Fibrotic function of bystander-activated PD-1high CD4 T cells in pulmonary fibrosis 2309544

Abstract Introduction Conventional CD4 T cell activation requires antigen recognition through the T cell receptor along with co-stimulatory signals and cytokines. Recent studies have demonstrated that cytokine signaling alone can activate CD4 T cells without antigen recognition, called bystander activation, and these CD4 T cells contribute to various inflammatory diseases. However, whether these bystander activated CD4 T cells contribute to pulmonary fibrosis remains unclear. Methods In this study, we investigated the unique function of fibrotic CD4 T cells using bleomycin-induced lung fibrosis model and analysis of public single-cell and spatial RNA sequencing data from lung tissue of human IPF patients, and explored the underlying mechanisms. Results We identified a subpopulation of PD-1high CD4 T cells which are markedly increased in pulmonary fibrosis mouse model and in lung tissue from IPF patients. These cells mainly express IL-33 receptor and amphiregulin and their activation is independent of specific antigen. Notably, PD-1high CD4 T cells are already present in lung tissue of steady-state mice, increased with aging and respond selectively to IL-33 and IL-25 stimulation in the absence of TCR engagement. Single-cell RNA sequencing revealed that IL-33 and IL-25 stimulated CD4 T cells upregulate Th2-related cytokines and amphiregulin with Batf identified as a key transcriptional regulator of this bystander response. Batf deficiency and chemical inhibition of Batf significantly reduced bystander-activation of CD4 T cells by IL-33 and IL-25 stimulation and markedly attenuated pulmonary fibrosis pathology. Conclusion Taken together, these findings identify a unrecognized population of bystander-activated, profibrotic PD-1high CD4 T cells in both mice and humans and establish the Batf/amphiregulin axis in CD4 T cells as an important mediator of pulmonary fibrosis progression. Funding Source This research was supported by the Basic Science Research Program of the National Research Foundation funded by the Korean government (RS-2024-00334289). Topic Categories Immune Response Regulation: Cellular Mechanisms (IRC)