The Mechanosensitive Ion Channel, Transient Receptor Potential Vanilloid 4 (TRPV4) in Macrophages Regulates the Host Defense Response to Bacterial Pneumonia

Macrophage phagocytosis and cytokine production are sensitive to the surrounding matrix and thereby mediate host defense and lung tissue injury. The consequences and mechanism of this matrix sensitivity are unknown. We have determined that the mechanosensitive channel, TRPV4, responds to extracellular matrix biophysical properties and thereby modulates the macrophage response to pathogens. We undertook this study to determine the in vivo consequences and the intracellular signaling pathway by which TRPV4 modulates macrophage responses. In order to evaluate the role of TRPV4 in chronic pneumonia/lung injury, WT and TRPV4 KO mice were administered agarose bead embedded-Pseudomonas aeruginosa. Loss of TRPV4 led to decreased phagocytosis by macrophages (6-fold), and increased lung injury as measured by inflammatory cell infiltration (≥ 80 ± 3%), vascular permeability (total protein ≥ 63 ± 6%), and cytokine secretion (IL-1β ≥ 71 ± 4%). In vivo, lung alveolar macrophages predominantly expressed TRPV4 and were the key phagocytic cell, as assessed by FACS and immunofluorescence. Known LPS signaling pathways were investigated in vitro. Loss of TRPV4 abrogates LPS-induced p38 activation and phagocytosis of E. coli particles. Additionally, loss of TRPV4 increased basal pro-/active IL-1β expression (2-fold), thereby enhancing IL-1β secretion independent of caspase 1 cleavage and blockade. These findings demonstrate that TRPV4 is important for bacterial clearance, lung injury and cytokine production in macrophages. TRPV4 mediates these effects through p38 and through upregulation of IL-1β protein expression in an inflammasome-independent manner. The results implicate macrophage TRPV4 as a key component of the host defense.