Notch Signaling Exacerbates Pulmonary Fibrosis by Regulating the Differentiation of CD4⁺ Tissue-Resident Memory T...

The involvement of the immune system in pulmonary fibrosis is established, the precise contributions of tissue-resident memory T (T_RM) cells are still poorly defined. This study sought to define the contribution of CD4⁺ T_RM cells to pulmonary fibrosis, their origin, and regulatory mechanisms. We combined bioinformatic analysis of human fibrotic lung single-cell RNA-sequencing data with experiments in a bleomycin-induced C57BL/6 mouse model. Flow cytometry, targeted in vivo depletion, lymphocyte trafficking blockade, cell co-culture, and pharmacological inhibition were employed. CD4⁺ T_RM cells were observed at higher frequencies within fibrotic lung tissue. Their presence correlated with disease severity, and they were found to exhibit a pro-inflammatory and pro-fibrotic phenotype. Their specific depletion alleviated fibrosis. These cells primarily originated from recruited circulating lymphocytes, as blocking this recruitment reduced T_RM accumulation and attenuated disease. Furthermore, the Notch signaling pathway was activated in fibrotic lung CD4⁺ T_RM cells, and its inhibition suppressed their differentiation and impaired their pro-fibrotic function. We conclude that CD4⁺ T_RM cells are pathogenic drivers in pulmonary fibrosis, originating from circulating precursors and being regulated by Notch signaling, underscoring their relevance for therapeutic intervention.