CD8 T cell independent immunity after single dose infection-treatment-vaccination (ITV) against Plasmodium yoelii

Abstract

Sporozoite vaccination of both humans and rodents elicits potent anti-malarial immunity, but the dose of sporozoites and the number of immunizations required varies with vaccination approach. Here we examine the immunological basis for superior protection afforded from single-dose vaccination with virulent sporozoites administered under prophylatic chloroquine-cover, referred to as infection-treatment-vaccination (ITV), compared to the well-studied approach of administering radiation-attenuated Plasmodium sporozoites (RAS). Earlier rodent studies utilizing ITV and RAS vaccination suggested a major role of CD8 T cells in reducing liver parasite burden after sporozoite challenge in a BALB/c mouse model. Consistent with this, we find that in C57Bl/6 mice ITV elicits substantially higher parasite-specific CD8 T cell responses than RAS vaccination and enhances immunity against P. yoelii infection. However, we show ITV-induced CD8 T cells are not necessary for protection following liver-stage sporozoite or blood-stage parasite challenge. Mechanistically, we found protection afforded from single-dose ITV is associated with low grade, transient parasitemia shortly following cessation of chloroquine treatment and generation of potent antibody responses to blood-stage parasites. Collectively, our data show the mechanistic basis for enhanced protective immunity against P. yoelli elicited by ITV in highly susceptible C57Bl/6 mice is independent of CD8 T cells. These studies may be relevant in understanding the potent immunity observed with ITV in humans.

Artesunate versus Chloroquine Infection–Treatment–Vaccination Defines Stage-Specific Immune Responses Associated with Prolonged Sterile Protection against Both Pre-erythrocytic and Erythrocytic Plasmodium yoelii Infection

“…Taken together, our data demonstrate that both CQ-and AS-ITV induced long-term sterile protection against pre-erythrocytic stages of malaria parasites. Moreover, CQ-ITV treatment also resulted in sterile protection against BS parasites, in agreement with previous reports (19). Interestingly, although unable to confer sterile protection, AS-ITV delayed the onset of BS infection, suggesting that although we were unable to detect BS in mice treated with AS, this regimen also induces some level of cross-stage immunity.…”

“…1B). Doubling the dose to 1.6 mg CQ did not abrogate this low-level burst of parasitemia, in agreement with a recent report (19). This result agrees with the known mechanism of action of CQ, which only affects late trophozoites and schizonts, allowing for the persistence of ring and early trophozoites in the iRBCs during the CQ treatment phase that become patent after drug withdrawal (15,19).…”

“…CD8 + T cells obtained from the livers of mice 6 d after the third course of CQ-or AS-ITV, which corresponds to the peak of CD8 + T response (19), were sorted by flow cytometry, and their purity was determined to be .95% (Supplemental Fig. 1).…”

“…More recently, it was shown in a rodent malaria model of infection that CQ-ITV leads to Ab-mediated immunity against BS Ags resulting from transient exposure to BS parasites during the treatment regimen (19). In general, immunization with irr-spz or early-arresting genetically attenuated parasites (GAPs) exposes the host to a restricted range of spz and early liver stage (LS) parasite Ags, whereas immunization with ITV or late-arresting GAPs likely broadens the repertoire of Ags to include those expressed by late LS and early BS parasites (3,20).…”

“…We identified a dose of AS-ITV that eliminates BSs, and compared it with a previously studied dose of CQ-ITV, which results in the persistence of BSs (8,19,23). This is the first time, to our knowledge, that these immunization strategies have been directly compared in terms of protection against different forms of the malaria parasite.…”