Potential contribution of alveolar epithelial type I cells to pulmonary fibrosis

Abstract

Pulmonary fibrosis (PF) is characterized by inflammation and fibrosis of the interstitium and destruction of alveolar histoarchitecture ultimately leading to a fatal impairment of lung function. Different concepts describe either a dominant role of inflammatory pathways or a disturbed remodeling of resident cells of the lung parenchyma during fibrogenesis. Further, a combination of both the mechanisms has been postulated. The present review emphasizes the particular involvement of alveolar epithelial type I cells in all these processes, their contribution to innate immune/inflammatory functions and maintenance of proper alveolar barrier functions. Amongst the different inflammatory and repair events the purinergic receptor P2X7, an ATP-gated cationic channel that regulates not only apoptosis, necrosis, autophagy, and NLPR3 inflammosome activation, but also the turnover of diverse tight junction (TJ) and water channel proteins, seems to be essential for the stability of alveolar barrier integrity and for the interaction with protective factors during lung injury.

Keywords: P2X7R; alveolar epithelial type I cell; pulmonary fibrosis.

Figure 1. Involvement of AECs in PF

Figure 2. Possible AECI related ion channels and junctional proteins contributing to PF

Figure 3. Presence of “intermediate” AECs in IPF lung samples

Immunohistochemical evidence of intermediate AECs in human IPF analyzed within a frame of a previous study [62]: co-localization of CD44v9, AECII specific (FITC) and Lycopersicon esculentum agglutinin, AECI specific (Texas Red) (A), E-cadherin, AECII specific (FITC) and RAGE, AECI specific (Texas Red) (B), and of E-cadherin, AECII specific (FITC) and ICAM-1, AECI specific (Texas Red) (C) in AECs of intermediate type. Arrows indicate double labeled cells.

Figure 4. Dual function of P2X7R as ion channel or signal molecule