Investigating sex differences in mouse models of malaria vaccination using acute, reversible hormone manipulation

Abstract

In many species, including mice and humans, biological sex impacts immunity. In general, females mount greater innate and adaptive immune responses, leading to sex differential vaccine outcomes. Prior research with mouse malaria models has shown female mice are better protected after vaccination and challenge than males. Prior studies further showed that castration significantly improved protection in males, suggesting that testosterone in intact males may be immunosuppressive. Here, we investigated the extent to which testosterone suppresses the formation of productive immune responses at vaccination compared to challenge in the context of malaria vaccination. We used a well-known GnRH antagonist, acyline, to suppress testosterone in BALB/cJ males. After dose optimization studies, mice were vaccinated with a two-dose vaccine and then challenged with wild type P. yoelii mouse malaria parasite. Acyline was injected subcutaneously on days 4 and 5 at a dose of 300 μg before either vaccination or challenge. Protection was measured using our standard endpoint assay, quantitative Plasmodium 18S rRNA RT-PCR. Male mice that had testosterone suppressed at challenge were better protected than those who had testosterone suppressed at vaccination. Adding exogenous testosterone abolished the protective effect of acyline. Taken together, this suggests that testosterone specific immunosuppression decreases protection to a greater extent during challenge compared to vaccination. Additionally, we also correlated suppressed peripheral testosterone levels with decreased androgen receptor expression in the liver as a surrogate measure for the liver hormone environment. Finally, the protective effect of suppressed testosterone during challenge was validated in another mouse and parasite model, C57BL/6 and P. berghei. Insight into the role of testosterone in vaccine response could help to engineer a better malaria vaccine, and further our understanding of sex differential outcomes in immunity.