ScRNA-seq analysis
PCA and t-SNE analyses were performed. Depending on the expression of maturation markers, the cells were classified into 11 different cell types based on cell type-specific gene expression (Table 1). Cells in the two datasets were separately arranged using the following threshold conditions: average expression proportion of markers in each clustering subgroup > 30% and average expression value > zero. Annotations with overlapping subgroups were selected for annotation with a greater proportion of marker expression.
The abundance of each cluster of control versus RIPF cells is shown in Fig. 1, which demonstrates the relative abundance of each cell type genotype.
In these scRNA-Seq data, the relative abundance of B and T cells in the RIPF group remarkably changed compared to that in the controls (Fig. 2); therefore, we initially analyzed these two cell types, and differentially expressed gene (DEG) analyses were performed on these data.
Table 1
Lung tissue cells are classified into 11 different cell types.
Cell types | Markers |
T cells | Cd3d, Cd3e, and Cd3g |
NK cells | Klrd1, Gzma, Nkg7, and Il2rb |
B cells | Ighd, Ms4a1, Cd79a, and Cd79b |
macrophages | Adgre1, Cd68, and Lyz2 |
monocytes | S100a8andS100a9 |
Dendritic cells | Cd83, Cd86, H2-Eb1, and H2-Ab1 |
platelets | Nrgn, Clec1b, Itga2b, and Itgb3 |
Epithelial cells | Scgb1a1, Sftpa1, Ager, and Krt7 |
Endothelial cells | Pecam1, Egfl7, Flt1, and Eng |
fibroblasts | Col1a1, Col1a2, and Col3a1 |
Analysis of DEGs of B cells
Analysis of differential genes in B cells between the disease and control groups revealed significant differential expression of the following biological processes: structural constituent of the eye lens, intermediate filament binding, immunoglobulin receptor binding, immunoglobulin complex, circulating immunoglobulin complex, euchromatin, DNA-directed DNA polymerase activity, DNA polymerase activity, DNA binding, and antigen binding (Fig. 3B). These genes were enriched in the following pathways: positive regulation of B-cell activation, plasma membrane invagination, phagocytosis, recognition, phagocytosis, engulfment, membrane invagination, immunoglobulin-mediated immune response, humoral immune response mediated by circulating immunoglobulin, complement activation, classical pathway, complement activation, and B-cell receptor signaling pathway (Fig. 3A). The analysis revealed positive regulatory activation of B cells in the disease group, resulting in a significant increase in the number of B cells and a high correlation with immunoglobulins (Fig. 3C). Ionizing radiation can induce the secretion of immunoglobulins by activating B cells. Immunoglobulins reportedly enhance inflammation and promote the production of extracellular matrix and fibrogenic cytokines (22). Therefore, we hypothesized that immunoglobulins could induce cytokine secretion, thereby promoting RIPF.
Analysis of DEG of T cells
The analysis of differential genes in T cells revealed that the following biological processes significantly differed: T cell differentiation, regulation of leukocyte differentiation, regulation of adaptive immune response, lymphocyte differentiation, antigen processing and presentation of peptide antigen, antigen processing and presentation of exogenous peptide antigen via MHC class II, antigen processing and presentation of exogenous peptide antigen, antigen processing and presentation of exogenous antigen, antigen processing and presentation, and adaptive immune response based on somatic recombination of immune receptors built from immunoglobulin superfamily domains (Fig. 4A). The top ten pathways with significant variation were as follows: T cell differentiation, regulation of T cell differentiation, regulation of T cell activation, regulation of lymphocyte differentiation, regulation of leukocyte differentiation, regulation of cell–cell adhesion, positive regulation of T cell activation, positive regulation of lymphocyte activation, positive regulation of leukocyte cell − cell adhesion, and lymphocyte differentiation (Fig. 4B). An apparent decrease in the number of T cells was observed in the lung tissue of rats following ionizing irradiation (Fig. 2). In summary, differentiation and activation signals in T cells and leukocytes and positive regulation of leukocyte cell–cell adhesion showed remarkable changes following ionizing radiation. Among the genes involved in these biological processes, we identified many cytokine-related genes, including CCL5, SMAD7, IL7r, IL18r1, TNFAIP3, and IL2rb, and immunoglobulin-related genes IGKC, IGHA, and IGHG2C (Fig. 4C). Considering the above, we speculated that RIPF may be closely related to cytokines.
RIPF and cytokines
The analysis of differences in gene expression between T cells and B cells indicated that a number of cytokines may be the underlying causative agent for the pathogenesis of RIPF. Consequently, RIPF single-cell transcriptome sequencing results were compared between those of disease and control groups, and the differential cytokines among them were selected for tSNE visualization. Four of these factors were visibly altered: CCL5, ICAM1, PF4, and TNFα (Fig. 5).
Lung histopathological sections indicated that the alveolar structure of non-irradiated rats was normal. After 30 Gy irradiation, the alveolar septum was thickened and infiltrated by numerous inflammatory cells. Masson staining results were consistent with the pathological alteration seen in HE staining. The results demonstrated pronounced fibrous exudation of lung tissue in the irradiated group, exhibiting signs of RIPF (Fig. 6A).
ELISA results showed that the levels of CCL5, ICAM1, and TNFα in the supernatant of BEAS-2B cells evidently increased following ionizing radiation (Fig. 6B).
IHC staining showed that CCL5, ICAM1, PF4, and TNFα levels were significantly upregulated in the lung tissue of rats in the RIPF group (Fig. 6C).