1. Composition and classification of immune cells in thymus tissue and peripheral blood of MG patients
To explore the immune pathogenesis and the immune microenvironment of MG, we performed scRNA-seq to study the transcriptomic profiles of thymus tissue and PBMCs ( MG-T and MG-B) in 2 patients with MG as well as PBMCs from 2 gender- and aged-matched healthy controls (HC-B). After quality control and cell filtering, 23 846 and 9 701 cells respectively originated from MG-T and MG-B, and 6 930 cells from HC-B were used for analysis. According to the classical genetic markers, all immune cells were divided into four categories: T cells, B cells, myeloid cells and NK cells (Fig. 1A, 1C), of which T cells accounted for the vast majority. Then, we compared the proportion of four immune cell groups in different samples. Since the difficulty of obtaining absolutely normal thymus tissue intraoperatively, we cited data from other studies in the literature on thymus tissue from healthy controls (HC-T) as a control group[13]. Obviously, each sample has a different frequency of cell distribution (Fig. 1B). The proportion of T cells in MG-T group was much higher than that in other groups including HC-T group. Compared with HC-B group, the proportions of T cells and B cells were higher and the proportions of myeloid cells and NK cells were lower than those in MG-B group. It can be seen that most of the differentially expressed genes were involved in T cell activation and differentiation signal pathways in both MG-T VS MG-B group and MG-B VS HC-B group, which further indicated that T cells are not only absolutely dominant in terms of numbers in the diseased tissues of MG, but also play a non-negligible role in terms of functions (Fig. 1D). We further clarified whether there was cell-specific expression of these MG-associated genes, and we found that the human leukocyte antigen (HLA) gene was the strongest associated risk factor for MG, similar to previous studies. Moreover, HLA-DQB1, HLA-DRA, HLA-DRB1, and HLA-DQA1 were predominantly expressed in B-cells and myeloid cells, suggesting a pathogenic antigen presentation function. Genes with the function of regulating T-cell activation were most highly expressed in T cells (e.g., CTLA4 gene), while the expression levels of genes such as AP2B1, HSPA8, and AP1B1 were significantly different in the MG-T, MG-B, and HC-B groups (Fig. 1E).
2. Characterization of T cells derived from scRNA-seq data
We regrouped the T cells from MG-T, MG-B, and HC-B into four populations, including CD4+T cells, CD8+T cells, double negative T cells (DNT) and double positive T cells (DPT). On this basis, we focus on the analysis of CD4+ T cells and CD8+ T cells, in which CD4+ T cells can be further divided into naïve, Th1/17, Treg, and Tfh cells, CD8+ T cells can be divided into naïve, cytotoxic and exhausted CD8+ T cells (Fig. 2A).
The proportions of T cell subsets in MG-B and HC-B were very similar, but the proportions of T cell subsets in MG-T were significantly different from those of MG-B and HC-T groups. In particular, the proportions of naïve CD8+ T cells and DPT in MG-T were much higher than those in MG-B and HC-T groups, indicating that there were a large number of immature T cells in the thymus of patients with MG, which were the immune reserve of the human body (Fig. 2B).
Figure 2C demonstrated that most of the marker genes were highly expressed only in their corresponding cluster. For instance, FOXP3 was a recognized marker gene in Treg cells, which was only highly expressed in CD4+ Treg cells, but low expressed in other clusters, which further proves the accuracy of cell clustering.
Then, we used the Monocle 3 method to construct the differentiation trajectory of CD4+ T cells and CD8+ T cells. With the development of the differentiation process, CD4+CD8- T cells gradually developed from naïve state to Th1/17, Treg, and Tfh states. However, almost all the CD4+CD8- T cells in MG-T remain naïve state, resulting in disruption of immune cell activation and differentiation processes (Fig. 2D).
In CD4-CD8+ T cells, trajectory analysis revealed that naïve CD8+ T cells differentiated into cytotoxic CD8+ T cells in MG-B, while in MG-T, a small number of naïve CD8+ T cells differentiated into exhausted CD8+ T cells. This may contribute to the imbalance of thymic immune function in MG patients. In addition, the KEGG enrichment analysis of up-regulated and down-regulated genes of MG-T VS MG-B showed CD4+ was highly correlated with NOD-like receptor signaling pathway and primary immunodeficiency, and CD8+ was highly correlated with NOD-like receptor signaling pathway and Fc gamma R-mediated phagocytosis, indicating that MG-T affected MG through many immune-related pathways (Fig. 2E, 2F)
3. Characterization of B cells derived from scRNA-seq data
Similarly, we subdivided B cells into 8 subgroups and named them according to the reported marker genes. These included 4 memory B cell clusters (0, 1, 2, 3), 3 naïve B cell clusters (4, 5, 6), and 2 plasma cell clusters (7, 8) (Fig. 3A, C).
Compared with HC-T, the proportion of memory B cells in MG-T increased significantly, while the number of naïve B cells decreased significantly. Similarly, compared with HC-B, the proportion of memory B cells in MG-B increased significantly, while the number of naïve B cells decreased significantly (Fig. 3B). Meanwhile, compared with HC-B, we identified 460 up-regulated and 399 down-regulated genes of B cells in MG-B (Fig. 3D). Furthermore, RNA velocity analysis of B cells across various samples showed that the differentiation velocity of cluster 8 in MG-T and MG-B was faster than that of HC-B (Fig. 3E).
4. Myeloid cell clustering analysis of MG patients and healthy controls
Next, myeloid cells were sub-grouped into 7 clusters, including three classical monocytes (clusters 0, 1, and 3), nonclassical monocytes (clusters 2), intermediate monocytes (clusters 5), cDC (clusters 4), and pDC (clusters 6) (Fig. 4A). And, there were significantly fewer myeloid cells cells in MG-T. The proportion of myeloid cell subtypes shows significant heterogeneity (Fig. 4B). The number of classical monocytes was significantly lower and the number of non-classical monocytes was significantly higher in MG-B compared to HC-B. DCs were predominant in HC-T, whereas monocytes and DCs each accounted for 50% of the total in MG-T.
Figure 4C demonstrated that the marker genes of monocytes and DCs were specifically highly expressed in the corresponding clusters. Compared with HC-B, we identified 854 up-regulated and 640 down-regulated genes of myeloid cells in MG-B. Moreover, the dys-regulated genes were mainly involved in the secretion of cytokines (Fig. 4D). The dys-regulated genes of MG-T VS MG-B were mainly involved in antigen presentation and monocyte differentiation (Fig. 4E). Furthermore, we analyzed the expression level of the MG-related genes in myeloid cell subpopulations and found that there was also cellular subpopulation heterogeneity (Fig. 4F). Genes such as HLA-DQA1, HLA-DQB1, HLA-DRA, and HLA-DRB1, whose expression was significantly up-regulated in MG-T, were predominantly expressed in intermediate monocytes, and genes such as AP1S3 and AP1G1, which were significantly over-expressed in the patients compared to controls, were predominantly distributed in plasmacytoid dendritic cells.
5. Cell-to-cell communication network analysis among samples
In addition to intracellular information, scRNA-seq can also explore interactions between cells by integrating ligand and receptor information. We analyzed the possible interactions between the major immune cells in the MG patient group and the control group and found that plasma cells interacted most strongly with other cells in the PBMC of MG patients (Fig. 5A). Whereas in MG-T samples, monocytes showed stronger interactions with DCs (Fig. 5B). Compared to HC-B, CXCL, GAS and CD30 signaling pathways were more enriched in MG-B (Fig. 5C). While, COMPLEMENT, XCR and IFN-II signaling pathways were more enriched in MG-T (Fig. 5D). Finally, we focused on analyzing the role of MG-related signaling pathways CD40 and Interleukin-16 (IL-16) pathway among various cell subpopulations. Compared with HC-B, Th1/17 cells and Treg cells interacted more strongly with NK cells and monocytes through CD40 signaling pathway, and this interaction of Treg cells was also reflected in MG-T. In MG-B and HC-B, the IL-16 signaling pathway is ubiquitous across cell populations, while in MG-T, this pathway communicates more strongly between T cells and B cells (Fig. 5E).