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

Abstract

؉ 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. Malaria infection with Plasmodium falciparum causes more than 600,000 deaths annually as well as significant morbidity worldwide (1). A range of efforts to control and treat malaria include public health measures such as insecticide-treated bed nets, indoor residual spraying, and widespread usage of antimalarial drugs. Despite the impact of these approaches, the most cost-effective solution to prevent infection and to ultimately control the malaria endemic is to develop a vaccine.Currently, the most advanced vaccine tested in humans against P. falciparum infection is RTS,S, which targets the circumsporozoite (CS) protein (CSP), the major and most abundant antigen expressed on the surfaces of infectious sporozoites. RTS,S given with the AS01 adjuvant (RTS,S/AS01) shows ϳ30% protection against clinical disease and severe malaria (2, 3). Thus, while these first results in phase III trials with RTS,S/AS01 are encouraging, there may be additional approaches for further optimizing the breadth, potency, and duration of immunity against the CSP using different immunogens or more-potent adjuvants. In terms of antigen design, RTS,S is comprised of a truncated form of CSP containing the central repeat region, NANP, which is a target for antibody-mediated neutralization, as well as CD8 ϩ and CD4 ϩ T cell epitopes at the C-terminal end. This truncated CS protein is then fused to the hepatitis B virus surface antigen, creating an immunogenic particle. Therefore, using a more-full-length CSP, including the N-terminal end and the R1 region of CSP as well as the minor repeat region (NVDP), might favor broader antibody responses than against the NANP repeat region alone (4-8). Moreover, a full-length CSP may provide additional T cell epitopes, leading to increased breadth of cellular immunity, which could also enhance protection. Another approach is to enhance the humoral and cellular immune responses by altering the type of adjuvant given with the full-length-CSP-based protein vaccine.Early studies in mice showed that protection was associated with high antibody titers (9-11). The next generation of malaria vaccines combined CSP with more-potent adjuvants, like Pseu-

Reversible Conformational Change in the Plasmodium falciparum Circumsporozoite Protein Masks Its Adhesion Domains

“…S2D in the supplemental material). These differences in immunogenicity between EcCSP and PpCSP are in agreement with the findings presented in a previous report on CSP immunogenicity (20).…”

Vaccines to combat river blindness: expression, selection and formulation of vaccines against infection with Onchocerca volvulus in a mouse model

“…Yet, mice immunized with P. pastoris expressed Ov -103 had a statistically significant reduction in parasite survival but not with E. coli expressed antigen; Ov -RAL-2 induced a statistically significant reduction in parasite survival only if produced by E. coli ; and Ov -CPI-2M produced in either Pichia or E. coli induced protective immunity. In other studies, no differences were observed in the immune responses induced by E. coli and Pichia expressed antigens (Giersing et al, 2005), whereas some antigens were more immunogenic if produced in E. coli compared with Pichia (Kastenmuller et al, 2013) and yet other antigens were more antigenic if produced in Pichia compared with E. coli (Yang et al, 2012). …”

Transgenic Parasites Stably Expressing Full-Length Plasmodium falciparum Circumsporozoite Protein as a Model for Vaccine Down-Selection in Mice Using Sterile Protection as an Endpoint

“…Additionally, it is noteworthy that several mice with low repeat-specific antibody titers were protected in the present study, as is often observed with RTS,S-immunized humans. Although cellular immune responses to CSP were not measured, T-cell epitopes in the N-and C-terminal regions of CSP might have contributed to protection (6,19), as observed by others using CSP-transgenic parasite models (11,23).…”

“…Several reports suggest that transgenic rodent parasites expressing the P. falciparum CSP gene are viable and infective in mice. One such parasite, in which the central repeat region of Plasmodium berghei CSP was exchanged with that of P. falciparum, has been used recently to evaluate the protective efficacy of P. falciparum CSP vaccines in mice (10,11). However, such parasites provide no information on the protective role of the Nand C-terminal epitopes of CSP.…”