A genome-wide screen in macrophages identifies PTEN as required for myeloid restriction of Listeria monocytogenes infection

Listeria monocytogenes (Lm) is an intracellular foodborne pathogen which causes the severe disease listeriosis in immunocompromised individuals. Macrophages play a dual role during Lm infection by both promoting dissemination of Lm from the gastrointestinal tract and limiting bacterial growth upon immune activation. Despite the relevance of macrophages to Lm infection, the mechanisms underlying phagocytosis of Lm by macrophages are not well understood. To identify host factors important for Lm infection of macrophages, we performed an unbiased CRISPR/Cas9 screen which revealed pathways that are specific to phagocytosis of Lm and those that are required for internalization of bacteria generally. Specifically, we discovered the tumor suppressor PTEN promotes macrophage phagocytosis of Lm and L. ivanovii, but not other Gram-positive bacteria. Additionally, we found that PTEN enhances phagocytosis of Lm via its lipid phosphatase activity by promoting adherence to macrophages. Using conditional knockout mice lacking Pten in myeloid cells, we show that PTEN-dependent phagocytosis is important for host protection during oral Lm infection. Overall, this study provides a comprehensive identification of macrophage factors involved in regulating Lm uptake and characterizes the function of one factor, PTEN, during Lm infection in vitro and in vivo. Importantly, these results demonstrate a role for opsonin-independent phagocytosis in Lm pathogenesis and suggest that macrophages play a primarily protective role during foodborne listeriosis. Author Summary Listeria monocytogenes (Lm) is a bacterial pathogen that causes the foodborne illness listeriosis primarily in immunocompromised, elderly, and pregnant individuals. Listeriosis is one of the deadliest bacterial infections known, with a mortality rate of ~30% even when treated with antibiotics. The high mortality rate of listeriosis is due to inefficient restriction of Lm by the immune system, and subsequent spread of bacteria beyond the gastrointestinal tract to internal organs such as the liver and brain. Macrophages are important for immune clearance of Lm but are also hypothesized to promote dissemination of intracellular Lm; thus, studies of Lm-macrophage interactions are critical for understanding the balance between bacterial growth and restriction by these phagocytes. We performed a forward genetic screen in macrophages and discovered that the tumor suppressor PTEN promotes phagocytosis of Lm by enhancing adherence to macrophages. These results demonstrate a novel function of macrophage PTEN, which canonically acts as a repressor of phagocytosis. In addition, we found that PTEN protects mice from severe disease and lowers bacterial burdens following oral inoculation of Lm. Our results demonstrate for the first time that macrophage phagocytosis is an important immune defense against invasive Lm during the foodborne route of infection.