STAT-3 is necessary for IL-27-mediated macrophage suppression but does not represent a therapeutic target for E....

Interleukin (IL)-27 is a heterodimeric immunoregulatory cytokine expressed at elevated levels early in life that compromises bacterial clearance and promotes severe outcomes during neonatal sepsis. In turn, IL-27Rα-deficient neonatal mice exhibit better control of bacteria, reduced systemic inflammation, and improved outcomes. IL-27 primarily activates and signals through either Signal transducer and activator of transcription (STAT)-1 or STAT-3 in macrophages. Targeted deletion of STAT-3 in macrophages has been reported to improve responsiveness to Lipopolysaccharide (LPS) and promote Th1 activity. As such, in the present study, we investigated the role of STAT-3 signaling in IL-27-mediated suppression of bacterial clearance and lysosomal activity in neonatal macrophages during in vitro infection and sepsis. Bone marrow-derived macrophages from myeloid-specific STAT-3 deletion in neonatal mice (LysMcreSTAT-3^fl/fl) showed improved control of intracellular bacteria and lysosomal acidification. Consistent with these findings, E. coli-infected neonatal LysMcreSTAT-3^fl/fl mice demonstrated improved bacterial clearance, but conversely, increased inflammatory response and mortality compared with neonatal mice with intact STAT-3 signaling. Pharmacological inhibition of STAT-3 in WT neonatal mice using S32-201 resulted in the inability to clear bacteria in either the blood or spleen relative to control mice. This study revealed that STAT-3 is necessary for IL-27 suppression of macrophage-mediated bacterial killing, but neither myeloid-specific nor global STAT-3 inhibition during neonatal sepsis achieves the same outcome as loss of IL-27 signaling. This suggests that STAT-3 is not a promising therapeutic target to mitigate IL-27 activity in early life infection.

The neonatal period is a time in which newborns have increased vulnerability and the highest risk of death from infection. This includes sepsis for which there is a considerable global burden of disease. We have determined that the cytokine interleukin (IL)-27 is expressed at elevated levels in the first days of life and continues to rise during experimental bacterial neonatal sepsis. Neonatal mice that cannot respond to IL-27 exhibit improved outcomes. In this work, we have investigated the influence of STAT-3 on control of bacteria and inflammation during IL-27 signaling in neonates. It is critical that we understand mechanisms that underlie neonatal susceptibility to infection so that we can identify new targets for therapeutic intervention. Here, we define the value of STAT-3 in approaches to targeted therapies for bacterial neonatal sepsis.