Protection from Experimental Cerebral Malaria with a Single Dose of Radiation-Attenuated, Blood-Stage Plasmodium berghei Parasites

Abstract

BackgroundWhole malaria parasites are highly effective in inducing immunity against malaria. Due to the limited success of subunit based vaccines in clinical studies, there has been a renewed interest in whole parasite-based malaria vaccines. Apart from attenuated sporozoites, there have also been efforts to use live asexual stage parasites as vaccine immunogens.Methodology and ResultsWe used radiation exposure to attenuate the highly virulent asexual blood stages of the murine malaria parasite P. berghei to a non-replicable, avirulent form. We tested the ability of the attenuated blood stage parasites to induce immunity to parasitemia and the symptoms of severe malaria disease. Depending on the mouse genetic background, a single high dose immunization without adjuvant protected mice from parasitemia and severe disease (CD1 mice) or from experimental cerebral malaria (ECM) (C57BL/6 mice). A low dose immunization did not protect against parasitemia or severe disease in either model after one or two immunizations. The protection from ECM was associated with a parasite specific antibody response and also with a lower level of splenic parasite-specific IFN-γ production, which is a mediator of ECM pathology in C57BL/6 mice. Surprisingly, there was no difference in the sequestration of CD8+ T cells and CD45+ CD11b+ macrophages in the brains of immunized, ECM-protected mice.ConclusionsThis report further demonstrates the effectiveness of a whole parasite blood-stage vaccine in inducing immunity to malaria and explicitly demonstrates its effectiveness against ECM, the most pathogenic consequence of malaria infection. This experimental model will be important to explore the formulation of whole parasite blood-stage vaccines against malaria and to investigate the immune mechanisms that mediate protection against parasitemia and cerebral malaria.

Chemical Attenuation of Plasmodium in the Liver Modulates Severe Malaria Disease Progression

“…This early peak appears to cause modulation of later inflammatory immunopathogenesis in the host, during the phase where splenic T cells would normally become activated and migrate to the brain at the crucial time point between days 8 and 10 postinfection (days 4-6 postblood infection) (3). This may be similar to other experimental ECM models in which early IFN-g production was associated with protection (18,19). Early NCM immunopathogenesis following CAT is marked by the early activation of the host immune system, possibly by isopentaquine-killed parasites, in turn leading to increased accumulation of splenic T effector cells and a pronounced intrahepatic and splenic Th1-environment.…”

“…Some experimental ECM models with "unnaturally early" IFN-g responses have been associated with protection from ECM (18,19), where inflammatory responses may diverge from their normal role of mediating splenic T cell activation and the development of pathology during the fulminant stage of disease (3). We believe that chemical attenuation in the NCM model causes protection via a similar dynamics (Fig.…”

“…One study demonstrated a correlation between protection from cerebral pathology and increased b 2 -microglobulin-specific IFN-g production at 24 h postinfection (18). The authors hypothesized that CD8 + T cells were the source of the IFN-g. A subsequent study demonstrated that when irradiated blood-stage parasites were administered as a vaccine, ECM did not develop upon further challenge with normal parasites (19). Similarly, protected animals produced early IFN-g, which the authors hypothesized is produced by splenic lymphocytes.…”

“…3A-D). CD69 is an early lymphocyte activation marker associated with active, tissue-invading cells (19), whereas CD25 forms part of the IL-2R and has been associated with several cell phenotypes, including early T cell activation (40) and regulatory T cells. These data suggest that early NCM immunopathogenesis Fig.…”

Induction of immunity following vaccination with a chemically attenuated malaria vaccine correlates with persistent antigenic stimulation

“…To exclude transfer of activated immune cells, which may have mediated protection, the blood was irradiated at 18 Gy prior to transfer. This dose is sufficient to prevent lymphocyte proliferation23 but over 50 times lower than the dose to adequately kill blood‐stage parasites 24. Other mice received the attenuated parasite vaccine at the same time, and controls received nRBC treated with TFA.…”