Abstract: Objective To assess the effect of neutralizing CD96 on natural killer (NK) cell functions in mice with pulmonary Chlamydia muridarum infection and explore the possible mechanism. Methods Male BALB/c mice were randomly divided into infection group (Cm group), anti-CD96 treatment group (anti-CD96 group) and control group (n=5). In the former two groups, C. muridarum was inoculated via intranasal administration to establish mouse models of pulmonary C. muridarum infection, and the mice in the control group received intranasal administration of the inhalation buffer. In anti-CD96 group, the mice were injected with anti-CD96 antibody intraperitoneally at the dose of 250 μg every 3 days after the infection; the mice in Cm group received intraperitoneal injections of saline. The body weight of the mice was recorded daily. The mice were sacrificed 5 days after C. muridarum infection, and CD96 expression was detected by quantitative real-time PCR and Western blotting. HE staining and pathological scores were used to evaluate pneumonia of the mice. The inclusion body forming units (IFUs) were detected in the lung tissue homogenates to assess lung tissue chlamydia load. Flow cytometry and ELISA were used to assess the capacity of the lung NK cells to produce interferon-γ (IFN-γ) and regulate macrophages and Th1 cells. Results C. muridarum infection inhibited CD96 expression in NK cells of the mice. Compared with those in Cm group, the mice in antiCD96 mice showed significantly milder lung inflammation (P<0.05) and reduced chlamydia load in the lung tissue (P<0.05). Neutralizing CD96 with anti-CD96 significantly enhanced IFN-γ secretion by the NK cells (P<0.05) and augmented the immunoregulatory effect of the NK cells shown by enhanced responses of the lung macrophages (P<0.05) and Th1 cells (P< 0.05). Conclusion Inhibition of CD96 alleviates pneumonia in C. muridarum-infected mice possibly by enhancing IFN-γ secretion by NK cells and augmenting the immunoregulatory effect of the NK cells on innate and adaptive immunity

Key words: C. muridarum, lung infection, CD96, natural killer cells, interferon-γ, immunoregulation