The importance of the tissue environment in regulating macrophage responses to type-2 inflammation

Abstract

Type-2 inflammation underpins both allergic disease and parasitic worm infections, where tissue macrophages have been shown to proliferate in situ and undergo alternative activation primarily in response to the type-2 cytokine IL-4. Additionally, dendritic cells (DCs) have been shown to express alternative activation markers in response to IL-4 and in type-2 inflammatory settings. However, it is currently unknown whether pulmonary macrophages and DCs also respond in this manner. In the work described in this thesis, we have addressed whether the unique environment of the lung modulates the ability of pulmonary macrophages and DCs to respond to IL-4 or type-2 inflammation. Using the reductionist model of administering IL-4 complex (IL-4c) in vivo, we have demonstrated that pulmonary macrophages, particularly alveolar macrophages, are poorly responsive to IL-4c, relative to interstitial macrophages, or macrophages from other tissue sites. Furthermore, conventional DCs (both cDC1s and cDC2s) isolated from the lungs were less responsive to IL-4c than peritoneal cavity DCs in their expression of M(IL-4) markers. The hyporesponsive pulmonary macrophage phenotype observed following IL-4c injection was also evident in more complex models of type-2 pulmonary inflammation, including Nippostrongylus brasiliensis infection, and exposure to house dust mite, Aspergillus fumigatus or rIL-33. Despite impaired proliferation and M(IL-4) activation, alveolar macrophages expressed the IL-4R-alpha and were able to phosphorylate STAT6 in response to in vivo rIL-4 exposure. Importantly, isolated alveolar macrophages gained the ability to respond to IL-4 when cultured ex vivo, strongly suggesting that their hyporesponsiveness depended upon the lung environment. Further supporting this hypothesis, peritoneal macrophage transferred intranasally into the lung airways rapidly lost the ability to respond to systemically delivered IL-4c, despite still expressing IL-4R-alpha. Impaired IL-4 responsiveness was not dependent on the host microbiota, as alveolar macrophages remained hyporesponsive in gnotobiotic mice. Additionally, alveolar macrophage hyporesponsiveness was independent of several previously described pulmonary regulatory mechanisms, including CD200/CD200R interactions, mucins and surfactant. Together, our data reveal a central role for the lung environment as a regulator of macrophage and DC responsiveness during type-2 inflammation in vivo. Additionally, they provide high-resolution characterisation of the pulmonary macrophage compartment, providing novel insight into these key innate cells and the fundamental mechanisms that control them during type-2 inflammation.

Date of Award	1 Aug 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Tracy Hussell (Supervisor) & Andrew MacDonald (Supervisor)