Search

Stable protective immunity can be achieved against malaria by the injection of radiation-attenuated sporozoites (gamma-spz) and is mediated by IFN-gamma producing CD8+ T cells targeting the pre-erythrocytic stages. An efficient malaria vaccine should mimic this immunity. We compared the immune response specific for the circumsporozoite protein (CSP) of Plasmodium berghei (P. berghei), an important target of this protective response, elicited in mice immunized with the long synthetic polypeptide (LSP) PbCS 242-310, representing the C-terminus of the CSP of P. berghei, with the adjuvant QS-21 or injected with gamma-spz. The ex vivo evaluation of the CD8+ T cell response by IFN-gamma ELISPOT assay revealed that the injection of LSP with QS-21 induced, compared to gamma-spz, a similar frequency of peptide-specific lymphocytes in the spleen but a eight-fold increase in the draining lymph-nodes. A very high frequency of CD8+ T cells, specific for the sequence PbCS 245-253, a H-2Kd-restricted CTL epitope, was obtained in the liver and spleen of mice immunized with the two regimens. Even though the frequency of H-2Kd PbCS 245-253 multimer+, CD8+ T cells was higher in gamma-spz immunized mice, the frequency of IFN-gamma producing CD8+ T cells was comparable. The phenotype of the CD8+ T cell responses was characterized with the help H-2Kd PbCS 245-253 multimer and most of the CSP-specific CD8+ T cells represented an intermediate subset between effector and central memory with CD44(high), CD45RB(high), CD62L(low) and CD122(high). The number of memory CD8+ T cells decreased after the last LSP immunization but could be boosted to higher level with live spz. The unique combination of LSP PbCS 242-310 and the adjuvant QS-21 induced an immune response that was comparable in terms of quality to the one generated with gamma-spz. This confirmed the potential of LSP as malaria vaccine candidates as well as for the study of the repertoire of targets of protective immunity in the gamma-spz vaccine model.

The goal of this project was the evaluation of a novel immunomodulatory adjuvant for human use, OM-174, which is a soluble adjuvant derived from Escherichia coli lipid A. For this study, we used a synthetic peptide, known for its safety and reproducibility and the murine model of BALB/c mice. The long peptide (PbCS 242-310) used corresponds to the C-terminal region of the circumsporozoite protein (CSP) that is the major protein on the surface of Plasmodium sporozoites. Subcutaneous injections of PbCS 242-310 in combination with soluble adjuvant OM-174 induced long lasting peptide-specific antibody titres comparable to those obtained by immunization with incomplete Freund's adjuvant (IFA). The ex vivo evaluation of the CD8(+) T cell response by IFN-gamma ELISPOT assay revealed that the injection of polypeptide with OM-174 adjuvant induced, compared to IFA, a similar and an eight-fold increased frequency of peptide-specific lymphocytes in the draining lymph-nodes and in the spleen, respectively. The CD8(+) T-cells are specific for the sequence PbCS 245-253, a well-known H-2K(d)-restricted CTL epitope, and are cytotoxic as shown in a chromium release assay. Immunization of BALB/c mice with this polypeptide in combination with adjuvant OM-174 conferred a protection after challenge with live Plasmodium berghei sporozoites.The strong antibody and CTL responses observed to a synthetic peptide in mice, the safety profile of the adjuvant and its extensive physico-chemical characterization suggest that OM-174 has a potential use in vaccine formulations for humans.