Defects in CD8+ T cell suppression by Foxp3-dE2 regulatory T cells

Introduction/Rationale: Regulatory T-cells (Tregs) prevent autoimmunity through suppressive functions which are largely programmed by the transcription factor FOXP3. Healthy humans express two major alternatively spliced isoforms of FOXP3: One full-length version containing all coding exons (FOXP3-FL) and one version lacking the second exon (FOXP3-ΔE2). Although healthy humans have Tregs programmed by either or both isoforms, humans with sole FOXP3-ΔE2 expression develop lethal autoimmunity. The overall goal of this study is to identify the defects in suppression by FOXP3-ΔE2 Tregs.

Methods: This study used novel mouse models including Foxp3-ΔE2 mice, influenza A viral infection, RNA-seq, ATAC-seq, spectral flow cytometry, confocal microscopy, and standard molecular biology techniques.

Results: Foxp3-ΔE2 Tregs have reduced expression of core Treg signature genes, including CD25 and IL-2/STAT5 signaling targets. Reduced CD25 expression on Foxp3-ΔE2 Tregs leads to lowered pSTAT5 responsiveness to IL-2 stimulation in vitro. Diminished regulation of IL-2 by Foxp3-ΔE2 Tregs associates with enhanced CD8+ T-cell expansion, activation, and memory formation in response to influenza viral infection. These defects in T cell restraint are intrinsic to Foxp3-ΔE2 Tregs as the phenotype is maintained in a Foxp3-FL-DTR/ΔE2 mouse model where WT Tregs are depleted immediately prior to infection. These phenotypic and functional defects in Foxp3-ΔE2 Tregs are putatively driven by impaired binding of the Foxp3-ΔE2 isoform to Runx1 which is the only transcription factor with enriched promoter binding motifs amongst differentially expressed genes and differentially accessible chromatin.

Conclusion: Reduced expression of key inhibitory molecules on Foxp3-ΔE2 Tregs results in diminished IL-2 regulation and enhanced CD8+ T-cell activation and memory formation. These findings are critical for understanding the defects in suppression by Foxp3-ΔE2 Tregs and how these cells may contribute to aberrant immune responses.