- In the present study, we examined the cellular origin and the expression levels of interleukin-18 binding protein (IL-18 BP), during normal maturation of murine testis. Immunohistochemical staining of testicular tissues from sexually mature mice, shows that testicular germ cells, at different stages of differentiation, express IL-18 BP. Leydig cells/interstitial cells and peritubular cells express higher levels of IL-18 BP, as compared to other testicular cells. The levels of IL-18 BP were similar in testicular tissues and homogenates from sexually immature and mature mice, as examined by western blot, ELISA and real time PCR analysis. Our results demonstrate, for the first time, the expression of IL-18 BP in murine testicular tissues and cells. Together with our previous studies, which showed over-expression of IL-18 in testicular tissues of immature mice as compared with mature mice, these results may indicate a role for IL-18 in testicular development, and also in the regulation of testicular functions under physiological conditions.