Interleukin-23 optimizes antimicrobial macrophage activity, which is reduced among persons harboring an IL-23 receptor variant that helps protect against inflammatory bowel disease (IBD), recent research has found.
“These [findings] highlight that the susceptibility to infections with therapeutic blockade of the IL-23/IL-12 pathways may be owing in part to the essential role for IL-23 in mediating antimicrobial functions in macrophages. They further highlight that carriers of the IL-23R–Q381 variant, who are relatively protected from IBD and other immune-mediated diseases, may be at increased risk for bacterial infection,” Rui Sun and Clara Abraham, MD, of Yale University, New Haven, Conn., wrote in Cellular and Molecular Gastroenterology and Hepatology.
IL-23 is key to the pathogenesis of IBD and is being studied as a therapeutic target, both alone and in combination with IL-12 blocking. Although human macrophages express low levels of IL-23 receptor, recent research reveals that IL-23R is up-regulated “within minutes of exposure to IL-23,” which promotes signaling and cytokine secretion, the investigators wrote. However, the extent to which IL-23 supports macrophage antimicrobial activity was unknown. To characterize protein expression, signaling, and bacterial uptake and clearance of bacteria by human macrophages derived from monocytes, the investigators tested these cells with Western blot, flow cytometry, and gentamicin protection, which involved coculturing human macrophages with bacteria, adding gentamicin solution, and then lysing and plating the cells onto agar to assess the extent to which the macrophages had taken up the bacteria.
After 48 hours of exposure to IL-23 or IL-12, macrophages increased their intracellular clearance of clinically relevant bacteria, including Enterococcus faecalis, adherent invasive Escherichia coli, and Salmonella typhimurium. Notably, this did not occur when the investigators reduced (“knocked down”) macrophage expression of either IL-23R or IL-12 receptor alpha 2. Additional investigations showed that in macrophages, IL-23 promotes bacterial uptake, clearance, and autophagy by inducing a pyruvate dehydrogenase kinase 1 (PDK1)–dependent pathway mediated by Janus kinase 2/tyrosine kinase 2 and by inducing reactive oxygen species (ROS) and reactive nitrogen species (RNS) pathways. IL-23 also activates two key proteins involved in autophagy (ATG5 and ATG16L1), the researchers reported. “ROS, RNS, and autophagy cooperate to mediate IL-23-induced bacterial clearance. Reduction of each ROS, RNS, and autophagy pathway partially reversed the enhanced bacterial clearance observed with chronic IL-23 treatment.”
Further tests found that IL-23 mediates antimicrobial pathways through the Janus kinase 2, tyrosine kinase 2, and STAT3 pathways, which “cooperate to mediate optimal IL-23-induced intracellular bacterial clearance in human macrophages.” Importantly, human macrophages showed less antimicrobial activity when transfected with the IL-23R–Q381 variant than with IL-23R–R381. The IL-23R-Q381 variant, which reduces susceptibility to IBD, “decreased IL-23-induced and NOD2-induced antimicrobial pathways and intracellular bacterial clearance in monocyte-derived macrophages,” the researchers explained. Evaluating actual carriers of these variants showed the same results – macrophages harboring IBD-protective IL-23R–R381/Q381 exhibited lower antimicrobial activity and less intracellular bacterial clearance compared with macrophages from carriers of IL-23R–R381/R381.
“Taken together, IL-23 promotes increased bacterial uptake and then induces a more rapid and effective clearance of these intracellular bacteria in human monocyte-derived macrophages,” the researchers wrote. “The reduced inflammatory responses observed in IL-23R Q381 carriers are associated with protection from multiple immune-mediated diseases. This would imply that loss-of-function observed with the common IL-23R–R381Q variant may lead to a disadvantage in select infectious diseases, including through [this variant’s] now identified role in promoting antimicrobial pathways in macrophages.”
The National Institutes of Health provided funding. The investigators reported having no conflicts of interest.
SOURCE: Sun R, Abraham C. Cell Molec Gastro Hepatol. 2020 May 28. doi.: 10.1016/j.jcmgh.2020.05.007.
