In Utero Cytokine Exposure Influences Postnatal Development of T Helper Cells

Background: Behavioral abnormalities in the offspring of mice that were immunologically challenged during pregnancy have been well documented in the literature. In these prenatal models, IL-6 induced by poly(I:C) is critical in mediating these abnormalities. While much has been published on the behaviors of offspring in this model, much less is known about how maternal immune stimulation during pregnancy affects the adaptive immune system of the offspring, and its possible role in the observed pathophysiology in neurodevelopmental disorders, such as autism.

Objectives: A major objective of this study is to optimize existing mouse models of autism (that currently use immunologically naïve pregnant dams) to more closely resemble the human scenario, where women develop immunological memory resulting from immunizations and natural exposure to antigens prior to pregnancy. Thus, a more robust mouse model of autism might be developed, and reveal factors that are significant in the etiology and/or pathogenesis of this disorder that are currently not considered in these animal models.  

Methods: Female C57BL/6 (B6) mice were immunized with allogeneic Balb/c spleen cells. One month later, immune and immunologically naïve wild-type (WT) B6 and IL-6 KO (IL-6^-/-) female mice were mated with WT B6 (IL-6^+/+) males. On gestational day 12, pregnant females were injected i.p. with PBS (control) or poly(I:C). Sera and amniotic fluids from dams were tested for the presence of multiple cytokines, using a bead-based multiplex Luminex platform, and lymphocyte phenotype/functional analyses were performed on their offspring.

Results: Overall, higher levels of pro-inflammatory cytokines IL-6, MCP-1, and GM-CSF were present in amniotic fluids than in sera in both immune and naïve poly(I:C)-injected pregnant dams. Levels of IL-6 trended higher in maternal sera of immune compared to naïve poly(I:C)-injected dams although they did not reach significance. FACS analysis of activated spleen cells from offspring of immune poly(I:C)-injected dams showed >5 fold increase in Th17 (CD4+ IL-17A+) cells compared to naïve poly(I:C)-injected dams. In mating between IL-6 KO dams and WT B6 males, significantly higher levels of IL-6 were found in the amniotic fluids compared to maternal sera. Analysis of supernatants of cultured placental and spleen cell preparations from these IL-6 KO dams demonstrated that IL-6 was produced from the heterozygous fetal (IL-6^+/-) component. These results indicate that immune challenge during pregnancy in females with immunological memory leads to development of a T helper cell repertoire in offspring that has been shown to mediate CNS pathology in autoimmune rodent models (e.g., EAE). 

Conclusions: The presence of immunological memory in dams prior to immune activation during pregnancy promoted development of pro-inflammatory Th17 cells in their offspring. This preferential differentiation could be due to differences in the cytokine profiles measured in amniotic fluids of immune vs naïve dams. Matings between IL-6 KO females and WT B6 males confirmed a significant fetal source of IL-6 production. Thus, although IL-6 may be a critical factor for the development of the behavioral abnormalities seen in offspring of poly(I:C)-injected pregnant dams, these abnormalities appear not to be solely dependent on its production from the maternal immune system.