Continuous expression of the transcription factor e2-2 maintains the cell fate of mature plasmacytoid dendritic cells

Abstract

The interferon-producing plasmacytoid dendritic cells (pDCs) share common progenitors with antigen-presenting classical dendritic cells (cDCs), yet they possess distinct morphology and molecular features resembling those of lymphocytes. It is unclear whether the unique cell fate of pDCs is actively maintained in the steady state. We report that the deletion of transcription factor E2-2 from mature peripheral pDCs caused their spontaneous differentiation into cells with cDC properties. This included the loss of pDC markers, increase in MHC class II expression and T cell priming capacity, acquisition of dendritic morphology, and induction of cDC signature genes. Genome-wide chromatin immunoprecipitation revealed direct binding of E2-2 to key pDC-specific and lymphoid genes, as well as to certain genes enriched in cDCs. Thus, E2-2 actively maintains the cell fate of mature pDCs and opposes the "default" cDC fate, in part through direct regulation of lineage-specific gene expression programs.

Figure 1. E2-2 deletion in the periphery causes spontaneous pDC differentiation

(A) Recombination efficiency in the pDCs from Itgax-Cre deleter strain, as measured by Cre-inducible EYFP reporter expression in Itgax-Cre⁺ Gt(ROSA)26Sor-EYFP⁺ mice. Shown are representative histograms of EYFP fluorescence in gated CD11c^lo Bst2⁺ pDCs from the BM or spleen, and in splenic CD11c^hi MHC II⁺ cDCs as a control. The positive threshold of EYFP fluorescence is indicated by a dashed line. (B) The pDC population within Cre reporter-positive cells in the Gt(ROSA)26Sor-EYFP⁺ Itgax-Cre⁺ Tcf4^flox/- conditional knockout (CKO) or Gt(ROSA)26Sor-EYFP⁺ Itgax-Cre⁺ control (Ctrl) mice. Shown are staining profiles of EYFP⁺ cells in the BM or spleen, with the fraction of CD11c^lo Bst2⁺ pDCs indicated. (C) DC populations within Cre reporter-positive splenocytes in control and CKO mice. Shown are staining profiles of EYFP⁺ cells; highlighted are CD11c^hi B220^- cDCs (blue), CD11c^lo B220⁺ pDCs (green) and the CD11c^hi B220^low cDC-like population (red). (D) The fraction among total splenocytes of the cell populations shown above (mean ± S.D. of 3 animals per genotype). N.s., not significant. (E) Cell surface phenotype of the B220^lo cDC-like population. Shown are staining histograms of EYFP⁺ splenocyte populations shown in (C), including pDCs from control mice and cDCs and the B220^low cDC-like cells from CKO mice. Representative of three CKO animals.

Figure 2. Inducible loss of E2-2 in mature pDCs causes differentiation into cDC-like cells

(A) Reduction of PDC numbers shortly 6 after E2-2 deletion. Tamoxifen (Tmx)-inducible Gt(ROSA)26Sor-CreER⁺ Tcf4^flox/flox CKO mice or Cre-negative littermate controls (Ctrl) were treated with Tmx and analyzed on day 6 after the last administration. Shown are absolute numbers of splenic cDC (CD11c^hi MHC II⁺) and PDC (CD11c^low Bst2⁺) in the individual CKO and control mice. N.s., not significant. (B) BrdU labeling of pre-existing pDCs. Tmx-inducible CKO and control mice were kept on BrdU for 14 days, followed by simultaneous 3-day Tmx treatment and BrdU withdrawal for 8 days. Shown is BrdU incorporation in the splenic CD8⁺ T cells, CD8⁺ CD11c^hi cDCs or CD11c^lo Bst2⁺ pDCs from control mice. The threshold of high-level BrdU staining is indicated by the dashed line. (C) The fraction of pDCs among BrdU^hi or BrdU^- splenocytes pooled from 3-4 control and CKO animals. (D) BrdU incorporation and B220 expression in the splenic CD8⁺ CD11c^hi MHC II⁺ cDCs from control and CKO mice. The percentages of B220⁺ BrdU^- and B220⁺ BrdU⁺ cells are indicated. (E) Cell surface phenotype of pDCs after Tmx-induced E2-2 deletion in PTCRA-EGFP reporter mice. Conditional PTCRA-EGFP⁺ Gt(ROSA)26Sor-CreER⁺ Tcf4^flox/flox (CKO) mice or Cre-negative littermate controls (Ctrl) were treated with Tmx and analyzed 6 days later. Shown are the fractions among total splenocytes and the staining profiles of B220⁺ EGFP⁺ pDCs, with the CD11c^high MHC II⁺ cDC gate indicated. Whereas the fraction of EGFP⁺ pDCs was variable due to partial EGFP expression in pDCs (Shigematsu et al., 2004), their phenotypic changes are representative of >5 CKO animals. (F) Staining histograms of gated B220⁺ EGFP⁺ pDCs from CKO and control mice (representative of 3 independent experiments).

Figure 3. E2-2-deficient pDCs acquire morphological and functional properties of cDCs

Conditional PTCRA-EGFP⁺ Gt(ROSA)26Sor-CreER⁺ Tcf4^flox/flox (CKO) mice or Cre-negative littermate controls (Ctrl) were treated with Tmx, and 6-7 days later splenic EGFP⁺ CD11c^lo/int pDCs and EGFP^- CD11c^hi cDCs were isolated by flow cytometry. (A) Representative confocal photomicrographs of pDCs stained for actin cytoskeleton (phalloidin, red) and nuclear DNA (DAPI, blue). For control cells, the typical pDC morphology (72% of all cells) and the most extreme example of actin remodeling are shown. For CKO cells, the examples of pDC morphology (33% of all cells), actin remodeling and dendrites are shown, along with sorted cDCs as a positive control. (B) Quantitative analysis of pDC morphology. Confocal microscopy images shown in panel (A) were scored for irregular cell shape, actin cytoskeleton and dendrites, and cumulative scores of individual control (n=55) and CKO (n=75) cells are shown. (C) T cell priming capacity of pDCs. The pDCs or cDCs were sorted as above and cultured alone or with purified allogeneic CD4⁺ T cells, and thymidine incorporation was determined 72 hr later (mean ± S.D. of triplicate cultures).

Figure 4. E2-2-deficient pDCs upregulate cDC-enriched genes

(A) Genes coordinately changed in pDCs on days 4 and 6 after E2-2 deletion. The pDCs were sorted from Gt(ROSA)26Sor-CreER⁺ Tcf4^flox/flox (CKO) mice or littermate controls 4 and 6 days after Tmx treatment and subjected to genome-wide expression profiling using microarrays. Principal component analysis (PCA) was used to identify probes up- or downregulated in CKO samples on both days. Top panels show expression graphs of individual probes (grey) and the principal component (red); lower panels show expression graphs for the indicated representative probes. Data represent average log hybridization intensity of duplicate array samples. (B) Expression profile of the genes upregulated in E2-2-deficient pDCs. For the 60 probes upregulated in pDCs after E2-2 deletion, normal expression values in major immune cell types were derived from GEO dataset GSE9810 and subjected to PCA. Shown are expression graphs for the two major principal components (cDC-enriched and cDC+B cell-enriched probes). (C) The cDC subset specificity of the upregulated genes. The 60 probes upregulated in pDCs after E2-2 deletion were queried for their expression in the normal CD8^- and CD8⁺ cDC subsets (GEO dataset GSE9810). Shown is log-ratio plot comparison of expression values in CD8^- and CD8⁺ cDCs, with the probes overexpressed >3-fold highlighted in red and green, respectively. The three probes for Cd8a are circled. (D) Quantitative analysis of gene expression in E2-2-deficient pDCs. The pDCs (CD11c^lo Bst2⁺) and cDCs (CD11c^hi Bst2^-) were sorted from control and CKO animals 5 days after Tmx treatment and analyzed by qRT-PCR for microarray target genes (top row), and downregulated (middle row) and upregulated (bottom row) transcription factors. Data represent normalized expression values relative to the control pDC sample (mean ± S.D. of triplicate reactions).

Figure 5. Direct binding of E2-2 to target genes in pDCs

(A) The binding of E2-2 to the 5’ regions of pDC-specific target genes in the human pDC cell line CAL-1. Chromatin immunoprecipitated with anti-E2-2 or control antibodies was labeled and hybridized to a microarray containing ~10 kb promoter regions of ~17,000 human genes. Shown are graphs of E2-2 binding probability (pXbar) over the 5’ gene regions, highlighting exons (filled boxes) and transcription start sites (arrows). Horizontal axis divisions correspond to 1 kb. Regions showing high background binding in control antibody hybridization (pXbar<0.1) are shaded in grey. Red dots represent the regions of binding determined by qPCR of unamplified chromatin (Cisse et al., 2008). (B) The binding of E2-2 to the 5’ regions of cDC-enriched target genes in CAL-1, shown as above.