Minimal Role for the Circumsporozoite Protein in the Induction of Sterile Immunity by Vaccination with Live Rodent Malaria Sporozoites▿ †

Immunization with live Plasmodium sporozoites under chloroquine prophylaxis (Spz plus CQ) induces sterile immunity against sporozoite challenge in rodents and, more importantly, in humans. Full protection is obtained with substantially fewer parasites than with the classic immunization with radiation-attenuated sporozoites. The sterile protection observed comprised a massive reduction in the hepatic parasite load and an additional effect at the blood stage level. Differences in the immune responses induced by the two protocols occur but are as yet little characterized. We have previously demonstrated that in mice immunized with irradiated sporozoites, immune responses against the circumsporozoite protein (CSP), the major component of the sporozoite's surface and the leading malaria vaccine candidate, were not essential for sterile protection. Here, we have employed transgenic Plasmodium berghei parasites in which the endogenous CSP was replaced by that of Plasmodium yoelii, another rodent malaria species, to assess the role of CSP in the sterile protection induced by the Spz-plus-CQ protocol. The data demonstrated that this role was minor because sterile immunity was obtained irrespective of the origin of CSP expressed by the parasites in this model of protection. The immunity was obtained through a single transient exposure of the host to the immunizing parasites (preerythrocytic and erythrocytic), a dose much smaller than that required for immunization with radiation-attenuated sporozoites

Comparison of the amino acid sequences of the <i>P. berghei</i> ANKA clone cy17 (34) and <i>P. falciparum</i> Welcome strain (35) CSP polypeptides.

<p>Black dots represent identical amino acid residues while bars represent amino acid residues with similar characteristics. The repeat regions (including the pre- and post-repeat sequences) are underlined.</p

Antibody reactivity induced by immunization with irradiated sporozoites.

<p>Pooled serum samples from groups of mice immunized with sporozoites from the different parasite lines were analyzed by ELISA to assay IgG and IgM responses induced against <i>P. berghei</i> CSP (A), or <i>P. falciparum</i> CSP (B), using long peptides covering the N-terminus or the C-terminus of the antigen, and short peptides that include some of the repeat units of the central repetitive region. Serums were also tested by IFAT against wet sporozoites to detect anti-CSP IgG and IgM antibodies(C). Titres are expressed as the log of the highest dilution of serum giving a positive staining.</p

Sterile protection in mice immunized with <i>P. berghei</i> irradiated sporozoites and challenged with <i>P. berghei</i> or <i>P. berghei</i> [<i>PfCS</i>] sporozoites.

<p>Mice were immunized with 1 or 3 injections of <i>P. berghei</i> (indicated on the left of the panel) before challenge with 5 000 <i>P. berghei</i> or <i>P. berghei</i> [<i>PfCS</i>] sporozoites. All naive control mice developed a patent blood-stage infection. The data are representative of those obtained in duplicate experiments.</p

Sterile protection in [BALB/c×C57BL/6] F1 mice immunized three times with <i>P. berghei</i> or <i>P. berghei</i> [<i>PfCS</i>] and challenged with 5000 <i>P. berghei</i> or <i>P. berghei</i> [<i>PfCS</i>] sporozoites.

<p>All animal groups (5 mice per group) were monitored for blood-stage infections by examination of Giemsa-stained blood smears obtained daily from day 2 to day 11 post-challenge. All naive control mice developed a patent blood-stage infection.</p