Influenza A(H1N1) vaccination during early pregnancy transiently promotes hippocampal neurogenesis and working memory. Involvement of Th1/Th2 balance.

The 2009 influenza A(H1N1) pandemic led to a particularly high risk of morbidity and mortality among pregnant women. Therefore, inactivated influenza vaccines have been widely recommended for women in any period of gestation. Recent studies have shown that the peripheral adaptive immune system plays an important role in the function of the central nervous system (CNS). The present study was conducted to explore if influenza vaccination, aiming to induce protective immune activation, affects maternal neurogenesis and cognitive ability. The results showed that A(H1N1) pregnant mice (AIV+Pre) had superior spatial working memory performance compared with pregnant controls (Pre). At the cellular level, a transient increase in both cell proliferation and neuronal differentiation in the dentate gyrus (DG) was found in the AIV+Pre group compared with the Pre group when BrdU was injected on gestational day 14 (G14). However, there were no obvious differences between A(H1N1) virgin mice (AIV+Vir) and virgin controls (Vir) in both hippocampal neurogenesis and working memory. Our findings further indicated that prolactin (PRL) concentrations were not overtly different between the AIV+Pre group and the Pre group at any time. Interestingly, IL-4 and IFN-γ levels were obviously increased both in the serum and hippocampus of the AIV+Pre group (with a T helper-1 like response; Th1) compared with the Pre group (with a T helper-2 like response; Th2) at G14, whereas the expression of IL-6 and TNF-α, the proinflammatory factors, was significantly reduced. Altogether, the results suggest that A(H1N1) vaccination during early pregnancy may contribute to adult hippocampal neurogenesis and spatial working memory and that the improvements were, at least in part, associated with Th1/Th2 balance.