TGF-β1 Directly Sensitizes Capsaicin-Sensitive Lung Vagal Neurons to Promote Airway Hypersensitivity

Transforming growth factor-β1 (TGF-β1) is a pleiotropic cytokine implicated in the pathogenesis of inflammatory airway diseases such as asthma. Airway hypersensitivity (AHS), arising from sensitization of capsaicin-sensitive lung vagal (CSLV) afferents, is a hallmark of these airway diseases. Although the immunoregulatory roles of TGF-β1 in airway diseases are well documented, its effects on pulmonary sensory neurons remain unexplored. Since TGF-β1 sensitizes dorsal root ganglion neurons, a somatic counterpart of CSLV neurons, we investigated whether TGF-β1 could sensitize CSLV afferents, thereby promoting AHS.

Male Brown-Norway rats were used to examine the effects of TGF-β1 on airway reflexes and the excitability of CSLV afferents in vivo using breathing pattern and single-fiber recordings, and its direct action on isolated CSLV neurons in vitro using Ca²⁺ imaging and immunofluorescence.

Intratracheal instillation of TGF-β1 enhanced the apneic responses and fiber activities of CSLV afferents elicited by capsaicin injections in vivo. The sensitizing effect emerged at 20 min and returned toward baseline by 90 min after TGF-β1 instillation. In isolated CSLV neurons, perfusion with TGF-β1 increased capsaicin- and ⍺,β-methylene ATP-evoked Ca²⁺ transients, indicating a direct sensitizing effect. Immunofluorescence demonstrated the expression of TGF-β receptors in these neurons. Pharmacological inhibition of TGF-β receptor I (TGF-βRI), Smad3, or transient receptor potential ankyrin 1 (TRPA1), scavenging of reactive oxygen species (ROS), and TRPA1-targeting small interfering RNA attenuated TGF-β1-induced enhancement of capsaicin-evoked responses.

TGF-β1 enhances CSLV-afferent excitability and directly sensitizes CSLV neurons, thereby promoting AHS. The cellular sensitizing effect involves TGF-βRI, Smad3, ROS, and TRPA1-associated mechanisms.