TCR-signals downstream adversely correlate with the survival signals of memory CD8+ T cells under homeostasis

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.

More time to kill: A longer liver stage increases T cell-mediated protection against pre-erythrocytic malaria

Liver stage (LS) Plasmodia mature in 2-2.5 days in rodents compared to 5-6 days in humans. Plasmodium-specific CD8+ T cell expansion differs across these varied timespans. To mimic the kinetics of CD8+ T cells of human Plasmodium infection, a two-dose challenge mouse model that achieved 4-5 days of LS antigen exposure was developed. In this model, mice were inoculated with a non-protective, low dose of late-arresting, genetically attenuated sporozoites to initiate T cell activation and then re-inoculated 2-3 days later with wild-type sporozoites. Vaccines that partially protected against traditional challenge completely protected against two-dose challenge. During the challenge period, CD8+ T cell frequencies increased in the livers of two-dose challenged mice but not in traditionally challenged mice, further suggesting that this model better recapitulates kinetics of CD8+ T cell expansion in humans during the P. falciparum LS. Vaccine development and antigen discovery efforts may be aided by using the two-dose challenge strategy.