B19-07 NGF-TrkA Signaling Drives Sympathetic Nerve-Dependent Pulmonary Fibrosis

Rationale Pulmonary fibrosis is characterized by relentless scar formation and accumulation of α-smooth muscle actin (αSMA)+ myofibroblasts. While fibroblast activation via TGF-β signaling drives extracellular matrix deposition, the role of neuro-fibrotic crosstalk remains poorly defined. We previously identified aberrant sympathetic innervation in fibrotic regions enriched with αSMA+ myofibroblasts, forming a distinct nerve-fibroblast niche. Nerve growth factor (NGF), a key mediator of sympathetic nerve growth, functions as a pleiotropic cytokine that influences multiple cell types. We tested the hypothesis that NGF promotes sympathetic nerve-dependent fibrogenesis through its high-affinity receptor, tropomyosin receptor kinase A (TrkA). Methods NGF expression was assessed in bleomycin-induced lung fibrosis in vivo and in normal human lung fibroblasts stimulated with TGF-β1 or co-cultured with sympathetic neurons derived from mouse superior cervical ganglia in vitro using RT-PCR, ELISA, and immunohistochemistry. Functional blockade of NGF was achieved by systemic administration of anti-NGF IgG in bleomycin-challenged wild-type mice and by generating mice with sympathetic nerve-specific deletion of TrkA (Th-Cre; Ntrk1f/f). Sympathetic innervation was analyzed using whole-lung tissue clearing and 3D light-sheet microscopy to identify TH+ nerves. Inflammation and fibrosis were quantified by bronchoalveolar lavage (BAL) white blood cell (WBC) counts, Sircol collagen assay, and Masson’s trichrome staining. Results In bleomycin-challenged lungs, αSMA+ myofibroblasts co-expressed NGF, while NGF concentrations were overall unchanged in BAL and tissue lysates. TGF-β1 stimulation increased NGF expression in human lung fibroblasts, which was further enhanced by neuronal co-culture. Neutralization of NGF with systemic anti-NGF IgG increased BAL WBC counts and exacerbated fibrosis, as evidenced by elevated collagen deposition and higher Modified Ashcroft scores. In contrast, sympathetic nerve-specific deletion of TrkA reversed bleomycin-induced aberrant sympathetic innervation, restored inflammatory homeostasis, and attenuated lung fibrosis, suggesting that TrkA signaling is a key driver of sympathetic nerve-dependent fibrogenesis. Conclusions Lung fibroblasts are a critical source of NGF within the profibrotic nerve-fibroblast niche. The deleterious effects of systemic NGF blockade exacerbate lung injury and fibrosis and underscore its context-dependent, pleiotropic regulatory role in tissue homeostasis. The finding that sympathetic nerve-specific TrkA deletion attenuates fibrosis positions NGF-TrkA signaling as a key mediator of the profibrotic nerve-fibroblast axis. Together, these findings reveal a previously unrecognized neuro-fibrotic signaling pathway with therapeutic potential in pulmonary fibrosis.