Heme-mediated STAT1 Dysfunction in Macrophages During Klebsiella pneumoniae Lung Infection

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PROJECT SUMMARY/ABSTRACT Macrophages are key effector immune cells in host defense against pathogen invasion, and understanding factors that cause macrophage dysfunction during severe infection remains a high priority in biomedical research. An acute rise in red blood cell engulfment by macrophages, called stressed erythrophagocytosis, occurs in pathologic conditions such as transfusion heme-iron overload, anemia of inflammation, certain hemolytic anemias, or severe sepsis, and involves the uptake of excess senescent red cells (sRBCs). Using a two-hit model of red cell transfusion and acute pulmonary infection, we recently uncovered a mechanism whereby stressed erythrophagocytosis led to a state of immunosuppression following Klebsiella pneumoniae (KP) infection in mice. This state of immunosuppression arose from a disruption in interferon signaling in the liver during infection, was caused by excess heme handling by macrophages, and led to a deficiency of STAT1, a master regulator of interferon responses. We previously established an experimental system in which macrophages are exposed to KP infection and concomitant sRBC delivery or excess hemin in vitro. In addition, we have developed a well-established bacterial pneumonia model system using KP that will be combined with direct delivery of sRBC or heme to the lung in order to examine alterations in macrophage phenotype and the immune cell repertoire in vivo. This proposal will test the hypothesis that excess heme in the lung, such as in conditions of local tissue damage during severe infection, can induce a state of STAT1-deficiency in macrophages, in part, by impairing STAT1 protein stability with deleterious consequences on host defense. Utilizing in vitro and in vivo model systems, I propose to investigate this hypothesis by pursuing the following aims: Aim 1. Determine whether excess heme induces a state of STAT1 deficiency in macrophages during KP infection by impairing STAT1 protein stability in vitro. Aim 2. Examine whether excess heme alters macrophage phenotype in the lung and impairs effective host defense during KP infection in vivo. Successful completion of this project will provide novel information regarding host-pathogen interactions during pathologic conditions of excess heme, and will address a gap in understanding of dysregulated host response in the lung during severe respiratory infection.