Interleukin-6 (IL6) is a determinant of the timing of parturition and birth in mice. We previously demonstrated that genetic IL6 deficiency delays parturition by ~24 h, and this is restored by administration of exogenous IL6. In this study, we have investigated whether IL6 influences the number or phenotypes of T cells or other leukocytes in uterine decidual tissue at the maternal–fetal interface. In late gestation, decidual leukocytes in Il6 null mutant (Il6−/−) mice exhibit an altered profile, characterized by reduced numbers of cells expressing the monocyte/macrophage marker F4/80 or the T-cell marker CD4, increased cells expressing the natural killer (NK) cell marker CD49b or the dendritic cell marker CD11c, but no change in cells expressing the neutrophil marker Ly6G. These changes are specific to late pregnancy, as similar differences in decidual leukocytes were not evident in mid-gestation Il6−/− mice. The IL6-regulated changes in decidual NK and dendritic cells appear secondary to local recruitment, as no comparable changes occurred in peripheral blood of Il6−/− mice. When exogenous IL6 was administered to restore normal timing of parturition, a partial reversal of the altered leukocyte profile was observed, with a 10% increase in the proportion of decidual CD4+ T cells, a notable 60% increase in CD8+ T cells including CD8+CD25+Foxp3+ regulatory T cells and a 60% reduction in CD4+IL9+ Th9 cells. Together these findings suggest that IL6-controlled accumulation of decidual CD4+ T cells and CD8+ regulatory T cells, with an associated decline in decidual Th9 cells, is instrumental for progressing parturition in mice.
