Malaria blood stage infection suppresses liver stage infection via host-induced interferons but not hepcidin

Malaria-causing Plasmodium parasites first replicate as liver stages (LS), which then seed symptomatic blood stage (BS) infection. Emerging evidence suggests that these stages impact each other via perturbation of host responses, and this influences the outcome of natural infection. We sought to understand whether the parasite stage interplay would affect live-attenuated whole parasite vaccination, since the efficacy of whole parasite vaccines strongly correlates with their extend of development in the liver. We thus investigated the impact of BS infection on LS development of genetically attenuated and wildtype parasites in female rodent malaria models and observed that for both, LS infection suffered severe suppression during concurrent BS infection. Strikingly and in contrast to previously published studies, we find that the BS-induced iron-regulating hormone hepcidin is not mediating suppression of LS development. Instead, we demonstrate that BS-induced host interferons are the main mediators of LS developmental suppression. The type of interferon involved depended on the BS-causing parasite species. Our study provides important mechanistic insights into the BS-mediated suppression of LS development. This has direct implications for understanding the outcomes of live-attenuated Plasmodium parasite vaccination in malaria-endemic areas and might impact the epidemiology of natural malaria infection.

Effective Stimulation for IL-12 p35 mRNA Accumulation and Bioactive IL-12 Production of Antigen-Presenting Cells Interacted with Th Cells

Bioactive IL-12 is composed of two subunits, p35 and p40. In the APC-Th cell interaction, p40 mRNA accumulation in APC was shown to be up-regulated by stimulation with CD40 ligand (CD40L) on Th cells. However, the CD40-CD40L interaction scarcely induced p35 mRNA accumulation in APC. In the present experiments, p35 mRNA accumulation was induced in splenic macrophages/dendritic cells by the interaction with paraformaldehyde-fixed Th1 cells in the presence of Ag, and the p35 mRNA accumulation was abrogated by the inclusion of anti-I-A in cultures to block TCR/MHC class II interaction. The accumulation was also induced by the stimulation with agonistic anti-I-A. These results indicate that the interaction of the MHC class II molecule with TCR evokes an activation signal for p35 mRNA accumulation in APC. Furthermore, the production of bioactive IL-12 in macrophages/dendritic cells stimulated with CD40L was enhanced by the inclusion of agonistic anti-I-A. The p35 mRNA accumulation and IL-12 production of macrophages/dendritic cells induced by stimulation with OVA-specific fixed Th1 clone expressing CD40L were also enhanced by adding OVA in cultures. These results indicate that the p35 mRNA accumulation induced by MHC class II stimulation plays a role in bioactive IL-12 production.