells than normal-derived cells were observed among these clusters (Fig. 5a), indicating that endothelial cells in interface zone were strongly affected by tumor progression.

Fig. 5
figure 5

Endothelial cell clusters. a tSNE plots of 1258 endothelial cells, color by relevant clusters (top) or sample type (bottom). b Color coding for expressions (gray to blue) of marker genes of lymphatic and vascular endothelial cells. ECs, endothelial cells. c Differences in the score of pathway activity of each cell by GSVA, between endothelial cells isolated from tumor or interface zone. d Violin plots displaying expressions of the selected genes in endothelial cells from tumor, interface and normal zones. e Differences in pathway activity of each cell scored by GSVA between zone-specific vascular endothelial cells (clusters 2, 4, 6 and 8). f Heatmap of single-cell regulon scores inferred by SCENIC. Number in brackets is the number of the target genes. g, genes

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Based on marker genes for every cluster (Additional file 2: Table S5), we revealed one set of lymphatic endothelial cells (cluster 9, TFF3 +) and nine sets of vascular endothelial cells (FLT1 + , Fig. 5b): One was mostly interface-derived (clusters 2; HLA-DQA2 +) and three were mostly tumor-derived (clusters 4, 6 and 8; ROBO1 +) (Additional file 1: Figure S2e). The remaining clusters had no zone-specific enrichment. In tumor zone, we found that multiple pathway signals, especially related to angiogenesis, were enhanced compared with those in interface zone (Fig. 5c). Interestingly, angiogenesis-related molecules HSPG2, FLT1, TIE1, HIF1A and KDR gradually decreased from the tumor to the interface to the normal zone (Fig. 5d), suggesting that breast tissue in breast cancer patients should not be simply divided into tumor and normal zones, and that the interface zone should be considered. Additionally, interface-derived endothelial cells had increased expressions of MHC class II molecules HLA-DRB1, DPA1, DRA, DPB1 and DQB1 (Fig. 5d) and enhanced pathway activities of allograft rejection, hedgehog signaling and interferon response than those in normal zone (Additional file 1: Figure S2f), indicating that interface-derived endothelial cells were active in presenting antigen epitopes.

Given that clusters 2, 4, 6 and 8 were interface-specific or tumor-specific enrichment, we analyzed pathway gene signature and found a diversity of pathway activities among these clusters (Fig. 5e). Wherein, interface-derived cluster 2 had overall weak pathway signals. For tumor-derived clusters 4, 6 and 8, they also showed different pathway signal activities: Cluster 6 had weak interferon responses, but its G2M checkpoint and E2F target pathways were relatively strong compared with clusters 4 and 8, suggesting cells in cluster 6 were under a strong proliferative state.

Finally, we analyzed which transcription factors cause differences between tumor-derived clusters 4, 6, 8 and interface-derived cluster 2 using SCENIC (Fig. 5f). This identified KLF2 as a candidate transcription factor underlying the difference between cluster 6 and others, whereas genes regulated by ETS1 and ETS2 were responsible for specific cell phenotypes of clusters 4 and 8. Previous studies reported KLF2 promote cell proliferation [45] and ETS1 and ETS2 contribute to angiogenesis [46], indicating tumor-promoting functions of these tumor-derived clusters. Notably, interface-derived cluster2 expressed high levels of MYC transcription factor that was essential for vasculogenesis during tumor progression [47]. Therefore, we believed that some endothelial cells in both interface and tumor zones provided necessary conditions for the survival of tumor cells.

Fibroblasts from interface zone share some characteristics with those from tumor zone

Although the function of fibroblasts in wound healing has been well understood, their role in cancer still needs to be explored [48, 49]. Here, a total of 7100 fibroblasts were detected, and reclustering revealed 11 subclusters (Fig. 6a). No interface-specific cluster was observed, but clusters 4, 7 and 10 significantly enriched in the tumor zone. Clusters 4, 7 and 10 preferentially expressed MMP11, COL11A1, NTM and KIF26B, whereas other clusters showed high expressions of CFD, VEGFD, CD34 and PDK4 (Fig. 6b). Average expression levels of the top 10 marker genes of clusters 4, 7 and 10 were confirmed in tumor and normal breast tissues based on TCGA database (Fig. 6c). We further verified this result by detecting the expression of a representative marker gene MMP11 in tissue sections through immunofluorescence staining (Fig. 6d). In additions, clusters 4, 7 and 10 expressed a variety of collagen proteins including COL1A1, COL1A2, COL3A1, COL6A1 and COL6A3 (Fig. 6e), which have been previously linked to abilities of proliferation, invasion, metastasis and drug resistance of tumor cells [50,51,52,53].

Fig. 6
figure 6

Fibroblast clusters. a tSNE plots of 7,100 fibroblasts, color by relevant clusters (top) or sample type (bottom). b Color coding for expression (gray to blue) of marker genes of fibroblasts. c Boxplots showing signature score of marker genes. The signature score is calculated by the average expression of log2(TPM + 1) of each gene in top 10 marker genes for cluster 4, 7 or 10 from breast cancer tissues (TCGA datasets, n = 1085) and normal breast tissues (TCGA and GTEx data, n = 291), **p < 0.01. d Target candidate staining of tumor, interface and normal zone. Nucleus stained by DAPI (blue), fibroblasts stained by DCN (green) and marker of interest depicted by MMP11 (red). DCN, decorin. MMP11, matrix metallopeptidase 11. Representative images were displayed. e Bubble plots displaying the expression of specific genes in fibroblast clusters. f Differences in pathway activity of each cell scored by GSVA between fibroblast clusters. g Violin plots displaying expressions of the selected genes in fibroblast clusters

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According to the analysis of pathway gene signatures, we observed a significant phenotypic diversity among all clusters (Fig. 6f). Clusters 4, 7 and 10 had an increased glycolysis, which may be responsible for their collagen synthesis [54]. PI3K/Akt/mTOR signaling, which has been reported to increase the migration of fibroblasts [55], was highly activated in clusters 4, 7 and 10. Besides, clusters 7 and 10 displayed high allograft rejection signature (Fig. 6f) and high expressions of MHC-related proteins B2M, TAP1 and TAP2 (Fig. 6g), indicating that they had roles of antigen presentation in TME and might induce expansion of regulatory T cells [56]. Although we did not find fibroblasts that only enriched in the interface zone, we observed a very small portion of interface-derived fibroblasts shared some characteristics with tumor zone-derived fibroblasts in clusters 4, 7 and 10 (Fig. 6a).

Epithelial cells are heterogeneous and have distinctive subtypes in interface zone

Epithelial cells were the most abundant cells among all isolated cells from nine tissues. Twenty clusters were obtained by reclustering (Fig. 7a). The expression levels of epithelial markers revealed distinct cell phenotypes of these clusters (Fig. 7b). These clusters were further classified into eight luminal groups L1-L8 and three basal groups B1-B3 based on lineage marker gene expression patterns (Fig. 7c), and the reliability of these patterns was confirmed in breast epithelial cell lines by analyzing the data in a previous study (Fig. 7d) [57]. IGFBP5, a marker of tumor-derived clusters 6 and 16 (Fig. 7e), was further examined in tissue sections by immunofluorescence, and the result confirmed presence of these clusters as separate cellular entities (Fig. 7f). In particular, there are a total of 13,310 epithelial cells from the interface zone, of which 6,031 cells are from patient 1, 3,992 cells are from patient 2 and 3287 cells are from patient 3. Due to individual differences, the subtypes of these cells have patient preferences, for example, cluster 10 mainly derived from patient 3, clusters 4 and 12 mainly derived from patient 2, and cluster 2 derived from all three patients (Fig. 1d).

Fig. 7
figure 7

Epithelial cell clusters. a tSNE plots of 42,235 epithelial cells, color by relevant clusters (top) or sample type (bottom). b tSNE plots of the selected gene expressions of epithelial cell clusters from all samples. c Heatmap of the selected gene expressions for the 20 epithelial clusters (left) and percentage and total number of cells from tumor, interface and normal zones for per cluster (right). d Heatmap of expressions of epithelial lineage markers in cell lines. e Violin plot displaying expressions of the selected gene in epithelial cell clusters. f Target candidate staining of tumor, interface and normal zones. Nucleus stained by DAPI (blue), epithelial cells stained by KRT19 (green), and marker of interest depicted by IGFBP5 (red). IGFBP5, insulin-like growth factor binding protein 5. KRT19, keratin 19. g Differences in pathway activity of each cell scored by GSVA between epithelial cell clusters. h Heatmap showing the number of potential ligand–receptor pairs predicted by CellphoneDB between cell types in normal tissues, interface tissues and tumor zones, respectively. i Bubble plots showing ligand–receptor pairs of TNF, HLA-F and LGALS9 between epithelial cells and other cell types in normal, interface or tumor zones

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The heatmap showed the expression patterns of specific genes in epithelial subclusters (Fig. 7c). Group B1 was characterized by expressions of KRT5, ACTA2 and TAGLN and displayed high levels of MKI67 indicating a robust proliferative state. Group B2 was characterized by expressions of KRT5, KRT14 and KRT17 and showed a strong activity of immunogenicity and proliferation as indicated by receptor expressions of HLA-DRA, HLA-DRB, MET and EGFR. Wherein, about 54% of group B2 cells derived from interface zone.

Group B3 was characterized by expressions of ACTA2 and TAGLN, and it only consisted of few cells with high expressions of proliferative markers CDKN1B and BCL2. In particular, the number of interface-derived cells was higher than that of tumor or normal-derived cells in group B1, B2 and B3, suggesting a unique property of the interface zone of breast tissues in breast cancer patients. Groups L1 and L2 were characterized by the expression of CDH1, a marker of luminal subtype. Wherein, cluster 11 was mature HER2 + luminal B subtypes with a phenotype of ESR1 + PGR + HER2 + while cluster 7 was mature HER2 + subtypes with a phenotype of HER2 + PGR-ESR1-. Group L2 phenotypes showed high levels of ESR1 and HER2, indicating HER2 + luminal B subtypes. Meanwhile, group L2 expressed ACTA2 and TAGLN representing an epithelial-to-mesenchymal transition (EMT) state. Groups L3 and L4 cells derived from the tumor zone and had low expressions of ESR1/PGR/HER2. Besides, group L4 presented EMT phenotypes as indicated by co-expressions of KRT8/18, ACTA2 and TAGLN and proliferative phenotypes as indicated by KI67 expression. About 89% of group L5 cells derived from the interface zone, and these cells showed high levels of ESR1 and MKI67, representing a phenotype of luminal B cells. Group L6 showed a phenotype of luminal A cells with ESR1 + HER2-MKI67- and expressed a low level of immunogenicity-related gene HLA-DRB. In group L6, cluster 4 was composed of 85% interface-derived cells while cluster 9 was composed of 99% normal-derived cells. Group L7 and L8 across the three zones co-expressed EPCAM and ITGA6, characteristics of progenitor cells [58].

In summary, luminal epithelial cells with low ESR1/PGR/HER2 were mainly enriched in tumor zone, and they preferentially expressed some pro-proliferation genes and EMT markers, while proliferative luminal B cells were mainly enriched in the interface zone, suggesting that breast tissues of breast cancer patients should not be simply divided into tumor or normal zone, and that the interface zone with a special microenvironment should not be ignored.

We further characterized the features of these groups with distinct phenotypes by analysis of pathway gene signatures (Fig. 7g). Results showed that most signal pathways in the basal groups were active, but G2M check point, E2F targets, notch signaling and oxidative phosphorylation pathway were not consistent indicating basal cells across the three zones were heterogeneous. For tumor-derived groups L3 and L4, their differences were obvious. Group L4 was enriched for hallmarks of cell cycle (e.g., G2M checkpoint and E2F targets) compared with group L3, which was consistent with the gene expression patterns analyzed above (Fig. 7c). For groups L5 and L6, interface-derived group L5 had higher G2M checkpoint and E2F target activities than group L6, suggesting that luminal cells were distinct under the influence of the microenvironment in the interface zone.

Considering the interaction between epithelial cells and stromal cells in the surrounding microenvironment, we further analyzed the cell–cell interaction network between all types in the normal zone, interface zone and tumor zone based on expressions of ligand–receptor pairs, respectively (Fig. 7h). Epithelial cells in the interface zone showed interactions with most cell types, and they showed especially strong interactions with macrophages and MSCs.

Meanwhile, macrophages and MSCs also showed strong interactions with epithelial cells in the tumor zone, while there is no such interaction in the normal zone. Based on these findings, we concluded that cell–cell interaction in the interface zone was under a more active state and some characteristics of the interface zone is shared with the tumor zone. Undoubtedly, cells in the tumor zone had their specific properties, for instance, fibroblasts that can be named CAFs, had stronger interactions with epithelial cells compared with other stromal cells.

Besides, the bubble plots displayed that the ligand–receptor pairs between the epithelial cells and the stromal cells are stronger in the interface and the tumor zones than that in the normal zone including TNF, HLA, collagen and their receptor families (Fig. 7i). Epithelial cells had a much greater effect on stromal cells than stromal cells have on epithelial cells in both the interface and tumor zones, which implied that epithelial cells play crucial roles in the formation of tumor or para-carcinoma microenvironment. For instance, interface- and tumor-derived epithelial cells expressed TNF, and endothelial cells expressed notch1, which was able to protect vascular endothelial cells from TNF-induced apoptosis [59], but endothelial cells almost had no effect on epithelial cells based on expressions of interaction pairs.

Functions of a selected gene BMPR1B differentially expressed in epithelial cells among three zones

We performed an in-depth study on a specific gene of BMPR1B that was highly expressed in cluster 4 (luminal A) and 12 (luminal B), which were mainly enriched in the interface zone (Fig. 7a and b). By analyzing the differential genes in cluster 12, we further found that the average expression level of BMPR1B gene in tumor-derived epithelial cells was higher than that in interface-derived epithelial cells (Fig. 8a, left). In addition, the average expression level of BMPR1B was higher in both the interface- and tumor-derived epithelial cells in cluster 12 than in all luminal epithelial cells in the normal zone (Fig. 8a, middle and right). BMPR1B had the same expression trend in cluster 4 as in cluster 12 (Additional file 1: Figure S3a). As a whole, the average expression level of BMPR1B gradually increased from the normal to the interface to the tumor-derived luminal cells (Fig. 8b). The expression of BMPR1B gene between tumor and normal zones is consistent with the TCGA database (Fig. 8c).

Fig. 8
figure 8

Expression and function of BMPR1B gene in epithelial cells. a Volcano plots of BMPR1B gene expression in cluster 12 from different zones here (left to right): Tumor zone versus interface zone, interface zone versus normal zone and tumor zone versus normal zone. b Heatmap of the average expression trend of BMPR1B gene in luminal cells across the three zones (normal, interface and tumor). c Box plots showing the log2(TPM + 1) value of BMPR1B gene in basal-like breast cancer (TCGA datasets, n = 135), HER2 + breast cancer (TCGA datasets, n = 66), luminal A breast cancer (TCGA datasets, n = 415), luminal B breast cancer (TCGA datasets, n = 194) and normal breast tissues (TCGA and GTEx data, n = 291), **p < 0.01. d BMPR1B overexpression in different types of epithelial cells was determined by RT-qPCR analysis. Matched cell lines with raw vector were used as control group, **p < 0.01 vs. control. e Line chart of cell proliferation measured at 450 nm, **p < 0.01. f Wound-healing assay of MCF10A and MCF-7 cells with or without BMPR1B overexpression. Left: Representative images of BMPR1B-overexpressed cells and control at three time point after wounding. Right: quantification of the reduction rate of scratch area (mean ± SD). g Photographs of cell clones observed under a microscope. BMPR1B gene promoted the colony forming ability of MCF7 cells. h Photographs of BALB/cNj-Foxn1nu/Gpt mice. BMPR1B-overexpressed MCF-7 cells and control cells were injected into subcutaneous location of left and right groin in nude mice, respectively. 15th day post-transplantation, mice were photographed, and tumors were removed surgically and weighed. i Statistical analysis of mice tumor weights (n = 3, *p < 0.05, t test)

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To explore functions of BMPR1B gene, we first overexpressed BMPR1B gene in non-tumorigenic MCF10A breast cell line (Fig. 8d) and found that BMPR1B overexpression inhibited the proliferation (Fig. 8e, up) and migration (Fig. 8f, up) of MCF10A cells. However, tumor-bearing experiment showed BMPR1B did not change the non-tumorigenic property of MCF10A cells in vivo.

Next, we investigated functions of BMPR1B gene in breast cancer cells with different phenotypes including MCF-7 (luminal type), SKBR3 (HER2 + type) and MDA-MB-231 (triple-negative breast cancer) by stably overexpressing BMPR1B gene (Fig. 8d). For MCF-7 cells, BMPR1B overexpression could enhance proliferation (Fig. 8e, down), migration (Fig. 8f, down) and clone formation (Fig. 8g) of the MCF7 cells in vitro and could promote tumor formation and growth in vivo (Fig. 8h and i). For MDA-MB-231 cells, BMPR1B overexpression promoted cell proliferation (Additional file 1: Figure S3b) and clone formation (Additional file 1: Figure S3c), but it did not affect cell migration (Additional file 1: Figure S3d) and tumorigenicity in vivo (Additional file 1: Figure S3e). For SKBR3 cells, BMPR1B overexpression did not change proliferation (Additional file 1: Figure S3b), clone formation (Additional file 1: Figure S3c) or migration of the SKBR3 cells (Additional file 1: Figure S3d). These findings suggest that BMPR1B gene may play crucial roles in the development of luminal breast carcinoma.

Discussion

Here we characterized a landscape of breast cancer tissue at single-cell resolution based on the concept of the existing three zones (i.e., tumor zone, normal zone and interface zone) in the breast tissue of breast cancer patients [13]. This landscape was a comprehensive exploration of breast TME where the interface zone is considered a unique functional and molecular zone between the tumor invasion front and the normal tissue zone. Although not all subtypes were fully described, we confirmed some key differences in each zone. By analyzing differences of major cell types in each zone, we confirmed a unique microenvironment of the interface zone. For instance, some specific epithelial cell subtypes only existed in the interface zone. This could provide a new reference for the study of tumor development and invasion and the determination of surgical excision boundary.

We found ten major stromal cell subtypes in three zones of breast tissue of breast cancer patients: endothelial cells, fibroblasts, macrophages, monocytes, DCs, MSCs, PSCs, progenitor cells, and T and B lymphoid cells. We described key phenotypes about these cell types. In terms of immune cells, we found that more regulatory T cells were distributed in the interface zone than in the normal zone, and they expressed strong angiogenesis-related molecules and high level of immune checkpoint molecules, which are conducive to the survival of tumor cells. Follicular B cells were abundant in the interface zone and exhibited T cell-independent antibody response. However, their functions across breast tissues are not significantly different after follicular B cells differentiate into plasma cells. Surprisingly, mast cells had functions of promoting angiogenesis in the interface zone. Macrophages were susceptible to the microenvironment and exhibited a variety of states in the interface zone. For instance, cluster 10 showed a tumor-promoting role as indicated by the high expression of EZH2, and cluster 0 had high expression level of the transcription factor PPARG that can regulate the production of M2-like macrophages. The expression pattern of these immune cells in the interface zone were similar but different from that in the tumor zone. This shows that immune cells of the interface zone can provide conditions for the occurrence or the development of the tumor, but they interestingly do not resemble the immune cells of the tumor zone.

Besides immune cells, we also analyzed other stromal cell types in breast tissues. For endothelial cells, expressions of angiogenesis-related genes gradually decreased from the tumor to the interface to the normal zone. In addition, expression levels of antigen presentation-related genes of the interface-derived endothelial cells were higher than those derived from the tumor and the normal zones, which indicated that the interface-derived endothelial cells actively participate in signal presentation between blood and tissues. Therefore, endothelial cells in neither the tumor nor the normal zone can represent those in the interface zone.

For fibroblasts, some clusters (clusters 4, 7 and 10) were mainly enriched in the tumor zone with a small portion of the interface-derived fibroblasts. These clusters represented cancer-associated fibroblasts (CAFs) based on the definition of CAFs that are stated as fibroblasts within or adjacent to a tumor [60]. This implied that some fibroblasts in the interface zone had been affected by the neighboring tumor because some interface-derived fibroblasts in clusters 4, 7 and 10 shared common characteristics with the tumor-derived fibroblasts. In addition, clusters 4, 7 and 10 were separated based on their gene expression profiles indicating a heterogenicity of CAFs.

Among all stem cell subpopulations, MSCs were mainly enriched in the interface and tumor zones and expressed collagens that could promote the growth and invasion of tumor cells. Notably, interface-derived epithelial progenitor cells (cluster 4) had the weakest DNA repair activities among all progenitor cells, which can easily lead to gene mutation [61], thus inducing cancer cell formation. This might provide a possibility that not all tumor infiltration processes are caused by the outward invasion of cancer cells from the tumor zone, but the continuous emergence of new cancer cells in the interface zone which may contribute to the formation of tumor infiltration.

For epithelial cells, they were grossly divided into two types luminal and basal populations. Group L7 and L8 that were progenitor cells based on their enrichment in the interface zone genes were clustered into the tSNE plot of epithelial cells. In each zone, there was a distribution of cells with specific phenotypes. For instance, luminal cells with low ESR1/PGR/HER2 were enriched in the tumor zone, and luminal B cells (L5) were enriched in the interface zone. These findings suggested that the interface zone has a special microenvironment that is different from the normal or tumor zones. We investigated the functions of a gene BMPR1B whose expression gradually increased in luminal cells from the normal to the interface to the tumor zone. We found that BMPR1B overexpression could enhance the malignant biological behaviors of MCF-7 breast cancer cells in both in vitro and in vivo experiments, suggesting that the interface zone possesses some characteristics like the tumor zone. In addition, we found that the proliferative luminal cells (KI67 +) in the interface zone of HER2 positive breast tumors highly expressed the BMPR1B gene and may develop into cancer cells in this specific microenvironment. This may suggest that it would be beneficial to remove the interface tissue zone during breast conservation surgery given the presence of proliferative luminal cells in HER2 positive breast tumors.

Conclusions

In summary, we described cellular and molecular profiles of human breast tissues based on a taxonomic concept of the existence of three zones in human breast tissue (a normal zone, an interface zone and a tumor zone) through single-cell RNA-seq analysis. This suggests interface zone could be easily affected by TME remodeling and possess unique characteristics. This single-cell landscape serves as a basis for studying the occurrence and invasion of breast cancer.