Interleukin 15 modulates the effects of poly I:C maternal immune activation on offspring behaviour

Abstract

Maternal infections during pregnancy are linked with an increased risk for disorders like Autism Spectrum Disorder and schizophrenia in the offspring. Although precise mechanisms are still unclear, clinical and preclinical evidence suggest a strong role for maternal immune activation (MIA) in the neurodevelopmental disruptions caused by maternal infection. Previously, studies using the Polyinosinic:Polycytidylic (Poly I:C) MIA preclinical model showed that cytokines like Interleukin 6 (Il6) are important mediators of MIA's effects. In this study, we hypothesized that Il15 may similarly act as a mediator of Poly I:C MIA, given its role in the antiviral immune response. To test this hypothesis, we induced Poly I:C MIA at gestational day 9.5 in wildtype (WT) and Il15 ^-/- rat dams and tested their offspring in adolescence and adulthood. Poly I:C MIA and Il15 knockout produced both independent and synergistic effects on offspring behaviour. Poly I:C MIA decreased startle reactivity in adult WT offspring but resulted in increased adolescent anxiety and decreased adult locomotor activity in Il15 ^-/- offspring. In addition, Poly I:C MIA led to genotype-independent effects on locomotor activity and prepulse inhibition. Finally, we showed that Il15 ^-/- offspring exhibit distinct phenotypes that were unrelated to Poly I:C MIA including altered startle reactivity, locomotion and signal transduction in the auditory brainstem. Overall, our findings indicate that the lack of Il15 can leave offspring either more or less susceptible to Poly I:C MIA, depending on the phenotype in question. Future studies should examine the contribution of fetal versus maternal Il15 in MIA to determine the precise developmental mechanisms underlying these changes.

Keywords: Age; Anxiety; Autism spectrum disorder; Interleukin 15; Maternal immune activation; Poly I:C; Pregnancy; Schizophrenia; Social behaviour; Startle.

Fig. 1

Overview of the present study including experimental timeline (A), offspring subgroups (B), and within-group testing timeline (C).

Fig. 2

WTbut notIl15^−/−Poly I:C offspring exhibit decreased startle reactivity in adulthood. Regardless of MIA, Il15^−/− adolescent offspring showed increased startle reactivity at 110, 115, 120 dB compared to WT (left graph; # Bonferroni adjusted p values of 0.001, 0.039, and 0.01, respectively). In adulthood, similar to adolescent data, Il15^−/− offspring had increased reactivity compared to WT, but this was only significant for 115 dB stimuli (right graph; # Bonferroni adjusted p value of 0.005). WT Poly I:C offspring, but not Il15^−/− Poly I:C offspring, showed increased startle reactivity at 85, 110, and 115 dB compared to respective saline controls (right graph; * Bonferroni adjusted p values of 0.031, 0.006, and 0.037, respectively). Data are shown as mean ± standard error. For animal numbers per group, please refer to Table 1.

Fig. 3

Wave I amplitudes of auditory brainstem responses,the number PnC giant neurons.andgiant neuronpresynaptic vGLUT2 immunoreactivity. a) Schematic showing a composite of the raw ABR trace created by averaging the individual measurements taken from all adult WT Saline animals in response 90 dB SPL click stimulus. The double headed arrow shows the measurement of ABR Wave I amplitude representing activity at the level of the auditory nerve, which relays information to the startle reflex circuit. b) In adolescence (left graph), there was no difference in ABR Wave I amplitude, measured at 5 different stimulus intensities in Poly I:C MIA offspring or in Il15^−/− animals. In adulthood (right graph), amplitudes across stimulus intensities were higher in Il15^−/− animals compared to WT (# Bonferroni adjusted p values: 50 dB p = 0.01, 65 dB p = 0.003, 75 dB p < 0.001, 85 dB p < 0.001, 90 dB p < 0.001). c) Individual dots represent individual animals. Left panel shows a visualization of the process of identifying giant neurons and the area surrounding each PnC giant neuron where vGlut2 staining was quantified. Neither giant neuron number nor vGlut2 density were affected by Poly I:C MIA or Il15 knockout. Data are shown as mean ± standard error. For animal numbers per group, please refer to Table 1.

Fig. 4

No differences in short-term habituation following Poly I:C MIA orIl15knockout. a) Visual summary of the short-term habituation experiment, highlighting the trials used to calculate STH scores. In b) and c), The two left panels show startle reactivity across 6 blocks of 5 trials each, and the right panels shows the habituation score calculated as a ratio of block 6 to block 1. b) In adolescence, neither genotype nor Poly I:C MIA alters the habituation curves or the habituation score. c) In adulthood, habituation score values suggest that none of the 4 groups habituated well (values overlapping with a score of 1) and there were no significant differences between the groups. Data are shown as mean ± standard error, individual dots represent individual animals. For animal numbers per group, please refer to Table 1.

Fig. 5

Poly I:C MIA leads to reduced PPI with a quiet prepulse in adolescentIl15^−/−offspring and increases PPI regardless of genotype in adulthood. a) Visual summary of the PPI experiment, highlighting the trials and trial types used to calculate PPI. PPI was measured across 4 conditions with 2 prepulses and 2 ISIs. b) In adolescence, Il15^−/− Poly I:C offspring showed reduced PPI with the 75 dB prepulse regardless of ISI (* Bonferroni adjusted p = 0.012). c) In adulthood, there was a main effect of Poly I:C MIA, where Poly I:C offspring had increased PPI regardless of genotype or prepulse-ISI condition (*p = 0.005). Data are shown as mean ± standard error, individual dots represent individual animals. For animal numbers per group, please refer to Table 1.

Fig. 6

Effects of Poly I:C and IL15 knockout on locomotor behaviour. a) In adolescence, total distance traveled (left panel) was increased in Il15^−/− (#p < 0.001) and decreased in Poly I:C offspring (*p = 0.003). The right panel shows that adolescent Il15^−/− offspring also generally showed increased centre time (#p = 0.045), however, Poly I:C reduced centre time in adolescent Il15^−/− but not WT offspring (* Bonferroni adjusted p = 0.001). b) In adulthood, Il15 knockout also increased both total distance and centre time (# total distance p = 0.004). Additionally, Il15^−/− Poly I:C offspring showed significantly increased locomotor activity compared to Il15^−/− saline offspring (* Bonferroni adjusted p = 0.016). Data are shown as mean ± standard error, individual dots represent individual animals. For animal numbers per group, please refer to Table 1.

Fig. 7

Neither Poly I:C MIA norIl15deficiency altered social behaviour in adolescence or adulthood. Social behaviour data is shown as sociability and social novelty scores. a) Neither genotype, Poly I:C MIA, nor an interaction between them influenced sociability and social novelty scores in adolescence. b) Adult data shows similar results as in adolescence with no significant differences between any of the groups. Data are shown as mean ± standard error, individual dots represent individual animals. For animal numbers per group, please refer to Table 1.