The synthetic, oxidized C-terminal fragment of the <i>Plasmodium berghei</i> circumsporozoite protein elicits a high protective response

A polypeptide of 69 amino acids (PbCS 242-310) encompassing the C-terminal region of the circumsporozoite protein of <i>Plasmodium berghei</i> (PbCS) was generated using solid-phase peptide synthesis. The immunological and protective properties of peptide PbCS 242-310 were studied in BALB/c mice (H-2^d). Two subcutaneous injections, in the presence of IFA at the base of the tail, generated (i) high titers of anti-peptide antibodies which also recognized the native <i>P. berghei</i> CS protein, (ii) cytolytic T cells specific for the K^d-restricted peptide PbCS 245-253 and (iii) partial CD8⁺-dependent protection against sporozoite-induced malaria. The same frequencies of peptide PbCS 245-253 specific CD8⁺ T cells were found by IFN-γ ELISPOT in the draining lymph nodes of animals immunized with the short optimal CTL peptide 245-253 or with the polypeptide 242-310, indicating that the longer polypeptide can be processed and presented <i>in vivo</i> in the context of MHC class I as efficiently as the short CTL peptide. Interestingly, higher levels of IFN-γ producing CD8⁺ T cells and protection were observed when the four cysteine residues present in the C-terminal peptide were fully oxidized. These findings underline the potential importance of the chemical nature of the C-terminal fragment on the activation of the immune system and concomitant protection.