Production of IL-18 Binding Protein by Radiosensitive and Radioresistant Cells in CpG-Induced Macrophage Activation Syndrome

IL-18 binding protein (IL-18BP) acts as a naturally occurring IL-18 decoy receptor. If the balance between IL-18 and IL-18BP is dysregulated, abnormal levels of free bioactive IL-18 are detected, such as in the sera of Il-18bp knockout (KO) mice with CpG-induced macrophage activation syndrome. To determine the cellular sources of Il-18bp in vivo, we selectively depleted Il-18bp expression in either radiosensitive or radioresistant cells using bone marrow transfer between wild-type (WT) and Il-18bp KO mice. Following repeated CpG injections, Il-18bp KO (donor) -> Il-18bp KO (recipient) chimeric mice exhibited more severe disease, with an enhanced Ifn-gamma signature and circulating free Il-18 levels, in comparison with WT -> WT chimeras. Interestingly, the phenotype of KO -> WT and WT -> KO mice did not differ from that of WT -> WT mice. Consistent with this finding, serum Il-18bp levels were similar in these three groups of mice. The contribution of radioresistant and radiosensitive cells to Il-18bp production varied markedly according to the organ examined, with a major contribution of radiosensitive cells in the spleen as opposed to a major contribution of radioresistant cells in the lung. Finally, Ifn-gamma blockade abrogated the CpG-induced but not the constitutive Il-18bp production. Our results demonstrate that circulating Il-18bp is induced in response to Ifn-gamma during CpG-induced macrophage activation syndrome and is present at high levels in the circulation to prevent the deleterious systemic effects of Il-18.