MLL1 Regulates T-Cell Phenotype via Notch Signaling During Abdominal Aortic Aneurysm Development

BACKGROUND: Abdominal aortic aneurysms (AAA) are characterized by progressive pathological vascular remodeling and a dysregulated immune microenvironment marked by an imbalance between anti-inflammatory regulatory T cells and proinflammatory T helper 17 (Th17) cells within the aortic wall. The molecular mechanisms governing CD4 + T-cell differentiation during AAA development remain incompletely defined. MLL1 (mixed lineage leukemia 1), a histone methyltransferase, has recently been shown to modulate T-cell lineage commitment through epigenetic regulation of transcriptional programs in multiple inflammatory diseases. We, therefore, sought to determine whether MLL1 promotes Th17 polarization during AAA development and to identify signaling pathways through which MLL1 regulates CD4 + T-cell infiltration and phenotype. METHODS: Single-cell RNA-sequencing was performed on human AAA and nonaneurysmal aortic tissues. Two murine AAA models were utilized: AngII (angiotensin II) infusion with adeno-associated virus–mediated PCSK9 (proprotein convertase subtilisin/kexin type 9) gain-of-function expression and topical porcine pancreatic elastase application in C57BL/6J mice. Mice with CD4 + T-cell–specific deletion of Notch signaling ( DNMAML fl/fl CD4 cre+ ), Notch1 ( Notch1 fl/fl CD4 cre+ ), or MLL1 ( Mll1 fl/fl CD4 cre+ ) were subjected to AngII infusion, and aneurysm diameter was quantified. CD4 + T cells were isolated by MACS for gene expression analysis and flow cytometric phenotyping. Chromatin immunoprecipitation assessed H3K4 trimethylation enrichment at target gene promoters. RESULTS: Single-cell sequencing of human AAA tissue demonstrated marked upregulation of MLL1 and Notch1 signaling in CD4 + T cells compared with controls. Both human AAA and murine AngII-induced AAA CD4 + T cells exhibited a predominant Th17 phenotype with increased MLL1 expression. ChIP analysis revealed enhanced MLL1-mediated H3K4 trimethylation enrichment at the Notch1 promoter. Mice deficient in CD4 + Th-cell Notch signaling ( DNMAML fl/fl CD4 cre+ or Notch1 fl/fl CD4 cre+ ) or MLL1 ( Mll1 fl/fl CD4 cre+ ) displayed a significant reduction in aneurysm formation, decreased Th17 activity, and increased regulatory T cell expression. CONCLUSIONS: MLL1 epigenetically regulates Notch1 signaling to drive pathogenic Th17 polarization during AAA development. Targeting the MLL1 axis may represent a novel therapeutic strategy to limit aneurysmal progression.