Secreted ISG15 induced by Chlamydia trachomatis infection exerts immunomodulatory effects on IFN-γ defense and inflammation

Interferon-γ (IFN-γ) is an immunoregulatory cytokine essential for cellular immunity against intracellular pathogens, including Chlamydia. Interferon-stimulated gene (ISG) 15, a member of the ubiquitin family, contributes to host resistance to viral and bacterial infections. ISG15 can exist either in an unconjugated form or covalently attached to host proteins through a process known as ISGylation. Here, we show that infection with Chlamydia trachomatis (Ct) induces the expression and secretion of ISG15 in human primary cells and mouse female genital tract (FGT) organoids. ISG15 secretion by genital tract epithelial cells resulted in increased IFN-γ release from natural killer (NK) cells. The production of IFN-γ by NK cells in response to ISG15 was completely abolished in NK cells lacking the interleukin-18 receptor alpha (IL-18Ra), demonstrating a co-stimulatory effect of ISG15 with IL-18 in enhancing IFN-γ release. ISG15 was secreted into the FGT and was involved in controlling bacterial load in a murine infection model. Furthermore, ISG15 reduced macrophage responsiveness to IFN-γ as an M1-polarizing signal for pro-inflammatory responses, potentially "shielding" macrophages from excessive IFN-γ. Evidence of uterine horn pathology and reduced IL-10 levels in the FGT of infected ISG15-/- mice further supports a critical dual function of ISG15 in controlling chlamydial infection and modulating the resulting inflammatory responses.