Pattern of interleukin 6 gene expression in vivo suggests a role for this cytokine in angiogenesis.

Interleukin 6 (IL-6) is a cytokine that acts on various cell types. Here we show that IL-6 mRNA is produced in vivo in two self-limiting physiologic angiogenic processes: (i) the formation of the vascular system accompanying development of ovarian follicles and (ii) the formation of a capillary network in the maternal decidua following embryonic implantation. In situ and RNA blot hybridization analyses detected transient expression of IL-6 mRNA in gonadotropin-primed hyperstimulated ovaries, with maximal mRNA levels coinciding with the period of formation of a capillary network around follicles. Expression of IL-6 mRNA was detected in the vasculature extending from the ovarian medulla to the forming capillary sheath in the thecal layer of individual growing follicles. No expression was detected in more-developed preovulatory follicles once angiogenesis had been completed. IL-6 mRNA was also detected in the uterus of pregnant mice 9.5 days postcoitum, and there was no appreciable IL-6 mRNA at later stages of embryonic development. Expression in the uterus was confined to cords of endothelial cells in the process of formation of an anastomosing network that traversed the maternal decidua towards the developing embryo. The expression of IL-6 mRNA in two independent physiological angiogenic processes and the transient nature of its expression in endothelial cells suggest a role for IL-6 in angiogenesis.