Murine Immune Responses to Liver-Stage Antigen 1 Protein FMP011, a Malaria Vaccine Candidate, Delivered with Adjuvant AS01B or AS02A

Abstract

Liver-stage antigen 1 (LSA1) is expressed byPlasmodium falciparum liver-stage antigen 1 (LSA1) is expressed exclusively in the liver stage of parasite development (17). The native LSA1 protein has a large central repeat region containing over 83 slightly degenerate 17-amino-acid repeat segments, flanked by two highly conserved N-and C-terminal regions. The lsa1 gene encodes a 230-kDa protein whose expression begins shortly after sporozoite invasion of the liver hepatocyte and increases rapidly with liver-stage development (13, 17). The LSA1 protein has been described as forming a flocculent mass surrounding the thousands of developing parasite nuclei and pseudocytomeres and is microscopically observed localized between the plasmalemma and the parasitophorous vacuole membranes inside the infected hepatocyte cytoplasm. Later in development the LSA1 protein has been detected adhering to emerging mature liver merozoites (13).It has been demonstrated that complete resistance to malaria infection can be induced in rodents and humans by prior injection, by needle or mosquito bite, of gamma-irradiated sporozoites (␥-spz) and that the mechanism of immunity is, in large part, due to major histocompatibility complex class I-restricted CD8 ϩ gamma interferon (IFN-␥)-secreting T cells directed against antigens expressed by malaria-infected hepatocytes (8-10, 12, 16, 17, 22, 23). Furthermore, the effector cells of this immune response target primarily liver-stage developing merozoites and not sporozoites before their entry into hepatocytes. In mice, the developing attenuated liver-stage parasites resulting from the irradiated sporozoites must be present at the time of normal sporozoite challenge for immunity to be manifested (16,22,27,39). In humans, the protection induced by the gamma-irradiated sporozoite model is effective against homologous and heterologous parasite strain challenge, indicating that the antigen(s) responsible is relatively conserved. The role of specific anti-LSA1 immune responses in this protection is evident. It has been reported that human volunteers exposed to a protective dose of P. falciparum irradiated sporozoites develop LSA1 peptide-specific proliferative T cells (17,20). In addition, epidemiologic studies have reinforced the association of LSA1 to disease resistance or severity among individuals living in areas where malaria is endemic. Studies of antibody (Ab) and CD4 ϩ or CD8 ϩ T-cell responses measured by antigenic specific cellular proliferation, cytokine production (primarily IFN-␥ and interleukin-10 [IL-10]), and presence of cytotoxic lymphocytes has been demonstrated in residents of various areas where malaria is endemic (6,11,25,35). Although these data should be interpreted cautiously given the differing ethnicities and ages of the subjects and variety of immunologic assays used, it appears that IFN-␥ and IL-10

Impact of Recombinant Adenovirus Serotype 35 Priming versus Boosting of aPlasmodium falciparumProtein: Characterization of T- and B-Cell Responses to Liver-Stage Antigen 1

“…This finding is not surprising, since recombinant protein vaccines formulated with adjuvants have been shown to generally elicit humoral and CD4 responses but poor or no CD8 ϩ T-cell responses (49). It should be mentioned that the number of CD4 ϩ IFN-␥-producing T cells upon immunization with the adjuvanted LSA-1 protein that we measured was lower than found in the previous studies (6,24). This discrepancy might be explained by the use of different adjuvants for the formulation of the LSA-1 protein and/or the use of protein instead of 15-mer peptides for the stimulation of splenocytes in the ELISPOT assay.…”

“…The T-cell epitopes identified in our study have been described earlier for the LSA-1 antigen in other studies (5,6,40,52).…”

“…Identification of CD4 ϩ and CD8 ϩ T-cell epitopes was performed for BALB/c (H-2 d ) mice 2 weeks after immunization with rAd35.LSA-1 and using 15-mer peptides arranged in a matrix as described previously (6,41). The different steps involved in the epitope mapping are depicted in Fig.…”

“…Recently, an adjuvanted recombinant LSA-1 protein vaccine was tested in a phase I/IIa clinical trial. Although the preclinical studies with mice demonstrated that this protein vaccine induces a potent antibody response and a detectable CD4 ϩ T-cell response (6,24), no protection against experimental challenge was observed for LSA-1-vaccinated volunteers (16). A reason for this failure to protect may be the poor induction of CD8 ϩ T-cell responses, a key factor for protection against preerythrocytic malaria.…”

“…In the current study, we aimed to determine whether the low cellular immunity induced with the recombinant LSA-1 protein (6,24) can be complemented with an rAd35 vector carrying the fully matching LSA-1 sequence, either as a priming or as a boosting vaccine.…”