Fetal inflammatory signals regulate maternal investment during marsupial pregnancy

Marsupial pregnancy is strikingly short: placental attachment in the gray short-tailed opossum Monodelphis domestica lasts only two days. The attachment period is characterized by a spike in inflammatory signaling, development of an expanded uterine capillary network, and exponential fetal growth. This brevity has historically been attributed to a maternal immune response to fetal contact that only eutherian mammals have evolved mechanisms to tolerate. However, several inflammatory cytokines, including interleukin-1A (IL-1A) and interleukin-6 (IL-6), are produced primarily by fetal cells. We hypothesized that placental cytokines function as solicitation signals that increase maternal investment. To test this, we treated pregnant opossums with inhibitors of IL-1 and IL-6 during the rapid growth phase. Inhibition of IL-1 and IL-6 signaling significantly increased average biomass per fetus (+14% and +12%), and as such these signals impose costs, rather than direct benefits, to intrauterine growth. However, controls showed greater surviving litter sizes than IL-1-inhibited animals, suggesting that IL-1A promotes offspring survival. Single-cell transcriptomes reveal that maternal vascular endothelial cells, perivascular cells, and fibroblasts are the primary targets of fetal IL-1A, and that maternal cells simultaneously up-regulate IL-1 antagonists IL1R2 and IL1RN late in gestation, suggesting maternal resistance to fetal signaling. Placental transcriptomics reveals that the cytokine surge is restricted to the final day of pregnancy when placental cells fuse to form syncytial knots, and that these cells produce additional vasomodulatory signals including a truncated isoform of VEGFA. We propose that marsupial fetuses co-opted inflammatory signaling to perform a novel solicitation function promoting their and their littermates' survival, possibly by altering maternal vascular development.