Single-cell RNA sequencing reveals T helper cells synthesizing steroids de novo to contribute to immune homeostasis

Abstract

T helper 2 (Th2) cells regulate helminth infections, allergic disorders, tumor immunity, and pregnancy by secreting various cytokines. It is likely that there are undiscovered Th2 signaling molecules. Although steroids are known to be immunoregulators, de novo steroid production from immune cells has not been previously characterized. Here, we demonstrate production of the steroid pregnenolone by Th2 cells in vitro and in vivo in a helminth infection model. Single-cell RNA sequencing and quantitative PCR analysis suggest that pregnenolone synthesis in Th2 cells is related to immunosuppression. In support of this, we show that pregnenolone inhibits Th cell proliferation and B cell immunoglobulin class switching. We also show that steroidogenic Th2 cells inhibit Th cell proliferation in a Cyp11a1 enzyme-dependent manner. We propose pregnenolone as a "lymphosteroid," a steroid produced by lymphocytes. We speculate that this de novo steroid production may be an intrinsic phenomenon of Th2-mediated immune responses to actively restore immune homeostasis.

Graphical abstract

Figure 1

Upregulation of a Steroid Synthesis Pathway in Th2 Cells In Vitro (A) Schematic representation of integrated steroid biosynthesis pathway with abbreviated metabolites (black) and protein factors (blue). Abbreviations are listed in the supplemental information. The first phase includes cholesterol biosynthesis and cholesterol uptake, the second phase is cholesterol transfer from the cytosol to the mitochondrial matrix, and the final phase includes steroid synthesis. mRNA expression of all the protein factors (blue) are shown as a color-coded map, where “white” indicates low expression and “red” indicates high expression level. FPKM (fragments per kilobase of transcript per million fragments mapped) values for each gene as obtained by RNA sequencing analysis were transformed to the color code (≤3 FPKM value as white and ≥15 FPKM value as maximum red). The comparison of the mRNA expression level between naive Th1 and Th2 cells is shown adjacent to the protein. The left-hand boxes represent color-coded expression levels in naive Th cells, middle is Th1, and right-hand boxes represent Th2 cells. (B) Differential upregulation of Cyp11a1 mRNA. Cyp11a1 mRNA expression levels obtained by RNA-seq were compared in different Th cell types (naive [N], Th1, and Th2). Data presented are averages of two independent repeats. Error bars are SDs from the mean. (C) Differential upregulation of Cyp11a1 protein. Comparison of Cyp11a1 protein expression in naive, Th1, and Th2 by western blotting.

Figure 2

The Steroid Synthesis Pathway in Th2 Cells Produces Pregnenolone: Steroid Production In Vitro during Th2 Polarization and In Vivo, in Response to Helminth Infection (A) Quantitative detection of pregnenolone by LC-MS/MS. Naive Th cells were cultured in Th1 or Th2 activation-differentiation conditions. After 3 days of stimulation, cells were rested for 2 days with equal cell density. Cell supernatants were extracted for steroid profiling. Bars represent the mean pregnenolone concentration ±SEM. p values were calculated by unpaired two-tailed t test. (B) Quantitative detection of pregnenolone by competitive ELISA. Th cells were stimulated for 3 days and rested for 3 days before supernatants were analyzed by competitive ELISA. Bars represent the mean concentration ±SEM from four independent experiments. p values were calculated using the unpaired two-tailed t test. (C) Th2 supernatants were analyzed by ELISA for pregnenolone production as described in (B) with or without the presence of Cyp11a1 inhibitor, aminoglutethimide (AG). p values were calculated by unpaired two-tailed t test. Error bars are SDs from the mean. (D) Schematic flowchart of in vivo experimental design. C57BL/6 mice were infected with nematode larvae (Nippostrongylus brasiliensis) for 10 days. On day 5 and day 10 postinfection, CD4⁺ Th cells were purified from spleens and mesenteric lymph nodes (MLN). They were then immediately analyzed by FACS for Cyp11a1 expression, or ex vivo cultured for 24 hr for detection of pregnenolone production by ELISA. (E) Cyp11a1 and GATA3 expression detected by FACS. Data shown were obtained from pooled samples of spleen and MLN of five mice and represent two experiments (n = 5 and n = 3 for each condition). p values were calculated using the unpaired two-tailed t test using data obtained from individual mice (n = [5 + 3] = 8). (F) Pregnenolone concentration of ex vivo cultured Th cell supernatant was measured by quantitative ELISA. Results shown are mean pregnenolone concentration ±SD from both spleen and MLN. Experiments were performed twice with eight mice per condition. p values were calculated using unpaired two-tailed t test.

Figure 3

Coexpression Pattern of Th2 Suppressor Cytokines with Cyp11a1 at Single-Cell Level (A) Gating strategy used to purify subpopulations of Th2 cells based on their cell generation and IL-13-GFP expression. Naive Th cells obtained from spleen of IL-13-GFP reporter mice were stained with CellTrace Violet dye and polarized for Th2 (72–96 hr). The fourth-generation cells that expressed IL-13-GFP (G4P) and second-generation cells that did not express IL-13-GFP (G2N) were FACS sorted and used for single-cell gene expression analysis by qPCR and for single-cell RNA sequencing. (B) Spearman correlation coefficient (r) of Th2-associated genes with Cyp11a1 at single-cell level. Th2 cells from the G2N and G4P groups, as shown in (A) were FACS sorted as single cells. mRNA expression of different protein factors in a single cell were analyzed by single-cell qPCR. (C) Hierarchical clustering of normalized mRNA expression data in single Th2 cell by qPCR for 73 selected genes. The clustering heatmap depicts patterns of coexpression among genes, where the green and red colors indicate the strength and direction of the gene-gene correlation (red meaning higher degrees of similarity, and green lower degrees of similarity). Hierarchical clustering was applied to group genes based on the similarity of their expression profile calculated by Spearman correlation across this data set. The gene cluster containing Cyp11a1 is boxed white, which contains Cyp11a1, IL-10, and TGF-β1.

Figure 4

Gene Expression Identity of Cyp11a1-Expressing Th2 Cells and Ly6C as a Surface Marker for Steroidogenic Cell Purification (A) Schematic summary of Cyp11a1-correlated genes as obtained from mRNA sequencing analysis of 91 single Th2 cells (52 cells from G4P and 39 cells from G2N, as shown in the Figure 3A). Genes are listed according to their correlation coefficient value, higher to lower, top to bottom in each category. Genes previously reported to be involved in immunosuppression are red and genes not coexpressed are crossed out. RNA sequencing and analysis are as described in Figures S3 and S4 and Tables S2, S3, and S4. (B) IL-4 mRNA expression level was plotted against Cyp11a1 mRNA expression as obtained from single-cell RNA sequencing. Each dot represents an individual cell. The color of a dot varies from yellow to black, representing the mRNA expression level of Gata3 in the cell. (C) Normalized mRNA expression level of Ly6C1 and Ly6C2 plotted against Cyp11a1 mRNA expression. Each dot represents an individual cell. (D) Ly6C⁺ cells were FACS sorted from in vitro differentiated Th1 (left panel) and Th2 (right panel) cell populations and FACS analyzed for Cyp11a1. Purified Th1 cells have a trace proportion of Cyp11a1⁺ cells, whereas almost all purified Th2 cells are Cyp11a1⁺. (E) Ly6C⁺ cells were FACS sorted from in vitro differentiated Th2 population and FACS analyzed for the expression of GATA3, FOXP3, and Cyp11a1.

Figure 5

Immunosuppression by Pregnenolone and Pregnenolone-Producing Cells (A–C) Pregnenolone inhibits Th cell proliferation. Naive Th cells were stained with CellTrace Violet and activated for Th1 (A) or Th2 (B) differentiation for 72 hr in the presence (blue histogram) or absence (red histogram) of pregnenolone (5 μM). The cell proliferation profile was captured by FACS based on dye decay. The histograms depicted are representative of three independent experiments with three to five mice each. (C) Mean division indices were calculated for the experiments described in (A) and (B). Division index is the average number of divisions that a cell (present in the starting population) has undergone. p values were calculated using unpaired two-tailed t test. (D) Ly6C⁺ Th2 cells inhibit Th cell proliferation. CellTrace-Violet-stained naive Th cells (“responder cells”) were activated by anti-CD3e and anti-CD28 in the presence of FACS-sorted Ly6C⁺ or Ly6C⁻ Th2 cells and anti-IL-10 antibody and/or AG as indicated. The proliferation profile of the responder Th cells grown in the absence of Th2 (blue) was captured by FACS on the third day of activation. This was compared to the proliferation profile of responder Th cells activated in the presence of Th2 only (Ly6C⁺ or Ly6C⁻) (red). The Ly6C⁺ cells were pretreated with neutralizing anti-IL-10 antibody (20 μg/ml) or with aminoglutethimide (AG, 250 μM) or with both for 24 hr and were used in the same culture conditions. The responder Th cell to Th2 (Ly6C⁺ or Ly6C⁻) cell ratio was 1:1. The histograms depicted are representative of three independent experiments with three mice in each experiment. Mean division indices ±SD are shown in the inset (right, bottom). p values were calculated using unpaired two-tailed t test. (E) Inhibition of Cyp11a1 activity negatively regulates IL-10 and TGF-β1 expression in Th2. In vitro polarized Th2 and Th1 cells (3 days activation, 2 days resting) in the presence or absence of aminoglutethimide (AG, 250 mM) were FACS analyzed after reactivation with PDBU/Ionomycin for intracellular cytokine (IL-10 and TGF-β1 for Th2; IFN-γ for Th1) expression. Data shown are representative of three independent experiments. (F) Effect of pregnenolone on B cell class switching to IgG1 and IgE. Naive resting B cells were stained with CellTrace Violet. Class switch recombination (CSR) was induced with LPS and IL-4 in the presence of different concentrations of pregnenolone. Cell-surface expression of IgG1 and IgE were analyzed by FACS on day 5 of stimulation. Data shown are representative of three independent experiments with three mice each. p values were calculated using unpaired two-tailed t test and were <0.05 when untreated groups were compared with 5 μM pregnenolone-treated groups.

Figure 6

Proposed Model for Lymphosteroid Production by Cyp11a1⁺ Th2 Cells and Its Functional Role in Type 2 Immune Response A subset of Th2 cells differentiates to become steroidogenic Th2 cells, which produce pregnenolone (PREG) as well as the suppressor cytokine IL-10. Together, these help to actively restore immune homeostasis, including through limiting Th cell proliferation and B cell differentiation.