The complex allergen house dust mite (HDM) acts directly on macrophages to stimulate noncanonical autophagy

Abstract

Abstract Asthma is a chronic inflammatory disease that results in narrowing of the airways. Inhalation of house dust mites (HDM) is a main cause of allergic asthma. Macrophages (Mf) are abundant in the lung and are one of the first cells exposed to HDM, however little is known about the effects of HDM on Mf. Autophagy has been linked to Mf phenotypes and asthma but the mechanism by which autophagy contributes to these processes is unclear. Autophagy is a homeostatic process involving degradation of intracellular components, including proteins, organelles, and foreign bodies. To determine whether HDM stimulates autophagy, we treated bone marrow-derived Mf (BMM) with HDM and evaluated autophagosome foci formation. HDM induced foci formation that was similar to treatment with the autophagy inducer Torin 1. Further, electron microscopy revealed an increase in double membraned vesicles in response to HDM and Torin 1, which could be due to an increase in autophagosome formation or inhibition of their fusion with lysosomes. To differentiate these hypotheses, we investigated markers of autophagy. During canonical autophagy, LC3-I is converted to membrane-associated LC3-II by lipidation, while the cargo adapter protein SQSTM1/p62 is degraded. Surprisingly, treatment with HDM caused a general increase in LC3-I and LC3-II without a substantial change in the LC3-II to LC3-I ratio. As expected, Torin 1 induced autophagic degradation demonstrated by reduction of SQSTM1/p62. However, SQSTM1/p62 increased dramatically in response to HDM, indicating a block in autophagic degradation. We observed a similar trend in THP-1 and human CD14+ Mf. These results suggest that HDM engages autophagy machinery but does not stimulate degradative autophagy.