THE ROLE OF IL-23 IN REGULATING THE FUNCTION OF PHAGOCYTIC CELLS DURING LISTERIA MONOCYTOGENES INFECTION

Purpose: Listeria monocytogenes (LM) is a Gram-positive intracellular pathogen that causes meningitis and septicemia in immunocompromised individuals, and spontaneous abortion in pregnant women. IL-23 is a pro-inflammatory cytokine that promotes the recruitment of immune cells during multiple infectious and autoimmune diseases. Using IL-23p19 knockout (KO) mice we have demonstrated, that IL-23 is required for resistance against LM, and for the efficient recruitment of neutrophils to the liver, and monocytes to the spleen. However, it is not known whether IL-23 can impact the function of phagocytic cells including monocytes, neutrophils, and macrophages during LM infection. Methods: Mice lacking IL-23p19 (IL-23p19 KO) and C57BL/6 (B6) mice were infected with LM. For the isolation of thioglycollate-elicited peritoneal macrophages, mice were intraperitoneally injected with 1 ml of 2% thioglycollate, three days prior to harvest. Flow cytometry based assays were performed to measure phagocytosis and production of reactive oxygen species (ROS) using the dyes hydroethidine (HE) and 2',7'-dichlorodihydrofluorescein diacetate (H2DCFDA). ELISAs were performed to measure the amounts of TNF-ɑ and IL-6. Results: The phagocytic ability of neutrophils and monocytes in the spleen was equivalent between IL-23p19 KO and B6 mice. IL-23p19 KO mice had reduced monocyte-specific ROS production in spleen in comparison to B6 mice. However, the production of monocyte-specific or neutrophil-specific ROS was not induced or enhanced by the addition of recombinant IL-23 to B6 splenocytes. Interestingly, there was a reduction in the production of TNF-ɑ and IL-6 from thioglycollate-elicited peritoneal macrophages from IL-23p19 KO mice compared to B6 mice. Conclusions: IL-23 does not mediate phagocytosis by neutrophils and monocytes. Optimal production of monocyte-derived ROS requires IL-23 during LM infection. However, IL-23 does not directly mediate the production of ROS. Lastly, IL-23 is required for enhancing the production of TNF-ɑ and IL-6 from thioglycollate-elicited peritoneal macrophages.