Full-length plasmodium falciparum circumsporozoite protein administered with long-chain poly(I·C) or the toll-like receptor 4 agonist glucopyranosyl lipid adjuvant-stable emulsion elicits potent antibody and CD4⁺ T cell immunity and protection in mice

The Plasmodium falciparum circumsporozoite (CS) protein (CSP) is a major vaccine target for preventing malaria infection. Thus, developing strong and durable antibody and T cell responses against CSP with novel immunogens and potent adjuvants may improve upon the success of current approaches. Here, we compare four distinct full-length P. falciparum CS proteins expressed in Escherichia coli or Pichia pastoris for their ability to induce immunity and protection in mice when administered with long-chain poly(I·C) [poly(I·C)LC] as an adjuvant. CS proteins expressed in E. coli induced high-titer antibody responses against the NANP repeat region and potent CSP-specific CD4⁺ T cell responses. Moreover, E. coli-derived CS proteins in combination with poly(I·C)LC induced potent multifunctional (interleukin 2-positive [IL-2⁺], tumor necrosis factor alpha-positive [TNF-α⁺], gamma interferon-positive [IFN-γ⁺]) CD4⁺ effector T cell responses in blood, in spleen, and particularly in liver. Using transgenic Plasmodium berghei expressing the repeat region of P. falciparum CSP [Pb-CS(Pf)], we showed that there was a 1-to 4-log decrease in malaria rRNA in the liver following a high-dose challenge and⁺50% sterilizing protection with a lowdose challenge compared to control levels. Protection was directly correlated with high-level antibody titers but not CD4⁺ T cell responses. Finally, protective immunity was also induced using the Toll-like receptor 4 agonist glucopyranosyl lipid adjuvantstable emulsion (GLA-SE) as the adjuvant, which also correlated with high antibody titers yet CD4⁺ T cell immunity that was significantly less potent than that with poly(I·C)LC. Overall, these data suggest that full-length CS proteins and poly(I·C)LC or GLA-SE offer a simple vaccine formulation to be used alone or in combination with other vaccines for preventing malaria infection.