Increasing evidence indicates that obesity is a risk factor for increased severity of influenza virus infection. However, its precise immunological mechanism is not fully understood. To investigate this, diet-induced obese (DIO) mice were established by feeding C57BL/6 male mice a high-fat diet for 16 weeks. DIO and lean control mice were infected intranasally with 3000 pfu of influenza A virus (IAV) (PR8/H1N1). Interestingly, we found adipose tissue located along the bronchus in naive DIO mice. In addition, the Nos2 level was significantly higher and Arg1 level was significantly lower in lung macrophages of naive DIO mice, consistent with an M1-skewed phenotype. The survival rate and body weight of DIO mice infected with IAV were significantly lower than those of lean control mice and associated with higher viral load in the lungs of DIO mice. Histopathological analysis demonstrated higher numbers of inflammatory cells in the lungs of DIO mice after IAV infection. Levels of cytokines, including TNF-alpha, IL-6, IL-10, and type I IFN (IFN-alpha and IFN-beta, in bronchoalveolar lavage fluid (BALF) were altered after IAV infection; in particular, IFN-alpha and IFN-beta levels were significantly suppressed in the BALF of DIO mice. In vitro, bone marrow-derived macrophages were stimulated with ligands of toll-like receptor (TLR) 7/8, a pattern recognition receptor for single-stranded RNA, and levels of TNF-alpha, IL-6, and IL-10 were similarly altered. In addition, levels of IFN-alpha and IFN-beta were significantly lower in culture supernatants of alveolar macrophages sorted from naive DIO mice and infected with IAV, compared to those in macrophages sorted from lean control mice. Collectively, these results suggest that macrophages may be the main contributors to poor outcomes of influenza virus infection in obesity. (C) 2019 Elsevier Inc. All rights reserved.