RUNX1 is a mediator of fibrotic activation and epigenetic memory in lung fibroblasts.

Repetitive injury is hypothesized to lead to progressive tissue fibrosis and end-stage organ failure. Whether tissue-resident mesenchymal cell populations retain epigenetic memory of prior injuries that contribute to this pathological process is unknown. Here we used a genetic lineage labeling approach to mark the lung mesenchyme prior to injury, then performed multimodal analyses on isolated lung mesenchyme during the initiation, progression, and resolution of the fibrotic response. Our results demonstrate the remarkable epigenetic and transcriptional plasticity of the lung mesenchyme during fibrotic activation and de-activation. Despite this plasticity, we also find that the lung mesenchyme exhibits an enhanced fibrotic program upon reinjury. We identify RUNX1 as a critical driver of both fibrotic activation and fibrotic memory. Comparison of fresh isolated and cultured lung mesenchyme demonstrates that RUNX1 is spontaneously activated in standard culture conditions, previously masking these roles of RUNX1. Targeted knockdown of RUNX1 dampens fibrotic mesenchymal cell activation immediately after cell isolation, but with reduced efficacy after only days of culture, confirming its functional importance to both early activation and long-term memory. Collectively, our findings implicate RUNX1 in the initiation and memory of fibrotic mesenchymal cell activation that together prime enhanced mesenchymal cell responses upon repeated injury.