Abstract] Objective To study the roles of IL-12 and IL-23 in the development of protective im-munity and pathological changes during chlamydial urogenital infection.Methods C57BL/6J wild type (wt) mice and mice deficient in IL-12p35 (IL-12p35 KO) or IL-12p40 (IL-12p40 KO)were inoculated in-travaginally with 1×104 IFU of live Chlamydia muridarum ( C.muridarum) organisms.Half mice of each group were reinfected on day 114 after primary infection.Vaginal swabs were taken every 3 or 4 days to mo-nitor live organism shedding.The mice were sacrificed after 114 or 143 days of primary infection and the va-ginal tract and kidney samples were collected for pathological analysis.The numbers of chlamydial inclusion bodies and bacteria in kidney homogenates were titrated after 100 days of primary infection.Results The infection time courses of mice deficient in either IL-12p35 or IL-12p40 were similar after primary infection, but were prolonged as compared with the wild type mice.All mice regardless of genotypes developed severe pathological damages in upper genital tracts with no significant difference among different groups.Almost all IL-12p40 KO mice and some IL-12p35 KO mice showed pathological changes in kidney samples.No obvious abnormality was observed in any of the kidneys from wild type mice.Neither the age-matched IL-12p35 KO nor IL-12p40 KO mice developed any gross pathological changes in kidney in the absence of chlamydial in-fection.C.muridarum inclusions were detected in kidney samples with gross pathological damages from IL-12p35 KO mice and IL-12p40 KO mice.No inclusions were ever detected in kidneys from the wild type mice.The numbers of chlamydial inclusions in the IL-12p40 KO mice were much higher than those of the IL-12p35 KO mice.Live bacteria were detected in mice deficient in either IL-12p35 or IL-12p40, but not in the wild type mice.No significant difference with the number of live bacteria was found between IL-12p35 KO mice and IL-12p40 KO mice.Conclusion IL-12 and IL-23 could inhibit the spread of C.muridarum in-fection from genital tract to kidney.The deficiency of IL-12 or IL-23 might relate to the renal lesions induced by Chlamydia infection.