View Abstract

A3847 - Aggravated Erythrophagocytosis Impairs IFNγ-Mediated Irg1 Expression in Response to Klebsiella Pneumoniae
Author Block: T. F. Olonisakin1, Z. Xiong2, J. Lee3; 1School of Medicine, Pittsburgh, PA, United States, 2Pulmonary Allergy Critical Care Medicine, Pittsburgh, PA, United States, 3Div of PACCM, University of Pittsburgh, Pittsburgh, PA, United States.
Rationale: Aggravated erythrophagocytosis — as occurs with transfusion of aged, damaged red blood cells (RBC) — results in an acute increase in mammalian iron recycling and a concurrent rise in transferrin bound and non-transferrin bound iron that may serve as nutrition for opportunistic extracellular pathogens such as Klebsiella pneumoniae. While enhanced delivery of effete RBC to macrophages may increase pathogen virulence by boosting nutritional iron source to the pathogen, heightened erythrophagocytosis may weaken immunity through dysregulated iron metabolism in the host cell. We hypothesized that aggravated erythrophagocytosis overwhelms physiologic RBC disposal and suppresses the host innate immune response to bacterial infection.
Methodology: 8 to 12-week-old male and female C57BL/6J mice were inoculated intratracheally with K. pneumoniae strain (ATCC 43816) and were transfused an hour later with 200 µL of syngeneic leukoreduced fresh RBC (0-1 day old) or aged RBC (11 day old). This transfusate volume approximates one unit of packed RBC in humans. 24h later, livers were excised and gene expression was evaluated by PCR Array and qPCR. Protein expression in liver tissue was evaluated by ELISA and immunoblotting. The murine macrophage cell line RAW 264.7 was utilized in vitro to delineate cell signaling.
Results: As the liver is the primary organ that mediates damaged RBC removal, we evaluated innate immune transcriptional responses in livers of mice transfused with either fresh or aged RBCs 24h following acute intrapulmonary K. pneumoniae infection. We observed selective suppression of Ifng, gamma-inducible Cxcl10, and immunoresponsive gene 1 (Irg1) that encodes the mitochondrial citric acid cycle intermediate, cis-aconitate decarboxylase, in livers of aged RBC-transfused mice. Suppression of IFNγ signaling pathway was validated by protein expression, as we observed attenuated IFNγ, phosphorylated STAT-1, and IRF-1 protein expression in livers of aged RBC-transfused mice. We further demonstrated that IFNγ augments intracellular killing of K. pneumoniae and that IFNγ induces Irg1 expression in macrophages.
Conclusions: Using an established model of K. pneumoniae infection and comparative damaged RBC, we demonstrate that aggravated erythrophagocytosis suppresses IFNγ and downstream Irg1 expression. Our findings invite the possibility that aged RBC-mediated suppression of Irg1, which is highly expressed in mammalian macrophages during bacterial infection and has been ascribed multiple roles in immunity, may be detrimental to the host during infection.