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

Abstract

ObjectivesBlood stage malaria parasites attenuated with seco‐cyclopropyl pyrrolo indole (CPI) analogues induce robust immunity in mice to homologous and heterologous malaria parasites and are being considered for the development of a human vaccine. However, it is not understood how attenuated parasites induce immunity. We showed that following vaccination, parasite DNA persisted in blood for several months, raising the possibility that ongoing immune stimulation may be critical. However, parasites were not seen microscopically beyond 24 h postvaccination. We aimed to provide a mechanistic understanding of immune induction.MethodsMice were vaccinated with chemically attenuated Plasmodium chabaudi parasites. PCR and adoptive transfer studies were used to determine the presence of parasites and antigen in vivo. In other experiments, Plasmodium falciparum parasitised red blood cells were attenuated in vitro and RNA and antigen expression studied.ResultsWe show that blood transferred from vaccinated mice into naïve mice activates T cells and induces complete protective immunity in the recipient mice strongly suggesting that there is persistence of parasite antigen postvaccination. This is supported by the presence of parasite RNA in vaccinated mice and both RNA and antigen expression in P. falciparum cultures treated with CPI drugs in vitro. In addition, drugs that block parasite growth also prevent the induction of immunity in vaccinated mice, indicating that some growth of attenuated parasites is required for immune induction.ConclusionsAttenuated parasites persist at submicroscopic levels in the blood of mice postvaccination with the ability to activate T cells and induce ongoing protective immune responses.

Live attenuated vaccines, a favorable strategy to provide long-term immunity against protozoan diseases

“…This protective response relies on the maintenance of either rapidly recruited or tissue-resident effector T cells [96], in addition to the coordinated functions of other immune cell types (reviewed in [22,67,97,98]). Evidence suggests that persistence of LAVs in very low numbers is needed to maintain these protective populations in the long term [1,14] Could it be that, although LAVs have safety drawbacks, these would be less serious than the adverse effects caused by adjuvants used for inactivated vaccines?…”

“…Hence, these cells generate a delayed cellular response that would be insufficient to hold parasites at the inoculation site. Conversely, as rapid effector T cells are short-lived, in order to achieve a long-term protective immunity the presence of small populations of live persistent parasites would be necessary to generate continuous waves of circulating effector T cells and then the maintenance of quick protective responses [1,13,14]. Accordingly, the elimination of persistent parasites by chemotherapy causes a loss of long-term immunity [15][16][17].…”

Vaccination with chemically attenuated Plasmodium falciparum asexual blood-stage parasites induces parasite-specific cellular immune responses in malaria-naïve volunteers: a pilot study

“…Rodent studies investigating chemically attenuated Plasmodium spp. suggest that persistence of low levels of parasite antigen may be important for inducing an antibody-independent protective immune response [ 20 ]. Although persisting parasites could not be detected in this current study beyond day 2 post-inoculation, it is possible that they were persisting at levels below the limit of detection of the qPCR.…”

“…Cyclopropylpyrolloindole analogues, such as centanamycin (CM) and tafuramycin-A (TF-A) have been used to successfully attenuate both sporozoite and asexual blood-stage malaria parasites [ 16 – 20 ]. These compounds bind covalently to poly-A regions of DNA [ 21 ].…”

Pre-clinical evaluation of a whole-parasite vaccine to control human babesiosis

“…However, Plasmodium spp. parasites that were chemically attenuated in vitro using the DNA-binding seco-cyclopropyl pyrrolo indole analogs were shown to induce significant protective immunity in mice (Good et al, 2013;Raja et al, 2016;Reiman et al, 2018) and when administered to human volunteers, they induced immune responses similar to those seen in mice protected by vaccination (Stanisic et al, 2018). Other whole-parasite vaccine approaches targeting the malaria parasite include killed organisms contained within liposomal membranes, mimicking the structure of the infected erythrocyte (Giddam et al, 2016), and irradiated sporozoite-stage parasites (Steinhardt et al, 2020).…”